# Intro This is a basic Linux firewall program. Look at your firewalls: > iptables -L We see the output of input, output and forwarding rules. # Forward I don't need any forwarding, so I'm going to drop all forwarding: > iptables -P FORWARD DROP # Input Let's 'A'dd, or 'A'ppend a rule with -A. Let's drop all input from a nearby IP > iptables -A INPUT -s 192.168.0.23 -j DROP Or we can block all input from a particular port on the full network. > iptables -A INPUT -s 192.168.0.0/24 -p tcp --destination-port 25 -j DROP > iptables -A INPUT --dport 80 -j ACCEPT This allows http traffic to an Apache web server over port 80. However, rules are accepted in order - so a packet cannot be rejected and then accepted. To delete rule 2 from the INPUT chain: > iptables -D INPUT 3 Alternatively, you can 'I'nsert a rule at the start, rather than 'A'ppending it. > iptables -I INPUT -s 192.168.0.13 DROP # Catchalls Catchall rules state that anything which is not permitted is forbidden. They must be allowed last. # -Jurice-Diction The -j flag accepts ACCEPT/REJECT/DROP. The last two are identical except that "REJECT" acknowledges the rejection. Flush all existing rules with: > iptables -F