Software Architect's Handbook

Denial-of-service

A denial-of-service (DoS) attack takes place whenever an attacker is able to deny service to valid users. An attacker can flood servers with packets to the point that the servers become unavailable or unusable. If a large number of bogus requests are sent to servers, they can be overloaded such that they cannot fulfill legitimate requests.

While a DoS attack may be conducted by a single computer, one type of DoS attack, known as a distributed denial-of-service (DDoS) attack, floods a victim from many different sources. This makes it much more difficult to block the source of the attack as well as to differentiate between legitimate traffic and traffic that is part of the attack.