A denial-of-service attack (DoS attack) is an attempt to deny service to legitimate users by overwhelming the server(s) and/or networks with a high number of requests. With some servers, a DoS can cause a system crash that may be classified as severe downtime.
When a DoS attack is orchestrated on a web server, the requests can elicit documents, images, videos, error pages, etc., which are intended to consume a high number of server and network resources to create the impression of an "outage."
For additional information about DoS attacks, see:
- Wikipedia entry: http://en.wikipedia.org/wiki/DoS
- The Hack FAQ; DoS attacks - http://www.nmrc.org/pub/faq/hackfaq/hackfaq-05.html
- CERT/CC; Denial of Service - http://www.cert.org/tech_tips/denial_of_service.html
- US-CERT; understanding DoS attacks - http://www.us-cert.gov/cas/tips/ST04-015.html