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DNS Implementation Technicalities

The DNS works as a ditributed database because of two fundamental ideas: replication and caching. We have already seen how caching works -- at any point in a query, if a nameserver has a current copy of the desired information, it can supply it instead of contacting other nameservers.
 
The DNS requires that all nameservers be replicated at least once -- that is, for each zone of authority there must be at least two authoritative nameservers -- a primary and one or more secondaries. The rules for replication are quite stringent -- at least one backup server must be located "off-site", attached to a different network, and with an independent power supply. So, for example, a secondary for sheoak.bendigo.latrobe.edu.au is luga.latrobe.edu.au, located at the Bundoora campus of La Trobe.
 
DNS queries and responses are an excellent example of an application where the reliable, connection-oriented transport mechanism of TCP is not necessary, and simply has too much overhead. In fact, queries are encapsulated in unreliable UDP datagrams, see later. If a resolver does not receive a reply from a nameserver, it usually either tries again, or tries the next nameserver for the same domain.
 

Extra infomation

Here's the definitive source of information on DNS. Telstra BigPond Direct has a good explanation of the various RR types in the DNS. Here's the bare bones of another lecture on DNS, with something of a Linux emphasis. The tutorial for this lecture is Tutorial #08.
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