Chapter 13.

Firewalls and

iptables

Linux comes with advanced tools for packet filtering   the process of controlling network packets

as they enter, move through, and exit the network stack within the kernel. Pre 2.4 kernels relied on

ipchains

for packet filtering and used lists of rules applied to packets at each step of the filtering

process. The introduction of the 2.4 kernel brought with it

iptables

(also called netfilter), which

is similar to

ipchains

but greatly expands on the scope and control available for filtering network

packets.

This chapter focuses on packet filtering basics, defines the differences between

ipchains

and

ipt 

ables

, explains various options available with

iptables

commands, and shows how filtering rules

can be preserved between system reboots.

If you require instructions for constructing

iptables

rules or setting up a firewall based on these

rules, please see Section 13.5 for more information.

Warning

The default firewall mechanism under the 2.4 kernel is iptables, but iptables cannot be used if

ipchains are already running. If ipchains are present at boot time, the kernel will issue an error

and fail to start iptables.

The functionality of ipchains is not affected by these errors.

13.1. Packet Filtering

Traffic moves through a network in packets. A network packet is collection of data in a specific size

and format. In order to transmit a file over a network, the sending computer must first break the file

into packets using the rules of the network protocol. Each of these packets holds a small part of the

file data. Upon receiving the transmission, the target computer reassembles the packets into the file.

Every packet contains information which helps it navigate the network and move toward its destina 

tion. The packet can tell computers along the way, as well as the destination machine, where it came

from, where it is going, and what type of packet it is, among other things. Most packets are designed to

carry data, although some protocols use packets in special ways. For example, the Transmission Con 

trol Protocol (TCP) uses a SYN packet, which contains no data, to initiate communication between

two systems.

The Linux kernel contains the built in ability to filter packets, allowing some of them into the system

while stopping others. The 2.4 kernel's netfilter has three built in tables or rules lists. They are as

follows:

  filter

  This is the default table for handling network packets.

  nat

  This table used to alter packets that create a new connection.

  mangle

  This table is used for specific types of packet alteration.

Each of these tables in turn have a group of built in chains which correspond to the actions performed

on the packet by the netfilter.

The built in chains for the

filter

table are as follows:

INPUT   This chain applies to packets received via a network interface.