Weekend Project: Secure Your System with Port Knocking

"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.

"Server-side setup

"The most popular port knocking package on Linux system is knockd. It works in concert with Linux's iptables firewall, running as a daemon that listens on a network interface for a pre-configured sequence of connection requests. When a matching sequence is detected, knockd executes an associated command from its configuration file — canonically, an iptables command that either opens or closes a particular port on which a service is already running."

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• An Easy Tutorial on IP Tables and Port Knocking(Feb 05, 2008)