By default, iptables rules disappear after a reboot. Use iptables-save or tools like iptables-persistent to keep them active. [Free Download] The Ultimate iptables Cheat Sheet

Here are some basic iptables commands:

iptables -A INPUT -m state --state INVALID -j DROP

iptables -A INPUT -p tcp --syn -m limit --limit 1/second --limit-burst 3 -j ACCEPT

Iptables might seem intimidating at first, but it is one of the most powerful tools in a Linux admin's toolkit. By implementing a "Default Drop" policy and carefully managing your ports, you can significantly reduce your server's attack surface.

Controls incoming traffic to your server (e.g., someone trying to SSH into your box).