[ILUG] bridge networking and KVM

[Mark McLoughlin]

On Mon, 2011-07-18 at 10:59 +0100, Gavin McCullagh wrote:
> Hi,
> 
> I'm looking to set up a KVM with bridge mode networking so that the VM is
> not NATted and is directly available on the local network alongside the
> host OS, eg host OS at 10.1.10.2/24 (on eth0) and VM at 10.1.10.3/24 (on
> its eth0, bridged onto the host eth0), both available from a remote host on
> the same subnet or on another subnet via a router.
> 
> I'm a bit new to bridge setup and the examples I've found all seem to miss
> out key details -- they just tell you arbitrary things to type but without
> explaining the background so I can tailor it.

Dunno whether you've seen e.g.

  http://wiki.libvirt.org/page/Networking
  http://wiki.libvirt.org/page/VirtualNetworking

There's some nice tips in there like to watch out for the fact that
iptables FORWARD chain applies to packets forwarded across the bridge

> I presume if I set br0 on the host OS to 10.1.10.3/24 I will encounter
> routing issues.  I guess I could use eg /31 or /32 netmasks which
> would avoid that.
> 
> Does one set IP addresses on the host OS bridge interface?  Do you use the
> same addresses or different to the VM interface?  What is the usual way of
> doing this?

Assuming eth0 on the host is currently assigned 10.1.10.2/24, what you
want to do is enslave eth0 to br0 and assign 10.1.10.2/24 to the br0
interface.

If you're using DHCP, then the bridge interface will use the MAC of the
enslaved eth0 and it will automatically be assigned 10.1.10.2/24

Cheers,
Mark.