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.