At first, I thought this was a bug, but then learned that RFCs 1112, 1122 and 1812 all specify that ICMP unreachables not be sent in response to multicast packets.
I'm struggling to grok the rationale behind not sending unreachables in response to multicast packets. It seems to me that our networks put IPv4 multicast speakers in a position where it's impossible for them to do the right thing.
For the exact same reason that replying to an ICMP Echo Request sent to your broadcast address is generally considered a Bad Idea.
The obvious solution is "Doctor, it hurts when I do that" "Don't do that anymore".
It's not as obvious to me as it is to you. I mean, v6 *requires* exactly this behavior, so it can't be all that bad, can it?
ICMP replies to multicast packets can cause ICMP "implosion". This is not a new discussion - see for instance http://mailman.nanog.org/pipermail/nanog/2012-June/048685.html Steinar Haug, Nethelp consulting, sthaug@nethelp.no