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fpdns - Fingerprinting DNS servers

A nameserver basically responds to a query. Interoperability is an
obvious requirement here. The standard protocol behaviour of different
DNS implementations is expected to be the same.

Requirements for protocol behaviour of DNS implementations is widely
documented in the case of 'common' dns messages. The DNS protocol
is over 20 years old and since its inception, there have been over
40 independent DNS implementations, while some implementations have
over 20 versions.

The methodology used to identify individual nameserver implementations
is based on "borderline" protocol behaviour. The DNS protocol offers
a multitude of message bits, response types, opcodes, classes, query
types and label types in a fashion that makes some mutually exclusive
while some are not used in a query messages at all. Not every
implementation offers the full set of features the DNS protocol set
currently has. Some implementations offer features outside the
protocol set, and there are implementations that do not conform to
standards.

WWW: http://www.rfc.se/fpdns/
Author: Roy Arends & Jakob Schlyter