diff options
| author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-27 07:24:22 +0000 |
|---|---|---|
| committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-27 07:24:22 +0000 |
| commit | 45d6379135504814ab723b57f0eb8be23393a51d (patch) | |
| tree | d4f2ec4acca824a8446387a758b0ce4238a4dffa /lib/dns/tests/testdata/dst | |
| parent | Initial commit. (diff) | |
| download | bind9-upstream.tar.xz bind9-upstream.zip | |
Adding upstream version 1:9.16.44.upstream/1%9.16.44upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'lib/dns/tests/testdata/dst')
| -rw-r--r-- | lib/dns/tests/testdata/dst/Ktest.+008+11349.key | 5 | ||||
| -rw-r--r-- | lib/dns/tests/testdata/dst/Ktest.+008+11349.private | 13 | ||||
| -rw-r--r-- | lib/dns/tests/testdata/dst/Ktest.+013+49130.key | 5 | ||||
| -rw-r--r-- | lib/dns/tests/testdata/dst/Ktest.+013+49130.private | 6 | ||||
| -rw-r--r-- | lib/dns/tests/testdata/dst/test1.data | 3077 | ||||
| -rw-r--r-- | lib/dns/tests/testdata/dst/test1.ecdsa256sig | 1 | ||||
| -rw-r--r-- | lib/dns/tests/testdata/dst/test1.rsasha256sig | 1 | ||||
| -rw-r--r-- | lib/dns/tests/testdata/dst/test2.data | 3077 |
8 files changed, 6185 insertions, 0 deletions
diff --git a/lib/dns/tests/testdata/dst/Ktest.+008+11349.key b/lib/dns/tests/testdata/dst/Ktest.+008+11349.key new file mode 100644 index 0000000..a1bd768 --- /dev/null +++ b/lib/dns/tests/testdata/dst/Ktest.+008+11349.key @@ -0,0 +1,5 @@ +; This is a zone-signing key, keyid 11349, for test. +; Created: 20181025090713 (Thu Oct 25 11:07:13 2018) +; Publish: 20181025090713 (Thu Oct 25 11:07:13 2018) +; Activate: 20181025090713 (Thu Oct 25 11:07:13 2018) +test. IN DNSKEY 256 3 8 AwEAAdqPwPScyURzeCUzEadKNYgQW50LPDV/ir9nWIbiSn2yMkymxiby BQH+Hk1neE9qa9X4XaEnKf5YZx7o14rRikmOb2lomtOkI9ovh1K/SvLO Zd1E3e61F29g1eCq52mMY3xAdEcBNqEq+6mgEwGmwl83+mAh5anxXNHa 2rcfdG+L diff --git a/lib/dns/tests/testdata/dst/Ktest.+008+11349.private b/lib/dns/tests/testdata/dst/Ktest.+008+11349.private new file mode 100644 index 0000000..5dfef79 --- /dev/null +++ b/lib/dns/tests/testdata/dst/Ktest.+008+11349.private @@ -0,0 +1,13 @@ +Private-key-format: v1.3 +Algorithm: 8 (RSASHA256) +Modulus: 2o/A9JzJRHN4JTMRp0o1iBBbnQs8NX+Kv2dYhuJKfbIyTKbGJvIFAf4eTWd4T2pr1fhdoScp/lhnHujXitGKSY5vaWia06Qj2i+HUr9K8s5l3UTd7rUXb2DV4KrnaYxjfEB0RwE2oSr7qaATAabCXzf6YCHlqfFc0dratx90b4s= +PublicExponent: AQAB +PrivateExponent: a4qmX/YxlmvWpz8spYr/MhcSbQCVPKGoLKv2RFBeZODknRDGmW0mh6d5U47hBPqRWvRdZak2oX7wJqZdQGIAT25bC09rLNMctfxXKtzwSaXFjXZGHGv+bDHcqIltvIYmRbb0pK/LinFaLZqfpVe0WOfKuT9BT03BlwSZV8GKgZE= +Prime1: 8oZLQoVpIqsiQw7bX5pTm/O0gEUnEzNOVEoLGsfIl68Lz/1CBm9ypTp8QOB0B9IpnH8vOS+NJM1az1d0RhqKow== +Prime2: 5rSbE6duWIb90uICkAUJn4OztHX0fkd9GKNYdsHVReFBH2poXGojVGkW6i/IaYl4NEXXr5Z89dWtR+RNH2Z9+Q== +Exponent1: 2IcuCmYyR9Gi9Vv+YIzYuRQMw7j5+hqEhJzW7UIRxdtzIG9s03INWZet9/5tmc35eM/Uyam6ynDN8vCRz0VDIQ== +Exponent2: vKcdVKIKWrvwXXzRaaGk79rLnZsDFiwxQG96TIpOczkyfpUNx9xHDaRtx4zRTnPKZrxiFkRx5LkZXHt1EWNHSQ== +Coefficient: pb9dFRZA2IRXDCGCM1ikp+QCs72wNn3hgURZLRLmtcBbQcYhP/dcp80SpInviwJPNRcKrfxninqygEARzfHtqQ== +Created: 20181025090713 +Publish: 20181025090713 +Activate: 20181025090713 diff --git a/lib/dns/tests/testdata/dst/Ktest.