Network Working Group                                     P. MockapetrisRequest for Comments: 1035                                           ISI                                                           November 1987Obsoletes: RFCs 882, 883, 973            DOMAIN NAMES - IMPLEMENTATION AND SPECIFICATION1. STATUS OF THIS MEMOThis RFC describes the details of the domain system and protocol, andassumes that the reader is familiar with the concepts discussed in acompanion RFC, "Domain Names - Concepts and Facilities" [RFC-1034].The domain system is a mixture of functions and data types which are anofficial protocol and functions and data types which are stillexperimental.  Since the domain system is intentionally extensible, newdata types and experimental behavior should always be expected in partsof the system beyond the official protocol.  The official protocol partsinclude standard queries, responses and the Internet class RR dataformats (e.g., host addresses).  Since the previous RFC set, severaldefinitions have changed, so some previous definitions are obsolete.Experimental or obsolete features are clearly marked in these RFCs, andsuch information should be used with caution.The reader is especially cautioned not to depend on the values whichappear in examples to be current or complete, since their purpose isprimarily 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                                        13Mockapetris                                                     [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                            42Mockapetris                                                     [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                                                              542. INTRODUCTION2.1. OverviewThe goal of domain names is to provide a mechanism for naming resourcesin 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 alocal agent, called a resolver, which retrieves information associatedwith the domain name.  Thus a user might ask for the host address ormail information associated with a particular domain name.  To enablethe user to request a particular type of information, an appropriatequery type is passed to the resolver with the domain name.  To the user,the domain tree is a single information space; the resolver isresponsible for hiding the distribution of data among name servers fromthe user.From the resolver's point of view, the database that makes up the domainspace is distributed among various name servers.  Different parts of thedomain space are stored in different name servers, although a particulardata item will be stored redundantly in two or more name servers.  Theresolver starts with knowledge of at least one name server.  When theresolver processes a user query it asks a known name server for theinformation; in return, the resolver either receives the desiredinformation or a referral to another name server.  Using thesereferrals, resolvers learn the identities and contents of other nameservers.  Resolvers are responsible for dealing with the distribution ofthe domain space and dealing with the effects of name server failure byconsulting redundant databases in other servers.Name servers manage two kinds of data.  The first kind of data held insets called zones; each zone is the complete database for a particular"pruned" subtree of the domain space.  This data is calledauthoritative.  A name server periodically checks to make sure that itszones are up to date, and if not, obtains a new copy of updated zonesMockapetris                                                     [Page 3]RFC 1035        Domain Implementation and Specification    November 1987from master files stored locally or in another name server.  The secondkind of data is cached data which was acquired by a local resolver.This data may be incomplete, but improves the performance of theretrieval process when non-local data is repeatedly accessed.  Cacheddata 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 thedatabase update and refresh problems in the name servers.2.2. Common configurationsA host can participate in the domain name system in a number of ways,depending on whether the host runs programs that retrieve informationfrom the domain system, name servers that answer queries from otherhosts, or various combinations of both functions.  The simplest, andperhaps 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; theformat of user queries and user responses is specific to the host andits operating system.  User queries will typically be operating systemcalls, and the resolver and its cache will be part of the host operatingsystem.  Less capable hosts may choose to implement the resolver as asubroutine to be linked in with every program that needs its services.Resolvers answer user queries with information they acquire via queriesto foreign name servers and the local cache.Note that the resolver may have to make several queries to severaldifferent foreign name servers to answer a particular user query, andhence the resolution of a user query may involve several networkaccesses and an arbitrary amount of time.  The queries to foreign nameservers and the corresponding responses have a standard format describedMockapetris                                                     [Page 4]RFC 1035        Domain Implementation and Specification    November 1987in this memo, and may be datagrams.Depending on its capabilities, a name server could be a stand aloneprogram on a dedicated machine or a process or processes on a largetimeshared 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 zonesby reading master files from its local file system, and answers queriesabout those zones that arrive from foreign resolvers.The DNS requires that all zones be redundantly supported by more thanone name server.  Designated secondary servers can acquire zones andcheck for updates from the primary server using the zone transferprotocol 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 avirtual circuit to a foreign name server to acquire a copy of a zone orto check that an existing copy has not changed.  The messages sent forMockapetris                                                     [Page 5]RFC 1035        Domain Implementation and Specification    November 1987these maintenance activities follow the same form as queries andresponses, but the message sequences are somewhat different.The information flow in a host that supports all aspects of the domainname 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 serverand resolver.  The contents of the shared database will typically be amixture of authoritative data maintained by the periodic refreshoperations of the name server and cached data from previous resolverrequests.  The structure of the domain data and the necessity forsynchronization between name servers and resolvers imply the generalcharacteristics of this database, but the actual format is up to thelocal implementor.Mockapetris                                                     [Page 6]RFC 1035        Domain Implementation and Specification    November 1987Information flow can also be tailored so that a group of hosts acttogether to optimize activities.  