Read more https://packetstormsecurity.com/files/177010/fwknop-2.6.11.tar.gz
Fwknop Port Knocking Utility 2.6.11
fwknop implements an authorization scheme that requires only a
single encrypted packet to communicate various pieces of
information, including desired access through a Netfilter policy
and/or specific commands to execute on the target system. The main
application of this program is to protect services such as SSH with
an additional layer of security in order to make the exploitation
of vulnerabilities much more difficult. The authorization server
works by passively monitoring authorization packets via libpcap.
