iptables

A firewall is a system or group of systems that establishes access control policies between two networks. In principle, a firewall can be thought of as a pair of mechanisms: the first blocks network traffic, the second allows network traffic. A firewall can be used to protect a network from external attacks, but a firewall cannot protect against attacks that don’t go through the firewall and attacks from someone on the same network, and a firewall cannot protect a network from poorly written application programs....