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Prior to writing this book, I had extensive discussions with the Sams.net editorial staff. In those discussions, one thing became immediately clear: Sams.net wanted a book that was valuable to all users, not just to a special class of them. An examination of earlier books on the subject proved instructive. The majority were well written and tastefully presented, but appealed primarily to UNIX or NT system administrators. I recognized that while this class of individuals is an important one, there are millions of average users yearning for basic knowledge of security. To accommodate that need, I aimed at creating an all-purpose Internet security book.

To do so, I had to break some conventions. Accordingly, this book probably differs from other Sams.net books in both content and form. Nevertheless, the book contains copious knowledge, and there are different ways to access it. This chapter briefly outlines how the reader can most effectively access and implement that knowledge.

Is this book of practical use? Absolutely. It can serve both as a reference book and a general primer. The key for each reader is to determine what information is most important to him or her. The book loosely follows two conventional designs common to books by Sams.net:

This book is a hybrid of both techniques. For example, the book examines services in the TCP/IP suite, then quickly progresses to how those services are integrated in modern browsers, how such services are compromised, and ultimately, how to secure against such compromises. In this respect, there is an evolutionary pattern to the book.

At the same time, the book begins with a general examination of the structure of the Internet and TCP/IP (which will seem light in comparison to later analyses of sniffing, where you examine the actual construct of an information packet). As you progress, the information becomes more and more advanced. In this respect, there is a developmental pattern to the book.

Different people will derive different benefits from this book, depending on their circumstances. I urge each reader to closely examine the following categories. The information will be most valuable to you whether you are

A system administrator is any person charged with managing a network or any portion of a network. Sometimes, people might not realize that they are a system administrator. In small companies, for example, programming duties and system administration are sometimes assigned to a single person. Thus, this person is a general, all-purpose technician. They keep the system running, add new accounts, and basically perform any task required on a day-to-day basis. This, for your purposes, is a system administrator.

This book presumes only basic knowledge of security from its system administrators, and I believe that this is reasonable. Many capable system administrators are not well versed in security, not because they are lazy or incompetent but because security was for them (until now) not an issue. For example, consider the sysad who lords over an internal LAN. One day, the powers that be decree that the LAN must establish a connection to the Net. Suddenly, that sysad is thrown into an entirely different (and hostile) environment. He or she might be exceptionally skilled at internal security but have little practical experience with the Internet. Today, numerous system administrators are faced with this dilemma. For many, additional funding to hire on-site security specialists is not available and thus, these people must go it alone. Not anymore. This book will serve such system administrators well as an introduction to Internet security.

Likewise, more experienced system administrators can effectively use this book to learn--or perhaps refresh their knowledge about--various aspects of Internet security that have been sparsely covered in books mass-produced for the general public.

For either class of sysad, this book will serve a fundamental purpose: It will assist them in protecting their network. Most importantly, this book shows the attack from both sides of the fence. It shows both how to attack and how to defend in a real-life, combat situation.

The term hacker refers to programmers and not to those who unlawfully breach the security of systems. A hacker is any person who investigates the integrity and security of an operating system. Most commonly, these individuals are programmers. They usually have advanced knowledge of both hardware and software and are capable of rigging (or hacking) systems in innovative ways. Often, hackers determine new ways to utilize or implement a network, ways that software manufacturers had not expressly intended.

This book presumes only basic knowledge of Internet security from its hackers and programmers. For them, this book will provide insight into the Net's most common security weaknesses. It will show how programmers must be aware of these weaknesses. There is an ever-increasing market for those who can code client/server applications, particularly for use on the Net. This book will help programmers make informed decisions about how to develop code safely and cleanly. As an added benefit, analysis of existing network utilities (and their deficiencies) may assist programmers in developing newer and perhaps more effective applications for the Internet.

A cracker is any individual who uses advanced knowledge of the Internet (or networks) to compromise network security. Historically, this activity involved cracking encrypted password files, but today, crackers employ a wide range of techniques. Hackers also sometimes test the security of networks, often with the identical tools and techniques used by crackers. To differentiate between these two groups on a trivial level, simply remember this: Crackers engage in such activities without authorization. As such, most cracking activity is unlawful, illegal, and therefore punishable by a term of imprisonment.

