Besides changing the default port for SSH, and using a key pair for authentication, port knocking can be used to further secure (or more accurately, obscure) your SSH server. It works by refusing connections to your SSH network port. This essentially hides the fact that you're running an SSH server until a sequence of connection attempts are made to predefined ports. Very secure and simple to implement, port knocking is one of the best ways to protect your server from malicious SSH connection at......