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+Network Working Group P. Mockapetris
+Request for Comments: 1035 ISI
+ November 1987
+Obsoletes: RFCs 882, 883, 973
+
+ DOMAIN NAMES - IMPLEMENTATION AND SPECIFICATION
+
+
+1. STATUS OF THIS MEMO
+
+This RFC describes the details of the domain system and protocol, and
+assumes that the reader is familiar with the concepts discussed in a
+companion RFC, "Domain Names - Concepts and Facilities" [RFC-4301].
+
+The domain system is a mixture of functions and data types which are an
+official protocol and functions and data types which are still
+experimental. Since the domain system is intentionally extensible, new
+data types and experimental behavior should always be expected in parts
+of the system beyond the official protocol. The official protocol parts
+include standard queries, responses and the Internet class RR data
+formats (e.g., host addresses). Since the previous RFC set, several
+definitions have changed, so some previous definitions are obsolete.
+
+Experimental or obsolete features are clearly marked in these RFCs, and
+such information should be used with caution.
+
+The reader is especially cautioned not to depend on the values which
+appear in examples to be current or complete, since their purpose is
+primarily pedagogical. Distribution of this memo is unlimited.
+
+ Table of Contents
+
+ 1. STATUS OF THIS MEMO 1
+ 2. INTRODUCTION 3
+ 2.1. Overview 3
+ 2.2. Common configurations 4
+ 2.3. Conventions 7
+ 2.3.1. Preferred name syntax 7
+ 2.3.2. Data Transmission Order 8
+ 2.3.3. Character Case 9
+ 2.3.4. Size limits 10
+ 3. DOMAIN NAME SPACE AND RR DEFINITIONS 10
+ 3.1. Name space definitions 10
+ 3.2. RR definitions 11
+ 3.2.1. Format 11
+ 3.2.2. TYPE values 12
+ 3.2.3. QTYPE values 12
+ 3.2.4. CLASS values 13
+
+
+
+Mockapetris [Page 1]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+ 3.2.5. QCLASS values 13
+ 3.3. Standard RRs 13
+ 3.3.1. CNAME RDATA format 14
+ 3.3.2. HINFO RDATA format 14
+ 3.3.3. MB RDATA format (EXPERIMENTAL) 14
+ 3.3.4. MD RDATA format (Obsolete) 15
+ 3.3.5. MF RDATA format (Obsolete) 15
+ 3.3.6. MG RDATA format (EXPERIMENTAL) 16
+ 3.3.7. MINFO RDATA format (EXPERIMENTAL) 16
+ 3.3.8. MR RDATA format (EXPERIMENTAL) 17
+ 3.3.9. MX RDATA format 17
+ 3.3.10. NULL RDATA format (EXPERIMENTAL) 17
+ 3.3.11. NS RDATA format 18
+ 3.3.12. PTR RDATA format 18
+ 3.3.13. SOA RDATA format 19
+ 3.3.14. TXT RDATA format 20
+ 3.4. ARPA Internet specific RRs 20
+ 3.4.1. A RDATA format 20
+ 3.4.2. WKS RDATA format 21
+ 3.5. IN-ADDR.ARPA domain 22
+ 3.6. Defining new types, classes, and special namespaces 24
+ 4. MESSAGES 25
+ 4.1. Format 25
+ 4.1.1. Header section format 26
+ 4.1.2. Question section format 28
+ 4.1.3. Resource record format 29
+ 4.1.4. Message compression 30
+ 4.2. Transport 32
+ 4.2.1. UDP usage 32
+ 4.2.2. TCP usage 32
+ 5. MASTER FILES 33
+ 5.1. Format 33
+ 5.2. Use of master files to define zones 35
+ 5.3. Master file example 36
+ 6. NAME SERVER IMPLEMENTATION 37
+ 6.1. Architecture 37
+ 6.1.1. Control 37
+ 6.1.2. Database 37
+ 6.1.3. Time 39
+ 6.2. Standard query processing 39
+ 6.3. Zone refresh and reload processing 39
+ 6.4. Inverse queries (Optional) 40
+ 6.4.1. The contents of inverse queries and responses 40
+ 6.4.2. Inverse query and response example 41
+ 6.4.3. Inverse query processing 42
+
+
+
+
+
+
+Mockapetris [Page 2]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+ 6.5. Completion queries and responses 42
+ 7. RESOLVER IMPLEMENTATION 43
+ 7.1. Transforming a user request into a query 43
+ 7.2. Sending the queries 44
+ 7.3. Processing responses 46
+ 7.4. Using the cache 47
+ 8. MAIL SUPPORT 47
+ 8.1. Mail exchange binding 48
+ 8.2. Mailbox binding (Experimental) 48
+ 9. REFERENCES and BIBLIOGRAPHY 50
+ Index 54
+
+2. INTRODUCTION
+
+2.1. Overview
+
+The goal of domain names is to provide a mechanism for naming resources
+in such a way that the names are usable in different hosts, networks,
+protocol families, internets, and administrative organizations.
+
+From the user's point of view, domain names are useful as arguments to a
+local agent, called a resolver, which retrieves information associated
+with the domain name. Thus a user might ask for the host address or
+mail information associated with a particular domain name. To enable
+the user to request a particular type of information, an appropriate
+query type is passed to the resolver with the domain name. To the user,
+the domain tree is a single information space; the resolver is
+responsible for hiding the distribution of data among name servers from
+the user.
+
+From the resolver's point of view, the database that makes up the domain
+space is distributed among various name servers. Different parts of the
+domain space are stored in different name servers, although a particular
+data item will be stored redundantly in two or more name servers. The
+resolver starts with knowledge of at least one name server. When the
+resolver processes a user query it asks a known name server for the
+information; in return, the resolver either receives the desired
+information or a referral to another name server. Using these
+referrals, resolvers learn the identities and contents of other name
+servers. Resolvers are responsible for dealing with the distribution of
+the domain space and dealing with the effects of name server failure by
+consulting redundant databases in other servers.
+
+Name servers manage two kinds of data. The first kind of data held in
+sets called zones; each zone is the complete database for a particular
+"pruned" subtree of the domain space. This data is called
+authoritative. A name server periodically checks to make sure that its
+zones are up to date, and if not, obtains a new copy of updated zones
+
+
+
+Mockapetris [Page 3]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+from master files stored locally or in another name server. The second
+kind of data is cached data which was acquired by a local resolver.
+This data may be incomplete, but improves the performance of the
+retrieval process when non-local data is repeatedly accessed. Cached
+data is eventually discarded by a timeout mechanism.
+
+This functional structure isolates the problems of user interface,
+failure recovery, and distribution in the resolvers and isolates the
+database update and refresh problems in the name servers.
+
+2.2. Common configurations
+
+A host can participate in the domain name system in a number of ways,
+depending on whether the host runs programs that retrieve information
+from the domain system, name servers that answer queries from other
+hosts, or various combinations of both functions. The simplest, and
+perhaps most typical, configuration is shown below:
+
+ Local Host | Foreign
+ |
+ +---------+ +----------+ | +--------+
+ | | user queries | |queries | | |
+ | User |-------------->| |---------|->|Foreign |
+ | Program | | Resolver | | | Name |
+ | |<--------------| |<--------|--| Server |
+ | | user responses| |responses| | |
+ +---------+ +----------+ | +--------+
+ | A |
+ cache additions | | references |
+ V | |
+ +----------+ |
+ | cache | |
+ +----------+ |
+
+User programs interact with the domain name space through resolvers; the
+format of user queries and user responses is specific to the host and
+its operating system. User queries will typically be operating system
+calls, and the resolver and its cache will be part of the host operating
+system. Less capable hosts may choose to implement the resolver as a
+subroutine to be linked in with every program that needs its services.
+Resolvers answer user queries with information they acquire via queries
+to foreign name servers and the local cache.
+
+Note that the resolver may have to make several queries to several
+different foreign name servers to answer a particular user query, and
+hence the resolution of a user query may involve several network
+accesses and an arbitrary amount of time. The queries to foreign name
+servers and the corresponding responses have a standard format described
+
+
+
+Mockapetris [Page 4]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+in this memo, and may be datagrams.
+
+Depending on its capabilities, a name server could be a stand alone
+program on a dedicated machine or a process or processes on a large
+timeshared host. A simple configuration might be:
+
+ Local Host | Foreign
+ |
+ +---------+ |
+ / /| |
+ +---------+ | +----------+ | +--------+
+ | | | | |responses| | |
+ | | | | Name |---------|->|Foreign |
+ | Master |-------------->| Server | | |Resolver|
+ | files | | | |<--------|--| |
+ | |/ | | queries | +--------+
+ +---------+ +----------+ |
+
+Here a primary name server acquires information about one or more zones
+by reading master files from its local file system, and answers queries
+about those zones that arrive from foreign resolvers.
+
+The DNS requires that all zones be redundantly supported by more than
+one name server. Designated secondary servers can acquire zones and
+check for updates from the primary server using the zone transfer
+protocol of the DNS. This configuration is shown below:
+
+ Local Host | Foreign
+ |
+ +---------+ |
+ / /| |
+ +---------+ | +----------+ | +--------+
+ | | | | |responses| | |
+ | | | | Name |---------|->|Foreign |
+ | Master |-------------->| Server | | |Resolver|
+ | files | | | |<--------|--| |
+ | |/ | | queries | +--------+
+ +---------+ +----------+ |
+ A |maintenance | +--------+
+ | +------------|->| |
+ | queries | |Foreign |
+ | | | Name |
+ +------------------|--| Server |
+ maintenance responses | +--------+
+
+In this configuration, the name server periodically establishes a
+virtual circuit to a foreign name server to acquire a copy of a zone or
+to check that an existing copy has not changed. The messages sent for
+
+
+
+Mockapetris [Page 5]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+these maintenance activities follow the same form as queries and
+responses, but the message sequences are somewhat different.
+
+The information flow in a host that supports all aspects of the domain
+name system is shown below:
+
+ Local Host | Foreign
+ |
+ +---------+ +----------+ | +--------+
+ | | user queries | |queries | | |
+ | User |-------------->| |---------|->|Foreign |
+ | Program | | Resolver | | | Name |
+ | |<--------------| |<--------|--| Server |
+ | | user responses| |responses| | |
+ +---------+ +----------+ | +--------+
+ | A |
+ cache additions | | references |
+ V | |
+ +----------+ |
+ | Shared | |
+ | database | |
+ +----------+ |
+ A | |
+ +---------+ refreshes | | references |
+ / /| | V |
+ +---------+ | +----------+ | +--------+
+ | | | | |responses| | |
+ | | | | Name |---------|->|Foreign |
+ | Master |-------------->| Server | | |Resolver|
+ | files | | | |<--------|--| |
+ | |/ | | queries | +--------+
+ +---------+ +----------+ |
+ A |maintenance | +--------+
+ | +------------|->| |
+ | queries | |Foreign |
+ | | | Name |
+ +------------------|--| Server |
+ maintenance responses | +--------+
+
+The shared database holds domain space data for the local name server
+and resolver. The contents of the shared database will typically be a
+mixture of authoritative data maintained by the periodic refresh
+operations of the name server and cached data from previous resolver
+requests. The structure of the domain data and the necessity for
+synchronization between name servers and resolvers imply the general
+characteristics of this database, but the actual format is up to the
+local implementor.
