Thanks for this, and the other posts - will take a look at them. I wasn't hard-coding anything originally, I was trying to hard-code en0 purely to test if this was the problem. It seems that multicast seems to work great if there's no other interface on the device, but as soon as there's a VPN it fails. Can you confirm if the 'ipsecN' interfaces with the 192.0.0.6 address are related to Apple Watch (as that's the only thing we can think of that's different here!) Can you see anything obvious in my code above (even though referencing en0 isn't recommended!) because even with that it doesn't seem to work as expected.