In this paper, it is advocated that defenders should take active action to stop DDoS attacks. We propose a new model based on Differential Games theory. Four main actors are included, Attacker, Defender, Victim, and Botnet. It is our belief that Victims who experience an attack should cooperate with Defender to defend for a DDoS-attack. The model indicates the minimum number of Bots that should blocked by Defender. A Differential Games model is used to determine how a Defender combats an Attacker and protect the servers. The feasibility and effectiveness of our approach is validated by measuring the performance of an experimental prototype against a series of attacks. The advantages of the scheme are discussed and further research directions are given.