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diff --git a/doc/arm/reference.rst b/doc/arm/reference.rst new file mode 100644 index 0000000..4bb477a --- /dev/null +++ b/doc/arm/reference.rst @@ -0,0 +1,7057 @@ +.. Copyright (C) Internet Systems Consortium, Inc. ("ISC") +.. +.. SPDX-License-Identifier: MPL-2.0 +.. +.. This Source Code Form is subject to the terms of the Mozilla Public +.. License, v. 2.0. If a copy of the MPL was not distributed with this +.. file, you can obtain one at https://mozilla.org/MPL/2.0/. +.. +.. See the COPYRIGHT file distributed with this work for additional +.. information regarding copyright ownership. + +.. Reference: + +BIND 9 Configuration Reference +============================== + +.. _configuration_file_elements: + +Configuration File Elements +--------------------------- + +Following is a list of elements used throughout the BIND configuration +file documentation: + +.. glossary:: + + ``acl_name`` + The name of an ``address_match_list`` as defined by the ``acl`` statement. + + ``address_match_list`` + A list of one or more ``ip_addr``, ``ip_prefix``, ``key_id``, or ``acl_name`` elements; see :ref:`address_match_lists`. + + ``remoteserver_list`` + A named list of one or more ``ip_addr`` with optional ``key_id`` and/or ``ip_port``. A ``remoteserver_list`` may include other ``remoteserver_list``. + + ``domain_name`` + A quoted string which is used as a DNS name; for example. ``my.test.domain``. + + ``namelist`` + A list of one or more ``domain_name`` elements. + + ``dotted_decimal`` + One to four integers valued 0 through 255 separated by dots (``.``), such as ``123.45.67`` or ``89.123.45.67``. + + ``ip4_addr`` + An IPv4 address with exactly four elements in ``dotted_decimal`` notation. + + ``ip6_addr`` + An IPv6 address, such as ``2001:db8::1234``. IPv6-scoped addresses that have ambiguity on their scope zones must be disambiguated by an appropriate zone ID with the percent character (``%``) as a delimiter. It is strongly recommended to use string zone names rather than numeric identifiers, to be robust against system configuration changes. However, since there is no standard mapping for such names and identifier values, only interface names as link identifiers are supported, assuming one-to-one mapping between interfaces and links. For example, a link-local address ``fe80::1`` on the link attached to the interface ``ne0`` can be specified as ``fe80::1%ne0``. Note that on most systems link-local addresses always have ambiguity and need to be disambiguated. + + ``ip_addr`` + An ``ip4_addr`` or ``ip6_addr``. + + ``ip_dscp`` + A ``number`` between 0 and 63, used to select a differentiated services code point (DSCP) value for use with outgoing traffic on operating systems that support DSCP. + + ``ip_port`` + An IP port ``number``. The ``number`` is limited to 0 through 65535, with values below 1024 typically restricted to use by processes running as root. In some cases, an asterisk (``*``) character can be used as a placeholder to select a random high-numbered port. + + ``ip_prefix`` + An IP network specified as an ``ip_addr``, followed by a slash (``/``) and then the number of bits in the netmask. Trailing zeros in an``ip_addr`` may be omitted. For example, ``127/8`` is the network ``127.0.0.0``with netmask ``255.0.0.0`` and ``1.2.3.0/28`` is network ``1.2.3.0`` with netmask ``255.255.255.240``. + When specifying a prefix involving a IPv6-scoped address, the scope may be omitted. In that case, the prefix matches packets from any scope. + + ``key_id`` + A ``domain_name`` representing the name of a shared key, to be used for transaction security. + + ``key_list`` + A list of one or more ``key_id``, separated by semicolons and ending with a semicolon. + + ``number`` + A non-negative 32-bit integer (i.e., a number between 0 and 4294967295, inclusive). Its acceptable value might be further limited by the context in which it is used. + + ``fixedpoint`` + A non-negative real number that can be specified to the nearest one-hundredth. Up to five digits can be specified before a decimal point, and up to two digits after, so the maximum value is 99999.99. Acceptable values might be further limited by the contexts in which they are used. + + ``path_name`` + A quoted string which is used as a pathname, such as ``zones/master/my.test.domain``. + + ``port_list`` + A list of an ``ip_port`` or a port range. A port range is specified in the form of ``range`` followed by two ``ip_port``s, ``port_low`` and ``port_high``, which represents port numbers from ``port_low`` through ``port_high``, inclusive. ``port_low`` must not be larger than ``port_high``. For example, ``range 1024 65535`` represents ports from 1024 through 65535. In either case an asterisk (``*``) character is not allowed as a valid ``ip_port``. + + ``size_spec`` + A 64-bit unsigned integer, or the keywords ``unlimited`` or ``default``. Integers may take values 0 <= value <= 18446744073709551615, though certain parameters (such as ``max-journal-size``) may use a more limited range within these extremes. In most cases, setting a value to 0 does not literally mean zero; it means "undefined" or "as big as possible," depending on the context. See the explanations of particular parameters that use ``size_spec`` for details on how they interpret its use. Numeric values can optionally be followed by a scaling factor: ``K`` or ``k`` for kilobytes, ``M`` or ``m`` for megabytes, and ``G`` or ``g`` for gigabytes, which scale by 1024, 1024*1024, and 1024*1024*1024 respectively. + ``unlimited`` generally means "as big as possible," and is usually the best way to safely set a very large number. + ``default`` uses the limit that was in force when the server was started. + + ``size_or_percent`` + A ``size_spec`` or integer value followed by ``%`` to represent percent. The behavior is exactly the same as ``size_spec``, but ``size_or_percent`` also allows specifying a positive integer value followed by the ``%`` sign to represent percent. + + ``yes_or_no`` + Either ``yes`` or ``no``. The words ``true`` and ``false`` are also accepted, as are the numbers ``1`` and ``0``. + + ``dialup_option`` + One of ``yes``, ``no``, ``notify``, ``notify-passive``, ``refresh``, or ``passive``. When used in a zone, ``notify-passive``, ``refresh``, and ``passive`` are restricted to secondary and stub zones. + + ``duration`` + A duration in BIND 9 can be written in three ways: as single number + representing seconds, as a string of numbers with TTL-style + time-unit suffixes, or in ISO 6801 duration format. + + Allowed TTL time-unit suffixes are: "W" (week), "D" (day), "H" (hour), + "M" (minute), and "S" (second). Examples: "1W" (1 week), "3d12h" + (3 days, 12 hours). + + ISO 8601 duration format consists of the letter "P", followed by an + optional series of numbers with unit suffixes "Y" (year), "M" (month), + "W" (week), and "D" (day); this may optionally be followed by the + letter "T", and another series of numbers with unit suffixes + "H" (hour), "M" (minute), and "S" (second). Examples: "P3M10D" + (3 months, 10 days), "P2WT12H" (2 weeks, 12 hours), "pt15m" + (15 minutes). For more information on ISO 8601 duration format, + see :rfc:`3339`, appendix A. + + Both TTL-style and ISO 8601 duration formats are case-insensitive. + +.. _address_match_lists: + +Address Match Lists +~~~~~~~~~~~~~~~~~~~ + +Syntax +^^^^^^ + +:: + + address_match_list = address_match_list_element ; ... + + address_match_list_element = [ ! ] ( ip_address | ip_prefix | + key key_id | acl_name | { address_match_list } ) + +Definition and Usage +^^^^^^^^^^^^^^^^^^^^ + +Address match lists are primarily used to determine access control for +various server operations. They are also used in the ``listen-on`` and +``sortlist`` statements. The elements which constitute an address match +list can be any of the following: + +- an IP address (IPv4 or IPv6) + +- an IP prefix (in ``/`` notation) + +- a key ID, as defined by the ``key`` statement + +- the name of an address match list defined with the ``acl`` statement + +- a nested address match list enclosed in braces + +Elements can be negated with a leading exclamation mark (``!``), and the +match list names "any", "none", "localhost", and "localnets" are +predefined. More information on those names can be found in the +description of the ``acl`` statement. + +The addition of the key clause made the name of this syntactic element +something of a misnomer, since security keys can be used to validate +access without regard to a host or network address. Nonetheless, the +term "address match list" is still used throughout the documentation. + +When a given IP address or prefix is compared to an address match list, +the comparison takes place in approximately O(1) time. However, key +comparisons require that the list of keys be traversed until a matching +key is found, and therefore may be somewhat slower. + +The interpretation of a match depends on whether the list is being used +for access control, defining ``listen-on`` ports, or in a ``sortlist``, +and whether the element was negated. + +When used as an access control list, a non-negated match allows access +and a negated match denies access. If there is no match, access is +denied. The clauses ``allow-notify``, ``allow-recursion``, +``allow-recursion-on``, ``allow-query``, ``allow-query-on``, +``allow-query-cache``, ``allow-query-cache-on``, ``allow-transfer``, +``allow-update``, ``allow-update-forwarding``, ``blackhole``, and +``keep-response-order`` all use address match lists. Similarly, the +``listen-on`` option causes the server to refuse queries on any of +the machine's addresses which do not match the list. + +Order of insertion is significant. If more than one element in an ACL is +found to match a given IP address or prefix, preference is given to +the one that came *first* in the ACL definition. Because of this +first-match behavior, an element that defines a subset of another +element in the list should come before the broader element, regardless +of whether either is negated. For example, in ``1.2.3/24; ! 1.2.3.13;`` +the 1.2.3.13 element is completely useless because the algorithm +matches any lookup for 1.2.3.13 to the 1.2.3/24 element. Using +``! 1.2.3.13; 1.2.3/24`` fixes that problem by blocking 1.2.3.13 +via the negation, but all other 1.2.3.\* hosts pass through. + +.. _comment_syntax: + +Comment Syntax +~~~~~~~~~~~~~~ + +The BIND 9 comment syntax allows comments to appear anywhere that +whitespace may appear in a BIND configuration file. To appeal to +programmers of all kinds, they can be written in the C, C++, or +shell/perl style. + +Syntax +^^^^^^ + +:: + + /* This is a BIND comment as in C */ + +:: + + // This is a BIND comment as in C++ + +:: + + # This is a BIND comment as in common Unix shells + # and perl + +Definition and Usage +^^^^^^^^^^^^^^^^^^^^ + +Comments may appear anywhere that whitespace may appear in a BIND +configuration file. + +C-style comments start with the two characters /\* (slash, star) and end +with \*/ (star, slash). Because they are completely delimited with these +characters, they can be used to comment only a portion of a line or to +span multiple lines. + +C-style comments cannot be nested. For example, the following is not +valid because the entire comment ends with the first \*/: + +:: + + /* This is the start of a comment. + This is still part of the comment. + /* This is an incorrect attempt at nesting a comment. */ + This is no longer in any comment. */ + +C++-style comments start with the two characters // (slash, slash) and +continue to the end of the physical line. They cannot be continued +across multiple physical lines; to have one logical comment span +multiple lines, each line must use the // pair. For example: + +:: + + // This is the start of a comment. The next line + // is a new comment, even though it is logically + // part of the previous comment. + +Shell-style (or perl-style) comments start with the +character ``#`` (number sign) and continue to the end of the physical +line, as in C++ comments. For example: + +:: + + # This is the start of a comment. The next line + # is a new comment, even though it is logically + # part of the previous comment. + +.. + +.. warning:: + + The semicolon (``;``) character cannot start a comment, unlike + in a zone file. The semicolon indicates the end of a + configuration statement. + +.. _Configuration_File_Grammar: + +Configuration File Grammar +-------------------------- + +A BIND 9 configuration consists of statements and comments. Statements +end with a semicolon; statements and comments are the only elements that +can appear without enclosing braces. Many statements contain a block of +sub-statements, which are also terminated with a semicolon. + +The following statements are supported: + + ``acl`` + Defines a named IP address matching list, for access control and other uses. + + ``controls`` + Declares control channels to be used by the ``rndc`` utility. + + ``dnssec-policy`` + Describes a DNSSEC key and signing policy for zones. See :ref:`dnssec-policy Grammar <dnssec_policy_grammar>` for details. + + ``include`` + Includes a file. + + ``key`` + Specifies key information for use in authentication and authorization using TSIG. + + ``logging`` + Specifies what information the server logs and where the log messages are sent. + + ``masters`` + Synonym for ``primaries``. + + ``options`` + Controls global server configuration options and sets defaults for other statements. + + ``parental-agents`` + Defines a named list of servers for inclusion in primary and secondary zones' ``parental-agents`` lists. + + ``primaries`` + Defines a named list of servers for inclusion in stub and secondary zones' ``primaries`` or ``also-notify`` lists. (Note: this is a synonym for the original keyword ``masters``, which can still be used, but is no longer the preferred terminology.) + + ``server`` + Sets certain configuration options on a per-server basis. + + ``statistics-channels`` + Declares communication channels to get access to ``named`` statistics. + + ``trust-anchors`` + Defines DNSSEC trust anchors: if used with the ``initial-key`` or ``initial-ds`` keyword, trust anchors are kept up-to-date using :rfc:`5011` trust anchor maintenance; if used with ``static-key`` or ``static-ds``, keys are permanent. + + ``managed-keys`` + Is identical to ``trust-anchors``; this option is deprecated in favor of ``trust-anchors`` with the ``initial-key`` keyword, and may be removed in a future release. + + ``trusted-keys`` + Defines permanent trusted DNSSEC keys; this option is deprecated in favor of ``trust-anchors`` with the ``static-key`` keyword, and may be removed in a future release. + + ``view`` + Defines a view. + + ``zone`` + Defines a zone. + +The ``logging`` and ``options`` statements may only occur once per +configuration. + +.. _acl_grammar: + +``acl`` Statement Grammar +~~~~~~~~~~~~~~~~~~~~~~~~~ + +.. include:: ../misc/acl.grammar.rst + +.. _acl: + +``acl`` Statement Definition and Usage +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The ``acl`` statement assigns a symbolic name to an address match list. +It gets its name from one of the primary uses of address match lists: Access +Control Lists (ACLs). + +The following ACLs are built-in: + + ``any`` + Matches all hosts. + + ``none`` + Matches no hosts. + + ``localhost`` + Matches the IPv4 and IPv6 addresses of all network interfaces on the system. When addresses are added or removed, the ``localhost`` ACL element is updated to reflect the changes. + + ``localnets`` + Matches any host on an IPv4 or IPv6 network for which the system has an interface. When addresses are added or removed, the ``localnets`` ACL element is updated to reflect the changes. Some systems do not provide a way to determine the prefix lengths of local IPv6 addresses; in such cases, ``localnets`` only matches the local IPv6 addresses, just like ``localhost``. + +.. _controls_grammar: + +``controls`` Statement Grammar +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +.. include:: ../misc/controls.grammar.rst + +.. _controls_statement_definition_and_usage: + +``controls`` Statement Definition and Usage +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The ``controls`` statement declares control channels to be used by +system administrators to manage the operation of the name server. These +control channels are used by the ``rndc`` utility to send commands to +and retrieve non-DNS results from a name server. + +An ``inet`` control channel is a TCP socket listening at the specified +``ip_port`` on the specified ``ip_addr``, which can be an IPv4 or IPv6 +address. An ``ip_addr`` of ``*`` (asterisk) is interpreted as the IPv4 +wildcard address; connections are accepted on any of the system's +IPv4 addresses. To listen on the IPv6 wildcard address, use an +``ip_addr`` of ``::``. If ``rndc`` is only used on the local host, +using the loopback address (``127.0.0.1`` or ``::1``) is recommended for +maximum security. + +If no port is specified, port 953 is used. The asterisk ``*`` cannot +be used for ``ip_port``. + +The ability to issue commands over the control channel is restricted by +the ``allow`` and ``keys`` clauses. Connections to the control channel +are permitted based on the ``address_match_list``. This is for simple IP +address-based filtering only; any ``key_id`` elements of the +``address_match_list`` are ignored. + +A ``unix`` control channel is a Unix domain socket listening at the +specified path in the file system. Access to the socket is specified by +the ``perm``, ``owner``, and ``group`` clauses. Note that on some platforms +(SunOS and Solaris), the permissions (``perm``) are applied to the parent +directory as the permissions on the socket itself are ignored. + +The primary authorization mechanism of the command channel is the +``key_list``, which contains a list of ``key_id``s. Each ``key_id`` in +the ``key_list`` is authorized to execute commands over the control +channel. See :ref:`admin_tools` for information about +configuring keys in ``rndc``. + +If the ``read-only`` clause is enabled, the control channel is limited +to the following set of read-only commands: ``nta -dump``, ``null``, +``status``, ``showzone``, ``testgen``, and ``zonestatus``. By default, +``read-only`` is not enabled and the control channel allows read-write +access. + +If no ``controls`` statement is present, ``named`` sets up a default +control channel listening on the loopback address 127.0.0.1 and its IPv6 +counterpart, ::1. In this case, and also when the ``controls`` statement +is present but does not have a ``keys`` clause, ``named`` attempts +to load the command channel key from the file ``rndc.key`` in ``/etc`` +(or whatever ``sysconfdir`` was specified when BIND was built). To +create an ``rndc.key`` file, run ``rndc-confgen -a``. + +To disable the command channel, use an empty ``controls`` statement: +``controls { };``. + +.. _include_grammar: + +``include`` Statement Grammar +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +:: + + include filename; + +.. _include_statement: + +``include`` Statement Definition and Usage +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The ``include`` statement inserts the specified file at the point where the +``include`` statement is encountered. The ``include`` statement facilitates +the administration of configuration files by permitting the reading or +writing of some things but not others. For example, the statement could +include private keys that are readable only by the name server. + +.. _key_grammar: + +``key`` Statement Grammar +~~~~~~~~~~~~~~~~~~~~~~~~~ + +.. include:: ../misc/key.grammar.rst + +.. _key_statement: + +``key`` Statement Definition and Usage +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The ``key`` statement defines a shared secret key for use with TSIG (see +:ref:`tsig`) or the command channel (see :ref:`controls_statement_definition_and_usage`). + +The ``key`` statement can occur at the top level of the configuration +file or inside a ``view`` statement. Keys defined in top-level ``key`` +statements can be used in all views. Keys intended for use in a +``controls`` statement (see :ref:`controls_statement_definition_and_usage`) +must be defined at the top level. + +The ``key_id``, also known as the key name, is a domain name that uniquely +identifies the key. It can be used in a ``server`` statement to cause +requests sent to that server to be signed with this key, or in address +match lists to verify that incoming requests have been signed with a key +matching this name, algorithm, and secret. + +The ``algorithm_id`` is a string that specifies a security/authentication +algorithm. The ``named`` server supports ``hmac-md5``, ``hmac-sha1``, +``hmac-sha224``, ``hmac-sha256``, ``hmac-sha384``, and ``hmac-sha512`` +TSIG authentication. Truncated hashes are supported by appending the +minimum number of required bits preceded by a dash, e.g., +``hmac-sha1-80``. The ``secret_string`` is the secret to be used by the +algorithm, and is treated as a Base64-encoded string. + +.. _logging_grammar: + +``logging`` Statement Grammar +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +.. include:: ../misc/logging.grammar.rst + +.. _logging_statement: + +``logging`` Statement Definition and Usage +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The ``logging`` statement configures a wide variety of logging options +for the name server. Its ``channel`` phrase associates output methods, +format options, and severity levels with a name that can then be used +with the ``category`` phrase to select how various classes of messages +are logged. + +Only one ``logging`` statement is used to define as many channels and +categories as desired. If there is no ``logging`` statement, the +logging configuration is: + +:: + + logging { + category default { default_syslog; default_debug; }; + category unmatched { null; }; + }; + +If ``named`` is started with the ``-L`` option, it logs to the specified +file at startup, instead of using syslog. In this case the logging +configuration is: + +:: + + logging { + category default { default_logfile; default_debug; }; + category unmatched { null; }; + }; + +The logging configuration is only established when the entire +configuration file has been parsed. When the server starts up, all +logging messages regarding syntax errors in the configuration file go to +the default channels, or to standard error if the ``-g`` option was +specified. + +.. _channel: + +The ``channel`` Phrase +^^^^^^^^^^^^^^^^^^^^^^ + +All log output goes to one or more ``channels``; there is no limit to +the number of channels that can be created. + +Every channel definition must include a destination clause that says +whether messages selected for the channel go to a file, go to a particular +syslog facility, go to the standard error stream, or are discarded. The definition can +optionally also limit the message severity level that is accepted +by the channel (the default is ``info``), and whether to include a +``named``-generated time stamp, the category name, and/or the severity level +(the default is not to include any). + +The ``null`` destination clause causes all messages sent to the channel +to be discarded; in that case, other options for the channel are +meaningless. + +The ``file`` destination clause directs the channel to a disk file. It +can include additional arguments to specify how large the file is +allowed to become before it is rolled to a backup file (``size``), how +many backup versions of the file are saved each time this happens +(``versions``), and the format to use for naming backup versions +(``suffix``). + +The ``size`` option is used to limit log file growth. If the file ever +exceeds the specified size, then ``named`` stops writing to the file +unless it has a ``versions`` option associated with it. If backup +versions are kept, the files are rolled as described below. If there is +no ``versions`` option, no more data is written to the log until +some out-of-band mechanism removes or truncates the log to less than the +maximum size. The default behavior is not to limit the size of the file. + +File rolling only occurs when the file exceeds the size specified with +the ``size`` option. No backup versions are kept by default; any +existing log file is simply appended. The ``versions`` option specifies +how many backup versions of the file should be kept. If set to +``unlimited``, there is no limit. + +The ``suffix`` option can be set to either ``increment`` or +``timestamp``. If set to ``timestamp``, then when a log file is rolled, +it is saved with the current timestamp as a file suffix. If set to +``increment``, then backup files are saved with incrementing numbers as +suffixes; older files are renamed when rolling. For example, if +``versions`` is set to 3 and ``suffix`` to ``increment``, then when +``filename.log`` reaches the size specified by ``size``, +``filename.log.1`` is renamed to ``filename.log.2``, ``filename.log.0`` +is renamed to ``filename.log.1``, and ``filename.log`` is renamed to +``filename.log.0``, whereupon a new ``filename.log`` is opened. + +Here is an example using the ``size``, ``versions``, and ``suffix`` options: + +:: + + channel an_example_channel { + file "example.log" versions 3 size 20m suffix increment; + print-time yes; + print-category yes; + }; + +The ``syslog`` destination clause directs the channel to the system log. +Its argument is a syslog facility as described in the ``syslog`` man +page. Known facilities are ``kern``, ``user``, ``mail``, ``daemon``, +``auth``, ``syslog``, ``lpr``, ``news``, ``uucp``, ``cron``, +``authpriv``, ``ftp``, ``local0``, ``local1``, ``local2``, ``local3``, +``local4``, ``local5``, ``local6``, and ``local7``; however, not all +facilities are supported on all operating systems. How ``syslog`` +handles messages sent to this facility is described in the +``syslog.conf`` man page. On a system which uses a very old +version of ``syslog``, which only uses two arguments to the ``openlog()`` +function, this clause is silently ignored. + +On Windows machines, syslog messages are directed to the EventViewer. + +The ``severity`` clause works like ``syslog``'s "priorities," except +that they can also be used when writing straight to a file rather +than using ``syslog``. Messages which are not at least of the severity +level given are not selected for the channel; messages of higher +severity levels are accepted. + +When using ``syslog``, the ``syslog.conf`` priorities +also determine what eventually passes through. For example, defining a +channel facility and severity as ``daemon`` and ``debug``, but only +logging ``daemon.warning`` via ``syslog.conf``, causes messages of +severity ``info`` and ``notice`` to be dropped. If the situation were +reversed, with ``named`` writing messages of only ``warning`` or higher, +then ``syslogd`` would print all messages it received from the channel. + +The ``stderr`` destination clause directs the channel to the server's +standard error stream. This is intended for use when the server is +running as a foreground process, as when debugging a +configuration, for example. + +The server can supply extensive debugging information when it is in +debugging mode. If the server's global debug level is greater than zero, +debugging mode is active. The global debug level is set either +by starting the ``named`` server with the ``-d`` flag followed by a +positive integer, or by running ``rndc trace``. The global debug level +can be set to zero, and debugging mode turned off, by running ``rndc +notrace``. All debugging messages in the server have a debug level; +higher debug levels give more detailed output. Channels that specify a +specific debug severity, for example: + +:: + + channel specific_debug_level { + file "foo"; + severity debug 3; + }; + +get debugging output of level 3 or less any time the server is in +debugging mode, regardless of the global debugging level. Channels with +``dynamic`` severity use the server's global debug level to determine +what messages to print. + +``print-time`` can be set to ``yes``, ``no``, or a time format +specifier, which may be one of ``local``, ``iso8601``, or +``iso8601-utc``. If set to ``no``, the date and time are not +logged. If set to ``yes`` or ``local``, the date and time are logged in +a human-readable format, using the local time zone. If set to +``iso8601``, the local time is logged in ISO 8601 format. If set to +``iso8601-utc``, the date and time are logged in ISO 8601 format, +with time zone set to UTC. The default is ``no``. + +``print-time`` may be specified for a ``syslog`` channel, but it is +usually pointless since ``syslog`` also logs the date and time. + +If ``print-category`` is requested, then the category of the message +is logged as well. Finally, if ``print-severity`` is on, then the +severity level of the message is logged. The ``print-`` options may +be used in any combination, and are always printed in the following +order: time, category, severity. Here is an example where all three +``print-`` options are on: + +``28-Feb-2000 15:05:32.863 general: notice: running`` + +If ``buffered`` has been turned on, the output to files is not +flushed after each log entry. By default all log messages are flushed. + +There are four predefined channels that are used for ``named``'s default +logging, as follows. If ``named`` is started with the ``-L`` option, then a fifth +channel, ``default_logfile``, is added. How they are used is described in +:ref:`the_category_phrase`. + +:: + + channel default_syslog { + // send to syslog's daemon facility + syslog daemon; + // only send priority info and higher + severity info; + }; + + channel default_debug { + // write to named.run in the working directory + // Note: stderr is used instead of "named.run" if + // the server is started with the '-g' option. + file "named.run"; + // log at the server's current debug level + severity dynamic; + }; + + channel default_stderr { + // writes to stderr + stderr; + // only send priority info and higher + severity info; + }; + + channel null { + // toss anything sent to this channel + null; + }; + + channel default_logfile { + // this channel is only present if named is + // started with the -L option, whose argument + // provides the file name + file "..."; + // log at the server's current debug level + severity dynamic; + }; + +The ``default_debug`` channel has the special property that it only +produces output when the server's debug level is non-zero. It normally +writes to a file called ``named.run`` in the server's working directory. + +For security reasons, when the ``-u`` command-line option is used, the +``named.run`` file is created only after ``named`` has changed to the +new UID, and any debug output generated while ``named`` is starting - +and still running as root - is discarded. To capture this +output, run the server with the ``-L`` option to specify a +default logfile, or the ``-g`` option to log to standard error which can +be redirected to a file. + +Once a channel is defined, it cannot be redefined. The +built-in channels cannot be altered directly, but the default logging +can be modified by pointing categories at defined channels. + +.. _the_category_phrase: + +The ``category`` Phrase +^^^^^^^^^^^^^^^^^^^^^^^ + +There are many categories, so desired logs can be sent anywhere +while unwanted logs are ignored. If +a list of channels is not specified for a category, log messages in that +category are sent to the ``default`` category instead. If no +default category is specified, the following "default default" is used: + +:: + + category default { default_syslog; default_debug; }; + +If ``named`` is started with the ``-L`` option, the default category +is: + +:: + + category default { default_logfile; default_debug; }; + +As an example, let's say a user wants to log security events to a file, but +also wants to keep the default logging behavior. They would specify the +following: + +:: + + channel my_security_channel { + file "my_security_file"; + severity info; + }; + category security { + my_security_channel; + default_syslog; + default_debug; + }; + +To discard all messages in a category, specify the ``null`` channel: + +:: + + category xfer-out { null; }; + category notify { null; }; + +The following are the available categories and brief descriptions of the +types of log information they contain. More categories may be added in +future BIND releases. + +.. include:: logging-categories.rst + +.. _query_errors: + +The ``query-errors`` Category +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The ``query-errors`` category is used to indicate why and how specific queries +resulted in responses which indicate an error. Normally, these messages are +logged at ``debug`` logging levels; note, however, that if query logging is +active, some are logged at ``info``. The logging levels are described below: + +At ``debug`` level 1 or higher - or at ``info`` when query logging is +active - each response with the rcode of SERVFAIL is logged as follows: + +``client 127.0.0.1#61502: query failed (SERVFAIL) for www.example.com/IN/AAAA at query.c:3880`` + +This means an error resulting in SERVFAIL was detected at line 3880 of source +file ``query.c``. Log messages of this level are particularly helpful in identifying +the cause of SERVFAIL for an authoritative server. + +At ``debug`` level 2 or higher, detailed context information about recursive +resolutions that resulted in SERVFAIL is logged. The log message looks +like this: + +:: + + fetch completed at resolver.c:2970 for www.example.com/A + in 10.000183: timed out/success [domain:example.com, + referral:2,restart:7,qrysent:8,timeout:5,lame:0,quota:0,neterr:0, + badresp:1,adberr:0,findfail:0,valfail:0] + +The first part before the colon shows that a recursive resolution for +AAAA records of www.example.com completed in 10.000183 seconds, and the +final result that led to the SERVFAIL was determined at line 2970 of +source file ``resolver.c``. + +The next part shows the detected final result and the latest result of +DNSSEC validation. The latter is always "success" when no validation attempt +was made. In this example, this query probably resulted in SERVFAIL because all +name servers are down or unreachable, leading to a timeout in 10 seconds. +DNSSEC validation was probably not attempted. + +The last part, enclosed in square brackets, shows statistics collected for this +particular resolution attempt. The ``domain`` field shows the deepest zone that +the resolver reached; it is the zone where the error was finally detected. The +meaning of the other fields is summarized in the following list. + +``referral`` + The number of referrals the resolver received throughout the resolution process. In the above ``example.com`` there are two. + +``restart`` + The number of cycles that the resolver tried remote servers at the ``domain`` zone. In each cycle, the resolver sends one query (possibly resending it, depending on the response) to each known name server of the ``domain`` zone. + +``qrysent`` + The number of queries the resolver sent at the ``domain`` zone. + +``timeout`` + The number of timeouts the resolver received since the last response. + +``lame`` + The number of lame servers the resolver detected at the ``domain`` zone. A server is detected to be lame either by an invalid response or as a result of lookup in BIND 9's address database (ADB), where lame servers are cached. + +``quota`` + The number of times the resolver was unable to send a query because it had exceeded the permissible fetch quota for a server. + +``neterr`` + The number of erroneous results that the resolver encountered in sending queries at the ``domain`` zone. One common case is when the remote server is unreachable and the resolver receives an "ICMP unreachable" error message. + +``badresp`` + The number of unexpected responses (other than ``lame``) to queries sent by the resolver at the ``domain`` zone. + +``adberr`` + Failures in finding remote server addresses of the``domain`` zone in the ADB. One common case of this is that the remote server's name does not have any address records. + +``findfail`` + Failures to resolve remote server addresses. This is a total number of failures throughout the resolution process. + +``valfail`` + Failures of DNSSEC validation. Validation failures are counted throughout the resolution process (not limited to the ``domain`` zone), but should only happen in ``domain``. + +At ``debug`` level 3 or higher, the same messages as those at +``debug`` level 1 are logged for errors other than +SERVFAIL. Note that negative responses such as NXDOMAIN are not errors, and are +not logged at this debug level. + +At ``debug`` level 4 or higher, the detailed context information logged at +``debug`` level 2 is logged for errors other than SERVFAIL and for negative +responses such as NXDOMAIN. + +.. _parental_agents_grammar: + +``parental-agents`` Statement Grammar +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +.. include:: ../misc/parental-agents.grammar.rst + +.. _parental_agents_statement: + +``parental-agents`` Statement Definition and Usage +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +``parental-agents`` lists allow for a common set of parental agents to be easily +used by multiple primary and secondary zones in their ``parental-agents`` lists. +A parental agent is the entity that the zone has a relationship with to +change its delegation information (defined in :rfc:`7344`). + +.. _primaries_grammar: + +``primaries`` Statement Grammar +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +.. include:: ../misc/primaries.grammar.rst + +.. _primaries_statement: + +``primaries`` Statement Definition and Usage +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +``primaries`` lists allow for a common set of primary servers to be easily +used by multiple stub and secondary zones in their ``primaries`` or +``also-notify`` lists. (Note: ``primaries`` is a synonym for the original +keyword ``masters``, which can still be used, but is no longer the +preferred terminology.) + +.. _options_grammar: + +``options`` Statement Grammar +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +This is the grammar of the ``options`` statement in the ``named.conf`` +file: + +.. include:: ../misc/options.grammar.rst + +.. _options: + +``options`` Statement Definition and Usage +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The ``options`` statement sets up global options to be used by BIND. +This statement may appear only once in a configuration file. If there is +no ``options`` statement, an options block with each option set to its +default is used. + +.. _attach-cache: + +``attach-cache`` + This option allows multiple views to share a single cache database. Each view has + its own cache database by default, but if multiple views have the + same operational policy for name resolution and caching, those views + can share a single cache to save memory, and possibly improve + resolution efficiency, by using this option. + + The ``attach-cache`` option may also be specified in ``view`` + statements, in which case it overrides the global ``attach-cache`` + option. + + The ``cache_name`` specifies the cache to be shared. When the ``named`` + server configures views which are supposed to share a cache, it + creates a cache with the specified name for the first view of these + sharing views. The rest of the views simply refer to the + already-created cache. + + One common configuration to share a cache is to allow all views + to share a single cache. This can be done by specifying + ``attach-cache`` as a global option with an arbitrary name. + + Another possible operation is to allow a subset of all views to share + a cache while the others retain their own caches. For example, if + there are three views A, B, and C, and only A and B should share a + cache, specify the ``attach-cache`` option as a view of A (or B)'s + option, referring to the other view name: + + :: + + view "A" { + // this view has its own cache + ... + }; + view "B" { + // this view refers to A's cache + attach-cache "A"; + }; + view "C" { + // this view has its own cache + ... + }; + + Views that share a cache must have the same policy on configurable + parameters that may affect caching. The current implementation + requires the following configurable options be consistent among these + views: ``check-names``, ``dnssec-accept-expired``, + ``dnssec-validation``, ``max-cache-ttl``, ``max-ncache-ttl``, + ``max-stale-ttl``, ``max-cache-size``, ``min-cache-ttl``, + ``min-ncache-ttl``, and ``zero-no-soa-ttl``. + + Note that there may be other parameters that may cause confusion if + they are inconsistent for different views that share a single cache. + For example, if these views define different sets of forwarders that + can return different answers for the same question, sharing the + answer does not make sense or could even be harmful. It is the + administrator's responsibility to ensure that configuration differences in + different views do not cause disruption with a shared cache. + +``directory`` + This sets the working directory of the server. Any non-absolute pathnames in + the configuration file are taken as relative to this directory. + The default location for most server output files (e.g., + ``named.run``) is this directory. If a directory is not specified, + the working directory defaults to ``"."