Ubuntu Server Cookbook

Securing a network with uncomplicated firewall

It is said that the best way to improve server security is to reduce the attack surface. Network communication in any system happens with the help of logical network ports, be it TCP ports or UDP ports. One part of the attack surface is the number of open ports that are waiting for connection to be established. It is always a good idea to block all unrequired ports. Any traffic coming to these ports can be filtered, that is, allowed or blocked with the help of a filtering system.

The Linux kernel provides a built-in packet filtering mechanism called netfilter, which is used to filter the traffic coming in or going out of the system. All modern Linux firewall systems use netfilter under the hood. Iptables ...