My overall goal is to not interrupt a production system while capturing HTTP Posts to a web application so that I can reverse engineer the telemetry coming from a closed application. I have control over the transmitter of the HTTP Posts but not the receiving web application.

It seems like I need a request "forking" proxy. Sort of a reverse proxy that pushes the request to 2 endpoints, a master and slave, only relaying the response from the master endpoint back to the requester. I am not a server geek so something like this may exist but I don't know the term of art for what I am looking for.

Another possibility could be a simple logging proxy. Capture a log of the web requests. Rewrite the log to target my "slave" web application. Playback the log with curl or something.

Thank you for your assistance.