What is a Denial of Service attack?


A Denial of Service attack (DoS) is an attack on a web site or service in order to make it unavailable or appear offline. A Distributed Denial of Service attack (DDoS) is where multiple computers in different locations are involved in the attack.

How a DoS attack works

The target web site or service is inundated with requests, such as to view a web page or to respond to a query. These requests are increased until they reach a point at which the targeted computer system is unable to handle further requests, and either crashes or appears offline, either temporarily or for a longer period.

Most computer systems are performance tested to handle a certain amount of traffic and requests, and what a DoS attack is doing is increasing the load on that system until it finds the upper limit that it can handle, then exceeds it in the hope of denying service to anyone wishing to use that service.

The affects of a DoS attack

A successful denial of service attack will have the affect of making a web site or service appear down or offline. Attempts to access it will result in time-outs or service not found responses. This denies the service that the website was providing to anyone that wished to access it, and may also cost the website owner in terms of lost revenue and in time and resources spent on bringing the service back on-line.

Who carries out DoS and DDoS attacks?

Denial of service attacks are usually carried out by experienced (malicious) computer users with a grudge against the owners of the system being targeted. Distributed denial of service attacks are carried out by multiple computers in various locations, or by the use of a botnet: a collection of computers infected with a trojan virus designed to assist in a DoS attack using hijacked computer resources, namely the infected machines and their internet connections.

Last word

Denial of Service attacks are a malicious attempt to knock a web site or service off-line by means of flooding the target computer system with so much traffic that it is unable to handle it and that it either crashes or fails to respond to legitimate requests.

There are security methods to limit a computer systems exposure to DoS attacks but they are not perfect and new methods to circumvent these methods are appearing all the time.

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Article date: 10th February 2011

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