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packet-forwarder.py

#!/usr/bin/python3 -u

"""
-> eth0: 192.168.17.36 -> 172.16.20.3 | protocol UDP | port 6000 | DATA |

<- eth1: 10.0.0.2 -> 10.0.0.1 | protocol UDP | port 6000 | DATA |


apt install --no-install-recommends python3-pcapy python3-scapy
"""

import sys
import pcapy
from scapy.layers.l2 import Ether
from scapy.layers.inet import IP
from scapy.sendrecv import send


def main():
    interface = "eth0"

    # Capture up to 65535 bytes.
    #
    # Pcapy does not support pcap_set_immediate_mode():
    # https://github.com/helpsystems/pcapy/issues/23
    # As a workaround, buffered mode with a short, 5ms read timeout is used.
    #
    cap = pcapy.open_live(interface, 65535, True, 5)
    cap.setfilter("udp and dst port 6000")

    print(f"Started to listen on {interface}")

    while True:
        # _pkthdr contains data like packet capture timestamp and
        # capture length.
        _pkthdr, frame = cap.next()

        try:
            eth_frame = Ether(frame)

            if eth_frame.haslayer("UDP"):
                udp_packet = eth_frame["UDP"]
                # print(udp_packet.show(dump=True))

                # Expects a route table entry for 10.0.0.1, e.g 10.0.0.2/30 on eth1.
                send(IP(dst="10.0.0.1") / udp_packet, verbose=False)

        except Exception as err:
            print(f"Error: {err!r}")
            continue


if __name__ == "__main__":
    try:
        main()
    except KeyboardInterrupt:
        sys.exit()