Sometimes this is done to offload lesscapable hosts so that they do not have to implement a full resolver.This can be appropriate for PCs or hosts which want to minimize theamount of new network code which is required.  This scheme can alsoallow a group of hosts can share a small number of caches rather thanmaintaining a large number of separate caches, on the premise that thecentralized caches will have a higher hit ratio.  In either case,resolvers are replaced with stub resolvers which act as front ends toresolvers located in a recursive server in one or more name serversknown 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 forreliability whenever possible.2.3. ConventionsThe domain system has several conventions dealing with low-level, butfundamental, issues.  While the implementor is free to violate theseconventions WITHIN HIS OWN SYSTEM, he must observe these conventions inALL behavior observed from other hosts.2.3.1. Preferred name syntaxThe DNS specifications attempt to be as general as possible in the rulesfor constructing domain names.  The idea is that the name of anyexisting object can be expressed as a domain name with minimal changes.Mockapetris                                                     [Page 7]RFC 1035        Domain Implementation and Specification    November 1987However, when assigning a domain name for an object, the prudent userwill select a name which satisfies both the rules of the domain systemand any existing rules for the object, whether these rules are publishedor implied by existing programs.For example, when naming a mail domain, the user should satisfy both therules 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 problemswhen old software is converted to use domain names.The following syntax will result in fewer problems with manyapplications 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 inupper case and a through z in lower case<digit> ::= any one of the ten digits 0 through 9Note that while upper and lower case letters are allowed in domainnames, no significance is attached to the case.  That is, two names withthe same spelling but different case are to be treated as if identical.The labels must follow the rules for ARPANET host names.  They muststart with a letter, end with a letter or digit, and have as interiorcharacters only letters, digits, and hyphen.  There are also somerestrictions 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.ARPA2.3.2. Data Transmission OrderThe order of transmission of the header and data described in thisdocument is resolved to the octet level.  Whenever a diagram shows aMockapetris                                                     [Page 8]RFC 1035        Domain Implementation and Specification    November 1987group of octets, the order of transmission of those octets is the normalorder in which they are read in English.  For example, in the followingdiagram, 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 inthe diagram is the high order or most significant bit.  That is, the bitlabeled 0 is the most significant bit.  For example, the followingdiagram 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 quantitythe left most bit of the whole field is the most significant bit.  Whena multi-octet quantity is transmitted the most significant octet istransmitted first.2.3.3. Character CaseFor all parts of the DNS that are part of the official protocol, allcomparisons between character strings (e.g., labels, domain names, etc.)are done in a case-insensitive manner.  At present, this rule is inforce throughout the domain system without exception.  However, futureadditions beyond current usage may need to use the full binary octetcapabilities in names, so attempts to store domain names in 7-bit ASCIIor use of special bytes to terminate labels, etc., should be avoided.When data enters the domain system, its original case should bepreserved whenever possible.  In certain circumstances this cannot bedone.  For example, if two RRs are stored in a database, one at x.y andone 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 thatcase can be discarded only when data is used to define structure in adatabase, and two names are identical when compared in a caseinsensitive manner.Mockapetris                                                     [Page 9]RFC 1035        Domain Implementation and Specification    November 1987Loss of case sensitive data must be minimized.  Thus while data for x.yand X.Y may both be stored under a single location x.y or X.Y, data fora.x and B.X would never be stored under A.x, A.X, b.x, or b.X.  Ingeneral, 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 shouldtake care to represent the data they supply to the domain system in acase-consistent manner if their system is case-sensitive.  The datadistribution system in the domain system will ensure that consistentrepresentations are preserved.2.3.4. Size limitsVarious objects and parameters in the DNS have size limits.  They arelisted below.  Some could be easily changed, others are morefundamental.labels          63 octets or lessnames           255 octets or lessTTL             positive values of a signed 32 bit number.UDP messages    512 octets or less3. DOMAIN NAME SPACE AND RR DEFINITIONS3.1. Name space definitionsDomain names in messages are expressed in terms of a sequence of labels.Each label is represented as a one octet length field followed by thatnumber of octets.  Since every domain name ends with the null label ofthe root, a domain name is terminated by a length byte of zero.  Thehigh order two bits of every length octet must be zero, and theremaining six bits of the length field limit the label to 63 octets orless.To simplify implementations, the total length of a domain name (i.e.,label octets and label length octets) is restricted to 255 octets orless.Although labels can contain any 8 bit values in octets that make up alabel, it is strongly recommended that labels follow the preferredsyntax described elsewhere in this memo, which is compatible withexisting host naming conventions.  Name servers and resolvers mustcompare labels in a case-insensitive manner (i.e., A=a), assuming ASCIIwith zero parity.  Non-alphabetic codes must match exactly.Mockapetris                                                    [Page 10]RFC 1035        Domain Implementation and Specification    November 19873.2. RR definitions3.2.1. FormatAll 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 1987RDATA           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 valuesTYPE fields are used in resource records.  