Linux Ethernet Bridging - Jan 2017 - [Bridge] [PATCH net-next] bridge: multicast to unicast

On Fri, Jan 06, 2017 at 01:47:52PM +0100, Johannes Berg
wrote:
> How does this compare and/or relate to the multicast-to-unicast feature
> we were going to add to the wifi stack, particularly mac80211? Do we
> perhaps not need that feature at all, if bridging will have it?
> 
> I suppose that the feature there could apply also to locally generated
> traffic when the AP interface isn't in a bridge, but I think I could
> live with requiring the AP to be put into a bridge to achieve a similar
> configuration?
> 
> Additionally, on an unrelated note, this seems to apply generically to
> all kinds of frames, losing information by replacing the address.
> Shouldn't it have similar limitations as the wifi stack feature has
> then, like only applying to ARP, IPv4, IPv6 and not general protocols?
(should all three be answered with Michael's and my reply to
Michael's mail, I think)
> 
> Also, it should probably come with the same caveat as we documented for
> the wifi feature:
> 
> ????Note that this may break certain expectations of the receiver,
> ????such as the ability to drop unicast IP packets received within
> ????multicast L2 frames, or the ability to not send ICMP destination
> ????unreachable messages for packets received in L2 multicast (which
> ????is required, but the receiver can't tell the difference if this
> ????new option is enabled.)
Actually, I do not quite understand that remark in the mac80211
multicast-to-unicast patch. IP should not care about the ethernet
header?

On Sat, 2017-01-07 at 16:15 +0100, Linus L?ssing wrote:
> Actually, I do not quite understand that remark in the mac80211
> multicast-to-unicast patch. IP should not care about the ethernet
> header?
But it does, for example RFC 1122 states:

?????????When a host sends a datagram to a link-layer broadcast address,
?????????the IP destination address MUST be a legal IP broadcast or IP
?????????multicast address.

 ????????A host SHOULD silently discard a datagram that is received via
?????????a link-layer broadcast (see Section 2.4) but does not specify
?????????an IP multicast or broadcast destination address.

You can probably find other examples too.

johannes