net/http: disabling HTTP/2 as per documentation does not work

[rittneje]

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

$ go version
go version go1.13.10 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/tmp/.gocache"
GOENV="/Users/jrittner/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/jrittner/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/jrittner/go1.13.10"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/jrittner/go1.13.10/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="0"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/qz/jbyzqww512dfd00_vfwk74l4zy5gvy/T/go-build718180347=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I set up an http.Transport with a non-nil, empty map for TLSNextProto in order to disable HTTP/2, as per the documentation.

https://golang.org/pkg/net/http/

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.

 transport := http.DefaultTransport.(*http.Transport).Clone()
 transport.TLSNextProto = make(map[string]func(authority string, c *tls.Conn) http.RoundTripper)
 httpClient := &http.Client{
   Transport: transport,
 }
 resp, err := httpClient.Get("https://google.com")
 if err != nil {
   panic(err)
 }
 defer resp.Body.Close()
 fmt.Println("Response status:", resp.Status)

What did you expect to see?

I expected it to do HTTP/1.1 over TLS.

What did you see instead?

panic: Get https://google.com: net/http: HTTP/1.x transport connection broken: malformed HTTP response "\x00\x00\x12\x04\x00\x00\x00\x00\x00\x00\x03\x00\x00\x00d\x00\x04\x00\x10\x00\x00\x00\x06\x00\x00@\x00\x00\x00\x04\b\x00\x00\x00\x00\x00\x00\x0f\x00\x01\x00\x00\x1e\a\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01http2_handshake_failed"

[andybons]

@FiloSottile @katiehockman @bradfitz @neild

[odeke-em]

Thank you for this report @rittneje!

It does work as advertised, except that you are using a http.DefaultTransport.Clone() which clones only the exported fields of that Transport and yet http.DefaultTransport is initialized with some internal variables. The problem you are encountering is an inconsistency in what's expected internally and the TLS handling function is returned as nil which causes that error.

You can ensure that this works by creating a fresh transport

package main

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

func main() {
	httpClient := &http.Client{
		Transport: &http.Transport{
			TLSNextProto: make(map[string]func(authority string, c *tls.Conn) http.RoundTripper),
		},
	}
	resp, err := httpClient.Get("https://google.com/")
	if err != nil {
		panic(err)
	}
	defer resp.Body.Close()
	fmt.Println("Response status:", resp.Status)
}

or alternatively if you want to entirely disable HTTP/2, you can set in your environment

GODEBUG=http2client=0

I am going to send a CL showing clear examples for what I've talked about and that'll perhaps serve as a reference for disabling HTTP/2.

[gopherbot]

Change https://golang.org/cl/235523 mentions this issue: net/http: add examples for disabling HTTP/2 in Transport

[odeke-em]

I forgot to mention, that if you also want to use a cloned http.DefaultTransport with HTTP/1, you'll need to ensure:
a) ForceAttemptHTTP2 = false
b) TLSClientConfig = new(tls.Config)

package main

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

func main() {
	tr := http.DefaultTransport.(*http.Transport).Clone()
	tr.ForceAttemptHTTP2 = false
	tr.TLSNextProto = make(map[string]func(authority string, c *tls.Conn) http.RoundTripper)
	tr.TLSClientConfig = &tls.Config{}
	httpClient := &http.Client{
		Transport: tr,
	}
	resp, err := httpClient.Get("https://google.com/")
	if err != nil {
		panic(err)
	}
	defer resp.Body.Close()
	fmt.Println("Response status:", resp.Status)
}

which will directly reflect in your programs.