Distributed denial-of-service (DDoS) defense is still a difficult problem though it has been extensively studied. The existing approaches are not capable of detecting various types of DDoS attacks. In particular, new emerging sophisticated DDoS attacks (e.g., Crossfire) constructed by low-rate and short-lived benign traffic are even more challenging to capture. Moreover, it is difficult to enforce realtime defense to throttle these detected attacks since the attack traffic can be concealed in benign traffic. Software defined networking (SDN) opens a new door to address these issues. In this paper, we propose Reinforcing Anti-DDoS Actions in Realtime (RADAR) to detect and throttle DDoS attacks via adaptive correlation analysis built upon unmodified commercial off-the-shelf SDN switches. It is a practical system to defend against a wide range of flooding-based DDoS attacks, e.g., link flooding (including Crossfire), SYN flooding, and UDP-based amplification attacks, while requiring neither modifications in SDN switches/protocols nor extra appliances. It accurately detects attacks by identifying attack features in suspicious flows, and locates attackers (or victims) to throttle the attack traffic by adaptive correlation analysis. We implement RADAR prototype using open source Floodlight controller, and evaluate its performance under various DDoS attacks by real hardware testbed based experiments. We observe that our scheme can successfully detect and effectively defend against various DDoS attacks with acceptable overhead.