repro.go

package main

import (
	"flag"
	"log"
	"io"
	"net"
	"golang.org/x/net/proxy"
)

var host = flag.String("host", "httpbin.org:80", "intercepting host")

func main() {
	flag.Parse()

	ln, err := net.Listen("tcp", ":1234")
	if err != nil {
		log.Println(err)
		return
	}

	log.Println("Listening...")
	for {
		conn, err := ln.Accept();
		if err != nil {
			log.Println(err)
		} else {
			switch conn.(type) {
			case *net.TCPConn:
				go handleConnection(conn.(*net.TCPConn))
			default:
				log.Println("Don't know how to handle conn:", conn)
			}
		}
	}
}

func handleConnection(conn *net.TCPConn) {
	log.Printf("CONNECTING: %s to %v\n", conn.RemoteAddr(), host)

	dialer, err := proxy.SOCKS5("tcp", "localhost:1080", nil, proxy.Direct)
	if err != nil {
		log.Println(err)
		conn.Close()
		return
	}

	_proxy, err := dialer.Dial("tcp", *host)
	if err != nil {
		log.Println(err)
		conn.Close()
		return
	}
	proxy := _proxy.(*net.TCPConn)

	go pipe(conn, proxy)
	go pipe(proxy, conn)
}

func pipe(from, to *net.TCPConn) {
	defer func() {
		log.Println("CLOSED WRITE:", to.RemoteAddr())
		to.CloseWrite()
	}()
	defer func() {
		log.Println("CLOSED READ:", from.RemoteAddr())
		from.CloseRead()
	}()

	const size = 64*1024
	var buf [size]byte
	for {
		n, err := from.Read(buf[0:size])
		if err != nil {
			if err != io.EOF {
				log.Println(err)
			}
			break
		} else {
			_, err := to.Write(buf[0:n])

			if err != nil {
				log.Println(err)
				break
			}
		}
	}
}