Presents a simple and robust mechanism called SYN-dog to sniff SYN flooding sources. We install SYN-dog as a software agent at leaf routers that connect stub networks to the Internet. The statelessness and low computation overhead of SYN-dog make itself immune to any flooding attacks. The core mechanism of SYN-dog is based on the protocol behavior of TCP SYN-SYN/ACK pairs, and is an instance of the sequential change detection. To make SYN-dog insensitive to site and access pattern, a non-parametric cumulative sum (CUSUM) method is applied, thus making SYN-dog much more generally applicable and its deployment much easier. Due to its proximity to the flooding sources, SYN-dog can trace the flooding sources without resorting to expensive IP traceback.