Published at LXer:

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. Yes, I’m going to explain & implement all in a while with an exact scenario. After that, you can also configure knockd on the server and protect your system from intruders.

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