Distributed Denial of Service Attack (DDoS) has emerged as a major threat to cyber space. A DDoS attack aims at exhausting the resources of the victim causing financial and reputational damages to it. The availability of free software make launching of DDoS attacks easy. The difficulty in differentiating a DDoS traffic from a legitimate traffic burst such as a flash crowd makes DDoS difficult to be identified. A wide range of techniques have been used in conventional networks to detect and mitigate DDoS attacks. Though the advent of Software Defined Networking (SDN) makes a network easy to be managed even SDN is vulnerable to DDoS attacks. In this case, the controller of the SDN gets overloaded with the incoming packets from the switches. In fact, a solution based on security analytics can be put in place to ward off this threat as a proactive security measure using the flow level statistics available from the SDN. Compared to the packet analysis used in traditional networks which is resource expensive the flow level statistics is relatively inexpensive. This paper focuses on the design and implementation of an attack detection system for detecting the flooding DDoS attacks TCP SYN flooding attacks, HTTP request flooding attacks, UDP flooding attacks and ICMP flooding attacks over SDN network traffic. The system uses various classification algorithms to classify a traffic into normal or attack. The feature sets for classification were arrived at using a feature selection module with ANOVA (Analysis of Variance) F-Test statistical method. Performance evaluation of each of the classifiers was carried out for the three feature sets obtained from the feature selection module using various performance measures and the results have been tabulated. The feature set which gives the best performance in detecting malicious traffic has been identified.