For the budding cracker, this book provides an incisive shortcut to knowledge of cracking that is difficult to acquire. All crackers start somewhere, many on the famous Usenet group alt.2600. As more new users flood the Internet, quality information about cracking (and security) becomes more difficult to find. The range of information is not well represented. Often, texts go from the incredibly fundamental to the excruciatingly technical. There is little material that is in between. This book will save the new cracker hundreds of hours of reading by digesting both the fundamental and the technical into a single (and I hope) well-crafted presentation.

For your purposes, business person refers to any individual who has established (or will establish) a commercial enterprise that uses the Internet as a medium. Hence, a business person--within the meaning employed in this book--is anyone who conducts commerce over the Internet by offering goods or services.

Businesses establish permanent connections each day. If yours is one of them, this book will help you in many ways, such as helping you make informed decisions about security. It will prepare you for unscrupulous security specialists, who may charge you thousands of dollars to perform basic, system-administration tasks. This book will also offer a basic framework for your internal security policies. You have probably read dozens of dramatic accounts about hackers and crackers, but these materials are largely sensationalized. (Commercial vendors often capitalize on your fear by spreading such stories.) The techniques that will be employed against your system are simple and methodical. Know them, and you will know at least the basics about how to protect your data.

A journalist is any party who is charged with reporting on the Internet. This can be someone who works for a wire news service or a college student writing for his or her university newspaper. The classification has nothing to do with how much money is paid for the reporting, nor where the reporting is published.

If you are a journalist, you know that security personnel rarely talk to the media. That is, they rarely provide an inside look at Internet security (and when they do, this usually comes in the form of assurances that might or might not have value). This book will assist journalists in finding good sources and solid answers to questions they might have. Moreover, this book will give the journalist who is new to security an overall view of the terrain. Technology writing is difficult and takes considerable research. My intent is to narrow that field of research for journalists who want to cover the Internet. In coming years, this type of reporting (whether by print or broadcast media) will become more prevalent.

A casual user is any individual who uses the Internet purely as a source of entertainment. Such users rarely spend more than 10 hours a week on the Net. They surf subjects that are of personal interest.

For the casual user, this book will provide an understanding of the Internet's innermost workings. It will prepare the reader for personal attacks of various kinds, not only from other, hostile users, but from the prying eyes of government. Essentially, this book will inform the reader that the Internet is not a toy, that one's identity can be traced and bad things can happen while using the Net. For the casual user, this book might well be retitled How to Avoid Getting Hijacked on the Information Superhighway.

A security specialist is anyone charged with securing one or more networks from attack. It is not necessary that they get paid for their services in order to qualify in this category. Some people do this as a hobby. If they do it, they are a specialist.

NOTE: In this book, the void refers to that portion of the Internet that exists beyond your router or modem. It is the dark, swirling mass of machines, services, and users beyond your computer or network. These are quantities that are unknown to you. This term is commonly used in security circles to refer to such quantities.

Much of the information covered here will be painfully familiar to the security specialist. Some of the material, however, might not be so familiar. (Most notably, some cross-platform materials for those maintaining networks with multiple operating systems.) Additionally, this book imparts a comprehensive view of security, encapsulated into a single text. (And naturally, the materials on the CD-ROM will provide convenience and utility.)

How you use this book is up to you. If you purchased or otherwise procured this book as a tool to facilitate illegal activities, so be it. You will not be disappointed, for the information contained within is well suited to such undertakings. However, note that this author does not suggest (nor does he condone) such activities. Those who unlawfully penetrate networks seldom do so for fun and often pursue destructive objectives. Considering how long it takes to establish a network, write software, configure hardware, and maintain databases, it is abhorrent to the hacking community that the cracking community should be destructive. Still, that is a choice and one choice--even a bad one--is better than no choice at all. Crackers serve a purpose within the scheme of security, too. They assist the good guys in discovering faults inherent within the network.

If you are charged with understanding in detail a certain aspect of security, follow the notes closely. Full citations appear in these notes, often showing multiple locations for a security document, RFC, FYI, or IDraft. Digested versions of such documents can never replace having the original, unabridged text.

The end of each chapter contains a small rehash of the information covered. For extremely handy reference, especially for those already familiar with the utilities and concepts discussed, this "Summary" portion of the chapter is quite valuable.

Certain examples contained within this book are available on the CD-ROM. Whenever you see the CD-ROM icon on the outside margin of a page, the resource is available on the CD. This might be source code, technical documents, an HTML presentation, system logs, or other valuable information.

