Go http request falls back to http2 even when force attempt is set to false

Question

There this weird thing that keeps happening every few requests where Go's http package throws the following error:

http2: server sent GOAWAY and closed the connection; LastStreamID=19999, ErrCode=NO_ERROR, debug=""

And so I realised that the default transport has ForceAttemptHTTP2 set to true(https://golang.org/src/net/http/transport.go line 48), so I manually set that to false like below:

    transport := &http.Transport{
        ForceAttemptHTTP2: false,
    }
    httpClient := &http.Client{Transport: transport}

But even after doing that, I still get the same http2 error instead of http1, which is not making sense to me?

I'm new to networking so I have a feeling I'm missing something that should be obvious here?

I guess the question would be how do I force the http package to only use http and not http2

Klemm

Not forcing HTTP/2 is not the same as disabling HTTP/2. – Mcgaha

Does this answer your question? https://mcmap.net/q/1444796/-how-to-disable-http-2-using-server-tlsnextproto/13860 – Mcgaha

Could you give me some more information on that? I'm not sure how I can apply that to an HTTP Client – Klemm

Yeah, good point. I'm not exactly sure how to translate that to the client, either. – Mcgaha

Answer 1

This seems to do it:

package main

import (
   "crypto/tls"
   "net/http"
)

func main() {
   client := &http.Client{
      Transport: &http.Transport{
         TLSNextProto: map[string]func(string, *tls.Conn)http.RoundTripper{},
      },
   }
   req, e := http.NewRequest("HEAD", "https://stackoverflow.com", nil)
   if e != nil {
      panic(e)
   }
   res, e := client.Do(req)
   if e != nil {
      panic(e)
   }
   println(res.Proto == "HTTP/1.1")
}

Programs that must disable HTTP/2 can do so by setting Transport.TLSNextProto (for clients) or Server.TLSNextProto (for servers) to a non-nil, empty map.

https://golang.org/pkg/net/http#pkg-overview

Admittedly, this is pretty awkward syntax, so you might prefer something like this instead:

package main

import (
   "net/http"
   "os"
)

func main() {
   os.Setenv("GODEBUG", "http2client=0")
   req, e := http.NewRequest("HEAD", "https://stackoverflow.com", nil)
   if e != nil {
      panic(e)
   }
   res, e := new(http.Client).Do(req)
   if e != nil {
      panic(e)
   }
   println(res.Proto == "HTTP/1.1")
}

Mnemonic

3rd (not less clunky) way: go build -tags nethttpomithttp2. – Kell

Thank you! This is what I was looking for! – Klemm