The defense against distributed denial of service (DDoS) attacks is one of the primary concerns on the Internet today. IP spoofing makes it difficult for the victim to determine the packet's origin. There is a need for a mechanism that could enable realtime traceback of the origins of the attacks. In this paper, we propose a novel protocol, fast autonomous system traceback (FAST) to traceback to the attack originating autonomous systems (AS). The multifold advantages of FAST include reconstruction requires just around 5 to 10 packets and reconstruction takes just a few seconds. We validate the performance through extensive simulations over the datasets obtained from traceroute.