knockd
A port-knocking server
It listens to all traffic on an ethernet (or PPP) interface, looking for special "knock" sequences of port-hits. A client (package knock) makes these port-hits by sending a TCP (or UDP) packet to a port on the server. This port need not be open -- since knockd listens at the link-layer level, it sees all traffic even if it's destined for a closed port. When the server detects a specific sequence of port-hits, it runs a command defined in its configuration file. This can be used to open up holes in a firewall for quick access.
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