In today's world Denial-of-Service attacks have huge impact on network security. DoS attacks are usually launched to make the service of a system unavailable to a person who is authorized to use it. Several methods were introduced for defending Denial-of-Service attacks earlier. In our paper we propose a client-puzzle mechanism approach to defend DoS attacks. Here, intermediate routers can be used for issuing and solving network puzzles of various difficulty levels depending on the intensity of the attack. The target server will be protected by using an intermediate firewall router for issuing the puzzles; this will reduce the load over the server. Likewise intermediate proxy routers can be used for solving the puzzle. But furthermore, this proxy can also be a target of attack. This problem can be overcome by using a hybrid traceback mechanism for the attacking client. This technique helps to find out the attacking node and the router through which the attack packet was forwarded.