Capturing UDP multicast packets in python on a apecific network adapter

Question

I have a sensor that sends udp packets on mulitcast group 224.0.2.2 to port 42102. It is connected to one of several network adapters in a computer, that has the address 10.13.1.100 assigned to it. The packets are visible as expected in Wireshark on the specified interface. The code I'm currently using:

import socket
import struct

# Create a UDP socket
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

# Bind the socket to the port
server_address = ('224.0.2.2', 42102)
sock.bind(server_address)

# Tell the kernel that we want to join a multicast group
# The address for the multicast group is the third parameter
group = socket.inet_aton('224.0.2.2')
mreq = struct.pack('4sL', group, socket.INADDR_ANY)
sock.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, mreq)
sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)

# Set the interface to listen on
interface = socket.inet_aton('10.13.1.100')
sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_IF, interface)

# Receive messages
while True:
    data, address = sock.recvfrom(1024)
    print('Received %s bytes from %s' % (len(data), address))
    print(data)

This reaches the recvfrom command and hangs, suggesting the socket is not receiving anything. I suspect there is some configuration error. Any help is appreciated

Answer 1

By joining the multicast group on interface socket.INADDR_ANY , you'll telling the OS to pick one interface to listen on, which may not be the one you want. You should set this to the specific interface IP.

group = socket.inet_aton('224.0.2.2')
interface = socket.inet_aton('10.13.1.100')
mreq = struct.pack('4sI', group, interface)
sock.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, mreq)

Setting IP_MULTICAST_IF does not specify which interface to listen on. It specifies which interface to send multicast traffic from.