FireWall KNock OPerator server side - C version

 The FireWall KNock OPerator implements an authorization scheme called
 Single Packet Authorization (SPA), based on Netfilter and libpcap.
 .
 Its main application is to protect services such as OpenSSH with
 an additional layer of security in order to make the exploitation of
 vulnerabilities (both 0-day and unpatched code) much more difficult.
 .
 The authorization server passively listens for authorization packets via
 libcap, thus preventing any connections from being processed on the
 traditional port. Access to a protected service is only granted after a
 valid encrypted and non-replayed packet is detected.