Fwknop: Single Packet Authorization and Port Knocking. Fwknop: Single Packet Authorization > Port Knocking fwknop stands for the "FireWall KNock OPerator", and implements an authorization scheme called Single Packet Authorization (SPA).
This method of authorization is based around a default-drop packet filter (fwknop supports iptables and firewalld on Linux, ipfw on FreeBSD and Mac OS X, and PF on OpenBSD) and libpcap. SPA is essentially next generation port knocking (more on this below).
Howto. Programming. Case.