Note that these types are asubset of QTYPEs.TYPE            value and meaningA               1 a host addressNS              2 an authoritative name serverMD              3 a mail destination (Obsolete - use MX)MF              4 a mail forwarder (Obsolete - use MX)CNAME           5 the canonical name for an aliasSOA             6 marks the start of a zone of authorityMB              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 descriptionPTR             12 a domain name pointerHINFO           13 host informationMINFO           14 mailbox or mail list informationMX              15 mail exchangeTXT             16 text strings3.2.3. QTYPE valuesQTYPE fields appear in the question part of a query.  QTYPES are asuperset of TYPEs, hence all TYPEs are valid QTYPEs.  In addition, thefollowing QTYPEs are defined:Mockapetris                                                    [Page 12]RFC 1035        Domain Implementation and Specification    November 1987AXFR            252 A request for a transfer of an entire zoneMAILB           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 records3.2.4. CLASS valuesCLASS fields appear in resource records.  The following CLASS mnemonicsand values are defined:IN              1 the InternetCS              2 the CSNET class (Obsolete - used only for examples in                some obsolete RFCs)CH              3 the CHAOS classHS              4 Hesiod [Dyer 87]3.2.5. QCLASS valuesQCLASS fields appear in the question section of a query.  QCLASS valuesare a superset of CLASS values; every CLASS is a valid QCLASS.  Inaddition to CLASS values, the following QCLASSes are defined:*               255 any class3.3. Standard RRsThe following RR definitions are expected to occur, at leastpotentially, in all classes.  In particular, NS, SOA, CNAME, and PTRwill be used in all classes, and have the same format in all classes.Because their RDATA format is known, all domain names in the RDATAsection of these RRs may be compressed.<domain-name> is a domain name represented as a series of labels, andterminated by a label with zero length.  <character-string> is a singlelength octet followed by that number of characters.  <character-string>is treated as binary information, and can be up to 256 characters inlength (including the length octet).Mockapetris                                                    [Page 13]RFC 1035        Domain Implementation and Specification    November 19873.3.1. 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 maychoose to restart the query at the canonical name in certain cases.  Seethe 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.  Themain use is for protocols such as FTP that can use special procedureswhen 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 1987MB records cause additional section processing which looks up an A typeRRs 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 typerecord corresponding to MADNAME.MD is obsolete.  See the definition of MX and [RFC-974] for details ofthe new scheme.  The recommended policy for dealing with MD RRs found ina master file is to reject them, or to convert them to MX RRs with apreference 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 typerecord corresponding to MADNAME.MF is obsolete.  See the definition of MX and [RFC-974] for details ofwthe new scheme.  The recommended policy for dealing with MD RRs found ina master file is to reject them, or to convert them to MX RRs with apreference of 10.Mockapetris                                                    [Page 15]RFC 1035        Domain Implementation and Specification    November 19873.3.6. 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 theserecords can be associated with a simple mailbox, they are usually usedwith a mailing list.Mockapetris                                                    [Page 16]RFC 1035        Domain Implementation and Specification    November 19873.3.8. 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 MRis as a forwarding entry for a user who has moved to a differentmailbox.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 hostspecified 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 octetsor less.Mockapetris                                                    [Page 17]RFC 1035        Domain Implementation and Specification    November 1987NULL records cause no additional section processing.  NULL RRs are notallowed in master files.  NULLs are used as placeholders in someexperimental 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 locatea type A record, and, when used in a referral, a special search of thezone in which they reside for glue information.The NS RR states that the named host should be expected to have a zonestarting at owner name of the specified class.  Note that the class maynot indicate the protocol family which should be used to communicatewith 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 usedin special domains to point to some other location in the domain space.These records are simple data, and don't imply any special processingsimilar to that performed by CNAME, which identifies aliases.  See thedescription of the IN-ADDR.ARPA domain for an example.Mockapetris                                                    [Page 18]RFC 1035        Domain Implementation and Specification    November 19873.3.13. 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 1987MINIMUM         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 maintenanceoperations.  However, MINIMUM is used in all query operations thatretrieve RRs from a zone.  Whenever a RR is sent in a response to aquery, the TTL field is set to the maximum of the TTL field from the RRand the MINIMUM field in the appropriate SOA.  Thus MINIMUM is a lowerbound on the TTL field for all RRs in a zone.  Note that this use ofMINIMUM should occur when the RRs are copied into the response and notwhen the zone is loaded from a master file or via a zone transfer.  Thereason for this provison is to allow future dynamic update facilities tochange 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 textdepends on the domain where it is found.3.4. Internet specific RRs3.4.1. A RDATA format    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+    |                    ADDRESS                    |    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+where:ADDRESS         A 32 bit Internet address.Hosts that have multiple Internet addresses will have multiple Arecords.Mockapetris                                                    [Page 20]RFC 1035        Domain Implementation and Specification    November 1987A records cause no additional section processing.  The RDATA section ofan A line in a master file is an Internet address expressed as fourdecimal 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 addressPROTOCOL        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 bya particular protocol on a particular internet address.  The PROTOCOLfield specifies an IP protocol number, and the bit map has one bit perport 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 aprotocol of interest, that bit is assumed zero.  The appropriate valuesand mnemonics for ports and protocols are specified in [RFC-1010].For example, if PROTOCOL=TCP (6), the 26th bit corresponds to TCP port25 (SMTP).  