[OpenWrt-Devel] Regarding bridge multicast-to-unicast patch

Linus Lüssing linus.luessing at c0d3.blue
Wed Jun 3 18:39:03 EDT 2015

On Wed, Jun 03, 2015 at 07:25:32PM +0200, Steven Barth wrote:
> >Steven, could you elaborate a little more on the scenario and
> >explain why it is a must to shut up automatically when having a
> >local user-space querier? If people are able to manually install and
> >configure a complex multicast router, aren't they capable of
> >manually disabling the bridge querier, too (there's even a netifd
> >option for that)?
> I'm not thinking about complex router setups but instead about simple SSM capable proxies (i.e. mcproxy and the like). I think thats a valid point and don't see why v6 behaves rather awkwardly compared to v4 which simply goes silent.

Hm, actually I would have thought the "simply going silent" a little
awkward because it's not what the IGMP/MRD standards mandate. And
that could lead to some down time if for instance two bridges
decide to shut up because of bad timing. The "lower address"
requirement in the querier election was supposed to take care of
that awkwardness. On the other hand using zero-source addresses
basically breaks the election mechanism too, so I guess it's ok to
shut up in that case...

> If i run a userspace querier the kernel should shut up otherwise the userspace querier might get utterly confused since its suddenly competing against "itself" in the election.

Ah, right, they use the same source address, good point! And for
that case, if the source addresses are the same then the bridge
already does that, both for IPv4 and IPv6 :). If receiving the
same source address as the own one, then it will select that
entity (no matter if from above the bridge or any bridge port),
as the new querier and will shut up. (compare

> I mean the kernel can still do the mc-> uc translation but simply let userspace do the querying. Though i guess if all else fails we need to do this manually using /proc writes in the init script hmmm...

Keep in mind that if we disabled the bridge querier by default
and if there is no userspace querier then we won't be able to
do mc->uc translation (or any multicast snooping). People wouldn't
be able to use their multicast streaming app with the OpenWRT
default configuration.

Cheers, Linus
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