Fwknop implements an authorization scheme known as Single Packet Authorization
(SPA) for Linux systems running Firewalld or Iptables. This mechanism requires
only a single encrypted and non-replayed packet to communicate various pieces
of information including desired access through a Firewalld or Iptables policy.
The main application of this program is to use Firewalld or Iptables in a
default-drop stance to protect services such as SSH with an additional layer of
security in order to make the exploitation of vulnerabilities (both 0-day and
not patched code) much more difficult.