"By hiding out DNS server version number you can improve server
security. fpdns is a program that remotely determines DNS server
versions. It does this by sending a series of borderline DNS
queries which are compared against a table of responses and server
versions. (just like nmap command's remote OS detection
"A nameserver basically responds to a query. Interoperability is
an obvious requirement here. The standard protocol behavior of
different DNS implementations is expected to be the same..."