[libvirt-users] Fake Network Interface

[Andrew Martin]

----- Original Message -----
> From: "Laine Stump" <laine at laine.org>
> To: libvirt-users at redhat.com
> Cc: "Andrew Martin" <amartin at 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