Reverse proxy listen_port not taking

I’ve set up a simple reverse proxy on Ubuntu: mitmproxy --mode reverse: --set block_global=false --set listen_port=80

Earlier, I was getting “Error starting proxy server: PermissionError(13, ‘Permission denied’)” for the above, which seems to have melted away; the current issue is that the listen_port is being ignored. I get RSTs back for requests sent on 80 (8080 is still in play)…

Is everyone else able to successfully configure the port the proxy is listening on? I could change my web server to 8080, but that misses the point…

Some/any information/suggestions would be greatly appreciated!

Am I asking a stupid/difficult/redundant question?

I’m afraid I can’t figure this out for myself; and I find what documentation there is incomplete/outdated/contains errors… what do people actually use?