Install knockd by entering the following commands in the terminal:
sudo apt update sudo apt install knockd
Description:
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.
Homepage: http://www.zeroflux.org/projects/knock
Version: 0.7-1ubuntu1
Section: universe/net