This How-To provides the details for securing a Linux gateway box with the IPTABLES firewall. This guide can be used for Kernels ranging from 2.4-2.6. Special rules for running Squid in transparent mode and providing Network Address Translation (NAT) are also covered in this guide.
The tool IPTABLES talks to the kernel and tells it what packets to filter.
The IPTABLES application operates at a high level by filtering TCP and UDP protocols before the data is passed onto the user applications that can be corrupted.
The IPTABLES tool inserts and deletes rules from the kernel’s packet filtering table.
What this means is that the rules you create in your Linux machine using IPTABLES are lost upon reboot.
The best way to use IPTABLES rules are to store them up in a simple shell script and use your Linux OS to load that script on boot up.
