configuring a firewall?

You must consider this major points when using ipchains in Linux (firewall):

Source address
Destination address
Port type (tcp/udp)
Port number
For more information read the article here at techrepublic (Linux) for using ipchains.

There are too many options and/or considerations to list them all in the limited space of this thread.

But here is a great link that will help you determine what is best for you.

http://www.firewallguide.com/

Good luck,

Max