Part II of this book is probably the most critical. It illustrates the basic design of the Internet. Each reader must understand this design before he or she can effectively grasp concepts in security. Topics include

In short, you will examine why and how the Internet was established, what services are available, the emergence of the WWW, why security might be difficult to achieve, and various techniques for living in a hostile computing environment.

Part III of this book examines the average toolbox of the hacker or cracker. It familiarizes the reader with Internet munitions, or weapons. It covers the proliferation of such weapons, who creates them, who uses them, how they work, and how the reader can use them. Some of the munitions covered are

Part IV of this book ventures into more complex territory, treating vulnerabilities inherent in certain operating systems or applications. At this point, the book forks, concentrating on issues relevant to particular classes of users. (For example, if you are a Novell user, you will naturally gravitate to the Novell chapter.)

Part V of this book examines who has the power on a given network. I will discuss the relationship between these authoritarian figures and their users, as well as abstract and philosophical views on Internet security. At this point, the material is most suited for those who will be living with security issues each day. Topics include

Part VI of this book concerns attacks: actual techniques to facilitate the compromise of a remote computer system. In it, I will discuss levels of attack, what these mean, and how one can prepare for them. You will examine various techniques in depth: so in depth that the average user can grasp--and perhaps implement--attacks of this nature. Part VI also examines complex subjects regarding the coding of safe CGI programs, weaknesses of various computer languages, and the relative strengths of certain authentication procedures. Topics discussed in this part include

The scope of this book is wide, but there are limitations on the usefulness of the information. Before examining these individually, I want to make something clear: Internet security is a complex subject. If you are charged with securing a network, relying solely upon this book is a mistake. No book has yet been written that can replace the experience, gut feeling, and basic savvy of a good system administrator. It is likely that no such book will ever be written. That settled, some points on this book's limitations include the following:

Timelines might or might not be a huge factor in the value of this book. I say might or might not for one reason only: Many people do not use the latest and the greatest in software or hardware. Economic and administrative realities often preclude this. Thus, there are LANs now operating on Windows for Workgroups that are permanently connected to the Net. Similarly, some individuals are using SPARCstation 1s running SunOS 4.1.3 for access. Because older software and hardware exist in the void, much of the material here will remain current. (Good examples are machines with fresh installs of an older operating system that has now been proven to contain numerous security bugs.)

Equally, I advise the reader to read carefully. Certain bugs examined in this book are common to a single version of software only (for example, Windows NT Server 3.51). The reader must pay particular attention to version information. One version of a given software might harbor a bug, whereas a later version does not. The security of the Internet is not a static thing. New holes are discovered at the rate of one per day. (Unfortunately, such holes often take much longer to fix.)

Be assured, however, that at the time of this writing, the information contained within this book was current. If you are unsure whether the information you need has changed, contact your vendor.

Although this book contains many practical examples, it is not a how-to for cracking Internet servers. True, I provide many examples of how cracking is done and even utilities with which to accomplish that task, but this book will not make the reader a master hacker or cracker. There is no substitute for experience, and this book cannot provide that.

What this book can provide is a strong background in Internet security, hacking, and cracking. A reader with little knowledge of these subjects will come away with enough information to crack the average server (by average, I mean a server maintained by individuals who have a working but somewhat imperfect knowledge of security).

Also, journalists will find this book bereft of the pulp style of sensationalist literature commonly associated with the subject. For this, I apologize. However, sagas of tiger teams and samurais are of limited value in the actual application of security. Security is a serious subject, and should be reported as responsibly as possible. Within a few years, many Americans will do their banking online. Upon the first instance of a private citizen losing his life savings to a cracker, the general public's fascination with pulp hacking stories will vanish and the fun will be over.

Lastly, bona fide security specialists might find that for them, only the last quarter of the book has significant value. As noted, I developed this book for all audiences. However, these gurus should keep their eyes open as they thumb through this book. They might be pleasantly surprised (or even downright outraged) at some of the information revealed in the last quarter of the text. Like a sleight-of-hand artist who breaks the magician's code, I have dropped some fairly decent baubles in the street.

It is of value to hackers, crackers, system administrators, business people, journalists, security specialists, and casual users. There is a high volume of information, the chapters move quickly, and (I hope) the book imparts the information in a clear and concise manner.

Equally, this book cannot make the reader a master hacker or cracker, nor can it suffice as your only source for security information. That said, let's move forward, beginning with a small primer on hackers and crackers.