Go httputil.ReverseProxy not overriding the Host header

Question

I'm basically trying to write a reverse proxy server so that when I curl localhost:8080/get it proxies the request to https://nghttp2.org/httpbin/get.

Note: the https://nghttp2.org/httpbin/get service listed above is http/2. But this behavior happens with http/1 as well, such as https://httpbin.org/get.

I'm using httputil.ReverseProxy for this and I'm rewriting the URL while customizing the Host header to not to leak the localhost:8080 to the actual backend.

However, the request still hits the backend with Host: localhost:8080 no matter how many times I set it on the header. Similarly, I used mitmproxy to snoop on the request and it looks like the net/http.Client sets the :authority pseudo-header to localhost:8080

Here's my source code:

package main

import (
    "log"
    "net/http"
    "net/http/httputil"
)

func main() {
    proxy := &httputil.ReverseProxy{
        Transport: roundTripper(rt),
        Director: func(req *http.Request) {
            req.URL.Scheme = "https"
            req.URL.Host = "nghttp2.org"
            req.URL.Path = "/httpbin" + req.URL.Path
            req.Header.Set("Host", "nghttp2.org") // <--- I set it here first
        },
    }
    log.Fatal(http.ListenAndServe(":8080", proxy))
}

func rt(req *http.Request) (*http.Response, error) {
    log.Printf("request received. url=%s", req.URL)
    req.Header.Set("Host", "nghttp2.org") // <--- I set it here as well
    defer log.Printf("request complete. url=%s", req.URL)

    return http.DefaultTransport.RoundTrip(req)
}


// roundTripper makes func signature a http.RoundTripper
type roundTripper func(*http.Request) (*http.Response, error)
func (f roundTripper) RoundTrip(req *http.Request) (*http.Response, error) { return f(req) }

When I query curl localhost:8080/get the request gets proxied to https://nghttp2.org/httpbin/get. The echoed response shows that clearly my directives setting the Host header didn't do anything:

{
  "headers": {
    "Accept": "*/*",
    "Accept-Encoding": "gzip",
    "Host": "localhost:8080",
    "User-Agent": "curl/7.54.0"
  },
  "origin": "2601:602:9c02:16c2:fca3:aaab:3914:4a71",
  "url": "https://localhost:8080/httpbin/get"
}

mitmproxy snooping also clearly shows that the request was made with :authority pseudo-header set to localhost:8080:

Answer 1

From http.Request docs:

        // For server requests, Host specifies the host on which the URL
        // is sought. Per RFC 7230, section 5.4, this is either the value
        // of the "Host" header or the host name given in the URL itself.
        // It may be of the form "host:port". For international domain
        // names, Host may be in Punycode or Unicode form. Use
        // golang.org/x/net/idna to convert it to either format if
        // needed.
        // To prevent DNS rebinding attacks, server Handlers should
        // validate that the Host header has a value for which the
        // Handler considers itself authoritative. The included
        // ServeMux supports patterns registered to particular host
        // names and thus protects its registered Handlers.
        //
        // For client requests, Host optionally overrides the Host
        // header to send. If empty, the Request.Write method uses
        // the value of URL.Host. Host may contain an international
        // domain name.
        Host string

So the value of URL.Host is only used in case request.Host is empty which is not the case. Setting request.Host should resolve the issue:

req.Host = "nghttp2.org"

Related issue discussed here.