Introduction

This book is essentially about creating a software-based firewall using Netfilter and iptables in the Linux operating system. Beyond the basics of a firewall, this book also looks at the firewall in the context of a networked computing environment. To that end, topics such as intrusion detection and system security are also covered.

Computer security is an expansive subject area. Volumes have been written about it and volumes will continue to be written about it. Computer security is centered around protection of data assets using three principles: confidentiality, integrity, and availability. Confidentiality means that data is accessible only by those who are authorized to access the data and no one else. Integrity ensures that the data is verifiably good and is not tainted. Availability means that the data can be accessed when it needs to be accessed. These three principles guide the discussion of computer security and provide the framework for this book.

In addition to the three principles of confidentiality, integrity, and availability, I subscribe to an in-depth, risk-assessed approach to computer security. This means that I don't consider any single option to be an endpoint when it comes to securing data, rather that each item such as a firewall or antivirus software plays a role in securing data. However, there is a cost involved with each measure of security. Therefore, each additional measure or layer of security must be assessed to ensure that the cost of that layer doesn't exceed the benefit of being protected from that risk.

Consider this example: I use two firewalls, a choke and gateway (see Chapter 6, "Packet Forwarding"), for my home network. I consider the benefit of having a dual-firewall approach to outweigh the cost of operating and maintaining the firewalls. Other people use a single firewall or no firewall at all. They consider the risk of their data or systems being unavailable or attacked to be less costly than running a dual-firewall setup or even a single firewall for some. Many more examples of this cost/benefit assessment could be done. Unfortunately, this analysis is often overlooked for many areas of security, not just computer security. For more information on this type of analysis and a good read on top of it, see Bruce Schneier's works Secrets and Lies and Beyond Fear.

The Purpose of This Book

The goal of this book is to give the reader enough information that they may configure a firewall using iptables in Linux. A secondary goal is to educate the reader about system and network security. However, because this isn't a book on system and network security, those topics are indeed secondary even though they do consume a large portion of the book. There are also topics in this book that I haven't seen (yet) in other books to any great degree.

You are reading the third revision of this book and the first revision with a new author, Steve Suehring. Bob Ziegler wrote the original material and also revised the work into its second revision in 2001. Bob did an excellent job and I've built upon his solid foundation to bring you the third revision. In addition, the previous revision had some material contributed by Carl B. Constantine. You'll find Carl's contribution, though updated, in Appendix C of this revision, "VPNs."

I learned much of what I know about Linux security while working at an Internet service provider (ISP), beginning in 1995. Resisting the temptation to recite a "back when I was young" tale, I'll just say that most of what I learned was done with security in mind. It had to be. By definition at an ISP, you must run publicly available services and those services must be available 24x7. Having publicly available services means that there's a constant threat (and frequent execution) of attacks against the network and the systems therein. If we wouldn't have considered security to be central to our operation, we simply could not have ensured the reliability that our customers demanded, nor could we have guaranteed the integrity of the data that we housed. None of this takes into account the general lack of security tools, software, and books like this back in 1995, either.

That background also helps to answer the question "Why Linux?" The answer was and is quite simple: Linux and open-source tools were the only solution when I was tasked with solving these problems. There simply was no other way to provide Internet services with anywhere near the reliability that Linux and open-source software provided. No other operating system provided the same set of reliability and security while at the same time keeping down the Total Cost of Ownership (TCO). The same can largely be said today. With a pure technological decision, Linux wins. Factor in TCO and the picture only gets better for Linux and open-source software, regardless of the results from funded and paid studies. Why Linux? Because it works.

Who Should Read This Book

I've usually found these "Who Should Read This Book" sections to be somewhat useless simply because the goal is to get you to think that you should read the book. Therefore, to satisfy the publisher I'll tell you that everyone should read this book. In fact, everyone should read this book multiple times, buying a separate copy each time.

In all seriousness, I can't tell you whether you should be reading this book, but I can tell you about the book.

This book assumes that you have already chosen a Linux distribution and that you've already installed it. This book also assumes that you're not looking for an introductory "HOWTO" on Linux or *nix security such as the chmod command. There are many great resources about those topics already, many of them on the Internet, and I feel as though coverage of those issues gets away from the focus of this book. However, this book does deal rather extensively with introductory material on network security, packet filtering, and the layers in the OSI model (if you're unfamiliar with the OSI model, it's explained in the book).

This book tries to be helpful to those who know nothing about firewalls as well as to those who know a bit about Linux and Linux security but want to carry that to the next level. This book could be used successfully by home users and enterprise security administrators alike.

To get the most out of this book, you should be comfortable with, or at least not afraid of, the Linux command line, or shell. You should know how to move about in the file system and perform basic shell commands.

Linux Distribution

Linux and open-source books need to be more distribution neutral or cover more than one distribution. This book does both. A Linux firewall is built using the iptables firewall administration program on top of the Netfilter core software that resides in the Linux kernel. As such, the Linux distribution you choose is largely irrelevant. The book does, however, cover some commands and issues as seen through the eyes of SUSE, Red Hat/Fedora, and Debian. Yes, there are other distributions, many of them very good. Favoring those three distributions is certainly not meant to take away from any other distribution.

The second edition of this book covered only Red Hat. However, I undertook an effort early on in the revision process to remove the distribution-centric tone where it did show up. This was not done to intentionally favor any one distribution or to reject another. Rather, this was a pragmatic decision to provide material applicable to a larger audience and to prevent confusion as to file and command locations if you don't happen to be using the same distribution as the author.

Errors in This Book

Although every effort is made to check facts and figures, files and syntax, some errors will inevitably slip through the writing, technical editing, copyediting, and review process. Let me apologize in advance for any such errors as exist within these pages. I invite the reader to visit my web site at http://www.braingia.org/ for updates and other information about this book. I also invite you to send me feedback at steve.suehring@braingia.com. Although I can't guarantee that I'll have the answer, I will definitely try to respond and point you in the right direction.

Companion Website

Visit http://www.braingia.org/ for up-to-date information on this book and links to interesting security articles. Included on the website are the latest versions of some of the same scripts you'll see within the text.

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