What version of Go are you using (go version)?

$ go version

go version go1.14.6 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

$ go env

What did you do?

What did you expect to see?

According to RFC 7230, absoluteURI should be passed when using proxy. When i set Host header, the host in absoluteURI is rewrote by the header, which is not acceptable. For example, using curl as follows:

curl -H '{Host:A}' http://B/some/page -x {proxy}

When capturing the traffic, we could get packet like this:

GET http://B/some/page HTTP 1.1\r\n
HOST A\r\n

While using http.Proxy in Go, i can set host by setting request.Host, but it cannot preserve original Host in absoluteURI.

u, _ := url.Parse(""proxy)
transport := &http.Transport{}
transport.Proxy = http.ProxyURL(u)
cli := &http.Client{Transport: transport}
req,  _ := http.NewRequest("GET", "http://B/some/page", nil)
req.Host="A"
cli.Do(req)

Packet captured is like this:

GET http://A/some/page.com 1.1\r\n
HOST A\r\n

Sometimes Host header can be used to verify the request, and Host is not equal with host domain in such case. I know proxy should rewrite Host according to RFC, but i think it's acceptable to distinguish host header and host in absoluteURI.

What did you see instead?

Maybe the user defined host won't rewrite the host in absoluteURI.
https://github.com/golang/go/blob/master/src/net/http/request.go#L585