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Firewalls and Internet Security: Repelling The Wily Hacker

Firewalls and Internet Security: Repelling The Wily Hacker

by William R. Cheswick, Steven M. Bellovin
- Cliff Stoll, author of The Cuckoo's Egg: Tracking a Spy Through the Maze of Computer Espionage

As a user of the Internet, you are fortunate to be tied into the world's greatest communication and information exchange - but not without a price. As a result of this connection, your computer, your organization's network, and everywhere that network


- Cliff Stoll, author of The Cuckoo's Egg: Tracking a Spy Through the Maze of Computer Espionage

As a user of the Internet, you are fortunate to be tied into the world's greatest communication and information exchange - but not without a price. As a result of this connection, your computer, your organization's network, and everywhere that network reaches are all vulnerable to potentially disastrous infiltration by hackers.

Written by the AT&T Bell Labs researchers who tracked the infamous "Berferd" hacker and also built the firewall gateway at Bell Labs, Firewalls and Internet Security gives you invaluable advice and practical tools for protecting your organization's computers from the very real threat of a hacker attack through the Internet. You will learn how to plan and execute a security strategy that will thwart the most determined and sophisticated of hackers - while still allowing you easy access to Internet services.

In particular, the authors show you a step-by-step plan for setting up a "firewall" gateway - a dedicated computer equipped with safeguards that acts as a single, more easily defended, Internet connection. They even include a description of their most recent gateway, the tools they used to build it, and the hacker attacks they devised to test it.

You will be fascinated by their first-hand account of one of the first documented hacker attacks, the "Berferd" case, in which Internet hackers created havoc for computer networks worldwide. In addition, you will find vital information on cryptography, a description of the tools used by hackers, and the legal implications of computer security.

With this book inhand, you will be well equipped to provide your organization with effective protection from the wily Internet hacker.

Editorial Reviews

The AT&T Bell Labs researchers who tracked the "Berferd" hacker and built the firewall gateway at Bell Labs, give advice and practical tools for protecting computers from the very real threat of a hacker attack through the Internet. They describe how to plan and execute a security strategy that will thwart the most determined and sophisticated hackers, while still allowing easy access to Internet services. A step-by-step plan for setting up a firewall gateway is described, as well as information on cryptography, tools used by hackers, and legal implications. Annotation c. Book News, Inc., Portland, OR (booknews.com)

Product Details

Addison Wesley Professional
Publication date:
Professional Computing Series
Edition description:
Older Edition
Product dimensions:
7.30(w) x 9.12(h) x 0.80(d)

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Read an Excerpt


It is easy to run a secure computer system. You merely have to disconnect all dial-up connections and permit only direct-wired terminals, put the machine and its terminals in a shielded room, and post a guard at the door.
- F.T. Grampp and R.H. Morris

For better or for worse, most computer systems are not run that way today. Security is, in general, a trade-off with convenience, and most people are not willing to forgo the convenience of remote access via networks to their computers. Inevitably, they suffer from some loss of security. It is our purpose here to discuss how to minimize the extent of that loss.

The situation is even worse for computers hooked up to some sort of network. Networks are risky for at least three major reasons. First, and most obvious, more points now exist from which an attack can be launched. Someone who cannot get to your computer cannot attack it; by adding more connection mechanisms for legitimate users, you are also adding more vulnerabilities.

A second reason is that you have extended the physical perimeter of your computer system. In a simple computer, everything is within one box. The CPU can fetch authentication data from memory, secure in the knowledge that no enemy can tamper with it or spy on it. Traditional mechanisms - mode bits, memory protection, and the like - can safeguard critical areas. This is not the case in a network. Messages received may be of uncertain provenance; messages sent are often exposed to all other systems on the net. Clearly, more caution is needed.

The third reason is more subtle, and deals with an essential distinction between an ordinary dial-up modem and a network.Modems, in general, offer one service, typically the ability to log in. When you connect, you're greeted with a login or Username prompt; the ability to do other things, such as sending mail, is mediated through this single choke point. There may be vulnerabilities in the login service, but it is a single service, and a comparatively simple one. Networked computers, on the other hand, offer many services: login, file transfer, disk access, remote execution, phone book, system status, etc. Thus, more points are in need of protection - points that are more complex and more difficult to protect. A networked file system, for example, cannot rely on a typed password for every transaction. Furthermore, many of these services were developed under the assumption that the extent of the network was comparatively limited. In an era of globe-spanning connectivity, that assumption has broken down, sometimes with severe consequences.

Networked computers have another peculiarity worth noting: they are generally not singular entities. That is, it is comparatively uncommon, in today's environment, to attach a computer to a network solely to talk to "strange" computers. More commonly, organizations own a number of computers, and these are connected to each other and to the outside world. This is both a bane and a blessing: a bane, because networked computers often need to trust their peers, to some extent, and a blessing, because the network may be configurable so that only one computer needs to talk to the outside world. Such dedicated computers, often called "firewall gateways," are at the heart of our suggested security strategy.

Our purpose here is twofold. First, we wish to show that this strategy is useful. That is, a firewall, if properly deployed against the expected threats, will provide an organization with greatly increased security. Second, we wish to show that such gateways are necessary, and that there is a real threat to be dealt with.


This book is written primarily for the network administrator who must protect an organization from unhindered exposure to the Internet. The typical reader should have a background in system administration and networking. Some portions necessarily get intensely technical. A number of chapters are of more general interest. Readers with a casual interest can safely skip the tough stuff and still enjoy the rest of the book.

