rfc882.txt

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      hence become available to users of the domain system.  A standard
      format for these files is given in [14].

      The standard format allows these files to be exchanged between
      hosts (via FTP, mail, or some other mechanism); this facility is
      useful when an organization wants a domain, but doesn't want to
      support a name server.  The organization can maintain the master
      files locally using a text editor, transfer them to a foreign host
      which runs a name server, and then arrange with the system
      administrator of the name server to get the files loaded.


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RFC 882                                                    November 1983
                                  Domain Names - Concepts and Facilities


      Each host's name servers and resolvers are configured by a local
      system administrator.  For a name server, this configuration data
      includes the identity of local master files and instructions on
      which non-local master files are to be loaded from foreign
      servers.  The name server uses the master files or copies to load
      its zones.  For resolvers, the configuration data identifies the
      name servers which should be the primary sources of information.

      The domain system defines procedures for accessing the data and
      for referrals to other name servers.  The domain system also
      defines procedures for caching retrieved data and for periodic
      refreshing of data defined by the system administrator.

      The system administrators provide:

         The definition of zone boundaries

         Master files of data

         Updates to master files

         Statements of the refresh policies desired

      The domain system provides:

         Standard formats for resource data

         Standard methods for querying the database

         Standard methods for name servers to refresh local data from
         foreign name servers

DOMAIN NAME SPACE

   Name space specifications and terminology

      The domain name space is a tree structure.  Each node and leaf on
      the tree corresponds to a resource set (which may be empty).  Each
      node and leaf has an associated label.  Labels are NOT guaranteed
      to be unique, with the exception of the root node, which has a
      null label.  The domain name of a node or leaf is the path from
      the root of the tree to the node or leaf.  By convention, the
      labels that compose a domain name are read left to right, from the
      most specific (lowest) to the least specific (highest).

      Internally, programs that manipulate domain names represent them
      as sequences of labels, where each label is a length octet
      followed by an octet string.  Because all domain names end at the
      root, which has a null string for a label, these internal


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RFC 882                                                    November 1983
                                  Domain Names - Concepts and Facilities


      representations can use a length byte of zero to terminate a
      domain name.  When domain names are printed, labels in a path are
      separated by dots (".").  The root label and its associated dot
      are omitted from printed domain names, but the root can be named
      by a null domain name (" " in this memo).

      To simplify implementations, the total number of octets that
      represent label octets and label lengths is limited to 255.  Thus
      a printed domain name can be up to 254 characters.

      A special label is defined that matches any other label.  This
      label is the asterisk or "*".  An asterisk matches a single label.
      Thus *.ARPA matches FOO.ARPA, but does not match FOO.BAR.ARPA.
      The asterisk is mainly used to create default resource records at
      the boundary between protocol families, and requires prudence in
      its use.

      A domain is identified by a domain name, and consists of that part
      of the domain name space that is at or below the domain name which
      specifies the domain.  A domain is a subdomain of another domain
      if it is contained within that domain.  This relationship can be
      tested by seeing if the subdomain's name has the containing
      domain's name as the right part of its name.  For example, A.B.C.D
      is a subdomain of B.C.D, C.D, D, and " ".

      This tree structure is intended to parallel the administrative
      organization and delegation of authority.  Potentially, each node
      or leaf on the tree can create new subdomains ad infinitum.  In
      practice, this delegation can be limited by the administrator of
      the name servers that manage the domain space and resource data.

      The following figure shows an example of a domain name space.

                                   |                            
                +------------------+------------------+         
                |                  |                  |         
              COLORS            FLAVORS             TRUTH       
                |                  |                            
          +-----+-----+            |                            
          |     |     |         NATURAL                         
         RED  BLUE  GREEN          |                            
                                   |                            
                   +---------------+---------------+            
                   |               |               |            
               CHOCOLATE        VANILLA        STRAWBERRY       

      In this example, the root domain has three immediate subdomains:
      COLORS, FLAVORS, and TRUTH.  The FLAVORS domain has one immediate
      subdomain named NATURAL.FLAVORS.  All of the leaves are also


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RFC 882                                                    November 1983
                                  Domain Names - Concepts and Facilities


      domains.  This domain tree has the names " "(the root), COLORS,
      RED.COLORS, BLUE.COLORS, GREEN.COLORS, FLAVORS, NATURAL.FLAVORS,
      CHOCOLATE.NATURAL.FLAVORS, VANILLA.NATURAL.FLAVORS,
      STRAWBERRY.NATURAL.FLAVORS, and TRUTH.  If we wished to add a new
      domain of ARTIFICIAL under FLAVORS, FLAVORS would typically be the
      administrative entity that would decide; if we wished to create
      CHIP and MOCHA names under CHOCOLATE, CHOCOLATE.NATURAL.FLAVORS
      would typically be the appropriate administrative entity.