If this bit is set, a SMTP server should be listening on TCPport 25; if zero, SMTP service is not supported on the specifiedaddress.The purpose of WKS RRs is to provide availability information forservers for TCP and UDP.  If a server supports both TCP and UDP, or hasmultiple 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 mnemonicsor decimal numbers.Mockapetris                                                    [Page 21]RFC 1035        Domain Implementation and Specification    November 19873.5. IN-ADDR.ARPA domainThe Internet uses a special domain to support gateway location andInternet address to host mapping.  Other classes may employ a similarstrategy in other domains.  The intent of this domain is to provide aguaranteed method to perform host address to host name mapping, and tofacilitate queries to locate all gateways on a particular network in theInternet.Note that both of these services are similar to functions that could beperformed by inverse queries; the difference is that this part of thedomain name space is structured according to address, and hence canguarantee that the appropriate data can be located without an exhaustivesearch of the domain space.The domain begins at IN-ADDR.ARPA and has a substructure which followsthe Internet addressing structure.Domain names in the IN-ADDR.ARPA domain are defined to have up to fourlabels in addition to the IN-ADDR.ARPA suffix.  Each label representsone octet of an Internet address, and is expressed as a character stringfor a decimal value in the range 0-255 (with leading zeros omittedexcept in the case of a zero octet which is represented by a singlezero).Host addresses are represented by domain names that have all four labelsspecified.  Thus data for Internet address 10.2.0.52 is located atdomain name 52.0.2.10.IN-ADDR.ARPA.  The reversal, though awkward toread, allows zones to be delegated which are exactly one network ofaddress space.  For example, 10.IN-ADDR.ARPA can be a zone containingdata for the ARPANET, while 26.IN-ADDR.ARPA can be a separate zone forMILNET.  Address nodes are used to hold pointers to primary host namesin the normal domain space.Network numbers correspond to some non-terminal nodes at various depthsin 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 primaryhost names of gateways attached to that network.  Since a gateway is, bydefinition, on more than one network, it will typically have two or morenetwork nodes which point at it.  Gateways will also have host levelpointers at their fully qualified addresses.Both the gateway pointers at network nodes and the normal host pointersat full address nodes use the PTR RR to point back to the primary domainnames of the corresponding hosts.For example, the IN-ADDR.ARPA domain will contain information about theISI gateway between net 10 and 26, an MIT gateway from net 10 to MIT'sMockapetris                                                    [Page 22]RFC 1035        Domain Implementation and Specification    November 1987net 18, and hosts A.ISI.EDU and MULTICS.MIT.EDU.  Assuming that ISIgateway 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 18.10.0.4and 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 originatea query of the form QTYPE=PTR, QCLASS=IN, QNAME=10.IN-ADDR.ARPA.  Itwould 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 ofthese gateways.A resolver which wanted to find the host name corresponding to Internethost 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 19873.6. Defining new types, classes, and special namespacesThe previously defined types and classes are the ones in use as of thedate of this memo.  New definitions should be expected.  This sectionmakes some recommendations to designers considering additions to theexisting facilities.  The mailing list NAMEDROPPERS@SRI-NIC.ARPA is theforum where general discussion of design issues takes place.In general, a new type is appropriate when new information is to beadded to the database about an existing object, or we need new dataformats for some totally new object.  Designers should attempt to definetypes and their RDATA formats that are generally applicable to allclasses, and which avoid duplication of information.  New classes areappropriate when the DNS is to be used for a new protocol, etc whichrequires new class-specific data formats, or when a copy of the existingname space is desired, but a separate management domain is necessary.New types and classes need mnemonics for master files; the format of themaster files requires that the mnemonics for type and class be disjoint.TYPE and CLASS values must be a proper subset of QTYPEs and QCLASSesrespectively.The present system uses multiple RRs to represent multiple values of atype rather than storing multiple values in the RDATA section of asingle RR.  This is less efficient for most applications, but does keepRRs shorter.  The multiple RRs assumption is incorporated in someexperimental work on dynamic update methods.The present system attempts to minimize the duplication of data in thedatabase in order to insure consistency.  Thus, in order to find theaddress of the host for a mail exchange, you map the mail domain name toa host name, then the host name to addresses, rather than a directmapping to host address.  This approach is preferred because it avoidsthe opportunity for inconsistency.In defining a new type of data, multiple RR types should not be used tocreate an ordering between entries or express different formats forequivalent bindings, instead this information should be carried in thebody of the RR and a single type used.  This policy avoids problems withcaching multiple types and defining QTYPEs to match multiple types.For example, the original form of mail exchange binding used two RRtypes one to represent a "closer" exchange (MD) and one to represent a"less close" exchange (MF).  The difficulty is that the presence of oneRR type in a cache doesn't convey any information about the otherbecause the query which acquired the cached information might have useda QTYPE of MF, MD, or MAILA (which matched both).  The redesignedMockapetris                                                    [Page 24]RFC 1035        Domain Implementation and Specification    November 1987service used a single type (MX) with a "preference" value in the RDATAsection which can order different RRs.  However, if any MX RRs are foundin the cache, then all should be there.4. MESSAGES4.1. FormatAll communications inside of the domain protocol are carried in a singleformat called a message.  The top level format of message is dividedinto 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 thatspecify which of the remaining sections are present, and also specifywhether the message is a query or a response, a standard query or someother opcode, etc.The names of the sections after the header are derived from their use instandard queries.  The question section contains fields that describe aquestion to a name server.  These fields are a query type (QTYPE), aquery class (QCLASS), and a query domain name (QNAME).  