We also hope that system and network designers will read the book. Many of the problems we discuss are the direct result of a lack of security-conscious design. We hope that newer protocols and systems will be inherently more secure.

Our examples and discussion unabashedly relate to Unix systems and programs. The majority of multiuser machines on the Internet run some version of the Unix operating system. Most application-level gateways are implemented in Unix. This is not to say that other operating systems are more secure; however, there are fewer of them on the Internet, and they are less popular as targets for that reason. But the principles and philosophy apply to network gateways built in other operating systems, or even to a run-time system like MS-DOS.

Our focus is on the TCP/IP protocol suite, especially as used on the Internet. Again, this is not because TCP/IP has more security problems than other protocol stacks - we doubt that very much - rather, it is a commentary on the success of TCP/IP. By far, it is the heterogeneous networking protocol of choice, not only on workstations, for which it is the native tongue, but on virtually all machines, ranging from desktop personal computers to the largest supercomputers. The Internet links most major universities in the United States (and many others around the world), research labs, many government agencies, and even a fair number of businesses. Our organization, AT&T Bell Laboratories, is on the Internet, and much of the advice we offer in this book is the result of our experiences with that connection. We believe that the lessons we have learned are applicable to any network with similar characteristics. We have read of serious attacks on computers attached to public X.25 data networks. Firewalls are useful there, too, although naturally they would differ in detail.

This is not a book on how to administer a system in a secure fashion, although we do make some suggestions along those lines. Numerous books on that topic already exist, such as Farrow, 1991, Garfinkel and Spafford, 1991, and Curry, 1992. Nor is this a cookbook to tell you how to administer various packaged firewall gateways. The technology is too new, and any such work would be obsolete before it was even published. Rather, it is a set of guidelines that, we hope, both defines the problem space and roughly sketches the boundaries of possible solution spaces. We also describe how we constructed our latest gateway, and why we made the decisions we did. Our design decisions are directly attributable to our experience in detecting and defending against attackers.

On occasion, we speak of "reports" that something has happened. We make apologies for the obscurity. Though we have made every effort to document our sources, some of our information comes from confidential discussions with other security administrators who do not want to be identified. Network security breaches can be very embarrassing, especially when they happen to organizations that should have known better.


Before we proceed further, it is worthwhile making one comment on terminology. We have chosen to call the attackers "hackers." To some, this choice is insulting, a slur by the mass media on the good name of many thousands of creative programmers. That is quite true. Nevertheless, the language has changed. Bruce Sterling expressed it very well Sterling, 1992, pages 55-56:

The term "hacking" is used routinely today by almost all law enforcement officials with any professional interest in computer fraud and abuse. American police describe almost any crime committed with, by, through, or against a computer as hacking.

Most important, "hacker" is what computer intruders choose to call themselves. Nobody who hacks into systems willingly describes himself (rarely, herself) as a "computer intruder," "computer trespasser," "cracker," "wormer," "dark-side hacker," or "high- tech street gangster." Several other demeaning terms have been invented in the hope that the press and public will leave the original sense of the word alone. But few people actually use these terms.


Our book begins by introducing the problem of security (Chapter 1) and surveying the important parts of the TCP/IP protocol suite (Chapter 2), with particular attention to security issues.

The second part of this book describes firewall construction in detail. We describe the several sorts of firewall gateways (Chapter 3) that have been built. Next, we present a comprehensive description of the construction of our third and newest gateway (Chapter 4), the variety of authentication strategies to choose from (Chapter 5), the other tools we used (Chapter 6), and the sorts of monitors we have installed (Chapter 7). We also describe the hacking tools we've built to test security (Chapter 8): you don't know if you're secure until someone has made a determined effort to breach your defenses. All of the information in this part is detailed enough to permit you to duplicate our work or to do it differently if your needs or priorities differ.

Security isn't just a matter of the present and future tenses. Chapter 9 is an attempt at a taxonomy of hacking, an analysis of different categories of attacks. Chapter 10 is quite concrete: we describe the single most determined (known?) attempt to hack our system, the so-called "Berferd" incident, and the fun we had during it. The next chapter summarizes the log data we and others have collected over the years.

In Chapter 12, we discuss the legal implications of computer security. The issues aren't always straightforward, it turns out. In Chapter 13, we show how encryption can be used in high-threat environments. Chapter 14 has some parting thoughts.

Errata to Be

Though we've tried our best, we suspect that a few errors have crept into this book. You'll be able to find an errata list, and perhaps further information, on ftp.research.att.com, in /dist/internet_security/firewall.book. Naturally, we'd appreciate word of any bugs you find, preferably by electronic mail to firewall-book@research.att.com.


There are many people who deserve our thanks for helping with this book. We thank in particular our reviewers: Betty Archer, Robert Bonomi, Jay Borkenhagen, Brent Chapman, Lorette Ellane Petersen Archer Cheswick, Steve Crocker, Dan Doernberg, Mark Eckenwiler, Jim Ellis, Ray Kaplan, Jeff Kellem, Brian Kernighan, Barbara T. Ling, Norma Loquendi, Barry Margolin Jeff Mogul, Gene Nelson, Craig Partridge, Marcus Ranum, Peter Weinberger, Norman Wilson, and of course our editor, John Wait, whose name almost, but not quite, fits into our ordering. Acting on all of the comments we received was painful, but has made this a better book. Of course, we bear the blame for any errors, not these intrepid readers.

Bill Cheswick

Steven M. Bellovin

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