May 26, 2018
Flexible port-knocking server and client
knockd is a port-knock server. It listens to all traffic on an ethernet or PPP interface, looking for special “knock” sequences of port-hits.
A client 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 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.