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The "UDP Send/Receive" program you mentioned is a program that can send and receive UDP unicast packets. To be able to receive multicast packets, a program must do either of:

  • Subscribe to the multicast group, this way Windows can tell the NIC to forward the multicast packets to the OS and then the OS can forward them to the application (in this case your application takes part as an endpoint in the multicast setup)
  • Capture packets of the wire using the WinPcap/Npcap library. It then needs to enable promiscuous mode on the NIC in order for it to send all the multicast traffic to the OS, even though no program subscribed to the multicast groups. (in this case your application is eavesdropping on the multicast group, just like Wireshark does)

Since Wireshark by default enables "Promiscuous mode" on a NIC that it starts capturing on, it will see the packets. Can Wireshark on your PC still see the UDP traffic when you disable "Promiscuous mode" in the Capture options before you start the capture?