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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/