[CentOS-virt] How to manually add a new interface to a bridge device?

Wed Dec 9 09:23:45 UTC 2015
Zoltan Frombach <zoltan at frombach.com>

You most definitely do not need to destroy and re-create a VM just to 
add a 2nd network interface.

I don't think those vnet interfaces got created by the host OS. I 
believe those are created by KVM (or libvirt) when you start a VM. I 
could be wrong though. But I just checked on my CentOS 6 KVM host 
machine and I see as many vnet interfaces as many VMs are currently 
running (or if one VM has two virtual network cards then of course that 
VM results two vnet interfaces shown on the host). Here the relevant 
part from one such VM's XML file (those XML files are under the 
/etc/libvirt/qemu directory on my CentOS host ) :

     <interface type='bridge'>
       <mac address='00:16:36:e2:20:ea'/>
       <source bridge='br0'/>
       <model type='virtio'/>
       <address type='pci' domain='0x0000' bus='0x00' slot='0x03' 
function='0x0'/>
     </interface>
     <interface type='bridge'>
       <mac address='00:16:36:e2:20:eb'/>
       <source bridge='br600'/>
       <model type='virtio'/>
       <address type='pci' domain='0x0000' bus='0x00' slot='0x06' 
function='0x0'/>
     </interface>

As you can see, there are two interfaces defined in this VM's XML file. 
One is connected to the host's br0 interface and the 2nd is connected to 
the host's br600 interface. You must make sure that all your mac 
addesses are unique on your network though!!

After editing a VM's XML file with your favorite editor, you need to 
execute this command:
virsh define /etc/libvirt/qemu//your-vm-name/.xml
or just simply use this editor:
virsh edit /your-vm-name/
(but I hate vi, so I use my favorite editor and when I finished editing 
I just run virsh define.)

I edited VM's XML files many times and I can add/remove virtual network 
cards without problems.

Zoltan

On 12/9/2015 9:55 AM, Howard Leadmon wrote:
>
> Tried that as well, but this has to be something that gets set at the 
> OS level and loaded, as if you look at dmesg output, you can see all 
> the vnet?? nodes as the OS comes online.    So the question is, what 
> is virt-install doing that creates the needed vnet interface that is 
> part of the bridge.   I really had to kill and reload the VM just to 
> load a second interface..
>
> ---
>
> Howard Leadmon
>
> *From:*centos-virt-bounces at centos.org 
> [mailto:centos-virt-bounces at centos.org] *On Behalf Of *Zoltan Frombach
> *Sent:* Wednesday, December 9, 2015 2:42 AM
> *To:* Discussion about the virtualization on CentOS 
> <centos-virt at centos.org>
> *Subject:* Re: [CentOS-virt] How to manually add a new interface to a 
> bridge device?
>
> I would stop the VM, edit its definition file (that's an XML file) and 
> then start it up. But be careful: After you edit the XML file, you 
> need to execute a command so KVM re-reads that file. I forgot that 
> command, but you can look it up on Google.
>
> On Dec 9, 2015 7:52 AM, "Howard Leadmon" <howard at leadmon.net 
> <mailto:howard at leadmon.net>> wrote:
>
>     Maybe my google-fu is failing me, but I have spent the past couple
>     hours looking at how to add a vnet? Device to my KVM host running
>     CentOS 6, and for the life of me I can’t get this going.
>
>     From all my research if I want to add a device I should just do
>     ‘brctl addif br1 vnet14’ if I want to add a vnet14 to bridge
>     br1.   When I do this, I get:
>
>     # brctl addif br0 vnet14
>
>     interface vnet14 does not exist!
>
>     If I run a ‘brctl show’ I get the following:
>
>     # brctl show
>
>     bridge name     bridge id               STP enabled interfaces
>
>     br0 8000.00237dd22a4c       no              eth0
>
>     vnet0
>
>     vnet10
>
>     vnet11
>
>     vnet13
>
>     vnet2
>
>                                                            vnet3
>
>     vnet4
>
>     vnet6
>
>     vnet8
>
>     br1        8000.00237dd22a50       no              eth1
>
>     vnet1
>
>     vnet12
>
>     vnet5
>
>                                            vnet7
>
>     vnet9
>
>     Needless to say the existing vnet?? Devices are in use on guest
>     VM’s currently.
>
>     When I create a new VM using virt-install, I usually add the
>     following to my command line:
>
>     --network=bridge:br0 --network=bridge:br1
>
>     I messed up building a new VM, and only added the br0 interface to
>     the VM, but need the br1 interface as well.  So my question is, or
>     a pointer to how I can add that br1 interface to my existing VM,
>     and create the needed vnet14 interface for it to attach to?
>
>     If anyone can explain how to do this, or give me a good pointer on
>     where the info is on how to do this, it would sure be a huge help..
>
>     Thanks…
>
>     ---
>
>     Howard Leadmon
>
>
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>
>
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