proxy.go

package main

import (
	"io"
	"log"
	"net"
	"sync"
	"time"
)

type CloseWriter interface {
	io.WriteCloser
	CloseWrite() error
}

func must(err error) {
	if err != nil {
		log.Fatal(err)
	}
}

func proxy(conn *net.TCPConn) {
	defer conn.Close()
	fwdAddr, err := net.ResolveTCPAddr("tcp", "127.0.0.1:6000")
	if err != nil {
		log.Println(err)
		return
	}

	fwd, err := net.DialTCP("tcp", nil, fwdAddr)
	if err != nil {
		log.Println(err)
		return
	}
	defer fwd.Close()

	wg := sync.WaitGroup{}
	wg.Add(2)

	transfer := func(d CloseWriter, s io.Reader) {
		if _, err := io.Copy(d, s); err != nil {
			log.Println(err)
		}
		if err := d.CloseWrite(); err != nil {
			log.Println(err)
		}
		wg.Done()
	}

	go transfer(fwd, conn)
	go transfer(conn, fwd)
	wg.Wait()
}

func main() {
	addr, err := net.ResolveTCPAddr("tcp", "127.0.0.1:5000")
	must(err)
	l, err := net.ListenTCP("tcp", addr)
	must(err)

	for {
		con, err := l.AcceptTCP()
		if err != nil {
			nerr, ok := err.(net.Error)
			if ok && nerr.Temporary() {
				time.Sleep(time.Millisecond * 10)
				continue
			}
			l.Close()
			log.Fatal(nerr)
		}
		proxy(con)
	}
}