Topology Poisoning can be easily performed by injecting a fake link between SDN switches using a silent relay attack. This attack is difficult to detect because 1) the attacker is a passive man-in-the-middle to relay control messages between the compromised ports on SDN switches, and 2) such messages are genuine for the SDN switches and controller. This poster proposes Silent Relay Detector (SRD) that tailormakes the SDN control messages to be processed normally by authentic switches but to make the attacker malfunction. The SRD implementation and experiment reveal how the silent relay attack is detected and fake link injection is prohibited ingeniously. © 2018 Association for Computing Machinery.