``, the directory from + which the server was started. The directory specified should be an + absolute path, and *must* be writable by the effective user ID of the + ``named`` process. + + The option takes effect only at the time that the configuration + option is parsed; if other files are being included before or after specifying the + new ``directory``, the ``directory`` option must be listed + before any other directive (like ``include``) that can work with relative + files. The safest way to include files is to use absolute file names. + +``dnstap`` + ``dnstap`` is a fast, flexible method for capturing and logging DNS + traffic. Developed by Robert Edmonds at Farsight Security, Inc., and + supported by multiple DNS implementations, ``dnstap`` uses + ``libfstrm`` (a lightweight high-speed framing library; see + https://github.com/farsightsec/fstrm) to send event payloads which + are encoded using Protocol Buffers (``libprotobuf-c``, a mechanism + for serializing structured data developed by Google, Inc.; see + https://developers.google.com/protocol-buffers/). + + To enable ``dnstap`` at compile time, the ``fstrm`` and + ``protobuf-c`` libraries must be available, and BIND must be + configured with ``--enable-dnstap``. + + The ``dnstap`` option is a bracketed list of message types to be + logged. These may be set differently for each view. Supported types + are ``client``, ``auth``, ``resolver``, ``forwarder``, and + ``update``. Specifying type ``all`` causes all ``dnstap`` + messages to be logged, regardless of type. + + Each type may take an additional argument to indicate whether to log + ``query`` messages or ``response`` messages; if not specified, both + queries and responses are logged. + + Example: To log all authoritative queries and responses, recursive + client responses, and upstream queries sent by the resolver, use: + + :: + + dnstap { + auth; + client response; + resolver query; + }; + + Logged ``dnstap`` messages can be parsed using the ``dnstap-read`` + utility (see :ref:`man_dnstap-read` for details). + + For more information on ``dnstap``, see http://dnstap.info. + + The fstrm library has a number of tunables that are exposed in + ``named.conf``, and can be modified if necessary to improve + performance or prevent loss of data. These are: + + - ``fstrm-set-buffer-hint``: The threshold number of bytes to + accumulate in the output buffer before forcing a buffer flush. The + minimum is 1024, the maximum is 65536, and the default is 8192. + + - ``fstrm-set-flush-timeout``: The number of seconds to allow + unflushed data to remain in the output buffer. The minimum is 1 + second, the maximum is 600 seconds (10 minutes), and the default + is 1 second. + + - ``fstrm-set-output-notify-threshold``: The number of outstanding + queue entries to allow on an input queue before waking the I/O + thread. The minimum is 1 and the default is 32. + + - ``fstrm-set-output-queue-model``: The queuing semantics + to use for queue objects. The default is ``mpsc`` (multiple + producer, single consumer); the other option is ``spsc`` (single + producer, single consumer). + + - ``fstrm-set-input-queue-size``: The number of queue entries to + allocate for each input queue. This value must be a power of 2. + The minimum is 2, the maximum is 16384, and the default is 512. + + - ``fstrm-set-output-queue-size``: The number of queue entries to + allocate for each output queue. The minimum is 2, the maximum is + system-dependent and based on ``IOV_MAX``, and the default is 64. + + - ``fstrm-set-reopen-interval``: The number of seconds to wait + between attempts to reopen a closed output stream. The minimum is + 1 second, the maximum is 600 seconds (10 minutes), and the default + is 5 seconds. For convenience, TTL-style time-unit suffixes may be + used to specify the value. + + Note that all of the above minimum, maximum, and default values are + set by the ``libfstrm`` library, and may be subject to change in + future versions of the library. See the ``libfstrm`` documentation + for more information. + +``dnstap-output`` + This configures the path to which the ``dnstap`` frame stream is sent + if ``dnstap`` is enabled at compile time and active. + + The first argument is either ``file`` or ``unix``, indicating whether + the destination is a file or a Unix domain socket. The second + argument is the path of the file or socket. (Note: when using a + socket, ``dnstap`` messages are only sent if another process such + as ``fstrm_capture`` (provided with ``libfstrm``) is listening on the + socket.) + + If the first argument is ``file``, then up to three additional + options can be added: ``size`` indicates the size to which a + ``dnstap`` log file can grow before being rolled to a new file; + ``versions`` specifies the number of rolled log files to retain; and + ``suffix`` indicates whether to retain rolled log files with an + incrementing counter as the suffix (``increment``) or with the + current timestamp (``timestamp``). These are similar to the ``size``, + ``versions``, and ``suffix`` options in a ``logging`` channel. The + default is to allow ``dnstap`` log files to grow to any size without + rolling. + + ``dnstap-output`` can only be set globally in ``options``. Currently, + it can only be set once while ``named`` is running; once set, it + cannot be changed by ``rndc reload`` or ``rndc reconfig``. + +``dnstap-identity`` + This specifies an ``identity`` string to send in ``dnstap`` messages. If + set to ``hostname``, which is the default, the server's hostname + is sent. If set to ``none``, no identity string is sent. + +``dnstap-version`` + This specifies a ``version`` string to send in ``dnstap`` messages. The + default is the version number of the BIND release. If set to + ``none``, no version string is sent. + +``geoip-directory`` + When ``named`` is compiled using the MaxMind GeoIP2 geolocation API, this + specifies the directory containing GeoIP database files. By default, the + option is set based on the prefix used to build the ``libmaxminddb`` module; + for example, if the library is installed in ``/usr/local/lib``, then the + default ``geoip-directory`` is ``/usr/local/share/GeoIP``. On Windows, + the default is the ``named`` working directory. See :ref:`acl` + for details about ``geoip`` ACLs. + +``key-directory`` + This is the directory where the public and private DNSSEC key files should be + found when performing a dynamic update of secure zones, if different + than the current working directory. (Note that this option has no + effect on the paths for files containing non-DNSSEC keys such as + ``bind.keys``, ``rndc.key``, or ``session.key``.) + +``lmdb-mapsize`` + When ``named`` is built with liblmdb, this option sets a maximum size + for the memory map of the new-zone database (NZD) in LMDB database + format. This database is used to store configuration information for + zones added using ``rndc addzone``. Note that this is not the NZD + database file size, but the largest size that the database may grow + to. + + Because the database file is memory-mapped, its size is limited by + the address space of the ``named`` process. The default of 32 megabytes + was chosen to be usable with 32-bit ``named`` builds. The largest + permitted value is 1 terabyte. Given typical zone configurations + without elaborate ACLs, a 32 MB NZD file ought to be able to hold + configurations of about 100,000 zones. + +``managed-keys-directory`` + This specifies the directory in which to store the files that track managed DNSSEC + keys (i.e., those configured using the ``initial-key`` or ``initial-ds`` + keywords in a ``trust-anchors`` statement). By default, this is the working + directory. The directory *must* be writable by the effective user ID of the + ``named`` process. + + If ``named`` is not configured to use views, managed keys for + the server are tracked in a single file called + ``managed-keys.bind``. Otherwise, managed keys are tracked in + separate files, one file per view; each file name is the view + name (or, if it contains characters that are incompatible with use as + a file name, the SHA256 hash of the view name), followed by the + extension ``.mkeys``. + + (Note: in earlier releases, file names for views always used the + SHA256 hash of the view name. To ensure compatibility after upgrading, + if a file using the old name format is found to exist, it is + used instead of the new format.) + +``max-ixfr-ratio`` + This sets the size threshold (expressed as a percentage of the size + of the full zone) beyond which ``named`` chooses to use an AXFR + response rather than IXFR when answering zone transfer requests. See + :ref:`incremental_zone_transfers`. + + The minimum value is ``1%``. The keyword ``unlimited`` disables ratio + checking and allows IXFRs of any size. The default is ``unlimited``. + +``new-zones-directory`` + This specifies the directory in which to store the configuration + parameters for zones added via ``rndc addzone``. By default, this is + the working directory. If set to a relative path, it is relative + to the working directory. The directory *must* be writable by the + effective user ID of the ``named`` process. + +``qname-minimization`` + When this is set to ``strict``, BIND follows the QNAME + minimization algorithm to the letter, as specified in :rfc:`7816`. + + Setting this option to ``relaxed`` causes BIND to fall back to + normal (non-minimized) query mode when it receives either NXDOMAIN or + other unexpected responses (e.g., SERVFAIL, improper zone cut, + REFUSED) to a minimized query. A resolver can use a leading + underscore, like ``_.example.com``, in an attempt to improve + interoperability. (See :rfc:`7816` section 3.) + + ``disabled`` disables QNAME minimization completely. + ``off`` is a synonym for ``disabled``. + + The current default is ``relaxed``, but it + may be changed to ``strict`` in a future release. + +``tkey-gssapi-keytab`` + This is the KRB5 keytab file to use for GSS-TSIG updates. If this option is + set and tkey-gssapi-credential is not set, updates are + allowed with any key matching a principal in the specified keytab. + +``tkey-gssapi-credential`` + This is the security credential with which the server should authenticate + keys requested by the GSS-TSIG protocol. Currently only Kerberos 5 + authentication is available; the credential is a Kerberos + principal which the server can acquire through the default system key + file, normally ``/etc/krb5.keytab``. The location of the keytab file can be + overridden using the ``tkey-gssapi-keytab`` option. Normally this + principal is of the form ``DNS/server.domain``. To use + GSS-TSIG, ``tkey-domain`` must also be set if a specific keytab is + not set with ``tkey-gssapi-keytab``. + +``tkey-domain`` + This domain is appended to the names of all shared keys generated with + ``TKEY``. When a client requests a ``TKEY`` exchange, it may or may + not specify the desired name for the key. If present, the name of the + shared key is ``client-specified part`` + ``tkey-domain``. + Otherwise, the name of the shared key is ``random hex digits`` + + ``tkey-domain``. In most cases, the ``domainname`` + should be the server's domain name, or an otherwise nonexistent + subdomain like ``_tkey.domainname``. If using GSS-TSIG, + this variable must be defined, unless a specific keytab + is specified using ``tkey-gssapi-keytab``. + +``tkey-dhkey`` + This is the Diffie-Hellman key used by the server to generate shared keys + with clients using the Diffie-Hellman mode of ``TKEY``. The server + must be able to load the public and private keys from files in the + working directory. In most cases, the ``key_name`` should be the + server's host name. + +``cache-file`` + This is for testing only. Do not use. + +``dump-file`` + This is the pathname of the file the server dumps the database to, when + instructed to do so with ``rndc dumpdb``. If not specified, the + default is ``named_dump.db``. + +``memstatistics-file`` + This is the pathname of the file the server writes memory usage statistics to + on exit. If not specified, the default is ``named.memstats``. + +``lock-file`` + This is the pathname of a file on which ``named`` attempts to acquire a + file lock when starting for the first time; if unsuccessful, the + server terminates, under the assumption that another server + is already running. If not specified, the default is + ``none``. + + Specifying ``lock-file none`` disables the use of a lock file. + ``lock-file`` is ignored if ``named`` was run using the ``-X`` + option, which overrides it. Changes to ``lock-file`` are ignored if + ``named`` is being reloaded or reconfigured; it is only effective + when the server is first started. + +``pid-file`` + This is the pathname of the file the server writes its process ID in. If not + specified, the default is ``/var/run/named/named.pid``. The PID file + is used by programs that send signals to the running name + server. Specifying ``pid-file none`` disables the use of a PID file; + no file is written and any existing one is removed. Note + that ``none`` is a keyword, not a filename, and therefore is not + enclosed in double quotes. + +``recursing-file`` + This is the pathname of the file where the server dumps the queries that are + currently recursing, when instructed to do so with ``rndc recursing``. + If not specified, the default is ``named.recursing``. + +``statistics-file`` + This is the pathname of the file the server appends statistics to, when + instructed to do so using ``rndc stats``. If not specified, the + default is ``named.stats`` in the server's current directory. The + format of the file is described in :ref:`statsfile`. + +``bindkeys-file`` + This is the pathname of a file to override the built-in trusted keys provided + by ``named``. See the discussion of ``dnssec-validation`` for + details. If not specified, the default is ``/etc/bind.keys``. + +``secroots-file`` + This is the pathname of the file the server dumps security roots to, when + instructed to do so with ``rndc secroots``. If not specified, the + default is ``named.secroots``. + +``session-keyfile`` + This is the pathname of the file into which to write a TSIG session key + generated by ``named`` for use by ``nsupdate -l``. If not specified, + the default is ``/var/run/named/session.key``. (See :ref:`dynamic_update_policies`, + and in particular the discussion of the ``update-policy`` statement's + ``local`` option, for more information about this feature.) + +``session-keyname`` + This is the key name to use for the TSIG session key. If not specified, the + default is ``local-ddns``. + +``session-keyalg`` + This is the algorithm to use for the TSIG session key. Valid values are + hmac-sha1, hmac-sha224, hmac-sha256, hmac-sha384, hmac-sha512, and + hmac-md5. If not specified, the default is hmac-sha256. + +``port`` + This is the UDP/TCP port number the server uses to receive and send DNS + protocol traffic. The default is 53. This option is mainly intended + for server testing; a server using a port other than 53 is not + able to communicate with the global DNS. + +``dscp`` + This is the global Differentiated Services Code Point (DSCP) value to + classify outgoing DNS traffic, on operating systems that support DSCP. + Valid values are 0 through 63. It is not configured by default. + +``random-device`` + This specifies a source of entropy to be used by the server; it is a + device or file from which to read entropy. If it is a file, + operations requiring entropy will fail when the file has been + exhausted. + + Entropy is needed for cryptographic operations such as TKEY + transactions, dynamic update of signed zones, and generation of TSIG + session keys. It is also used for seeding and stirring the + pseudo-random number generator which is used for less critical + functions requiring randomness, such as generation of DNS message + transaction IDs. + + If ``random-device`` is not specified, or if it is set to ``none``, + entropy is read from the random number generation function + supplied by the cryptographic library with which BIND was linked + (i.e. OpenSSL or a PKCS#11 provider). + + The ``random-device`` option takes effect during the initial + configuration load at server startup time and is ignored on + subsequent reloads. + +``preferred-glue`` + If specified, the listed type (A or AAAA) is emitted before + other glue in the additional section of a query response. The default + is to prefer A records when responding to queries that arrived via + IPv4 and AAAA when responding to queries that arrived via IPv6. + +.. _root-delegation-only: + +``root-delegation-only`` + This turns on enforcement of delegation-only in TLDs (top-level domains) + and root zones with an optional exclude list. + + DS queries are expected to be made to and be answered by delegation-only + zones. Such queries and responses are treated as an exception to + delegation-only processing and are not converted to NXDOMAIN + responses, provided a CNAME is not discovered at the query name. + + If a delegation-only zone server also serves a child zone, it is not + always possible to determine whether an answer comes from the + delegation-only zone or the child zone. SOA NS and DNSKEY records are + apex-only records and a matching response that contains these records + or DS is treated as coming from a child zone. RRSIG records are also + examined to see whether they are signed by a child zone, and the + authority section is examined to see if there is evidence that + the answer is from the child zone. Answers that are determined to be + from a child zone are not converted to NXDOMAIN responses. Despite + all these checks, there is still a possibility of false negatives when + a child zone is being served. + + Similarly, false positives can arise from empty nodes (no records at + the name) in the delegation-only zone when the query type is not ``ANY``. + + Note that some TLDs are not delegation-only; e.g., "DE", "LV", "US", and + "MUSEUM". This list is not exhaustive. + + :: + + options { + root-delegation-only exclude { "de"; "lv"; "us"; "museum"; }; + }; + +``disable-algorithms`` + This disables the specified DNSSEC algorithms at and below the specified + name. Multiple ``disable-algorithms`` statements are allowed. Only + the best-match ``disable-algorithms`` clause is used to + determine the algorithms. + + If all supported algorithms are disabled, the zones covered by the + ``disable-algorithms`` setting are treated as insecure. + + Configured trust anchors in ``trust-anchors`` (or ``managed-keys`` or + ``trusted-keys``) that match a disabled algorithm are ignored and treated + as if they were not configured. + +``disable-ds-digests`` + This disables the specified DS digest types at and below the specified + name. Multiple ``disable-ds-digests`` statements are allowed. Only + the best-match ``disable-ds-digests`` clause is used to + determine the digest types. + + If all supported digest types are disabled, the zones covered by + ``disable-ds-digests`` are treated as insecure. + +``dnssec-must-be-secure`` + This specifies hierarchies which must be or may not be secure (signed and + validated). If ``yes``, then ``named`` only accepts answers if + they are secure. If ``no``, then normal DNSSEC validation applies, + allowing insecure answers to be accepted. The specified domain + must be defined as a trust anchor, for instance in a ``trust-anchors`` + statement, or ``dnssec-validation auto`` must be active. + +``dns64`` + This directive instructs ``named`` to return mapped IPv4 addresses to + AAAA queries when there are no AAAA records. It is intended to be + used in conjunction with a NAT64. Each ``dns64`` defines one DNS64 + prefix. Multiple DNS64 prefixes can be defined. + + Compatible IPv6 prefixes have lengths of 32, 40, 48, 56, 64, and 96, per + :rfc:`6052`. Bits 64..71 inclusive must be zero, with the most significant bit + of the prefix in position 0. + + In addition, a reverse IP6.ARPA zone is created for the prefix + to provide a mapping from the IP6.ARPA names to the corresponding + IN-ADDR.ARPA names using synthesized CNAMEs. ``dns64-server`` and + ``dns64-contact`` can be used to specify the name of the server and + contact for the zones. These can be set at the view/options + level but not on a per-prefix basis. + + Each ``dns64`` supports an optional ``clients`` ACL that determines + which clients are affected by this directive. If not defined, it + defaults to ``any;``. + + Each ``dns64`` supports an optional ``mapped`` ACL that selects which + IPv4 addresses are to be mapped in the corresponding A RRset. If not + defined, it defaults to ``any;``. + + Normally, DNS64 does not apply to a domain name that owns one or more + AAAA records; these records are simply returned. The optional + ``exclude`` ACL allows specification of a list of IPv6 addresses that + are ignored if they appear in a domain name's AAAA records; + DNS64 is applied to any A records the domain name owns. If not + defined, ``exclude`` defaults to ::ffff:0.0.0.0/96. + + An optional ``suffix`` can also be defined to set the bits trailing + the mapped IPv4 address bits. By default these bits are set to + ``::``. The bits matching the prefix and mapped IPv4 address must be + zero. + + If ``recursive-only`` is set to ``yes``, the DNS64 synthesis only + happens for recursive queries. The default is ``no``. + + If ``break-dnssec`` is set to ``yes``, the DNS64 synthesis happens + even if the result, if validated, would cause a DNSSEC validation + failure. If this option is set to ``no`` (the default), the DO is set + on the incoming query, and there are RRSIGs on the applicable + records, then synthesis does not happen. + + :: + + acl rfc1918 { 10/8; 192.168/16; 172.16/12; }; + + dns64 64:FF9B::/96 { + clients { any; }; + mapped { !rfc1918; any; }; + exclude { 64:FF9B::/96; ::ffff:0000:0000/96; }; + suffix ::; + }; + +``dnssec-loadkeys-interval`` + When a zone is configured with ``auto-dnssec maintain;``, its key + repository must be checked periodically to see if any new keys have + been added or any existing keys' timing metadata has been updated + (see :ref:`man_dnssec-keygen` and :ref:`man_dnssec-settime`). + The ``dnssec-loadkeys-interval`` option + sets the frequency of automatic repository checks, in minutes. The + default is ``60`` (1 hour), the minimum is ``1`` (1 minute), and + the maximum is ``1440`` (24 hours); any higher value is silently + reduced. + +``dnssec-policy`` + This specifies which key and signing policy (KASP) should be used for this + zone. This is a string referring to a ``dnssec-policy`` statement. There + are three built-in policies: ``default``, which uses the default policy, + ``insecure``, to be used when you want to gracefully unsign your zone, and + ``none``, which means no DNSSEC policy. The default is ``none``. + See :ref:`dnssec-policy Grammar <dnssec_policy_grammar>` for more details. + +``dnssec-update-mode`` + If this option is set to its default value of ``maintain`` in a zone + of type ``primary`` which is DNSSEC-signed and configured to allow + dynamic updates (see :ref:`dynamic_update_policies`), and if ``named`` has access + to the private signing key(s) for the zone, then ``named`` + automatically signs all new or changed records and maintains signatures + for the zone by regenerating RRSIG records whenever they approach + their expiration date. + + If the option is changed to ``no-resign``, then ``named`` signs + all new or changed records, but scheduled maintenance of signatures + is disabled. + + With either of these settings, ``named`` rejects updates to a + DNSSEC-signed zone when the signing keys are inactive or unavailable + to ``named``. (A planned third option, ``external``, will disable all + automatic signing and allow DNSSEC data to be submitted into a zone + via dynamic update; this is not yet implemented.) + +``nta-lifetime`` + This specifies the default lifetime, in seconds, for + negative trust anchors added via ``rndc nta``. + + A negative trust anchor selectively disables DNSSEC validation for + zones that are known to be failing because of misconfiguration, rather + than an attack. When data to be validated is at or below an active + NTA (and above any other configured trust anchors), ``named`` + aborts the DNSSEC validation process and treats the data as insecure + rather than bogus. This continues until the NTA's lifetime has + elapsed. NTAs persist across ``named`` restarts. + + For convenience, TTL-style time-unit suffixes can be used to specify the NTA + lifetime in seconds, minutes, or hours. It also accepts ISO 8601 duration + formats. + + ``nta-lifetime`` defaults to one hour; it cannot exceed one week. + +``nta-recheck`` + This specifies how often to check whether negative trust anchors added via + ``rndc nta`` are still necessary. + + A negative trust anchor is normally used when a domain has stopped + validating due to operator error; it temporarily disables DNSSEC + validation for that domain. In the interest of ensuring that DNSSEC + validation is turned back on as soon as possible, ``named`` + periodically sends a query to the domain, ignoring negative trust + anchors, to find out whether it can now be validated. If so, the + negative trust anchor is allowed to expire early. + + Validity checks can be disabled for an individual NTA by using + ``rndc nta -f``, or for all NTAs by setting ``nta-recheck`` to zero. + + For convenience, TTL-style time-unit suffixes can be used to specify the NTA + recheck interval in seconds, minutes, or hours. It also accepts ISO 8601 + duration formats. + + The default is five minutes. It cannot be longer than ``nta-lifetime``, which + cannot be longer than a week. + +``max-zone-ttl`` + + This should now be configured as part of ``dnssec-policy``. + Use of this option in ``options``, ``view`` and ``zone`` blocks has no + effect on any zone for which a ``dnssec-policy`` has also been configured. + + ``max-zone-ttl`` specifies a maximum permissible TTL value in seconds. + For convenience, TTL-style time-unit suffixes may be used to specify the + maximum value. When a zone file is loaded, any record encountered with a + TTL higher than ``max-zone-ttl`` causes the zone to be rejected. + + This is useful in DNSSEC-signed zones because when rolling to a new + DNSKEY, the old key needs to remain available until RRSIG records + have expired from caches. The ``max-zone-ttl`` option guarantees that + the largest TTL in the zone is no higher than the set value. + + (Note: because ``map``-format files load directly into memory, this + option cannot be used with them.) + + The default value is ``unlimited``. Setting ``max-zone-ttl`` to zero is + equivalent to ``unlimited``. + +``stale-answer-ttl`` + This specifies the TTL to be returned on stale answers. The default is 30 + seconds. The minimum allowed is 1 second; a value of 0 is updated silently + to 1 second. + + For stale answers to be returned, they must be enabled, either in the + configuration file using ``stale-answer-enable`` or via + ``rndc serve-stale on``. + +``serial-update-method`` + Zones configured for dynamic DNS may use this option to set the + update method to be used for the zone serial number in the SOA + record. + + With the default setting of ``serial-update-method increment;``, the + SOA serial number is incremented by one each time the zone is + updated. + + When set to ``serial-update-method unixtime;``, the SOA serial number + is set to the number of seconds since the Unix epoch, unless the + serial number is already greater than or equal to that value, in + which case it is simply incremented by one. + + When set to ``serial-update-method date;``, the new SOA serial number + is the current date in the form "YYYYMMDD", followed by two + zeroes, unless the existing serial number is already greater than or + equal to that value, in which case it is incremented by one. + +``zone-statistics`` + If ``full``, the server collects statistical data on all zones, + unless specifically turned off on a per-zone basis by specifying + ``zone-statistics terse`` or ``zone-statistics none`` in the ``zone`` + statement. The statistical data includes, for example, DNSSEC signing + operations and the number of authoritative answers per query type. The + default is ``terse``, providing minimal statistics on zones + (including name and current serial number, but not query type + counters). + + These statistics may be accessed via the ``statistics-channel`` or + using ``rndc stats``, which dumps them to the file listed in the + ``statistics-file``. See also :ref:`statsfile`. + + For backward compatibility with earlier versions of BIND 9, the + ``zone-statistics`` option can also accept ``yes`` or ``no``; ``yes`` + has the same meaning as ``full``. As of BIND 9.10, ``no`` has the + same meaning as ``none``; previously, it was the same as ``terse``. + +.. _boolean_options: + +Boolean Options +^^^^^^^^^^^^^^^ + +``automatic-interface-scan`` + If ``yes`` and supported by the operating system, this automatically rescans + network interfaces when the interface addresses are added or removed. The + default is ``yes``. This configuration option does not affect the time-based + ``interface-interval`` option; it is recommended to set the time-based + ``interface-interval`` to 0 when the operator confirms that automatic + interface scanning is supported by the operating system. + + The ``automatic-interface-scan`` implementation uses routing sockets for the + network interface discovery; therefore, the operating system must + support the routing sockets for this feature to work. + +``allow-new-zones`` + If ``yes``, then zones can be added at runtime via ``rndc addzone``. + The default is ``no``. + + Newly added zones' configuration parameters are stored so that they + can persist after the server is restarted. The configuration + information is saved in a file called ``viewname.nzf`` (or, if + ``named`` is compiled with liblmdb, in an LMDB database file called + ``viewname.nzd``). "viewname" is the name of the view, unless the view + name contains characters that are incompatible with use as a file + name, in which case a cryptographic hash of the view name is used + instead. + + Configurations for zones added at runtime are stored either in + a new-zone file (NZF) or a new-zone database (NZD), depending on + whether ``named`` was linked with liblmdb at compile time. See + :ref:`man_rndc` for further details about ``rndc addzone``. + +``auth-nxdomain`` + If ``yes``, then the ``AA`` bit is always set on NXDOMAIN responses, + even if the server is not actually authoritative. The default is + ``no``. + +``memstatistics`` + This writes memory statistics to the file specified by + ``memstatistics-file`` at exit. The default is ``no`` unless ``-m + record`` is specified on the command line, in which case it is ``yes``. + +``dialup`` + If ``yes``, then the server treats all zones as if they are doing + zone transfers across a dial-on-demand dialup link, which can be + brought up by traffic originating from this server. Although this setting has + different effects according to zone type, it concentrates the zone + maintenance so that everything happens quickly, once every + ``heartbeat-interval``, ideally during a single call. It also + suppresses some normal zone maintenance traffic. The default + is ``no``. + + If specified in the ``view`` and + ``zone`` statements, the ``dialup`` option overrides the global ``dialup`` + option. + + If the zone is a primary zone, the server sends out a NOTIFY + request to all the secondaries (default). This should trigger the zone + serial number check in the secondary (providing it supports NOTIFY), + allowing the secondary to verify the zone while the connection is active. + The set of servers to which NOTIFY is sent can be controlled by + ``notify`` and ``also-notify``. + + If the zone is a secondary or stub zone, the server suppresses + the regular "zone up to date" (refresh) queries and only performs them + when the ``heartbeat-interval`` expires, in addition to sending NOTIFY + requests. + + Finer control can be achieved by using ``notify``, which only sends + NOTIFY messages; ``notify-passive``, which sends NOTIFY messages and + suppresses the normal refresh queries; ``refresh``, which suppresses + normal refresh processing and sends refresh queries when the + ``heartbeat-interval`` expires; and ``passive``, which disables + normal refresh processing. + + +--------------------+-----------------+-----------------+-----------------+ + | dialup mode | normal refresh | heart-beat | heart-beat | + | | | refresh | notify | + +--------------------+-----------------+-----------------+-----------------+ + | ``no`` | yes | no | no | + | (default) | | | | + +--------------------+-----------------+-----------------+-----------------+ + | ``yes`` | no | yes | yes | + +--------------------+-----------------+-----------------+-----------------+ + | ``notify`` | yes | no | yes | + +--------------------+-----------------+-----------------+-----------------+ + | ``refresh`` | no | yes | no | + +--------------------+-----------------+-----------------+-----------------+ + | ``passive`` | no | no | no | + +--------------------+-----------------+-----------------+-----------------+ + | ``notify-passive`` | no | no | yes | + +--------------------+-----------------+-----------------+-----------------+ + + Note that normal NOTIFY processing is not affected by ``dialup``. + +``flush-zones-on-shutdown`` + When the name server exits upon receiving SIGTERM, flush or do not + flush any pending zone writes. The default is + ``flush-zones-on-shutdown no``. + +``geoip-use-ecs`` + This option was part of an experimental implementation of the EDNS + CLIENT-SUBNET for authoritative servers, but is now obsolete. + +``root-key-sentinel`` + If ``yes``, respond to root key sentinel probes as described in + draft-ietf-dnsop-kskroll-sentinel-08. The default is ``yes``. + +``reuseport`` + This option enables kernel load-balancing of sockets on systems which support + it, including Linux (SO_REUSEPORT) and FreeBSD (SO_REUSEPORT_LB). This + instructs the kernel to distribute incoming socket connections among the + networking threads based on a hashing scheme. For more information, see the + receive network flow classification options (``rx-flow-hash``) section in the + ``ethtool`` manual page. The default is ``yes``. + + Enabling ``reuseport`` significantly increases general throughput when + incoming traffic is distributed uniformly onto the threads by the + operating system. However, in cases where a worker thread is busy with a + long-lasting operation, such as processing a Response Policy Zone (RPZ) or + Catalog Zone update or an unusually large zone transfer, incoming traffic + that hashes onto that thread may be delayed. On servers where these events + occur frequently, it may be preferable to disable socket load-balancing so + that other threads can pick up the traffic that would have been sent to the + busy thread. + + Note: this option can only be set when ``named`` first starts. + Changes will not take effect during reconfiguration; the server + must be restarted. + +``message-compression`` + If ``yes``, DNS name compression is used in responses to regular + queries (not including AXFR or IXFR, which always use compression). + Setting this option to ``no`` reduces CPU usage on servers and may + improve throughput. However, it increases response size, which may + cause more queries to be processed using TCP; a server with + compression disabled is out of compliance with :rfc:`1123` Section + 6.1.3.2. The default is ``yes``. + +``minimal-responses`` + This option controls the addition of records to the authority and + additional sections of responses. Such records may be included in + responses to be helpful to clients; for example, MX records may + have associated address records included in the additional section, + obviating the need for a separate address lookup. However, adding + these records to responses is not mandatory and requires additional + database lookups, causing extra latency when marshalling responses. + + Responses to DNSKEY, DS, CDNSKEY, and CDS requests will never have + optional additional records added. Responses to NS requests will + always have additional section