HTTP proxy with CONNECT

gistfile1.txt

package main

import (
	"flag"
	"fmt"
	"io"
	"log"
	"net"
	"net/http"
)

// Hop-by-hop headers. These are removed when sent to the backend.
// http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html
var hopHeaders = []string{
	"Connection",
	"Keep-Alive",
	"Proxy-Authenticate",
	"Proxy-Authorization",
	"Te", // canonicalized version of "TE"
	"Trailers",
	"Transfer-Encoding",
	"Upgrade",
}

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

func delHopHeaders(header http.Header) {
	for _, h := range hopHeaders {
		header.Del(h)
	}
}

type proxy struct {
}

func (p *proxy) handleConnect(w http.ResponseWriter, req *http.Request) error {
	hj, ok := w.(http.Hijacker)
	if !ok {
		return fmt.Errorf("writer %T doesn't support hijacking", w)
	}

	conn, _, err := hj.Hijack()
	if err != nil {
		return err
	}
	defer conn.Close()

	remoteConn, err := net.Dial("tcp", req.URL.Host)
	if err != nil {
		return err
	}
	defer remoteConn.Close()

	_, err = conn.Write([]byte("HTTP/1.0 200 OK\r\n\r\n"))
	if err != nil {
		return err
	}

	done := make(chan error, 2)
	go func() {
		_, err := io.Copy(conn, remoteConn)
		done <- err
	}()
	go func() {
		_, err := io.Copy(remoteConn, conn)
		done <- err
	}()

	return <-done
}

func (p *proxy) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	log.Printf("%s: %s %s", req.RemoteAddr, req.Method, req.URL)

	if req.Method == "CONNECT" {
		err := p.handleConnect(w, req)
		if err != nil {
			log.Printf("%s: %s", req.RemoteAddr, err)
		}
		return
	}

	if req.URL.Scheme != "http" && req.URL.Scheme != "https" {
		msg := "unsupported protocal scheme " + req.URL.Scheme
		http.Error(w, msg, http.StatusBadRequest)
		log.Println(msg)
		return
	}

	delHopHeaders(req.Header)
	req.RequestURI = "" // http: Request.RequestURI can't be set in client requests.

	client := &http.Client{}
	resp, err := client.Do(req)
	if err != nil {
		http.Error(w, "Server Error", http.StatusInternalServerError)
		log.Fatal("ServeHTTP:", err)
	}
	defer resp.Body.Close()

	log.Println(req.RemoteAddr, " ", resp.Status)

	delHopHeaders(resp.Header)

	copyHeader(w.Header(), resp.Header)
	w.WriteHeader(resp.StatusCode)
	io.Copy(w, resp.Body)
}

func main() {
	var addr = flag.String("addr", "127.0.0.1:8080", "The addr of the application.")
	flag.Parse()

	log.Println("Starting proxy server on", *addr)
	log.Fatal(http.ListenAndServe(*addr, &proxy{}))
}