+
+
+
+
+Mockapetris [Page 6]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+Information flow can also be tailored so that a group of hosts act
+together to optimize activities. Sometimes this is done to offload less
+capable hosts so that they do not have to implement a full resolver.
+This can be appropriate for PCs or hosts which want to minimize the
+amount of new network code which is required. This scheme can also
+allow a group of hosts can share a small number of caches rather than
+maintaining a large number of separate caches, on the premise that the
+centralized caches will have a higher hit ratio. In either case,
+resolvers are replaced with stub resolvers which act as front ends to
+resolvers located in a recursive server in one or more name servers
+known to perform that service:
+
+ Local Hosts | Foreign
+ |
+ +---------+ |
+ | | responses |
+ | Stub |<--------------------+ |
+ | Resolver| | |
+ | |----------------+ | |
+ +---------+ recursive | | |
+ queries | | |
+ V | |
+ +---------+ recursive +----------+ | +--------+
+ | | queries | |queries | | |
+ | Stub |-------------->| Recursive|---------|->|Foreign |
+ | Resolver| | Server | | | Name |
+ | |<--------------| |<--------|--| Server |
+ +---------+ responses | |responses| | |
+ +----------+ | +--------+
+ | Central | |
+ | cache | |
+ +----------+ |
+
+In any case, note that domain components are always replicated for
+reliability whenever possible.
+
+2.3. Conventions
+
+The domain system has several conventions dealing with low-level, but
+fundamental, issues. While the implementor is free to violate these
+conventions WITHIN HIS OWN SYSTEM, he must observe these conventions in
+ALL behavior observed from other hosts.
+
+2.3.1. Preferred name syntax
+
+The DNS specifications attempt to be as general as possible in the rules
+for constructing domain names. The idea is that the name of any
+existing object can be expressed as a domain name with minimal changes.
+
+
+
+Mockapetris [Page 7]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+However, when assigning a domain name for an object, the prudent user
+will select a name which satisfies both the rules of the domain system
+and any existing rules for the object, whether these rules are published
+or implied by existing programs.
+
+For example, when naming a mail domain, the user should satisfy both the
+rules of this memo and those in RFC-822. When creating a new host name,
+the old rules for HOSTS.TXT should be followed. This avoids problems
+when old software is converted to use domain names.
+
+The following syntax will result in fewer problems with many
+
+applications that use domain names (e.g., mail, TELNET).
+
+<domain> ::= <subdomain> | " "
+
+<subdomain> ::= <label> | <subdomain> "." <label>
+
+<label> ::= <letter> [ [ <ldh-str> ] <let-dig> ]
+
+<ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>
+
+<let-dig-hyp> ::= <let-dig> | "-"
+
+<let-dig> ::= <letter> | <digit>
+
+<letter> ::= any one of the 52 alphabetic characters A through Z in
+upper case and a through z in lower case
+
+<digit> ::= any one of the ten digits 0 through 9
+
+Note that while upper and lower case letters are allowed in domain
+names, no significance is attached to the case. That is, two names with
+the same spelling but different case are to be treated as if identical.
+
+The labels must follow the rules for ARPANET host names. They must
+start with a letter, end with a letter or digit, and have as interior
+characters only letters, digits, and hyphen. There are also some
+restrictions on the length. Labels must be 63 characters or less.
+
+For example, the following strings identify hosts in the Internet:
+
+A.ISI.EDU XX.LCS.MIT.EDU SRI-NIC.ARPA
+
+2.3.2. Data Transmission Order
+
+The order of transmission of the header and data described in this
+document is resolved to the octet level. Whenever a diagram shows a
+
+
+
+Mockapetris [Page 8]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+group of octets, the order of transmission of those octets is the normal
+order in which they are read in English. For example, in the following
+diagram, the octets are transmitted in the order they are numbered.
+
+ 0 1
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | 1 | 2 |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | 3 | 4 |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | 5 | 6 |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+Whenever an octet represents a numeric quantity, the left most bit in
+the diagram is the high order or most significant bit. That is, the bit
+labeled 0 is the most significant bit. For example, the following
+diagram represents the value 170 (decimal).
+
+ 0 1 2 3 4 5 6 7
+ +-+-+-+-+-+-+-+-+
+ |1 0 1 0 1 0 1 0|
+ +-+-+-+-+-+-+-+-+
+
+Similarly, whenever a multi-octet field represents a numeric quantity
+the left most bit of the whole field is the most significant bit. When
+a multi-octet quantity is transmitted the most significant octet is
+transmitted first.
+
+2.3.3. Character Case
+
+For all parts of the DNS that are part of the official protocol, all
+comparisons between character strings (e.g., labels, domain names, etc.)
+are done in a case-insensitive manner. At present, this rule is in
+force throughout the domain system without exception. However, future
+additions beyond current usage may need to use the full binary octet
+capabilities in names, so attempts to store domain names in 7-bit ASCII
+or use of special bytes to terminate labels, etc., should be avoided.
+
+When data enters the domain system, its original case should be
+preserved whenever possible. In certain circumstances this cannot be
+done. For example, if two RRs are stored in a database, one at x.y and
+one at X.Y, they are actually stored at the same place in the database,
+and hence only one casing would be preserved. The basic rule is that
+case can be discarded only when data is used to define structure in a
+database, and two names are identical when compared in a case
+insensitive manner.
+
+
+
+
+Mockapetris [Page 9]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+Loss of case sensitive data must be minimized. Thus while data for x.y
+and X.Y may both be stored under a single location x.y or X.Y, data for
+a.x and B.X would never be stored under A.x, A.X, b.x, or b.X. In
+general, this preserves the case of the first label of a domain name,
+but forces standardization of interior node labels.
+
+Systems administrators who enter data into the domain database should
+take care to represent the data they supply to the domain system in a
+case-consistent manner if their system is case-sensitive. The data
+distribution system in the domain system will ensure that consistent
+representations are preserved.
+
+2.3.4. Size limits
+
+Various objects and parameters in the DNS have size limits. They are
+listed below. Some could be easily changed, others are more
+fundamental.
+
+labels 63 octets or less
+
+names 255 octets or less
+
+TTL positive values of a signed 32 bit number.
+
+UDP messages 512 octets or less
+
+3. DOMAIN NAME SPACE AND RR DEFINITIONS
+
+3.1. Name space definitions
+
+Domain names in messages are expressed in terms of a sequence of labels.
+Each label is represented as a one octet length field followed by that
+number of octets. Since every domain name ends with the null label of
+the root, a domain name is terminated by a length byte of zero. The
+high order two bits of every length octet must be zero, and the
+remaining six bits of the length field limit the label to 63 octets or
+less.
+
+To simplify implementations, the total length of a domain name (i.e.,
+label octets and label length octets) is restricted to 255 octets or
+less.
+
+Although labels can contain any 8 bit values in octets that make up a
+label, it is strongly recommended that labels follow the preferred
+syntax described elsewhere in this memo, which is compatible with
+existing host naming conventions. Name servers and resolvers must
+compare labels in a case-insensitive manner (i.e., A=a), assuming ASCII
+with zero parity. Non-alphabetic codes must match exactly.
+
+
+
+Mockapetris [Page 10]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+3.2. RR definitions
+
+3.2.1. Format
+
+All RRs have the same top level format shown below:
+
+ 1 1 1 1 1 1
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | |
+ / /
+ / NAME /
+ | |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | TYPE |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | CLASS |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | TTL |
+ | |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | RDLENGTH |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--|
+ / RDATA /
+ / /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+
+where:
+
+NAME an owner name, i.e., the name of the node to which this
+ resource record pertains.
+
+TYPE two octets containing one of the RR TYPE codes.
+
+CLASS two octets containing one of the RR CLASS codes.
+
+TTL a 32 bit signed integer that specifies the time interval
+ that the resource record may be cached before the source
+ of the information should again be consulted. Zero
+ values are interpreted to mean that the RR can only be
+ used for the transaction in progress, and should not be
+ cached. For example, SOA records are always distributed
+ with a zero TTL to prohibit caching. Zero values can
+ also be used for extremely volatile data.
+
+RDLENGTH an unsigned 16 bit integer that specifies the length in
+ octets of the RDATA field.
+
+
+
+Mockapetris [Page 11]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+RDATA a variable length string of octets that describes the
+ resource. The format of this information varies
+ according to the TYPE and CLASS of the resource record.
+
+3.2.2. TYPE values
+
+TYPE fields are used in resource records. Note that these types are a
+subset of QTYPEs.
+
+TYPE value and meaning
+
+A 1 a host address
+
+NS 2 an authoritative name server
+
+MD 3 a mail destination (Obsolete - use MX)
+
+MF 4 a mail forwarder (Obsolete - use MX)
+
+CNAME 5 the canonical name for an alias
+
+SOA 6 marks the start of a zone of authority
+
+MB 7 a mailbox domain name (EXPERIMENTAL)
+
+MG 8 a mail group member (EXPERIMENTAL)
+
+MR 9 a mail rename domain name (EXPERIMENTAL)
+
+NULL 10 a null RR (EXPERIMENTAL)
+
+WKS 11 a well known service description
+
+PTR 12 a domain name pointer
+
+HINFO 13 host information
+
+MINFO 14 mailbox or mail list information
+
+MX 15 mail exchange
+
+TXT 16 text strings
+
+3.2.3. QTYPE values
+
+QTYPE fields appear in the question part of a query. QTYPES are a
+superset of TYPEs, hence all TYPEs are valid QTYPEs. In addition, the
+following QTYPEs are defined:
+
+
+
+Mockapetris [Page 12]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+AXFR 252 A request for a transfer of an entire zone
+
+MAILB 253 A request for mailbox-related records (MB, MG or MR)
+
+MAILA 254 A request for mail agent RRs (Obsolete - see MX)
+
+* 255 A request for all records
+
+3.2.4. CLASS values
+
+CLASS fields appear in resource records. The following CLASS mnemonics
+and values are defined:
+
+IN 1 the Internet
+
+CS 2 the CSNET class (Obsolete - used only for examples in
+ some obsolete RFCs)
+
+CH 3 the CHAOS class
+
+HS 4 Hesiod [Dyer 87]
+
+3.2.5. QCLASS values
+
+QCLASS fields appear in the question section of a query. QCLASS values
+are a superset of CLASS values; every CLASS is a valid QCLASS. In
+addition to CLASS values, the following QCLASSes are defined:
+
+* 255 any class
+
+3.3. Standard RRs
+
+The following RR definitions are expected to occur, at least
+potentially, in all classes. In particular, NS, SOA, CNAME, and PTR
+will be used in all classes, and have the same format in all classes.
+Because their RDATA format is known, all domain names in the RDATA
+section of these RRs may be compressed.
+
+<domain-name> is a domain name represented as a series of labels, and
+terminated by a label with zero length. <character-string> is a single
+length octet followed by that number of characters. <character-string>
+is treated as binary information, and can be up to 256 characters in
+length (including the length octet).
+
+
+
+
+
+
+
+
+Mockapetris [Page 13]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+3.3.1. CNAME RDATA format
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ / CNAME /
+ / /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+CNAME A <domain-name> which specifies the canonical or primary
+ name for the owner. The owner name is an alias.
