On 1/29/2018 3:01 PM, Keller, Jacob E wrote:
Hi,
I'm currently investigating how macvlan devices behave in regards to vlan
support, and found some interesting behavior that I am not sure how best to
correct, or what the right path forward is.
If I create a macvlan device:
ip link add link ens0 name macvlan0 type macvlan:
and then add a VLAN to it:
ip link add link macvlan0 name vlan10 type vlan id 10
This works to pass VLAN 10 traffic over the macvlan device. This seems like
expected behavior.
However, if I then also add vlan 10 to the lowerdev:
ip link add link ens0 name lowervlan10 type vlan id 10
Then traffic stops flowing to the VLAN on the macvlan device.
This happens, as far as I can tell, because of how the VLAN traffic is filtered
first, and then forwarded to the VLAN device, which doesn't know about how the
macvlan device exists.
It seems, essentially, that vlan stacked on top of a macvlan shouldn't work.
Because the vlan code basically expects each vlan to apply to every MAC
address, and the macvlan device works by putting its MAC address into the
unicast address list, there's no way for a device driver to know when or how to
apply the vlan.
This gets a bit more confusing when we add in the l2 fwd hardware offload.
Currently, at least for the Intel network parts, this isn't supported, because
of a bug in which the device drivers don't apply the VLANs to the macvlan
accelerated addresses. If we fix this, at least for fm10k, the behavior is
slightly better, because of how the hardware filtering at the MAC address
happens first, and we direct the traffic to the proper device regardless of
VLAN.
In addition to this peculiarity of VLANs on both the macvlan and lowerdev, is
that when a macvlan device adds a VLAN, the lowerdev gets an indication to add
the vlan via its .ndo_vlan_rx_add_vid(), which doesn't distinguish between
which addresses the VLAN might apply to. It thus simply, depending on hardware
design, enables the VLAN for all its unicast and multicast addresses. Some
hardware could theoretically support MAC+VLAN pairs, where it could distinguish
that a VLAN should only be added for some subset of addresses. Other hardware
might not be so lucky..
Unfortunately, this has the weird consequence that if we have the following
stack of devices:
vlan10@macvlan0
macvlan0@ens0
ens0
Then ens0 will receive VLAN10 traffic on every address. So VLAN 10 traffic
destined to the MAC of the lowerdev will be received, instead of dropped.
If we add VLAN 10 to the lowerdev so we have both the above stack and also
lowervlan10@ens0
ens0 (mac gg:hh:ii:jj:kk)
then all vlan 10 traffic will be received on the lowerdev VLAN 10, without any
being forwarded to the VLAN10 attached to the macvlan.
However, if we add two macvlans, and each add the vlan10, so we have the
following:
avlan10@macvlan0
macvlan0@ens0
ens0
bvlan10@macvlan1
macvlan1@ens0
ens0
In this case, it does appear that traffic is sorted out correctly. It seems
that only if the lowerdev gets the VLAN does it end up breaking. If I remove
bvlan10 from macvlan1, the traffic associated with vlan10 is still received by
macvlan1, even though in principle it should no longer be.
What is the correct behavior here? Should this just be "administrators should know
better"? I don't think that's a great argument, and either way we're still
essentially leaking VLANs across the macvlan interfaces, which I don't think is ideal.
I see two possible solutions:
1) modify macvlan driver so that it is marked as VLAN_CHALLENGED, and thus
indicate it cannot handle VLAN traffic on top of it.
a. In order to get the VLANs associated, administrator could instead add the
VLAN first, and then add the macvlan on top. This I think is a better
configuration.
b. that doesn't work in the offload case, unless/until we fix the VLAN
interface to forward the l2_dfwd_add_station() along with a vid.
c. this could appear as loss of functionality, since in some cases these
VLAN on top of macvlan work today (with the interesting caveats listed above).
2) modify how VLANs interact with MAC addresses, so that the lowerdev can
explicitly be aware of which VLANs are tied to which address groups, in order
to allow for the explicit configuration of which MAC+VLAN pairs are actually
allowed.
a. this is a much more invasive change to driver interface, and more
difficult to get right
b. possibly other configurations of stacked devices might have a similar
problem, so we could solve more here? Or create more problems.. I'm not really
certain.
I think the correct solution is (1) but I wasn't sure what others thought, and
whether anyone else has encountered the problems I mention and outline above. I
cc'd Alex who I discussed with offline when I first heard of and began
investigating this, in case he has anything further to add.
Regards,
Jake
Hi Jake,
The current behavior seems logical to me, but I suppose Alex might argue
differently. The macvlan was put onto the default lowerdev assuming the
lowerdev will hand it all the default traffic, and then the macvlan
splits out its own vlan traffic. As soon as the lowerdev assumption
changes, it is going to change what gets pushed up to the macvlan dev.
If the lowerdev is separating the vlan traffic out of the "default" flow
headed to the macvlan, then the initial assumption has changed and the
vlan traffic has been vectored off before it can be delivered up the
stack to the macvlan.
There's an argument that the lowerdev shouldn't know anything about the
upperdev's routing, just deliver to the upperdev and let the upperdev
worry about it. But perhaps this becomes is a question of precedence:
does the lowerdev split traffic first by mac address or by vlan tag.
I don't like your option 1: as you point out, it breaks current
functionality, likely depended upon in some containers that are using
macvlans to manage their traffic. We don't know what's going on inside
that container and I don't think we want to break its ability to split
its own vlans.
Like I said, I think the current behavior is mostly correct, but a
version of option 2 might be good to help support offload of the
mac+vlan pair into a macvlan channel.
sln
