UDP Multicast on Host-Only LAN blocked on host

Discussion in 'Installation and Configuration' started by ThierryL4, Feb 6, 2022.

  1. ThierryL4

    ThierryL4 Bit Poster

    My problem: The macOS host cannot communicate with any VM using UDP multicast in a "host-only" virtual LAN.
    Using Parallels Destop 17.1.1 on macOS 12.1 (Intel), all VM running Linux (various distros, tests done with Fedora and Ubuntu).
    • Sending/receiving multicast from/to the macOS system works when communicating over Ethernet with an external physical Linux system (a Raspberry Pi). So, in general, multicast works on this macOS system. The problem is specific to the communication between the host and a Linux guest using Parallels.
    • On the macOS host, the firewall allows incoming connections. I even tried with the firewall temporarily disabled and it did not change the problem. And it works anyway with external physical systems, so it cannot be a macOS firewall issue.
    • The various Linux VM's successfully communicate between each other using the same interface on the "host-only" virtual LAN. The problem is specific to the communication between the host and any guest.
    • Running Wireshark everywhere (the host and all guests) at the same time shows the same display on all Wireshark instances. So, the same data are present on all interfaces of the virtual LAN.
    • Now, concentrating on the Wireshark instance running on the host:
      • When a Linux guest receives, we see the IGMP "join group" requests on Wireshark.
      • When a Linux guest sends, all multicast data packets are seen on Wireshark.
      • When the host receives, we do not see the IGMP join requests on Wireshark and the application never receives the data packets (which are displayed by Wireshark on the same host).
      • When the host sends, we do not see the outgoing data packets on Wireshark. The guests consequently never receive the data packets.
    • So, it seems that there is something blocking any multicast traffic on the macOS host, between the application and the Wireshark probing point. This happens only on the Parallels virtual network interface ("bridge101"). This blocage does not happen on the physical network interface of the Mac ("en0").
    Any idea?

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