What is distributed denial-of-service attack (DDoS) ? - Security Term

A distributed denial-of-service (DDoS) attack is one in which a multitude of compromised systems attack a single target, thereby causing denial of service for users of the targeted system. The flood of incoming messages to the target system essentially forces it to shut down, thereby denying service to the system to legitimate users.

In a typical DDoS attack, a hacker (or, if you prefer, cracker) begins by exploiting a vulnerability in one computer system and making it the DDoS master. It is from the master system that the intruder identifies and communicates with other systems that can be compromised. The intruder loads cracking tools available on the Internet on multiple -- sometimes thousands of -- compromised systems. With a single command, the intruder instructs the controlled machines to launch one of many flood attacks against a specified target. The inundation of packets to the target causes a denial of service.

While the press tends to focus on the target of DDoS attacks as the victim, in reality there are many victims in a DDoS attack -- the final target and as well the systems controlled by the intruder. Although the owners of co-opted computers are typically unaware that their computers have been compromised, they are nevertheless likely to suffer degradation of service and malfunction. Both owners and users of targeted sites are affected by a denial of service. Yahoo, Buy.com, RIAA and the United States Copyright Office are among the victims of DDoS attacks. DDoS attacks can also create more widespread disruption. In October 2010, for example, a massive DDoS attack took the entire country of Myanmar offline.

A computer under the control of an intruder is known as a zombie or bot. A group of co-opted computers is known as a botnet or a zombie army. Both Kaspersky Labs and Symantec have identified botnets -- not spam, viruses, or worms -- as the biggest threat to Internet security.

source: searchsecurity.techtarget