Binary package “knockd” in ubuntu focal

small port-knock daemon

 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.