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

tun-ping-linux.go

package main

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

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

	cmd, err := exec.Run("/sbin/ifconfig",
		[]string{"ifconfig", "tun0", "192.168.7.1",
			"pointopoint", "192.168.7.2", "up"},
		nil, ".", 0, 1, 2)
	if err != nil {
		log.Exitf("error exec.Run(): %v\n", err)
	}
	cmd.Wait(0)

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

Normally TCPIP stack are handled by OS kernel. But you get layer 2 packet directly. If you want to handle this like generate HTTP request, you have to use userspace implemented tcp/ip stack such as gvisor/vpp etc or construct your own tcp/ip stack and handle it yourself.

Here is an example: https://github.com/WireGuard/wireguard-go/blob/master/tun/netstack/examples/http_server.go , it serves a http server under wireguard which is a layer3 VPN. In this case it handle layer3 packet directly to serve a http server.

In your case you get layer2 packet but convert it to layer3 is pretty easy, you need to fill/remove the layer2 header such as source MAC address and Destination MAC address so that you can convert a layer2 packet to layer3 packet.

Layer 3 to layer 4 you have to construct tcp/ip stack or use gvisor.