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A caching-only server saves data in a cache file until the data expires. Expiration occurs based on a ``time-to-live'' field attached to data received from another server.
A caching-only server answers data from its cache if it has the information, or requests it from authoritative servers if it does not.
Caching-only servers may make use of forwarders, servers with connections to outside networks that build up a large database of data. In this case, the caching-only server first queries a forwarder (rather than following its default resolution process), which in turn queries another server, if necessary, to find the desired data. This limits traffic outside the zone to traffic to and from the forwarders. If the forwarder does not answer, the caching-only server tries the primary server for its zone.
Root servers are used by the caching-only server to help find other zones.
See also: