[TriLUG] Ubuntu and KVM

Brian McCullough bdmc at bdmcc-us.com
Wed Aug 19 15:33:07 EDT 2009

On Wed, Aug 19, 2009 at 02:12:59PM -0400, Jason Herr wrote:
> I ran into this earlier...   Here's what I used for Ubuntu:
> https://help.ubuntu.com/community/KVM/Networking

Yup.  Been there several times.

> Basically, you set up a bridge interface and run your server's network 
> through it.  

That has been done in several ways.  I'll talk about them later.

> You can then configure the vm's xml file to use it and/or 
> put the change into the template file for libvirt.   There are 
> side-effects.  Root access is needed to bring up the tap access for the 
> bridging.

I'm not certain that that is correct.  As I said, I am inheriting this.

I did make a copy of the libvirt.xml ( was that the name ) file and
removed references to the "default" network, but I suspect that that
isn't being used yet.

> There were assumptions made in that document that I'm still not sure I 
> get.  NetworkManager fights against that process pretty hard, so you'll 
> need to disable it (my update to the wiki).  

Yes, it's already been removed from the machine.

> Also, I'd suggest doing 
> this configuration change locally or via a IP-based KVM switch since 
> you're so far away.  My server got knocked off the net doing this the 
> first time.  

Yup.  Been there, did that a couple of times.  Fortunately, except for
this last time, it recovered and I was able to get back in.

OK, back to /etc/network/interfaces.  In the "host" machine, I found it
with br0 and br1, as well as several br0:x interfaces defined.

According to the wiki page, only br0 should be necessary, or is that
just an "example" simplification?

Every time that I have got close to the example configuration, the host
machine's communications have got very shakey.

This last time, I put in exactly that example configuration ( with my
own IP, netmast, etc. ) and it really went away.  I am just waiting for
someone on the left coast to get into the colo and attach and read the 
console for me.

If I had it working, should the system automatically assign a br0:99 for
the new guest ( child ) when it starts?

That's the behaviour that I remember from when I was first building my
Xen machines.  You don't do something frequently enough, and you lose

And, as I suspect you are going to say, if I made sure that the
libvirt.xml file ONLY said Bridge, this would be true.

Thank you for the guidance,

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