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Hide your secret SSH server (or whatever) and only open the port after the secret knock. This is a clever solution for private or login-required servers. [This article has been up a few months but is well worth a look. -Ed.]
Open ports remain a necessary vulnerability: they allow connections to applications but also may turn into open doors for attack. This article presents a new security system, termed port knocking, in which trusted users manipulate firewall rules by transmitting information across closed ports.
Briefly, users make connection attempts to sequences of closed ports. The failed connections are logged by the server-side packet filtering firewall and detected by a dæmon that monitors the firewall log file. When a properly formatted knock sequence, playing the role of the secret used in the authentication, is received, firewall rules are manipulated based on the information content of the sequence. This user-based authentication system is both robust, being mediated by the kernel firewall, and stealthy--it's not possible to detect whether a networked machine is listening for port knocks. Port knocking does not require any open ports, and it can be extended to transmit any type of information encoded in a port sequence.
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