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.

Homepage:
https://github.com/mrash/fwknop/
