This paper studies the traditional client puzzle scheme and proposes an adaptive scheme which erforms a lightweight client-server interaction to flexibly adjust the puzzle difficulty according to the eal-time statuses of both client and server. To evaluate the applicability, the authors combine the two schemes and develop an adaptive DoS-resistant security framework for Peer-to-Peer networks. The theoretical analyses and experimental results show that the adaptive client puzzle scheme can ffectively defend against various DoS attacks without significantly influencing legitimate clients’ experiences even in a highly malicious environment.