flooding-based search mechanism is often used in unstructured P2P systems. Although a flooding-based search mechanism is simple and easy to implement, it is vulnerable to overlay distributed denial-of-service (DDoS) attacks. Most previous security techniques protect networks from network-layer DDoS attacks, but cannot be applied to overlay DDoS attacks. Overlay flooding-based DDoS attacks can be more damaging in that a small number of messages are inherently propagated to consume a large amount of bandwidth and computation resources. We propose a distributed and scalable method, DD-POLICE, to detect malicious nodes in order to defend P2P systems from overlay flooding-based DDoS attacks. We show the effectiveness of DD-POLICE by comprehensive simulation studies. We believe that deploying DD-POLICE will make P2P systems more scalable and robust.