> This required some hacking in mrouted and the kernel in order to
> decouple the notion of "IP address" from "interface identifier".

I'd like to ask you a very simple question.

Why don't you define another vif flag, e.g., VIFF_LOGICAL, instead of
an obsolete VIFF_TUNNEL?
gif, gre etc. have each if_type, then you can distinguish the
interface is physical or logical (virtual) interface.
If you distinguish the interface's attribute, then you can skip
routines of unneed check for a logical interface.

Something unwilling?
--
Hitoshi Asaeda