Linux Ethernet Bridging - Aug 2023 - [Bridge] Getting + maintaining a consistent MDB view in userspace?

On Tue, 8 Aug 2023 02:29:09 +0200
Linus L?ssing <linus.luessing at c0d3.blue> wrote:
> Hi,
> 
> Is there a recommended way to get and monitor the MDB state from
> userspace, without potential synchronization races?
> 
> I can use "bridge monitor mdb" to get changes or "bridge mdb
show"
> to get the current state (as well as use the underlying netlink
> calls from my own application without the "bridge" command).
> 
> But if I were first calling "bridge mdb show" and then
"bridge
> monitor mdb" then there is a brief moment where I might miss some
> changes. Or if I were doing it the other way around then a change
> might wrongly override a valid entry? Or the full dump might not
> be as up to date as the change event I got before?
> 
> Ideally I'd be looking for a variant of "bridge monitor mdb"
which
> first dumps the whole, consistent mdb state and after that
> provides updates in the correct order.
> 
> Regards, Linus
Same issue as dumping routing table which routing daemons deal with.
Most applications end up with two netlink sockets, one for monitor
and one for dumping.

You really can't do it from the bridge utility, need to build your
own tools.

On Mon, Aug 07, 2023 at 06:20:51PM -0700, Stephen Hemminger
wrote:
> Same issue as dumping routing table which routing daemons deal with.
> Most applications end up with two netlink sockets, one for monitor
> and one for dumping.
> 
> You really can't do it from the bridge utility, need to build your
> own tools.
Thanks for the reply, ok.

For the old MLDv1/IGMPv2 output when having
"multicast_mld_version" set to 1 the output of "bridge mdb
show"
and "bridge monitor mdb" looks quite straightforward. And I can
piece something together with that (or its netlink messages underneath).
And I see symmetrical netlink events for additions and deletions.

When setting a bridge's "multicast_mld_version" to 2 it looks to
me like
there is quite a bit of redundancy in both "bridge mdb show"
and "bridge monitor mdb" output. And addition and deletion
notifications
don't look that symmetric to me then anymore. There doesn't
happen to exist some documentation about these (netlink specific)
additions that came with the IGMPv3/MLDv2 implementation, or does
it?

Also, are there tools people can recommend for Linux to test SSM
with various kinds of include/exclude settings?
The only tool I found for SSM so far is mcjoin:
https://github.com/troglobit/mcjoin
But it does not seem to support SSM in exclude mode yet.


The reason I'm interested in this is I'm currently experimenting
with creating proxied IGMP/MLD behaviour between bridge ports,
setup from a daemon in userspace. Which utilizes dummy network interfaces
and tc and is supposed to replicate multicast listeners.
Very early prototype: https://github.com/T-X/brmldproxy/tree/main