Middleboxes in private networks have been known to change packets in many ways, making it hard to design protocol extensions that work for the large majority of Internet users. Addressing the need to know what such middleboxes do, we introduce a tool called fling ("flexible ping"). fling can carry out (almost) any kind of protocol dialogue between a server and a client based on a simple specification in a json and a pcap file, and identify what middleboxes do to the packets of the dialogue. This fills a gap in the state of the art, where other tools that control both ends of a path are either limited in some form or have to be updated for every new test. We present results from small-scale tests that prove the flexibility of fling, which is a prerequisite for our next step: development of a large-scale measurement platform.