Denial of Service

A denial of service attack is an effort to make one or more computer systems unavailable. It is typically targeted at web servers, but it can also be used on mail servers, name servers, and any other type of computer system.

Denial of service (DoS) attacks may be initiated from a single machine, but they typically use many computers to carry out an attack. Since most servers have firewalls and other security software installed, it is easy to lock out individual systems. Therefore, distributed denial of service (DDoS) attacks are often used to coordinate multiple systems in a simultaneous attack.

A distributed denial of service attack tells all coordinated systems to send a stream of requests to a specific server at the same time. These requests may be a simple ping or a more complex series of packets. If the server cannot respond to the large number of simultaneous requests, incoming requests will eventually become queued. This backlog of requests may result in a slow response time or a no response at all. When the server is unable to respond to legitimate requests, the denial of service attack has succeeded.

DoS attacks are a common method hackers use to attack websites. Since flooding a server with requests does not require any authentication, even a highly secured server is vulnerable. However, a single system is typically not capable of carrying out a successful DoS attack. Therefore, a hacker may create a botnet to control multiple computers at once. A botnet can be used to carry out a DDoS attack, which is far more effective than an attack from a single computer.

Denial of service attacks can be problematic, especially when they cause large websites to be unavailable during high-traffic times. Fortunately, security software has been developed to detect DoS attacks and limit their effectiveness. While many well-known websites, like Google, Twitter, and WordPress, have all been targets of denial of service attacks in the past, they have been able to update their security systems and prevent further service interruptions.

Updated August 11, 2011

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