Linux operating systems are very popular within the Cyber Security competition space for several reason, the foremost likely being that its free. Nonetheless, this abundance often calls for competitors to have at least a basic understanding of how to defend Linux systems. Arguable the first step in defending a Linux system is strong passwords, but a strong firewall is defiantly the second.

First of all, make a bash script for your firewall configuration, run it often (I like rc.local, cron, and init), and hide it well (I like a places I would frequent as a defender, but not as an attacker, like /etc/apache2/sites-available). This iptables script should at the very least include an IN and OUT filter for ssh and the port(s) core service(s). A basic script might look like the following.

#!/bin/bash
iptables -F #clear the table
#iptables tricks
iptables -A INPUT -i lo -j ACCEPT #accept connections on our loop back
iptables -A INPUT -p tcp --tcp-flags ALL NONE -j DROP #drop bull packets
iptables -A INPUT -p tcp ! --syn -m state --state NEW -j DROP #drop syn floods (couple with syn.cookies)
iptables -A INPUT -p tcp --tcp-flags ALL ALL -j DROP #drop xmas packets cause this isn't the 90s
iptables -A INPUT -p tcp --dport 80,443 -m limit --limit 25/minute --limit-burst 100 -j ACCEPT #limit incoming connections
#IN Chain
iptables -A INPUT -p tcp --dport 22 -m state --state NEW,ESTABLISHED -j ACCEPT #open ssh
iptables -A INPUT -p tcp --dport 80 -m state --state NEW,ESTABLISHED -j ACCEPT #open http
iptables -A INPUT -p tcp --dport 443 -m state --state NEW,ESTABLISHED -j ACCEPT #open https
#OUT Chain
iptables -A OUTPUT -p tcp --sport 22 -m state --state ESTABLISHED -j ACCEPT #allow ssh out
iptables -A OUTPUT -p tcp --sport 80 -m state --state ESTABLISHED -j ACCEPT #allow http out
iptables -A OUTPUT -p tcp --sport 443 -m state --state ESTABLISHED -j ACCEPT #allow https out
iptables -P OUTPUT DROP #drop other out
iptables -P INPUT DROP #drop other in

Allow this is likely not all the rules one might require throughout a competition, I’ve found its a good place to start. I hope to add more examples and useful rules as I learn of them in my career. If anyone has any questions feel free to hit me up.