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DNS Servers and Resolvers
A nameserver provides domain-name-to-IP-address
mappings (and a few other functions, but "looking up" IP addresses
is the most common) for one or more zones, which
are sub-trees of the domain name space. For example,
sheoak is a nameserver for the zone
bendigo.latrobe.edu.au. This means that if I want to
look up a particular IP address in that zone, I can ask
sheoak.
Exactly which server is responsible for a particular zone is
specified in start of authority (SOA) RRs. An SOA
RR specifies, for the particular name server, the zones for which
it has authority. It also has the email address of the site
administrator, a unique serial number and various
other bits and pieces.
The DNS system forms a distributed database of
domain information.
A resolver is a library function[1] which queries the nameserver when called
from a user program. It can check the local cache of names and, if
necessary, request a RR from a nameserver (privately caching the
response). In other words, a resolver is software which asks a
nameserver for information.
[1] Such as is
implemented in the Unix library function
gethostbyname(3).
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