The last threesections have the same format: a possibly empty list of concatenatedresource records (RRs).  The answer section contains RRs that answer thequestion; the authority section contains RRs that point toward anauthoritative name server; the additional records section contains RRswhich relate to the query, but are not strictly answers for thequestion.Mockapetris                                                    [Page 25]RFC 1035        Domain Implementation and Specification    November 19874.1.1. Header section formatThe 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 useAA              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 bitMockapetris                                                    [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., zoneMockapetris                                                    [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 formatThe question section is used to carry the "question" in most queries,i.e., the parameters that define what is being asked.  The sectioncontains 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 1987QCLASS          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 formatThe answer, authority, and additional sections all share the sameformat: a variable number of resource records, where the number ofrecords 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 1987RDLENGTH        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 compressionIn order to reduce the size of messages, the domain system utilizes acompression scheme which eliminates the repetition of domain names in amessage.  In this scheme, an entire domain name or a list of labels atthe end of a domain name is replaced with a pointer to a prior occuranceof 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 distinguishedfrom a label, since the label must begin with two zero bits becauselabels are restricted to 63 octets or less.  (The 10 and 01 combinationsare reserved for future use.)  The OFFSET field specifies an offset fromthe start of the message (i.e., the first octet of the ID field in thedomain header).  A zero offset specifies the first byte of the ID field,etc.The compression scheme allows a domain name in a message to berepresented as either:   - a sequence of labels ending in a zero octet   - a pointer   - a sequence of labels ending with a pointerPointers can only be used for occurances of a domain name where theformat is not class specific.  If this were not the case, a name serveror 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 RDATAformats.If a domain name is contained in a part of the message subject to alength field (such as the RDATA section of an RR), and compression isMockapetris                                                    [Page 30]RFC 1035        Domain Implementation and Specification    November 1987used, the length of the compressed name is used in the lengthcalculation, 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 thatcontain 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 themessage, 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 nameFOO.F.ISI.ARPA is shown at offset 40; this definition uses a pointer toconcatenate a label for FOO to the previously defined F.ISI.ARPA.  Thedomain name ARPA is defined at offset 64 using a pointer to the ARPAcomponent of the name F.ISI.ARPA at 20; note that this pointer relies onARPA being the last label in the string at 20.  The root domain name isMockapetris                                                    [Page 31]RFC 1035        Domain Implementation and Specification    November 1987defined by a single octet of zeros at 92; the root domain name has nolabels.4.2. TransportThe DNS assumes that messages will be transmitted as datagrams or in abyte stream carried by a virtual circuit.  While virtual circuits can beused for any DNS activity, datagrams are preferred for queries due totheir lower overhead and better performance.  Zone refresh activitiesmust use virtual circuits because of the need for reliable transfer.The Internet supports name server access using TCP [RFC-793] on serverport 53 (decimal) as well as datagram access using UDP [RFC-768] on UDPport 53 (decimal).4.2.1. UDP usageMessages sent using UDP user server port 53 (decimal).Messages carried by UDP are restricted to 512 bytes (not counting the IPor UDP headers).  Longer messages are truncated and the TC bit is set inthe header.UDP is not acceptable for zone transfers, but is the recommended methodfor standard queries in the Internet.  Queries sent using UDP may belost, and hence a retransmission strategy is required.  Queries or theirresponses may be reordered by the network, or by processing in nameservers, so resolvers should not depend on them being returned in order.The optimal UDP retransmission policy will vary with performance of theInternet 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 befound in the resolver section of this memo.4.2.2. TCP usageMessages sent over TCP connections use server port 53 (decimal).  Themessage is prefixed with a two byte length field which gives the messageMockapetris                                                    [Page 32]RFC 1035        Domain Implementation and Specification    November 1987length, excluding the two byte length field.  This length field allowsthe low-level processing to assemble a complete message before beginningto 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 FILESMaster files are text files that contain RRs in text form.  Since thecontents of a zone can be expressed in the form of a list of RRs amaster file is most often used to define a zone, though it can be usedto list a cache's contents.  Hence, this section first discusses theformat of RRs in a master file, and then the special considerations whena master file is used to create a zone in some name server.5.1. FormatThe format of these files is a sequence of entries.  Entries arepredominantly line-oriented, though parentheses can be used to continuea list of items across a line boundary, and text literals can containCRLF within the text.  Any combination of tabs and spaces act as adelimiter between the separate items that make up an entry.  The end ofany line in the master file can end with a comment.  The comment startswith 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 isfollowed by a domain name, and resets the current origin for relativedomain names to the stated name.  $INCLUDE inserts the named file intothe current file, and may optionally specify a domain name that sets therelative domain name origin for the included file.  $INCLUDE may alsohave a comment.  Note that a $INCLUDE entry never changes the relativeorigin of the parent file, regardless of changes to the relative originmade within the included file.The last two forms represent RRs.  If an entry for an RR begins with ablank, then the RR is assumed to be owned by the last stated owner.  Ifan 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 andRDATA field appropriate to the type and class.  