+----------+         |            
                                | 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.









 

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.




 

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   
                                                     |