processing. + + ``minimal-responses`` takes one of four values: + + - ``no``: the server is as complete as possible when generating + responses. + - ``yes``: the server only adds records to the authority and additional + sections when such records are required by the DNS protocol (for + example, when returning delegations or negative responses). This + provides the best server performance but may result in more client + queries. + - ``no-auth``: the server omits records from the authority section except + when they are required, but it may still add records to the + additional section. + - ``no-auth-recursive``: the same as ``no-auth`` when recursion is requested + in the query (RD=1), or the same as ``no`` if recursion is not requested. + + ``no-auth`` and ``no-auth-recursive`` are useful when answering stub + clients, which usually ignore the authority section. + ``no-auth-recursive`` is meant for use in mixed-mode servers that + handle both authoritative and recursive queries. + + The default is ``no-auth-recursive``. + +``glue-cache`` + When set to ``yes``, a cache is used to improve query performance + when adding address-type (A and AAAA) glue records to the additional + section of DNS response messages that delegate to a child zone. + + The glue cache uses memory proportional to the number of delegations + in the zone. The default setting is ``yes``, which improves + performance at the cost of increased memory usage for the zone. To avoid + this, set it to ``no``. + +``minimal-any`` + If set to ``yes``, the server replies with only one of + the RRsets for the query name, and its covering RRSIGs if any, + when generating a positive response to a query of type ANY over UDP, + instead of replying with all known RRsets for the name. Similarly, a + query for type RRSIG is answered with the RRSIG records covering + only one type. This can reduce the impact of some kinds of attack + traffic, without harming legitimate clients. (Note, however, that the + RRset returned is the first one found in the database; it is not + necessarily the smallest available RRset.) Additionally, + ``minimal-responses`` is turned on for these queries, so no + unnecessary records are added to the authority or additional + sections. The default is ``no``. + +``notify`` + If set to ``yes`` (the default), DNS NOTIFY messages are sent when a + zone the server is authoritative for changes; see :ref:`notify`. + The messages are sent to the servers listed in the zone's NS records + (except the primary server identified in the SOA MNAME field), and to + any servers listed in the ``also-notify`` option. + + If set to ``primary-only`` (or the older keyword ``master-only``), + notifies are only sent for primary zones. If set to ``explicit``, + notifies are sent only to servers explicitly listed using + ``also-notify``. If set to ``no``, no notifies are sent. + + The ``notify`` option may also be specified in the ``zone`` + statement, in which case it overrides the ``options notify`` + statement. It would only be necessary to turn off this option if it + caused secondary zones to crash. + +``notify-to-soa`` + If ``yes``, do not check the name servers in the NS RRset against the + SOA MNAME. Normally a NOTIFY message is not sent to the SOA MNAME + (SOA ORIGIN), as it is supposed to contain the name of the ultimate + primary server. Sometimes, however, a secondary server is listed as the SOA MNAME in + hidden primary configurations; in that case, the + ultimate primary should be set to still send NOTIFY messages to all the name servers + listed in the NS RRset. + +``recursion`` + If ``yes``, and a DNS query requests recursion, then the server + attempts to do all the work required to answer the query. If recursion + is off and the server does not already know the answer, it + returns a referral response. The default is ``yes``. Note that setting + ``recursion no`` does not prevent clients from getting data from the + server's cache; it only prevents new data from being cached as an + effect of client queries. Caching may still occur as an effect of the + server's internal operation, such as NOTIFY address lookups. + +``request-nsid`` + If ``yes``, then an empty EDNS(0) NSID (Name Server Identifier) + option is sent with all queries to authoritative name servers during + iterative resolution. If the authoritative server returns an NSID + option in its response, then its contents are logged in the ``nsid`` + category at level ``info``. The default is ``no``. + +``request-sit`` + This experimental option is obsolete. + +``require-server-cookie`` + If ``yes``, require a valid server cookie before sending a full response to a UDP + request from a cookie-aware client. BADCOOKIE is sent if there is a + bad or nonexistent server cookie. + + The default is ``no``. + + Users wishing to test that DNS COOKIE clients correctly handle + BADCOOKIE, or who are getting a lot of forged DNS requests with DNS COOKIES + present, should set this to ``yes``. Setting this to ``yes`` results in a reduced amplification effect + in a reflection attack, as the BADCOOKIE response is smaller than a full + response, while also requiring a legitimate client to follow up with a second + query with the new, valid, cookie. + +``answer-cookie`` + When set to the default value of ``yes``, COOKIE EDNS options are + sent when applicable in replies to client queries. If set to ``no``, + COOKIE EDNS options are not sent in replies. This can only be set + at the global options level, not per-view. + + ``answer-cookie no`` is intended as a temporary measure, for use when + ``named`` shares an IP address with other servers that do not yet + support DNS COOKIE. A mismatch between servers on the same address is + not expected to cause operational problems, but the option to disable + COOKIE responses so that all servers have the same behavior is + provided out of an abundance of caution. DNS COOKIE is an important + security mechanism, and should not be disabled unless absolutely + necessary. + +``send-cookie`` + If ``yes``, then a COOKIE EDNS option is sent along with the query. + If the resolver has previously communicated with the server, the COOKIE + returned in the previous transaction is sent. This is used by the + server to determine whether the resolver has talked to it before. A + resolver sending the correct COOKIE is assumed not to be an off-path + attacker sending a spoofed-source query; the query is therefore + unlikely to be part of a reflection/amplification attack, so + resolvers sending a correct COOKIE option are not subject to response + rate limiting (RRL). Resolvers which do not send a correct COOKIE + option may be limited to receiving smaller responses via the + ``nocookie-udp-size`` option. + + The default is ``yes``. + +``stale-answer-enable`` + If ``yes``, enable the returning of "stale" cached answers when the name + servers for a zone are not answering and the ``stale-cache-enable`` option is + also enabled. The default is not to return stale answers. + + Stale answers can also be enabled or disabled at runtime via + ``rndc serve-stale on`` or ``rndc serve-stale off``; these override + the configured setting. ``rndc serve-stale reset`` restores the + setting to the one specified in ``named.conf``. Note that if stale + answers have been disabled by ``rndc``, they cannot be + re-enabled by reloading or reconfiguring ``named``; they must be + re-enabled with ``rndc serve-stale on``, or the server must be + restarted. + + Information about stale answers is logged under the ``serve-stale`` + log category. + +``stale-answer-client-timeout`` + This option defines the amount of time (in milliseconds) that ``named`` + waits before attempting to answer the query with a stale RRset from cache. + If a stale answer is found, ``named`` continues the ongoing fetches, + attempting to refresh the RRset in cache until the + ``resolver-query-timeout`` interval is reached. + + This option is off by default, which is equivalent to setting it to + ``off`` or ``disabled``. It also has no effect if ``stale-answer-enable`` + is disabled. + + The maximum value for this option is ``resolver-query-timeout`` minus + one second. The minimum value, ``0``, causes a cached (stale) RRset to be + immediately returned if it is available while still attempting to + refresh the data in cache. :rfc:`8767` recommends a value of ``1800`` + (milliseconds). + +``stale-cache-enable`` + If ``yes``, enable the retaining of "stale" cached answers. Default ``yes``. + +``stale-refresh-time`` + If the name servers for a given zone are not answering, this sets the time + window for which ``named`` will promptly return "stale" cached answers for + that RRSet being requested before a new attempt in contacting the servers + is made. For convenience, TTL-style time-unit suffixes may be used to + specify the value. It also accepts ISO 8601 duration formats. + + The default ``stale-refresh-time`` is 30 seconds, as :rfc:`8767` recommends + that attempts to refresh to be done no more frequently than every 30 + seconds. A value of zero disables the feature, meaning that normal + resolution will take place first, if that fails only then ``named`` will + return "stale" cached answers. + +``nocookie-udp-size`` + This sets the maximum size of UDP responses that are sent to queries + without a valid server COOKIE. A value below 128 is silently + raised to 128. The default value is 4096, but the ``max-udp-size`` + option may further limit the response size as the default for + ``max-udp-size`` is 4096. + +``sit-secret`` + This experimental option is obsolete. + +``cookie-algorithm`` + This sets the algorithm to be used when generating the server cookie; the options are + "aes" or "siphash24". The default is "siphash24". The "aes" option remains for legacy + purposes. + +``cookie-secret`` + If set, this is a shared secret used for generating and verifying + EDNS COOKIE options within an anycast cluster. If not set, the system + generates a random secret at startup. The shared secret is + encoded as a hex string and needs to be 128 bits for either "siphash24" + or "aes". + + If there are multiple secrets specified, the first one listed in + ``named.conf`` is used to generate new server cookies. The others + are only used to verify returned cookies. + +``response-padding`` + The EDNS Padding option is intended to improve confidentiality when + DNS queries are sent over an encrypted channel, by reducing the + variability in packet sizes. If a query: + + 1. contains an EDNS Padding option, + 2. includes a valid server cookie or uses TCP, + 3. is not signed using TSIG or SIG(0), and + 4. is from a client whose address matches the specified ACL, + + then the response is padded with an EDNS Padding option to a multiple + of ``block-size`` bytes. If these conditions are not met, the + response is not padded. + + If ``block-size`` is 0 or the ACL is ``none;``, this feature is + disabled and no padding occurs; this is the default. If + ``block-size`` is greater than 512, a warning is logged and the value + is truncated to 512. Block sizes are ordinarily expected to be powers + of two (for instance, 128), but this is not mandatory. + +``trust-anchor-telemetry`` + This causes ``named`` to send specially formed queries once per day to + domains for which trust anchors have been configured via, e.g., + ``trust-anchors`` or ``dnssec-validation auto``. + + The query name used for these queries has the form + ``_ta-xxxx(-xxxx)(...).<domain>``, where each "xxxx" is a group of four + hexadecimal digits representing the key ID of a trusted DNSSEC key. + The key IDs for each domain are sorted smallest to largest prior to + encoding. The query type is NULL. + + By monitoring these queries, zone operators are able to see which + resolvers have been updated to trust a new key; this may help them + decide when it is safe to remove an old one. + + The default is ``yes``. + +``use-ixfr`` + *This option is obsolete*. To disable IXFR to a + particular server or servers, see the information on the + ``provide-ixfr`` option in :ref:`server_statement_definition_and_usage`. + See also :ref:`incremental_zone_transfers`. + +``provide-ixfr`` + See the description of ``provide-ixfr`` in :ref:`server_statement_definition_and_usage`. + +``request-ixfr`` + See the description of ``request-ixfr`` in :ref:`server_statement_definition_and_usage`. + +``request-expire`` + See the description of ``request-expire`` in :ref:`server_statement_definition_and_usage`. + +``match-mapped-addresses`` + If ``yes``, then an IPv4-mapped IPv6 address matches any + address-match list entries that match the corresponding IPv4 address. + + This option was introduced to work around a kernel quirk in some + operating systems that causes IPv4 TCP connections, such as zone + transfers, to be accepted on an IPv6 socket using mapped addresses. + This caused address-match lists designed for IPv4 to fail to match. + However, ``named`` now solves this problem internally. The use of + this option is discouraged. + +``ixfr-from-differences`` + When ``yes`` and the server loads a new version of a primary zone from + its zone file or receives a new version of a secondary file via zone + transfer, it compares the new version to the previous one and + calculates a set of differences. The differences are then logged in + the zone's journal file so that the changes can be transmitted to + downstream secondaries as an incremental zone transfer. + + By allowing incremental zone transfers to be used for non-dynamic + zones, this option saves bandwidth at the expense of increased CPU + and memory consumption at the primary server. In particular, if the new + version of a zone is completely different from the previous one, the + set of differences is of a size comparable to the combined size + of the old and new zone versions, and the server needs to + temporarily allocate memory to hold this complete difference set. + + ``ixfr-from-differences`` also accepts ``primary`` + and ``secondary`` at the view and options levels, + which causes ``ixfr-from-differences`` to be enabled for all primary + or secondary zones, respectively. It is off for all zones by default. + + Note: if inline signing is enabled for a zone, the user-provided + ``ixfr-from-differences`` setting is ignored for that zone. + +``multi-master`` + This should be set when there are multiple primary servers for a zone and the + addresses refer to different machines. If ``yes``, ``named`` does not + log when the serial number on the primary is less than what ``named`` + currently has. The default is ``no``. + +``auto-dnssec`` + Zones configured for dynamic DNS may use this option to allow varying + levels of automatic DNSSEC key management. There are three possible + settings: + + ``auto-dnssec allow;`` permits keys to be updated and the zone fully + re-signed whenever the user issues the command ``rndc sign zonename``. + + ``auto-dnssec maintain;`` includes the above, but also + automatically adjusts the zone's DNSSEC keys on a schedule, according + to the keys' timing metadata (see :ref:`man_dnssec-keygen` and + :ref:`man_dnssec-settime`). The command ``rndc sign zonename`` + causes ``named`` to load keys from the key repository and sign the + zone with all keys that are active. ``rndc loadkeys zonename`` + causes ``named`` to load keys from the key repository and schedule + key maintenance events to occur in the future, but it does not sign + the full zone immediately. Note: once keys have been loaded for a + zone the first time, the repository is searched for changes + periodically, regardless of whether ``rndc loadkeys`` is used. The + recheck interval is defined by ``dnssec-loadkeys-interval``. + + ``auto-dnssec off;`` does not allow for DNSSEC key management. + This is the default setting. + + This option may only be activated at the zone level; if configured + at the view or options level, it must be set to ``off``. + + The DNSSEC records are written to the zone's filename set in ``file``, + unless ``inline-signing`` is enabled. + +``dnssec-enable`` + This option is obsolete and has no effect. + +.. _dnssec-validation-option: + +``dnssec-validation`` + This option enables DNSSEC validation in ``named``. + + If set to ``auto``, DNSSEC validation is enabled and a default trust + anchor for the DNS root zone is used. This trust anchor is provided + as part of BIND and is kept up-to-date using :ref:`rfc5011.support` key + management. + + If set to ``yes``, DNSSEC validation is enabled, but a trust anchor must be + manually configured using a ``trust-anchors`` statement (or the + ``managed-keys`` or ``trusted-keys`` statements, both deprecated). If + there is no configured trust anchor, validation does not take place. + + If set to ``no``, DNSSEC validation is disabled. + + The default is ``auto``, unless BIND is built with + ``configure --disable-auto-validation``, in which case the default is + ``yes``. + + The default root trust anchor is stored in the file ``bind.keys``. + ``named`` loads that key at startup if ``dnssec-validation`` is + set to ``auto``. A copy of the file is installed along with BIND 9, + and is current as of the release date. If the root key expires, a new + copy of ``bind.keys`` can be downloaded from + https://www.isc.org/bind-keys. + + (To prevent problems if ``bind.keys`` is not found, the current trust + anchor is also compiled in ``named``. Relying on this is not + recommended, however, as it requires ``named`` to be recompiled with + a new key when the root key expires.) + + .. note:: ``named`` loads *only* the root key from ``bind.keys``. The file + cannot be used to store keys for other zones. The root key in + ``bind.keys`` is ignored if ``dnssec-validation auto`` is not in + use. + + Whenever the resolver sends out queries to an EDNS-compliant + server, it always sets the DO bit indicating it can support DNSSEC + responses, even if ``dnssec-validation`` is off. + +``validate-except`` + This specifies a list of domain names at and beneath which DNSSEC + validation should *not* be performed, regardless of the presence of a + trust anchor at or above those names. This may be used, for example, + when configuring a top-level domain intended only for local use, so + that the lack of a secure delegation for that domain in the root zone + does not cause validation failures. (This is similar to setting a + negative trust anchor except that it is a permanent configuration, + whereas negative trust anchors expire and are removed after a set + period of time.) + +``dnssec-accept-expired`` + This accepts expired signatures when verifying DNSSEC signatures. The + default is ``no``. Setting this option to ``yes`` leaves ``named`` + vulnerable to replay attacks. + +``querylog`` + Query logging provides a complete log of all incoming queries and all query + errors. This provides more insight into the server's activity, but with a + cost to performance which may be significant on heavily loaded servers. + + The ``querylog`` option specifies whether query logging should be active when + ``named`` first starts. If ``querylog`` is not specified, then query logging + is determined by the presence of the logging category ``queries``. Query + logging can also be activated at runtime using the command ``rndc querylog + on``, or deactivated with ``rndc querylog off``. + +``check-names`` + This option is used to restrict the character set and syntax of + certain domain names in primary files and/or DNS responses received + from the network. The default varies according to usage area. For + ``primary`` zones the default is ``fail``. For ``secondary`` zones the + default is ``warn``. For answers received from the network + (``response``), the default is ``ignore``. + + The rules for legal hostnames and mail domains are derived from + :rfc:`952` and :rfc:`821` as modified by :rfc:`1123`. + + ``check-names`` applies to the owner names of A, AAAA, and MX records. + It also applies to the domain names in the RDATA of NS, SOA, MX, and + SRV records. It further applies to the RDATA of PTR records where the + owner name indicates that it is a reverse lookup of a hostname (the + owner name ends in IN-ADDR.ARPA, IP6.ARPA, or IP6.INT). + +``check-dup-records`` + This checks primary zones for records that are treated as different by + DNSSEC but are semantically equal in plain DNS. The default is to + ``warn``. Other possible values are ``fail`` and ``ignore``. + +``check-mx`` + This checks whether the MX record appears to refer to an IP address. The + default is to ``warn``. Other possible values are ``fail`` and + ``ignore``. + +``check-wildcard`` + This option is used to check for non-terminal wildcards. The use of + non-terminal wildcards is almost always as a result of a lack of + understanding of the wildcard matching algorithm (:rfc:`1034`). This option + affects primary zones. The default (``yes``) is to check for + non-terminal wildcards and issue a warning. + +``check-integrity`` + This performs post-load zone integrity checks on primary zones. It checks + that MX and SRV records refer to address (A or AAAA) records and that + glue address records exist for delegated zones. For MX and SRV + records, only in-zone hostnames are checked (for out-of-zone hostnames, + use ``named-checkzone``). For NS records, only names below top-of-zone + are checked (for out-of-zone names and glue consistency checks, use + ``named-checkzone``). The default is ``yes``. + + The use of the SPF record to publish Sender Policy Framework is + deprecated, as the migration from using TXT records to SPF records was + abandoned. Enabling this option also checks that a TXT Sender Policy + Framework record exists (starts with "v=spf1") if there is an SPF + record. Warnings are emitted if the TXT record does not exist; they can + be suppressed with ``check-spf``. + +``check-mx-cname`` + If ``check-integrity`` is set, then fail, warn, or ignore MX records + that refer to CNAMES. The default is to ``warn``. + +``check-srv-cname`` + If ``check-integrity`` is set, then fail, warn, or ignore SRV records + that refer to CNAMES. The default is to ``warn``. + +``check-sibling`` + When performing integrity checks, also check that sibling glue + exists. The default is ``yes``. + +``check-spf`` + If ``check-integrity`` is set, check that there is a TXT Sender + Policy Framework record present (starts with "v=spf1") if there is an + SPF record present. The default is ``warn``. + +``zero-no-soa-ttl`` + If ``yes``, when returning authoritative negative responses to SOA queries, set + the TTL of the SOA record returned in the authority section to zero. + The default is ``yes``. + +``zero-no-soa-ttl-cache`` + If ``yes``, when caching a negative response to an SOA query set the TTL to zero. + The default is ``no``. + +``update-check-ksk`` + When set to the default value of ``yes``, check the KSK bit in each + key to determine how the key should be used when generating RRSIGs + for a secure zone. + + Ordinarily, zone-signing keys (that is, keys without the KSK bit set) + are used to sign the entire zone, while key-signing keys (keys with + the KSK bit set) are only used to sign the DNSKEY RRset at the zone + apex. However, if this option is set to ``no``, then the KSK bit is + ignored; KSKs are treated as if they were ZSKs and are used to sign + the entire zone. This is similar to the ``dnssec-signzone -z`` + command-line option. + + When this option is set to ``yes``, there must be at least two active + keys for every algorithm represented in the DNSKEY RRset: at least + one KSK and one ZSK per algorithm. If there is any algorithm for + which this requirement is not met, this option is ignored for + that algorithm. + +``dnssec-dnskey-kskonly`` + When this option and ``update-check-ksk`` are both set to ``yes``, + only key-signing keys (that is, keys with the KSK bit set) are + used to sign the DNSKEY, CDNSKEY, and CDS RRsets at the zone apex. + Zone-signing keys (keys without the KSK bit set) are used to sign + the remainder of the zone, but not the DNSKEY RRset. This is similar + to the ``dnssec-signzone -x`` command-line option. + + The default is ``no``. If ``update-check-ksk`` is set to ``no``, this + option is ignored. + +``try-tcp-refresh`` + If ``yes``, try to refresh the zone using TCP if UDP queries fail. The default is + ``yes``. + +``dnssec-secure-to-insecure`` + This allows a dynamic zone to transition from secure to insecure (i.e., + signed to unsigned) by deleting all of the DNSKEY records. The + default is ``no``. If set to ``yes``, and if the DNSKEY RRset at the + zone apex is deleted, all RRSIG and NSEC records are removed from + the zone as well. + + If the zone uses NSEC3, it is also necessary to delete the + NSEC3PARAM RRset from the zone apex; this causes the removal of + all corresponding NSEC3 records. (It is expected that this + requirement will be eliminated in a future release.) + + Note that if a zone has been configured with ``auto-dnssec maintain`` + and the private keys remain accessible in the key repository, + the zone will be automatically signed again the next time ``named`` + is started. + +``synth-from-dnssec`` + This option synthesizes answers from cached NSEC, NSEC3, and other RRsets that have been + proved to be correct using DNSSEC. The default is ``no``, but it will become + ``yes`` again in future releases. + + .. note:: DNSSEC validation must be enabled for this option to be effective. + This initial implementation only covers synthesis of answers from + NSEC records; synthesis from NSEC3 is planned for the future. This + will also be controlled by ``synth-from-dnssec``. + +Forwarding +^^^^^^^^^^ + +The forwarding facility can be used to create a large site-wide cache on +a few servers, reducing traffic over links to external name servers. It +can also be used to allow queries by servers that do not have direct +access to the Internet, but wish to look up exterior names anyway. +Forwarding occurs only on those queries for which the server is not +authoritative and does not have the answer in its cache. + +``forward`` + This option is only meaningful if the forwarders list is not empty. A + value of ``first`` is the default and causes the server to query the + forwarders first; if that does not answer the question, the + server then looks for the answer itself. If ``only`` is + specified, the server only queries the forwarders. + +``forwarders`` + This specifies a list of IP addresses to which queries are forwarded. The + default is the empty list (no forwarding). Each address in the list can be + associated with an optional port number and/or DSCP value, and a default port + number and DSCP value can be set for the entire list. + +Forwarding can also be configured on a per-domain basis, allowing for +the global forwarding options to be overridden in a variety of ways. +Particular domains can be set to use different forwarders, or have a +different ``forward only/first`` behavior, or not forward at all; see +:ref:`zone_statement_grammar`. + +.. _dual_stack: + +Dual-stack Servers +^^^^^^^^^^^^^^^^^^ + +Dual-stack servers are used as servers of last resort, to work around +problems in reachability due to the lack of support for either IPv4 or IPv6 +on the host machine. + +``dual-stack-servers`` + This specifies host names or addresses of machines with access to both + IPv4 and IPv6 transports. If a hostname is used, the server must be + able to resolve the name using only the transport it has. If the + machine is dual-stacked, the ``dual-stack-servers`` parameter has no + effect unless access to a transport has been disabled on the command + line (e.g., ``named -4``). + +.. _access_control: + +Access Control +^^^^^^^^^^^^^^ + +Access to the server can be restricted based on the IP address of the +requesting system. See :ref:`address_match_lists` +for details on how to specify IP address lists. + +``allow-notify`` + This ACL specifies which hosts may send NOTIFY messages to inform + this server of changes to zones for which it is acting as a secondary + server. This is only applicable for secondary zones (i.e., type + ``secondary`` or ``slave``). + + If this option is set in ``view`` or ``options``, it is globally + applied to all secondary zones. If set in the ``zone`` statement, the + global value is overridden. + + If not specified, the default is to process NOTIFY messages only from + the configured ``primaries`` for the zone. ``allow-notify`` can be used + to expand the list of permitted hosts, not to reduce it. + +``allow-query`` + This specifies which hosts are allowed to ask ordinary DNS questions. + ``allow-query`` may also be specified in the ``zone`` statement, in + which case it overrides the ``options allow-query`` statement. If not + specified, the default is to allow queries from all hosts. + + .. note:: ``allow-query-cache`` is used to specify access to the cache. + +``allow-query-on`` + This specifies which local addresses can accept ordinary DNS questions. + This makes it possible, for instance, to allow queries on + internal-facing interfaces but disallow them on external-facing ones, + without necessarily knowing the internal network's addresses. + + Note that ``allow-query-on`` is only checked for queries that are + permitted by ``allow-query``. A query must be allowed by both ACLs, + or it is refused. + + ``allow-query-on`` may also be specified in the ``zone`` statement, + in which case it overrides the ``options allow-query-on`` statement. + + If not specified, the default is to allow queries on all addresses. + + .. note:: ``allow-query-cache`` is used to specify access to the cache. + +``allow-query-cache`` + This specifies which hosts are allowed to get answers from the cache. If + ``allow-recursion`` is not set, BIND checks to see if the following parameters + are set, in order: ``allow-query-cache`` and ``allow-query`` (unless ``recursion no;`` is set). + If neither of those parameters is set, the default (localnets; localhost;) is used. + +``allow-query-cache-on`` + This specifies which local addresses can send answers from the cache. If + ``allow-query-cache-on`` is not set, then ``allow-recursion-on`` is + used if set. Otherwise, the default is to allow cache responses to be + sent from any address. Note: both ``allow-query-cache`` and + ``allow-query-cache-on`` must be satisfied before a cache response + can be sent; a client that is blocked by one cannot be allowed by the + other. + +``allow-recursion`` + This specifies which hosts are allowed to make recursive queries through + this server. BIND checks to see if the following parameters are set, in + order: ``allow-query-cache`` and ``allow-query``. If neither of those parameters + is set, the default (localnets; localhost;) is used. + +``allow-recursion-on`` + This specifies which local addresses can accept recursive queries. If + ``allow-recursion-on`` is not set, then ``allow-query-cache-on`` is + used if set; otherwise, the default is to allow recursive queries on + all addresses. Any client permitted to send recursive queries can + send them to any address on which ``named`` is listening. Note: both + ``allow-recursion`` and ``allow-recursion-on`` must be satisfied + before recursion is allowed; a client that is blocked by one cannot + be allowed by the other. + +``allow-update`` + When set in the ``zone`` statement for a primary zone, this specifies which + hosts are allowed to submit Dynamic DNS updates to that zone. The + default is to deny updates from all hosts. + + Note that allowing updates based on the requestor's IP address is + insecure; see :ref:`dynamic_update_security` for details. + + In general, this option should only be set at the ``zone`` level. + While a default value can be set at the ``options`` or ``view`` level + and inherited by zones, this could lead to some zones unintentionally + allowing updates. + + Updates are written to the zone's filename that is set in ``file``. + +``allow-update-forwarding`` + When set in the ``zone`` statement for a secondary zone, this specifies which + hosts are allowed to submit Dynamic DNS updates and have them be + forwarded to the primary. The default is ``{ none; }``, which means + that no update forwarding is performed. + + To enable update forwarding, specify + ``allow-update-forwarding { any; };`` in the ``zone`` statement. + Specifying values other than ``{ none; }`` or ``{ any; }`` is usually + counterproductive; the responsibility for update access control + should rest with the primary server, not the secondary. + + Note that enabling the update forwarding feature on a secondary server + may expose primary servers to attacks if they rely on insecure + IP-address-based access control; see :ref:`dynamic_update_security` for more details. + + In general this option should only be set at the ``zone`` level. + While a default value can be set at the ``options`` or ``view`` level + and inherited by zones, this can lead to some zones unintentionally + forwarding updates. + +.. _allow-transfer-access: + +``allow-transfer`` + This specifies which hosts are allowed to receive zone transfers from the + server. ``allow-transfer`` may also be specified in the ``zone`` + statement, in which case it overrides the ``allow-transfer`` + statement set in ``options`` or ``view``. If not specified, the + default is to allow transfers to all hosts. + +``blackhole`` + This specifies a list of addresses which the server does not accept queries + from or use to resolve a query. Queries from these addresses are not + responded to. The default is ``none``. + +``keep-response-order`` + This specifies a list of addresses to which the server sends responses + to TCP queries, in the same order in which they were received. This + disables the processing of TCP queries in parallel. The default is + ``none``. + +``no-case-compress`` + This specifies a list of addresses which require responses to use + case-insensitive compression. This ACL can be used when ``named`` + needs to work with clients that do not comply with the requirement in + :rfc:`1034` to use case-insensitive name comparisons when checking for + matching domain names. + + If left undefined, the ACL defaults to ``none``: case-insensitive + compression is used for all clients. If the ACL is defined and + matches a client, case is ignored when compressing domain + names in DNS responses sent to that client. + + This can result in slightly smaller responses; if a response contains + the names "example.com" and "example.COM", case-insensitive + compression treats the second one as a duplicate. It also + ensures that the case of the query name exactly matches the case of + the owner names of returned records, rather than matches the case of + the records entered in the zone file. This allows responses to + exactly match the query, which is required by some clients due to + incorrect use of case-sensitive comparisons. + + Case-insensitive compression is *always* used in AXFR and IXFR + responses, regardless of whether the client matches this ACL. + + There are circumstances in which ``named`` does not preserve the case + of owner names of records: if a zone file defines records of + different types with the same name, but the capitalization of the + name is different (e.g., "www.example.com/A" and + "WWW.EXAMPLE.COM/AAAA"), then all responses for that name use + the *first* version of the name that was used in the zone file. This + limitation may be addressed in a future release. However, domain + names specified in the rdata of resource records (i.e., records of + type NS, MX, CNAME, etc.) always have their case preserved unless + the client matches this ACL. + +``resolver-query-timeout`` + This is the amount of time in milliseconds that the resolver spends + attempting to resolve a recursive query before failing. The default + and minimum is ``10000`` and the maximum is ``30000``. Setting it to + ``0`` results in the default being used. + + This value was originally specified in seconds. Values less than or + equal to 300 are treated as seconds and converted to + milliseconds before applying the above limits. + +Interfaces +^^^^^^^^^^ + +The interfaces and ports that the server answers queries from may be +specified using the ``listen-on`` option. ``listen-on`` takes an +optional port and an ``address_match_list`` of IPv4 addresses. (IPv6 +addresses are ignored, with a logged warning.) The server listens on +all interfaces allowed by the address match list. If a port is not +specified, port 53 is used. + +Multiple ``listen-on`` statements are allowed. For example: + +:: + + listen-on { 5.6.7.8; }; + listen-on port 1234 { !1.2.3.4; 1.2/16; }; + +enables the name server on port 53 for the IP address 5.6.7.8, and +on port 1234 of an address on the machine in net 1.2 that is not +1.2.3.4. + +If no ``listen-on`` is specified, the server listens on port 53 on +all IPv4 interfaces. + +The ``listen-on-v6`` option is used to specify the interfaces and the +ports on which the server listens for incoming queries sent using +IPv6. If not specified, the server listens on port 53 on all IPv6 +interfaces. + +Multiple ``listen-on-v6`` options can be used. For example: + +:: + + listen-on-v6 { any; }; + listen-on-v6 port 1234 { !2001:db8::/32; any; }; + +enables the name server on port 53 for any IPv6 addresses (with a +single wildcard socket), and on port 1234 of IPv6 addresses that are not +in the prefix 2001:db8::/32 (with separate sockets for each matched +address). + +To instruct the server not to listen on any IPv6 address, use: + +:: + + listen-on-v6 { none; }; + +.. _query_address: + +Query Address +^^^^^^^^^^^^^ + +If the server does not know the answer to a question, it queries other +name servers. ``query-source`` specifies the address and port used for +such queries. For queries sent over IPv6, there is a separate +``query-source-v6`` option. If ``address`` is ``*`` (asterisk) or is +omitted, a wildcard IP address (``INADDR_ANY``) is used. + +If ``port`` is ``*`` or is omitted, a random port number from a +pre-configured range is picked up and used for each query. The +port range(s) is specified in the ``use-v4-udp-ports`` (for IPv4) +and ``use-v6-udp-ports`` (for IPv6) options, excluding the ranges +specified in the ``avoid-v4-udp-ports`` and ``avoid-v6-udp-ports`` +options, respectively. + +The defaults of the ``query-source`` and ``query-source-v6`` options +are: + +:: + + query-source address * port *; + query-source-v6 address * port *; + +If ``use-v4-udp-ports`` or ``use-v6-udp-ports`` is unspecified, +``named`` checks whether the operating system provides a programming +interface to retrieve the system's default range for ephemeral ports. If +such an interface is available, ``named`` uses the corresponding +system default range; otherwise, it uses its own defaults: + +:: + + use-v4-udp-ports { range 1024 65535; }; + use-v6-udp-ports { range 1024 65535; }; + +.. note:: Make sure