Distributed Denial-of-Service (DDoS) attacks pose a serious threat to internet security. Most current research focuses on detection and prevention methods on the victim server or source side. To date, there has been no work on defenses using valuable information from the innocent client whose IP has been used in attacking packets. In this paper, the authors propose a novel cooperative system for producing warning of a DDoS attack. The system consists of a client detector and a server detector.