Software defined networking (SDN) has created new opportunities for both the research community and the networking industry, but SDN also has created new challenges. One of the most fundamental SDN security issues we have observed is the absence of data plane authentication. We present a definition of this problem and a controller and switch design called Mynah that makes a first step towards addressing this problem. We then present a prototype experiment results and show that Mynah solves the issue very effectively with a trivial overhead around 4.5 percent of overall communication latency.