You all know these old gangster films where a guy uses a knock sequence on a door to get in? Port Knocking is exactly that, just for your server. Installing port knocking on Ubuntu is easy. I will show you in this article how to install and set up port knocking. The steps from this tutorials should work for Debian 8 as well.
Original URL: https://www.howtoforge.com/tutorial/how-to-use-port-knocking-to-hide-the-ssh-port-from-attackers-on-ubuntu/