net/http: How can I cancel a Dial in net/http/Transport DialContext or "how can http client use only tcp6."

[imfht]

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

go version go1.11 darwin/amd64

Does this issue reproduce with the latest release?

yes

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

GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/jinxufang/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/jinxufang/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/Cellar/go/1.11/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.11/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
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 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/5_/2mhh8j2n4tdgndp9_9b0stzm0000gn/T/go-build163807452=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I create my own DialContext because want I to detect if a domain is ipv6-only. code below

package main
import (
	"errors"
	"fmt"
	"net"
	"net/http"
	"syscall"
	"time"
)
func ModifiedTransport() {
	var MyTransport = &http.Transport{
		DialContext: (&net.Dialer{
			Timeout:   30 * time.Second,
			KeepAlive: 30 * time.Second,
			DualStack: false,
			Control: func(network, address string, c syscall.RawConn) error {
				if network == "ipv4" {
					// cancel connection here because the site could connect via IPv6  and client.Get("http://myexample.com") return a non-nil err.
					return errors.New("you should not use ipv4")
				}
				return nil
			},
		}).DialContext,
		MaxIdleConns:          100,
		IdleConnTimeout:       90 * time.Second,
		TLSHandshakeTimeout:   10 * time.Second,
		ExpectContinueTimeout: 1 * time.Second,
	}
	var myClient = http.Client{Transport: MyTransport}
	resp, myerr := myClient.Get("http://www.github.com")
	if myerr != nil {
		fmt.Println("request error")
		return 
	}
	var buffer = make([]byte, 1000)
	resp.Body.Read(buffer)
	fmt.Println(string(buffer))
}
func main(){
	ModifiedTransport();
}

What did you expect to see?

I want to cancel the request if domain can not connect via IPv6.

What did you see instead?

I do not now how to close the request even when I can enter network == "ipv4".

addition

Python can solve the question via https://stackoverflow.com/questions/33046733/force-requests-to-use-ipv4-ipv6. I do not know how to do it in golang. Could someone one help me?
Thanks so much!

[katiehockman]

I'm closing this issue. Please use a forum, where there will be more people who can help you. See https://golang.org/wiki/Questions . Thanks.