[libvirt] how do I stop libvirt futzing with my network configuration?

Nix nix at esperi.org.uk
Sat Nov 28 22:06:16 UTC 2009

On 28 Nov 2009, Ian Woodstock spake thusly:
> I've been running with this configuration for many months on dozens of hosts.
> - Created a bridge (the old fashion way) in /etc/sysconfig/network-scripts
> Bridge called br0 with one device eth1.
> - Created a VM in virt manager (or edit existing)
> Picked "Shared Physical Device"  Device "eth1 (Bridge br0)"  in the GUI.

That's the mystery. I did it (directly via brctl, as it happens, 'cos I'm
making several with particular properties and want to enforce them):

spindle:/etc/libvirt/qemu# brctl show
bridge name     bridge id               STP enabled     interfaces
linux-net               8000.06eb4e4985df       no              dummy0

52: linux-net: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN
    link/ether 06:eb:4e:49:85:df brd ff:ff:ff:ff:ff:ff
    inet scope global linux-net

(hm, the state UNKNOWN is sort of bizarre. It's up...)

(dummy0 is a member of this bridge because I've previously noted that a
bridge with no members at all isn't picked up by the GUI).

It appears in the GUI, all right: as 'host device linux-net (not bridged)',
greyed out and unselectable. Calling a bridge 'not bridged' is more than
slightly bizarre.

Perhaps the netcf thing means I'm *required* to stick stuff in
/etc/sysconfig/network-scripts even though I'm not using RH so it would
never otherwise be used? From my reading of the source, that's not so...

I suppose I should just have it check for the bridge and rip out all this
other stuff, but it seems bizarre that I have to do it.

> Or just add it to the VMs XML

That's too late to run the installer if it needs to access the network :/
I suppose I could continue doing the VM installation qemu run by hand...

> Works like a charm and there's certainly no configuration in libvirt
> for this interface, ie. nothing in /var/run/libvirt/network, and
> *nothing* set up in Virt Manager under "Host Details->Virtual networks
> You do need to make sure that you disable netfilter on the bridge or
> setup the appropriate iptables rules ( see
> http://wiki.libvirt.org/page/Networking#Bridged_networking_.28aka_.22shared_physical_device.22.29)

There's no iptables at all on this particular box (at least not yet,
although it may turn up later on when I put Windows guests on here: I'm
not having *them* running around free).

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