A distributed denial-of-service (DDoS) attack is one in which a bunch of compromised systems attack the target machine/server, 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.
The most common method of attack is to send a mass saturation of requests for external communication to the target server. These systems are flooded with requests for information from non-users, and often non-visitors to the website. The goal of this attack is to create a large enough presence of false traffic such that legitimate web traffic intended for actual web users is slowed down and delayed. If this type of service becomes too slow, time sensitive information such as live video footage may be rendered entirely useless to legitimate end users.
For more info on the linux commands and methods to mitigate DDOS Visit :
http://ndimensionz.com/kb/ddos-attck-prevention-in-linux-servers/
