mitmproxy/doc-src/features/upstreamproxy.html

In this mode, mitmproxy accepts proxy requests and unconditionally forwards all
requests to a specified upstream proxy server. This is in contrast to <a
href="@!urlTo("reverseproxy.html")!@">reverse proxy mode</a>, in which
mitmproxy forwards ordinary HTTP requests to an upstream server.

<table class="table">
    <tbody>
        <tr>
            <th width="20%">command-line</th> <td>-U http[s]://hostname[:port]</td>
        </tr>
        <tr>
            <th>mitmproxy shortcut</th> <td><b>U</b></td>
        </tr>
    </tbody>
</table>