Port knocking is an authentication system that allows a server to keep ports closed by default, and open them up only when clients send a pre-determined sequence of connection requests aimed at particular TCP or UDP ports. as a result, you can, for example, keep SSH both invisible and inaccessible to passersby, but still allow clients armed with the secret knock to connect. setting up port knocking on your Linux system is easy, but make sure you set aside time to familiarize yourself with the security and practical risks.

More information at: Linux.com