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In software-defined networking (SDN), a controller program updates the forwarding rules installed on network packet-processing devices in response to events. Such programs are often physically distributed, running on several nodes of the network, and this distributed setting makes programming and debugging especially difficult. Furthermore, bugs in these programs can lead to serious problems such as packet loss and security violations. In this paper, we propose a program synthesis approach that makes it easier to write distributed controller programs. The programmer can specify each sequential process, and add a declarative specification of paths that packets are allowed to take. The synthesizer then inserts enough synchronization among the distributed controller processes such that the declarative specification will be satisfied by all packets traversing the network. Our key technical contribution is …