[PATCH] network: drop use of dummy tap device in bridges

Laine Stump laine at redhat.com
Thu Sep 3 15:11:18 UTC 2020

On 9/3/20 10:58 AM, Daniel P. Berrangé wrote:
> On Thu, Sep 03, 2020 at 10:56:25AM -0400, Laine Stump wrote:
>> On 9/3/20 9:34 AM, Daniel P. Berrangé wrote:
>>> A long time ago we introduced a dummy tap device (e.g. virbr0-nic) that
>>> we attached to the bridge device created for virtual networks:
>>>     commit 5754dbd56d4738112a86776c09e810e32f7c3224
>>>     Author: Laine Stump <laine at redhat.com>
>>>     Date:   Wed Feb 9 03:28:12 2011 -0500
>>>       Give each virtual network bridge its own fixed MAC address
>>> This was a hack to workaround a Linux kernel bug where it would not
>>> honour any attempt to set a MAC address on a bridge. Instead the
>>> bridge would adopt the numerically lowest MAC address of all NICs
>>> attached to the bridge. This lead to the MAC addrss of the bridge
>>> changing over time as NICs were attached/detached.
>>> The Linux bug was actually fixed 3 years before the libvirt
>>> workaround was added in:
>>>     commit 92c0574f11598c8036f81e27d2e8bdd6eed7d76d
>>>     Author: Stephen Hemminger <shemminger at vyatta.com>
>>>     Date:   Tue Jun 17 16:10:06 2008 -0700
>>>       bridge: make bridge address settings sticky
>>>       Normally, the bridge just chooses the smallest mac address as the
>>>       bridge id and mac address of bridge device. But if the administrator
>>>       has explictly set the interface address then don't change it.
>>>       Signed-off-by: Stephen Hemminger <shemminger at vyatta.com>
>>>       Signed-off-by: David S. Miller <davem at davemloft.net>
>>> but libvirt needed to support RHEL-5 kernels at that time, so
>>> none the less added the workaround.
>>> We have long since dropped support for RHEL-5 vintage distros,
>>> so there's no reason to keep the dummy tap device for the purpose
>>> of setting the bridge MAC address.
>>> Later the dummy TAP device was used for a second purpose related
>>> to IPv6 DAD (Duplicate Address Detection) in:
>>>     commit db488c79173b240459c7754f38c3c6af9b432970
>>>     Author: Benjamin Cama <benoar at dolka.fr>
>>>     Date:   Wed Sep 26 21:02:20 2012 +0200
>>>       network: fix dnsmasq/radvd binding to IPv6 on recent kernels
>>> This was again dealing with a regression in the Linux kernel, where
>>> if there were no devices attached to the bridge in the UP state,
>>> IPv6 DAD would not be performed. The virbr0-nic was attached but
>>> in the DOWN state, so the above libvirt fix tenporarily brought
>>> the NIC online. The Linux commit causing the problem was in v2.6.38
>>>     commit 1faa4356a3bd89ea11fb92752d897cff3a20ec0e
>>>     Author: stephen hemminger <shemminger at vyatta.com>
>>>     Date:   Mon Mar 7 08:34:06 2011 +0000
>>>       bridge: control carrier based on ports online
>>> A short while later Linux was tweaked so that DAD would still occur
>>> if the bridge had no attached devices at all in 3.1:
>>>     commit b64b73d7d0c480f75684519c6134e79d50c1b341
>>>     Author: stephen hemminger <shemminger at vyatta.com>
>>>     Date:   Mon Oct 3 18:14:45 2011 +0000
>>>       bridge: leave carrier on for empty bridge
>>> IOW, the only reason we need the DAD hack of bringing virbr0-nic
>>> online is because virbr0-nic exists. Once it doesn't exist, then
>>> we hit the "empty bridge" case which works in Linux.
>> Ooh, cool! Having it used for DAD was the only reason I didn't remove
>> virbrX-nic after I put in the patch to set the bridge MAC directly:
>> commit 13ec827052fcd79a4350f499aab5f4aa20ea83fa
>> Author: Laine Stump <laine at redhat.com>
>> Date:   Thu Oct 17 21:12:30 2019 -0400
>>      util: set bridge device MAC address explicitly during
>>      virNetDevBridgeCreate
>> (I even comment about this, but didn't dig down to learn that it was no
>> longer required for DAD - I just thought that was the way bridges + DAD were
>> designed to work).
>> Nice detective work! Did some problem lead you to this, or did you just get
>> tired of always seeing that useless device hanging around?
> The bug report from a user mentioned in the commit message prompted it

Sigh. Only one cup of coffee so far...

>> Reviewed-by: Laine Stump <laine at redhat.com>
>> (I give this assuming the veracity of all the info you've cited.)
>> One thing that comes to mind - do we still need to wait for DAD to finish in
>> networkStartNetworkVirtual(). I think maybe we were waiting for that only so
>> that we could set the dummy interface offline (there might be some other
>> reason I'm not thinking of though).
> I'm not sure, and I don't currently have an IPv6 deployment to test
> this, so I'm loathe to touch it myself. Perhaps someone else can do the
> change and validate it though...

Well, I have IPv6 running on my internal network, and my ISP connection 
has IPv6 connectivity, and I usually have at least one libvirt network 
using IPv6. But I'm never really sure what to look for to make sure that 
something is working for everyone, and not just me :-)

Based on you saying you don't have an IPv6 deployment to test, I guess 
maybe I should install a build with this patch and match I can do some 
rudimentary things with it...

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