the ranges are sufficiently large for security. A + desirable size depends on several parameters, but we generally recommend + it contain at least 16384 ports (14 bits of entropy). Note also that the + system's default range when used may be too small for this purpose, and + that the range may even be changed while ``named`` is running; the new + range is automatically applied when ``named`` is reloaded. Explicit + configuration of ``use-v4-udp-ports`` and ``use-v6-udp-ports`` is encouraged, + so that the ranges are sufficiently large and are reasonably + independent from the ranges used by other applications. + +.. note:: The operational configuration where ``named`` runs may prohibit + the use of some ports. For example, Unix systems do not allow + ``named``, if run without root privilege, to use ports less than 1024. + If such ports are included in the specified (or detected) set of query + ports, the corresponding query attempts will fail, resulting in + resolution failures or delay. It is therefore important to configure the + set of ports that can be safely used in the expected operational + environment. + +The defaults of the ``avoid-v4-udp-ports`` and ``avoid-v6-udp-ports`` +options are: + +:: + + avoid-v4-udp-ports {}; + avoid-v6-udp-ports {}; + +.. note:: BIND 9.5.0 introduced the ``use-queryport-pool`` option to support + a pool of such random ports, but this option is now obsolete because + reusing the same ports in the pool may not be sufficiently secure. For + the same reason, it is generally strongly discouraged to specify a + particular port for the ``query-source`` or ``query-source-v6`` options; + it implicitly disables the use of randomized port numbers. + +``use-queryport-pool`` + This option is obsolete. + +``queryport-pool-ports`` + This option is obsolete. + +``queryport-pool-updateinterval`` + This option is obsolete. + +.. note:: The address specified in the ``query-source`` option is used for both + UDP and TCP queries, but the port applies only to UDP queries. TCP + queries always use a random unprivileged port. + +.. warning:: Specifying a single port is discouraged, as it removes a layer of + protection against spoofing errors. + +.. warning:: The configured ``port`` must not be same as the listening port. + +.. note:: See also ``transfer-source``, ``notify-source`` and ``parental-source``. + +.. _zone_transfers: + +Zone Transfers +^^^^^^^^^^^^^^ + +BIND has mechanisms in place to facilitate zone transfers and set limits +on the amount of load that transfers place on the system. The following +options apply to zone transfers. + +``also-notify`` + This option defines a global list of IP addresses of name servers that are also + sent NOTIFY messages whenever a fresh copy of the zone is loaded, in + addition to the servers listed in the zone's NS records. This helps + to ensure that copies of the zones quickly converge on stealth + servers. Optionally, a port may be specified with each + ``also-notify`` address to send the notify messages to a port other + than the default of 53. An optional TSIG key can also be specified + with each address to cause the notify messages to be signed; this can + be useful when sending notifies to multiple views. In place of + explicit addresses, one or more named ``primaries`` lists can be used. + + If an ``also-notify`` list is given in a ``zone`` statement, it + overrides the ``options also-notify`` statement. When a + ``zone notify`` statement is set to ``no``, the IP addresses in the + global ``also-notify`` list are not sent NOTIFY messages for that + zone. The default is the empty list (no global notification list). + +``max-transfer-time-in`` + Inbound zone transfers running longer than this many minutes are + terminated. The default is 120 minutes (2 hours). The maximum value + is 28 days (40320 minutes). + +``max-transfer-idle-in`` + Inbound zone transfers making no progress in this many minutes are + terminated. The default is 60 minutes (1 hour). The maximum value + is 28 days (40320 minutes). + +``max-transfer-time-out`` + Outbound zone transfers running longer than this many minutes are + terminated. The default is 120 minutes (2 hours). The maximum value + is 28 days (40320 minutes). + +``max-transfer-idle-out`` + Outbound zone transfers making no progress in this many minutes are + terminated. The default is 60 minutes (1 hour). The maximum value + is 28 days (40320 minutes). + +``notify-rate`` + This specifies the rate at which NOTIFY requests are sent during normal zone + maintenance operations. (NOTIFY requests due to initial zone loading + are subject to a separate rate limit; see below.) The default is 20 + per second. The lowest possible rate is one per second; when set to + zero, it is silently raised to one. + +``startup-notify-rate`` + This is the rate at which NOTIFY requests are sent when the name server + is first starting up, or when zones have been newly added to the + name server. The default is 20 per second. The lowest possible rate is + one per second; when set to zero, it is silently raised to one. + +``serial-query-rate`` + Secondary servers periodically query primary servers to find out if + zone serial numbers have changed. Each such query uses a minute + amount of the secondary server's network bandwidth. To limit the amount + of bandwidth used, BIND 9 limits the rate at which queries are sent. + The value of the ``serial-query-rate`` option, an integer, is the + maximum number of queries sent per second. The default is 20 per + second. The lowest possible rate is one per second; when set to zero, + it is silently raised to one. + +``transfer-format`` + Zone transfers can be sent using two different formats, + ``one-answer`` and ``many-answers``. The ``transfer-format`` option + is used on the primary server to determine which format it sends. + ``one-answer`` uses one DNS message per resource record transferred. + ``many-answers`` packs as many resource records as possible into one + message. ``many-answers`` is more efficient; the default is ``many-answers``. + ``transfer-format`` may be overridden on a per-server basis by using + the ``server`` statement. + +``transfer-message-size`` + This is an upper bound on the uncompressed size of DNS messages used + in zone transfers over TCP. If a message grows larger than this size, + additional messages are used to complete the zone transfer. + (Note, however, that this is a hint, not a hard limit; if a message + contains a single resource record whose RDATA does not fit within the + size limit, a larger message will be permitted so the record can be + transferred.) + + Valid values are between 512 and 65535 octets; any values outside + that range are adjusted to the nearest value within it. The + default is ``20480``, which was selected to improve message + compression; most DNS messages of this size will compress to less + than 16536 bytes. Larger messages cannot be compressed as + effectively, because 16536 is the largest permissible compression + offset pointer in a DNS message. + + This option is mainly intended for server testing; there is rarely + any benefit in setting a value other than the default. + +``transfers-in`` + This is the maximum number of inbound zone transfers that can run + concurrently. The default value is ``10``. Increasing + ``transfers-in`` may speed up the convergence of secondary zones, but it + also may increase the load on the local system. + +``transfers-out`` + This is the maximum number of outbound zone transfers that can run + concurrently. Zone transfer requests in excess of the limit are + refused. The default value is ``10``. + +``transfers-per-ns`` + This is the maximum number of inbound zone transfers that can concurrently + transfer from a given remote name server. The default value is + ``2``. Increasing ``transfers-per-ns`` may speed up the convergence + of secondary zones, but it also may increase the load on the remote name + server. ``transfers-per-ns`` may be overridden on a per-server basis + by using the ``transfers`` phrase of the ``server`` statement. + +``transfer-source`` + ``transfer-source`` determines which local address is bound to + IPv4 TCP connections used to fetch zones transferred inbound by the + server. It also determines the source IPv4 address, and optionally + the UDP port, used for the refresh queries and forwarded dynamic + updates. If not set, it defaults to a system-controlled value which + is usually the address of the interface "closest to" the remote + end. This address must appear in the remote end's ``allow-transfer`` + option for the zone being transferred, if one is specified. This + statement sets the ``transfer-source`` for all zones, but can be + overridden on a per-view or per-zone basis by including a + ``transfer-source`` statement within the ``view`` or ``zone`` block + in the configuration file. + + .. warning:: Specifying a single port is discouraged, as it removes a layer of + protection against spoofing errors. + + .. warning:: The configured ``port`` must not be same as the listening port. + +``transfer-source-v6`` + This option is the same as ``transfer-source``, except zone transfers are performed + using IPv6. + +``alt-transfer-source`` + This indicates an alternate transfer source if the one listed in ``transfer-source`` + fails and ``use-alt-transfer-source`` is set. + + .. note:: To avoid using the alternate transfer source, + set ``use-alt-transfer-source`` appropriately and + do not depend upon getting an answer back to the first refresh + query. + +``alt-transfer-source-v6`` + This indicates an alternate transfer source if the one listed in + ``transfer-source-v6`` fails and ``use-alt-transfer-source`` is set. + +``use-alt-transfer-source`` + This indicates whether the alternate transfer sources should be used. If views are specified, + this defaults to ``no``; otherwise, it defaults to ``yes``. + +``notify-source`` + ``notify-source`` determines which local source address, and + optionally UDP port, is used to send NOTIFY messages. This + address must appear in the secondary server's ``primaries`` zone clause or + in an ``allow-notify`` clause. This statement sets the + ``notify-source`` for all zones, but can be overridden on a per-zone + or per-view basis by including a ``notify-source`` statement within + the ``zone`` or ``view`` block in the configuration file. + + .. warning:: Specifying a single port is discouraged, as it removes a layer of + protection against spoofing errors. + + .. warning:: The configured ``port`` must not be same as the listening port. + +``notify-source-v6`` + This option acts like ``notify-source``, but applies to notify messages sent to IPv6 + addresses. + +.. _port_lists: + +UDP Port Lists +^^^^^^^^^^^^^^ + +``use-v4-udp-ports``, ``avoid-v4-udp-ports``, ``use-v6-udp-ports``, and +``avoid-v6-udp-ports`` specify a list of IPv4 and IPv6 UDP ports that +are or are not used as source ports for UDP messages. See +:ref:`query_address` about how the available ports are +determined. For example, with the following configuration: + +:: + + use-v6-udp-ports { range 32768 65535; }; + avoid-v6-udp-ports { 40000; range 50000 60000; }; + +UDP ports of IPv6 messages sent from ``named`` are in one of the +following ranges: 32768 to 39999, 40001 to 49999, and 60001 to 65535. + +``avoid-v4-udp-ports`` and ``avoid-v6-udp-ports`` can be used to prevent +``named`` from choosing as its random source port a port that is blocked +by a firewall or a port that is used by other applications; if a +query went out with a source port blocked by a firewall, the answer +would not pass through the firewall and the name server would have to query +again. Note: the desired range can also be represented only with +``use-v4-udp-ports`` and ``use-v6-udp-ports``, and the ``avoid-`` +options are redundant in that sense; they are provided for backward +compatibility and to possibly simplify the port specification. + +.. _resource_limits: + +Operating System Resource Limits +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The server's usage of many system resources can be limited. Scaled +values are allowed when specifying resource limits. For example, ``1G`` +can be used instead of ``1073741824`` to specify a limit of one +gigabyte. ``unlimited`` requests unlimited use, or the maximum available +amount. ``default`` uses the limit that was in force when the server was +started. See the description of ``size_spec`` in :ref:`configuration_file_elements`. + +The following options set operating system resource limits for the name +server process. Some operating systems do not support some or any of the +limits; on such systems, a warning is issued if an unsupported +limit is used. + +``coresize`` + This sets the maximum size of a core dump. The default is ``default``. + +``datasize`` + This sets the maximum amount of data memory the server may use. The default is + ``default``. This is a hard limit on server memory usage; if the + server attempts to allocate memory in excess of this limit, the + allocation will fail, which may in turn leave the server unable to + perform DNS service. Therefore, this option is rarely useful as a way + to limit the amount of memory used by the server, but it can be + used to raise an operating system data size limit that is too small + by default. To limit the amount of memory used by the + server, use the ``max-cache-size`` and ``recursive-clients`` options + instead. + +``files`` + This sets the maximum number of files the server may have open concurrently. + The default is ``unlimited``. + +``stacksize`` + This sets the maximum amount of stack memory the server may use. The default is + ``default``. + +.. _server_resource_limits: + +Server Resource Limits +^^^^^^^^^^^^^^^^^^^^^^ + +The following options set limits on the server's resource consumption +that are enforced internally by the server rather than by the operating +system. + +``max-journal-size`` + This sets a maximum size for each journal file (see :ref:`journal`), + expressed in bytes or, if followed by an + optional unit suffix ('k', 'm', or 'g'), in kilobytes, megabytes, or + gigabytes. When the journal file approaches the specified size, some + of the oldest transactions in the journal are automatically + removed. The largest permitted value is 2 gigabytes. Very small + values are rounded up to 4096 bytes. It is possible to specify ``unlimited``, + which also means 2 gigabytes. If the limit is set to ``default`` or + left unset, the journal is allowed to grow up to twice as large + as the zone. (There is little benefit in storing larger journals.) + + This option may also be set on a per-zone basis. + +``max-records`` + This sets the maximum number of records permitted in a zone. The default is + zero, which means the maximum is unlimited. + +``recursive-clients`` + This sets the maximum number (a "hard quota") of simultaneous recursive lookups + the server performs on behalf of clients. The default is + ``1000``. Because each recursing client uses a fair bit of memory (on + the order of 20 kilobytes), the value of the ``recursive-clients`` + option may have to be decreased on hosts with limited memory. + + ``recursive-clients`` defines a "hard quota" limit for pending + recursive clients; when more clients than this are pending, new + incoming requests are not accepted, and for each incoming request + a previous pending request is dropped. + + A "soft quota" is also set. When this lower quota is exceeded, + incoming requests are accepted, but for each one, a pending request + is dropped. If ``recursive-clients`` is greater than 1000, the + soft quota is set to ``recursive-clients`` minus 100; otherwise it is + set to 90% of ``recursive-clients``. + +``tcp-clients`` + This is the maximum number of simultaneous client TCP connections that the + server accepts. The default is ``150``. + +.. _clients-per-query: + +``clients-per-query``; ``max-clients-per-query`` + These set the initial value (minimum) and maximum number of recursive + simultaneous clients for any given query (<qname,qtype,qclass>) that + the server accepts before dropping additional clients. ``named`` + attempts to self-tune this value and changes are logged. The + default values are 10 and 100. + + This value should reflect how many queries come in for a given name + in the time it takes to resolve that name. If the number of queries + exceeds this value, ``named`` assumes that it is dealing with a + non-responsive zone and drops additional queries. If it gets a + response after dropping queries, it raises the estimate. The + estimate is then lowered in 20 minutes if it has remained + unchanged. + + If ``clients-per-query`` is set to zero, there is no limit on + the number of clients per query and no queries are dropped. + + If ``max-clients-per-query`` is set to zero, there is no upper + bound other than that imposed by ``recursive-clients``. + +``fetches-per-zone`` + This sets the maximum number of simultaneous iterative queries to any one + domain that the server permits before blocking new queries for + data in or beneath that zone. This value should reflect how many + fetches would normally be sent to any one zone in the time it would + take to resolve them. It should be smaller than + ``recursive-clients``. + + When many clients simultaneously query for the same name and type, + the clients are all attached to the same fetch, up to the + ``max-clients-per-query`` limit, and only one iterative query is + sent. However, when clients are simultaneously querying for + *different* names or types, multiple queries are sent and + ``max-clients-per-query`` is not effective as a limit. + + Optionally, this value may be followed by the keyword ``drop`` or + ``fail``, indicating whether queries which exceed the fetch quota for + a zone are dropped with no response, or answered with SERVFAIL. + The default is ``drop``. + + If ``fetches-per-zone`` is set to zero, there is no limit on the + number of fetches per query and no queries are dropped. The + default is zero. + + The current list of active fetches can be dumped by running + ``rndc recursing``. The list includes the number of active fetches + for each domain and the number of queries that have been passed + (allowed) or dropped (spilled) as a result of the ``fetches-per-zone`` + limit. (Note: these counters are not cumulative over time; + whenever the number of active fetches for a domain drops to zero, + the counter for that domain is deleted, and the next time a fetch + is sent to that domain, it is recreated with the counters set + to zero.) + +``fetches-per-server`` + This sets the maximum number of simultaneous iterative queries that the server + allows to be sent to a single upstream name server before + blocking additional queries. This value should reflect how many + fetches would normally be sent to any one server in the time it would + take to resolve them. It should be smaller than + ``recursive-clients``. + + Optionally, this value may be followed by the keyword ``drop`` or + ``fail``, indicating whether queries are dropped with no + response or answered with SERVFAIL, when all of the servers + authoritative for a zone are found to have exceeded the per-server + quota. The default is ``fail``. + + If ``fetches-per-server`` is set to zero, there is no limit on + the number of fetches per query and no queries are dropped. The + default is zero. + + The ``fetches-per-server`` quota is dynamically adjusted in response + to detected congestion. As queries are sent to a server and either are + answered or time out, an exponentially weighted moving average + is calculated of the ratio of timeouts to responses. If the current + average timeout ratio rises above a "high" threshold, then + ``fetches-per-server`` is reduced for that server. If the timeout + ratio drops below a "low" threshold, then ``fetches-per-server`` is + increased. The ``fetch-quota-params`` options can be used to adjust + the parameters for this calculation. + +``fetch-quota-params`` + This sets the parameters to use for dynamic resizing of the + ``fetches-per-server`` quota in response to detected congestion. + + The first argument is an integer value indicating how frequently to + recalculate the moving average of the ratio of timeouts to responses + for each server. The default is 100, meaning that BIND recalculates the + average ratio after every 100 queries have either been answered or + timed out. + + The remaining three arguments represent the "low" threshold + (defaulting to a timeout ratio of 0.1), the "high" threshold + (defaulting to a timeout ratio of 0.3), and the discount rate for the + moving average (defaulting to 0.7). A higher discount rate causes + recent events to weigh more heavily when calculating the moving + average; a lower discount rate causes past events to weigh more + heavily, smoothing out short-term blips in the timeout ratio. These + arguments are all fixed-point numbers with precision of 1/100; at + most two places after the decimal point are significant. + +``reserved-sockets`` + This sets the number of file descriptors reserved for TCP, stdio, etc. This + needs to be big enough to cover the number of interfaces ``named`` + listens on plus ``tcp-clients``, as well as to provide room for + outgoing TCP queries and incoming zone transfers. The default is + ``512``. The minimum value is ``128`` and the maximum value is + ``128`` fewer than maxsockets (-S). This option may be removed in the + future. + + This option has little effect on Windows. + +``max-cache-size`` + This sets the maximum amount of memory to use for an individual cache + database and its associated metadata, in bytes or percentage of total + physical memory. By default, each view has its own separate cache, + which means the total amount of memory required for cache data is the + sum of the cache database sizes for all views (unless the + :ref:`attach-cache <attach-cache>` option is used). + + When the amount of data in a cache database reaches the configured + limit, ``named`` starts purging non-expired records (following an + LRU-based strategy). + + The default size limit for each individual cache is: + + - 90% of physical memory for views with ``recursion`` set to + ``yes`` (the default), or + + - 2 MB for views with ``recursion`` set to ``no``. + + Any positive value smaller than 2 MB is ignored and reset to 2 MB. + The keyword ``unlimited``, or the value ``0``, places no limit on the + cache size; records are then purged from the cache only when they + expire (according to their TTLs). + + .. note:: + + For configurations which define multiple views with separate + caches and recursion enabled, it is recommended to set + ``max-cache-size`` appropriately for each view, as using the + default value of that option (90% of physical memory for each + individual cache) may lead to memory exhaustion over time. + + .. note:: + + ``max-cache-size`` does not work reliably for the maximum + amount of memory of 100 MB or lower. + + Upon startup and reconfiguration, caches with a limited size + preallocate a small amount of memory (less than 1% of + ``max-cache-size`` for a given view). This preallocation serves as an + optimization to eliminate extra latency introduced by resizing + internal cache structures. + + On systems where detection of the amount of physical memory is not + supported, percentage-based values fall back to ``unlimited``. Note + that the amount of physical memory available is only detected on + startup, so ``named`` does not adjust the cache size limits if the + amount of physical memory is changed at runtime. + +``tcp-listen-queue`` + This sets the listen-queue depth. The default and minimum is 10. If the kernel + supports the accept filter "dataready", this also controls how many + TCP connections are queued in kernel space waiting for some + data before being passed to accept. Non-zero values less than 10 are + silently raised. A value of 0 may also be used; on most platforms + this sets the listen-queue length to a system-defined default value. + +``tcp-initial-timeout`` + This sets the amount of time (in units of 100 milliseconds) that the server waits on + a new TCP connection for the first message from the client. The + default is 300 (30 seconds), the minimum is 25 (2.5 seconds), and the + maximum is 1200 (two minutes). Values above the maximum or below the + minimum are adjusted with a logged warning. (Note: this value + must be greater than the expected round-trip delay time; otherwise, no + client will ever have enough time to submit a message.) This value + can be updated at runtime by using ``rndc tcp-timeouts``. + +``tcp-idle-timeout`` + This sets the amount of time (in units of 100 milliseconds) that the server waits on + an idle TCP connection before closing it, when the client is not using + the EDNS TCP keepalive option. The default is 300 (30 seconds), the + maximum is 1200 (two minutes), and the minimum is 1 (one-tenth of a + second). Values above the maximum or below the minimum are + adjusted with a logged warning. See ``tcp-keepalive-timeout`` for + clients using the EDNS TCP keepalive option. This value can be + updated at runtime by using ``rndc tcp-timeouts``. + +``tcp-keepalive-timeout`` + This sets the amount of time (in units of 100 milliseconds) that the server waits on + an idle TCP connection before closing it, when the client is using the + EDNS TCP keepalive option. The default is 300 (30 seconds), the + maximum is 65535 (about 1.8 hours), and the minimum is 1 (one-tenth + of a second). Values above the maximum or below the minimum are + adjusted with a logged warning. This value may be greater than + ``tcp-idle-timeout`` because clients using the EDNS TCP keepalive + option are expected to use TCP connections for more than one message. + This value can be updated at runtime by using ``rndc tcp-timeouts``. + +``tcp-advertised-timeout`` + This sets the timeout value (in units of 100 milliseconds) that the server sends + in responses containing the EDNS TCP keepalive option, which informs a + client of the amount of time it may keep the session open. The + default is 300 (30 seconds), the maximum is 65535 (about 1.8 hours), + and the minimum is 0, which signals that the clients must close TCP + connections immediately. Ordinarily this should be set to the same + value as ``tcp-keepalive-timeout``. This value can be updated at + runtime by using ``rndc tcp-timeouts``. + +``update-quota`` + This is the maximum number of simultaneous DNS UPDATE messages that + the server will accept for updating local authoritiative zones or + forwarding to a primary server. The default is ``100``. + +.. _intervals: + +Periodic Task Intervals +^^^^^^^^^^^^^^^^^^^^^^^ + +``cleaning-interval`` + This option is obsolete. + +``heartbeat-interval`` + The server performs zone maintenance tasks for all zones marked + as ``dialup`` whenever this interval expires. The default is 60 + minutes. Reasonable values are up to 1 day (1440 minutes). The + maximum value is 28 days (40320 minutes). If set to 0, no zone + maintenance for these zones occurs. + +``interface-interval`` + The server scans the network interface list every ``interface-interval`` + minutes. The default is 60 minutes; the maximum value is 28 days (40320 + minutes). If set to 0, interface scanning only occurs when the configuration + file is loaded, or when ``automatic-interface-scan`` is enabled and supported + by the operating system. After the scan, the server begins listening for + queries on any newly discovered interfaces (provided they are allowed by the + ``listen-on`` configuration), and stops listening on interfaces that have + gone away. For convenience, TTL-style time-unit suffixes may be used to + specify the value. It also accepts ISO 8601 duration formats. + +.. _the_sortlist_statement: + +The ``sortlist`` Statement +^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The response to a DNS query may consist of multiple resource records +(RRs) forming a resource record set (RRset). The name server +normally returns the RRs within the RRset in an indeterminate order (but +see the ``rrset-order`` statement in :ref:`rrset_ordering`). The client resolver code should +rearrange the RRs as appropriate: that is, using any addresses on the +local net in preference to other addresses. However, not all resolvers +can do this or are correctly configured. When a client is using a local +server, the sorting can be performed in the server, based on the +client's address. This only requires configuring the name servers, not +all the clients. + +The ``sortlist`` statement (see below) takes an ``address_match_list`` and +interprets it in a special way. Each top-level statement in the ``sortlist`` +must itself be an explicit ``address_match_list`` with one or two elements. The +first element (which may be an IP address, an IP prefix, an ACL name, or a nested +``address_match_list``) of each top-level list is checked against the source +address of the query until a match is found. When the addresses in the first +element overlap, the first rule to match is selected. + +Once the source address of the query has been matched, if the top-level +statement contains only one element, the actual primitive element that +matched the source address is used to select the address in the response +to move to the beginning of the response. If the statement is a list of +two elements, then the second element is interpreted as a topology +preference list. Each top-level element is assigned a distance, and the +address in the response with the minimum distance is moved to the +beginning of the response. + +In the following example, any queries received from any of the addresses +of the host itself get responses preferring addresses on any of the +locally connected networks. Next most preferred are addresses on the +192.168.1/24 network, and after that either the 192.168.2/24 or +192.168.3/24 network, with no preference shown between these two +networks. Queries received from a host on the 192.168.1/24 network +prefer other addresses on that network to the 192.168.2/24 and +192.168.3/24 networks. Queries received from a host on the 192.168.4/24 +or the 192.168.5/24 network only prefer other addresses on their +directly connected networks. + +:: + + sortlist { + // IF the local host + // THEN first fit on the following nets + { localhost; + { localnets; + 192.168.1/24; + { 192.168.2/24; 192.168.3/24; }; }; }; + // IF on class C 192.168.1 THEN use .1, or .2 or .3 + { 192.168.1/24; + { 192.168.1/24; + { 192.168.2/24; 192.168.3/24; }; }; }; + // IF on class C 192.168.2 THEN use .2, or .1 or .3 + { 192.168.2/24; + { 192.168.2/24; + { 192.168.1/24; 192.168.3/24; }; }; }; + // IF on class C 192.168.3 THEN use .3, or .1 or .2 + { 192.168.3/24; + { 192.168.3/24; + { 192.168.1/24; 192.168.2/24; }; }; }; + // IF .4 or .5 THEN prefer that net + { { 192.168.4/24; 192.168.5/24; }; + }; + }; + +The following example illlustrates reasonable behavior for the local host +and hosts on directly connected networks. Responses sent to queries from the +local host favor any of the directly connected networks. Responses +sent to queries from any other hosts on a directly connected network +prefer addresses on that same network. Responses to other queries +are not sorted. + +:: + + sortlist { + { localhost; localnets; }; + { localnets; }; + }; + +.. _rrset_ordering: + +RRset Ordering +^^^^^^^^^^^^^^ + +.. note:: + + While alternating the order of records in a DNS response between + subsequent queries is a known load distribution technique, certain + caveats apply (mostly stemming from caching) which usually make it a + suboptimal choice for load balancing purposes when used on its own. + +The ``rrset-order`` statement permits configuration of the ordering of +the records in a multiple-record response. See also: +:ref:`the_sortlist_statement`. + +Each rule in an ``rrset-order`` statement is defined as follows: + +:: + + [class <class_name>] [type <type_name>] [name "<domain_name>"] order <ordering> + +The default qualifiers for each rule are: + + - If no ``class`` is specified, the default is ``ANY``. + - If no ``type`` is specified, the default is ``ANY``. + - If no ``name`` is specified, the default is ``*`` (asterisk). + +``<domain_name>`` only matches the name itself, not any of its +subdomains. To make a rule match all subdomains of a given name, a +wildcard name (``*.<domain_name>``) must be used. Note that +``*.<domain_name>`` does *not* match ``<domain_name>`` itself; to +specify RRset ordering for a name and all of its subdomains, two +separate rules must be defined: one for ``<domain_name>`` and one for +``*.<domain_name>``. + +The legal values for ``<ordering>`` are: + +``fixed`` + Records are returned in the order they are defined in the zone file. + +.. note:: + + The ``fixed`` option is only available if BIND is configured with + ``--enable-fixed-rrset`` at compile time. + +``random`` + Records are returned in a random order. + +``cyclic`` + Records are returned in a cyclic round-robin order, rotating by one + record per query. + +``none`` + Records are returned in the order they were retrieved from the + database. This order is indeterminate, but remains consistent as + long as the database is not modified. + +The default RRset order used depends on whether any ``rrset-order`` +statements are present in the configuration file used by ``named``: + + - If no ``rrset-order`` statement is present in the configuration + file, the implicit default is to return all records in ``random`` + order. + + - If any ``rrset-order`` statements are present in the configuration + file, but no ordering rule specified in these statements matches a + given RRset, the default order for that RRset is ``none``. + +Note that if multiple ``rrset-order`` statements are present in the +configuration file (at both the ``options`` and ``view`` levels), they +are *not* combined; instead, the more-specific one (``view``) replaces +the less-specific one (``options``). + +If multiple rules within a single ``rrset-order`` statement match a +given RRset, the first matching rule is applied. + +Example: + +:: + + rrset-order { + type A name "foo.isc.org" order random; + type AAAA name "foo.isc.org" order cyclic; + name "bar.isc.org" order fixed; + name "*.bar.isc.org" order random; + name "*.baz.isc.org" order cyclic; + }; + +With the above configuration, the following RRset ordering is used: + +=================== ======== =========== +QNAME QTYPE RRset Order +=================== ======== =========== +``foo.isc.org`` ``A`` ``random`` +``foo.isc.org`` ``AAAA`` ``cyclic`` +``foo.isc.org`` ``TXT`` ``none`` +``sub.foo.isc.org`` all ``none`` +``bar.isc.org`` all ``fixed`` +``sub.bar.isc.org`` all ``random`` +``baz.isc.org`` all ``none`` +``sub.baz.isc.org`` all ``cyclic`` +=================== ======== =========== + +.. _tuning: + +Tuning +^^^^^^ + +``lame-ttl`` + This is always set to 0. More information is available in the + `security advisory for CVE-2021-25219 + <https://kb.isc.org/docs/cve-2021-25219>`_. + +``servfail-ttl`` + This sets the number of seconds to cache a SERVFAIL response due to DNSSEC + validation failure or other general server failure. If set to ``0``, + SERVFAIL caching is disabled. The SERVFAIL cache is not consulted if + a query has the CD (Checking Disabled) bit set; this allows a query + that failed due to DNSSEC validation to be retried without waiting + for the SERVFAIL TTL to expire. + + The maximum value is ``30`` seconds; any higher value is + silently reduced. The default is ``1`` second. + +``min-ncache-ttl`` + To reduce network traffic and increase performance, the server stores + negative answers. ``min-ncache-ttl`` is used to set a minimum + retention time for these answers in the server, in seconds. For + convenience, TTL-style time-unit suffixes may be used to specify the + value. It also accepts ISO 8601 duration formats. + + The default ``min-ncache-ttl`` is ``0`` seconds. ``min-ncache-ttl`` cannot + exceed 90 seconds and is truncated to 90 seconds if set to a greater + value. + +``min-cache-ttl`` + This sets the minimum time for which the server caches ordinary (positive) + answers, in seconds. For convenience, TTL-style time-unit suffixes may be used + to specify the value. It also accepts ISO 8601 duration formats. + + The default ``min-cache-ttl`` is ``0`` seconds. ``min-cache-ttl`` cannot + exceed 90 seconds and is truncated to 90 seconds if set to a greater + value. + +``max-ncache-ttl`` + To reduce network traffic and increase performance, the server stores + negative answers. ``max-ncache-ttl`` is used to set a maximum retention time + for these answers in the server, in seconds. For convenience, TTL-style + time-unit suffixes may be used to specify the value. It also accepts ISO 8601 + duration formats. + + The default ``max-ncache-ttl`` is 10800 seconds (3 hours). ``max-ncache-ttl`` + cannot exceed 7 days and is silently truncated to 7 days if set to a + greater value. + +``max-cache-ttl`` + This sets the maximum time for which the server caches ordinary (positive) + answers, in seconds. For convenience, TTL-style time-unit suffixes may be used + to specify the value. It also accepts ISO 8601 duration formats. + + The default ``max-cache-ttl`` is 604800 (one week). A value of zero may cause + all queries to return SERVFAIL, because of lost caches of intermediate RRsets + (such as NS and glue AAAA/A records) in the resolution process. + +``max-stale-ttl`` + If retaining stale RRsets in cache is enabled, and returning of stale cached + answers is also enabled, ``max-stale-ttl`` sets the maximum time for which + the server retains records past their normal expiry to return them as stale + records, when the servers for those records are not reachable. The default + is 1 day. The minimum allowed is 1 second; a value of 0 is updated silently + to 1 second. + + For stale answers to be returned, the retaining of them in cache must be + enabled via the configuration option ``stale-cache-enable``, and returning + cached answers must be enabled, either in the configuration file using the + ``stale-answer-enable`` option or by calling ``rndc serve-stale on``. + + When ``stale-cache-enable`` is set to ``no``, setting the ``max-stale-ttl`` + has no effect, the value of ``max-cache-ttl`` will be ``0`` in such case. + +``resolver-nonbackoff-tries`` + This specifies how many retries occur before exponential backoff kicks in. The + default is ``3``. + +``resolver-retry-interval`` + This sets the base retry interval in milliseconds. The default is ``800``. + +``sig-validity-interval`` + this specifies the upper bound of the number of days that RRSIGs + generated by ``named`` are valid; the default is ``30`` days, + with a maximum of 3660 days (10 years). The optional second value + specifies the minimum bound on those RRSIGs and also determines + how long before expiry ``named`` starts regenerating those RRSIGs. + The default value for the lower bound is 1/4 of the upper bound; + it is expressed in days if the upper bound is greater than 7, + and hours if it is less than or equal to 7 days. + + When new RRSIGs are generated, the length of time is randomly + chosen between these two limits, to spread out the re-signing + load. When RRSIGs are re-generated, the upper bound is used, with + a small amount of jitter added. New RRSIGs are generated by a + number of processes, including the processing of UPDATE requests + (ref:`dynamic_update`), the addition and removal of records via + in-line signing, and the initial signing of a zone. + + The signature inception time is unconditionally set to one hour + before the current time, to allow for a limited amount of clock skew. + + The ``sig-validity-interval`` can be overridden for DNSKEY records by + setting ``dnskey-sig-validity``. + + The ``sig-validity-interval`` should be at least several multiples + of the SOA expire interval, to allow for reasonable interaction + between the various timer and expiry dates. + +``dnskey-sig-validity`` + This specifies the number of days into the future when DNSSEC signatures + that are automatically generated for DNSKEY RRsets as a result of + dynamic updates (:ref:`dynamic_update`) will expire. + If set to a non-zero value, this overrides the value set by + ``sig-validity-interval``. The default is zero, meaning + ``sig-validity-interval`` is used. The maximum value is 3660 days (10 + years), and higher values are rejected. + +``sig-signing-nodes`` + This specifies the maximum number of nodes to be examined in each quantum, + when signing a zone with a new DNSKEY. The default is ``100``. + +``sig-signing-signatures`` + This specifies a threshold number of signatures that terminates + processing a quantum, when signing a zone with a new DNSKEY. The + default is ``10``. + +``sig-signing-type`` + This specifies a private RDATA type to be used when generating signing-state + records. The default is ``65534``. + + This parameter may be removed in a future version, + once there is a standard type. + + Signing-state records are used internally by ``named`` to track + the current state of a zone-signing process, i.e., whether it is + still active or has been completed. The records can be inspected + using the command ``rndc signing -list zone``. Once ``named`` has + finished signing a zone with a particular key, the signing-state + record associated with that key can be removed from the zone by + running ``rndc signing -clear keyid/algorithm zone``. To clear all of + the completed signing-state records for a zone, use + ``rndc signing -clear all zone``. + +``min-refresh-time``; ``max-refresh-time``; ``min-retry-time``; ``max-retry-time`` + These options control the server's behavior on refreshing a zone + (querying for SOA changes) or retrying failed transfers. Usually the + SOA values for the zone are used, up to a hard-coded maximum expiry + of 24 weeks. However, these values are set by the primary, giving + secondary server administrators little control over their contents. + + These options allow the administrator to set a minimum and maximum + refresh and retry time in seconds per-zone, per-view, or globally. + These options are valid for secondary and stub zones, and clamp the SOA + refresh and retry times to the specified values. + + The following defaults apply: ``min-refresh-time`` 300 seconds, + ``max-refresh-time`` 2419200 seconds (4 weeks), ``min-retry-time`` + 500 seconds, and ``max-retry-time`` 1209600 seconds (2 weeks). + +``edns-udp-size`` + This sets the maximum advertised EDNS UDP buffer size, in bytes, to control + the size of packets received from authoritative servers in response + to recursive queries. Valid values are 512 to 4096; values outside + this range are silently adjusted to the nearest value within it. + The default value is 1232. + + The usual reason for setting ``edns-udp-size`` to a non-default value + is to get UDP answers to pass through broken firewalls that block + fragmented packets and/or block UDP DNS packets that are greater than + 512 bytes. + + When ``named`` first queries a remote server, it advertises a UDP + buffer size of 512, as this has the greatest chance of success on the + first try. + + If the initial query is successful with EDNS advertising a buffer size of + 512, then ``named`` will advertise progressively larger buffer sizes on + successive queries, until responses begin timing out or ``edns-udp-size`` is + reached. + + The default buffer sizes used by ``named`` are 512, 1232, 1432, and + 4096, but never exceeding ``edns-udp-size``. (The values 1232 and + 1432 are chosen to allow for an IPv4-/IPv6-encapsulated UDP message + to be sent without fragmentation at the minimum MTU sizes for + Ethernet and IPv6 networks.) + + According to the measurements done by multiple parties the default value + should not be causing the fragmentation as most of the Internet "core" is able to + cope with IP message sizes between 1400-1500 bytes, the 1232 size was picked + as a conservative minimal number that could be changed by the DNS operator to + a estimated path MTU minus the estimated header space. In practice, the + smallest MTU witnessed in the operational DNS community is 1500 octets, the + Ethernet maximum payload size, so a a useful default for maximum DNS/UDP + payload size on **reliable** networks would be 1432. + + Any server-specific ``edns-udp-size`` setting has precedence over all + the above rules. + +``max-udp-size`` + This sets the maximum EDNS UDP message size that ``named`` sends, in bytes. + Valid values are 512 to 4096; values outside this range are + silently adjusted to the nearest value within it. The default value + is 1232. + + This value applies to responses sent by a server; to set the + advertised buffer size in queries, see ``edns-udp-size``. + + The usual reason for setting ``max-udp-size`` to a non-default value + is to allow UDP answers to pass through broken firewalls that block + fragmented packets and/or block UDP packets that are greater than 512 + bytes. This is independent of the advertised receive buffer + (``edns-udp-size``). + + Setting this to a low value encourages additional TCP traffic to + the name server. + +``masterfile-format`` + This specifies the file format of zone files (see :ref:`zonefile_format` + for details). The default value is ``text``, which is the standard + textual representation, except for secondary zones, in which the default + value is ``raw``. Files in formats other than ``text`` are typically + expected to be generated by the ``named-compilezone`` tool, or dumped by + ``named``. + + Note that when a zone file in a format other than ``text`` is loaded, + ``named`` may omit some of the checks which are performed for a file in + ``text`` format. For example, ``check-names`` only applies when loading + zones in ``text`` format, and ``max-zone-ttl`` only applies to ``text`` + and ``raw``. Zone files in binary formats should be generated with the + same check level as that specified in the ``named`` configuration file. + + ``map`` format files are loaded directly into memory via memory mapping, + with only minimal validity checking. Because they are not guaranteed to + be compatible from one version of BIND 9 to another, and are not + compatible from one system architecture to another, they should be used + with caution. See :ref:`zonefile_format` for further discussion. + + When configured in ``options``, this statement sets the + ``masterfile-format`` for all zones, but it can be overridden on a + per-zone or per-view basis by including a ``masterfile-format`` + statement within the ``zone`` or ``view`` block in the configuration + file. + +``masterfile-style`` + This specifies the formatting of zone files during dump, when the + ``masterfile-format`` is ``text``. This option is ignored with any + other ``masterfile-format``. + + When set to ``relative``, records are printed in a multi-line format, + with owner names expressed relative to a shared origin. When set to + ``full``, records are printed in a single-line format with absolute + owner names. The ``full`` format is most suitable when a zone file + needs to be processed automatically by a script. The ``relative`` + format is more human-readable, and is thus suitable when a zone is to + be edited by hand. The default is ``relative``. + +``max-recursion-depth`` + This sets the maximum number of levels of recursion that are permitted at + any one time while servicing a recursive query. Resolving a name may + require looking up a name server address, which in turn requires + resolving another name, etc.; if the number of recursions exceeds + this value, the recursive query is terminated and returns SERVFAIL. + The default is 7. + +``max-recursion-queries`` + This sets the maximum number of iterative queries that may be sent while + servicing a recursive query. If more queries are sent, the recursive + query is terminated and returns SERVFAIL. The default is 100. + +``notify-delay`` + This sets the delay, in seconds, between sending sets of NOTIFY messages + for a zone. Whenever a NOTIFY message is sent for a zone, a timer will + be set for this duration. If the zone is updated again before the timer + expires, the NOTIFY for that update will be postponed. The default is 5 + seconds. + + The overall rate at which NOTIFY messages are sent for all zones is + controlled by ``notify-rate``. + +``max-rsa-exponent-size`` + This sets the maximum RSA exponent size, in bits, that is accepted when + validating. Valid values are 35 to 4096 bits. The default, zero, is + also accepted and is equivalent to 4096. + +``prefetch`` + When a query is received for cached data which is to expire shortly, + ``named`` can refresh the data from the authoritative server + immediately, ensuring that the cache always has an answer available. + + ``prefetch`` specifies the "trigger" TTL value at which prefetch + of the current query takes place; when a cache record with a + lower or equal TTL value is encountered during query processing, it is + refreshed. Valid trigger TTL values are 1 to 10 seconds. Values + larger than 10 seconds are silently reduced to 10. Setting a + trigger TTL to zero causes prefetch to be disabled. The default + trigger TTL is ``2``. + + An optional second argument specifies the "eligibility" TTL: the + smallest *original* TTL value that is accepted for a record to + be eligible for prefetching. The eligibility TTL must be at least six + seconds longer than the trigger TTL; if not, ``named`` + silently adjusts it upward. The default eligibility TTL is ``9``. + +``v6-bias`` + When determining the next name server to try, this indicates by how many + milliseconds to prefer IPv6 name servers. The default is ``50`` + milliseconds. + +.. _builtin: + +Built-in Server Information Zones +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The server provides some helpful diagnostic information through a number +of built-in zones under the pseudo-top-level-domain ``bind`` in the +``CHAOS`` class. These zones are part of a built-in view +(see :ref:`view_statement_grammar`) of class ``CHAOS``, which is +separate from the default view of class ``IN``. Most global +configuration options (``allow-query``, etc.) apply to this view, +but some are locally overridden: ``notify``, ``recursion``, and +``allow-new-zones`` are always set to ``no``, and ``rate-limit`` is set +to allow three responses per second. + +To disable these zones, use the options below or hide the +built-in ``CHAOS`` view by defining an explicit view of class ``CHAOS`` +that matches all clients. + +``version`` + This is the version the server should report via a query of the name + ``version.bind`` with type ``TXT`` and class ``CHAOS``. The default is + the real version number of this server. Specifying ``version none`` + disables processing of the queries. + + Setting ``version`` to any value (including ``none``) also disables + queries for ``authors.bind TXT CH``. + +``hostname`` + This is the hostname the server should report via a query of the name + ``hostname.bind`` with type ``TXT`` and class ``CHAOS``. This defaults + to the hostname of the machine hosting the name server, as found by + the ``gethostname()`` function. The primary purpose of such queries is to + identify which of a group of anycast servers is actually answering + the queries. Specifying ``hostname none;`` disables processing of + the queries. + +``server-id`` + This is the ID the server should report when receiving a Name Server + Identifier (NSID) query, or a query of the name ``ID.SERVER`` with + type ``TXT`` and class ``CHAOS``. The primary purpose of such queries is + to identify which of a group of anycast servers is actually answering + the queries. Specifying ``server-id none;`` disables processing of + the queries. Specifying ``server-id hostname;`` causes ``named`` + to use the hostname as found by the ``gethostname()`` function. The + default ``server-id`` is ``none``. + +.. _empty: + +Built-in Empty Zones +^^^^^^^^^^^^^^^^^^^^ + +The ``named`` server has some built-in empty zones, for SOA and NS records +only. These are for zones that should normally be answered locally and for +which queries should not be sent to the Internet's root servers. The +official servers that cover these namespaces return NXDOMAIN responses +to these queries. In particular, these cover the reverse namespaces for +addresses from :rfc:`1918`, :rfc:`4193`, :rfc:`5737`, and :rfc:`6598`. They also +include the reverse namespace for the IPv6 local address (locally assigned), +IPv6 link local addresses, the IPv6 loopback address, and the IPv6 +unknown address. + +The server attempts to determine if a built-in zone already exists +or is active (covered by a forward-only forwarding declaration) and does +not create an empty zone if either is true. + +The current list of empty zones is: + +- 10.IN-ADDR.ARPA +- 16.172.IN-ADDR.ARPA +- 17.172.IN-ADDR.ARPA +- 18.172.IN-ADDR.ARPA +- 19.172.IN-ADDR.ARPA +- 20.172.IN-ADDR.ARPA +- 21.172.IN-ADDR.ARPA +- 22.172.IN-ADDR.ARPA +- 23.172.IN-ADDR.ARPA +- 24.172.IN-ADDR.ARPA +- 25.172.IN-ADDR.ARPA +- 26.172.IN-ADDR.ARPA +- 27.172.IN-ADDR.ARPA +- 28.172.IN-ADDR.ARPA +- 29.172.IN-ADDR.ARPA +- 30.172.IN-ADDR.ARPA +- 31.172.IN-ADDR.ARPA +- 168.192.IN-ADDR.ARPA +- 64.100.IN-ADDR.ARPA +- 65.100.IN-ADDR.ARPA +- 66.100.IN-ADDR.ARPA +- 67.100.IN-ADDR.ARPA +- 68.100.IN-ADDR.ARPA +- 69.100.IN-ADDR.ARPA +- 70.100.IN-ADDR.ARPA +- 71.100.IN-ADDR.ARPA +- 72.100.IN-ADDR.ARPA +- 73.100.IN-ADDR.ARPA +- 74.100.IN-ADDR.ARPA +- 75.100.IN-ADDR.ARPA +- 76.100.IN-ADDR.ARPA +- 77.100.IN-ADDR.ARPA +- 78.100.IN-ADDR.ARPA +- 79.100.IN-ADDR.ARPA +- 80.100.IN-ADDR.ARPA +- 81.100.IN-ADDR.ARPA +- 82.100.IN-ADDR.ARPA +- 83.100.IN-ADDR.ARPA +- 84.100.IN-ADDR.ARPA +- 85.100.IN-ADDR.ARPA +- 86.100.IN-ADDR.ARPA +- 87.100.IN-ADDR.ARPA +- 88.100.IN-ADDR.ARPA +- 89.100.IN-ADDR.ARPA +- 90.100.IN-ADDR.ARPA +- 91.100.IN-ADDR.ARPA +- 92.100.IN-ADDR.ARPA +- 93.100.IN-ADDR.ARPA +- 94.100.IN-ADDR.ARPA +- 95.100.IN-ADDR.ARPA +- 96.100.IN-ADDR.ARPA +- 97.100.IN-ADDR.ARPA +- 98.100.IN-ADDR.ARPA +- 99.100.IN-ADDR.ARPA +- 100.100.IN-ADDR.ARPA +- 101.100.IN-ADDR.ARPA +- 102.100.IN-ADDR.ARPA +- 103.100.IN-ADDR.ARPA +- 104.100.IN-ADDR.ARPA +- 105.100.IN-ADDR.ARPA +- 106.100.IN-ADDR.ARPA +- 107.100.IN-ADDR.ARPA +- 108.100.IN-ADDR.ARPA +- 109.100.IN-ADDR.ARPA +- 110.100.IN-ADDR.ARPA +- 111.100.IN-ADDR.ARPA +- 112.100.IN-ADDR.ARPA +- 113.100.IN-ADDR.ARPA +- 114.100.IN-ADDR.ARPA +- 115.100.IN-ADDR.ARPA +- 116.100.IN-ADDR.ARPA +- 117.100.IN-ADDR.ARPA +- 118.100.IN-ADDR.ARPA +- 119.100.IN-ADDR.ARPA +- 120.100.IN-ADDR.ARPA +- 121.100.IN-ADDR.ARPA +- 122.100.IN-ADDR.ARPA +- 123.100.IN-ADDR.ARPA +- 124.100.IN-ADDR.ARPA +- 125.100.IN-ADDR.ARPA +- 126.100.IN-ADDR.ARPA +- 127.100.IN-ADDR.ARPA +- 0.IN-ADDR.ARPA +- 127.IN-ADDR.ARPA +- 254.169.IN-ADDR.ARPA +- 2.0.192.IN-ADDR.ARPA +- 100.51.198.IN-ADDR.ARPA +- 113.0.203.IN-ADDR.ARPA +- 255.255.255.255.IN-ADDR.ARPA +- 0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA +- 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA +- 8.B.D.0.1.0.0.2.IP6.ARPA +- D.F.IP6.ARPA +- 8.E.F.IP6.ARPA +- 9.E.F.IP6.ARPA +- A.E.F.IP6.ARPA +- B.E.F.IP6.ARPA +- EMPTY.AS112.ARPA +- HOME.ARPA + +Empty zones can be set at the view level and only apply to views of +class IN. Disabled empty zones are only inherited from options if there +are no disabled empty zones specified at the view level. To override the +options list of disabled zones, disable the root zone at the +view level. For example: + +:: + + disable-empty-zone "."; + +If using the address ranges covered here, +reverse zones covering the addresses should already be in place. In practice this +appears to not be the case, with many queries being made to the +infrastructure servers for names in these spaces. So many, in fact, that +sacrificial servers had to be deployed to channel the query load +away from the infrastructure servers. + +.. note:: + + The real parent servers for these zones should disable all empty zones + under the parent zone they serve. For the real root servers, this is + all built-in empty zones. This enables them to return referrals + to deeper in the tree. + +``empty-server`` + This specifies the server name that appears in the returned SOA record for + empty zones. If none is specified, the zone's name is used. + +``empty-contact`` + This specifies the contact name that appears in the returned SOA record for + empty zones. If none is specified, "." is used. + +``empty-zones-enable`` + This enables or disables all empty zones. By default, they are enabled. + +``disable-empty-zone`` + This disables individual empty zones. By default, none are disabled. This + option can be specified multiple times. + +.. _content_filtering: + +Content Filtering +^^^^^^^^^^^^^^^^^ + +BIND 9 provides the ability to filter out responses from external +DNS servers containing certain types of data in the answer section. +Specifically, it can reject address (A or AAAA) records if the +corresponding IPv4 or IPv6 addresses match the given +``address_match_list`` of the ``deny-answer-addresses`` option. It can +also reject CNAME or DNAME records if the "alias" name (i.e., the CNAME +alias or the substituted query name due to DNAME) matches the given +``namelist`` of the ``deny-answer-aliases`` option, where "match" means +the alias name is a subdomain of one of the ``name_list`` elements. If +the optional ``namelist`` is specified with ``except-from``, records +whose query name matches the list are accepted regardless of the +filter setting. Likewise, if the alias name is a subdomain of the +corresponding zone, the ``deny-answer-aliases`` filter does not apply; +for example, even if "example.com" is specified for +``deny-answer-aliases``, + +:: + + www.example.com. CNAME xxx.example.com. + +returned by an "example.com" server is accepted. + +In the ``address_match_list`` of the ``deny-answer-addresses`` option, +only ``ip_addr`` and ``ip_prefix`` are meaningful; any ``key_id`` is +silently ignored. + +If a response message is rejected due to the filtering, the entire +message is discarded without being cached, and a SERVFAIL error is +returned to the client. + +This filtering is intended to prevent "DNS rebinding attacks," in which +an attacker, in response to a query for a domain name the attacker +controls, returns an IP address within the user's own network or an alias name +within the user's own domain. A naive web browser or script could then serve +as an unintended proxy, allowing the attacker to get access to an +internal node of the local network that could not be externally accessed +otherwise. See the paper available at +https://dl.acm.org/doi/10.1145/1315245.1315298 for more details +about these attacks. + +For example, with a domain named "example.net" and an internal +network using an IPv4 prefix 192.0.2.0/24, an administrator might specify the +following rules: + +:: + + deny-answer-addresses { 192.0.2.0/24; } except-from { "example.net"; }; + deny-answer-aliases { "example.net"; }; + +If an external attacker let a web browser in the local network look up +an IPv4 address of "attacker.example.com", the attacker's DNS server +would return a response like this: + +:: + + attacker.example.com. A 192.0.2.1 + +in the answer section. Since the rdata of this record (the IPv4 address) +matches the specified prefix 192.0.2.0/24, this response would be +ignored. + +On the other hand, if the browser looked up a legitimate internal web +server "www.example.net" and the following response were returned to the +BIND 9 server: + +:: + + www.example.net. A 192.0.2.2 + +it would be accepted, since the owner name "www.example.net" matches the +``except-from`` element, "example.net". + +Note that this is not really an attack on the DNS per se. In fact, there +is nothing wrong with having an "external" name mapped to an "internal" +IP address or domain name from the DNS point of view; it might actually +be provided for a legitimate purpose, such as for debugging. As long as +the mapping is provided by the correct owner, it either is not possible or does +not make sense to detect whether the intent of the mapping is legitimate +within the DNS. The "rebinding" attack must primarily be +protected at the application that uses the DNS. For a large site, +however, it may be difficult to protect all possible applications at +once. This filtering feature is provided only to help such an +operational environment; turning it on is generally discouraged +unless there is no other choice and the attack is a +real threat to applications. + +Care should be particularly taken if using this option for +addresses within 127.0.0.0/8. These addresses are obviously "internal," +but many applications conventionally rely on a DNS mapping from some +name to such an address. Filtering out DNS records containing this +address spuriously can break such applications. + +.. _rpz: + +Response Policy Zone (RPZ) Rewriting +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +BIND 9 includes a limited mechanism to modify DNS responses for requests +analogous to email anti-spam DNS rejection lists. Responses can be changed to +deny the existence of domains (NXDOMAIN), deny the existence of IP +addresses for domains (NODATA), or contain other IP addresses or data. + +Response policy zones are named in the ``response-policy`` option for +the view, or among the global options if there is no ``response-policy`` +option for the view. Response policy zones are ordinary DNS zones +containing RRsets that can be queried normally if allowed. It is usually +best to restrict those queries with something like +``allow-query { localhost; };``. Note that zones using +``masterfile-format map`` cannot be used as policy zones. + +A ``response-policy`` option can support multiple policy zones. To +maximize performance, a radix tree is used to quickly identify response +policy zones containing triggers that match the current query. This +imposes an upper limit of 64 on the number of policy zones in a single +``response-policy`` option; more than that is a configuration error. + +Rules encoded in response policy zones are processed after those defined in +:ref:`access_control`. All queries from clients which are not permitted access +to the resolver are answered with a status code of REFUSED, regardless of +configured RPZ rules. + +Five policy triggers can be encoded in RPZ records. + +``RPZ-CLIENT-IP`` + IP records are triggered by the IP address of the DNS client. Client + IP address triggers are encoded in records that have owner names that + are subdomains of ``rpz-client-ip``, relativized to the policy zone + origin name, and that encode an address or address block. IPv4 addresses + are represented as ``prefixlength.B4.B3.B2.B1.rpz-client-ip``. The + IPv4 prefix length must be between 1 and 32. All four bytes - B4, B3, + B2, and B1 - must be present. B4 is the decimal value of the least + significant byte of the IPv4 address as in IN-ADDR.ARPA. + + IPv6 addresses are encoded in a format similar to the standard IPv6 + text representation, + ``prefixlength.W8.W7.W6.W5.W4.W3.W2.W1.rpz-client-ip``. Each of + W8,...,W1 is a one- to four-digit hexadecimal number representing 16 + bits of the IPv6 address as in the standard text representation of + IPv6 addresses, but reversed as in IP6.ARPA. (Note that this + representation of IPv6 addresses is different from IP6.ARPA, where each + hex digit occupies a label.) All 8 words must be present except when + one set of consecutive zero words is replaced with ``.zz.``, analogous + to double colons (::) in standard IPv6 text encodings. The IPv6 + prefix length must be between 1 and 128. + +``QNAME`` + QNAME policy records are triggered by query names of requests and + targets of CNAME records resolved to generate the response. The owner + name of a QNAME policy record is the query name relativized to the + policy zone. + +``RPZ-IP`` + IP triggers are IP addresses in an A or AAAA record in the ANSWER + section of a response. They are encoded like client-IP triggers, + except as subdomains of ``rpz-ip``. + +``RPZ-NSDNAME`` + NSDNAME triggers match names of authoritative servers for the query name, a + parent of the query name, a CNAME for the query name, or a parent of a CNAME. + They are encoded as subdomains of ``rpz-nsdname``, relativized + to the RPZ origin name. NSIP triggers match IP addresses in A and AAAA + RRsets for domains that can be checked against NSDNAME policy records. The + ``nsdname-enable`` phrase turns NSDNAME triggers off or on for a single + policy zone or for all zones. + + If authoritative name servers for the query name are not yet known, ``named`` + recursively looks up the authoritative servers for the query name before + applying an RPZ-NSDNAME rule, which can cause a processing delay. + +``RPZ-NSIP`` + NSIP triggers match the IP addresses of authoritative servers. They + are encoded like IP triggers, except as subdomains of ``rpz-nsip``. + NSDNAME and NSIP triggers are checked only for names with at least + ``min-ns-dots`` dots. The default value of ``min-ns-dots`` is 1, to + exclude top-level domains. The ``nsip-enable`` phrase turns NSIP + triggers off or on for a single policy zone or for all zones. + + If a name server's IP address is not yet known, ``named`` + recursively looks up the IP address before applying an RPZ-NSIP rule, + which can cause a processing delay. To speed up processing at the cost + of precision, the ``nsip-wait-recurse`` option can be used; when set + to ``no``, RPZ-NSIP rules are only applied when a name server's + IP address has already been looked up and cached. If a server's IP + address is not in the cache, the RPZ-NSIP rule is ignored, + but the address is looked up in the background and the rule + is applied to subsequent queries. The default is ``yes``, + meaning RPZ-NSIP rules are always applied, even if an address + needs to be looked up first. + +The query response is checked against all response policy zones, so two +or more policy records can be triggered by a response. Because DNS +responses are rewritten according to at most one policy record, a single +record encoding an action (other than ``DISABLED`` actions) must be +chosen. Triggers, or the records that encode them, are chosen for +rewriting in the following order: + +1. Choose the triggered record in the zone that appears first in the + response-policy option. +2. Prefer CLIENT-IP to QNAME to IP to NSDNAME to NSIP triggers in a + single zone. +3. Among NSDNAME triggers, prefer the trigger that matches the smallest + name under the DNSSEC ordering. +4. Among IP or NSIP triggers, prefer the trigger with the longest + prefix. +5. Among triggers with the same prefix length, prefer the IP or NSIP + trigger that matches the smallest IP address. + +When the processing of a response is restarted to resolve DNAME or CNAME +records and a policy record set has not been triggered, all response +policy zones are again consulted for the DNAME or CNAME names and +addresses. + +RPZ record sets are any types of DNS record, except DNAME or DNSSEC, that +encode actions or responses to individual queries. Any of the policies +can be used with any of the triggers. For example, while the +``TCP-only`` policy is commonly used with ``client-IP`` triggers, it can +be used with any type of trigger to force the use of TCP for responses +with owner names in a zone. + +``PASSTHRU`` + The auto-acceptance policy is specified by a CNAME whose target is + ``rpz-passthru``. It causes the response to not be rewritten and is + most often used to "poke holes" in policies for CIDR blocks. + +``DROP`` + The auto-rejection policy is specified by a CNAME whose target is + ``rpz-drop``. It causes the response to be discarded. Nothing is sent + to the DNS client. + +``TCP-Only`` + The "slip" policy is specified by a CNAME whose target is + ``rpz-tcp-only``. It changes UDP responses to short, truncated DNS + responses that require the DNS client to try again with TCP. It is + used to mitigate distributed DNS reflection attacks. + +``NXDOMAIN`` + The "domain undefined" response is encoded by a CNAME whose target is + the root domain (.). + +``NODATA`` + The empty set of resource records is specified by a CNAME whose target + is the wildcard top-level domain (``*.``). It rewrites the response to + NODATA or ANCOUNT=0. + +``Local Data`` + A set of ordinary DNS records can be used to answer queries. Queries + for record types not in the set are answered with NODATA. + + A special form of local data is a CNAME whose target is a wildcard + such as \*.example.com. It is used as if an ordinary CNAME after + the asterisk (\*) has been replaced with the query name. + This special form is useful for query logging in the walled garden's + authoritative DNS server. + +All of the actions specified in all of the individual records in a +policy zone can be overridden with a ``policy`` clause in the +``response-policy`` option. An organization using a policy zone provided +by another organization might use this mechanism to redirect domains to +its own walled garden. + +``GIVEN`` + The placeholder policy says "do not override but perform the action + specified in the zone." + +``DISABLED`` + The testing override policy causes policy zone records to do nothing + but log what they would have done if the policy zone were not + disabled. The response to the DNS query is written (or not) + according to any triggered policy records that are not disabled. + Disabled policy zones should appear first, because they are often + not logged if a higher-precedence trigger is found first. + +``PASSTHRU``; ``DROP``; ``TCP-Only``; ``NXDOMAIN``; ``NODATA`` + These settings each override the corresponding per-record policy. + +``CNAME domain`` + This causes all RPZ policy records to act as if they were "cname domain" + records. + +By default, the actions encoded in a response policy zone are applied +only to queries that ask for recursion (RD=1). That default can be +changed for a single policy zone, or for all response policy zones in a view, +with a ``recursive-only no`` clause. This feature is useful for serving +the same zone files both inside and outside an :rfc:`1918` cloud and using +RPZ to delete answers that would otherwise contain :rfc:`1918` values on +the externally visible name server or view. + +Also by default, RPZ actions are applied only to DNS requests that +either do not request DNSSEC metadata (DO=0) or when no DNSSEC records +are available for the requested name in the original zone (not the response +policy zone). This default can be changed for all response policy zones +in a view with a ``break-dnssec yes`` clause. In that case, RPZ actions +are applied regardless of DNSSEC. The name of the clause option reflects +the fact that results rewritten by RPZ actions cannot verify. + +No DNS records are needed for a QNAME or Client-IP trigger; the name or +IP address itself is sufficient, so in principle the query name need not +be recursively resolved. However, not resolving the requested name can +leak the fact that response policy rewriting is in use, and that the name +is listed in a policy zone, to operators of servers for listed names. To +prevent that information leak, by default any recursion needed for a +request is done before any policy triggers are considered. Because +listed domains often have slow authoritative servers, this behavior can +cost significant time. The ``qname-wait-recurse no`` option overrides +the default and enables that behavior when recursion cannot change a +non-error response. The option does not affect QNAME or client-IP +triggers in policy zones listed after other zones containing IP, NSIP, +and NSDNAME triggers, because those may depend on the A, AAAA, and NS +records that would be found during recursive resolution. It also does +not affect DNSSEC requests (DO=1) unless ``break-dnssec yes`` is in use, +because the response would depend on whether RRSIG records were +found during resolution. Using this option can cause error responses +such as SERVFAIL to appear to be rewritten, since no recursion is being +done to discover problems at the authoritative server. + +The ``dnsrps-enable yes`` option turns on the DNS Response Policy Service +(DNSRPS) interface, if it has been compiled in ``named`` using +``configure --enable-dnsrps``. + +The ``dnsrps-options`` block provides additional RPZ configuration +settings, which are passed through to the DNSRPS provider library. +Multiple DNSRPS settings in an ``dnsrps-options`` string should be +separated with semi-colons (;). The DNSRPS provider, librpz, is passed a +configuration string consisting of the ``dnsrps-options`` text, +concatenated with settings derived from the ``response-policy`` +statement. + +Note: the ``dnsrps-options`` text should only include configuration +settings that are specific to the DNSRPS provider. For example, the +DNSRPS provider from Farsight Security takes options such as +``dnsrpzd-conf``, ``dnsrpzd-sock``, and ``dnzrpzd-args`` (for details of +these options, see the ``librpz`` documentation). Other RPZ +configuration settings could be included in ``dnsrps-options`` as well, +but if ``named`` were switched back to traditional RPZ by setting +``dnsrps-enable`` to "no", those options would be ignored. + +The TTL of a record modified by RPZ policies is set from the TTL of the +relevant record in the policy zone. It is then limited to a maximum value. +The ``max-policy-ttl`` clause changes the maximum number of seconds from its +default of 5. For convenience, TTL-style time-unit suffixes may be used +to specify the value. It also accepts ISO 8601 duration formats. + +For example, an administrator might use this option statement: + +:: + + response-policy { zone "badlist"; }; + +and this zone statement: + +:: + + zone "badlist" {type primary; file "primary/badlist"; allow-query {none;}; }; + +with this zone file: + +:: + + $TTL 1H + @ SOA LOCALHOST. named-mgr.example.com (1 1h 15m 30d 2h) + NS LOCALHOST. + + ; QNAME policy records. There are no periods (.) after the owner names. + nxdomain.domain.com CNAME . ; NXDOMAIN policy + *.nxdomain.domain.com CNAME . ; NXDOMAIN policy + nodata.domain.com CNAME *. ; NODATA policy + *.nodata.domain.com CNAME *. ; NODATA policy + bad.domain.com A 10.0.0.1 ; redirect to a walled garden + AAAA 2001:2::1 + bzone.domain.com CNAME garden.example.com. + + ; do not rewrite (PASSTHRU) OK.DOMAIN.COM + ok.domain.com CNAME rpz-passthru. + + ; redirect x.bzone.domain.com to x.bzone.domain.com.garden.example.com + *.bzone.domain.com CNAME *.garden.example.com. + + ; IP policy records that rewrite all responses containing A records in 127/8 + ; except 127.0.0.1 + 8.0.0.0.127.rpz-ip CNAME . + 32.1.0.0.127.rpz-ip CNAME rpz-passthru. + + ; NSDNAME and NSIP policy records + ns.domain.com.rpz-nsdname CNAME . + 48.zz.2.2001.rpz-nsip CNAME . + + ; auto-reject and auto-accept some DNS clients + 112.zz.2001.rpz-client-ip CNAME rpz-drop. + 8.0.0.0.127.rpz-client-ip CNAME rpz-drop. + + ; force some DNS clients and responses in the example.com zone to TCP + 16.0.0.1.10.rpz-client-ip CNAME rpz-tcp-only. + example.com CNAME rpz-tcp-only. + *.example.com CNAME rpz-tcp-only. + +RPZ can affect server performance. Each configured response policy zone +requires the server to perform one to four additional database lookups +before a query can be answered. For example, a DNS server with four +policy zones, each with all four kinds of response triggers (QNAME, IP, +NSIP, and NSDNAME), requires a total of 17 times as many database lookups +as a similar DNS server with no response policy zones. A BIND 9 server +with adequate memory and one response policy zone with QNAME and IP +triggers might achieve a maximum queries-per-second (QPS) rate about 20% +lower. A server with four response policy zones with QNAME and IP +triggers might have a maximum QPS rate about 50% lower. + +Responses rewritten by RPZ are counted in the ``RPZRewrites`` +statistics. + +The ``log`` clause can be used to optionally turn off rewrite logging +for a particular response policy zone. By default, all rewrites are +logged. + +The ``add-soa`` option controls whether the RPZ's SOA record is added to +the section for traceback of changes from this zone. +This can be set at the individual policy zone level or at the +response-policy level. The default is ``yes``. + +Updates to RPZ zones are processed asynchronously; if there is more than +one update pending they are bundled together. If an update to a RPZ zone +(for example, via IXFR) happens less than ``min-update-interval`` +seconds after the most recent update, the changes are not +carried out until this interval has elapsed. The default is ``60`` +seconds. For convenience, TTL-style time-unit suffixes may be used to +specify the value. It also accepts ISO 8601 duration formats. + +.. _rrl: + +Response Rate Limiting +^^^^^^^^^^^^^^^^^^^^^^ + +Excessive, almost-identical UDP *responses* can be controlled by +configuring a ``rate-limit`` clause in an ``options`` or ``view`` +statement. This mechanism keeps authoritative BIND 9 from being used to +amplify reflection denial-of-service (DoS) attacks. Short BADCOOKIE errors or +truncated (TC=1) responses can be sent to provide rate-limited responses to +legitimate clients within a range of forged, attacked IP addresses. +Legitimate clients react to dropped responses by retrying, +to BADCOOKIE errors by including a server cookie when retrying, +and to truncated responses by switching to TCP. + +This mechanism is intended for authoritative DNS servers. It can be used +on recursive servers, but can slow applications such as SMTP servers +(mail receivers) and HTTP clients (web browsers) that repeatedly request +the same domains. When possible, closing "open" recursive servers is +better. + +Response