tproxy is a single threaded multiplexed proxy and a relay with trigger and logging capabilities. Suppose your client-server application has a server running on port 8000. If you modify the server to run on port 8001 and you put tproxy in the middle, it will relay all traffic from 8000 to 8001. Doing so makes tproxy function as a user level tap. tproxy lets you increment debug counters so that you can use standard Unix tools to trove through logs after the execution and testing time.