Simple TCP forwarding implementation, part of the code comes from shadowsocks/shadowsocks-go

leakybuf.go

// https://github.com/shadowsocks/shadowsocks-go/blob/master/LICENSE

package main

type LeakyBuf struct {
	bufSize  int
	freeList chan []byte
}

const leakyBufSize = 4096
const maxNBuf = 2048

var leakyBuf = NewLeakyBuf(maxNBuf, leakyBufSize)

func NewLeakyBuf(n, bufSize int) *LeakyBuf {
	return &LeakyBuf{
		bufSize:  bufSize,
		freeList: make(chan []byte, n),
	}
}

func (lb *LeakyBuf) Get() (b []byte) {
	select {
	case b = <-lb.freeList:
	default:
		b = make([]byte, lb.bufSize)
	}
	return
}

func (lb *LeakyBuf) Put(b []byte) {
	if len(b) != lb.bufSize {
		panic("invalid buffer size that's put into leaky buffer")
	}
	select {
	case lb.freeList <- b:
	default:
	}
	return
}

main.go

package main

import (
	"flag"
	"log"
	"net"
)

var localAddr, remoteAddr string

func main() {
	flag.StringVar(&localAddr, "l", "0.0.0.0:80", "local addr")
	flag.StringVar(&remoteAddr, "r", "127.0.0.1:8080", "remote addr")
	flag.Parse()

	tcpAddr, err := net.ResolveTCPAddr("tcp", localAddr)
	if err != nil {
		log.Println(err)
		return
	}
	tcpListener, err := net.ListenTCP("tcp", tcpAddr)
	if err != nil {
		log.Println(err)
		return
	}
	for {
		tcpConn, err := tcpListener.AcceptTCP()
		if err != nil {
			log.Println(err)
			continue
		}
		go tcpHandler(tcpConn)
	}
}

func tcpHandler(tcpConn *net.TCPConn) {
	dstConn, err := net.Dial("tcp", remoteAddr)
	if err != nil {
		log.Println(err)
		return
	}
	go pipe(tcpConn, dstConn)
	pipe(dstConn, tcpConn)
}

func pipe(src, dst net.Conn) {
	defer dst.Close()
	buf := leakyBuf.Get()
	defer leakyBuf.Put(buf)
	for {
		n, err := src.Read(buf)
		if n > 0 {
			if _, err := dst.Write(buf[0:n]); err != nil {
				log.Println(err)
				break
			}
		}
		if err != nil {
			break
		}
	}
	return
}