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Building Internet Firewalls

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1.2 What Are You Trying To Protect Against?

What's out there to worry about? What types of attacks are you likely to face on the Internet, and what types of attackers are likely to be carrying them out? And what about simple accidents or stupidity? In the sections below, we touch on these topics but we don't go into any technical detail; later chapters describe different kinds of attacks in some detail and explain how firewalls can help protect against them.

1.2.1 Types of Attacks

There are many types of attacks on systems, and many ways of categorizing these attacks. In this section, we break attacks down into three basic categories: intrusion, denial of service, and information theft. Intrusion

The most common attacks on your systems are intrusions; with intrusions, people are actually able to use your computers. Most attackers want to use your computers as if they were legitimate users.

Attackers have dozens of ways to get access. They range from social engineering attacks (you figure out the name of somebody high up in the company; you call a system administrator, claiming to be that person and claiming to need your password changed right now, so that you can get important work done), to simple guesswork (you try account name and password combinations until one works), to intricate ways to get in without needing to know an account name and password.

As we describe in this book, firewalls help prevent intrusions in a number of ways. Ideally, they block all ways to get into a system without knowing an account name and password. Properly configured, they reduce the number of accounts accessible from the outside that are therefore vulnerable to guesswork or social engineering. Most people configure their firewalls to use one-time passwords that prevent guessing attacks. Even if you don't use these passwords, which we describe in Chapter 10, Authentication and Inbound Services, a firewall will give you a controlled place to log attempts to get into your system, and, in this way, they help you detect guessing attacks. Denial of Service

A denial of service attack is one that's aimed entirely at preventing you from using your own computers.

In late 1994, writers Josh Quittner and Michelle Slatalla were the target of an "electronic mail bomb." Apparently in retaliation for an article on the cracker community they'd published in Wired magazine, someone broke into IBM, Sprint, and the writers' network provider, and modified programs so their email and telephone service was disrupted. A flood of email messages so overwhelmed their network service that other messages couldn't get through; eventually, their Internet connection was shut down entirely. Their phone service also fell victim to the intruders, who reprogrammed the service so that callers were routed to an out-of state number where they heard an obscene recording.

Although some cases of electronic sabotage involve the actual destruction or shutting down of equipment or data, more often they follow the pattern of flooding seen in the Quittner-Slatalla case or in the case of the Internet worm. An intruder so floods a system or network - with messages, processes, or network requests - that no real work can be done. The system or network spends all its time responding to messages and requests, and can't satisfy any of them.

While flooding is the simplest and most common way to carry out a denial of service attack, a cleverer attacker can also disable services, reroute them, or replace them. For example, the phone attack in the Quittner-Slatalla case denied phone service by rerouting their phone calls elsewhere; it's possible to mount the same kind of attack against Internet services.

It's close to impossible to avoid all denial of service attacks. Sometimes it's a "heads, I win; tails, you lose" situation for attackers. For example, many sites set accounts up to become unusable after a certain number of failed login attempts. This prevents attackers from simply trying passwords until they find the right one. On the other hand, it gives the attackers an easy way to mount a denial of service attack: they lock any user's account simply by trying to log in a few times.

Most often, the risk of denial of service attacks is unavoidable. If you accept things from the external universe - electronic mail, telephone calls, or packages - it's possible to get flooded. The notorious college prank of ordering a pizza or two from every pizzeria in town to be delivered to your least favorite person is a form of denial of service; it's hard to do much else while arguing with 42 pizza deliverers. In the electronic world, denial of service is as likely to happen by accident as on purpose (have you ever had a persistent fax machine try to fax something to your voice line?). The most important thing is to set up services so that if one of them is flooded, the rest of your site keeps functioning while you find and fix the problem.

Fortunately, deliberate denial of service attacks are not terribly popular. They're so easy that they're considered "unsporting" by many attackers; they tend to be simple to trace back and are therefore risky to the attacker; and they don't provide the attacker with the information or the ability to use your computers (the payoff for most other attacks). Intentional denial of service attacks are the work of people who are angry at your site in particular, and at most sites such people are quite rare.

You are far more likely to encounter unintentional denial of service problems, as we discuss in the section on "Stupidity and Accidents" later in this chapter. Information Theft

Some types of attacks allow an attacker to get data without ever having to directly use your computers. Usually these attacks exploit Internet services that are intended to give out information, inducing the services to give out more information than was intended, or to give it out to the wrong people. Many Internet services are designed for use on local area networks, and don't have the type or degree of security that would allow them to be used safely across the Internet.

