Distributed denial of service (DDoS) is a major threat to the availability of Internet services. The anonymity allowed by IP networking, together with the distributed, large scale nature of the Internet, makes DDoS attacks stealthy and difficult to counter. As various attack tools become widely available and require minimum knowledge to operate, automated anti-DDoS systems are increasingly important. This paper studies the problem of providing an anti-DoS service (called AID) for general-purpose TCP-based public servers. We design a random peer-to-peer (RP2P) network that connects the registered client networks with the registered servers. RP2P is easy to manage and its longest path length is just three hops. The AID service ensures that the registered client networks can always access the registered servers even when they are under DoS attacks. It creates the financial incentive for commercial companies to provide the service, and meets the need for enterprises without the expertise to outsource their anti-DoS operations.