#!/usr/bin/env python3

# NOTE: as explained in 3d, I was working on this when something was wrong with
# the network, and as a result, didn't get the point of this exercise.
# This code does *not* work. I am quite confident I could get it to work, but
# don't have the time to redo everything. 

import socket
import struct
import math

mac_1 = b'\x00\x0f\xc9\x0c\xee\xed'
mac_2 = b'\x00\x0f\xc9\x0c\xf7\x93'

def parse_tcp(packet):
    header_length = packet[12] * 4
    header = packet[:14] # We don't care about the variable length options
    data = packet[header_length:]
    src_port, dst_port, seqn, ackn, flags = struct.unpack("!HHIIxB", header)
    return src_port, dst_port, seqn, ackn, flags, data

def parse_udp(packet):
    header_length = 8
    header = packet[:header_length]
    data = packet[header_length:]
    src_port, dst_port, data_len, checksum = struct.unpack("!HHHH", header)
    return src_port, dst_port, data_len, data, checksum

def parse_ip(packet):
    header_length_in_bytes = (packet[0] & 0x0f) * 4
    header = packet[:20]
    data = packet[header_length_in_bytes:]
    length, protocol, src, dst = struct.unpack("!xxHxxxxxBxx4s4s", header)
    header = {'length': length,
            'protocol': protocol,
            'source': src,
            'destination': dst}
    return header_length_in_bytes, header, data

def format_ip(addr):
    return '.'.join('%d'%i for i in addr)

def parse_eth(packet):
    if (packet[13:14] == b'\x81\x00'):
        dst, src, typecode = struct.unpack("!6s6sxxxx2s", packet[:18])
        data = packet[18:]
    else:
        dst, src, typecode = struct.unpack("!6s6s2s", packet[:14])
        data = packet[14:]
    return dst, src, typecode, data

def format_mac(addr):
    return ':'.join('%02x'%i for i in addr)

def main():
    s = socket.socket(socket.AF_PACKET, socket.SOCK_RAW, socket.ntohs(0x0003))
    while True:
        raw, address = s.recvfrom(2 ** 16 - 1)

        eth_dst, eth_src, eth_type, eth_data = parse_eth(raw)

        if eth_type == b'\x08\x00': # IP
            print("ETH:  {} --> {} ({})".format(
                format_mac(eth_dst), format_mac(eth_src), eth_type))

            ip_header_len, ip_header, ip_payload = parse_ip(eth_data)
            print("IP:   {} --> {} ({:04x})".format(
                format_ip(ip_header['source']), 
                format_ip(ip_header['destination']),
                ip_header['protocol']))

            if ip_header['protocol'] == 0x11: # UDP
                src_port, dst_port, _, udp_data, _ = parse_udp(ip_payload)
                print("UDP:  :{} --> :{}".format(
                    src_port, dst_port))
            elif ip_header['protocol'] == 0x06: # TCP
                src_port, dst_port, seqn, ackn, flags, tcp_data = parse_tcp(
                        ip_payload)
                print("TCP:  :{} --> :{} / SEQ:{} ACK:{} ({:#02x})".format(
                    src_port, dst_port, seqn, ackn, flags))

                if eth_src == mac_1:
                    eth_dst = mac_2
                    s.sendto(eth_dst + eth_src + eth_type + eth_data, ip_header['destination'])
                    print("Forwarded to {}".format(mac_2))
                elif eth_src == mac_2:
                    eth_dst = mac_1
                    s.sendto(eth_dst + eth_src + eth_type + eth_data, ip_header['destination'])
                    print("Forwarded to {}".format(mac_1))



        print()

if __name__ == "__main__":
    main()