Bonjour is built on top of DNS. You don't need a layer 2 tunnel to make it work.
However, it normally does rely on multicast. Rather than trying to bridge broadcast domains (which is going to cause performance issues), a more efficient option is to setup an Avahi mDNS reflector on either end of the tunnel to rebroadcast mDNS packets.
Alternatively, there's also a Wide-Area Bonjour service that works over unicast and doesn't need any special packet forwarding, provided you run a Bonjour-aware DNS server:
You are technically correct (best kind of correct) however, in reality, I see folks using L2 tunnels to solve for bonjour etc all the time. Usually those without networking knowledge to solve the forwarding.
Yeah, you can do it the right way...or you can just tunnel layer 2 and forget about it. I see it done a fair bit for both Bonjour/Avahi and DHCP (why?).
However, it normally does rely on multicast. Rather than trying to bridge broadcast domains (which is going to cause performance issues), a more efficient option is to setup an Avahi mDNS reflector on either end of the tunnel to rebroadcast mDNS packets.
See, for example: https://www.reddit.com/r/WireGuard/comments/g80bxf/comment/h...
Alternatively, there's also a Wide-Area Bonjour service that works over unicast and doesn't need any special packet forwarding, provided you run a Bonjour-aware DNS server:
http://www.dns-sd.org/serversetup.html
https://help.dyn.com/bonjour-and-dns-discovery/
(More generally, Layer 2 tunnels are best avoided unless you really need them for something arcane, like IPX or NetBIOS.)