This paper presents a method for DDoS attack detection in source network. This method is based on BPF packet filtering to monitor packets on the gateway of the source network. The information entropy of the destination IP address of the network packets is used to detect the suspicious traffic. Finally, the connection tracing technique is used to judge the suspicious traffic. This method can effectively identify the traffic of SYN Flood, and directly intercepts and saves the computing resources in the source network.