Chapter 9. TCP/IP Firewall

Table of Contents
Methods of Attack
What Is a Firewall?
What Is IP Filtering?
Setting Up Linux for Firewalling
Three Ways We Can Do Filtering
Original IP Firewall (2.0 Kernels)
IP Firewall Chains (2.2 Kernels)
Netfilter and IP Tables (2.4 Kernels)
TOS Bit Manipulation
Testing a Firewall Configuration
A Sample Firewall Configuration

Security is increasingly important for companies and individuals alike. The Internet has provided them with a powerful tool to distribute information about themselves and obtain information from others, but it has also exposed them to dangers that they have previously been exempt from. Computer crime, information theft, and malicious damage are all potential dangers.

An unauthorized and unscrupulous person who gains access to a computer system may guess system passwords or exploit the bugs and idiosyncratic behavior of certain programs to obtain a working account on that machine. Once they are able to log in to the machine, they may have access to information that may be damaging, such as commercially sensitive information like marketing plans, new project details, or customer information databases. Damaging or modifying this type of data can cause severe setbacks to the company.

The safest way to avoid such widespread damage is to prevent unauthorized people from gaining network access to the machine. This is where firewalls come in.


Constructing secure firewalls is an art. It involves a good understanding of technology, but equally important, it requires an understanding of the philosophy behind firewall designs. We won't cover everything you need to know in this book; we strongly recommend you do some additional research before trusting any particular firewall design, including any we present here.

There is enough material on firewall configuration and design to fill a whole book, and indeed there are some good resources that you might like to read to expand your knowledge on the subject. Two of these are:

Building Internet Firewalls

by D. Chapman and E. Zwicky (O'Reilly). A guide explaining how to design and install firewalls for Unix, Linux, and Windows NT, and how to configure Internet services to work with the firewalls.

Firewalls and Internet Security

by W. Cheswick and S. Bellovin (Addison Wesley). This book covers the philosophy of firewall design and implementation.

We will focus on the Linux-specific technical issues in this chapter. Later we will present a sample firewall configuration that should serve as a useful starting point in your own configuration, but as with all security-related matters, trust no one. Double check the design, make sure you understand it, and then modify it to suit your requirements. To be safe, be sure.

Methods of Attack

As a network administrator, it is important that you understand the nature of potential attacks on computer security. We'll briefly describe the most important types of attacks so that you can better understand precisely what the Linux IP firewall will protect you against. You should do some additional reading to ensure that you are able to protect your network against other types of attacks. Here are some of the more important methods of attack and ways of protecting yourself against them:

Unauthorized access

This simply means that people who shouldn't use your computer services are able to connect and use them. For example, people outside your company might try to connect to your company accounting machine or to your NFS server.

There are various ways to avoid this attack by carefully specifying who can gain access through these services. You can prevent network access to all except the intended users.

Exploitation of known weaknesses in programs

Some programs and network services were not originally designed with strong security in mind and are inherently vulnerable to attack. The BSD remote services (rlogin, rexec, etc.) are an example.

The best way to protect yourself against this type of attack is to disable any vulnerable services or find alternatives. With Open Source, it is sometimes possible to repair the weaknesses in the software.

Denial of service

Denial of service attacks cause the service or program to cease functioning or prevent others from making use of the service or program. These may be performed at the network layer by sending carefully crafted and malicious datagrams that cause network connections to fail. They may also be performed at the application layer, where carefully crafted application commands are given to a program that cause it to become extremely busy or stop functioning.

Preventing suspicious network traffic from reaching your hosts and preventing suspicious program commands and requests are the best ways of minimizing the risk of a denial of service attack. It's useful to know the details of the attack method, so you should educate yourself about each new attack as it gets publicized.


This type of attack causes a host or application to mimic the actions of another. Typically the attacker pretends to be an innocent host by following IP addresses in network packets. For example, a well-documented exploit of the BSD rlogin service can use this method to mimic a TCP connection from another host by guessing TCP sequence numbers.

To protect against this type of attack, verify the authenticity of datagrams and commands. Prevent datagram routing with invalid source addresses. Introduce unpredictablility into connection control mechanisms, such as TCP sequence numbers and the allocation of dynamic port addresses.


This is the simplest type of attack. A host is configured to "listen" to and capture data not belonging to it. Carefully written eavesdropping programs can take usernames and passwords from user login network connections. Broadcast networks like Ethernet are especially vulnerable to this type of attack.

To protect against this type of threat, avoid use of broadcast network technologies and enforce the use of data encryption.

IP firewalling is very useful in preventing or reducing unauthorized access, network layer denial of service, and IP spoofing attacks. It not very useful in avoiding exploitation of weaknesses in network services or programs and eavesdropping.