Sniff and send UDP traffic using Scapy

Question

I followed the tutorial below to implement a packet sniffer in Python:

http://www.binarytides.com/python-packet-sniffer-code-linux/

On receiving each UDP packet, I would like to send an already saved pcap file (test.pcap). The following snippet shows my implementation:

# receive a packet
while True:
  packet = s.recvfrom(65565)

  #packet string from tuple
  packet = packet[0]

  #parse ethernet header
  eth_length = 14

  eth_header = packet[:eth_length]
  eth = unpack('!6s6sH' , eth_header)
  eth_protocol = socket.ntohs(eth[2])
  print 'Destination MAC : ' + eth_addr(packet[0:6]) + ' Source MAC : ' + 
    eth_addr(packet[6:12]) + ' Protocol : ' + str(eth_protocol)

  if eth_addr(packet[6:12]) != my_MAC_address:

      #Parse IP packets, IP Protocol number = 8
      if eth_protocol == 8 :
      #Parse IP header
      #take first 20 characters for the ip header
      ip_header = packet[eth_length:20+eth_length]

      #now unpack them :)
      iph = unpack('!BBHHHBBH4s4s' , ip_header)

      version_ihl = iph[0]
      version = version_ihl >> 4
      ihl = version_ihl & 0xF

      iph_length = ihl * 4

      ttl = iph[5]
      protocol = iph[6]
      s_addr = socket.inet_ntoa(iph[8]);
      d_addr = socket.inet_ntoa(iph[9]);

      print 'Version : ' + str(version) + ' IP Header Length : ' + str(ihl) + ' TTL : ' + str(ttl) + ' Protocol : ' + str(protocol) + ' Source Address : ' + str(s_addr) + ' Destination Address : ' + str(d_addr)


      #UDP packets
      if protocol == 17 :
         u = iph_length + eth_length
         udph_length = 8
         udp_header = packet[u:u+8]

         #now unpack them :)
         udph = unpack('!HHHH' , udp_header)

         source_port = udph[0]
         dest_port = udph[1]
         length = udph[2]
         checksum = udph[3]

         print 'Source Port : ' + str(source_port) + ' Dest Port : ' + str(dest_port) + ' Length : ' + str(length) + ' Checksum : ' + str(checksum)

         h_size = eth_length + iph_length + udph_length
         data_size = len(packet) - h_size

         #get data from the packet
         data = packet[h_size:]

         print 'Data : ' + data
         my_pkt = rdpcap("test.pcap")
         sendp(my_pkt)

Test.pcap contains a UDP packet with UDP_src=7777 and UDP_dest=9999.

Traffic is generated using netcat as follows:

nc -u -p 7777 ip_dst_addr 9999

The sniffer can receive only first netcat msg and send test.pcap in response. But subsequent netcat msgs are not received at all. However, using any other combination of UDP ports in netcat, the sniffer works fine. For example: running netcat as:

nc -u -p 8888 ip_dst_addr 9999

there is no problem and I am able to send test.pcap in response to each UDP packet/msg.

Any help would be greatly appreciated!

Sniff and send UDP traffic using Scapy

1 Answers