What is a Distributed Denial-of-Service attack?

A Distributed Denial-of-Service (DDoS) is a type of DoS attack that is launched from compromised hosts distributed within or across networks. A DDoS attack is coordinated across many systems, all controlled by a single attacker known as a master. Prior to the attack, the master compromises a large number of hosts, typically without their owners’ knowledge, and installs software that will later enable the coordinated attack. These compromised hosts called zombies are then used to perform the actual attack. Zombies are also called daemons, agents, slaves, or bots. When the master is ready to launch the attack, every available zombie is contacted and instructed to attack a single victim. The master is not a part of the attack; hence tracing the true origin of a DDoS attack is very difficult. As with a DoS attack, packets sent from each zombie might be spoofed to fool the victim into accepting data from the trusted source. DDoS allows the attackers to utilize the network to multiplex low-volume sources into a high-volume stream to overwhelm the targets. Through the master-zombie communication, the real attackers can potentially hide their identities behind the zombies.