Transparent socks5 server with route mark

socks5.go

package main

import (
	"flag"
	"fmt"
	"log"
	"net"
	"os"
	"syscall"

	"github.com/armon/go-socks5"
)

func setSocketMark(fd int, value int) (e error) {
	return syscall.SetsockoptInt(fd, syscall.SOL_SOCKET, syscall.SO_MARK, value)
}

func main() {
	listenaddr := flag.String("l", ":6488", "socks5 proxy listen address")
	mark := flag.Int("m", 0, "route mark")
	flag.Parse()
	dialer := &net.Dialer{
		Control: func(_, _ string, c syscall.RawConn) error {
			return c.Control(func(fd uintptr) {
				ex := setSocketMark(int(fd), *mark)
				if ex != nil {
					fmt.Printf("net dialer set mark error: %s", ex)
				}
			})
		},
	}
	// Create a SOCKS5 server
	config := &socks5.Config{
		Logger: log.New(os.Stdout, "SOCKS5Server", log.LstdFlags),
		Dial:   dialer.DialContext,
	}
	server, err := socks5.New(config)
	if err != nil {
		panic(err)
	}

	fmt.Printf("Listening on %s...\n", *listenaddr)
	fmt.Print("Press Ctrl+C to interrupt: ")

	// Create SOCKS5 proxy on localhost port 1080
	if err := server.ListenAndServe("tcp", *listenaddr); err != nil {
		panic(err)
	}
}