I have STM3220G-Eval board with STM32F207 MCU. I've loaded UDP Echo Server lwIP based sample application (from CubeMX archive). This app used port #7. I've tried to use iperf3 in client mode (Windows OS), but it failed to work with the board (though Echotool successfully worked as a client). Can iperf3 work with custom UDP echo server?
Short answer: Not really. The iperf3 client and server need to communicate with each other over a control channel that is set up before the test starts. This allows them to exchange test parameters, ending conditions, and so on. If you wanted to make an iperf3 server on your embedded system, it would need to speak the (not very well documented) control protocol used by the iperf3 client.
iperf version 2 doesn't use a control channel; it might work for your application if all you need to do is send UDP packets to your board.
Bruce.