Denial-of-service attack (DoS attack) is a cyber-attack where the perpetrator seeks to make a machine or network resource unavailable to its intended users by temporarily or indefinitely disrupting services of a host connected to the Internet.
In a distributed denial-of-service (DDoS) attack, the incoming traffic flooding the victim originates from many different sources – potentially hundreds of thousands or more. This effectively makes it impossible to stop the attack simply by blocking a single IP address; plus, it is very difficult to distinguish legitimate user traffic from attack traffic when spread across so many points of origin. There are two general forms of DoS attacks: those that crash services and those that flood services. The most serious attacks are distributed. Many attacks involve forging of IP sender addresses (IP address spoofing) so that the location of the attacking machines cannot easily be identified and so that the attack cannot be easily defeated using ingress filtering.
Court testimony shows us the first demostration of DoS attack was made by Khan C. Smith in 1997 during a Defcon event disrupting Internet access to the Las Vegas Strip for over an hour and the release of sample code during the event led to the online attack of Sprint, Earthlink, eTrade, and other major corportations in the year to follow.