Debian/Ubuntu: Configure knockd on Server for Port Knocking
When I was young, my mother asked me not to open the door unless the door knocked in a series of four which was only known to us. I think you all have done in their childhood or later.
That same concept I want to use in my Linux system, you are pondering what he is speaking? How and where will be a door on my system, and how to knock? I do not understand it; explain to me this abstract concept.