Network firewall is system which limits network access to and from a network. The job of firewall is similar to that of physical firewall that separates prevents the spreading of fire between buildings, substation transformers, or within an aircraft.They are classified into different types according to their protection mechanism. The classification of firewall helps to select the appropriate one depending on the security requirements of an organization. The various classifications are as follows :
- Packet Filtering Firewall
- Stateful Packet Inspection Firewall
- Application-Level Proxy
Linux is one of the well known open source operating system. The latest stable Linux Kernel is 3.0.1. The kernel contains a framework known as netfilter that handles the processing and manipulation of network packets within the Linux kernel. The netfilter framework is embedded within the Linux 2.4.x and 2.6.x kernels. Netfilter/iptables framework it is composed of two components. One component is kernel space known as netfilter which is the core element and the other component is user space known as iptables. It is a tool which makes easy way to add, edit and delete rules in the tables built inside the core.
More information about netfilter iptables can be found at http://www.netfilter.org/