fwknop stands for the FireWall KNock OPerator, and implements an authorization scheme called Single Packet Authorization (SPA) that based around Netfilter and libpcap.
SPA requires only a single encrypted packet in order to communicate various pieces of information including desired access through a Netfilter policy and/or complete commands to execute on the target system. By using Netfilter to maintain a “default drop” stance, the main application of this program 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 monitors authorization packets via libcap and hence there is no “server” to which to connect in the traditional sense. Access to a protected service is only granted after a valid encrypted and non-replayed packet is monitored.