[ovirt-users] vlan-tagging on non-tagged network

[Felix Pepinghege]

Hi Ido, hi everybody,

sorry that I kept you waiting for two months, I only just found the time 
to go back to this problem.

You were completely right with your guess. The ethernet frames do appear 
on the vnet-interface, but not on the bridge. The dropped-counter seems 
to be independent from these losses, though.

However, while this tells me *where* the problem is, I still don't know 
*what* the problem is. I've done some research, but couldn't find 
anything particularly helpful.
An interesting point may be that this problem is mono-directional. That 
is, the bridge happily passes vlan-tagged frames from the ethernet 
device to the vnet, but not the other way around. Untagged ethernet 
frames make their way through the brigde, no matter where they come from.

The vlan module is loaded, as to the versioning questions:
# cat /etc/centos-release ; uname -s -v -r
CentOS Linux release 7.1.1503 (Core)
Linux 3.10.0-229.7.2.el7.x86_64 #1 SMP Tue Jun 23 22:06:11 UTC 2015

The guest OS is an up-to-date Debian Jessie, which should not matter, 
though, as the frames get lost on the doorstep of the bridge on the host.

Again, any suggestions are much appreciated!

Regards,
Felix


Am 16.06.2015 um 08:27 schrieb Ido Barkan:
> Hey Felix.
>
> IIUC your frames are dropped by the bridge. Ovirt uses Linux Bridges
> To connect virtual machines to 'networks'. The guest connects to the bridge
> using a tap device which usually is called 'vnet<number>'.
>
> So, just to verify, can you please tcpdump both on the bridge device and on the tap device?
> The bridge can be quite noisy so I suggest filtering traffic using the guest's MAC
> address. So I am not sure what protocol you use for tunneling but applying
> a filter similar to this one should do the job:
>           tcpdump -n -i vnet0 -vvv -s 1500 'udp[38:4]=0x001a4aaeec8e'
>
> My guess is that you will observe traffic on the tap device, but not on the bridge.
> You didn't specify which centOS version you use but I do remember seeing people
> complaining about Linux bridges discarding their tagged frames.
> You can -maybe- also observe the 'dropped' counter increases on the bridge by running:
>           'ip -s link show dev trunk'
>
> There were a few bugs on rhel6/7 about this, specifically I remember
> https://bugzilla.redhat.com/show_bug.cgi?id=1174291
> and
> https://bugzilla.redhat.com/show_bug.cgi?id=1200275#c20
>
> Also, is the vlan module loaded on your host?
> 'lsmod |grep 8021q'
>
> Thanks,
> Ido
>
> ----- Original Message -----
> From: "Felix Pepinghege" <pepinghege at ira.uka.de>
> To: Users at ovirt.org
> Sent: Monday, June 15, 2015 11:33:39 AM
> Subject: [ovirt-users] vlan-tagging on non-tagged network
>
> Hi everybody!
>
> I am experiencing a behaviour of ovirt, of which I don't know whether it
> is expected or not. My setup is as follows:
> A virtual machine has a logical network attached to it, which is
> configured without vlan-tagging and listens to the name 'trunk'.
> The VM is running an openvpn server. It is a patched openvpn version,
> including vlan-tagging. That is, openvpn clients get a vlan tag. This
> should not really be an issue but should satisfy the "why do you want to
> do it in the first place"-questions.
> Anyhow, effectively, the VM simply puts vlan-tagged ethernet-frames on
> the virtual network. These frames, however, never make it to the host's
> network bridge, which represents the logical network.
> My observations are: According to tcpdump, the vlan-tagged packages
> arrive at the "eth1"-interface inside the VM (which *is* the correct
> interface). Again, according to tcpdump, these packages never arrive at
> the corresponding network-bridge (i.e., the interface 'trunk') on the host.
> I know that the setup itself is feasible with KVM---I have it working on
> a proxmox-machine. Therefore, my conclusion is, that ovirt doesn't like
> vlan-tagged ethernet-frames on non-tagged logical networks, and somehow
> filters them out, though I don't really see on what "level" that would
> happen (Handling the ethernet frames should be a concern of
> KVM/QEMU/Linux only, once ovirt has started the VM).
> So this problem could be a CentOS issue, but I really don't see why
> CentOS should act differently than debian does (proxmox is debian-based).
> Is this a known/wanted/expected behaviour of ovirt, and can I somehow
> prevent or elude it?
>
> Any help is much appreciated! Of course I am happy to provide more
> information if that helps helping me :)
>
> Regards,
> Felix
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