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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