rate limiting uses a "credit" or "token bucket" scheme. Each +combination of identical response and client has a conceptual "account" +that earns a specified number of credits every second. A prospective +response debits its account by one. Responses are dropped or truncated +while the account is negative. Responses are tracked within a rolling +window of time which defaults to 15 seconds, but which can be configured with +the ``window`` option to any value from 1 to 3600 seconds (1 hour). The +account cannot become more positive than the per-second limit or more +negative than ``window`` times the per-second limit. When the specified +number of credits for a class of responses is set to 0, those responses +are not rate-limited. + +The notions of "identical response" and "DNS client" for rate limiting +are not simplistic. All responses to an address block are counted as if +to a single client. The prefix lengths of address blocks are specified +with ``ipv4-prefix-length`` (default 24) and ``ipv6-prefix-length`` +(default 56). + +All non-empty responses for a valid domain name (qname) and record type +(qtype) are identical and have a limit specified with +``responses-per-second`` (default 0 or no limit). All valid wildcard +domain names are interpreted as the zone's origin name concatenated to +the "*" name. All empty (NODATA) responses for a valid domain, +regardless of query type, are identical. Responses in the NODATA class +are limited by ``nodata-per-second`` (default ``responses-per-second``). +Requests for any and all undefined subdomains of a given valid domain +result in NXDOMAIN errors, and are identical regardless of query type. +They are limited by ``nxdomains-per-second`` (default +``responses-per-second``). This controls some attacks using random +names, but can be relaxed or turned off (set to 0) on servers that +expect many legitimate NXDOMAIN responses, such as from anti-spam +rejection lists. Referrals or delegations to the server of a given +domain are identical and are limited by ``referrals-per-second`` +(default ``responses-per-second``). + +Responses generated from local wildcards are counted and limited as if +they were for the parent domain name. This controls flooding using +random.wild.example.com. + +All requests that result in DNS errors other than NXDOMAIN, such as +SERVFAIL and FORMERR, are identical regardless of requested name (qname) +or record type (qtype). This controls attacks using invalid requests or +distant, broken authoritative servers. By default the limit on errors is +the same as the ``responses-per-second`` value, but it can be set +separately with ``errors-per-second``. + +Many attacks using DNS involve UDP requests with forged source +addresses. Rate limiting prevents the use of BIND 9 to flood a network +with responses to requests with forged source addresses, but could let a +third party block responses to legitimate requests. There is a mechanism +that can answer some legitimate requests from a client whose address is +being forged in a flood. Setting ``slip`` to 2 (its default) causes +every other UDP request without a valid server cookie to be answered with +a small response. The small size and reduced frequency, and resulting lack of +amplification, of "slipped" responses make them unattractive for +reflection DoS attacks. ``slip`` must be between 0 and 10. A value of 0 +does not "slip"; no small responses are sent due to rate limiting. Rather, +all responses are dropped. A value of 1 causes every response to slip; +values between 2 and 10 cause every nth response to slip. + +If the request included a client cookie, then a "slipped" response is +a BADCOOKIE error with a server cookie, which allows a legitimate client +to include the server cookie to be exempted from the rate limiting +when it retries the request. +If the request did not include a cookie, then a "slipped" response is +a truncated (TC=1) response, which prompts a legitimate client to +switch to TCP and thus be exempted from the rate limiting. Some error +responses, including REFUSED and SERVFAIL, cannot be replaced with +truncated responses and are instead leaked at the ``slip`` rate. + +(Note: dropped responses from an authoritative server may reduce the +difficulty of a third party successfully forging a response to a +recursive resolver. The best security against forged responses is for +authoritative operators to sign their zones using DNSSEC and for +resolver operators to validate the responses. When this is not an +option, operators who are more concerned with response integrity than +with flood mitigation may consider setting ``slip`` to 1, causing all +rate-limited responses to be truncated rather than dropped. This reduces +the effectiveness of rate-limiting against reflection attacks.) + +When the approximate query-per-second rate exceeds the ``qps-scale`` +value, the ``responses-per-second``, ``errors-per-second``, +``nxdomains-per-second``, and ``all-per-second`` values are reduced by +the ratio of the current rate to the ``qps-scale`` value. This feature +can tighten defenses during attacks. For example, with +``qps-scale 250; responses-per-second 20;`` and a total query rate of +1000 queries/second for all queries from all DNS clients including via +TCP, then the effective responses/second limit changes to (250/1000)*20, +or 5. Responses to requests that included a valid server cookie, +and responses sent via TCP, are not limited but are counted to compute +the query-per-second rate. + +Communities of DNS clients can be given their own parameters or no +rate limiting by putting ``rate-limit`` statements in ``view`` statements +instead of in the global ``option`` statement. A ``rate-limit`` statement +in a view replaces, rather than supplements, a ``rate-limit`` +statement among the main options. DNS clients within a view can be +exempted from rate limits with the ``exempt-clients`` clause. + +UDP responses of all kinds can be limited with the ``all-per-second`` +phrase. This rate limiting is unlike the rate limiting provided by +``responses-per-second``, ``errors-per-second``, and +``nxdomains-per-second`` on a DNS server, which are often invisible to +the victim of a DNS reflection attack. Unless the forged requests of the +attack are the same as the legitimate requests of the victim, the +victim's requests are not affected. Responses affected by an +``all-per-second`` limit are always dropped; the ``slip`` value has no +effect. An ``all-per-second`` limit should be at least 4 times as large +as the other limits, because single DNS clients often send bursts of +legitimate requests. For example, the receipt of a single mail message +can prompt requests from an SMTP server for NS, PTR, A, and AAAA records +as the incoming SMTP/TCP/IP connection is considered. The SMTP server +can need additional NS, A, AAAA, MX, TXT, and SPF records as it +considers the SMTP ``Mail From`` command. Web browsers often repeatedly +resolve the same names that are duplicated in HTML <IMG> tags in a page. +``all-per-second`` is similar to the rate limiting offered by firewalls +but is often inferior. Attacks that justify ignoring the contents of DNS +responses are likely to be attacks on the DNS server itself. They +usually should be discarded before the DNS server spends resources making +TCP connections or parsing DNS requests, but that rate limiting must be +done before the DNS server sees the requests. + +The maximum size of the table used to track requests and rate-limit +responses is set with ``max-table-size``. Each entry in the table is +between 40 and 80 bytes. The table needs approximately as many entries +as the number of requests received per second. The default is 20,000. To +reduce the cold start of growing the table, ``min-table-size`` (default 500) +can set the minimum table size. Enable ``rate-limit`` category +logging to monitor expansions of the table and inform choices for the +initial and maximum table size. + +Use ``log-only yes`` to test rate-limiting parameters without actually +dropping any requests. + +Responses dropped by rate limits are included in the ``RateDropped`` and +``QryDropped`` statistics. Responses that are truncated by rate limits are +included in ``RateSlipped`` and ``RespTruncated``. + +NXDOMAIN Redirection +^^^^^^^^^^^^^^^^^^^^ + +``named`` supports NXDOMAIN redirection via two methods: + +- Redirect zone (:ref:`zone_statement_grammar`) +- Redirect namespace + +With either method, when ``named`` gets an NXDOMAIN response it examines a +separate namespace to see if the NXDOMAIN response should be replaced +with an alternative response. + +With a redirect zone (``zone "." { type redirect; };``), the data used +to replace the NXDOMAIN is held in a single zone which is not part of +the normal namespace. All the redirect information is contained in the +zone; there are no delegations. + +With a redirect namespace (``option { nxdomain-redirect <suffix> };``), +the data used to replace the NXDOMAIN is part of the normal namespace +and is looked up by appending the specified suffix to the original +query name. This roughly doubles the cache required to process +NXDOMAIN responses, as both the original NXDOMAIN response and the +replacement data (or an NXDOMAIN indicating that there is no +replacement) must be stored. + +If both a redirect zone and a redirect namespace are configured, the +redirect zone is tried first. + +.. _server_statement_grammar: + +``server`` Statement Grammar +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +.. include:: ../misc/server.grammar.rst + +.. _server_statement_definition_and_usage: + +``server`` Statement Definition and Usage +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The ``server`` statement defines characteristics to be associated with a +remote name server. If a prefix length is specified, then a range of +servers is covered. Only the most specific server clause applies, +regardless of the order in ``named.conf``. + +The ``server`` statement can occur at the top level of the configuration +file or inside a ``view`` statement. If a ``view`` statement contains +one or more ``server`` statements, only those apply to the view and any +top-level ones are ignored. If a view contains no ``server`` statements, +any top-level ``server`` statements are used as defaults. + +If a remote server is giving out bad data, marking it +as bogus prevents further queries to it. The default value of +``bogus`` is ``no``. + +The ``provide-ixfr`` clause determines whether the local server, acting +as primary, responds with an incremental zone transfer when the given +remote server, a secondary, requests it. If set to ``yes``, incremental +transfer is provided whenever possible. If set to ``no``, all +transfers to the remote server are non-incremental. If not set, the +value of the ``provide-ixfr`` option in the view or global options block +is used as a default. + +The ``request-ixfr`` clause determines whether the local server, acting +as a secondary, requests incremental zone transfers from the given +remote server, a primary. If not set, the value of the ``request-ixfr`` +option in the view or global options block is used as a default. It may +also be set in the zone block; if set there, it overrides the +global or view setting for that zone. + +IXFR requests to servers that do not support IXFR automatically +fall back to AXFR. Therefore, there is no need to manually list which +servers support IXFR and which ones do not; the global default of +``yes`` should always work. The purpose of the ``provide-ixfr`` and +``request-ixfr`` clauses is to make it possible to disable the use of +IXFR even when both primary and secondary claim to support it: for example, if +one of the servers is buggy and crashes or corrupts data when IXFR is +used. + +The ``request-expire`` clause determines whether the local server, when +acting as a secondary, requests the EDNS EXPIRE value. The EDNS EXPIRE +value indicates the remaining time before the zone data expires and +needs to be refreshed. This is used when a secondary server transfers +a zone from another secondary server; when transferring from the +primary, the expiration timer is set from the EXPIRE field of the SOA +record instead. The default is ``yes``. + +The ``edns`` clause determines whether the local server attempts to +use EDNS when communicating with the remote server. The default is +``yes``. + +The ``edns-udp-size`` option sets the EDNS UDP size that is advertised +by ``named`` when querying the remote server. Valid values are 512 to +4096 bytes; values outside this range are silently adjusted to the +nearest value within it. This option is useful when +advertising a different value to this server than the value advertised +globally: for example, when there is a firewall at the remote site that +is blocking large replies. Note: currently, this sets a single UDP size +for all packets sent to the server; ``named`` does not deviate from this +value. This differs from the behavior of ``edns-udp-size`` in +``options`` or ``view`` statements, where it specifies a maximum value. +The ``server`` statement behavior may be brought into conformance with +the ``options``/``view`` behavior in future releases. + +The ``edns-version`` option sets the maximum EDNS VERSION that is +sent to the server(s) by the resolver. The actual EDNS version sent is +still subject to normal EDNS version-negotiation rules (see :rfc:`6891`), +the maximum EDNS version supported by the server, and any other +heuristics that indicate that a lower version should be sent. This +option is intended to be used when a remote server reacts badly to a +given EDNS version or higher; it should be set to the highest version +the remote server is known to support. Valid values are 0 to 255; higher +values are silently adjusted. This option is not needed until +higher EDNS versions than 0 are in use. + +The ``max-udp-size`` option sets the maximum EDNS UDP message size +``named`` sends. Valid values are 512 to 4096 bytes; values outside +this range are silently adjusted. This option is useful when +there is a firewall that is blocking large replies from +``named``. + +The ``padding`` option adds EDNS Padding options to outgoing messages, +increasing the packet size to a multiple of the specified block size. +Valid block sizes range from 0 (the default, which disables the use of +EDNS Padding) to 512 bytes. Larger values are reduced to 512, with a +logged warning. Note: this option is not currently compatible with no +TSIG or SIG(0), as the EDNS OPT record containing the padding would have +to be added to the packet after it had already been signed. + +The ``tcp-only`` option sets the transport protocol to TCP. The default +is to use the UDP transport and to fallback on TCP only when a truncated +response is received. + +The ``tcp-keepalive`` option adds EDNS TCP keepalive to messages sent +over TCP. Note that currently idle timeouts in responses are ignored. + +The server supports two zone transfer methods. The first, +``one-answer``, uses one DNS message per resource record transferred. +``many-answers`` packs as many resource records as possible into a single +message, which is more efficient. +It is possible to specify which method to use for a server via the +``transfer-format`` option; if not set there, the +``transfer-format`` specified by the ``options`` statement is used. + +``transfers`` is used to limit the number of concurrent inbound zone +transfers from the specified server. If no ``transfers`` clause is +specified, the limit is set according to the ``transfers-per-ns`` +option. + +The ``keys`` clause identifies a ``key_id`` defined by the ``key`` +statement, to be used for transaction security (see :ref:`tsig`) +when talking to the remote server. When a request is sent to the remote +server, a request signature is generated using the key specified +here and appended to the message. A request originating from the remote +server is not required to be signed by this key. + +Only a single key per server is currently supported. + +The ``transfer-source`` and ``transfer-source-v6`` clauses specify the +IPv4 and IPv6 source address, respectively, to be used for zone transfer with the +remote server. For an IPv4 remote server, only +``transfer-source`` can be specified. Similarly, for an IPv6 remote +server, only ``transfer-source-v6`` can be specified. For more details, +see the description of ``transfer-source`` and ``transfer-source-v6`` in +:ref:`zone_transfers`. + +The ``notify-source`` and ``notify-source-v6`` clauses specify the IPv4 +and IPv6 source address, respectively, to be used for notify messages sent to remote +servers. For an IPv4 remote server, only ``notify-source`` +can be specified. Similarly, for an IPv6 remote server, only +``notify-source-v6`` can be specified. + +The ``query-source`` and ``query-source-v6`` clauses specify the IPv4 +and IPv6 source address, respectively, to be used for queries sent to remote servers. +For an IPv4 remote server, only ``query-source`` can be +specified. Similarly, for an IPv6 remote server, only +``query-source-v6`` can be specified. + +The ``request-nsid`` clause determines whether the local server adds +an NSID EDNS option to requests sent to the server. This overrides +``request-nsid`` set at the view or option level. + +The ``send-cookie`` clause determines whether the local server adds +a COOKIE EDNS option to requests sent to the server. This overrides +``send-cookie`` set at the view or option level. The ``named`` server +may determine that COOKIE is not supported by the remote server and not +add a COOKIE EDNS option to requests. + +.. _statschannels: + +``statistics-channels`` Statement Grammar +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +.. include:: ../misc/statistics-channels.grammar.rst + +.. _statistics_channels: + +``statistics-channels`` Statement Definition and Usage +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The ``statistics-channels`` statement declares communication channels to +be used by system administrators to get access to statistics information +on the name server. + +This statement is intended to be flexible to support multiple communication +protocols in the future, but currently only HTTP access is supported. It +requires that BIND 9 be compiled with libxml2 and/or json-c (also known +as libjson0); the ``statistics-channels`` statement is still accepted +even if it is built without the library, but any HTTP access fails +with an error. + +An ``inet`` control channel is a TCP socket listening at the specified +``ip_port`` on the specified ``ip_addr``, which can be an IPv4 or IPv6 +address. An ``ip_addr`` of ``*`` (asterisk) is interpreted as the IPv4 +wildcard address; connections are accepted on any of the system's +IPv4 addresses. To listen on the IPv6 wildcard address, use an +``ip_addr`` of ``::``. + +If no port is specified, port 80 is used for HTTP channels. The asterisk +(``*``) cannot be used for ``ip_port``. + +Attempts to open a statistics channel are restricted by the +optional ``allow`` clause. Connections to the statistics channel are +permitted based on the ``address_match_list``. If no ``allow`` clause is +present, ``named`` accepts connection attempts from any address. Since +the statistics may contain sensitive internal information, the source of +connection requests must be restricted appropriately so that only +trusted parties can access the statistics channel. + +Gathering data exposed by the statistics channel locks various subsystems in +``named``, which could slow down query processing if statistics data is +requested too often. + +An issue in the statistics channel would be considered a security issue +only if it could be exploited by unprivileged users circumventing the access +control list. In other words, any issue in the statistics channel that could be +used to access information unavailable otherwise, or to crash ``named``, is +not considered a security issue if it can be avoided through the +use of a secure configuration. + +If no ``statistics-channels`` statement is present, ``named`` does not +open any communication channels. + +The statistics are available in various formats and views, depending on +the URI used to access them. For example, if the statistics channel is +configured to listen on 127.0.0.1 port 8888, then the statistics are +accessible in XML format at http://127.0.0.1:8888/ or +http://127.0.0.1:8888/xml. A CSS file is included, which can format the +XML statistics into tables when viewed with a stylesheet-capable +browser, and into charts and graphs using the Google Charts API when +using a JavaScript-capable browser. + +Broken-out subsets of the statistics can be viewed at +http://127.0.0.1:8888/xml/v3/status (server uptime and last +reconfiguration time), http://127.0.0.1:8888/xml/v3/server (server and +resolver statistics), http://127.0.0.1:8888/xml/v3/zones (zone +statistics), http://127.0.0.1:8888/xml/v3/net (network status and socket +statistics), http://127.0.0.1:8888/xml/v3/mem (memory manager +statistics), http://127.0.0.1:8888/xml/v3/tasks (task manager +statistics), and http://127.0.0.1:8888/xml/v3/traffic (traffic sizes). + +The full set of statistics can also be read in JSON format at +http://127.0.0.1:8888/json, with the broken-out subsets at +http://127.0.0.1:8888/json/v1/status (server uptime and last +reconfiguration time), http://127.0.0.1:8888/json/v1/server (server and +resolver statistics), http://127.0.0.1:8888/json/v1/zones (zone +statistics), http://127.0.0.1:8888/json/v1/net (network status and +socket statistics), http://127.0.0.1:8888/json/v1/mem (memory manager +statistics), http://127.0.0.1:8888/json/v1/tasks (task manager +statistics), and http://127.0.0.1:8888/json/v1/traffic (traffic sizes). + +.. _trust_anchors: + +``trust-anchors`` Statement Grammar +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +.. include:: ../misc/trust-anchors.grammar.rst + +.. _trust-anchors: + +``trust-anchors`` Statement Definition and Usage +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The ``trust-anchors`` statement defines DNSSEC trust anchors. DNSSEC is +described in :ref:`DNSSEC`. + +A trust anchor is defined when the public key or public key digest for a non-authoritative +zone is known but cannot be securely obtained through DNS, either +because it is the DNS root zone or because its parent zone is unsigned. +Once a key or digest has been configured as a trust anchor, it is treated as if it +has been validated and proven secure. + +The resolver attempts DNSSEC validation on all DNS data in subdomains of +configured trust anchors. Validation below specified names can be +temporarily disabled by using ``rndc nta``, or permanently disabled with +the ``validate-except`` option. + +All keys listed in ``trust-anchors``, and their corresponding zones, are +deemed to exist regardless of what parent zones say. Only keys +configured as trust anchors are used to validate the DNSKEY RRset for +the corresponding name. The parent's DS RRset is not used. + +``trust-anchors`` may be set at the top level of ``named.conf`` or within +a view. If it is set in both places, the configurations are additive; +keys defined at the top level are inherited by all views, but keys +defined in a view are only used within that view. + +The ``trust-anchors`` statement can contain +multiple trust-anchor entries, each consisting of a +domain name, followed by an "anchor type" keyword indicating +the trust anchor's format, followed by the key or digest data. + +If the anchor type is ``static-key`` or +``initial-key``, then it is followed with the +key's flags, protocol, and algorithm, plus the Base64 representation +of the public key data. This is identical to the text +representation of a DNSKEY record. Spaces, tabs, newlines, and +carriage returns are ignored in the key data, so the +configuration may be split into multiple lines. + +If the anchor type is ``static-ds`` or +``initial-ds``, it is followed with the +key tag, algorithm, digest type, and the hexadecimal +representation of the key digest. This is identical to the +text representation of a DS record. Spaces, tabs, newlines, +and carriage returns are ignored. + +Trust anchors configured with the +``static-key`` or ``static-ds`` +anchor types are immutable, while keys configured with +``initial-key`` or ``initial-ds`` +can be kept up-to-date automatically, without intervention from the resolver operator. +(``static-key`` keys are identical to keys configured using the +deprecated ``trusted-keys`` statement.) + +Suppose, for example, that a zone's key-signing key was compromised, and +the zone owner had to revoke and replace the key. A resolver which had +the original key +configured using ``static-key`` or +``static-ds`` would be unable to validate +this zone any longer; it would reply with a SERVFAIL response +code. This would continue until the resolver operator had +updated the ``trust-anchors`` statement with +the new key. + +If, however, the trust anchor had been configured using +``initial-key`` or ``initial-ds`` +instead, the zone owner could add a "stand-by" key to +the zone in advance. ``named`` would store +the stand-by key, and when the original key was revoked, +``named`` would be able to transition smoothly +to the new key. It would also recognize that the old key had +been revoked and cease using that key to validate answers, +minimizing the damage that the compromised key could do. +This is the process used to keep the ICANN root DNSSEC key +up-to-date. + +Whereas ``static-key`` and +``static-ds`` trust anchors continue +to be trusted until they are removed from +``named.conf``, an +``initial-key`` or ``initial-ds`` +is only trusted *once*: for as long as it +takes to load the managed key database and start the +:rfc:`5011` key maintenance process. + +It is not possible to mix static with initial trust anchors +for the same domain name. + +The first time ``named`` runs with an +``initial-key`` or ``initial-ds`` +configured in ``named.conf``, it fetches the +DNSKEY RRset directly from the zone apex, +and validates it +using the trust anchor specified in ``trust-anchors``. +If the DNSKEY RRset is validly signed by a key matching +the trust anchor, then it is used as the basis for a new +managed-keys database. + +From that point on, whenever ``named`` runs, it sees the ``initial-key`` or ``initial-ds`` +listed in ``trust-anchors``, checks to make sure :rfc:`5011` key maintenance +has already been initialized for the specified domain, and if so, +simply moves on. The key specified in the ``trust-anchors`` statement is +not used to validate answers; it is superseded by the key or keys stored +in the managed-keys database. + +The next time ``named`` runs after an ``initial-key`` or ``initial-ds`` has been *removed* +from the ``trust-anchors`` statement (or changed to a ``static-key`` or ``static-ds``), the +corresponding zone is removed from the managed-keys database, and +:rfc:`5011` key maintenance is no longer used for that domain. + +In the current implementation, the managed-keys database is stored as a +master-format zone file. + +On servers which do not use views, this file is named +``managed-keys.bind``. When views are in use, there is a separate +managed-keys database for each view; the filename is the view name +(or, if a view name contains characters which would make it illegal as a +filename, a hash of the view name), followed by the suffix ``.mkeys``. + +When the key database is changed, the zone is updated. As with any other +dynamic zone, changes are written into a journal file, e.g., +``managed-keys.bind.jnl`` or ``internal.mkeys.jnl``. Changes are +committed to the primary file as soon as possible afterward, +usually within 30 seconds. Whenever ``named`` is using +automatic key maintenance, the zone file and journal file can be +expected to exist in the working directory. (For this reason, among +others, the working directory should be always be writable by +``named``.) + +If the ``dnssec-validation`` option is set to ``auto``, ``named`` +automatically initializes an ``initial-key`` for the root zone. The key +that is used to initialize the key-maintenance process is stored in +``bind.keys``; the location of this file can be overridden with the +``bindkeys-file`` option. As a fallback in the event no ``bind.keys`` +can be found, the initializing key is also compiled directly into +``named``. + +.. _dnssec_policy_grammar: + +``dnssec-policy`` Statement Grammar +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +.. include:: ../misc/dnssec-policy.grammar.rst + +.. _dnssec_policy: + +``dnssec-policy`` Statement Definition and Usage +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The ``dnssec-policy`` statement defines a key and signing policy (KASP) +for zones. + +A KASP determines how one or more zones are signed with DNSSEC. For +example, it specifies how often keys should roll, which cryptographic +algorithms to use, and how often RRSIG records need to be refreshed. + +Keys are not shared among zones, which means that one set of keys per +zone is generated even if they have the same policy. If multiple views +are configured with different versions of the same zone, each separate +version uses the same set of signing keys. + +Multiple key and signing policies can be configured. To attach a policy +to a zone, add a ``dnssec-policy`` option to the ``zone`` statement, +specifying the name of the policy that should be used. + +The ``dnssec-policy`` statement requires dynamic DNS to be set up, or +``inline-signing`` to be enabled. + +If ``inline-signing`` is enabled, this means that a signed version of the +zone is maintained separately and is written out to a different file on disk +(the zone's filename plus a ``.signed`` extension). + +If the zone is dynamic because it is configured with an ``update-policy`` or +``allow-update``, the DNSSEC records are written to the filename set in the +original zone's ``file``, unless ``inline-signing`` is explicitly set. + +Key rollover timing is computed for each key according to the key +lifetime defined in the KASP. The lifetime may be modified by zone TTLs +and propagation delays, to prevent validation failures. When a key +reaches the end of its lifetime, ``named`` generates and publishes a new +key automatically, then deactivates the old key and activates the new +one; finally, the old key is retired according to a computed schedule. + +Zone-signing key (ZSK) rollovers require no operator input. Key-signing +key (KSK) and combined-signing key (CSK) rollovers require action to be +taken to submit a DS record to the parent. Rollover timing for KSKs and +CSKs is adjusted to take into account delays in processing and +propagating DS updates. + +There are two predefined ``dnssec-policy`` names: ``none`` and +``default``. Setting a zone's policy to ``none`` is the same as not +setting ``dnssec-policy`` at all; the zone is not signed. Policy +``default`` causes the zone to be signed with a single combined-signing +key (CSK) using algorithm ECDSAP256SHA256; this key has an unlimited +lifetime. (A verbose copy of this policy may be found in the source +tree, in the file ``doc/misc/dnssec-policy.default.conf``.) + +.. note:: The default signing policy may change in future releases. + This could require changes to a signing policy when upgrading to a + new version of BIND. Check the release notes carefully when + upgrading to be informed of such changes. To prevent policy changes + on upgrade, use an explicitly defined ``dnssec-policy``, rather than + ``default``. + +If a ``dnssec-policy`` statement is modified and the server restarted or +reconfigured, ``named`` attempts to change the policy smoothly from the +old one to the new. For example, if the key algorithm is changed, then +a new key is generated with the new algorithm, and the old algorithm is +retired when the existing key's lifetime ends. + +.. note:: Rolling to a new policy while another key rollover is already + in progress is not yet supported, and may result in unexpected + behavior. + +The following options can be specified in a ``dnssec-policy`` statement: + + ``dnskey-ttl`` + This indicates the TTL to use when generating DNSKEY resource + records. The default is 1 hour (3600 seconds). + + ``keys`` + This is a list specifying the algorithms and roles to use when + generating keys and signing the zone. Entries in this list do not + represent specific DNSSEC keys, which may be changed on a regular + basis, but the roles that keys play in the signing policy. For + example, configuring a KSK of algorithm RSASHA256 ensures that the + DNSKEY RRset always includes a key-signing key for that algorithm. + + Here is an example (for illustration purposes only) of some possible + entries in a ``keys`` list: + + :: + + keys { + ksk key-directory lifetime unlimited algorithm rsasha256 2048; + zsk lifetime 30d algorithm 8; + csk lifetime P6MT12H3M15S algorithm ecdsa256; + }; + + This example specifies that three keys should be used in the zone. + The first token determines which role the key plays in signing + RRsets. If set to ``ksk``, then this is a key-signing key; it has + the KSK flag set and is only used to sign DNSKEY, CDS, and CDNSKEY + RRsets. If set to ``zsk``, this is a zone-signing key; the KSK flag + is unset, and the key signs all RRsets *except* DNSKEY, CDS, and + CDNSKEY. If set to ``csk``, the key has the KSK flag set and is + used to sign all RRsets. + + An optional second token determines where the key is stored. + Currently, keys can only be stored in the configured + ``key-directory``. This token may be used in the future to store + keys in hardware security modules or separate directories. + + The ``lifetime`` parameter specifies how long a key may be used + before rolling over. For convenience, TTL-style time-unit suffixes + can be used to specify the key lifetime. It also accepts ISO 8601 + duration formats. + + In the example above, the first key has an + unlimited lifetime, the second key may be used for 30 days, and the + third key has a rather peculiar lifetime of 6 months, 12 hours, 3 + minutes, and 15 seconds. A lifetime of 0 seconds is the same as + ``unlimited``. + + Note that the lifetime of a key may be extended if retiring it too + soon would cause validation failures. For example, if the key were + configured to roll more frequently than its own TTL, its lifetime + would automatically be extended to account for this. + + The ``algorithm`` parameter specifies the key's algorithm, expressed + either as a string ("rsasha256", "ecdsa384", etc.) or as a decimal + number. An optional second parameter specifies the key's size in + bits. If it is omitted, as shown in the example for the second and + third keys, an appropriate default size for the algorithm is used. + Each KSK/ZSK pair must have the same algorithm. A CSK combines the + functionality of a ZSK and a KSK. + + ``purge-keys`` + This is the time after when DNSSEC keys that have been deleted from + the zone can be removed from disk. If a key still determined to have + presence (for example in some resolver cache), ``named`` will not + remove the key files. + + The default is ``P90D`` (90 days). Set this option to ``0`` to never + purge deleted keys. + + ``publish-safety`` + This is a margin that is added to the pre-publication interval in + rollover timing calculations, to give some extra time to cover + unforeseen events. This increases the time between when keys are + published and when they become active. The default is ``PT1H`` (1 + hour). + + ``retire-safety`` + This is a margin that is added to the post-publication interval in + rollover timing calculations, to give some extra time to cover + unforeseen events. This increases the time a key remains published + after it is no longer active. The default is ``PT1H`` (1 hour). + + ``signatures-refresh`` + This determines how frequently an RRSIG record needs to be + refreshed. The signature is renewed when the time until the + expiration time is less than the specified interval. The default is + ``P5D`` (5 days), meaning signatures that expire in 5 days or sooner + are refreshed. + + ``signatures-validity`` + This indicates the validity period of an RRSIG record (subject to + inception offset and jitter). The default is ``P2W`` (2 weeks). + + ``signatures-validity-dnskey`` + This is similar to ``signatures-validity``, but for DNSKEY records. + The default is ``P2W`` (2 weeks). + + ``max-zone-ttl`` + + This specifies the maximum permissible TTL value for the zone. When + a zone file is loaded, any record encountered with a TTL higher than + ``max-zone-ttl`` causes the zone to be rejected. + + This ensures that when rolling to a new DNSKEY, the old key will remain + available until RRSIG records have expired from caches. The + ``max-zone-ttl`` option guarantees that the largest TTL in the + zone is no higher than a known and predictable value. + + .. note:: Because ``map``-format files load directly into memory, + this option cannot be used with them. + + The default value ``PT24H`` (24 hours). A value of zero is treated + as if the default value were in use. + + + ``nsec3param`` + Use NSEC3 instead of NSEC, and optionally set the NSEC3 parameters. + + Here is an example of an ``nsec3`` configuration: + + :: + + nsec3param iterations 5 optout no salt-length 8; + + The default is to use NSEC. The ``iterations``, ``optout``, and + ``salt-length`` parts are optional, but if not set, the values in + the example above are the default NSEC3 parameters. Note that the + specific salt string is not specified by the user; :iscman:`named` creates a salt + of the indicated length. + + .. warning:: + Do not use extra :term:`iterations <Iterations>`, :term:`salt <Salt>`, and + :term:`opt-out <Opt-out>` unless their implications are fully understood. + A higher number of iterations causes interoperability problems and opens + servers to CPU-exhausting DoS attacks. + + ``zone-propagation-delay`` + This is the expected propagation delay from the time when a zone is + first updated to the time when the new version of the zone is served + by all secondary servers. The default is ``PT5M`` (5 minutes). + + ``parent-ds-ttl`` + This is the TTL of the DS RRset that the parent zone uses. The + default is ``P1D`` (1 day). + + ``parent-propagation-delay`` + This is the expected propagation delay from the time when the parent + zone is updated to the time when the new version is served by all of + the parent zone's name servers. The default is ``PT1H`` (1 hour). + +Automated KSK Rollovers +^^^^^^^^^^^^^^^^^^^^^^^ + +BIND has mechanisms in place to facilitate automated KSK rollovers. It +publishes CDS and CDNSKEY records that can be used by the parent zone to +publish or withdraw the zone's DS records. BIND will query the parental +agents to see if the new DS is actually published before withdrawing the +old DNSSEC