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servers and the corresponding responses have a standard format described <BR> <BR> <BR> <BR>Mockapetris [Page 4] <BR> <BR>RFC 1035 Domain Implementation and Specification November 1987 <BR> <BR> <BR>in this memo, and may be datagrams. <BR> <BR>Depending on its capabilities, a name server could be a stand alone <BR>program on a dedicated machine or a process or processes on a large <BR>timeshared host. A simple configuration might be: <BR> <BR> Local Host | Foreign <BR> | <BR> +---------+ | <BR> / /| | <BR> +---------+ | +----------+ | +--------+ <BR> | | | | |responses| | | <BR> | | | | Name |---------|->|Foreign | <BR> | Master |-------------->| Server | | |Resolver| <BR> | files | | | |<--------|--| | <BR> | |/ | | queries | +--------+ <BR> +---------+ +----------+ | <BR> <BR>Here a primary name server acquires information about one or more zones <BR>by reading master files from its local file system, and answers queries <BR>about those zones that arrive from foreign resolvers. <BR> <BR>The DNS requires that all zones be redundantly supported by more than <BR>one name server. Designated secondary servers can acquire zones and <BR>check for updates from the primary server using the zone transfer <BR>protocol of the DNS. This configuration is shown below: <BR> <BR> Local Host | Foreign <BR> | <BR> +---------+ | <BR> / /| | <BR> +---------+ | +----------+ | +--------+ <BR> | | | | |responses| | | <BR> | | | | Name |---------|->|Foreign | <BR> | Master |-------------->| Server | | |Resolver| <BR> | files | | | |<--------|--| | <BR> | |/ | | queries | +--------+ <BR> +---------+ +----------+ | <BR> A |maintenance | +--------+ <BR> | +------------|->| | <BR> | queries | |Foreign | <BR> | | | Name | <BR> +------------------|--| Server | <BR> maintenance responses | +--------+ <BR> <BR>In this configuration, the name server periodically establishes a <BR>virtual circuit to a foreign name server to acquire a copy of a zone or <BR>to check that an existing copy has not changed. The messages sent for <BR> <BR> <BR> <BR>Mockapetris [Page 5] <BR> <BR>RFC 1035 Domain Implementation and Specification November 1987 <BR> <BR> <BR>these maintenance activities follow the same form as queries and <BR>responses, but the message sequences are somewhat different. <BR> <BR>The information flow in a host that supports all aspects of the domain <BR>name system is shown below: <BR> <BR> Local Host | Foreign <BR> | <BR> +---------+ +----------+ | +--------+ <BR> | | user queries | |queries | | | <BR>
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