It now appears to be completely random with regard to when server will respond to the multicast packets and when it won’t. But, it works great with an old usb nic I found.
Definitely an issue with the nic not seeing the multicast packets. The clue there is when tcp dump shifts the box into promiscuous mode it works.
Also, the randomness is probably because when the server first start, it gratuitously pre-caches mdns info for about ten minutes or so.
So when you start it, even if it can’t see queries, the queries get answered and it populates iTunes browses because of the gratuitous mdns pre-caching.
Well, and the “other nic works” thing is kind of a dead giveaway too.