Wouldn't that effectively multihome the machine, and bring with it all those related hassles?
Yes, as I (mis)understand it, it does.
However I have never experienced any of the problems that others say exist with multihomed DCs or boxen. (I'm not saying that they don't exist, just that I've not run in to them.)
The algorithm for the NetLogon service (in very broad view) would be a simple three-stage affair: Find all of the network interfaces on the machine; find all of the IP addresses bound to each interface; send the DDNS Update requests to register them.
I would be quite surprised if there wasn't an exclusion, either of network interfaces that are marked as loopback interfaces (in the first stage) or of IP addresses in 127.0.0.0/8 (in the second stage), preventing the registration of IP addresses and network interfaces that are not routable outside of the machine itself. This is the sensible thing to do, and it would be surprising to learn that it wasn't being done. There's a relatively easy way and documented way to do perform this sort of exclusion using the SIO_ADDRESS_LIST_QUERY WinSock ioctl
, which filters out machine-local and link-local IP addresses that are bound to software loopback network interfaces.
The existence of such a filter would explain why you've never had a problem, here.