rfc883.txt

RFC 相关的技术文档

Network Working Group                                     P. Mockapetris
Request for Comments:  883                                           ISI
                                                           November 1983

            DOMAIN NAMES - IMPLEMENTATION and SPECIFICATION

        +-----------------------------------------------------+
        |                                                     |
        | This memo discusses the implementation of domain    |
        | name servers and resolvers, specifies the format of |
        | transactions, and discusses the use of domain names |
        | in the context of existing mail systems and other   |
        | network software.                                   |
        |                                                     |
        | This memo assumes that the reader is familiar with  |
        | RFC 882, "Domain Names - Concepts and Facilities"   |
        | which discusses the basic principles of domain      |
        | names and their use.                                |
        |                                                     |
        | The algorithms and internal data structures used in |
        | this memo are offered as suggestions rather than    |
        | requirements; implementers are free to design their |
        | own structures so long as the same external         |
        | behavior is achieved.                               |
        |                                                     |
        +-----------------------------------------------------+

   
   

           +-----------------------------------------------+
           |                                               |
           |             *****  WARNING  *****             |
           |                                               |
           | This RFC contains format specifications which |
           | are preliminary and are included for purposes |
           | of explanation only.  Do not attempt to use   |
           | this information for actual implementations.  |
           |                                               |
           +-----------------------------------------------+















Mockapetris                                                     [Page i]


RFC 883                                                    November 1983
                         Domain Names - Implementation and Specification


TABLE OF CONTENTS
   INTRODUCTION........................................................3
      Overview.........................................................3
      Implementation components........................................2
      Conventions......................................................6
      Design philosophy................................................8
   NAME SERVER TRANSACTIONS...........................................11
      Introduction....................................................11
      Query and response transport....................................11
      Overall message format..........................................13
      The contents of standard queries and responses..................15
      Standard query and response example.............................15
      The contents of inverse queries and responses...................17
      Inverse query and response example..............................18
      Completion queries and responses................................19
      Completion query and response example...........................22
      Recursive Name Service..........................................24
      Header section format...........................................26
      Question section format.........................................29
      Resource record format..........................................30
      Domain name representation and compression......................31
      Organization of the Shared database.............................33
      Query processing................................................36
      Inverse query processing........................................37
      Completion query processing.....................................38
   NAME SERVER MAINTENANCE............................................39
      Introduction....................................................39
      Conceptual model of maintenance operations......................39
      Name server data structures and top level logic.................41
      Name server file loading........................................43
      Name server file loading example................................45
      Name server remote zone transfer................................47
   RESOLVER ALGORITHMS................................................50
      Operations......................................................50
   DOMAIN SUPPORT FOR MAIL............................................52
      Introduction....................................................52
      Agent binding...................................................53
      Mailbox binding.................................................54
   Appendix 1 - Domain Name Syntax Specification......................56
   Appendix 2 - Field formats and encodings...........................57
      TYPE values.....................................................57
      QTYPE values....................................................57
      CLASS values....................................................58
      QCLASS values...................................................58
      Standard resource record formats................................59
   Appendix 3 - Internet specific field formats and operations........67
   REFERENCES and BIBLIOGRAPHY........................................72
   INDEX..............................................................73
   


Mockapetris                                                    [Page ii]


RFC 883                                                    November 1983
                         Domain Names - Implementation and Specification


INTRODUCTION

   Overview

      The goal of domain names is to provide a mechanism for naming
      resources in such a way that the names are usable in different
      hosts, networks, protocol families, internets, and administrative
      organizations.

      From the user's point of view, domain names are useful as
      arguments to a local agent, called a resolver, which retrieves
      information associated with the domain name.  Thus a user might
      ask for the host address or mail information associated with a
      particular domain name.  To enable the user to request a
      particular type of information, an appropriate query type is
      passed to the resolver with the domain name.  To the user, the
      domain tree is a single information space.

      From the resolver's point of view, the database that makes up the
      domain space is distributed among various name servers.  Different
      parts of the domain space are stored in different name servers,
      although a particular data item will usually be stored redundantly
      in two or more name servers.  The resolver starts with knowledge
      of at least one name server.  When the resolver processes a user
      query it asks a known name server for the information; in return,
      the resolver either receives the desired information or a referral
      to another name server.  Using these referrals, resolvers learn
      the identities and contents of other name servers.  Resolvers are
      responsible for dealing with the distribution of the domain space
      and dealing with the effects of name server failure by consulting
      redundant databases in other servers.

      Name servers manage two kinds of data.  The first kind of data
      held in sets called zones; each zone is the complete database for
      a particular subtree of the domain space.  This data is called
      authoritative.  A name server periodically checks to make sure
      that its zones are up to date, and if not obtains a new copy of
      updated zones from master files stored locally or in another name
      server.  The second kind of data is cached data which was acquired
      by a local resolver.  This data may be incomplete but improves the
      performance of the retrieval process when non-local data is
      repeatedly accessed.  Cached data is eventually discarded by a
      timeout mechanism.

