Port Knocking ¶

A niche practice mostly found during CTFs is called port knocking. Port knocking servers, such as knockd, while watch over every port and will run a command when ports are knocked in a given order.

For instance, if the sequence is 1111 5555 7777, we could knock using:

$ sudo apt install -y knockd
$ knock IP -v 1111 5555 7777

We would also use netcat:

$ nc IP 1111
$ nc IP 5555
$ nc IP 7777

📚 It could be used to only show some ports to a user after they knocked the ports following the secret sequence.