+
+CNAME RRs cause no additional section processing, but name servers may
+choose to restart the query at the canonical name in certain cases. See
+the description of name server logic in [RFC-1034] for details.
+
+3.3.2. HINFO RDATA format
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ / CPU /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ / OS /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+CPU A <character-string> which specifies the CPU type.
+
+OS A <character-string> which specifies the operating
+ system type.
+
+Standard values for CPU and OS can be found in [RFC-1010].
+
+HINFO records are used to acquire general information about a host. The
+main use is for protocols such as FTP that can use special procedures
+when talking between machines or operating systems of the same type.
+
+3.3.3. MB RDATA format (EXPERIMENTAL)
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ / MADNAME /
+ / /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+MADNAME A <domain-name> which specifies a host which has the
+ specified mailbox.
+
+
+
+Mockapetris [Page 14]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+MB records cause additional section processing which looks up an A type
+RRs corresponding to MADNAME.
+
+3.3.4. MD RDATA format (Obsolete)
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ / MADNAME /
+ / /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+MADNAME A <domain-name> which specifies a host which has a mail
+ agent for the domain which should be able to deliver
+ mail for the domain.
+
+MD records cause additional section processing which looks up an A type
+record corresponding to MADNAME.
+
+MD is obsolete. See the definition of MX and [RFC-974] for details of
+the new scheme. The recommended policy for dealing with MD RRs found in
+a master file is to reject them, or to convert them to MX RRs with a
+preference of 0.
+
+3.3.5. MF RDATA format (Obsolete)
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ / MADNAME /
+ / /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+MADNAME A <domain-name> which specifies a host which has a mail
+ agent for the domain which will accept mail for
+ forwarding to the domain.
+
+MF records cause additional section processing which looks up an A type
+record corresponding to MADNAME.
+
+MF is obsolete. See the definition of MX and [RFC-974] for details ofw
+the new scheme. The recommended policy for dealing with MD RRs found in
+a master file is to reject them, or to convert them to MX RRs with a
+preference of 10.
+
+
+
+
+
+
+
+Mockapetris [Page 15]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+3.3.6. MG RDATA format (EXPERIMENTAL)
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ / MGMNAME /
+ / /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+MGMNAME A <domain-name> which specifies a mailbox which is a
+ member of the mail group specified by the domain name.
+
+MG records cause no additional section processing.
+
+3.3.7. MINFO RDATA format (EXPERIMENTAL)
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ / RMAILBX /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ / EMAILBX /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+RMAILBX A <domain-name> which specifies a mailbox which is
+ responsible for the mailing list or mailbox. If this
+ domain name names the root, the owner of the MINFO RR is
+ responsible for itself. Note that many existing mailing
+ lists use a mailbox X-request for the RMAILBX field of
+ mailing list X, e.g., Msgroup-request for Msgroup. This
+ field provides a more general mechanism.
+
+
+EMAILBX A <domain-name> which specifies a mailbox which is to
+ receive error messages related to the mailing list or
+ mailbox specified by the owner of the MINFO RR (similar
+ to the ERRORS-TO: field which has been proposed). If
+ this domain name names the root, errors should be
+ returned to the sender of the message.
+
+MINFO records cause no additional section processing. Although these
+records can be associated with a simple mailbox, they are usually used
+with a mailing list.
+
+
+
+
+
+
+
+
+Mockapetris [Page 16]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+3.3.8. MR RDATA format (EXPERIMENTAL)
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ / NEWNAME /
+ / /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+NEWNAME A <domain-name> which specifies a mailbox which is the
+ proper rename of the specified mailbox.
+
+MR records cause no additional section processing. The main use for MR
+is as a forwarding entry for a user who has moved to a different
+mailbox.
+
+3.3.9. MX RDATA format
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | PREFERENCE |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ / EXCHANGE /
+ / /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+PREFERENCE A 16 bit integer which specifies the preference given to
+ this RR among others at the same owner. Lower values
+ are preferred.
+
+EXCHANGE A <domain-name> which specifies a host willing to act as
+ a mail exchange for the owner name.
+
+MX records cause type A additional section processing for the host
+specified by EXCHANGE. The use of MX RRs is explained in detail in
+[RFC-974].
+
+3.3.10. NULL RDATA format (EXPERIMENTAL)
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ / <anything> /
+ / /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+Anything at all may be in the RDATA field so long as it is 65535 octets
+or less.
+
+
+
+
+Mockapetris [Page 17]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+NULL records cause no additional section processing. NULL RRs are not
+allowed in master files. NULLs are used as placeholders in some
+experimental extensions of the DNS.
+
+3.3.11. NS RDATA format
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ / NSDNAME /
+ / /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+NSDNAME A <domain-name> which specifies a host which should be
+ authoritative for the specified class and domain.
+
+NS records cause both the usual additional section processing to locate
+a type A record, and, when used in a referral, a special search of the
+zone in which they reside for glue information.
+
+The NS RR states that the named host should be expected to have a zone
+starting at owner name of the specified class. Note that the class may
+not indicate the protocol family which should be used to communicate
+with the host, although it is typically a strong hint. For example,
+hosts which are name servers for either Internet (IN) or Hesiod (HS)
+class information are normally queried using IN class protocols.
+
+3.3.12. PTR RDATA format
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ / PTRDNAME /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+PTRDNAME A <domain-name> which points to some location in the
+ domain name space.
+
+PTR records cause no additional section processing. These RRs are used
+in special domains to point to some other location in the domain space.
+These records are simple data, and don't imply any special processing
+similar to that performed by CNAME, which identifies aliases. See the
+description of the IN-ADDR.ARPA domain for an example.
+
+
+
+
+
+
+
+
+Mockapetris [Page 18]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+3.3.13. SOA RDATA format
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ / MNAME /
+ / /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ / RNAME /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | SERIAL |
+ | |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | REFRESH |
+ | |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | RETRY |
+ | |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | EXPIRE |
+ | |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | MINIMUM |
+ | |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+MNAME The <domain-name> of the name server that was the
+ original or primary source of data for this zone.
+
+RNAME A <domain-name> which specifies the mailbox of the
+ person responsible for this zone.
+
+SERIAL The unsigned 32 bit version number of the original copy
+ of the zone. Zone transfers preserve this value. This
+ value wraps and should be compared using sequence space
+ arithmetic.
+
+REFRESH A 32 bit time interval before the zone should be
+ refreshed.
+
+RETRY A 32 bit time interval that should elapse before a
+ failed refresh should be retried.
+
+EXPIRE A 32 bit time value that specifies the upper limit on
+ the time interval that can elapse before the zone is no
+ longer authoritative.
+
+
+
+
+
+Mockapetris [Page 19]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+MINIMUM The unsigned 32 bit minimum TTL field that should be
+ exported with any RR from this zone.
+
+SOA records cause no additional section processing.
+
+All times are in units of seconds.
+
+Most of these fields are pertinent only for name server maintenance
+operations. However, MINIMUM is used in all query operations that
+retrieve RRs from a zone. Whenever a RR is sent in a response to a
+query, the TTL field is set to the maximum of the TTL field from the RR
+and the MINIMUM field in the appropriate SOA. Thus MINIMUM is a lower
+bound on the TTL field for all RRs in a zone. Note that this use of
+MINIMUM should occur when the RRs are copied into the response and not
+when the zone is loaded from a master file or via a zone transfer. The
+reason for this provison is to allow future dynamic update facilities to
+change the SOA RR with known semantics.
+
+
+3.3.14. TXT RDATA format
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ / TXT-DATA /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+TXT-DATA One or more <character-string>s.
+
+TXT RRs are used to hold descriptive text. The semantics of the text
+depends on the domain where it is found.
+
+3.4. Internet specific RRs
+
+3.4.1. A RDATA format
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | ADDRESS |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+ADDRESS A 32 bit Internet address.
+
+Hosts that have multiple Internet addresses will have multiple A
+records.
+
+
+
+
+
+Mockapetris [Page 20]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+A records cause no additional section processing. The RDATA section of
+an A line in a master file is an Internet address expressed as four
+decimal numbers separated by dots without any embedded spaces (e.g.,
+"10.2.0.52" or "192.0.5.6").
+
+3.4.2. WKS RDATA format
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | ADDRESS |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | PROTOCOL | |
+ +--+--+--+--+--+--+--+--+ |
+ | |
+ / <BIT MAP> /
+ / /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+ADDRESS An 32 bit Internet address
+
+PROTOCOL An 8 bit IP protocol number
+
+<BIT MAP> A variable length bit map. The bit map must be a
+ multiple of 8 bits long.
+
+The WKS record is used to describe the well known services supported by
+a particular protocol on a particular internet address. The PROTOCOL
+field specifies an IP protocol number, and the bit map has one bit per
+port of the specified protocol. The first bit corresponds to port 0,
+the second to port 1, etc. If the bit map does not include a bit for a
+protocol of interest, that bit is assumed zero. The appropriate values
+and mnemonics for ports and protocols are specified in [RFC-1010].
+
+For example, if PROTOCOL=TCP (6), the 26th bit corresponds to TCP port
+25 (SMTP). If this bit is set, a SMTP server should be listening on TCP
+port 25; if zero, SMTP service is not supported on the specified
+address.
+
+The purpose of WKS RRs is to provide availability information for
+servers for TCP and UDP. If a server supports both TCP and UDP, or has
+multiple Internet addresses, then multiple WKS RRs are used.
+
+WKS RRs cause no additional section processing.
+
+In master files, both ports and protocols are expressed using mnemonics
+or decimal numbers.
+
+
+
+
+Mockapetris [Page 21]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+3.5. IN-ADDR.ARPA domain
+
+The Internet uses a special domain to support gateway location and
+Internet address to host mapping. Other classes may employ a similar
+strategy in other domains. The intent of this domain is to provide a
+guaranteed method to perform host address to host name mapping, and to
+facilitate queries to locate all gateways on a particular network in the
+Internet.
+
+Note that both of these services are similar to functions that could be
+performed by inverse queries; the difference is that this part of the
+domain name space is structured according to address, and hence can
+guarantee that the appropriate data can be located without an exhaustive
+search of the domain space.
+
+The domain begins at IN-ADDR.ARPA and has a substructure which follows
+the Internet addressing structure.
+
+Domain names in the IN-ADDR.ARPA domain are defined to have up to four
+labels in addition to the IN-ADDR.ARPA suffix. Each label represents
+one octet of an Internet address, and is expressed as a character string
+for a decimal value in the range 0-255 (with leading zeros omitted
+except in the case of a zero octet which is represented by a single
+zero).
+
+Host addresses are represented by domain names that have all four labels
+specified. Thus data for Internet address 10.2.0.52 is located at
+domain name 52.0.2.10.IN-ADDR.ARPA. The reversal, though awkward to
+read, allows zones to be delegated which are exactly one network of
+address space. For example, 10.IN-ADDR.ARPA can be a zone containing
+data for the ARPANET, while 26.IN-ADDR.ARPA can be a separate zone for
+MILNET. Address nodes are used to hold pointers to primary host names
+in the normal domain space.
