FADM: DDoS Flooding Attack Detection and Mitigation System in Software-Defined Networking. Distributed Denial-of-Service (DDoS) flooding attack is one of the most serious threats to network security. Software-Defined Networking (SDN) has recently emerged as a new network management platform, and its centralized control architecture brings many new opportunities for defending against network attacks. In this paper, we propose FADM, an efficient and lightweight framework to detect and mitigate DDoS attacks in SDN. Firstly, the network traffic information is collected through the SDN controller and sFlow agents. Then an entropy-based method is used to measure network features, and the SVM classifier is applied to identify network anomalies. By adopting these methods together, the timeliness and accuracy of attack detection are effectively improved. To keep the major network functionality working, we propose an efficient attack mitigation mechanism based on the white-list and traffic migration. By introducing the mitigation agent to the network, attack traffic can be timely blocked while benign traffic can be forwarded as usual, which prevents the controller resources from being exhausted and ensures that legitimate users can access the network normally. The experimental results show that multiple DDoS attacks can be accurately detected and effectively mitigated by FADM, which enables the network to recover in a short time.