[libvirt-users] Use libvirt to create an NIC on the guest machine and connect to an existing TAP interface

[Laine Stump]

On 08/29/2016 03:37 AM, Marina Danial wrote:
> Hello,
>
> I am facing the following issue.
>
> I have several TAP interfaces that are created by the following method 
> on OpenVSwitch:
>
> /
> /
> /ovs-vsctl add-br br1
> /
> /
> /
> /sudo /usr/sbin/tunctl -t tap01 -p -u root/
> /
> /
> /sudo /sbin/ip link set tap01 up
> /
> /
> /
> /# Disable multicast/
> /sudo /sbin/ifconfig tap01 -multicast/
> /
> /
> /ovs-vsctl add-port br1 tap01/


Aside from the part about disabling multicast, this is all handled 
(including creation of the tap device, and destroying it when the guest 
is terminated) by libvirt's <interface type='bridge'> with a 
<virtualport type='openvswitch'> subelement. This page has more details 
and an example:

   http://www.libvirt.org/formatdomain.html#elementsNICSBridge

As far as turning off multicast, this is the first time I've seen anyone 
request that. Unfortunately the network hooks 
(https://www.libvirt.org/hooks.html ) that are run for each interface 
happen prior to the creation of the tap devices, so you can't just add a 
network hook script to do it. If you *really* need multicast disabled, 
with current libvirt you would need to add a *qemu* hook script that 
looked for the tap device name in the guest's xml and set -multicast, or 
you would need to use "<interface type='ethernet'> (which just creates a 
tap device, then calls a script to "connect it to the network") with a 
"<script path='some script that sets -multicast *and* adds the tap to 
the ovs bridge'/>". type='ethernet' and its script are also described a 
bit further down in the same link I posted above.



> /
> /
> /
> /
> /
> /
> /
> /
>
> I need to launch a qemu VMs and have its NIC card that is created 
> using libvirt  to be connected to tap01 or other taps that are created 
> with the above method.
>
> I need a way that libvirt only creates the new NIC on either eth0 or 
> eht1 on the guest machine and connect it to tap01.
>
> Please help me in the best way to do so whether using the C++ APIs 
> that libvirt provides or use the domain and/or network XMLs to achieve 
> this.
>
>
> Thanks a lot
> Marina
>
>
>
>
>
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