Software-Defined Networking (SDN) controllers are considered as Network Operating Systems (NOSs) and often viewed as a single point of failure. Detecting which SDN controller is managing a target network is a big step for an attacker to launch specific/effective attacks against it. In this paper, we demonstrate the feasibility of fingerpirinting SDN controllers. We propose techniques allowing an attacker placed in the data plane, which is supposed to be physically separate from the control plane, to detect which controller is managing the network. To the best of our knowledge, this is the first work on fingerprinting SDN controllers, with as primary goal to emphasize the necessity to highly secure the controller. We focus on OpenFlow-based SDN networks since OpenFlow is currently the most deployed SDN technology by hardware and software vendors.