Thanks 2 http://www.cipherdyne.org/fwknop/
I found it on cnsea
fwknop stands for "Firewall Knock Operator" and is an upcoming piece of software that will be released at the DEFCON 12 conference in July, 2004 in Las Vegas.
fwknop implements network access controls (via iptables) based on a flexible port knocking mini-language, but with a twist; it combines port knocking and passive operating system fingerprinting to make it possible to do things like only allow, say, Linux-2.4/2.6 systems to connect to your SSH daemon.
fwknop supports shared, multi-protocol port knock sequences along with both relative and absolute timeouts, and coded port knock sequences encrypted with the Rijndael block cipher.
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