Website: http://cipherdyne.org/fwknop
From the FAQ:
fwknop implements an authorization scheme known as Single Packet Authorization (SPA) that requires only a single encrypted packet to communicate various pieces of information including desired access through a Netfilter policy and/or specific commands to execute on the target system. The main application of this program is to protect services such as SSH with an additional layer of security in order to make the exploitation of vulnerabilities (both 0-day and unpatched code) much more difficult. The authorization server passively monitors authorization packets via libpcap and hence there is no "server" to which to connect in the traditional sense. Any service protected by fwknop is inaccessible (by using Netfilter to intercept packets within the Linux kernel) before authenticating; anyone scanning for the service will not be able to detect that it is even listening. This authorization scheme offers many advantages over port knocking, include being non-replayable, much more data can be communicated, and the scheme cannot be broken by simply connecting to extraneous ports on the server in an effort to break knock sequences. The authorization packets can easily be spoofed as well, and this makes it possible to make it appear as though, say, www.yahoo.com is trying to authenticate to a target system but in reality the actual connection will come from a seemingly unrelated IP. Although the default data collection method is to use libpcap to sniff packets off the wire, fwknop can also read packets out of a file that is written by the Netfilter ulogd pcap writer.
One more thing. You don't allow any service in iptables like ssh, ...
fwknopd inserts the appropriate iptables rules when you send the SPA to it with the port you want to allow, temporarily, for you to connect.
