Simple tcp forwarding daemon

fwd.go

// Copyright (c) 2013, Moritz Bitsch <moritzbitsch@googlemail.com>
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice appear in all copies.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
// ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
// ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
// OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

package main

import (
	"bitbucket.org/madmo/daemonize"
	"flag"
	"fmt"
	"io"
	"net"
	"os"
	"os/signal"
	"syscall"
)

var (
	optListenAddress  = flag.String("l", ":443", "Set the listen address")
	optConnectAddress = flag.String("c", "127.0.0.1:22", "Connect address")
	optNoDaemon       = flag.Bool("D", false, "Do not fork into background")
)

func dropRoot() {
	if syscall.Getuid() == 0 {
		syscall.Setgid(65534)
		syscall.Setuid(65534)
	}
	if syscall.Getuid() == 0 {
		panic("still root")
	}
}

func HandleConnection(c net.Conn) {
	defer c.Close()
	cc, err := net.Dial("tcp", *optConnectAddress)
	if err != nil {
		fmt.Fprintf(os.Stderr, "Can't connect: %v\n", cc)
		return
	}
	defer cc.Close()

	go io.Copy(c, cc)
	io.Copy(cc, c)
}

func main() {
	flag.Parse()

	if !*optNoDaemon {
		p, err := daemonize.Daemonize(false, false)
		if err != nil {
			fmt.Fprintf(os.Stderr, "Can't daemonize: %v\n")
			return
		}
		if p != nil {
			return
		}
	}

	sig := make(chan os.Signal)

	signal.Notify(sig, os.Interrupt)

	l, err := net.Listen("tcp", *optListenAddress)
	if err != nil {
		fmt.Fprintf(os.Stderr, "Can't listen: %v\n", err)
		return
	}
	defer l.Close()

	dropRoot()

	go func() {
		for {
			c, err := l.Accept()
			if err != nil {
				fmt.Fprintf(os.Stderr, "Can't accept: %v\n", err)
				return
			}
			HandleConnection(c)
		}
	}()

	<-sig
	fmt.Println("Got Signal, exiting")
}