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

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

Catchall rules state that anything which is not permitted is forbidden. They must be allowed last.

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