Class and type use thestandard mnemonics, TTL is a decimal integer.  Omitted class and TTLvalues are default to the last explicitly stated values.  Since type andclass mnemonics are disjoint, the parse is unique.  (Note that thisorder is different from the order used in examples and the order used inthe 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 andseparated by dots.  Quoting conventions allow arbitrary characters to bestored in domain names.  Domain names that end in a dot are calledabsolute, and are taken as complete.  Domain names which do not end in adot are called relative; the actual domain name is the concatenation ofthe relative part with an origin specified in a $ORIGIN, $INCLUDE, or asan argument to the master file loading routine.  A relative name is anerror 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 setof characters without interior spaces, or as a string beginning with a "and ending with a ".  Inside a " delimited string any character canoccur, except for a " itself, which must be quoted using \ (back slash).Because these files are text files several special encodings arenecessary 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 zonesWhen a master file is used to load a zone, the operation should besuppressed if any errors are encountered in the master file.  Therationale for this is that a single error can have widespreadconsequences.  For example, suppose that the RRs defining a delegationhave syntax errors; then the server will return authoritative nameerrors for all names in the subzone (except in the case where thesubzone is also present on the server).Several other validity checks that should be performed in addition toinsuring 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 exampleThe following is an example file which might be used to define theISI.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      VAXAA       A       26.3.0.103VENERA  A       10.1.0.52        A       128.9.0.32VAXA    A       10.2.0.27        A       128.9.0.33$INCLUDE <SUBSYS>ISI-MAILBOXES.TXTWhere 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      CURLEYNote the use of the \ character in the SOA RR to specify the responsibleperson mailbox "Action.domains@E.ISI.EDU".Mockapetris                                                    [Page 36]RFC 1035        Domain Implementation and Specification    November 19876. NAME SERVER IMPLEMENTATION6.1. ArchitectureThe optimal structure for the name server will depend on the hostoperating system and whether the name server is integrated with resolveroperations, either by supporting recursive service, or by sharing itsdatabase with a resolver.  This section discusses implementationconsiderations for a name server which shares a database with aresolver, but most of these concerns are present in any name server.6.1.1. ControlA name server must employ multiple concurrent activities, whether theyare implemented as separate tasks in the host's OS or multiplexinginside a single name server program.  It is simply not acceptable for aname server to block the service of UDP requests while it waits for TCPdata for refreshing or query activities.  Similarly, a name servershould not attempt to provide recursive service without processing suchrequests in parallel, though it may choose to serialize requests from asingle client, or to regard identical requests from the same client asduplicates.  A name server should not substantially delay requests whileit reloads a zone from master files or while it incorporates a newlyrefreshed zone into its database.6.1.2. DatabaseWhile name server implementations are free to use any internal datastructures they choose, the suggested structure consists of three majorparts:   - 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 treestructure format, with different data chained off the nodes in differentparts: in the catalog the data is pointers to zones, while in the zoneand cache data structures, the data will be RRs.  In designing the treeframework the designer should recognize that query processing will needto traverse the tree using case-insensitive label comparisons; and thatMockapetris                                                    [Page 37]RFC 1035        Domain Implementation and Specification    November 1987in real data, a few nodes have a very high branching factor (100-1000 ormore), 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 nodein two pieces: a standardized-case representation of the label where allASCII characters are in a single case, together with a bit mask thatdenotes which characters are actually of a different case.  Thebranching factor diversity can be handled using a simple linked list fora node until the branching factor exceeds some threshold, andtransitioning to a hash structure after the threshold is exceeded.  Inany case, hash structures used to store tree sections must insure thathash functions and procedures preserve the casing conventions of theDNS.The use of separate structures for the different parts of the databaseis 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 allowsthe name server to deal with crashes of the name server's host.  Stateinformation which a name server should save across system crashesMockapetris                                                    [Page 38]RFC 1035        Domain Implementation and Specification    November 1987includes the catalog structure (including the state of refreshing foreach zone) and the zone data itself.6.1.3. TimeBoth the TTL data for RRs and the timing data for refreshing activitiesdepends on 32 bit timers in units of seconds.  Inside the database,refresh timers and TTLs for cached data conceptually "count down", whiledata in the zone stays with constant TTLs.A recommended implementation strategy is to store time in two ways:  asa relative increment and as an absolute time.  One way to do this is touse positive 32 bit numbers for one type and negative numbers for theother.  The RRs in zones use relative times; the refresh timers andcache data use absolute times.  Absolute numbers are taken with respectto some known origin and converted to relative values when placed in theresponse to a query.  When an absolute TTL is negative after conversionto relative, then the data is expired and should be ignored.6.2. Standard query processingThe major algorithm for standard query processing is presented in[RFC-1034].When processing queries with QCLASS=*, or some other QCLASS whichmatches multiple classes, the response should never be authoritativeunless the server can guarantee that the response covers all classes.When composing a response, RRs which are to be inserted in theadditional section, but duplicate RRs in the answer or authoritysections, may be omitted from the additional section.When a response is so long that truncation is required, the truncationshould start at the end of the response and work forward in thedatagram.  Thus if there is any data for the authority section, theanswer section is guaranteed to be unique.The MINIMUM value in the SOA should be used to set a floor on the TTL ofdata distributed from a zone.  