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Building Internet Firewalls

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1. Why Internet Firewalls?

What Are You Trying to Protect?
What Are You Trying To Protect Against?
How Can You Protect Your Site?
What Is an Internet Firewall?

It is scarcely possible to enter a bookstore, read a magazine or a newspaper, or listen to a news broadcast without seeing or hearing something about the Internet in some guise. It's become so popular that it no longer requires explanations when mentioned in nontechnical publications, and it gets mentioned plenty, in magazines ranging from The New Yorker to Bead and Button. While nontechnical publications are obsessed with the Internet, the technical publications have moved on and are obsessed with security. It's a logical progression; once the first excitement of having a superhighway in your neighborhood wears off, you're bound to notice that not only does it let you travel, it lets a very large number of strangers show up where you are, and not all of them are people you would have invited.

Both views are true: The Internet is a marvelous technological advance that provides access to information, and the ability to publish information, in revolutionary ways. But it's also a major danger that provides the ability to pollute and destroy information in revolutionary ways. This book is about one way to balance the advantages and the risks - to take part in the Internet while still protecting yourself.

Later in this chapter, we describe different models of security people have used to protect their data and resources on the Internet. Our emphasis in this book is on the network security model and, in particular, the use of Internet firewalls. A firewall is a form of protection that allows a network to connect to the Internet while maintaining a degree of security. The section later in this chapter called "What is an Internet Firewall?" describes the basics of firewalls and summarize what they can - and cannot - do to help make your site secure. Before we discuss what you can do with a firewall, though, we want to describe briefly why you need one. What are you protecting on your systems? What types of attacks and attackers are we seeing today? What types of security can you use to protect your site?

1.1 What Are You Trying to Protect?

A firewall is basically a protective device. If you are building a firewall, the first thing you need to worry about is what you're trying to protect. When you connect to the Internet, you're putting three things at risk:

1.1.1 Your Data

Your data has three separate characteristics that need to be protected:

  • Secrecy: you might not want other people to know it.

  • Integrity: you probably don't want other people to change it.

  • Availability: you almost certainly want to be able to use it yourself.

People tend to focus on the risks associated with secrecy, and it's true that those are usually large risks. Many organizations have some of their most important secrets - the designs for their products, their financial records, or student records - on their computers. On the other hand, you may find that at your site it is relatively easy to separate the machines containing this kind of highly secret data from the machines that connect to the Internet.

Suppose that you can separate your data in this way, and that none of the information that is Internet accessible is secret. In that case, why should you worry about security? Because secrecy isn't the only thing you're trying to protect. You still need to worry about integrity and availability. After all, if your data isn't secret, and if you don't mind its being changed, and if you don't care whether or not anybody can get to it, why are you wasting disk space on it?

Even if your data isn't particularly secret, you'll suffer the consequences if it's destroyed or modified. Some of these consequences have readily calculable costs: if you lose data, you'll have to pay to have it reconstructed; if you were planning to sell that data in some form, you'll have lost sales regardless of whether the data is something you sell directly, the designs you use to build things from, or the code for a software product. There are also intangible costs associated with any security incident. The most serious is the loss of confidence (user confidence, customer confidence, investor confidence, staff confidence, student confidence, public confidence) in your systems and data and, consequently, a loss of confidence in your organization.

1.1.2 Your Resources

Even if you have data you don't care about - even if you enjoy reinstalling your operating system every week because it exercises the disks, or something like that - if other people are going to use your computers, you probably would like to benefit from this use in some way. Most people want to use their own computers, or they want to charge other people for using them. Even people who give away computer time and disk space usually expect to get good publicity and thanks for it; they aren't going to get it from intruders. You spend good time and money on your computing resources, and it is your right to determine how they are used.

Intruders often argue that they are using only excess resources; as a consequence, their intrusions don't cost their victims anything. There are two problems with this argument.

First, it's impossible for an intruder to determine successfully what resources are excess and use only those. It may look as if your system has oceans of empty disk space and hours of unused computing time; in fact, though, you might be just about to start computing animation sequences that are going to use every bit and every microsecond. An intruder can't give back your resources when you want them. (Along the same lines, I don't usually use my car between midnight and 6 A.M., but that doesn't mean I'm willing to lend it to you without being asked. What if I have an early-morning flight the next day, or what if I'm called out to deal with an emergency?)

Second, it's your right to use your resources the way you want to, even if you merely feel some sort of Zen joy at the sight of empty disk space, or if you like the way the blinky lights look when nothing's happening on your computer. Computing resources are not natural resources, nor are they limited resources that are wasted or destroyed if they're not used.

1.1.3 Your Reputation

An intruder appears on the Internet with your identity. Anything he does appears to come from you. What are the consequences?

Most of the time, the consequences are simply that other sites - or law enforcement agencies - start calling you to ask why you're trying to break into their systems. (This isn't as rare an occurrence as it may seem. One site got serious about security when its system administration staff added a line item to their time cards for conversations with the FBI about break-in attempts originating from their site.)

Sometimes, such impostors cost you a lot more than lost time. An intruder who actively dislikes you, or simply takes pleasure in making life difficult for strangers, may send electronic mail or post news messages that purport to come from you. Generally, people who choose to do this aim for maximum hatefulness, rather than believability, but even if only a few people believe these messages, the cleanup can be long and humiliating. Anything even remotely believable can do permanent damage to your reputation.

A few years ago, an impostor posing as a Texas A&M professor sent out hate email containing racist comments to thousands of recipients. The impostor was never found, and the professor is still dealing with the repercussions of the forged messages. In another case, a student at Dartmouth sent out email over the signature of a professor late one night during exam period. Claiming a family emergency, the forged mail canceled the next day's exam, and only a few students showed up.

It's possible to forge electronic mail or news without gaining access to a site, but it's much easier to show that a message is a forgery if it's generated from outside the forged site. The messages coming from an intruder who has gained access to your site will look exactly like yours because they are yours. An intruder will also have access to all kinds of details that an external forger won't. For example, an intruder has all of your mailing lists available and knows exactly who you send mail to.

Even if an intruder doesn't use your identity, a break-in at your site isn't good for your reputation. It shakes people's confidence in your organization. In addition, most intruders will attempt to go from your machines to others, which is going to make their next victims think of your site as a platform for computer criminals. Many intruders will also use compromised sites as distribution sites for pirated software and/or pornography, which is not going to endear you to many folks either. Whether or not it's your fault, having your name linked to other intrusions, software piracy, and pornography is hard to recover from.

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I. Network SecurityBook Index1.2 What Are You Trying To Protect Against?