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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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Copyright © 2001 by
Philip Scott, La Trobe
University.
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