      This functional structure isolates the problems of user interface,
      failure recovery, and distribution in the resolvers and isolates
      the database update and refresh problems in the name servers.




Mockapetris                                                     [Page 1]


RFC 883                                                    November 1983
                         Domain Names - Implementation and Specification


   Implementation components

      A host can participate in the domain name system in a number of
      ways, depending on whether the host runs programs that retrieve
      information from the domain system, name servers that answer
      queries from other hosts, or various combinations of both
      functions.  The simplest, and perhaps most typical, configuration
      is shown below:

                   Local Host                        |  Foreign   
                                                     |            
      +---------+               +----------+         |  +--------+
      |         | user queries  |          |queries  |  |        |
      |  User   |-------------->|          |---------|->|Foreign |
      | Program |               | Resolver |         |  |  Name  |
      |         |<--------------|          |<--------|--| Server |
      |         | user responses|          |responses|  |        |
      +---------+               +----------+         |  +--------+
                                  |     A            |            
                  cache additions |     | references |            
                                  V     |            |            
                                +----------+         |            
                                | database |         |            
                                +----------+         |            

      User programs interact with the domain name space through
      resolvers; the format of user queries and user responses is
      specific to the host and its operating system.  User queries will
      typically be operating system calls, and the resolver and its
      database will be part of the host operating system.  Less capable
      hosts may choose to implement the resolver as a subroutine to be
      linked in with every program that needs its services.

      Resolvers answer user queries with information they acquire via
      queries to foreign name servers, and may also cache or reference
      domain information in the local database.

      Note that the resolver may have to make several queries to several
      different foreign name servers to answer a particular user query,
      and hence the resolution of a user query may involve several
      network accesses and an arbitrary amount of time.  The queries to
      foreign name servers and the corresponding responses have a
      standard format described in this memo, and may be datagrams.








Mockapetris                                                     [Page 2]


RFC 883                                                    November 1983
                         Domain Names - Implementation and Specification


      Depending on its capabilities, a name server could be a stand
      alone program on a dedicated machine or a process or processes on
      a large timeshared host.  A simple configuration might be:

                   Local Host                        |  Foreign   
                                                     |            
        +---------+                                  |            
       /         /|                                  |            
      +---------+ |             +----------+         |  +--------+
      |         | |             |          |responses|  |        |
      |         | |             |   Name   |---------|->|Foreign |
      |  Master |-------------->|  Server  |         |  |Resolver|
      |  files  | |             |          |<--------|--|        |
      |         |/              |          | queries |  +--------+
      +---------+               +----------+         |            

      Here the name server acquires information about one or more zones
      by reading master files from its local file system, and answers
      queries about those zones that arrive from foreign resolvers.

      A more sophisticated name server might acquire zones from foreign
      name servers as well as local master files.  This configuration is
      shown below:

                   Local Host                        |  Foreign   
                                                     |            
        +---------+                                  |            
       /         /|                                  |            
      +---------+ |             +----------+         |  +--------+
      |         | |             |          |responses|  |        |
      |         | |             |   Name   |---------|->|Foreign |
      |  Master |-------------->|  Server  |         |  |Resolver|
      |  files  | |             |          |<--------|--|        |
      |         |/              |          | queries |  +--------+
      +---------+               +----------+         |            
                                  A     |maintenance |  +--------+
                                  |     \------------|->|        |
                                  |      queries     |  |Foreign |
                                  |                  |  |  Name  |
                                  \------------------|--| Server |
                               maintenance responses |  +--------+

      In this configuration, the name server periodically establishes a
      virtual circuit to a foreign name server to acquire a copy of a
      zone or to check that an existing copy has not changed.  The
      messages sent for these maintenance activities follow the same
      form as queries and responses, but the message sequences are
      somewhat different.



Mockapetris                                                     [Page 3]


RFC 883                                                    November 1983
                         Domain Names - Implementation and Specification


      The information flow in a host that supports all aspects of the
      domain name system is shown below:

                   Local Host                        |  Foreign   
                                                     |            
      +---------+               +----------+         |  +--------+
      |         | user queries  |          |queries  |  |        |
      |  User   |-------------->|          |---------|->|Foreign |
      | Program |               | Resolver |         |  |  Name  |
      |         |<--------------|          |<--------|--| Server |
      |         | user responses|          |responses|  |        |
      +---------+               +----------+         |  +--------+
                                  |     A            |            
                  cache additions |     | references |            
                                  V     |            |            
                                +----------+         |            
                                |  Shared  |         |            
                                | database |         |            
                                +----------+         |            
                                  A     |            |            
        +---------+     refreshes |     | references |            
       /         /|               |     V            |            
      +---------+ |             +----------+         |  +--------+
      |         | |             |          |responses|  |        |
      |         | |             |   Name   |---------|->|Foreign |
      |  Master |-------------->|  Server  |         |  |Resolver|
      |  files  | |             |          |<--------|--|        |
      |         |/              |          | queries |  +--------+