Re: [libvirt-users] Fake Network Interface

15 Jan 2014

      ----- Original Message -----
...
From: "Laine Stump" <laine@laine.org>
To: libvirt-users@redhat.com
Cc: "Andrew Martin" <amartin@xes-inc.com>
Sent: Tuesday, January 14, 2014 3:54:51 AM
Subject: Re: [libvirt-users] Fake Network Interface
On 12/30/2013 11:25 AM, Andrew Martin wrote:
...
Is there a supported method for creating a fake network interface in a VM's
configuration file? I was using the below construct, however it is no
longer
working for me in recent versions of libvirt (libvirt 1.0.2 with qemu-kvm
1.4.0).
I assume this wasn't working for you either because you previously had a
network named "fake" in your config, and that network wasn't present
when you tested with the newer libvirt, or because the <address> element
created a conflict with an existing device on the same guest that was
already using the same slot?
...
Is there a different, preferred method for creating a fake virtual
interface on the guest which does not exist on the host?
    <interface type='network'>
      <mac address='aa:aa:aa:aa:aa:aa'/>
      <source network='fake'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03'
      function='0x0'/>
    </interface>
On 01/14/2014 12:54 AM, Andrew Martin wrote:
...
I found the answer to this question:
1. create an XML file with a fake configuration (including a subnet not in
use elsewhere):
<network>
  <name>fake</name>
  <bridge name='virbr1' stp='on' delay='0' />
  <ip address='192.168.0.1' netmask='255.255.0.0'></ip>
</network>
1) you don't need to (and probably shouldn't) specify the <bridge>
element when defining the new network - libvirt will autogenerate a
unique bridge name if one isn't provided, and stp defaults to on with a
delay of 0 anyway.
2) If this network is just a parking place, then you don't need to
specify an <ip> element either. This will save you from having an extra
dnsmasq instance started on the host to answer dns queries to
192.168.0.1 (and has the added advantage that a guest will be unable to
use this "parked" network connection to erroneously communicate with the
host).
In other words, all you really need in the XML file is this:
<network>
    <name>fake</name>
  </network>
...
2. define the network: virsh net-define /path/to/fake.xml
3. autostart the network: virsh net-autostart fake
4. start the network now: virsh net-start fake
Once done, you can add this block of XML to your VM config with "virsh edit
<vm_name>":
     <interface type='network'>
       <mac address='aa:aa:aa:aa:aa:aa'/>
       <source network='fake'/>
       <address type='pci' domain='0x0000' bus='0x00' slot='0x03'
       function='0x0'/>
     </interface>
Note that you should adjust the "slot" and "mac" arguments to avoid
conflicts.
It's actually easier and less error prone to just omit the <mac> and
<address> elements completely - libvirt will auto-generate
non-conflicting values for both if they aren't provided. So the
interface xml would look like this:
<interface type='network'>
     <source network='fake'/>
   </interface>
(you may want to add a <model type='virtio'/> to use a better emulated
device than the default rtl8139).
Laine,

Thanks for the suggestions on the best practices for how to do this.

Andrew