Distributed denial-of-service attack (DDoS attack) is an attempt by a hacker to make a machine or network resource unavailable for its intended users.

The target of an automated denial of service DDos attack gets flooded with dummy requests, which take up all resources and overwhelm the system's ability to serve additional visitors.

Acting preemptively against DDOs is a challenge. There is virtually little that can be done to avoid being a victim of a DDoS attack. There are some load-balancing tools that can distribute requests among many servers. A load-balanced system can handle more requests but it can become a victim as well. Other mechanisms try to automatically analyze the origins of the attach and rate limit or fully block the sources. Despite all measures a large scale DDoS attack can hardly pass unnoticed by legitimate users.