Re: macvtap with disconnected physical interface

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On Tue, May 03, 2022 at 11:15:26PM +0200, Gionatan Danti wrote:
> Il 2022-05-03 14:58 Laine Stump ha scritto:
> > I can't say that I've ever tried this, since my only reason for using
> > macvtap is to provide the guests with direct connectivity to the
> > physical network, and unplugging the physdev negates that. The
> > behavior you describe doesn't surprise me all that much though, since
> > the physical device in the case of a host bridge isn't an integral
> > part of the bridge (it's just one more device attached to a port),
> > while the physical device and macvlan bridge a much more closely
> > associated.
> Yeah, it sound very plausible.
> > I'm Cc'ing Michael Tsirkin to see if he has more authoritative
> > information on whether or not the macvtaps connected to a macvlan
> > bridge can communicate amongst themselves when the physdev is
> > disconnected.
> Thanks.
> > In the meantime, is there a reason you don't want to just use a
> > standard host bridge that's not connected to any physdev? The one
> > thing I can think of is that you might not want to allow communication
> > between the host and guests, but as long as the bridge itself isn't
> > given an IP address, that won't be possible (at least at the level of
> > IP).
> I generally use plain bridge for my KVM setup. Specifically, when using
> VLANs I setup the following:
> eth -> eth.xx -> bridge -> vnet
> This time, however, I need *both* a trunk-enabled VM (a virtual firewall)
> and other segregated virtual machines. A "plain" bridge setup would be
> something as:
> eth -> bridge -> bridge.xx -> bridge -> vnet
> Notice the two bridges, needed because bridge.xx is a VLAN interface when no
> vnet can be directly attached. To avoid the double bridges, I tried the
> following:
> eth -> bridge -> bridge.xx -> macvtap
> It seems to work very well but, during testing, I discovered that if the
> interface under the macvtap one (in this case the bridge itself) goes down,
> inter-guest networking is lost. As a side note, in the specific scenario I
> described above, such issues can not really happen: as a vnet interface is
> going to be always bound to the first bridge, it will be *always* up due to
> the vnet interface itself being always up (irrespective of the physical link
> status) and forcing the bridge up.

well this is kind of by design. if we don't force guest's link
down it will think it can access internet, with all packets
being lost.
I see how you might want that in some cases - maybe we
should consider an option to disable this behaviour.

> However, working so well, I thought to change my classical bridge setup with
> a macvtap based one even for simpler installation. In short, going from:
> eth -> bridge -> vnet
> to:
> eth -> macvtap
> But this very simple setup is going deny all guest traffic should the
> physical interface become disconnected. A very crude solution would be to
> issues "ip link set macvtap0 protodown off" when the physical link goes
> down, but I wonder if a better solution exists.
> That said, is replacing classical bridges with macvtap interfaces a bad
> idea? Anything I should know before doing that?
> Regards.
> -- 
> Danti Gionatan
> Supporto Tecnico
> Assyoma S.r.l. -
> email: g.danti@xxxxxxxxxx - info@xxxxxxxxxx
> GPG public key ID: FF5F32A8

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