>I did a test of DAD (actually, what I was really doing was getting
>tcpdump output of DAD occurring, to use in an example).

	DAD code actually works.  i'm sure about this because, in some cases,
	ethernet driver's bug (IFF_SUMPLEX flag is set but does not behave as
	IFF_SIMPLEX) triggers DAD failure.

	in your case i suspect there's a problem in ethernet driver's
	multicast filter that causes DAD fail to work.  i suspect that
	"b4" machine did not receive NA from "ad" machine.

	try running tcpdump on "b4" machine (so that "b4" would receive all
	packets) and perform the same test again, then you'll see
	fe80::dead:beef marked as "duplicated".

itojun