UDP relayer. Modified from https://github.com/EtiennePerot/misc-scripts/blob/master/udp-relay.py

udp-relay.python

#!/usr/bin/env python3
# Super simple script that listens to a local UDP port and relays all packets to an arbitrary remote host.
# Packets that the host sends back will also be relayed to the local UDP client.
# Works with Python 2 and 3
import ipaddress
import socket
import sys

# Whether or not to print the IP address and port of each packet received
debug = False


def fail(reason):
    sys.stderr.write(reason + '\n')
    sys.exit(1)


def addressMatches(a, b: (str, str, str, str)) -> bool:
    # convert embedded ipv4 to ipv4
    ip_a = ipaddress.ip_address(a[0])
    ip_b = ipaddress.ip_address(b[0])
    if ip_a.version == 6 and ip_a.ipv4_mapped is not None:
        ip_a = ip_a.ipv4_mapped
    if ip_b.version == 6 and ip_b.ipv4_mapped is not None:
        ip_b = ip_b.ipv4_mapped
    return ip_a == ip_b


if len(sys.argv) != 4:
    fail('Usage: udp-relay.py localPort remoteHost remotePort')

localPort, remoteHost, remotePort = sys.argv[1], sys.argv[2], sys.argv[3]

try:
    localPort = int(localPort)
except:
    fail('Invalid port number: ' + str(localPort))
try:
    remotePort = int(remotePort)
except:
    fail('Invalid port number: ' + str(remotePort))

try:
    s = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM)
    s.bind(('::', localPort))
except:
    fail('Failed to bind on port ' + str(localPort))

knownClient = None
knownServer = (remoteHost, remotePort, 0, 0)
print("known server: " + str(knownServer))
sys.stdout.write('All set, listening on ' + str(localPort) + '.\n')
while True:
    data, addr = s.recvfrom(32768)
    if knownClient is None or not addressMatches(addr, knownServer):
        if debug:
            print("")
        knownClient = addr

    if debug:
        print("Packet received from " + str(addr))

    if addr == knownClient:
        if debug:
            print("\tforwarding to " + str(knownServer))

        s.sendto(data, knownServer)
    else:
        if debug:
            print("\tforwarding to " + str(knownClient))
        s.sendto(data, knownClient)