Get started learning about Iptables Chains, how to add/update/delete iptables rules, and start with a good basic set of rules for any server.
By default Iptables rules will not survive through a server reboot! Find out how to accomplish saving your firewall rules (along with some handy commands) on Debian/Ubuntu and CentOS/RedHat servers.
Iptables chains typically default to ACCEPT traffic. We'll see how to default to DROP traffic instead, and then we'll see how Iptables chains can work together to help protect your system. We'll install Fail2Ban to monitor authentication attempts, which makes use of Iptables chains.
^ Ad space to help offset hosting costs :D