Reading/Writing Linux's TUN/TAP device in Go.

tun-ping-linux.go

package main

import (
	"log"
	"os"
	"os/exec"
	"syscall"
	"unsafe"
)

func main() {
	file, err := os.OpenFile("/dev/net/tun", os.O_RDWR, 0)
	if err != nil {
		log.Fatalf("error os.Open(): %v\n", err)
	}

	ifr := make([]byte, 18)
	copy(ifr, []byte("tun0"))
	ifr[16] = 0x01
	ifr[17] = 0x10
	_, _, errno := syscall.Syscall(syscall.SYS_IOCTL, uintptr(file.Fd()),
		uintptr(0x400454ca), uintptr(unsafe.Pointer(&ifr[0])))
	if errno != 0 {
		log.Fatalf("error syscall.Ioctl(): %v\n", syscall.Errno(errno).Error())
	}

	// use ip instead of ifconfig to config the interface
	cmd := exec.Command("/sbin/ip")
	cmd.Args = []string{
		"addr", "add", "192.168.7.1/24", "peer", "192.168.7.2", "dev", "tun0",
	}

	if err := cmd.Run(); err != nil {
		log.Fatalf("error exec.Run(): %v\n", err)
	}

	// Set interface up explicitly if needed
	cmdUp := exec.Command("/sbin/ip")
	cmdUp.Args = []string{"link", "set", "dev", "tun0", "up"}
	if err := cmdUp.Run(); err != nil {
		log.Fatalf("error setting interface up: %v\n", err)
	}

	for {
		buf := make([]byte, 2048)
		read, err := file.Read(buf)
		if err != nil {
			log.Fatalf("error os.Read(): %v\n", err)
		}

		for i := 0; i < 4; i++ {
			buf[i+12], buf[i+16] = buf[i+16], buf[i+12]
		}
		buf[20] = 0
		buf[22] = 0
		buf[23] = 0
		var checksum uint16
		for i := 20; i < read; i += 2 {
			checksum += uint16(buf[i])<<8 + uint16(buf[i+1])
		}
		checksum = ^(checksum + 4)
		buf[22] = byte(checksum >> 8)
		buf[23] = byte(checksum & ((1 << 8) - 1))

		_, err = file.Write(buf)
		if err != nil {
			log.Fatalf("error os.Write(): %v\n", err)
		}
	}
}