+
+Network numbers correspond to some non-terminal nodes at various depths
+in the IN-ADDR.ARPA domain, since Internet network numbers are either 1,
+2, or 3 octets. Network nodes are used to hold pointers to the primary
+host names of gateways attached to that network. Since a gateway is, by
+definition, on more than one network, it will typically have two or more
+network nodes which point at it. Gateways will also have host level
+pointers at their fully qualified addresses.
+
+Both the gateway pointers at network nodes and the normal host pointers
+at full address nodes use the PTR RR to point back to the primary domain
+names of the corresponding hosts.
+
+For example, the IN-ADDR.ARPA domain will contain information about the
+ISI gateway between net 10 and 26, an MIT gateway from net 10 to MIT's
+
+
+
+Mockapetris [Page 22]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+net 18, and hosts A.ISI.EDU and MULTICS.MIT.EDU. Assuming that ISI
+gateway has addresses 10.2.0.22 and 26.0.0.103, and a name MILNET-
+GW.ISI.EDU, and the MIT gateway has addresses 10.0.0.77 and 18.10.0.4
+and a name GW.LCS.MIT.EDU, the domain database would contain:
+
+ 10.IN-ADDR.ARPA. PTR MILNET-GW.ISI.EDU.
+ 10.IN-ADDR.ARPA. PTR GW.LCS.MIT.EDU.
+ 18.IN-ADDR.ARPA. PTR GW.LCS.MIT.EDU.
+ 26.IN-ADDR.ARPA. PTR MILNET-GW.ISI.EDU.
+ 22.0.2.10.IN-ADDR.ARPA. PTR MILNET-GW.ISI.EDU.
+ 103.0.0.26.IN-ADDR.ARPA. PTR MILNET-GW.ISI.EDU.
+ 77.0.0.10.IN-ADDR.ARPA. PTR GW.LCS.MIT.EDU.
+ 4.0.10.18.IN-ADDR.ARPA. PTR GW.LCS.MIT.EDU.
+ 103.0.3.26.IN-ADDR.ARPA. PTR A.ISI.EDU.
+ 6.0.0.10.IN-ADDR.ARPA. PTR MULTICS.MIT.EDU.
+
+Thus a program which wanted to locate gateways on net 10 would originate
+a query of the form QTYPE=PTR, QCLASS=IN, QNAME=10.IN-ADDR.ARPA. It
+would receive two RRs in response:
+
+ 10.IN-ADDR.ARPA. PTR MILNET-GW.ISI.EDU.
+ 10.IN-ADDR.ARPA. PTR GW.LCS.MIT.EDU.
+
+The program could then originate QTYPE=A, QCLASS=IN queries for MILNET-
+GW.ISI.EDU. and GW.LCS.MIT.EDU. to discover the Internet addresses of
+these gateways.
+
+A resolver which wanted to find the host name corresponding to Internet
+host address 10.0.0.6 would pursue a query of the form QTYPE=PTR,
+QCLASS=IN, QNAME=6.0.0.10.IN-ADDR.ARPA, and would receive:
+
+ 6.0.0.10.IN-ADDR.ARPA. PTR MULTICS.MIT.EDU.
+
+Several cautions apply to the use of these services:
+ - Since the IN-ADDR.ARPA special domain and the normal domain
+ for a particular host or gateway will be in different zones,
+ the possibility exists that that the data may be inconsistent.
+
+ - Gateways will often have two names in separate domains, only
+ one of which can be primary.
+
+ - Systems that use the domain database to initialize their
+ routing tables must start with enough gateway information to
+ guarantee that they can access the appropriate name server.
+
+ - The gateway data only reflects the existence of a gateway in a
+ manner equivalent to the current HOSTS.TXT file. It doesn't
+ replace the dynamic availability information from GGP or EGP.
+
+
+
+Mockapetris [Page 23]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+3.6. Defining new types, classes, and special namespaces
+
+The previously defined types and classes are the ones in use as of the
+date of this memo. New definitions should be expected. This section
+makes some recommendations to designers considering additions to the
+existing facilities. The mailing list NAMEDROPPERS@SRI-NIC.ARPA is the
+forum where general discussion of design issues takes place.
+
+In general, a new type is appropriate when new information is to be
+added to the database about an existing object, or we need new data
+formats for some totally new object. Designers should attempt to define
+types and their RDATA formats that are generally applicable to all
+classes, and which avoid duplication of information. New classes are
+appropriate when the DNS is to be used for a new protocol, etc which
+requires new class-specific data formats, or when a copy of the existing
+name space is desired, but a separate management domain is necessary.
+
+New types and classes need mnemonics for master files; the format of the
+master files requires that the mnemonics for type and class be disjoint.
+
+TYPE and CLASS values must be a proper subset of QTYPEs and QCLASSes
+respectively.
+
+The present system uses multiple RRs to represent multiple values of a
+type rather than storing multiple values in the RDATA section of a
+single RR. This is less efficient for most applications, but does keep
+RRs shorter. The multiple RRs assumption is incorporated in some
+experimental work on dynamic update methods.
+
+The present system attempts to minimize the duplication of data in the
+database in order to insure consistency. Thus, in order to find the
+address of the host for a mail exchange, you map the mail domain name to
+a host name, then the host name to addresses, rather than a direct
+mapping to host address. This approach is preferred because it avoids
+the opportunity for inconsistency.
+
+In defining a new type of data, multiple RR types should not be used to
+create an ordering between entries or express different formats for
+equivalent bindings, instead this information should be carried in the
+body of the RR and a single type used. This policy avoids problems with
+caching multiple types and defining QTYPEs to match multiple types.
+
+For example, the original form of mail exchange binding used two RR
+types one to represent a "closer" exchange (MD) and one to represent a
+"less close" exchange (MF). The difficulty is that the presence of one
+RR type in a cache doesn't convey any information about the other
+because the query which acquired the cached information might have used
+a QTYPE of MF, MD, or MAILA (which matched both). The redesigned
+
+
+
+Mockapetris [Page 24]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+service used a single type (MX) with a "preference" value in the RDATA
+section which can order different RRs. However, if any MX RRs are found
+in the cache, then all should be there.
+
+4. MESSAGES
+
+4.1. Format
+
+All communications inside of the domain protocol are carried in a single
+format called a message. The top level format of message is divided
+into 5 sections (some of which are empty in certain cases) shown below:
+
+ +---------------------+
+ | Header |
+ +---------------------+
+ | Question | the question for the name server
+ +---------------------+
+ | Answer | RRs answering the question
+ +---------------------+
+ | Authority | RRs pointing toward an authority
+ +---------------------+
+ | Additional | RRs holding additional information
+ +---------------------+
+
+The header section is always present. The header includes fields that
+specify which of the remaining sections are present, and also specify
+whether the message is a query or a response, a standard query or some
+other opcode, etc.
+
+The names of the sections after the header are derived from their use in
+standard queries. The question section contains fields that describe a
+question to a name server. These fields are a query type (QTYPE), a
+query class (QCLASS), and a query domain name (QNAME). The last three
+sections have the same format: a possibly empty list of concatenated
+resource records (RRs). The answer section contains RRs that answer the
+question; the authority section contains RRs that point toward an
+authoritative name server; the additional records section contains RRs
+which relate to the query, but are not strictly answers for the
+question.
+
+
+
+
+
+
+
+
+
+
+
+
+Mockapetris [Page 25]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+4.1.1. Header section format
+
+The header contains the following fields:
+
+ 1 1 1 1 1 1
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | ID |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ |QR| Opcode |AA|TC|RD|RA| Z | RCODE |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | QDCOUNT |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | ANCOUNT |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | NSCOUNT |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | ARCOUNT |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+ID A 16 bit identifier assigned by the program that
+ generates any kind of query. This identifier is copied
+ the corresponding reply and can be used by the requester
+ to match up replies to outstanding queries.
+
+QR A one bit field that specifies whether this message is a
+ query (0), or a response (1).
+
+OPCODE A four bit field that specifies kind of query in this
+ message. This value is set by the originator of a query
+ and copied into the response. The values are:
+
+ 0 a standard query (QUERY)
+
+ 1 an inverse query (IQUERY)
+
+ 2 a server status request (STATUS)
+
+ 3-15 reserved for future use
+
+AA Authoritative Answer - this bit is valid in responses,
+ and specifies that the responding name server is an
+ authority for the domain name in question section.
+
+ Note that the contents of the answer section may have
+ multiple owner names because of aliases. The AA bit
+
+
+
+Mockapetris [Page 26]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+ corresponds to the name which matches the query name, or
+ the first owner name in the answer section.
+
+TC TrunCation - specifies that this message was truncated
+ due to length greater than that permitted on the
+ transmission channel.
+
+RD Recursion Desired - this bit may be set in a query and
+ is copied into the response. If RD is set, it directs
+ the name server to pursue the query recursively.
+ Recursive query support is optional.
+
+RA Recursion Available - this be is set or cleared in a
+ response, and denotes whether recursive query support is
+ available in the name server.
+
+Z Reserved for future use. Must be zero in all queries
+ and responses.
+
+RCODE Response code - this 4 bit field is set as part of
+ responses. The values have the following
+ interpretation:
+
+ 0 No error condition
+
+ 1 Format error - The name server was
+ unable to interpret the query.
+
+ 2 Server failure - The name server was
+ unable to process this query due to a
+ problem with the name server.
+
+ 3 Name Error - Meaningful only for
+ responses from an authoritative name
+ server, this code signifies that the
+ domain name referenced in the query does
+ not exist.
+
+ 4 Not Implemented - The name server does
+ not support the requested kind of query.
+
+ 5 Refused - The name server refuses to
+ perform the specified operation for
+ policy reasons. For example, a name
+ server may not wish to provide the
+ information to the particular requester,
+ or a name server may not wish to perform
+ a particular operation (e.g., zone
+
+
+
+Mockapetris [Page 27]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+ transfer) for particular data.
+
+ 6-15 Reserved for future use.
+
+QDCOUNT an unsigned 16 bit integer specifying the number of
+ entries in the question section.
+
+ANCOUNT an unsigned 16 bit integer specifying the number of
+ resource records in the answer section.
+
+NSCOUNT an unsigned 16 bit integer specifying the number of name
+ server resource records in the authority records
+ section.
+
+ARCOUNT an unsigned 16 bit integer specifying the number of
+ resource records in the additional records section.
+
+4.1.2. Question section format
+
+The question section is used to carry the "question" in most queries,
+i.e., the parameters that define what is being asked. The section
+contains QDCOUNT (usually 1) entries, each of the following format:
+
+ 1 1 1 1 1 1
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | |
+ / QNAME /
+ / /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | QTYPE |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | QCLASS |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+QNAME a domain name represented as a sequence of labels, where
+ each label consists of a length octet followed by that
+ number of octets. The domain name terminates with the
+ zero length octet for the null label of the root. Note
+ that this field may be an odd number of octets; no
+ padding is used.
+
+QTYPE a two octet code which specifies the type of the query.
+ The values for this field include all codes valid for a
+ TYPE field, together with some more general codes which
+ can match more than one type of RR.
+
+
+
+Mockapetris [Page 28]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+QCLASS a two octet code that specifies the class of the query.
+ For example, the QCLASS field is IN for the Internet.
