Chapter 2. Understanding Denial of Service


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 192.0.34.166). 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.



Internet Denial of Service. Attack and Defense Mechanisms
Internet Denial of Service: Attack and Defense Mechanisms
ISBN: 0131475738
EAN: 2147483647
Year: 2003
Pages: 126

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