I would recommend to:
- google for some firewall examples using iptables
- have 'man iptables' at hand
- write your own firewall (no joke!)
Otherwise, you will never know, what your firewall is
really doing.
(iptables look a little bit confusing in the beginning; but there are
many examples for firewalls on the web which help a lot in the
beginning; you can just build your own firewall by extending resp.
modifying an existing example.)