+
+4.1.3. Resource record format
+
+The answer, authority, and additional sections all share the same
+format: a variable number of resource records, where the number of
+records is specified in the corresponding count field in the header.
+Each resource record has the following format:
+ 1 1 1 1 1 1
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | |
+ / /
+ / NAME /
+ | |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | TYPE |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | CLASS |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | TTL |
+ | |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | RDLENGTH |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--|
+ / RDATA /
+ / /
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+where:
+
+NAME a domain name to which this resource record pertains.
+
+TYPE two octets containing one of the RR type codes. This
+ field specifies the meaning of the data in the RDATA
+ field.
+
+CLASS two octets which specify the class of the data in the
+ RDATA field.
+
+TTL a 32 bit unsigned integer that specifies the time
+ interval (in seconds) that the resource record may be
+ cached before it should be discarded. Zero values are
+ interpreted to mean that the RR can only be used for the
+ transaction in progress, and should not be cached.
+
+
+
+
+
+Mockapetris [Page 29]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+RDLENGTH an unsigned 16 bit integer that specifies the length in
+ octets of the RDATA field.
+
+RDATA a variable length string of octets that describes the
+ resource. The format of this information varies
+ according to the TYPE and CLASS of the resource record.
+ For example, the if the TYPE is A and the CLASS is IN,
+ the RDATA field is a 4 octet ARPA Internet address.
+
+4.1.4. Message compression
+
+In order to reduce the size of messages, the domain system utilizes a
+compression scheme which eliminates the repetition of domain names in a
+message. In this scheme, an entire domain name or a list of labels at
+the end of a domain name is replaced with a pointer to a prior occurrence
+of the same name.
+
+The pointer takes the form of a two octet sequence:
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | 1 1| OFFSET |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+The first two bits are ones. This allows a pointer to be distinguished
+from a label, since the label must begin with two zero bits because
+labels are restricted to 63 octets or less. (The 10 and 01 combinations
+are reserved for future use.) The OFFSET field specifies an offset from
+the start of the message (i.e., the first octet of the ID field in the
+domain header). A zero offset specifies the first byte of the ID field,
+etc.
+
+The compression scheme allows a domain name in a message to be
+represented as either:
+
+ - a sequence of labels ending in a zero octet
+
+ - a pointer
+
+ - a sequence of labels ending with a pointer
+
+Pointers can only be used for occurrences of a domain name where the
+format is not class specific. If this were not the case, a name server
+or resolver would be required to know the format of all RRs it handled.
+As yet, there are no such cases, but they may occur in future RDATA
+formats.
+
+If a domain name is contained in a part of the message subject to a
+length field (such as the RDATA section of an RR), and compression is
+
+
+
+Mockapetris [Page 30]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+used, the length of the compressed name is used in the length
+calculation, rather than the length of the expanded name.
+
+Programs are free to avoid using pointers in messages they generate,
+although this will reduce datagram capacity, and may cause truncation.
+However all programs are required to understand arriving messages that
+contain pointers.
+
+For example, a datagram might need to use the domain names F.ISI.ARPA,
+FOO.F.ISI.ARPA, ARPA, and the root. Ignoring the other fields of the
+message, these domain names might be represented as:
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ 20 | 1 | F |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ 22 | 3 | I |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ 24 | S | I |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ 26 | 4 | A |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ 28 | R | P |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ 30 | A | 0 |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ 40 | 3 | F |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ 42 | O | O |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ 44 | 1 1| 20 |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ 64 | 1 1| 26 |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ 92 | 0 | |
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+The domain name for F.ISI.ARPA is shown at offset 20. The domain name
+FOO.F.ISI.ARPA is shown at offset 40; this definition uses a pointer to
+concatenate a label for FOO to the previously defined F.ISI.ARPA. The
+domain name ARPA is defined at offset 64 using a pointer to the ARPA
+component of the name F.ISI.ARPA at 20; note that this pointer relies on
+ARPA being the last label in the string at 20. The root domain name is
+
+
+
+Mockapetris [Page 31]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+defined by a single octet of zeros at 92; the root domain name has no
+labels.
+
+4.2. Transport
+
+The DNS assumes that messages will be transmitted as datagrams or in a
+byte stream carried by a virtual circuit. While virtual circuits can be
+used for any DNS activity, datagrams are preferred for queries due to
+their lower overhead and better performance. Zone refresh activities
+must use virtual circuits because of the need for reliable transfer.
+
+The Internet supports name server access using TCP [RFC-793] on server
+port 53 (decimal) as well as datagram access using UDP [RFC-768] on UDP
+port 53 (decimal).
+
+4.2.1. UDP usage
+
+Messages sent using UDP user server port 53 (decimal).
+
+Messages carried by UDP are restricted to 512 bytes (not counting the IP
+or UDP headers). Longer messages are truncated and the TC bit is set in
+the header.
+
+UDP is not acceptable for zone transfers, but is the recommended method
+for standard queries in the Internet. Queries sent using UDP may be
+lost, and hence a retransmission strategy is required. Queries or their
+responses may be reordered by the network, or by processing in name
+servers, so resolvers should not depend on them being returned in order.
+
+The optimal UDP retransmission policy will vary with performance of the
+Internet and the needs of the client, but the following are recommended:
+
+ - The client should try other servers and server addresses
+ before repeating a query to a specific address of a server.
+
+ - The retransmission interval should be based on prior
+ statistics if possible. Too aggressive retransmission can
+ easily slow responses for the community at large. Depending
+ on how well connected the client is to its expected servers,
+ the minimum retransmission interval should be 2-5 seconds.
+
+More suggestions on server selection and retransmission policy can be
+found in the resolver section of this memo.
+
+4.2.2. TCP usage
+
+Messages sent over TCP connections use server port 53 (decimal). The
+message is prefixed with a two byte length field which gives the message
+
+
+
+Mockapetris [Page 32]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+length, excluding the two byte length field. This length field allows
+the low-level processing to assemble a complete message before beginning
+to parse it.
+
+Several connection management policies are recommended:
+
+ - The server should not block other activities waiting for TCP
+ data.
+
+ - The server should support multiple connections.
+
+ - The server should assume that the client will initiate
+ connection closing, and should delay closing its end of the
+ connection until all outstanding client requests have been
+ satisfied.
+
+ - If the server needs to close a dormant connection to reclaim
+ resources, it should wait until the connection has been idle
+ for a period on the order of two minutes. In particular, the
+ server should allow the SOA and AXFR request sequence (which
+ begins a refresh operation) to be made on a single connection.
+ Since the server would be unable to answer queries anyway, a
+ unilateral close or reset may be used instead of a graceful
+ close.
+
+5. MASTER FILES
+
+Master files are text files that contain RRs in text form. Since the
+contents of a zone can be expressed in the form of a list of RRs a
+master file is most often used to define a zone, though it can be used
+to list a cache's contents. Hence, this section first discusses the
+format of RRs in a master file, and then the special considerations when
+a master file is used to create a zone in some name server.
+
+5.1. Format
+
+The format of these files is a sequence of entries. Entries are
+predominantly line-oriented, though parentheses can be used to continue
+a list of items across a line boundary, and text literals can contain
+CRLF within the text. Any combination of tabs and spaces act as a
+delimiter between the separate items that make up an entry. The end of
+any line in the master file can end with a comment. The comment starts
+with a ";" (semicolon).
+
+The following entries are defined:
+
+ <blank>[<comment>]
+
+
+
+
+Mockapetris [Page 33]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+ $ORIGIN <domain-name> [<comment>]
+
+ $INCLUDE <file-name> [<domain-name>] [<comment>]
+
+ <domain-name><rr> [<comment>]
+
+ <blank><rr> [<comment>]
+
+Blank lines, with or without comments, are allowed anywhere in the file.
+
+Two control entries are defined: $ORIGIN and $INCLUDE. $ORIGIN is
+followed by a domain name, and resets the current origin for relative
+domain names to the stated name. $INCLUDE inserts the named file into
+the current file, and may optionally specify a domain name that sets the
+relative domain name origin for the included file. $INCLUDE may also
+have a comment. Note that a $INCLUDE entry never changes the relative
+origin of the parent file, regardless of changes to the relative origin
+made within the included file.
+
+The last two forms represent RRs. If an entry for an RR begins with a
+blank, then the RR is assumed to be owned by the last stated owner. If
+an RR entry begins with a <domain-name>, then the owner name is reset.
+
+<rr> contents take one of the following forms:
+
+ [<TTL>] [<class>] <type> <RDATA>
+
+ [<class>] [<TTL>] <type> <RDATA>
+
+The RR begins with optional TTL and class fields, followed by a type and
+RDATA field appropriate to the type and class. Class and type use the
+standard mnemonics, TTL is a decimal integer. Omitted class and TTL
+values are default to the last explicitly stated values. Since type and
+class mnemonics are disjoint, the parse is unique. (Note that this
+order is different from the order used in examples and the order used in
+the actual RRs; the given order allows easier parsing and defaulting.)
+
+<domain-name>s make up a large share of the data in the master file.
+The labels in the domain name are expressed as character strings and
+separated by dots. Quoting conventions allow arbitrary characters to be
+stored in domain names. Domain names that end in a dot are called
+absolute, and are taken as complete. Domain names which do not end in a
+dot are called relative; the actual domain name is the concatenation of
+the relative part with an origin specified in a $ORIGIN, $INCLUDE, or as
+an argument to the master file loading routine. A relative name is an
+error when no origin is available.
+
+
+
+
+
+Mockapetris [Page 34]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+<character-string> is expressed in one or two ways: as a contiguous set
+of characters without interior spaces, or as a string beginning with a "
+and ending with a ". Inside a " delimited string any character can
+occur, except for a " itself, which must be quoted using \ (back slash).
+
+Because these files are text files several special encodings are
+necessary to allow arbitrary data to be loaded. In particular:
+
+ of the root.
+
+@ A free standing @ is used to denote the current origin.
+
+\X where X is any character other than a digit (0-9), is
+ used to quote that character so that its special meaning
+ does not apply. For example, "\." can be used to place
+ a dot character in a label.
+
+\DDD where each D is a digit is the octet corresponding to
+ the decimal number described by DDD. The resulting
+ octet is assumed to be text and is not checked for
+ special meaning.
+
+( ) Parentheses are used to group data that crosses a line
+ boundary. In effect, line terminations are not
+ recognized within parentheses.
+
+; Semicolon is used to start a comment; the remainder of
+ the line is ignored.
+
+5.2. Use of master files to define zones
+
+When a master file is used to load a zone, the operation should be
+suppressed if any errors are encountered in the master file. The
+rationale for this is that a single error can have widespread
+consequences. For example, suppose that the RRs defining a delegation
+have syntax errors; then the server will return authoritative name
+errors for all names in the subzone (except in the case where the
+subzone is also present on the server).
+
+Several other validity checks that should be performed in addition to
+insuring that the file is syntactically correct:
+
+ 1. All RRs in the file should have the same class.
+
+ 2. Exactly one SOA RR should be present at the top of the zone.
+
+ 3. If delegations are present and glue information is required,
+ it should be present.
+
+
+
+Mockapetris [Page 35]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+ 4. Information present outside of the authoritative nodes in the
+ zone should be glue information, rather than the result of an
+ origin or similar error.
