rfc1035.txt
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- Network Working Group P. Mockapetris
- Request for Comments: 1035 ISI
- November 1987
- Obsoletes: RFCs 882, 883, 973
- DOMAIN NAMES - IMPLEMENTATION AND SPECIFICATION
- 1. STATUS OF THIS MEMO
- This RFC describes the details of the domain system and protocol, and
- assumes that the reader is familiar with the concepts discussed in a
- companion RFC, "Domain Names - Concepts and Facilities" [RFC-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 imbedded 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 occurance
- 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 occurances 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.
- Superceeded by this memo.
- [RFC-883] P. Mockapetris, "Domain names - Implementation and
- Specification," RFC-883, USC/Information Sciences
- Institute, November 1983.
- Superceeded 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]