key. + + .. note:: + The DS response is not validated so it is recommended to set up a + trust relationship with the parental agent. For example, use TSIG to + authenticate the parental agent, or point to a validating resolver. + +The following options apply to DS queries sent to ``parental-agents``: + +``parental-source`` + ``parental-source`` determines which local source address, and optionally + UDP port, is used to send parental DS queries. This statement sets the + ``parental-source`` for all zones, but can be overridden on a per-zone or + per-view basis by including a ``parental-source`` statement within the + ``zone`` or ``view`` block in the configuration file. + + .. warning:: Specifying a single port is discouraged, as it removes a layer of + protection against spoofing errors. + + .. warning:: The configured ``port`` must not be same as the listening port. + +``parental-source-v6`` + This option acts like ``parental-source``, but applies to parental DS + queries sent to IPv6 addresses. + +.. _managed-keys: + +``managed-keys`` Statement Grammar +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +.. include:: ../misc/managed-keys.grammar.rst + +.. _managed_keys: + +``managed-keys`` Statement Definition and Usage +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The ``managed-keys`` statement has been +deprecated in favor of :ref:`trust_anchors` +with the ``initial-key`` keyword. + +.. _trusted-keys: + +``trusted-keys`` Statement Grammar +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +.. include:: ../misc/trusted-keys.grammar.rst + +.. _trusted_keys: + +``trusted-keys`` Statement Definition and Usage +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The ``trusted-keys`` statement has been deprecated in favor of +:ref:`trust_anchors` with the ``static-key`` keyword. + +.. _view_statement_grammar: + +``view`` Statement Grammar +~~~~~~~~~~~~~~~~~~~~~~~~~~ + +:: + + view view_name [ class ] { + match-clients { address_match_list } ; + match-destinations { address_match_list } ; + match-recursive-only yes_or_no ; + [ view_option ; ... ] + [ zone_statement ; ... ] + } ; + +.. _view_statement: + +``view`` Statement Definition and Usage +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The ``view`` statement is a powerful feature of BIND 9 that lets a name +server answer a DNS query differently depending on who is asking. It is +particularly useful for implementing split DNS setups without having to +run multiple servers. + +Each ``view`` statement defines a view of the DNS namespace that is +seen by a subset of clients. A client matches a view if its source IP +address matches the ``address_match_list`` of the view's +``match-clients`` clause, and its destination IP address matches the +``address_match_list`` of the view's ``match-destinations`` clause. If +not specified, both ``match-clients`` and ``match-destinations`` default +to matching all addresses. In addition to checking IP addresses, +``match-clients`` and ``match-destinations`` can also take ``keys`` +which provide an mechanism for the client to select the view. A view can +also be specified as ``match-recursive-only``, which means that only +recursive requests from matching clients match that view. The order +of the ``view`` statements is significant; a client request is +resolved in the context of the first ``view`` that it matches. + +Zones defined within a ``view`` statement are only accessible to +clients that match the ``view``. By defining a zone of the same name in +multiple views, different zone data can be given to different clients: +for example, "internal" and "external" clients in a split DNS setup. + +Many of the options given in the ``options`` statement can also be used +within a ``view`` statement, and then apply only when resolving queries +with that view. When no view-specific value is given, the value in the +``options`` statement is used as a default. Also, zone options can have +default values specified in the ``view`` statement; these view-specific +defaults take precedence over those in the ``options`` statement. + +Views are class-specific. If no class is given, class IN is assumed. +Note that all non-IN views must contain a hint zone, since only the IN +class has compiled-in default hints. + +If there are no ``view`` statements in the config file, a default view +that matches any client is automatically created in class IN. Any +``zone`` statements specified on the top level of the configuration file +are considered to be part of this default view, and the ``options`` +statement applies to the default view. If any explicit ``view`` +statements are present, all ``zone`` statements must occur inside +``view`` statements. + +Here is an example of a typical split DNS setup implemented using +``view`` statements: + +:: + + view "internal" { + // This should match our internal networks. + match-clients { 10.0.0.0/8; }; + + // Provide recursive service to internal + // clients only. + recursion yes; + + // Provide a complete view of the example.com + // zone including addresses of internal hosts. + zone "example.com" { + type primary; + file "example-internal.db"; + }; + }; + + view "external" { + // Match all clients not matched by the + // previous view. + match-clients { any; }; + + // Refuse recursive service to external clients. + recursion no; + + // Provide a restricted view of the example.com + // zone containing only publicly accessible hosts. + zone "example.com" { + type primary; + file "example-external.db"; + }; + }; + +.. _zone_statement_grammar: + +``zone`` Statement Grammar +~~~~~~~~~~~~~~~~~~~~~~~~~~ + +.. include:: ../misc/master.zoneopt.rst +.. include:: ../misc/slave.zoneopt.rst +.. include:: ../misc/mirror.zoneopt.rst +.. include:: ../misc/hint.zoneopt.rst +.. include:: ../misc/stub.zoneopt.rst +.. include:: ../misc/static-stub.zoneopt.rst +.. include:: ../misc/forward.zoneopt.rst +.. include:: ../misc/redirect.zoneopt.rst +.. include:: ../misc/delegation-only.zoneopt.rst +.. include:: ../misc/in-view.zoneopt.rst + +.. _zone_statement: + +``zone`` Statement Definition and Usage +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +.. _zone_types: + +Zone Types +^^^^^^^^^^ + +The ``type`` keyword is required for the ``zone`` configuration unless +it is an ``in-view`` configuration. Its acceptable values are: +``primary`` (or ``master``), ``secondary`` (or ``slave``), ``mirror``, +``hint``, ``stub``, ``static-stub``, ``forward``, ``redirect``, +or ``delegation-only``. + +``primary`` + A primary zone has a master copy of the data for the zone and is able + to provide authoritative answers for it. Type ``master`` is a synonym + for ``primary``. + +``secondary`` + A secondary zone is a replica of a primary zone. Type ``slave`` is a + synonym for ``secondary``. The ``primaries`` list specifies one or more IP + addresses of primary servers that the secondary contacts to update + its copy of the zone. Primaries list elements can + also be names of other primaries lists. By default, + transfers are made from port 53 on the servers; + this can be changed for all servers by specifying + a port number before the list of IP addresses, + or on a per-server basis after the IP address. + Authentication to the primary can also be done with + per-server TSIG keys. If a file is specified, then the + replica is written to this file + whenever the zone + is changed, and reloaded from this file on a server + restart. Use of a file is recommended, since it + often speeds server startup and eliminates a + needless waste of bandwidth. Note that for large + numbers (in the tens or hundreds of thousands) of + zones per server, it is best to use a two-level + naming scheme for zone filenames. For example, + a secondary server for the zone + ``example.com`` might place + the zone contents into a file called + ``ex/example.com``, where + ``ex/`` is just the first two + letters of the zone name. (Most operating systems + behave very slowly if there are 100000 files in a single directory.) + +``mirror`` + A mirror zone is similar to a zone of type ``secondary``, except its + data is subject to DNSSEC validation before being used in answers. + Validation is applied to the entire zone during the zone transfer + process, and again when the zone file is loaded from disk upon + restarting ``named``. If validation of a new version of a mirror zone + fails, a retransfer is scheduled; in the meantime, the most recent + correctly validated version of that zone is used until it either + expires or a newer version validates correctly. If no usable zone + data is available for a mirror zone, due to either transfer failure + or expiration, traditional DNS recursion is used to look up the + answers instead. Mirror zones cannot be used in a view that does not + have recursion enabled. + + Answers coming from a mirror zone look almost exactly like answers + from a zone of type ``secondary``, with the notable exceptions that + the AA bit ("authoritative answer") is not set, and the AD bit + ("authenticated data") is. + + Mirror zones are intended to be used to set up a fast local copy of + the root zone (see :rfc:`8806`). A default list of primary servers + for the IANA root zone is built into ``named``, so its mirroring can + be enabled using the following configuration: + + :: + + zone "." { + type mirror; + }; + + Mirror zone validation always happens for the entire zone contents. + This ensures that each version of the zone used by the resolver is + fully self-consistent with respect to DNSSEC. For incoming mirror + zone IXFRs, every revision of the zone contained in the IXFR sequence + is validated independently, in the order in which the zone revisions + appear on the wire. For this reason, it might be useful to force use + of AXFR for mirror zones by setting ``request-ixfr no;`` for the + relevant zone (or view). Other, more efficient zone verification + methods may be added in the future. + + To make mirror zone contents persist between ``named`` restarts, use + the :ref:`file <file-option>` option. + + Mirroring a zone other than root requires an explicit list of primary + servers to be provided using the ``primaries`` option (see + :ref:`primaries_grammar` for details), and a key-signing key (KSK) + for the specified zone to be explicitly configured as a trust anchor + (see :ref:`trust-anchors`). + + When configuring NOTIFY for a mirror zone, only ``notify no;`` and + ``notify explicit;`` can be used at the zone level; any other + ``notify`` setting at the zone level is a configuration error. Using + any other ``notify`` setting at the ``options`` or ``view`` level + causes that setting to be overridden with ``notify explicit;`` for + the mirror zone. The global default for the ``notify`` option is + ``yes``, so mirror zones are by default configured with ``notify + explicit;``. + + Outgoing transfers of mirror zones are disabled by default but may be + enabled using :ref:`allow-transfer <allow-transfer-access>`. + + .. note:: + Use of this zone type with any zone other than the root should be + considered *experimental* and may cause performance issues, + especially for zones that are large and/or frequently updated. + +``hint`` + The initial set of root name servers is specified using a hint zone. + When the server starts, it uses the root hints to find a root name + server and get the most recent list of root name servers. If no hint zone + is specified for class IN, the server uses a compiled-in default set of + root servers hints. Classes other than IN have no built-in default hints. + +``stub`` + A stub zone is similar to a secondary zone, except that it replicates only + the NS records of a primary zone instead of the entire zone. Stub zones + are not a standard part of the DNS; they are a feature specific to the + BIND implementation. + + Stub zones can be used to eliminate the need for a glue NS record in a parent + zone, at the expense of maintaining a stub zone entry and a set of name + server addresses in ``named.conf``. This usage is not recommended for + new configurations, and BIND 9 supports it only in a limited way. If a BIND 9 + primary, serving a parent zone, has child stub + zones configured, all the secondary servers for the parent zone also need to + have the same child stub zones configured. + + Stub zones can also be used as a way to force the resolution of a given + domain to use a particular set of authoritative servers. For example, the + caching name servers on a private network using :rfc:`1918` addressing may be + configured with stub zones for ``10.in-addr.arpa`` to use a set of + internal name servers as the authoritative servers for that domain. + +``static-stub`` + A static-stub zone is similar to a stub zone, with the following + exceptions: the zone data is statically configured, rather than + transferred from a primary server; and when recursion is necessary for a query + that matches a static-stub zone, the locally configured data (name server + names and glue addresses) is always used, even if different authoritative + information is cached. + + Zone data is configured via the ``server-addresses`` and ``server-names`` + zone options. + + The zone data is maintained in the form of NS and (if necessary) glue A or + AAAA RRs internally, which can be seen by dumping zone databases with + ``rndc dumpdb -all``. The configured RRs are considered local configuration + parameters rather than public data. Non-recursive queries (i.e., those + with the RD bit off) to a static-stub zone are therefore prohibited and + are responded to with REFUSED. + + Since the data is statically configured, no zone maintenance action takes + place for a static-stub zone. For example, there is no periodic refresh + attempt, and an incoming notify message is rejected with an rcode + of NOTAUTH. + + Each static-stub zone is configured with internally generated NS and (if + necessary) glue A or AAAA RRs. + +``forward`` + A forward zone is a way to configure forwarding on a per-domain basis. + A ``zone`` statement of type ``forward`` can contain a ``forward`` and/or + ``forwarders`` statement, which applies to queries within the domain + given by the zone name. If no ``forwarders`` statement is present, or an + empty list for ``forwarders`` is given, then no forwarding is done + for the domain, canceling the effects of any forwarders in the ``options`` + statement. Thus, to use this type of zone to change the + behavior of the global ``forward`` option (that is, "forward first" to, + then "forward only", or vice versa), but use the same servers as set + globally, re-specify the global forwarders. + +``redirect`` + Redirect zones are used to provide answers to queries when normal + resolution would result in NXDOMAIN being returned. Only one redirect zone + is supported per view. ``allow-query`` can be used to restrict which + clients see these answers. + + If the client has requested DNSSEC records (DO=1) and the NXDOMAIN response + is signed, no substitution occurs. + + To redirect all NXDOMAIN responses to 100.100.100.2 and + 2001:ffff:ffff::100.100.100.2, configure a type ``redirect`` zone + named ".", with the zone file containing wildcard records that point to + the desired addresses: ``*. IN A 100.100.100.2`` and + ``*. IN AAAA 2001:ffff:ffff::100.100.100.2``. + + As another example, to redirect all Spanish names (under .ES), use similar entries + but with the names ``*.ES.`` instead of ``*.``. To redirect all commercial + Spanish names (under COM.ES), use wildcard entries + called ``*.COM.ES.``. + + Note that the redirect zone supports all possible types; it is not + limited to A and AAAA records. + + If a redirect zone is configured with a ``primaries`` option, then it is + transferred in as if it were a secondary zone. Otherwise, it is loaded from a + file as if it were a primary zone. + + Because redirect zones are not referenced directly by name, they are not + kept in the zone lookup table with normal primary and secondary zones. To reload + a redirect zone, use ``rndc reload -redirect``; to retransfer a + redirect zone configured as a secondary, use ``rndc retransfer -redirect``. + When using ``rndc reload`` without specifying a zone name, redirect + zones are reloaded along with other zones. + +``delegation-only`` + This zone type is used to enforce the delegation-only status of infrastructure + zones (e.g., COM, NET, ORG). Any answer that is received without an + explicit or implicit delegation in the authority section is treated + as NXDOMAIN. This does not apply to the zone apex, and should not be + applied to leaf zones. + + ``delegation-only`` has no effect on answers received from forwarders. + + See caveats in :ref:`root-delegation-only <root-delegation-only>`. + +``in-view`` + When using multiple views, a ``primary`` or ``secondary`` zone configured + in one view can be referenced in a subsequent view. This allows both views + to use the same zone without the overhead of loading it more than once. This + is configured using a ``zone`` statement, with an ``in-view`` option + specifying the view in which the zone is defined. A ``zone`` statement + containing ``in-view`` does not need to specify a type, since that is part + of the zone definition in the other view. + + See :ref:`multiple_views` for more information. + +Class +^^^^^ + +The zone's name may optionally be followed by a class. If a class is not +specified, class ``IN`` (for ``Internet``) is assumed. This is correct +for the vast majority of cases. + +The ``hesiod`` class is named for an information service from MIT's +Project Athena. It was used to share information about various systems +databases, such as users, groups, printers, and so on. The keyword ``HS`` +is a synonym for hesiod. + +Another MIT development is Chaosnet, a LAN protocol created in the +mid-1970s. Zone data for it can be specified with the ``CHAOS`` class. + +.. _zone_options: + +Zone Options +^^^^^^^^^^^^ + +``allow-notify`` + See the description of ``allow-notify`` in :ref:`access_control`. + +``allow-query`` + See the description of ``allow-query`` in :ref:`access_control`. + +``allow-query-on`` + See the description of ``allow-query-on`` in :ref:`access_control`. + +``allow-transfer`` + See the description of ``allow-transfer`` in :ref:`access_control`. + +``allow-update`` + See the description of ``allow-update`` in :ref:`access_control`. + +``update-policy`` + This specifies a "Simple Secure Update" policy. See :ref:`dynamic_update_policies`. + +``allow-update-forwarding`` + See the description of ``allow-update-forwarding`` in :ref:`access_control`. + +``also-notify`` + This option is only meaningful if ``notify`` is active for this zone. The set of + machines that receive a ``DNS NOTIFY`` message for this zone is + made up of all the listed name servers (other than the primary) + for the zone, plus any IP addresses specified with + ``also-notify``. A port may be specified with each ``also-notify`` + address to send the notify messages to a port other than the default + of 53. A TSIG key may also be specified to cause the ``NOTIFY`` to be + signed by the given key. ``also-notify`` is not meaningful for stub + zones. The default is the empty list. + +``check-names`` + This option is used to restrict the character set and syntax of + certain domain names in primary files and/or DNS responses received + from the network. The default varies according to zone type. For + ``primary`` zones the default is ``fail``; for ``secondary`` zones the + default is ``warn``. It is not implemented for ``hint`` zones. + +``check-mx`` + See the description of ``check-mx`` in :ref:`boolean_options`. + +``check-spf`` + See the description of ``check-spf`` in :ref:`boolean_options`. + +``check-wildcard`` + See the description of ``check-wildcard`` in :ref:`boolean_options`. + +``check-integrity`` + See the description of ``check-integrity`` in :ref:`boolean_options`. + +``check-sibling`` + See the description of ``check-sibling`` in :ref:`boolean_options`. + +``zero-no-soa-ttl`` + See the description of ``zero-no-soa-ttl`` in :ref:`boolean_options`. + +``update-check-ksk`` + See the description of ``update-check-ksk`` in :ref:`boolean_options`. + +``dnssec-loadkeys-interval`` + See the description of ``dnssec-loadkeys-interval`` in :ref:`options`. + +``dnssec-update-mode`` + See the description of ``dnssec-update-mode`` in :ref:`options`. + +``dnssec-dnskey-kskonly`` + See the description of ``dnssec-dnskey-kskonly`` in :ref:`boolean_options`. + +``try-tcp-refresh`` + See the description of ``try-tcp-refresh`` in :ref:`boolean_options`. + +``database`` + This specifies the type of database to be used to store the zone data. + The string following the ``database`` keyword is interpreted as a + list of whitespace-delimited words. The first word identifies the + database type, and any subsequent words are passed as arguments to + the database to be interpreted in a way specific to the database + type. + + The default is ``rbt``, BIND 9's native in-memory red-black tree + database. This database does not take arguments. + + Other values are possible if additional database drivers have been + linked into the server. Some sample drivers are included with the + distribution but none are linked in by default. + +``dialup`` + See the description of ``dialup`` in :ref:`boolean_options`. + +``delegation-only`` + This flag only applies to forward, hint, and stub zones. If set to + ``yes``, then the zone is treated as if it is also a + delegation-only type zone. + + See caveats in :ref:`root-delegation-only <root-delegation-only>`. + +.. _file-option: + +``file`` + This sets the zone's filename. In ``primary``, ``hint``, and ``redirect`` + zones which do not have ``primaries`` defined, zone data is loaded from + this file. In ``secondary``, ``mirror``, ``stub``, and ``redirect`` zones + which do have ``primaries`` defined, zone data is retrieved from + another server and saved in this file. This option is not applicable + to other zone types. + +``forward`` + This option is only meaningful if the zone has a forwarders list. The ``only`` value + causes the lookup to fail after trying the forwarders and getting no + answer, while ``first`` allows a normal lookup to be tried. + +``forwarders`` + This is used to override the list of global forwarders. If it is not + specified in a zone of type ``forward``, no forwarding is done for + the zone and the global options are not used. + +``journal`` + This allows the default journal's filename to be overridden. The default is + the zone's filename with "``.jnl``" appended. This is applicable to + ``primary`` and ``secondary`` zones. + +``max-ixfr-ratio`` + See the description of ``max-ixfr-ratio`` in :ref:`options`. + +``max-journal-size`` + See the description of ``max-journal-size`` in :ref:`server_resource_limits`. + +``max-records`` + See the description of ``max-records`` in :ref:`server_resource_limits`. + +``max-transfer-time-in`` + See the description of ``max-transfer-time-in`` in :ref:`zone_transfers`. + +``max-transfer-idle-in`` + See the description of ``max-transfer-idle-in`` in :ref:`zone_transfers`. + +``max-transfer-time-out`` + See the description of ``max-transfer-time-out`` in :ref:`zone_transfers`. + +``max-transfer-idle-out`` + See the description of ``max-transfer-idle-out`` in :ref:`zone_transfers`. + +``notify`` + See the description of ``notify`` in :ref:`boolean_options`. + +``notify-delay`` + See the description of ``notify-delay`` in :ref:`tuning`. + +``notify-to-soa`` + See the description of ``notify-to-soa`` in :ref:`boolean_options`. + +``zone-statistics`` + See the description of ``zone-statistics`` in :ref:`options`. + +``server-addresses`` + This option is only meaningful for static-stub zones. This is a list of IP addresses + to which queries should be sent in recursive resolution for the zone. + A non-empty list for this option internally configures the apex + NS RR with associated glue A or AAAA RRs. + + For example, if "example.com" is configured as a static-stub zone + with 192.0.2.1 and 2001:db8::1234 in a ``server-addresses`` option, + the following RRs are internally configured: + + :: + + example.com. NS example.com. + example.com. A 192.0.2.1 + example.com. AAAA 2001:db8::1234 + + These records are used internally to resolve names under the + static-stub zone. For instance, if the server receives a query for + "www.example.com" with the RD bit on, the server initiates + recursive resolution and sends queries to 192.0.2.1 and/or + 2001:db8::1234. + +``server-names`` + This option is only meaningful for static-stub zones. This is a list of domain names + of name servers that act as authoritative servers of the static-stub + zone. These names are resolved to IP addresses when ``named`` + needs to send queries to these servers. For this supplemental + resolution to be successful, these names must not be a subdomain of the + origin name of the static-stub zone. That is, when "example.net" is the + origin of a static-stub zone, "ns.example" and "master.example.com" + can be specified in the ``server-names`` option, but "ns.example.net" + cannot; it is rejected by the configuration parser. + + A non-empty list for this option internally configures the apex + NS RR with the specified names. For example, if "example.com" is + configured as a static-stub zone with "ns1.example.net" and + "ns2.example.net" in a ``server-names`` option, the following RRs + are internally configured: + + :: + + example.com. NS ns1.example.net. + example.com. NS ns2.example.net. + + These records are used internally to resolve names under the + static-stub zone. For instance, if the server receives a query for + "www.example.com" with the RD bit on, the server initiates recursive + resolution, resolves "ns1.example.net" and/or "ns2.example.net" to IP + addresses, and then sends queries to one or more of these addresses. + +``sig-validity-interval`` + See the description of ``sig-validity-interval`` in :ref:`tuning`. + +``sig-signing-nodes`` + See the description of ``sig-signing-nodes`` in :ref:`tuning`. + +``sig-signing-signatures`` + See the description of ``sig-signing-signatures`` in + :ref:`tuning`. + +``sig-signing-type`` + See the description of ``sig-signing-type`` in :ref:`tuning`. + +``transfer-source`` + See the description of ``transfer-source`` in :ref:`zone_transfers`. + +``transfer-source-v6`` + See the description of ``transfer-source-v6`` in :ref:`zone_transfers`. + +``alt-transfer-source`` + See the description of ``alt-transfer-source`` in :ref:`zone_transfers`. + +``alt-transfer-source-v6`` + See the description of ``alt-transfer-source-v6`` in :ref:`zone_transfers`. + +``use-alt-transfer-source`` + See the description of ``use-alt-transfer-source`` in :ref:`zone_transfers`. + +``notify-source`` + See the description of ``notify-source`` in :ref:`zone_transfers`. + +``notify-source-v6`` + See the description of ``notify-source-v6`` in :ref:`zone_transfers`. + +``min-refresh-time``; ``max-refresh-time``; ``min-retry-time``; ``max-retry-time`` + See the descriptions in :ref:`tuning`. + +``ixfr-from-differences`` + See the description of ``ixfr-from-differences`` in :ref:`boolean_options`. + (Note that the ``ixfr-from-differences`` choices of ``primary`` and ``secondary`` + are not available at the zone level.) + +``key-directory`` + See the description of ``key-directory`` in :ref:`options`. + +``auto-dnssec`` + See the description of ``auto-dnssec`` in :ref:`options`. + +``serial-update-method`` + See the description of ``serial-update-method`` in :ref:`options`. + +``inline-signing`` + If ``yes``, BIND 9 maintains a separate signed version of the zone. + An unsigned zone is transferred in or loaded from disk and the signed + version of the zone is served with, possibly, a different serial + number. The signed version of the zone is stored in a file that is + the zone's filename (set in ``file``) with a ``.signed`` extension. + This behavior is disabled by default. + +``multi-master`` + See the description of ``multi-master`` in :ref:`boolean_options`. + +``masterfile-format`` + See the description of ``masterfile-format`` in :ref:`tuning`. + +``max-zone-ttl`` + See the description of ``max-zone-ttl`` in :ref:`options`. + +``dnssec-secure-to-insecure`` + See the description of ``dnssec-secure-to-insecure`` in :ref:`boolean_options`. + +.. _dynamic_update_policies: + +Dynamic Update Policies +^^^^^^^^^^^^^^^^^^^^^^^ + +BIND 9 supports two methods of granting clients the right to +perform dynamic updates to a zone: + +- ``allow-update`` - a simple access control list +- ``update-policy`` - fine-grained access control + +In both cases, BIND 9 writes the updates to the zone's filename +set in ``file``. + +In the case of a DNSSEC zone, DNSSEC records are also written to +the zone's filename, unless ``inline-signing`` is enabled. + + .. note:: The zone file can no longer be manually updated while ``named`` + is running; it is now necessary to perform :option:`rndc freeze`, edit, + and then perform :option:`rndc thaw`. Comments and formatting + in the zone file are lost when dynamic updates occur. + +The ``allow-update`` clause is a simple access control list. Any client +that matches the ACL is granted permission to update any record in the +zone. + +The ``update-policy`` clause allows more fine-grained control over which +updates are allowed. It specifies a set of rules, in which each rule +either grants or denies permission for one or more names in the zone to +be updated by one or more identities. Identity is determined by the key +that signed the update request, using either TSIG or SIG(0). In most +cases, ``update-policy`` rules only apply to key-based identities. There +is no way to specify update permissions based on the client source address. + +``update-policy`` rules are only meaningful for zones of type +``primary``, and are not allowed in any other zone type. It is a +configuration error to specify both ``allow-update`` and +``update-policy`` at the same time. + +A pre-defined ``update-policy`` rule can be switched on with the command +``update-policy local;``. ``named`` automatically +generates a TSIG session key when starting and stores it in a file; +this key can then be used by local clients to update the zone while +``named`` is running. By default, the session key is stored in the file +``/var/run/named/session.key``, the key name is "local-ddns", and the +key algorithm is HMAC-SHA256. These values are configurable with the +``session-keyfile``, ``session-keyname``, and ``session-keyalg`` options, +respectively. A client running on the local system, if run with +appropriate permissions, may read the session key from the key file and +use it to sign update requests. The zone's update policy is set to +allow that key to change any record within the zone. Assuming the key +name is "local-ddns", this policy is equivalent to: + +:: + + update-policy { grant local-ddns zonesub any; }; + +with the additional restriction that only clients connecting from the +local system are permitted to send updates. + +Note that only one session key is generated by ``named``; all zones +configured to use ``update-policy local`` accept the same key. + +The command ``nsupdate -l`` implements this feature, sending requests to +localhost and signing them using the key retrieved from the session key +file. + +Other rule definitions look like this: + +:: + + ( grant | deny ) identity ruletype name types + +Each rule grants or denies privileges. Rules are checked in the order in +which they are specified in the ``update-policy`` statement. Once a +message has successfully matched a rule, the operation is immediately +granted or denied, and no further rules are examined. There are 13 types +of rules; the rule type is specified by the ``ruletype`` field, and the +interpretation of other fields varies depending on the rule type. + +In general, a rule is matched when the key that signed an update request +matches the ``identity`` field, the name of the record to be updated +matches the ``name`` field (in the manner specified by the ``ruletype`` +field), and the type of the record to be updated matches the ``types`` +field. Details for each rule type are described below. + +The ``identity`` field must be set to a fully qualified domain name. In +most cases, this represents the name of the TSIG or SIG(0) key that +must be used to sign the update request. If the specified name is a +wildcard, it is subject to DNS wildcard expansion, and the rule may +apply to multiple identities. When a TKEY exchange has been used to +create a shared secret, the identity of the key used to authenticate the +TKEY exchange is used as the identity of the shared secret. Some +rule types use identities matching the client's Kerberos principal (e.g, +``"host/machine@REALM"``) or Windows realm (``machine$@REALM``). + +The ``name`` field also specifies a fully qualified domain name. This often +represents the name of the record to be updated. Interpretation of this +field is dependent on rule type. + +If no ``types`` are explicitly specified, then a rule matches all types +except RRSIG, NS, SOA, NSEC, and NSEC3. Types may be specified by name, +including ``ANY``; ANY matches all types except NSEC and NSEC3, which can +never be updated. Note that when an attempt is made to delete all +records associated with a name, the rules are checked for each existing +record type. + +The ruletype field has 16 values: ``name``, ``subdomain``, ``zonesub``, ``wildcard``, +``self``, ``selfsub``, ``selfwild``, ``ms-self``, ``ms-selfsub``, ``ms-subdomain``, +``krb5-self``, ``krb5-selfsub``, ``krb5-subdomain``, +``tcp-self``, ``6to4-self``, and ``external``. + +``name`` + With exact-match semantics, this rule matches when the name being updated is identical to the contents of the ``name`` field. + +``subdomain`` + This rule matches when the name being updated is a subdomain of, or identical to, the contents of the ``name`` field. + +``zonesub`` + This rule is similar to subdomain, except that it matches when the name being updated is a subdomain of the zone in which the ``update-policy`` statement appears. This obviates the need to type the zone name twice, and enables the use of a standard ``update-policy`` statement in multiple zones without modification. + When this rule is used, the ``name`` field is omitted. + +``wildcard`` + The ``name`` field is subject to DNS wildcard expansion, and this rule matches when the name being updated is a valid expansion of the wildcard. + +``self`` + This rule matches when the name of the record being updated matches the contents of the ``identity`` field. The ``name`` field is ignored. To avoid confusion, it is recommended that this field be set to the same value as the ``identity`` field or to "." + The ``self`` rule type is most useful when allowing one key per name to update, where the key has the same name as the record to be updated. In this case, the ``identity`` field can be specified as ``*`` (asterisk). + +``selfsub`` + This rule is similar to ``self``, except that subdomains of ``self`` can also be updated. + +``selfwild`` + This rule is similar to ``self``, except that only subdomains of ``self`` can be updated. + +``ms-self`` + When a client sends an UPDATE using a Windows machine principal (for example, ``machine$@REALM``), this rule allows records with the absolute name of ``machine.REALM`` to be updated. + + The realm to be matched is specified in the ``identity`` field. + + The ``name`` field has no effect on this rule; it should be set to "." as a placeholder. + + For example, ``grant EXAMPLE.COM ms-self . A AAAA`` allows any machine with a valid principal in the realm ``EXAMPLE.COM`` to update its own address records. + +``ms-selfsub`` + This is similar to ``ms-self``, except it also allows updates to any subdomain of the name specified in the Windows machine principal, not just to the name itself. + +``ms-subdomain`` + When a client sends an UPDATE using a Windows machine principal (for example, ``machine$@REALM``), this rule allows any machine in the specified realm to update any record in the zone or in a specified subdomain of the zone. + + The realm to be matched is specified in the ``identity`` field. + + The ``name`` field specifies the subdomain that may be updated. If set to "." or any other name at or above the zone apex, any name in the zone can be updated. + + For example, if ``update-policy`` for the zone "example.com" includes ``grant EXAMPLE.COM ms-subdomain hosts.example.com. AA AAAA``, any machine with a valid principal in the realm ``EXAMPLE.COM`` is able to update address records at or below ``hosts.example.com``. + +``krb5-self`` + When a client sends an UPDATE using a Kerberos machine principal (for example, ``host/machine@REALM``), this rule allows records with the absolute name of ``machine`` to be updated, provided it has been authenticated by REALM. This is similar but not identical to ``ms-self``, due to the ``machine`` part of the Kerberos principal being an absolute name instead of an unqualified name. + + The realm to be matched is specified in the ``identity`` field. + + The ``name`` field has no effect on this rule; it should be set to "." as a placeholder. + + For example, ``grant EXAMPLE.COM krb5-self . A AAAA`` allows any machine with a valid principal in the realm ``EXAMPLE.COM`` to update its own address records. + +``krb5-selfsub`` + This is similar to ``krb5-self``, except it also allows updates to any subdomain of the name specified in the ``machine`` part of the Kerberos principal, not just to the name itself. + +``krb5-subdomain`` + This rule is identical to ``ms-subdomain``, except that it works with Kerberos machine principals (i.e., ``host/machine@REALM``) rather than Windows machine principals. + +``tcp-self`` + This rule allows updates that have been sent via TCP and for which the standard mapping from the client's IP address into the ``in-addr.arpa`` and ``ip6.arpa`` namespaces matches the name to be updated. The ``identity`` field must match that name. The ``name`` field should be set to ".". Note that, since identity is based on the client's IP address, it is not necessary for update request messages to be signed. + + .. note:: + It is theoretically possible to spoof these TCP sessions. + +``6to4-self`` + This allows the name matching a 6to4 IPv6 prefix, as specified in :rfc:`3056`, to be updated by any TCP connection from either the 6to4 network or from the corresponding IPv4 address. This is intended to allow NS or DNAME RRsets to be added to the ``ip6.arpa`` reverse tree. + + The ``identity`` field must match the 6to4 prefix in ``ip6.arpa``. The ``name`` field should be set to ".". Note that, since identity is based on the client's IP address, it is not necessary for update request messages to be signed. + + In addition, if specified for an ``ip6.arpa`` name outside of the ``2.0.0.2.ip6.arpa`` namespace, the