A distributed denial-of-service (DDoS) attack is an attack wherein multiple compromised computer systems flood the bandwidth and/or resources of a target, such as a server, website or other network resource, and cause a denial of service for users of the targeted resource. The flood of incoming messages, connection requests or malformed packets to the target system forces it to slow down or even crash and shut down, thereby denying service to legitimate users or systems. This paper presents a literature review of DDoS attacks and the common defense mechanisms available. It also presents a literature review of the defenses for low-rate DDoS attacks that have not been handled effectively hitherto.