This works okay on my home network, but it's a little more magical than
I wanted - I can't force it to pick up the wifi interface. If the
Ethernet is plugged in, the laptop always and only picks that, even
if I know the server only asked the Ethernet interface.
This is fine, but only because my Ethernet happens to be faster than
my Wifi. I'm not sure how it will behave at work, where WiFi and
Ethernet may be separate networks.
At least the error messages are better now, so I can figure out why
it wasn't auto-starting with systemd.
turns out you can join multiple multicast groups on the same socket.
And then I think the OS or the network somehow decides how to route
packets to you. Without touching anything else, if I'm plugged into
Ethernet, it picks that address (when running client on the desktop)
and when I unplug it, it picks the laptop's WiFi.
_