fwknop: Single Packet Authorization and Port Knocking , Linux Firewall

CipherDyne released fwknop-2.0 ,fully written in C and functions on embedded systems, OpenBSD, and more.

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 both iptables on Linux systems and ipfw on FreeBSD and Mac OS X systems) and libpcap. SPA is essentially next generation port knocking .

Download it from here:

http://www.cipherdyne.org/fwknop/