corresponding /48 reverse name can be updated. For example, TCP/IPv6 connections from 2001:DB8:ED0C::/48 can update records at ``C.0.D.E.8.B.D.0.1.0.0.2.ip6.arpa``. + + .. note:: + It is theoretically possible to spoof these TCP sessions. + +``external`` + This rule allows ``named`` to defer the decision of whether to allow a given update to an external daemon. + + The method of communicating with the daemon is specified in the ``identity`` field, the format of which is "``local:``\ path", where "path" is the location of a Unix-domain socket. (Currently, "local" is the only supported mechanism.) + + Requests to the external daemon are sent over the Unix-domain socket as datagrams with the following format: + + :: + + Protocol version number (4 bytes, network byte order, currently 1) + Request length (4 bytes, network byte order) + Signer (null-terminated string) + Name (null-terminated string) + TCP source address (null-terminated string) + Rdata type (null-terminated string) + Key (null-terminated string) + TKEY token length (4 bytes, network byte order) + TKEY token (remainder of packet) + + The daemon replies with a four-byte value in network byte order, containing either 0 or 1; 0 indicates that the specified update is not permitted, and 1 indicates that it is. + + .. warning:: The external daemon must not delay communication. This policy is evaluated synchronously; any wait period negatively affects :iscman:`named` performance. + +.. _multiple_views: + +Multiple Views +^^^^^^^^^^^^^^ + +When multiple views are in use, a zone may be referenced by more than +one of them. Often, the views contain different zones with the same +name, allowing different clients to receive different answers for the +same queries. At times, however, it is desirable for multiple views to +contain identical zones. The ``in-view`` zone option provides an +efficient way to do this; it allows a view to reference a zone that was +defined in a previously configured view. For example: + +:: + + view internal { + match-clients { 10/8; }; + + zone example.com { + type primary; + file "example-external.db"; + }; + }; + + view external { + match-clients { any; }; + + zone example.com { + in-view internal; + }; + }; + +An ``in-view`` option cannot refer to a view that is configured later in +the configuration file. + +A ``zone`` statement which uses the ``in-view`` option may not use any +other options, with the exception of ``forward`` and ``forwarders``. +(These options control the behavior of the containing view, rather than +change the zone object itself.) + +Zone-level ACLs (e.g., allow-query, allow-transfer), and other +configuration details of the zone, are all set in the view the referenced +zone is defined in. Be careful to ensure that ACLs are wide +enough for all views referencing the zone. + +An ``in-view`` zone cannot be used as a response policy zone. + +An ``in-view`` zone is not intended to reference a ``forward`` zone. + +.. _zone_file: + +Zone File +--------- + +.. _types_of_resource_records_and_when_to_use_them: + +Types of Resource Records and When to Use Them +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +This section, largely borrowed from :rfc:`1034`, describes the concept of a +Resource Record (RR) and explains when each type is used. Since the +publication of :rfc:`1034`, several new RRs have been identified and +implemented in the DNS. These are also included. + +Resource Records +^^^^^^^^^^^^^^^^ + +A domain name identifies a node. Each node has a set of resource +information, which may be empty. The set of resource information +associated with a particular name is composed of separate RRs. The order +of RRs in a set is not significant and need not be preserved by name +servers, resolvers, or other parts of the DNS. However, sorting of +multiple RRs is permitted for optimization purposes: for example, to +specify that a particular nearby server be tried first. See +:ref:`the_sortlist_statement` and :ref:`rrset_ordering`. + +The components of a Resource Record are: + +owner name + The domain name where the RR is found. + +type + An encoded 16-bit value that specifies the type of the resource record. + +TTL + The time-to-live of the RR. This field is a 32-bit integer in units of seconds, and is primarily used by resolvers when they cache RRs. The TTL describes how long a RR can be cached before it should be discarded. + +class + An encoded 16-bit value that identifies a protocol family or an instance of a protocol. + +RDATA + The resource data. The format of the data is type- and sometimes class-specific. + +For a complete list of *types* of valid RRs, including those that have been obsoleted, please refer to https://en.wikipedia.org/wiki/List_of_DNS_record_types. + +The following *classes* of resource records are currently valid in the +DNS: + +IN + The Internet. + +CH + Chaosnet, a LAN protocol created at MIT in the mid-1970s. It was rarely used for its historical purpose, but was reused for BIND's built-in server information zones, e.g., ``version.bind``. + +HS + Hesiod, an information service developed by MIT's Project Athena. It was used to share information about various systems databases, such as users, groups, printers, etc. + +The owner name is often implicit, rather than forming an integral part +of the RR. For example, many name servers internally form tree or hash +structures for the name space, and chain RRs off nodes. The remaining RR +parts are the fixed header (type, class, TTL), which is consistent for +all RRs, and a variable part (RDATA) that fits the needs of the resource +being described. + +The TTL field is a time limit on how long an RR can be +kept in a cache. This limit does not apply to authoritative data in +zones; that also times out, but follows the refreshing policies for the +zone. The TTL is assigned by the administrator for the zone where the +data originates. While short TTLs can be used to minimize caching, and a +zero TTL prohibits caching, the realities of Internet performance +suggest that these times should be on the order of days for the typical +host. If a change is anticipated, the TTL can be reduced prior to +the change to minimize inconsistency, and then +increased back to its former value following the change. + +The data in the RDATA section of RRs is carried as a combination of +binary strings and domain names. The domain names are frequently used as +"pointers" to other data in the DNS. + +.. _rr_text: + +Textual Expression of RRs +^^^^^^^^^^^^^^^^^^^^^^^^^ + +RRs are represented in binary form in the packets of the DNS protocol, +and are usually represented in highly encoded form when stored in a name +server or resolver. In the examples provided in :rfc:`1034`, a style +similar to that used in primary files was employed in order to show the +contents of RRs. In this format, most RRs are shown on a single line, +although continuation lines are possible using parentheses. + +The start of the line gives the owner of the RR. If a line begins with a +blank, then the owner is assumed to be the same as that of the previous +RR. Blank lines are often included for readability. + +Following the owner are listed the TTL, type, and class of the RR. Class +and type use the mnemonics defined above, and TTL is an integer before +the type field. To avoid ambiguity in parsing, type and class +mnemonics are disjoint, TTLs are integers, and the type mnemonic is +always last. The IN class and TTL values are often omitted from examples +in the interest of clarity. + +The resource data or RDATA section of the RR is given using knowledge +of the typical representation for the data. + +For example, the RRs carried in a message might be shown as: + + +---------------------+---------------+--------------------------------+ + | ``ISI.EDU.`` | ``MX`` | ``10 VENERA.ISI.EDU.`` | + +---------------------+---------------+--------------------------------+ + | | ``MX`` | ``10 VAXA.ISI.EDU`` | + +---------------------+---------------+--------------------------------+ + | ``VENERA.ISI.EDU`` | ``A`` | ``128.9.0.32`` | + +---------------------+---------------+--------------------------------+ + | | ``A`` | ``10.1.0.52`` | + +---------------------+---------------+--------------------------------+ + | ``VAXA.ISI.EDU`` | ``A`` | ``10.2.0.27`` | + +---------------------+---------------+--------------------------------+ + | | ``A`` | ``128.9.0.33`` | + +---------------------+---------------+--------------------------------+ + +The MX RRs have an RDATA section which consists of a 16-bit number +followed by a domain name. The address RRs use a standard IP address +format to contain a 32-bit Internet address. + +The above example shows six RRs, with two RRs at each of three domain +names. + +Here is another possible example: + + +----------------------+---------------+-------------------------------+ + | ``XX.LCS.MIT.EDU.`` | ``IN A`` | ``10.0.0.44`` | + +----------------------+---------------+-------------------------------+ + | | ``CH A`` | ``MIT.EDU. 2420`` | + +----------------------+---------------+-------------------------------+ + +This shows two addresses for ``XX.LCS.MIT.EDU``, each of a +different class. + +.. _mx_records: + +Discussion of MX Records +~~~~~~~~~~~~~~~~~~~~~~~~ + +As described above, domain servers store information as a series of +resource records, each of which contains a particular piece of +information about a given domain name (which is usually, but not always, +a host). The simplest way to think of an RR is as a typed pair of data, a +domain name matched with a relevant datum and stored with some +additional type information, to help systems determine when the RR is +relevant. + +MX records are used to control delivery of email. The data specified in +the record is a priority and a domain name. The priority controls the +order in which email delivery is attempted, with the lowest number +first. If two priorities are the same, a server is chosen randomly. If +no servers at a given priority are responding, the mail transport agent +falls back to the next largest priority. Priority numbers do not +have any absolute meaning; they are relevant only respective to other +MX records for that domain name. The domain name given is the machine to +which the mail is delivered. It *must* have an associated address +record (A or AAAA); CNAME is not sufficient. + +For a given domain, if there is both a CNAME record and an MX record, +the MX record is in error and is ignored. Instead, the mail is +delivered to the server specified in the MX record pointed to by the +CNAME. For example: + + +------------------------+--------+--------+--------------+------------------------+ + | ``example.com.`` | ``IN`` | ``MX`` | ``10`` | ``mail.example.com.`` | + +------------------------+--------+--------+--------------+------------------------+ + | | ``IN`` | ``MX`` | ``10`` | ``mail2.example.com.`` | + +------------------------+--------+--------+--------------+------------------------+ + | | ``IN`` | ``MX`` | ``20`` | ``mail.backup.org.`` | + +------------------------+--------+--------+--------------+------------------------+ + | ``mail.example.com.`` | ``IN`` | ``A`` | ``10.0.0.1`` | | + +------------------------+--------+--------+--------------+------------------------+ + | ``mail2.example.com.`` | ``IN`` | ``A`` | ``10.0.0.2`` | | + +------------------------+--------+--------+--------------+------------------------+ + +Mail delivery is attempted to ``mail.example.com`` and +``mail2.example.com`` (in any order); if neither of those succeeds, +delivery to ``mail.backup.org`` is attempted. + +.. _Setting_TTLs: + +Setting TTLs +~~~~~~~~~~~~ + +The time-to-live (TTL) of the RR field is a 32-bit integer represented in +units of seconds, and is primarily used by resolvers when they cache +RRs. The TTL describes how long an RR can be cached before it should be +discarded. The following three types of TTLs are currently used in a zone +file. + +SOA + The last field in the SOA is the negative caching TTL. This controls how long other servers cache no-such-domain (NXDOMAIN) responses from this server. + + The maximum time for negative caching is 3 hours (3h). + +$TTL + The $TTL directive at the top of the zone file (before the SOA) gives a default TTL for every RR without a specific TTL set. + +RR TTLs + Each RR can have a TTL as the second field in the RR, which controls how long other servers can cache it. + +All of these TTLs default to units of seconds, though units can be +explicitly specified: for example, ``1h30m``. + +.. _ipv4_reverse: + +Inverse Mapping in IPv4 +~~~~~~~~~~~~~~~~~~~~~~~ + +Reverse name resolution (that is, translation from IP address to name) +is achieved by means of the ``in-addr.arpa`` domain and PTR records. +Entries in the in-addr.arpa domain are made in least-to-most significant +order, read left to right. This is the opposite order to the way IP +addresses are usually written. Thus, a machine with an IP address of +10.1.2.3 would have a corresponding in-addr.arpa name of +3.2.1.10.in-addr.arpa. This name should have a PTR resource record whose +data field is the name of the machine or, optionally, multiple PTR +records if the machine has more than one name. For example, in the +``example.com`` domain: + + +--------------+-------------------------------------------------------+ + | ``$ORIGIN`` | ``2.1.10.in-addr.arpa`` | + +--------------+-------------------------------------------------------+ + | ``3`` | ``IN PTR foo.example.com.`` | + +--------------+-------------------------------------------------------+ + +.. note:: + + The ``$ORIGIN`` line in this example is only to provide context; + it does not necessarily appear in the actual + usage. It is only used here to indicate that the example is + relative to the listed origin. + +.. _zone_directives: + +Other Zone File Directives +~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The DNS "master file" format was initially defined in :rfc:`1035` and has +subsequently been extended. While the format itself is class-independent, +all records in a zone file must be of the same class. + +Master file directives include ``$ORIGIN``, ``$INCLUDE``, and ``$TTL.`` + +.. _atsign: + +The ``@`` (at-sign) +^^^^^^^^^^^^^^^^^^^ + +When used in the label (or name) field, the asperand or at-sign (@) +symbol represents the current origin. At the start of the zone file, it +is the <``zone_name``>, followed by a trailing dot (.). + +.. _origin_directive: + +The ``$ORIGIN`` Directive +^^^^^^^^^^^^^^^^^^^^^^^^^ + +Syntax: ``$ORIGIN`` domain-name [comment] + +``$ORIGIN`` sets the domain name that is appended to any +unqualified records. When a zone is first read, there is an implicit +``$ORIGIN`` <``zone_name``>``.``; note the trailing dot. The +current ``$ORIGIN`` is appended to the domain specified in the +``$ORIGIN`` argument if it is not absolute. + +:: + + $ORIGIN example.com. + WWW CNAME MAIN-SERVER + +is equivalent to + +:: + + WWW.EXAMPLE.COM. CNAME MAIN-SERVER.EXAMPLE.COM. + +.. _include_directive: + +The ``$INCLUDE`` Directive +^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Syntax: ``$INCLUDE`` filename [origin] [comment] + +This reads and processes the file ``filename`` as if it were included in the +file at this point. The ``filename`` can be an absolute path, or a relative +path. In the latter case it is read from ``named``'s working directory. If +``origin`` is specified, the file is processed with ``$ORIGIN`` set to that +value; otherwise, the current ``$ORIGIN`` is used. + +The origin and the current domain name revert to the values they had +prior to the ``$INCLUDE`` once the file has been read. + +.. note:: + + :rfc:`1035` specifies that the current origin should be restored after + an ``$INCLUDE``, but it is silent on whether the current domain name + should also be restored. BIND 9 restores both of them. This could be + construed as a deviation from :rfc:`1035`, a feature, or both. + +.. _ttl_directive: + +The ``$TTL`` Directive +^^^^^^^^^^^^^^^^^^^^^^ + +Syntax: ``$TTL`` default-ttl [comment] + +This sets the default Time-To-Live (TTL) for subsequent records with undefined +TTLs. Valid TTLs are of the range 0-2147483647 seconds. + +``$TTL`` is defined in :rfc:`2308`. + +.. _generate_directive: + +BIND Primary File Extension: the ``$GENERATE`` Directive +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Syntax: ``$GENERATE`` range owner [ttl] [class] type rdata [comment] + +``$GENERATE`` is used to create a series of resource records that only +differ from each other by an iterator. + +``range`` + This can be one of two forms: start-stop or start-stop/step. + If the first form is used, then step is set to 1. "start", + "stop", and "step" must be positive integers between 0 and + (2^31)-1. "start" must not be larger than "stop". + +``owner`` + This describes the owner name of the resource records to be created. + + The ``owner`` string may include one or more ``$`` (dollar sign) + symbols, which will be replaced with the iterator value when + generating records; see below for details. + +``ttl`` + This specifies the time-to-live of the generated records. If + not specified, this is inherited using the normal TTL inheritance + rules. + + ``class`` and ``ttl`` can be entered in either order. + +``class`` + This specifies the class of the generated records. This must + match the zone class if it is specified. + + ``class`` and ``ttl`` can be entered in either order. + +``type`` + This can be any valid type. + +``rdata`` + This is a string containing the RDATA of the resource record + to be created. As with ``owner``, the ``rdata`` string may + include one or more ``$`` symbols, which are replaced with the + iterator value. ``rdata`` may be quoted if there are spaces in + the string; the quotation marks do not appear in the generated + record. + + Any single ``$`` (dollar sign) symbols within the ``owner`` or + ``rdata`` strings are replaced by the iterator value. To get a ``$`` + in the output, escape the ``$`` using a backslash ``\\``, e.g., + ``\$``. (For compatibility with earlier versions, ``$$`` is also + recognized as indicating a literal ``$`` in the output.) + + The ``$`` may optionally be followed by modifiers which change + the offset from the iterator, field width, and base. Modifiers + are introduced by a ``{`` (left brace) immediately following + the ``$``, as in ``${offset[,width[,base]]}``. For example, + ``${-20,3,d}`` subtracts 20 from the current value and prints + the result as a decimal in a zero-padded field of width 3. + Available output forms are decimal (``d``), octal (``o``), + hexadecimal (``x`` or ``X`` for uppercase), and nibble (``n`` + or ``N`` for uppercase). The modfiier cannot contain whitespace + or newlines. + + The default modifier is ``${0,0,d}``. If the ``owner`` is not + absolute, the current ``$ORIGIN`` is appended to the name. + + In nibble mode, the value is treated as if it were a reversed + hexadecimal string, with each hexadecimal digit as a separate + label. The width field includes the label separator. + +Examples: + +``$GENERATE`` can be used to easily generate the sets of records required +to support sub-/24 reverse delegations described in :rfc:`2317`: + +:: + + $ORIGIN 0.0.192.IN-ADDR.ARPA. + $GENERATE 1-2 @ NS SERVER$.EXAMPLE. + $GENERATE 1-127 $ CNAME $.0 + +is equivalent to + +:: + + 0.0.0.192.IN-ADDR.ARPA. NS SERVER1.EXAMPLE. + 0.0.0.192.IN-ADDR.ARPA. NS SERVER2.EXAMPLE. + 1.0.0.192.IN-ADDR.ARPA. CNAME 1.0.0.0.192.IN-ADDR.ARPA. + 2.0.0.192.IN-ADDR.ARPA. CNAME 2.0.0.0.192.IN-ADDR.ARPA. + ... + 127.0.0.192.IN-ADDR.ARPA. CNAME 127.0.0.0.192.IN-ADDR.ARPA. + +This example creates a set of A and MX records. Note the MX's ``rdata`` +is a quoted string; the quotes are stripped when ``$GENERATE`` is processed: + +:: + + $ORIGIN EXAMPLE. + $GENERATE 1-127 HOST-$ A 1.2.3.$ + $GENERATE 1-127 HOST-$ MX "0 ." + +is equivalent to + +:: + + HOST-1.EXAMPLE. A 1.2.3.1 + HOST-1.EXAMPLE. MX 0 . + HOST-2.EXAMPLE. A 1.2.3.2 + HOST-2.EXAMPLE. MX 0 . + HOST-3.EXAMPLE. A 1.2.3.3 + HOST-3.EXAMPLE. MX 0 . + ... + HOST-127.EXAMPLE. A 1.2.3.127 + HOST-127.EXAMPLE. MX 0 . + + +This example generates A and AAAA records using modifiers; the AAAA +``owner`` names are generated using nibble mode: + +:: + + $ORIGIN EXAMPLE. + $GENERATE 0-2 HOST-${0,4,d} A 1.2.3.${1,0,d} + $GENERATE 1024-1026 ${0,3,n} AAAA 2001:db8::${0,4,x} + +is equivalent to: + +:: + + HOST-0000.EXAMPLE. A 1.2.3.1 + HOST-0001.EXAMPLE. A 1.2.3.2 + HOST-0002.EXAMPLE. A 1.2.3.3 + 0.0.4.EXAMPLE. AAAA 2001:db8::400 + 1.0.4.EXAMPLE. AAAA 2001:db8::401 + 2.0.4.EXAMPLE. AAAA 2001:db8::402 + +The ``$GENERATE`` directive is a BIND extension and not part of the +standard zone file format. + +.. _zonefile_format: + +Additional File Formats +~~~~~~~~~~~~~~~~~~~~~~~ + +In addition to the standard text format, BIND 9 supports the ability +to read or dump to zone files in other formats. + +The ``raw`` format is a binary representation of zone data in a manner +similar to that used in zone transfers. Since it does not require +parsing text, load time is significantly reduced. + +An even faster alternative is the ``map`` format, which is an image of a +BIND 9 in-memory zone database; it can be loaded directly into memory via +the ``mmap()`` function and the zone can begin serving queries almost +immediately. Because records are not indivdually processed when loading a +``map`` file, zones using this format cannot be used in ``response-policy`` +statements. + +For a primary server, a zone file in ``raw`` or ``map`` format is expected +to be generated from a text zone file by the ``named-compilezone`` command. +For a secondary server or a dynamic zone, the zone file is automatically +generated when ``named`` dumps the zone contents after zone transfer or +when applying prior updates, if one of these formats is specified by the +``masterfile-format`` option. + +If a zone file in a binary format needs manual modification, it first must +be converted to ``text`` format by the ``named-compilezone`` command, +then converted back after editing. For example: + +:: + named-compilezone -f map -F text -o zonefile.text <origin> zonefile.map + [edit zonefile.text] + named-compilezone -f text -F map -o zonefile.map <origin> zonefile.text + +Note that the ``map`` format is highly architecture-specific. A ``map`` +file *cannot* be used on a system with different pointer size, endianness, +or data alignment than the system on which it was generated, and should in +general be used only inside a single system. + +The ``map`` format is also dependent on the internal memory representation +of a zone database, which may change from one release of BIND 9 to another. +``map`` files are never compatible across major releases, and may not be +compatible across minor releases; any upgrade to BIND 9 may cause ``map`` +files to be rejected when loading. If a ``map`` file is being used for a +primary zone, it will need to be regenerated from text before restarting +the server. If it used for a secondary zone, this is unnecessary; the +rejection of the file will trigger a retransfer of the zone from the +primary. (To avoid a spike in traffic upon restart, it may be desirable in +some cases to convert ``map`` files to ``text`` format using +``named-compilezone`` before an upgrade, then back to ``map`` format with +the new version of ``named-compilezone`` afterward.) + +The use of ``map`` format may also be limited by operating system +mmap(2) limits like ``sysctl vm.max_map_count``. For Linux, this +defaults to 65536, which limits the number of mapped zones that can +be used without increasing ``vm.max_map_count``. + +``raw`` format uses network byte order and avoids architecture- +dependent data alignment so that it is as portable as possible, but it is +still primarily expected to be used inside the same single system. To +export a zone file in either ``raw`` or ``map`` format, or make a portable +backup of such a file, conversion to ``text`` format is recommended. + +.. _statistics: + +BIND 9 Statistics +----------------- + +BIND 9 maintains lots of statistics information and provides several +interfaces for users to access those statistics. The available +statistics include all statistics counters that are meaningful in BIND 9, +and other information that is considered useful. + +The statistics information is categorized into the following sections: + +Incoming Requests + The number of incoming DNS requests for each OPCODE. + +Incoming Queries + The number of incoming queries for each RR type. + +Outgoing Queries + The number of outgoing queries for each RR type sent from the internal + resolver, maintained per view. + +Name Server Statistics + Statistics counters for incoming request processing. + +Zone Maintenance Statistics + Statistics counters regarding zone maintenance operations, such as zone + transfers. + +Resolver Statistics + Statistics counters for name resolutions performed in the internal resolver, + maintained per view. + +Cache DB RRsets + Statistics counters related to cache contents, maintained per view. + + The "NXDOMAIN" counter is the number of names that have been cached as + nonexistent. Counters named for RR types indicate the number of active + RRsets for each type in the cache database. + + If an RR type name is preceded by an exclamation point (!), it represents the + number of records in the cache which indicate that the type does not exist + for a particular name; this is also known as "NXRRSET". If an RR type name + is preceded by a hash mark (#), it represents the number of RRsets for this + type that are present in the cache but whose TTLs have expired; these RRsets + may only be used if stale answers are enabled. If an RR type name is + preceded by a tilde (~), it represents the number of RRsets for this type + that are present in the cache database but are marked for garbage collection; + these RRsets cannot be used. + +Socket I/O Statistics + Statistics counters for network-related events. + +A subset of Name Server Statistics is collected and shown per zone for +which the server has the authority, when ``zone-statistics`` is set to +``full`` (or ``yes``), for backward compatibility. See the description of +``zone-statistics`` in :ref:`options` for further details. + +These statistics counters are shown with their zone and view names. The +view name is omitted when the server is not configured with explicit +views. + +There are currently two user interfaces to get access to the statistics. +One is in plain-text format, dumped to the file specified by the +``statistics-file`` configuration option; the other is remotely +accessible via a statistics channel when the ``statistics-channels`` +statement is specified in the configuration file (see :ref:`statschannels`.) + +.. _statsfile: + +The Statistics File +~~~~~~~~~~~~~~~~~~~ + +The text format statistics dump begins with a line, like: + +``+++ Statistics Dump +++ (973798949)`` + +The number in parentheses is a standard Unix-style timestamp, measured +in seconds since January 1, 1970. Following that line is a set of +statistics information, which is categorized as described above. Each +section begins with a line, like: + +``++ Name Server Statistics ++`` + +Each section consists of lines, each containing the statistics counter +value followed by its textual description; see below for available +counters. For brevity, counters that have a value of 0 are not shown in +the statistics file. + +The statistics dump ends with the line where the number is identical to +the number in the beginning line; for example: + +``--- Statistics Dump --- (973798949)`` + +.. _statistics_counters: + +Statistics Counters +~~~~~~~~~~~~~~~~~~~ + +The following lists summarize the statistics counters that BIND 9 provides. +For each counter, the abbreviated +symbol name is given; these symbols are shown in the statistics +information accessed via an HTTP statistics channel. +The description of the counter is also shown in the +statistics file but, in this document, may be slightly +modified for better readability. + +.. _stats_counters: + +Name Server Statistics Counters +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +``Requestv4`` + This indicates the number of IPv4 requests received. Note: this also counts non-query requests. + +``Requestv6`` + This indicates the number of IPv6 requests received. Note: this also counts non-query requests. + +``ReqEdns0`` + This indicates the number of requests received with EDNS(0). + +``ReqBadEDN SVer`` + This indicates the number of requests received with an unsupported EDNS version. + +``ReqTSIG`` + This indicates the number of requests received with TSIG. + +``ReqSIG0`` + This indicates the number of requests received with SIG(0). + +``ReqBadSIG`` + This indicates the number of requests received with an invalid (TSIG or SIG(0)) signature. + +``ReqTCP`` + This indicates the number of TCP requests received. + +``AuthQryRej`` + This indicates the number of rejected authoritative (non-recursive) queries. + +``RecQryRej`` + This indicates the number of rejected recursive queries. + +``XfrRej`` + This indicates the number of rejected zone transfer requests. + +``UpdateRej`` + This indicates the number of rejected dynamic update requests. + +``Response`` + This indicates the number of responses sent. + +``RespTruncated`` + This indicates the number of truncated responses sent. + +``RespEDNS0`` + This indicates the number of responses sent with EDNS(0). + +``RespTSIG`` + This indicates the number of responses sent with TSIG. + +``RespSIG0`` + This indicates the number of responses sent with SIG(0). + +``QrySuccess`` + This indicates the number of queries that resulted in a successful answer, meaning queries which return a NOERROR response with at least one answer RR. This corresponds to the ``success`` counter of previous versions of BIND 9. + +``QryAuthAns`` + This indicates the number of queries that resulted in an authoritative answer. + +``QryNoauthAns`` + This indicates the number of queries that resulted in a non-authoritative answer. + +``QryReferral`` + This indicates the number of queries that resulted in a referral answer. This corresponds to the ``referral`` counter of previous versions of BIND 9. + +``QryNxrrset`` + This indicates the number of queries that resulted in NOERROR responses with no data. This corresponds to the ``nxrrset`` counter of previous versions of BIND 9. + +``QrySERVFAIL`` + This indicates the number of queries that resulted in SERVFAIL. + +``QryFORMERR`` + This indicates the number of queries that resulted in FORMERR. + +``QryNXDOMAIN`` + This indicates the number of queries that resulted in NXDOMAIN. This corresponds to the ``nxdomain`` counter of previous versions of BIND 9. + +``QryRecursion`` + This indicates the number of queries that caused the server to perform recursion in order to find the final answer. This corresponds to the ``recursion`` counter of previous versions of BIND 9. + +``QryDuplicate`` + This indicates the number of queries which the server attempted to recurse but for which it discovered an existing query with the same IP address, port, query ID, name, type, and class already being processed. This corresponds to the ``duplicate`` counter of previous versions of BIND 9. + +``QryDropped`` + This indicates the number of recursive queries for which the server discovered an excessive number of existing recursive queries for the same name, type, and class, and which were subsequently dropped. This is the number of dropped queries due to the reason explained with the ``clients-per-query`` and ``max-clients-per-query`` options (see :ref:`clients-per-query <clients-per-query>`). This corresponds to the ``dropped`` counter of previous versions of BIND 9. + +``QryFailure`` + This indicates the number of query failures. This corresponds to the ``failure`` counter of previous versions of BIND 9. Note: this counter is provided mainly for backward compatibility with previous versions; normally, more fine-grained counters such as ``AuthQryRej`` and ``RecQryRej`` that would also fall into this counter are provided, so this counter is not of much interest in practice. + +``QryNXRedir`` + This indicates the number of queries that resulted in NXDOMAIN that were redirected. + +``QryNXRedirRLookup`` + This indicates the number of queries that resulted in NXDOMAIN that were redirected and resulted in a successful remote lookup. + +``XfrReqDone`` + This indicates the number of requested and completed zone transfers. + +``UpdateReqFwd`` + This indicates the number of forwarded update requests. + +``UpdateRespFwd`` + This indicates the number of forwarded update responses. + +``UpdateFwdFail`` + This indicates the number of forwarded dynamic updates that failed. + +``UpdateDone`` + This indicates the number of completed dynamic updates. + +``UpdateFail`` + This indicates the number of failed dynamic updates. + +``UpdateBadPrereq`` + This indicates the number of dynamic updates rejected due to a prerequisite failure. + +``UpdateQuota`` + This indicates the number of times a dynamic update or update + forwarding request was rejected because the number of pending + requests exceeded ``update-quota``. + +``RateDropped`` + This indicates the number of responses dropped due to rate limits. + +``RateSlipped`` + This indicates the number of responses truncated by rate limits. + +``RPZRewrites`` + This indicates the number of response policy zone rewrites. + +.. _zone_stats: + +Zone Maintenance Statistics Counters +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +``NotifyOutv4`` + This indicates the number of IPv4 notifies sent. + +``NotifyOutv6`` + This indicates the number of IPv6 notifies sent. + +``NotifyInv4`` + This indicates the number of IPv4 notifies received. + +``NotifyInv6`` + This indicates the number of IPv6 notifies received. + +``NotifyRej`` + This indicates the number of incoming notifies rejected. + +``SOAOutv4`` + This indicates the number of IPv4 SOA queries sent. + +``SOAOutv6`` + This indicates the number of IPv6 SOA queries sent. + +``AXFRReqv4`` + This indicates the number of requested IPv4 AXFRs. + +``AXFRReqv6`` + This indicates the number of requested IPv6 AXFRs. + +``IXFRReqv4`` + This indicates the number of requested IPv4 IXFRs. + +``IXFRReqv6`` + This indicates the number of requested IPv6 IXFRs. + +``XfrSuccess`` + This indicates the number of successful zone transfer requests. + +``XfrFail`` + This indicates the number of failed zone transfer requests. + +.. _resolver_stats: + +Resolver Statistics Counters +^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +``Queryv4`` + This indicates the number of IPv4 queries sent. + +``Queryv6`` + This indicates the number of IPv6 queries sent. + +``Responsev4`` + This indicates the number of IPv4 responses received. + +``Responsev6`` + This indicates the number of IPv6 responses received. + +``NXDOMAIN`` + This indicates the number of NXDOMAINs received. + +``SERVFAIL`` + This indicates the number of SERVFAILs received. + +``FORMERR`` + This indicates the number of FORMERRs received. + +``OtherError`` + This indicates the number of other errors received. + +``EDNS0Fail`` + This indicates the number of EDNS(0) query failures. + +``Mismatch`` + This indicates the number of mismatched responses received, meaning the DNS ID, response's source address, and/or the response's source port does not match what was expected. (The port must be 53 or as defined by the ``port`` option.) This may be an indication of a cache poisoning attempt. + +``Truncated`` + This indicates the number of truncated responses received. + +``Lame`` + This indicates the number of lame delegations received. + +``Retry`` + This indicates the number of query retries performed. + +``QueryAbort`` + This indicates the number of queries aborted due to quota control. + +``QuerySockFail`` + This indicates the number of failures in opening query sockets. One common reason for such failures is due to a limitation on file descriptors. + +``QueryTimeout`` + This indicates the number of query timeouts. + +``GlueFetchv4`` + This indicates the number of IPv4 NS address fetches invoked. + +``GlueFetchv6`` + This indicates the number of IPv6 NS address fetches invoked. + +``GlueFetchv4Fail`` + This indicates the number of failed IPv4 NS address fetches. + +``GlueFetchv6Fail`` + This indicates the number of failed IPv6 NS address fetches. + +``ValAttempt`` + This indicates the number of attempted DNSSEC validations. + +``ValOk`` + This indicates the number of successful DNSSEC validations. + +``ValNegOk`` + This indicates the number of successful DNSSEC validations on negative information. + +``ValFail`` + This indicates the number of failed DNSSEC validations. + +``QryRTTnn`` + This provides a frequency table on query round-trip times (RTTs). Each ``nn`` specifies the corresponding frequency. In the sequence of ``nn_1``, ``nn_2``, ..., ``nn_m``, the value of ``nn_i`` is the number of queries whose RTTs are between ``nn_(i-1)`` (inclusive) and ``nn_i`` (exclusive) milliseconds. For the sake of convenience, we define ``nn_0`` to be 0. The last entry should be represented as ``nn_m+``, which means the number of queries whose RTTs are equal to or greater than ``nn_m`` milliseconds. + +.. _socket_stats: + +Socket I/O Statistics Counters +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Socket I/O statistics counters are defined per socket type, which are +``UDP4`` (UDP/IPv4), ``UDP6`` (UDP/IPv6), ``TCP4`` (TCP/IPv4), ``TCP6`` +(TCP/IPv6), ``Unix`` (Unix Domain), and ``FDwatch`` (sockets opened +outside the socket module). In the following list, ``<TYPE>`` represents +a socket type. Not all counters are available for all socket types; +exceptions are noted in the descriptions. + +``<TYPE>Open`` + This indicates the number of sockets opened successfully. This counter does not apply to the ``FDwatch`` type. + +``<TYPE>OpenFail`` + This indicates the number of failures to open sockets. This counter does not apply to the ``FDwatch`` type. + +``<TYPE>Close`` + This indicates the number of closed sockets. + +``<TYPE>BindFail`` + This indicates the number of failures to bind sockets. + +``<TYPE>ConnFail`` + This indicates the number of failures to connect sockets. + +``<TYPE>Conn`` + This indicates the number of connections established successfully. + +``<TYPE>AcceptFail`` + This indicates the number of failures to accept incoming connection requests. This counter does not apply to the ``UDP`` and ``FDwatch`` types. + +``<TYPE>Accept`` + This indicates the number of incoming connections successfully accepted. This counter does not apply to the ``UDP`` and ``FDwatch`` types. + +``<TYPE>SendErr`` + This indicates the number of errors in socket send operations. + +``<TYPE>RecvErr`` + This indicates the number of errors in socket receive operations, including errors of send operations on a connected UDP socket, notified by an ICMP error message. |