proxy.go

package main

import (
	"crypto/tls"
	"flag"
	"fmt"
	"io"
	"log"
	"net"
	"net/http"
	"time"
)

func handleTunneling(w http.ResponseWriter, r *http.Request) {
	targetConnection, err := net.DialTimeout("tcp", r.Host, 10*time.Second)
	if err != nil {
		http.Error(w, err.Error(), http.StatusServiceUnavailable)
		return
	}
	w.WriteHeader(http.StatusOK)
	hijacker, ok := w.(http.Hijacker)
	if !ok {
		http.Error(w, "Hijacking not supported", http.StatusInternalServerError)
		return
	}
	clientConnection, _, err := hijacker.Hijack()
	if err != nil {
		http.Error(w, err.Error(), http.StatusServiceUnavailable)
	}
	go transfer(targetConnection, clientConnection)
	go transfer(clientConnection, targetConnection)
}

func transfer(target io.WriteCloser, source io.ReadCloser) {
	defer target.Close()
	defer source.Close()
	io.Copy(target, source)
}

func handleHTTP(w http.ResponseWriter, req *http.Request) {
	resp, err := http.DefaultTransport.RoundTrip(req)
	if err != nil {
		http.Error(w, err.Error(), http.StatusServiceUnavailable)
		return
	}
	defer resp.Body.Close()
	copyHeader(w.Header(), resp.Header)
	w.WriteHeader(resp.StatusCode)
	io.Copy(w, resp.Body)
}

func copyHeader(dst, src http.Header) {
	for k, vv := range src {
		for _, v := range vv {
			dst.Add(k, v)
		}
	}
}

func handlePing(w http.ResponseWriter, req *http.Request) {
	w.WriteHeader(http.StatusOK)
}

func main() {
	var port int
	flag.IntVar(&port, "port", 8888, "port")
	flag.Parse()

	server := &http.Server{
		Addr: fmt.Sprintf("0.0.0.0:%d", port),
		Handler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			if r.Method == http.MethodGet && r.URL.Path == "/ping" {
				handlePing(w, r)
				return
			}
			if r.Method == http.MethodConnect {
				handleTunneling(w, r)
			} else {
				handleHTTP(w, r)
			}
		}),
		// Disable HTTP/2.
		TLSNextProto: make(map[string]func(*http.Server, *tls.Conn, http.Handler)),
	}

	log.Fatal(server.ListenAndServe())

}