Send custom ethernet packet using raw socket

Question

I searched how to send a raw ethernet packet using a custom ethertype but a lot of people talks abouts about tcp and udp protocols. I need to open a raw socket, take all the packets that have my custom ether type, read the payload and send back a packet with a different custom ether type.

func main() {
    //set promiscuos mode
    cmd := exec.Command("ifconfig", "eth0", "promisc")
    err := cmd.Run()
    if err != nil {
        fmt.Println("Error: " + err.Error())
        return
    }
    //open raw socket with custom ethertype_1 and bind to interface
    fd, err := syscall.Socket(syscall.AF_PACKET, syscall.SOCK_RAW, Htons(ETHER_TYPE_1))
    if err != nil {
        fmt.Println("Error: " + err.Error())
        return
    }
    err = syscall.BindToDevice(fd, "eth0")
    if err != nil {
        fmt.Println("Error: " + err.Error())
        return
    }
    f := os.NewFile(uintptr(fd), fmt.Sprintf("fd %d", fd))

    for {
        buf := make([]byte, 1024)
        numRead, err := f.Read(buf)
        if err != nil {
            fmt.Println(err)
        }
        go ReadSocket(buf, numRead)
    }

func ReadSocket(buf []byte, numRead int) {
    packet := BufToEthPacket(buf, numRead)

    fmt.Printf("Destination MAC: % X\n", packet.dstMac)
    fmt.Printf("Source MAC:      % X\n", packet.srcMac)
    fmt.Printf("ether type:      %X\n", packet.ethType)
    fmt.Printf("Payload:         % X\n", packet.payload)

    var myPacket EthPacket
    myPacket.srcMac = packet.dstMac
    myPacket.dstMac = packet.srcMac
    myPacket.ethType = ETHER_TYPE_2
    myPacket.payload = packet.payload

    var myBuf = EthPacketToBuf(myPacket)

    //open raw socket with custom ethertype_2 and bind to interface
    fd, err := syscall.Socket(syscall.AF_PACKET, syscall.SOCK_RAW, Htons(ETHER_TYPE_2))
    if err != nil {
        fmt.Println("Error: " + err.Error())
        return
    }
    err = syscall.BindToDevice(fd, "eth0")
    if err != nil {
        fmt.Println("Error: " + err.Error())
        return
    }
    f := os.NewFile(uintptr(fd), fmt.Sprintf("fd %d", fd))

    n, err := f.Write(myBuf)
    if err != nil {
        fmt.Println("Error: " + err.Error())
        return
    }
    if n != numRead {
        fmt.Println("Error: byte length not equal")
        return
    }
}

I received the packet but the f.Write(myBuf) return me the following error: Error: write fd 5: no such device or address

What I'm doing wrong?

Answer 1

Using os.Write will execute a write syscall in the background, which is meant for actual files. To "write" data to a network socket you need to use the sendto syscall.

The following example sends data with a custom ether type. So just an ethernet packet with some data.

package main

import (
    "log"
    "net"
    "os"
    "syscall"
)

func main() {
    ifname := os.Args[1]
    iface, err := net.InterfaceByName(ifname)
    if err != nil {
        log.Fatal("get link by name:", err)
    }

    srcMac := iface.HardwareAddr
    if len(srcMac) == 0 {
        srcMac = []byte{0, 0, 0, 0, 0, 0}
    }
    dstMac := []byte{0x00, 0x01, 0x02, 0x03, 0x04, 0x05}

    fd, _ := syscall.Socket(syscall.AF_PACKET, syscall.SOCK_RAW, int(htons(syscall.ETH_P_ALL)))
    addr := syscall.SockaddrLinklayer{
        Ifindex: iface.Index,
        Halen:   6, // Ethernet address length is 6 bytes
        Addr: [8]uint8{
            dstMac[0],
            dstMac[1],
            dstMac[2],
            dstMac[3],
            dstMac[4],
            dstMac[5],
        },
    }

    ethHeader := []byte{
        dstMac[0], dstMac[1], dstMac[2], dstMac[3], dstMac[4], dstMac[5],
        srcMac[0], srcMac[1], srcMac[2], srcMac[3], srcMac[4], srcMac[5],
        0x12, 0x34, // your custom ethertype
    }

    // Your custom data
    p := append(ethHeader, []byte("Hello World")...)

    err = syscall.Sendto(fd, p, 0, &addr)
    if err != nil {
        log.Fatal("Sendto:", err)
    }
}

// htons converts a short (uint16) from host-to-network byte order.
func htons(i uint16) uint16 {
    return (i<<8)&0xff00 | i>>8
}