Go RoundTrip/Transport Proxy Address

Question

I understand that the Proxy field of http.Transport asks for a function that generates proxy server addresses. So this is my roundtripper:

roundtripper := &http.Transport{
    Proxy: proxyrouter.Calculateproxy,
...
}

So the type of Proxy is func(*Request) (*url.URL, error). This gets linked to the server and is later on called with:

response := roundtripper.RoundTrip(request)

Which returns the response. Now is there any way to know what proxy address was used to get this response? (since my Calculateproxy function just takes random addresses)

Answer 1

Have the Proxy function add a header to record the proxy server used:

Transport{
    Proxy: func(req *Request) (*url.URL, error) {
        p, err := proxyrouter.Calculateproxy(req)
        if err != nil {
            return err
        }
        req.Header.Set("X-Proxy-Addr", p.String())
        return req, nil
    },
}

The http.Response has a reference to the originating request

proxy := resp.Request.Header.Get("X-Proxy-Addr")