Help File Library: Iptables Basics
Written By: Prince_KenshiI’m sure many of you have been wondering how to use iptables to set up a basic firewall. I was wondering the same thing for a long time until I recently figured it out. I’ll try to explain the basics to at least get you started.
First you need to know how the firewall treats packets leaving, entering, or passing through your computer. Basically there is a chain for each of these. Any packet entering your computer goes through the INPUT chain. Any packet that your computer sends out to the network goes through the OUTPUT chain. Any packet that your computer picks up on one network and sends to another goes through the FORWARD chain. The chains are half of the logic behind iptables themselves.
Now the way that iptables works is that you set up certain rules in each of these chains that decide what happens to packets of data that pass through them. For instance, if your computer was to send out a packet to www.yahoo.com to request an HTML page, it would first pass through the OUTPUT chain. The kernel would look through the rules in the chain and see if any of them match. The first one that matches will decide the outcome of that packet. If none of the rules match, then the policy of the whole chain will be the final decision maker. Then whatever reply Yahoo! sent back would pass through the INPUT chain. It’s no more complicated than that.