+013+49130.key b/lib/dns/tests/testdata/dst/Ktest.+013+49130.key new file mode 100644 index 0000000..e3ff931 --- /dev/null +++ b/lib/dns/tests/testdata/dst/Ktest.+013+49130.key @@ -0,0 +1,5 @@ +; This is a zone-signing key, keyid 49130, for test. +; Created: 20181025090718 (Thu Oct 25 11:07:18 2018) +; Publish: 20181025090718 (Thu Oct 25 11:07:18 2018) +; Activate: 20181025090718 (Thu Oct 25 11:07:18 2018) +test. IN DNSKEY 256 3 13 uP04fwB/DuBBqdjPLseIoFT7vgtP8Lr/be1NhRBvibwQ+Hr+3GQhIKIK XbamgOUxXJ9JDjWFAT2KXw0V3sAN9w== diff --git a/lib/dns/tests/testdata/dst/Ktest.+013+49130.private b/lib/dns/tests/testdata/dst/Ktest.+013+49130.private new file mode 100644 index 0000000..754d9f9 --- /dev/null +++ b/lib/dns/tests/testdata/dst/Ktest.+013+49130.private @@ -0,0 +1,6 @@ +Private-key-format: v1.3 +Algorithm: 13 (ECDSAP256SHA256) +PrivateKey: feGDRABRCbcsCqssKK5B5518y95smrv/cJnz2pa/UVA= +Created: 20181025090718 +Publish: 20181025090718 +Activate: 20181025090718 diff --git a/lib/dns/tests/testdata/dst/test1.data b/lib/dns/tests/testdata/dst/test1.data new file mode 100644 index 0000000..cf84e9f --- /dev/null +++ b/lib/dns/tests/testdata/dst/test1.data @@ -0,0 +1,3077 @@ +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-1034]. + +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] + diff --git a/lib/dns/tests/testdata/dst/test1.ecdsa256sig b/lib/dns/tests/testdata/dst/test1.ecdsa256sig new file mode 100644 index 0000000..dcae9d1 --- /dev/null +++ b/lib/dns/tests/testdata/dst/test1.ecdsa256sig @@ -0,0 +1 @@ +72E0998732EECAE2BEA12A278DFDEE14DB09A43C1E646A08BB0A6EEB90C5B75F9B359BEC1580313BFA8012C1DC15D34D1B227C71AD23161E2757AEB162AE3D99 diff --git a/lib/dns/tests/testdata/dst/test1.rsasha256sig b/lib/dns/tests/testdata/dst/test1.rsasha256sig new file mode 100644 index 0000000..36e0b09 --- /dev/null +++ b/lib/dns/tests/testdata/dst/test1.rsasha256sig @@ -0,0 +1 @@ +C5CC8AB9FB5C0B4F03650456C993A868EB674ACBF2A867E023DC00F17D240CEDCADB8714981B7B48CF6CF86722632610FF312063B5E6D20EF441B89F02BC6813A35F9C6F045D017DB75C8724DBAA0C55A0D4EA850339944C75890B4DD0382AFA3E30E1CAA7B190C1B1FB17B5DD2279C0DF1049911E64198B3376070A34F38F4B diff --git a/lib/dns/tests/testdata/dst/test2.data b/lib/dns/tests/testdata/dst/test2.data new file mode 100644 index 0000000..a323bb3 --- /dev/null +++ b/lib/dns/tests/testdata/dst/test2.data @@ -0,0 +1,3077 @@ +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] + |