This floor function should be done whenthe data is copied into a response.  This will allow future dynamicupdate protocols to change the SOA MINIMUM field without ambiguoussemantics.6.3. Zone refresh and reload processingIn spite of a server's best efforts, it may be unable to load zone datafrom a master file due to syntax errors, etc., or be unable to refresh azone within the its expiration parameter.  In this case, the name serverMockapetris                                                    [Page 39]RFC 1035        Domain Implementation and Specification    November 1987should 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 createdduring the transfer, the master should continue to send the old versionif possible.  In any case, it should never send part of one version andpart of another.  If completion is not possible, the master should resetthe 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 notrequired to support any form of inverse queries.  If a name serverreceives an inverse query that it does not support, it returns an errorresponse with the "Not Implemented" error set in the header.  Whileinverse query support is optional, all name servers must be at leastable to return the error response.6.4.1. The contents of inverse queries and responses          Inversequeries reverse the mappings performed by standard query operations;while a standard query maps a domain name to a resource, an inversequery maps a resource to a domain name.  For example, a standard querymight bind a domain name to a host address; the corresponding inversequery binds the host address to a domain name.Inverse queries take the form of a single RR in the answer section ofthe message, with an empty question section.  The owner name of thequery RR and its TTL are not significant.  The response carriesquestions in the question section which identify all names possessingthe query RR WHICH THE NAME SERVER KNOWS.  Since no name server knowsabout all of the domain name space, the response can never be assumed tobe complete.  Thus inverse queries are primarily useful for databasemanagement and debugging activities.  Inverse queries are NOT anacceptable method of mapping host addresses to host names; use the IN-ADDR.ARPA domain instead.Where possible, name servers should provide case-insensitive comparisonsfor 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" shouldproduce the same result as an inverse query for "IBM-pc unix".  However,this cannot be guaranteed because name servers may possess RRs thatcontain character strings but the name server does not know that thedata 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 sectionMockapetris                                                    [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, theowner name and TTL of the RR in the answer section which defines theinverse query is modified to exactly match an RR found at the firstQNAME.RRs returned in the inverse queries cannot be cached using the samemechanism as is used for the replies to standard queries.  One reasonfor this is that a name might have multiple RRs of the same type, andonly one would appear.  For example, an inverse query for a singleaddress of a multiply homed host might create the impression that onlyone address existed.6.4.2. Inverse query and response example          The overall structureof an inverse query for retrieving the domain name that corresponds toInternet 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 styleaddress 10.1.0.52.  Since the owner name is not known, any domain namecan be used as a placeholder (and is ignored).  A single octet of zero,signifying the root, is usually used because it minimizes the length ofthe message.  The TTL of the RR is not significant.  The response tothis 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 theTYPE field in the answer section of the inverse query.  Responses toinverse queries may contain multiple questions when the inverse is notunique.  If the question section in the response is not empty, then theRR in the answer section is modified to correspond to be an exact copyof an RR at the first QNAME.6.4.3. Inverse query processingName servers that support inverse queries can support these operationsthrough exhaustive searches of their databases, but this becomesimpractical as the size of the database increases.  An alternativeapproach 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 aparticular zone is changed during a refresh, only its inversions need tobe redone.Support for transfer of this type of inversion may be included in futureversions of the domain system, but is not supported in this version.6.5. Completion queries and responsesThe optional completion services described in RFC-882 and RFC-883 havebeen deleted.  Redesigned services may become available in the future.Mockapetris                                                    [Page 42]RFC 1035        Domain Implementation and Specification    November 19877. RESOLVER IMPLEMENTATIONThe top levels of the recommended resolver algorithm are discussed in[RFC-1034].  This section discusses implementation details assuming thedatabase structure suggested in the name server implementation sectionof this memo.7.1. Transforming a user request into a queryThe first step a resolver takes is to transform the client's request,stated in a format suitable to the local OS, into a search specificationfor RRs at a specific name which match a specific QTYPE and QCLASS.Where possible, the QTYPE and QCLASS should correspond to a single typeand a single class, because this makes the use of cached data muchsimpler.  The reason for this is that the presence of data of one typein a cache doesn't confirm the existence or non-existence of data ofother types, hence the only way to be sure is to consult anauthoritative source.  If QCLASS=* is used, then authoritative answerswon't be available.Since a resolver must be able to multiplex multiple requests if it is toperform its function efficiently, each pending request is usuallyrepresented in some block of state information.  This state block willtypically 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 theMockapetris                                                    [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 queriesAs described in [RFC-1034], the basic task of the resolver is toformulate a query which will answer the client's request and direct thatquery to name servers which can provide the information.  The resolverwill usually only have very strong hints about which servers to ask, inthe form of NS RRs, and may have to revise the query, in response toCNAMEs, or revise the set of name servers the resolver is asking, inresponse to delegation responses which point the resolver to nameservers closer to the desired information.  