Not Seeing UDP Multicast Messages From Another Device

I have a Windows machine where I have two scripts that send and receive messages via UDP multicast (on the same machine). I have a C and Python3 implementation of this. The Python3

Solution 1:

As covered in https://www.tldp.org/HOWTO/Multicast-HOWTO-6.html, the sockets API requires that you identify the interface as well as the multicast address/port you want to use.

By not specifying this in your sample code, you have left this down to the OS to pick and it has picked a VirtualBox Host-Only Network. Unfortunately this type of network is limited to VMs running on the Windows machines.

To fix it, you need to identify the interface that you want to use for the multicast and pass that in to your sending and receving code. For example:

sender.py

sock.setsockopt(socket.SOL_IP, socket.IP_MULTICAST_IF, socket.inet_aton(MCAST_IF_IP))

receiver.py

mreq = struct.pack('4s4s', group, socket.inet_aton(MCAST_IF_IP))

where MCAST_IF_IP is the IP address of the interface that you want to use.