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). The design decisions that guide the development of fwknop can be found in the blog post "Single Packet...