   Resource set information

      A domain name identifies a set of resource information.  The set
      of resource information associated with a particular name is
      composed of separate resource records (RRs).

      Each resource record has the following major components:

         The domain name which identifies resource set that holds this
         record, and hence the "owner" of the information.  For example,
         a RR that specifies a host address has a domain name the
         specifies the host having that address.  Thus F.ISI.ARPA might
         be the owner of a RR which specified an address field of
         10.2.0.52.  Since name servers typically store their resource
         information in tree structures paralleling the organization of
         the domain space, this information can usually be stored
         implicitly in the database; however it is always included in
         each resource record carried in a message.

         Other information used to manage the RR, such as length fields,
         timeouts, etc.  This information is omitted in much of this
         memo, but is discussed in [14].

         A resource type field that specifies the type of the resource
         in this resource record.  Types refer to abstract resources
         such as host addresses or mail delivery agents.  The type field
         is two octets long and uses an encoding that is standard
         throughout the domain name system.

         A class field identifies the format of the resource data, such
         as the ARPA Internet format (IN) or the Computer Science
         Network format (CSNET), for certain RR types (such as address
         data).  Note that while the class may separate different
         protocol families, networks, etc. it does not do so in all
         cases.  For example, the IN class uses 32 bit IP addresses
         exclusively, but the CSNET class uses 32 bit IP addresses, X.25
         addresses, and phone numbers.  Thus the class field should be
         used as a guide for interpreting the resource data.  The class
         field is two octets long and uses an encoding that is standard
         throughout the domain name system.


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RFC 882                                                    November 1983
                                  Domain Names - Concepts and Facilities


         Resource data that describes the resource.  The format of this
         data can be determined given the type and class fields, but
         always starts with a two octet length field that allows a name
         server or resolver to determine the boundaries of the resource
         data in any transaction, even if it cannot "understand" the
         resource data itself.  Thus name servers and resolvers can hold
         and pass on records which they cannot interpret.  The format of
         the internal data is restricted only by the maximum length of
         65535 octets; for example the host address record might specify
         a fixed 32 bit number for one class, and a variable length list
         of addresses in another class.

      While the class field in effect partitions the resource data in
      the domain name system into separate parallel sections according
      to class, services can span class boundaries if they use
      compatible resource data formats.  For example, the domain name
      system uses compatible formats for structure information, and the
      mail data decouples mail agent identification from details of how
      to contact the agent (e.g. host addresses).

      This memo uses the following types in its examples:

         A     - the host address associated with the domain name

         MF    - identifies a mail forwarder for the domain

         MD    - identifies a mail destination for the domain

         NS    - the authoritative name server for the domain

         SOA   - identifies the start of a zone of authority

         CNAME - identifies the canonical name of an alias

      This memo uses the following classes in its examples:

         IN - the ARPA Internet system

         CS - the CSNET system

      The first type of resource record holds a host name to host
      address binding.  Its fields are:

  +--------+--------+--------+--------------//----------------------+
  |<owner> |   A    | <class>| <class specific address>information  |
  +--------+--------+--------+--------------//----------------------+





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RFC 882                                                    November 1983
                                  Domain Names - Concepts and Facilities


      The content of the class specific information varies according to
      the value in the CLASS field; for the ARPA Internet, it is the 32
      bit ARPA Internet address of the host, for the CSNET it might be
      the phone number of the host.  For example, F.ISI.ARPA might have
      two A records of the form:

       +----------+--------+--------+----------------------------+
       |F.ISI.ARPA|   A    |   IN   |          10.2.0.52         |
       +----------+--------+--------+----------------------------+
                                  and
       +----------+--------+--------+----------------------------+
       |F.ISI.ARPA|   A    |   CS   |         213-822-2112       |
       +----------+--------+--------+----------------------------+

      Note that the data formats for the A type are class dependent, and
      the Internet address and phone number formats shown above are for
      purposes of illustration only.  The actual data formats are
      specified in [14].  For example, CS class data for type A records
      might actually be a list of Internet addresses, phone numbers and
      TELENET addresses.

      The mail forwarder (MF) and mail delivery (MD) records have the
      following format:

        +--------+--------+--------+----------------------------+