+
+5.3. Master file example
+
+The following is an example file which might be used to define the
+ISI.EDU zone.and is loaded with an origin of ISI.EDU:
+
+@ IN SOA VENERA Action\.domains (
+ 20 ; SERIAL
+ 7200 ; REFRESH
+ 600 ; RETRY
+ 3600000; EXPIRE
+ 60) ; MINIMUM
+
+ NS A.ISI.EDU.
+ NS VENERA
+ NS VAXA
+ MX 10 VENERA
+ MX 20 VAXA
+
+A A 26.3.0.103
+
+VENERA A 10.1.0.52
+ A 128.9.0.32
+
+VAXA A 10.2.0.27
+ A 128.9.0.33
+
+
+$INCLUDE <SUBSYS>ISI-MAILBOXES.TXT
+
+Where the file <SUBSYS>ISI-MAILBOXES.TXT is:
+
+ MOE MB A.ISI.EDU.
+ LARRY MB A.ISI.EDU.
+ CURLEY MB A.ISI.EDU.
+ STOOGES MG MOE
+ MG LARRY
+ MG CURLEY
+
+Note the use of the \ character in the SOA RR to specify the responsible
+person mailbox "Action.domains@E.ISI.EDU".
+
+
+
+
+
+
+
+Mockapetris [Page 36]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+6. NAME SERVER IMPLEMENTATION
+
+6.1. Architecture
+
+The optimal structure for the name server will depend on the host
+operating system and whether the name server is integrated with resolver
+operations, either by supporting recursive service, or by sharing its
+database with a resolver. This section discusses implementation
+considerations for a name server which shares a database with a
+resolver, but most of these concerns are present in any name server.
+
+6.1.1. Control
+
+A name server must employ multiple concurrent activities, whether they
+are implemented as separate tasks in the host's OS or multiplexing
+inside a single name server program. It is simply not acceptable for a
+name server to block the service of UDP requests while it waits for TCP
+data for refreshing or query activities. Similarly, a name server
+should not attempt to provide recursive service without processing such
+requests in parallel, though it may choose to serialize requests from a
+single client, or to regard identical requests from the same client as
+duplicates. A name server should not substantially delay requests while
+it reloads a zone from master files or while it incorporates a newly
+refreshed zone into its database.
+
+6.1.2. Database
+
+While name server implementations are free to use any internal data
+structures they choose, the suggested structure consists of three major
+parts:
+
+ - A "catalog" data structure which lists the zones available to
+ this server, and a "pointer" to the zone data structure. The
+ main purpose of this structure is to find the nearest ancestor
+ zone, if any, for arriving standard queries.
+
+ - Separate data structures for each of the zones held by the
+ name server.
+
+ - A data structure for cached data. (or perhaps separate caches
+ for different classes)
+
+All of these data structures can be implemented an identical tree
+structure format, with different data chained off the nodes in different
+parts: in the catalog the data is pointers to zones, while in the zone
+and cache data structures, the data will be RRs. In designing the tree
+framework the designer should recognize that query processing will need
+to traverse the tree using case-insensitive label comparisons; and that
+
+
+
+Mockapetris [Page 37]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+in real data, a few nodes have a very high branching factor (100-1000 or
+more), but the vast majority have a very low branching factor (0-1).
+
+One way to solve the case problem is to store the labels for each node
+in two pieces: a standardized-case representation of the label where all
+ASCII characters are in a single case, together with a bit mask that
+denotes which characters are actually of a different case. The
+branching factor diversity can be handled using a simple linked list for
+a node until the branching factor exceeds some threshold, and
+transitioning to a hash structure after the threshold is exceeded. In
+any case, hash structures used to store tree sections must insure that
+hash functions and procedures preserve the casing conventions of the
+DNS.
+
+The use of separate structures for the different parts of the database
+is motivated by several factors:
+
+ - The catalog structure can be an almost static structure that
+ need change only when the system administrator changes the
+ zones supported by the server. This structure can also be
+ used to store parameters used to control refreshing
+ activities.
+
+ - The individual data structures for zones allow a zone to be
+ replaced simply by changing a pointer in the catalog. Zone
+ refresh operations can build a new structure and, when
+ complete, splice it into the database via a simple pointer
+ replacement. It is very important that when a zone is
+ refreshed, queries should not use old and new data
+ simultaneously.
+
+ - With the proper search procedures, authoritative data in zones
+ will always "hide", and hence take precedence over, cached
+ data.
+
+ - Errors in zone definitions that cause overlapping zones, etc.,
+ may cause erroneous responses to queries, but problem
+ determination is simplified, and the contents of one "bad"
+ zone can't corrupt another.
+
+ - Since the cache is most frequently updated, it is most
+ vulnerable to corruption during system restarts. It can also
+ become full of expired RR data. In either case, it can easily
+ be discarded without disturbing zone data.
+
+A major aspect of database design is selecting a structure which allows
+the name server to deal with crashes of the name server's host. State
+information which a name server should save across system crashes
+
+
+
+Mockapetris [Page 38]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+includes the catalog structure (including the state of refreshing for
+each zone) and the zone data itself.
+
+6.1.3. Time
+
+Both the TTL data for RRs and the timing data for refreshing activities
+depends on 32 bit timers in units of seconds. Inside the database,
+refresh timers and TTLs for cached data conceptually "count down", while
+data in the zone stays with constant TTLs.
+
+A recommended implementation strategy is to store time in two ways: as
+a relative increment and as an absolute time. One way to do this is to
+use positive 32 bit numbers for one type and negative numbers for the
+other. The RRs in zones use relative times; the refresh timers and
+cache data use absolute times. Absolute numbers are taken with respect
+to some known origin and converted to relative values when placed in the
+response to a query. When an absolute TTL is negative after conversion
+to relative, then the data is expired and should be ignored.
+
+6.2. Standard query processing
+
+The major algorithm for standard query processing is presented in
+[RFC-1034].
+
+When processing queries with QCLASS=*, or some other QCLASS which
+matches multiple classes, the response should never be authoritative
+unless the server can guarantee that the response covers all classes.
+
+When composing a response, RRs which are to be inserted in the
+additional section, but duplicate RRs in the answer or authority
+sections, may be omitted from the additional section.
+
+When a response is so long that truncation is required, the truncation
+should start at the end of the response and work forward in the
+datagram. Thus if there is any data for the authority section, the
+answer section is guaranteed to be unique.
+
+The MINIMUM value in the SOA should be used to set a floor on the TTL of
+data distributed from a zone. This floor function should be done when
+the data is copied into a response. This will allow future dynamic
+update protocols to change the SOA MINIMUM field without ambiguous
+semantics.
+
+6.3. Zone refresh and reload processing
+
+In spite of a server's best efforts, it may be unable to load zone data
+from a master file due to syntax errors, etc., or be unable to refresh a
+zone within the its expiration parameter. In this case, the name server
+
+
+
+Mockapetris [Page 39]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+should answer queries as if it were not supposed to possess the zone.
+
+If a master is sending a zone out via AXFR, and a new version is created
+during the transfer, the master should continue to send the old version
+if possible. In any case, it should never send part of one version and
+part of another. If completion is not possible, the master should reset
+the connection on which the zone transfer is taking place.
+
+6.4. Inverse queries (Optional)
+
+Inverse queries are an optional part of the DNS. Name servers are not
+required to support any form of inverse queries. If a name server
+receives an inverse query that it does not support, it returns an error
+response with the "Not Implemented" error set in the header. While
+inverse query support is optional, all name servers must be at least
+able to return the error response.
+
+6.4.1. The contents of inverse queries and responses Inverse
+queries reverse the mappings performed by standard query operations;
+while a standard query maps a domain name to a resource, an inverse
+query maps a resource to a domain name. For example, a standard query
+might bind a domain name to a host address; the corresponding inverse
+query binds the host address to a domain name.
+
+Inverse queries take the form of a single RR in the answer section of
+the message, with an empty question section. The owner name of the
+query RR and its TTL are not significant. The response carries
+questions in the question section which identify all names possessing
+the query RR WHICH THE NAME SERVER KNOWS. Since no name server knows
+about all of the domain name space, the response can never be assumed to
+be complete. Thus inverse queries are primarily useful for database
+management and debugging activities. Inverse queries are NOT an
+acceptable method of mapping host addresses to host names; use the IN-
+ADDR.ARPA domain instead.
+
+Where possible, name servers should provide case-insensitive comparisons
+for inverse queries. Thus an inverse query asking for an MX RR of
+"Venera.isi.edu" should get the same response as a query for
+"VENERA.ISI.EDU"; an inverse query for HINFO RR "IBM-PC UNIX" should
+produce the same result as an inverse query for "IBM-pc unix". However,
+this cannot be guaranteed because name servers may possess RRs that
+contain character strings but the name server does not know that the
+data is character.
+
+When a name server processes an inverse query, it either returns:
+
+ 1. zero, one, or multiple domain names for the specified
+ resource as QNAMEs in the question section
+
+
+
+Mockapetris [Page 40]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+ 2. an error code indicating that the name server doesn't support
+ inverse mapping of the specified resource type.
+
+When the response to an inverse query contains one or more QNAMEs, the
+owner name and TTL of the RR in the answer section which defines the
+inverse query is modified to exactly match an RR found at the first
+QNAME.
+
+RRs returned in the inverse queries cannot be cached using the same
+mechanism as is used for the replies to standard queries. One reason
+for this is that a name might have multiple RRs of the same type, and
+only one would appear. For example, an inverse query for a single
+address of a multiply homed host might create the impression that only
+one address existed.
+
+6.4.2. Inverse query and response example The overall structure
+of an inverse query for retrieving the domain name that corresponds to
+Internet address 10.1.0.52 is shown below:
+
+ +-----------------------------------------+
+ Header | OPCODE=IQUERY, ID=997 |
+ +-----------------------------------------+
+ Question | <empty> |
+ +-----------------------------------------+
+ Answer | <anyname> A IN 10.1.0.52 |
+ +-----------------------------------------+
+ Authority | <empty> |
+ +-----------------------------------------+
+ Additional | <empty> |
+ +-----------------------------------------+
+
+This query asks for a question whose answer is the Internet style
+address 10.1.0.52. Since the owner name is not known, any domain name
+can be used as a placeholder (and is ignored). A single octet of zero,
+signifying the root, is usually used because it minimizes the length of
+the message. The TTL of the RR is not significant. The response to
+this query might be:
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Mockapetris [Page 41]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+ +-----------------------------------------+
+ Header | OPCODE=RESPONSE, ID=997 |
+ +-----------------------------------------+
+ Question |QTYPE=A, QCLASS=IN, QNAME=VENERA.ISI.EDU |
+ +-----------------------------------------+
+ Answer | VENERA.ISI.EDU A IN 10.1.0.52 |
+ +-----------------------------------------+
+ Authority | <empty> |
+ +-----------------------------------------+
+ Additional | <empty> |
+ +-----------------------------------------+
+
+Note that the QTYPE in a response to an inverse query is the same as the
+TYPE field in the answer section of the inverse query. Responses to
+inverse queries may contain multiple questions when the inverse is not
+unique. If the question section in the response is not empty, then the
+RR in the answer section is modified to correspond to be an exact copy
+of an RR at the first QNAME.
