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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 (caching the response). In other words, a resolver is software which asks a nameserver for information.
 
[1] Such as is built-in to the Unix library function gethostbyname(3).
Lecture 9: The Domain Name System (DNS) Copyright © 2003 P.Scott, La Trobe University Bendigo.

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