Variable for the real port of a proxy?

[Mark Reginald James]

Hi,

Is there any way to set a header that identifies the real
port of the proxy that's handing a request? That is, if I
have a round-robin proxy-set, a request forwarded to the
proxy at port 8000 will have the header "X-Real-Port: 8000".

I've tried both the $proxy_port and the $server_port
variables, but they're both 80.

Thanks.