Network Security at the Kernel Level

iptables allows you to define rules for what traffic can enter or leave your server.

Commands:

• List rules: sudo iptables -L -n -v.
• Allow established connections: sudo iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT.
• Block everything else (Default DROP policy): sudo iptables -P INPUT DROP.

Warning: Ensure you have allowed your SSH port before setting the default policy to DROP, or you will be locked out!