In addition to theinformation requested by the client, the resolver may have to call uponits own services to determine the address of name servers it wishes tocontact.In any case, the model used in this memo assumes that the resolver ismultiplexing attention between multiple requests, some from the client,and some internally generated.  Each request is represented by somestate information, and the desired behavior is that the resolvertransmit queries to name servers in a way that maximizes the probabilitythat the request is answered, minimizes the time that the request takes,and avoids excessive transmissions.  The key algorithm uses the stateinformation of the request to select the next name server address toquery, and also computes a timeout which will cause the next actionshould a response not arrive.  The next action will usually be atransmission to some other server, but may be a temporary error to theMockapetris                                                    [Page 44]RFC 1035        Domain Implementation and Specification    November 1987client.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 ancestorzone that the resolver knows about.  To avoid startup problems, theresolver should have a set of default servers which it will ask shouldit have no current NS RRs which are appropriate.  The resolver then addsto SLIST all of the known addresses for the name servers, and may startparallel requests to acquire the addresses of the servers when theresolver has the name, but no addresses, for the name servers.To complete initialization of SLIST, the resolver attaches whateverhistory information it has to the each address in SLIST.  This willusually consist of some sort of weighted averages for the response timeof the address, and the batting average of the address (i.e., how oftenthe address responded at all to the request).  Note that thisinformation should be kept on a per address basis, rather than on a pername server basis, because the response time and batting average of aparticular server may vary considerably from address to address.  Notealso that this information is actually specific to a resolver address /server address pair, so a resolver with multiple addresses may wish tokeep separate histories for each of its addresses.  Part of this stepmust deal with addresses which have no such history; in this case anexpected round trip time of 5-10 seconds should be the worst case, withlower estimates for the same local network, etc.Note that whenever a delegation is followed, the resolver algorithmreinitializes SLIST.The information establishes a partial ranking of the available nameserver addresses.  Each time an address is chosen and the state shouldbe altered to prevent its selection again until all other addresses havebeen tried.  The timeout for each transmission should be 50-100% greaterthan 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 responsesThe first step in processing arriving response datagrams is to parse theresponse.  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 IDfield in the domain header, and then to verify that the question sectioncorresponds to the information currently desired.  This requires thatthe transmission algorithm devote several bits of the domain ID field toa 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 19877.4. Using the cacheIn general, we expect a resolver to cache all data which it receives inresponses 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 aresponse, and wants to cache the RRs, it should check its cache foralready existing RRs.  Depending on the circumstances, either the datain the response or the cache is preferred, but the two should never becombined.  If the data in the response is from authoritative data in theanswer section, it is always preferred.8. MAIL SUPPORTThe domain system defines a standard for mapping mailboxes into domainnames, and two methods for using the mailbox information to derive mailrouting information.  The first method is called mail exchange bindingand the other method is mailbox binding.  The mailbox encoding standardand mail exchange binding are part of the DNS official protocol, and arethe recommended method for mail routing in the Internet.  Mailboxbinding is an experimental feature which is still under development andsubject to change.Mockapetris                                                    [Page 47]RFC 1035        Domain Implementation and Specification    November 1987The mailbox encoding standard assumes a mailbox name of the form"<local-part>@<mail-domain>".  While the syntax allowed in each of thesesections varies substantially between the various mail internets, thepreferred 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 domainname 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, itsrepresentation in a master file will require the use of backslashquoting to ensure that the domain name is properly encoded.  Forexample, the mailbox Action.domains@ISI.EDU would be represented asAction\.domains.ISI.EDU.8.1. Mail exchange bindingMail exchange binding uses the <mail-domain> part of a mailboxspecification to determine where mail should be sent.  The <local-part>is not even consulted.  [RFC-974] specifies this method in detail, andshould be consulted before attempting to use mail exchange support.One of the advantages of this method is that it decouples maildestination naming from the hosts used to support mail service, at thecost 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 domainname to locate type MX RRs which list hosts willing to accept mail for<mail-domain>, together with preference values which rank the hostsaccording to an order specified by the administrators for <mail-domain>.In this memo, the <mail-domain> ISI.EDU is used in examples, togetherwith the hosts VENERA.ISI.EDU and VAXA.ISI.EDU as mail exchanges forISI.EDU.  If a mailer had a message for Mockapetris@ISI.EDU, it wouldroute it by looking up MX RRs for ISI.EDU.  The MX RRs at ISI.EDU nameVENERA.ISI.EDU and VAXA.ISI.EDU, and type A queries can find the hostaddresses.8.2. Mailbox binding (Experimental)In mailbox binding, the mailer uses the entire mail destinationspecification to construct a domain name.  The encoded domain name forthe 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 islegal with a MB.  The MINFO RR identifies two mailboxes.  One of theseidentifies a responsible person for the original mailbox name.  Thismailbox 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 shouldreceive error messages for mail failures.  This is particularlyappropriate for mailing lists when errors in member names should bereported to a person other than the one who sends a message to the list.Mockapetris                                                    [Page 49]RFC 1035        Domain Implementation and Specification    November 1987New 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 1987Index          *   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, 18Mockapetris                                                    [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   12Mockapetris                                                    [Page 55]