Handling System Overload Resulting from DDoS Attacks and Flash Crowd Events

This paper presents a system that provides mitigation for DDoS attacks as a service, and is capable of handling flash crowd events at the same time. Providing DDoS protection as a service represents an important solution especially for Websites that have limited resources with no infrastructure in place for defense against these attacks. The proposed system is composed of two main components: (i) The distributed CAPTCHA service, which comprises a large number of powerful nodes geographically and suitably distributed in the Internet acting as a large distributed firewall, and (ii) The HTTPredirect module, which is a stateless HTTP server that redirects Web requests destined to the targeted Webserver to one of the CAPTCHA nodes. The CAPTCHA node can then segregate legitimate clients from automated attacks by requiring them to solve a challenge. Upon successful response, legitimate clients (humans) are forwarded through a given CAPTCHA node to the Webserver.