Preface

This book is a practical guide to building your own firewall. It provides step-by-step explanations of how to design and install a firewall at your site and how to configure Internet services such as electronic mail, FTP, the World Wide Web, and others to work with a firewall. Firewalls are complex, though, and we can't boil everything down to simple rules. Too much depends on exactly what hardware, operating system, and networking you are using at your site, and what you want your users to be able to do and not do. We've tried to give you enough rules, examples, and resources here so you'll be able to do the rest on your own.

What is a firewall, and what does it do for you? A firewall is a way to restrict access between the Internet and your internal network. You typically install a firewall at the point of maximum leverage, the point where your network connects to the Internet. The existence of a firewall at your site can greatly reduce the odds that outside attackers will penetrate your internal systems and networks. The firewall can also keep your own users from compromising your systems by sending dangerous information -- unencrypted passwords and sensitive data -- to the outside world.

The attacks on Internet-connected systems we are seeing today are more serious and more technically complex than those in the past. To keep these attacks from compromising our systems, we need all the help we can get. Firewalls are a highly effective way of protecting sites from these attacks. For that reason, we strongly recommend you include a firewall in your site's overall Internet security plan. However, a firewall should be only one component in that plan. It's also vital that you establish a security policy, that you implement strong host security, and that you consider the use of authentication and encryption devices that work with the firewalls you install. This book will touch on each of these topics while maintaining its focus on firewalls.

0.1. Scope of This Book

This book is divided into five parts.

Part I, "Network Security", explores the problem of Internet security and focuses on firewalls as part of an effective strategy to address that problem.

Chapter 1, "Why Internet Firewalls?", introduces the major risks associated with using the Internet today; discusses what to protect, and what to protect against; discusses various security models; and introduces firewalls in the context of what they can and can't do for your site's security.
Chapter 2, "Internet Services", outlines the services users want and need from the Internet, and summarizes the security problems posed by those services.
Chapter 3, "Security Strategies", outlines the basic security principles an organization needs to understand before it adopts a security policy and invests in specific security mechanisms.

Part II, "Building Firewalls", describes how to build firewalls.

Chapter 4, "Packets and Protocols ", describes the basic network concepts firewalls work with.
Chapter 5, "Firewall Technologies", explains the terms and technologies used in building firewalls.
Chapter 6, "Firewall Architectures", describes the major architectures used in constructing firewalls, and the situations they are best suited to.
Chapter 7, "Firewall Design", presents the process of designing a firewall.
Chapter 8, "Packet Filtering" describes how packet filtering systems work, and discusses what you can and can't accomplish with them in building a firewall.
Chapter 9, "Proxy Systems", describes how proxy clients and servers work, and how to use these systems in building a firewall.
Chapter 10, "Bastion Hosts", presents a general overview of the process of designing and building the bastion hosts used in many firewall configurations.
Chapter 11, "Unix and Linux Bastion Hosts", presents the details of designing and building a Unix or Linux bastion host.
Chapter 12, "Windows NT and Windows 2000 Bastion Hosts ", presents the details of designing and building a Windows NT bastion host.

Part III, "Internet Services", describes how to configure services in the firewall environment.

Chapter 13, "Internet Services and Firewalls", describes the general issues involved in selecting and configuring services in the firewall environment.
Chapter 14, "Intermediary Protocols", discusses basic protocols that are used by multiple services.
Chapter 15, "The World Wide Web", discusses the Web and related services.
Chapter 16, "Electronic Mail and News", discusses services used for transferring electronic mail and Usenet news.
Chapter 17, "File Transfer, File Sharing, and Printing", discusses the services used for moving files from one place to another.
Chapter 18, "Remote Access to Hosts", discusses services that allow you to use one computer from another computer.
Chapter 19, "Real-Time Conferencing Services", discusses services that allow people to interact with each other online.
Chapter 20, "Naming and Directory Services", discusses the services used to distribute information about hosts and users.
Chapter 21, "Authentication and Auditing Services", discusses services used to identify users before they get access to resources, to keep track of what sort of access they should have, and to keep records of who accessed what and when.
Chapter 22, "Administrative Services", discusses other services used to administer machines and networks.
Chapter 23, "Databases and Games", discusses the remaining two major classes of popular Internet services, databases and games.
Chapter 24, "Two Sample Firewalls", presents two sample configurations for basic firewalls.

Part IV, "Keeping Your Site Secure", describes how to establish a security policy for your site, maintain your firewall, and handle the security problems that may occur with even the most effective firewalls.

Chapter 25, "Security Policies", discusses the importance of having a clear and well-understood security policy for your site, and what that policy should and should not contain. It also discusses ways of getting management and users to accept the policy.
Chapter 26, "Maintaining Firewalls", describes how to maintain security at your firewall over time and how to keep yourself aware of new Internet security threats and technologies.
Chapter 27, "Responding to Security Incidents", describes what to do when a break-in occurs, or when you suspect that your security is being breached.

Part V, "Appendixes", consists of the following summary appendixes:

Appendix A, "Resources", contains a list of places you can go for further information and help with Internet security: World Wide Web pages, FTP sites, mailing lists, newsgroups, response teams, books, papers, and conferences.
Appendix B, "Tools", summarizes the best freely available firewall tools and how to get them.
Appendix C, "Cryptography", contains background information on cryptography that is useful to anyone trying to decrypt the marketing materials for security products.