Use UFW (Uncomplicated Firewall) to manage your firewall on Ubuntu, Debian, or Arch Linux; this guide contains instructions for setting up default rules, adding/removing rules, setting up logging, and some advanced features.
This guide will introduce you to FirewallD, its notions of zones and services, and show you some basic configuration steps.
Enhance your server's security through the use of TCP wrappers
Use iptables to manage Netfilter rules.
Protecting access to your Linode with the arno-iptables-firewall package.