knockd - 0.8-2+b6 main

A port-knock server that listens to all traffic on a given network
interface (only Ethernet and PPP are currently supported), looking for
a special "knock" sequences of port-hits. A remote system
makes these port-hits by sending a TCP (or UDP) packet to a port on the
server. 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.