Information theft doesn't need to be active or particularly technical. People who want to find out personal information could simply call you and ask (perhaps pretending to be somebody who had a right to know): this is an active information theft. Or they could tap your telephone: a passive information theft. Similarly, people who want to gather electronic information could actively query for it (perhaps pretending to be a machine or a user with valid access) or could passively tap the network and wait for it to flow by.

Most people who steal information try to get access to your computers; they're looking for usernames and passwords. Fortunately for them, and unfortunately for everybody else, that's the easiest kind of information to get when tapping a network. Username and password information occurs quite predictably at the beginning of many network interactions, and such information can be reused in the same form.

How would you proceed if you want to find out how somebody answers her telephone? Installing a tap would be an easy and reliable way to get that information, and a tap at a central point in the telephone system would yield you the telephone greetings of hundreds or thousands of people in a short period of time.

On the other hand, what if you want to know how somebody spells his last name, or what the names and ages of his children are? In this case, a telephone tap is a slow and unreliable way to get that information. A telephone tap at a central point in the system will probably yield that information about some people, and it will certainly yield some secret information you could use in interesting ways, but the information is going to be buried among the conversations of hundreds of people setting up lunch dates and chatting about the weather.

Similarly, network taps, which are usually called sniffers, are very effective at finding password information, but are rarely used to gather other kinds of information. Getting more specific information about a site requires either extreme dedication and patience, or the knowledge that the information you want will reliably pass through a given place at a given time. For example, if you know that somebody calls his bank to transfer money between his checking and savings accounts at 2 P.M. every other Friday, it's worth tapping that phone call to find out the person's access codes and account numbers. However, it's probably not worth tapping somebody else's phone, on the off chance that they too will do such a transfer, because most people don't transfer money over the phone at all.

Network sniffing is much easier than tapping a telephone line. Traditionally, the connectors used to hook a computer up to a network are known as network taps (that's why the term tapping isn't used for spying on a network), and the connectors behave like taps too. In most common network technologies, such as Ethernet and token ring, any computer on a local area network is capable of seeing all the traffic that passes across that local area network. Traffic that crosses the Internet may cross any number of local area networks, any one of which can be a point of compromise. Network service providers and public-access systems are very popular targets for intrusions; sniffers placed there can be extremely successful because so much traffic passes through these networks.

There are several types of protection against information theft. A properly configured firewall will protect you against people who are trying to get more information than you intended to give. Once you've decided to give information out across the Internet, however, it's very difficult to protect against that information reaching an unintended audience, either through misauthentication (somebody claiming to be authorized, when they're not) or through sniffing (somebody simply reading information as it crosses a correctly authorized channel). Although these risks are outside of the protection a firewall can give (because they occur once information has intentionally been allowed to go outside your network), we do discuss them, and the methods used to reduce them, as appropriate in this book.

1.2.2 Types of Attackers

This section very briefly describes the types of attackers who are out there on the Internet. There are many ways to categorize these attackers; we can't really do justice to the many variants of attackers we've seen over the years, and any quick summary of this kind necessarily presents a rather stereotyped view. Nevertheless, this summary may be useful in distinguishing the main categories of attackers.

All attackers share certain characteristics. They don't want to be caught, so they try to conceal themselves. If they gain access to your system, they will certainly attempt to preserve that access, if possible, by building in extra ways to get access (and they hope you won't notice these access routes even if you find the attackers themselves). Most of them have some contact with other people who have the same kinds of interests ("the underground" is not hard to find), and most will share the information they get from attacking your system. Joyriders

Joyriders are bored people looking for amusement. They break in because they think you might have interesting data, or because it would be amusing to use your computers, or because they have nothing better to do. They might be out to learn about the kind of computer you have or about the data you have. They're curious, but not actively malicious; however, they often damage the system through ignorance or in trying to cover their tracks. Joyriders are particularly attracted to well-known sites and uncommon computers. Vandals

Vandals are out to do damage, either because they get their kicks from destroying things, or because they don't like you. When one gets to you, you'll know it.

Vandals are a big problem if you're somebody that the Internet underground might think of as The Enemy (for example, the phone company or the government) or if you tend to annoy people who have computers and time (for example, you're a university with failing students, or a computer company with annoyed customers, or you have an aggressively commercial presence on the Internet). You can also become a target simply by being large and visible; if you put a big white wall up in certain neighborhoods, people will put graffiti on it no matter how they feel about you.