+
+6.4.3. Inverse query processing
+
+Name servers that support inverse queries can support these operations
+through exhaustive searches of their databases, but this becomes
+impractical as the size of the database increases. An alternative
+approach is to invert the database according to the search key.
+
+For name servers that support multiple zones and a large amount of data,
+the recommended approach is separate inversions for each zone. When a
+particular zone is changed during a refresh, only its inversions need to
+be redone.
+
+Support for transfer of this type of inversion may be included in future
+versions of the domain system, but is not supported in this version.
+
+6.5. Completion queries and responses
+
+The optional completion services described in RFC-882 and RFC-883 have
+been deleted. Redesigned services may become available in the future.
+
+
+
+
+
+
+
+
+
+
+
+
+
+Mockapetris [Page 42]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+7. RESOLVER IMPLEMENTATION
+
+The top levels of the recommended resolver algorithm are discussed in
+[RFC-1034]. This section discusses implementation details assuming the
+database structure suggested in the name server implementation section
+of this memo.
+
+7.1. Transforming a user request into a query
+
+The first step a resolver takes is to transform the client's request,
+stated in a format suitable to the local OS, into a search specification
+for RRs at a specific name which match a specific QTYPE and QCLASS.
+Where possible, the QTYPE and QCLASS should correspond to a single type
+and a single class, because this makes the use of cached data much
+simpler. The reason for this is that the presence of data of one type
+in a cache doesn't confirm the existence or non-existence of data of
+other types, hence the only way to be sure is to consult an
+authoritative source. If QCLASS=* is used, then authoritative answers
+won't be available.
+
+Since a resolver must be able to multiplex multiple requests if it is to
+perform its function efficiently, each pending request is usually
+represented in some block of state information. This state block will
+typically contain:
+
+ - A timestamp indicating the time the request began.
+ The timestamp is used to decide whether RRs in the database
+ can be used or are out of date. This timestamp uses the
+ absolute time format previously discussed for RR storage in
+ zones and caches. Note that when an RRs TTL indicates a
+ relative time, the RR must be timely, since it is part of a
+ zone. When the RR has an absolute time, it is part of a
+ cache, and the TTL of the RR is compared against the timestamp
+ for the start of the request.
+
+ Note that using the timestamp is superior to using a current
+ time, since it allows RRs with TTLs of zero to be entered in
+ the cache in the usual manner, but still used by the current
+ request, even after intervals of many seconds due to system
+ load, query retransmission timeouts, etc.
+
+ - Some sort of parameters to limit the amount of work which will
+ be performed for this request.
+
+ The amount of work which a resolver will do in response to a
+ client request must be limited to guard against errors in the
+ database, such as circular CNAME references, and operational
+ problems, such as network partition which prevents the
+
+
+
+Mockapetris [Page 43]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+ resolver from accessing the name servers it needs. While
+ local limits on the number of times a resolver will retransmit
+ a particular query to a particular name server address are
+ essential, the resolver should have a global per-request
+ counter to limit work on a single request. The counter should
+ be set to some initial value and decremented whenever the
+ resolver performs any action (retransmission timeout,
+ retransmission, etc.) If the counter passes zero, the request
+ is terminated with a temporary error.
+
+ Note that if the resolver structure allows one request to
+ start others in parallel, such as when the need to access a
+ name server for one request causes a parallel resolve for the
+ name server's addresses, the spawned request should be started
+ with a lower counter. This prevents circular references in
+ the database from starting a chain reaction of resolver
+ activity.
+
+ - The SLIST data structure discussed in [RFC-1034].
+
+ This structure keeps track of the state of a request if it
+ must wait for answers from foreign name servers.
+
+7.2. Sending the queries
+
+As described in [RFC-1034], the basic task of the resolver is to
+formulate a query which will answer the client's request and direct that
+query to name servers which can provide the information. The resolver
+will usually only have very strong hints about which servers to ask, in
+the form of NS RRs, and may have to revise the query, in response to
+CNAMEs, or revise the set of name servers the resolver is asking, in
+response to delegation responses which point the resolver to name
+servers closer to the desired information. In addition to the
+information requested by the client, the resolver may have to call upon
+its own services to determine the address of name servers it wishes to
+contact.
+
+In any case, the model used in this memo assumes that the resolver is
+multiplexing attention between multiple requests, some from the client,
+and some internally generated. Each request is represented by some
+state information, and the desired behavior is that the resolver
+transmit queries to name servers in a way that maximizes the probability
+that the request is answered, minimizes the time that the request takes,
+and avoids excessive transmissions. The key algorithm uses the state
+information of the request to select the next name server address to
+query, and also computes a timeout which will cause the next action
+should a response not arrive. The next action will usually be a
+transmission to some other server, but may be a temporary error to the
+
+
+
+Mockapetris [Page 44]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+client.
+
+The resolver always starts with a list of server names to query (SLIST).
+This list will be all NS RRs which correspond to the nearest ancestor
+zone that the resolver knows about. To avoid startup problems, the
+resolver should have a set of default servers which it will ask should
+it have no current NS RRs which are appropriate. The resolver then adds
+to SLIST all of the known addresses for the name servers, and may start
+parallel requests to acquire the addresses of the servers when the
+resolver has the name, but no addresses, for the name servers.
+
+To complete initialization of SLIST, the resolver attaches whatever
+history information it has to the each address in SLIST. This will
+usually consist of some sort of weighted averages for the response time
+of the address, and the batting average of the address (i.e., how often
+the address responded at all to the request). Note that this
+information should be kept on a per address basis, rather than on a per
+name server basis, because the response time and batting average of a
+particular server may vary considerably from address to address. Note
+also that this information is actually specific to a resolver address /
+server address pair, so a resolver with multiple addresses may wish to
+keep separate histories for each of its addresses. Part of this step
+must deal with addresses which have no such history; in this case an
+expected round trip time of 5-10 seconds should be the worst case, with
+lower estimates for the same local network, etc.
+
+Note that whenever a delegation is followed, the resolver algorithm
+reinitializes SLIST.
+
+The information establishes a partial ranking of the available name
+server addresses. Each time an address is chosen and the state should
+be altered to prevent its selection again until all other addresses have
+been tried. The timeout for each transmission should be 50-100% greater
+than the average predicted value to allow for variance in response.
+
+Some fine points:
+
+ - The resolver may encounter a situation where no addresses are
+ available for any of the name servers named in SLIST, and
+ where the servers in the list are precisely those which would
+ normally be used to look up their own addresses. This
+ situation typically occurs when the glue address RRs have a
+ smaller TTL than the NS RRs marking delegation, or when the
+ resolver caches the result of a NS search. The resolver
+ should detect this condition and restart the search at the
+ next ancestor zone, or alternatively at the root.
+
+
+
+
+
+Mockapetris [Page 45]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+ - If a resolver gets a server error or other bizarre response
+ from a name server, it should remove it from SLIST, and may
+ wish to schedule an immediate transmission to the next
+ candidate server address.
+
+7.3. Processing responses
+
+The first step in processing arriving response datagrams is to parse the
+response. This procedure should include:
+
+ - Check the header for reasonableness. Discard datagrams which
+ are queries when responses are expected.
+
+ - Parse the sections of the message, and insure that all RRs are
+ correctly formatted.
+
+ - As an optional step, check the TTLs of arriving data looking
+ for RRs with excessively long TTLs. If a RR has an
+ excessively long TTL, say greater than 1 week, either discard
+ the whole response, or limit all TTLs in the response to 1
+ week.
+
+The next step is to match the response to a current resolver request.
+The recommended strategy is to do a preliminary matching using the ID
+field in the domain header, and then to verify that the question section
+corresponds to the information currently desired. This requires that
+the transmission algorithm devote several bits of the domain ID field to
+a request identifier of some sort. This step has several fine points:
+
+ - Some name servers send their responses from different
+ addresses than the one used to receive the query. That is, a
+ resolver cannot rely that a response will come from the same
+ address which it sent the corresponding query to. This name
+ server bug is typically encountered in UNIX systems.
+
+ - If the resolver retransmits a particular request to a name
+ server it should be able to use a response from any of the
+ transmissions. However, if it is using the response to sample
+ the round trip time to access the name server, it must be able
+ to determine which transmission matches the response (and keep
+ transmission times for each outgoing message), or only
+ calculate round trip times based on initial transmissions.
+
+ - A name server will occasionally not have a current copy of a
+ zone which it should have according to some NS RRs. The
+ resolver should simply remove the name server from the current
+ SLIST, and continue.
+
+
+
+
+Mockapetris [Page 46]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+7.4. Using the cache
+
+In general, we expect a resolver to cache all data which it receives in
+responses since it may be useful in answering future client requests.
+However, there are several types of data which should not be cached:
+
+ - When several RRs of the same type are available for a
+ particular owner name, the resolver should either cache them
+ all or none at all. When a response is truncated, and a
+ resolver doesn't know whether it has a complete set, it should
+ not cache a possibly partial set of RRs.
+
+ - Cached data should never be used in preference to
+ authoritative data, so if caching would cause this to happen
+ the data should not be cached.
+
+ - The results of an inverse query should not be cached.
+
+ - The results of standard queries where the QNAME contains "*"
+ labels if the data might be used to construct wildcards. The
+ reason is that the cache does not necessarily contain existing
+ RRs or zone boundary information which is necessary to
+ restrict the application of the wildcard RRs.
+
+ - RR data in responses of dubious reliability. When a resolver
+ receives unsolicited responses or RR data other than that
+ requested, it should discard it without caching it. The basic
+ implication is that all sanity checks on a packet should be
+ performed before any of it is cached.
+
+In a similar vein, when a resolver has a set of RRs for some name in a
+response, and wants to cache the RRs, it should check its cache for
+already existing RRs. Depending on the circumstances, either the data
+in the response or the cache is preferred, but the two should never be
+combined. If the data in the response is from authoritative data in the
+answer section, it is always preferred.
+
+8. MAIL SUPPORT
+
+The domain system defines a standard for mapping mailboxes into domain
+names, and two methods for using the mailbox information to derive mail
+routing information. The first method is called mail exchange binding
+and the other method is mailbox binding. The mailbox encoding standard
+and mail exchange binding are part of the DNS official protocol, and are
+the recommended method for mail routing in the Internet. Mailbox
+binding is an experimental feature which is still under development and
+subject to change.
+
+
+
+
+Mockapetris [Page 47]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+The mailbox encoding standard assumes a mailbox name of the form
+"<local-part>@<mail-domain>". While the syntax allowed in each of these
+sections varies substantially between the various mail internets, the
+preferred syntax for the ARPA Internet is given in [RFC-822].
+
+The DNS encodes the <local-part> as a single label, and encodes the
+<mail-domain> as a domain name. The single label from the <local-part>
+is prefaced to the domain name from <mail-domain> to form the domain
+name corresponding to the mailbox. Thus the mailbox HOSTMASTER@SRI-
+NIC.ARPA is mapped into the domain name HOSTMASTER.SRI-NIC.ARPA. If the
+<local-part> contains dots or other special characters, its
+representation in a master file will require the use of backslash
+quoting to ensure that the domain name is properly encoded. For
+example, the mailbox Action.domains@ISI.EDU would be represented as
+Action\.domains.ISI.EDU.
