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Understanding Denial of Service

Denial of Service (DoS) attacks aren't quite like other malicious network traffic. Instead of gaining some benefit, the perpetrator of a DoS attack seeks only to do harm. This chapter explains the hows, whys, and whos of DoS attacks, and what you can do about them.
This chapter is from the book

A denial-of-service attack is different in goal, form, and effect than most of the attacks that are launched at networks and computers. Most attackers involved in cybercrime seek to break into a system, extract its secrets, or fool it into providing a service that they should not be allowed to use. Attackers commonly try to steal credit card numbers or proprietary information, gain control of machines to install their software or save their data, deface Web pages, or alter important content on victim machines. Frequently, compromised machines are valued by attackers as resources that can be turned to whatever purpose they currently deem important.

In DDoS attacks, breaking into a large number of computers and gaining malicious control of them is just the first step. The attacker then moves on to the DoS attack itself, which has a different goal—to prevent victim machines or networks from offering service to their legitimate users. No data is stolen, nothing is altered on the victim machines, and no unauthorized access occurs. The victim simply stops offering service to normal clients because it is preoccupied with handling the attack traffic. While no unauthorized access to the victim of the DDoS flood occurs, a large number of other hosts have previously been compromised and controlled by the attacker, who uses them as attack weapons. In most cases, this is unauthorized access, by the legal definition of that term.

While the denial-of-service effect on the victim may sound relatively benign, especially when one considers that it usually lasts only as long as the attack is active, for many network users it can be devastating. Use of Internet services has become an important part of our daily lives. The Internet is increasingly being used to conduct business and even to provide some critical services. Following are some examples of the damaging effects of DoS attacks.

  • Sites that offer services to users through online orders make money only when users can access those services. For example, a large book-selling site cannot sell books to its customers if they cannot browse the site's Web pages and order products online. A DoS attack on such sites means a severe loss of revenue for as long as the attack lasts. Prolonged or frequent attacks also inflict long-lasting damage to a site's reputation—customers who were unable to access the desired service are likely to take their business to the competition. Sites whose reputations were damaged may have trouble attracting new customers or investor funding in the future.
  • Large news sites and search engines are paid by marketers to present their advertisements to the public. The revenue depends on the number of users that view the site's Web page. A DoS attack on such a site means a direct loss of revenue from the marketers, and may have the long-lasting effect of driving the customers to more easily accessible sites. Loss of popularity translates to a direct loss of advertisers' business.
  • Some sites offer a critical free service to Internet users. For example, the Internet's Domain Name System (DNS) provides the necessary information to translate human-readable Web addresses (such as www.example.com) into Internet Protocol (IP) addresses (such as All Web browsers and numerous other applications depend on DNS to be able to fetch information requested by the users. If DNS servers are under a DoS attack and cannot respond due to overload, many sites may become unreachable because their addresses cannot be resolved, even though those sites are online and fully capable of handling traffic. This makes DNS a part of the critical infrastructure, and other equally important pieces of the Internet's infrastructure are also vulnerable.
  • Numerous businesses have come to depend on the Internet for critical daily activities. A DoS attack may interrupt an important videoconference meeting or a large customer order. It may prevent a company from sending out an important document for a rapidly approaching deadline or interfere with its bid for a large contract.
  • The Internet is increasingly being used to facilitate management of public services, such as water, power, and sewage, and to deliver critical information for important activities, such as weather and traffic reports for docking ships. A DoS attack that disrupts these critical services will directly affect even people whose activities are not related to computers or the Internet. It may even endanger human lives.
  • A vast number of people use the Internet on a daily basis for entertainment or for communicating with friends and family. While a DoS attack that disrupts these activities may not cause them any serious damage, it is certainly an unpleasant experience that they wish to avoid. If such disruptions occur frequently, people are likely to stop using the Internet for these purposes, in favor of more reliable technologies.

2.1 The Ulterior Motive

Why do attackers seek to deny service? This act, very disruptive in nature, is not always an end in and of itself. What could be the ultimate goal then?

Some of the early DoS attacks were largely proofs of concept or simple pranks played by hackers. The ultimate goal was to prove that something could be done, such as taking a large, popular Web site offline. Such a major achievement brings an attacker recognition in the underground community.

Frequently, attackers would also fight each other for supremacy via denial of service. Internet chat channels were and still are a sought-after resource by the attackers. They are used to coordinate multiple attacking machines and to trade code and illegal information with other attackers. The user who created the channel controls the access to it, and is called a moderator, operator, or owner. The easy way to take over the channel (and along with it all the attack machines that are controlled via this channel) and to dominate all the communications is to perform a DoS attack on its current moderator. When a moderator's machine goes offline, another user can take over the channel. Besides supremacy, attackers also sought revenge through denial of service. A hacker whose machines were knocked offline by DoS would "return the favor" by attacking the perpetrator. People who dared to speak ill of hackers in public have also felt DoS revenge.

Another frequent motive of DoS attacks is self-described as being political. Individuals or groups who disagree with views or actions of a certain organization (an online media site, a corporation, or a government) have been known to launch DoS attacks against computers and networks owned by this organization.

If the target of the attack is a company, a conceivable motive can be a competitor's wish to gain an edge in the market. So far, no attacks have been proved to have this motive. However, there is a major lack of data on perpetrators and motives of DoS attacks. The vast majority of attacks are not reported, let alone investigated. Of those that do undergo detailed investigation, only a few contain enough evidence to establish the motive. It is thus quite possible that some companies may resort to such illegal means of driving the competition out of the market.

Recently, a number of attacks have appeared as part of extortion attempts [ZDn04]. The attackers threaten an online business with a denial of service, and a payment is requested for "protection." Sites that refuse the payment are being "persuaded" by small-scale attacks.

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