Fortunately, vandals are fairly rare. People don't like them, even people in the underground who have nothing against breaking into computers in general. Vandals also tend to inspire people to go to great lengths to find them and stop them. Unlike more mundane intruders, vandals have short but splashy careers. Most of them also go for straightforward destruction, which is unpleasant but is relatively easily detected and repaired. In most circumstances, deleting your data, or even ruining your computer equipment, is not the worst thing somebody could do to you, but it is what vandals do. (Actually, introducing subtle but significant changes in programs or financial data would be much harder to detect and fix.)

Unfortunately, it's close to impossible to stop a determined vandal; somebody with a true vendetta against your site is going to get you, sooner or later. Certain attacks are attractive to vandals but not to other types of attackers. For example, denial of service attacks are not attractive to joyriders; while joyriders are around in your system, they are just as interested as you are in having your computers up, running, and available to the Internet. Score Keepers

Many intruders are engaging in an updated version of an ancient tradition. They're gaining bragging rights, based on the number and types of systems they've broken into.

Like joyriders and vandals, score keepers may prefer sites of particular interest. Breaking into something well-known, well-defended, or otherwise especially neat is usually worth more points to them. However, they'll also attack anything they can get at; they're going for quantity as well as quality. They don't have to want anything you've got, or care in the least about the characteristics of your site. They may or may not do damage on the way through. They'll certainly gather information and keep it for later use (perhaps using it to barter with other attackers). They'll probably try to leave themselves ways to get back in later. And, if at all possible, they'll use your machines as a platform to attack others.

These people are the ones you discover long after they've broken in to your system. You may find out slowly, because something's odd about your machine. Or you'll find out when another site or a law enforcement agency calls up because your system is being used to attack other places. Or you'll find out when somebody sends you a copy of your own private data, which they've found on a cracked system on the other side of the world. Spies (Industrial and Otherwise)

Most people who break into computers do so for the same reason people climb mountains - because they're there. While these people are not above theft, they usually steal things that are directly convertible into money or further access (e.g., credit card, telephone, or network access information). If they find secrets they think they can sell, they may try to do so, but that's not their main business.

As far as anybody knows, serious computer-based espionage is much rarer, outside of traditional espionage circles. (That is, if you're a professional spy, there are probably other professional spies watching you and your computers). Espionage is much more difficult to detect than run-of-the-mill break-ins, however. Information theft need not leave any traces at all, and even intrusions are relatively rarely detected immediately. Somebody who breaks in, copies data, and leaves without disturbing anything is quite likely get away with it at most sites.

In practical terms, most organizations can't prevent spies from succeeding. The precautions that governments take to protect sensitive information on computers are complex, expensive, and cumbersome; therefore they are used on only the most critical resources. These precautions include electromagnetic shielding, careful access controls, and absolutely no connections to unsecured networks.

What can you do to protect against attackers of this kind? You can ensure that your Internet connection isn't the easiest way for a spy to gather information. You don't want some kid to break into your computers and find something that immediately appears to be worth trying to sell to spies; you don't want your competitors to be trivially able to get to your data; and you do want to make it expensive and risky to spy on you. Some people say it's unreasonable to protect data from network access when somebody could get it easily by coming to your site physically. We don't agree; physical access is generally more expensive and more risky for an attacker than network access.

1.2.3 Stupidity and Accidents

Most disasters are not caused through ill will; they're accidents or stupid mistakes. One recent study estimates that 55% of all security incidents actually result from naive or untrained users doing things they shouldn't.[1]

[1] Richard Power, Current and Future Danger: A CSI Primer on Computer Crime and Information Warfare, San Francisco, CA: Computer Security Institute, 1995.

Denial of service incidents, for example, frequently aren't attacks at all. Apple's corporate electronic mail was rendered nonfunctional for several days (and their network provider was severely inconvenienced) by an accident involving a single mail message sent from a buggy mail server to a large mailing list. The mail resulted in a cascade of hundreds of thousands of error messages. The only hostile person involved was the system administrator, who wasn't hostile until he had to clean up the resulting mess.

Similarly, it's not uncommon for companies to destroy their own data, or release it to the world, by accident. Firewalls aren't designed to deal with this kind of problem. In fact there is no known way to protect yourself from either accidents or stupidity. Unfortunately, whether people are attacking you on purpose, or are simply making mistakes, the results are quite similar. (Hence the saying, "Never ascribe to malice that which can adequately be explained by stupidity.") When you protect yourself against evil-doers, you also help protect yourself against the more common, but equally devastating, unintentional or well-intentioned error.

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