The Distributed Denial of Service attack (DDoS) is an important variation of the DoS attack. With a DDoS attack, computers compromised by the attacker participate in a DoS attack against a single target. Even if the target computer is not directly hit by the incoming DoS packets, the effects of thousands (or even tens of thousands) of requests made for resources will overwhelm the target network, ultimately causing a crash. Some examples of DDoS attacks include:

• Smurf
• Tribe Flood Network (TFN)
• Stacheldraht

DDoS attacks require an initial intrusion phase where automated tools compromise large numbers of systems to use in an attack. The initial intrusion phase is followed by the actual attack. The large number of compromised computers is called a botnet; the compromised computers are called zombies.