#! /usr/bin/env python

import argparse
from scapy.all import *
from scapy.contrib.gtp_v2 import *

def isValidIPv4(ipv4):
    try:
        socket.inet_aton(ipv4)
    except socket.error:
        return False
    return True

def get_args():
    """ Get User Arguements.  """

    desc = """
    Used to receive and parse Packet.
    """

    parser = argparse.ArgumentParser(description = desc)
    parser.add_argument('--filter', action = 'store', type = str, dest = 'filter', default = None, help = "Filter string used for sniff")

    usr_args =  parser.parse_args()

    return usr_args

def pkt_handler(pkt):
    try:
        msg = None

        if pkt[Ether].type == 0x800 and pkt[IP].proto == 17 and pkt[UDP].dport == 2152:

            """ Try to parse the packet as GTP packet.  """
            pkt[UDP].decode_payload_as(GTPHeader)
            pkt[GTPHeader].decode_payload_as(IP)
            if isValidIPv4(pkt[GTPHeader][IP].src) and isValidIPv4(pkt[GTPHeader][IP].dst):
                msg = pkt.sprintf(''
                    'Outer IP:: %IP.src% -> %IP.dst% \n'
                    '\t Outer UDP: %UDP.sport% : %UDP.dport% \n'
                    '\t GTP:: version: %GTPHeader.version%, teid: %GTPHeader.teid%, seq: %GTPHeader.seq%, type: %GTPHeader.gtp_type% \n'
                    '')
                msg += pkt[GTPHeader].sprintf(''
                    '\t Inner IP:: %IP.src% -> %IP.dst% \n'
                    '{ICMP: \t Inner ICMP:: type: %ICMP.type%, code: %ICMP.code%, seq: %ICMP.seq% \n}'
                    '{TCP: \t Inner TCP:: %TCP.sport% : %TCP.dport% \n'
                    '{Raw: \t %Raw.load% \n}}'
                    '{UDP: \t Innere UDP:: %UDP.sport% : %UDP.dport% \n'
                    '{Raw: \t %Raw.load% \n}}'
                    '\n')
        if msg is None:
            """ For all other packets.  """
            msg = pkt.sprintf(''
                'IP:: %IP.src% -> %IP.dst% \n'
                '{ICMP: \t ICMP:: type: %ICMP.type%, code: %ICMP.code%, seq: %ICMP.seq% \n}'
                '{UDP: \t UDP:: %UDP.sport% : %UDP.dport% \n'
                '{Raw: \t %Raw.load% \n}}'
                '{TCP: \t TCP:: %TCP.sport% : %TCP.dport% \n'
                '{Raw: \t %Raw.load% \n}}'
                '\n')
    except:
        print "[Warn] Unhandled packet.\n"
        pkt.summary()
    else:
        return msg

if __name__ == '__main__':
    usr_args = get_args()

    sniff(prn = pkt_handler, filter = usr_args.filter)