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)
		}
	}
}

are there any way to pass the request to TCPIP stack?

are there any way to pass the request to TCPIP stack?

not sure what you mean

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.