+
+8.1. Mail exchange binding
+
+Mail exchange binding uses the <mail-domain> part of a mailbox
+specification to determine where mail should be sent. The <local-part>
+is not even consulted. [RFC-974] specifies this method in detail, and
+should be consulted before attempting to use mail exchange support.
+
+One of the advantages of this method is that it decouples mail
+destination naming from the hosts used to support mail service, at the
+cost of another layer of indirection in the lookup function. However,
+the addition layer should eliminate the need for complicated "%", "!",
+etc encodings in <local-part>.
+
+The essence of the method is that the <mail-domain> is used as a domain
+name to locate type MX RRs which list hosts willing to accept mail for
+<mail-domain>, together with preference values which rank the hosts
+according to an order specified by the administrators for <mail-domain>.
+
+In this memo, the <mail-domain> ISI.EDU is used in examples, together
+with the hosts VENERA.ISI.EDU and VAXA.ISI.EDU as mail exchanges for
+ISI.EDU. If a mailer had a message for Mockapetris@ISI.EDU, it would
+route it by looking up MX RRs for ISI.EDU. The MX RRs at ISI.EDU name
+VENERA.ISI.EDU and VAXA.ISI.EDU, and type A queries can find the host
+addresses.
+
+8.2. Mailbox binding (Experimental)
+
+In mailbox binding, the mailer uses the entire mail destination
+specification to construct a domain name. The encoded domain name for
+the mailbox is used as the QNAME field in a QTYPE=MAILB query.
+
+Several outcomes are possible for this query:
+
+
+
+Mockapetris [Page 48]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+ 1. The query can return a name error indicating that the mailbox
+ does not exist as a domain name.
+
+ In the long term, this would indicate that the specified
+ mailbox doesn't exist. However, until the use of mailbox
+ binding is universal, this error condition should be
+ interpreted to mean that the organization identified by the
+ global part does not support mailbox binding. The
+ appropriate procedure is to revert to exchange binding at
+ this point.
+
+ 2. The query can return a Mail Rename (MR) RR.
+
+ The MR RR carries new mailbox specification in its RDATA
+ field. The mailer should replace the old mailbox with the
+ new one and retry the operation.
+
+ 3. The query can return a MB RR.
+
+ The MB RR carries a domain name for a host in its RDATA
+ field. The mailer should deliver the message to that host
+ via whatever protocol is applicable, e.g., b,SMTP.
+
+ 4. The query can return one or more Mail Group (MG) RRs.
+
+ This condition means that the mailbox was actually a mailing
+ list or mail group, rather than a single mailbox. Each MG RR
+ has a RDATA field that identifies a mailbox that is a member
+ of the group. The mailer should deliver a copy of the
+ message to each member.
+
+ 5. The query can return a MB RR as well as one or more MG RRs.
+
+ This condition means the the mailbox was actually a mailing
+ list. The mailer can either deliver the message to the host
+ specified by the MB RR, which will in turn do the delivery to
+ all members, or the mailer can use the MG RRs to do the
+ expansion itself.
+
+In any of these cases, the response may include a Mail Information
+(MINFO) RR. This RR is usually associated with a mail group, but is
+legal with a MB. The MINFO RR identifies two mailboxes. One of these
+identifies a responsible person for the original mailbox name. This
+mailbox should be used for requests to be added to a mail group, etc.
+The second mailbox name in the MINFO RR identifies a mailbox that should
+receive error messages for mail failures. This is particularly
+appropriate for mailing lists when errors in member names should be
+reported to a person other than the one who sends a message to the list.
+
+
+
+Mockapetris [Page 49]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+New fields may be added to this RR in the future.
+
+
+9. REFERENCES and BIBLIOGRAPHY
+
+[Dyer 87] S. Dyer, F. Hsu, "Hesiod", Project Athena
+ Technical Plan - Name Service, April 1987, version 1.9.
+
+ Describes the fundamentals of the Hesiod name service.
+
+[IEN-116] J. Postel, "Internet Name Server", IEN-116,
+ USC/Information Sciences Institute, August 1979.
+
+ A name service obsoleted by the Domain Name System, but
+ still in use.
+
+[Quarterman 86] J. Quarterman, and J. Hoskins, "Notable Computer Networks",
+ Communications of the ACM, October 1986, volume 29, number
+ 10.
+
+[RFC-742] K. Harrenstien, "NAME/FINGER", RFC-742, Network
+ Information Center, SRI International, December 1977.
+
+[RFC-768] J. Postel, "User Datagram Protocol", RFC-768,
+ USC/Information Sciences Institute, August 1980.
+
+[RFC-793] J. Postel, "Transmission Control Protocol", RFC-793,
+ USC/Information Sciences Institute, September 1981.
+
+[RFC-799] D. Mills, "Internet Name Domains", RFC-799, COMSAT,
+ September 1981.
+
+ Suggests introduction of a hierarchy in place of a flat
+ name space for the Internet.
+
+[RFC-805] J. Postel, "Computer Mail Meeting Notes", RFC-805,
+ USC/Information Sciences Institute, February 1982.
+
+[RFC-810] E. Feinler, K. Harrenstien, Z. Su, and V. White, "DOD
+ Internet Host Table Specification", RFC-810, Network
+ Information Center, SRI International, March 1982.
+
+ Obsolete. See RFC-952.
+
+[RFC-811] K. Harrenstien, V. White, and E. Feinler, "Hostnames
+ Server", RFC-811, Network Information Center, SRI
+ International, March 1982.
+
+
+
+
+Mockapetris [Page 50]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+ Obsolete. See RFC-953.
+
+[RFC-812] K. Harrenstien, and V. White, "NICNAME/WHOIS", RFC-812,
+ Network Information Center, SRI International, March
+ 1982.
+
+[RFC-819] Z. Su, and J. Postel, "The Domain Naming Convention for
+ Internet User Applications", RFC-819, Network
+ Information Center, SRI International, August 1982.
+
+ Early thoughts on the design of the domain system.
+ Current implementation is completely different.
+
+[RFC-821] J. Postel, "Simple Mail Transfer Protocol", RFC-821,
+ USC/Information Sciences Institute, August 1980.
+
+[RFC-830] Z. Su, "A Distributed System for Internet Name Service",
+ RFC-830, Network Information Center, SRI International,
+ October 1982.
+
+ Early thoughts on the design of the domain system.
+ Current implementation is completely different.
+
+[RFC-882] P. Mockapetris, "Domain names - Concepts and
+ Facilities," RFC-882, USC/Information Sciences
+ Institute, November 1983.
+
+ Superseded by this memo.
+
+[RFC-883] P. Mockapetris, "Domain names - Implementation and
+ Specification," RFC-883, USC/Information Sciences
+ Institute, November 1983.
+
+ Superseded by this memo.
+
+[RFC-920] J. Postel and J. Reynolds, "Domain Requirements",
+ RFC-920, USC/Information Sciences Institute,
+ October 1984.
+
+ Explains the naming scheme for top level domains.
+
+[RFC-952] K. Harrenstien, M. Stahl, E. Feinler, "DoD Internet Host
+ Table Specification", RFC-952, SRI, October 1985.
+
+ Specifies the format of HOSTS.TXT, the host/address
+ table replaced by the DNS.
+
+
+
+
+
+Mockapetris [Page 51]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+[RFC-953] K. Harrenstien, M. Stahl, E. Feinler, "HOSTNAME Server",
+ RFC-953, SRI, October 1985.
+
+ This RFC contains the official specification of the
+ hostname server protocol, which is obsoleted by the DNS.
+ This TCP based protocol accesses information stored in
+ the RFC-952 format, and is used to obtain copies of the
+ host table.
+
+[RFC-973] P. Mockapetris, "Domain System Changes and
+ Observations", RFC-973, USC/Information Sciences
+ Institute, January 1986.
+
+ Describes changes to RFC-882 and RFC-883 and reasons for
+ them.
+
+[RFC-974] C. Partridge, "Mail routing and the domain system",
+ RFC-974, CSNET CIC BBN Labs, January 1986.
+
+ Describes the transition from HOSTS.TXT based mail
+ addressing to the more powerful MX system used with the
+ domain system.
+
+[RFC-1001] NetBIOS Working Group, "Protocol standard for a NetBIOS
+ service on a TCP/UDP transport: Concepts and Methods",
+ RFC-1001, March 1987.
+
+ This RFC and RFC-1002 are a preliminary design for
+ NETBIOS on top of TCP/IP which proposes to base NetBIOS
+ name service on top of the DNS.
+
+[RFC-1002] NetBIOS Working Group, "Protocol standard for a NetBIOS
+ service on a TCP/UDP transport: Detailed
+ Specifications", RFC-1002, March 1987.
+
+[RFC-1010] J. Reynolds, and J. Postel, "Assigned Numbers", RFC-1010,
+ USC/Information Sciences Institute, May 1987.
+
+ Contains socket numbers and mnemonics for host names,
+ operating systems, etc.
+
+[RFC-1031] W. Lazear, "MILNET Name Domain Transition", RFC-1031,
+ November 1987.
+
+ Describes a plan for converting the MILNET to the DNS.
+
+[RFC-1032] M. Stahl, "Establishing a Domain - Guidelines for
+ Administrators", RFC-1032, November 1987.
+
+
+
+Mockapetris [Page 52]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+ Describes the registration policies used by the NIC to
+ administer the top level domains and delegate subzones.
+
+[RFC-1033] M. Lottor, "Domain Administrators Operations Guide",
+ RFC-1033, November 1987.
+
+ A cookbook for domain administrators.
+
+[Solomon 82] M. Solomon, L. Landweber, and D. Neuhengen, "The CSNET
+ Name Server", Computer Networks, vol 6, nr 3, July 1982.
+
+ Describes a name service for CSNET which is independent
+ from the DNS and DNS use in the CSNET.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Mockapetris [Page 53]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+Index
+
+ * 13
+
+ ; 33, 35
+
+ <character-string> 35
+ <domain-name> 34
+
+ @ 35
+
+ \ 35
+
+ A 12
+
+ Byte order 8
+
+ CH 13
+ Character case 9
+ CLASS 11
+ CNAME 12
+ Completion 42
+ CS 13
+
+ Hesiod 13
+ HINFO 12
+ HS 13
+
+ IN 13
+ IN-ADDR.ARPA domain 22
+ Inverse queries 40
+
+ Mailbox names 47
+ MB 12
+ MD 12
+ MF 12
+ MG 12
+ MINFO 12
+ MINIMUM 20
+ MR 12
+ MX 12
+
+ NS 12
+ NULL 12
+
+ Port numbers 32
+ Primary server 5
+ PTR 12, 18
+
+
+
+Mockapetris [Page 54]
+
+RFC 1035 Domain Implementation and Specification November 1987
+
+
+ QCLASS 13
+ QTYPE 12
+
+ RDATA 12
+ RDLENGTH 11
+
+ Secondary server 5
+ SOA 12
+ Stub resolvers 7
+
+ TCP 32
+ TXT 12
+ TYPE 11
+
+ UDP 32
+
+ WKS 12
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Mockapetris [Page 55]
+
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