[libvirt] configuring a disconnected <interface>

[Dan Kenigsberg]

Hi List,

Like most of us, I've bought my actual computer with an Ethernet
interface card, that I can connect to a wall jack at will. It's quite
easy to disconnect the Ethernet cable from the wall, as well.

I would like to expose this behavior to virtual computers, too. Via
libvirt, of course. That's useful, since an admin may need to disconnect
a running VM from a network temporarily, without resorting to
hot-unplugging its nic.

How should the domxml represent this situation? An interface that is
connected to a specific LAN1 bridge is

    <interface type='bridge'>
        <source bridge='LAN1'>
        <target dev='vnet1'/>
        <model type='virtio'/>
    </interface>

Would the following make sense for an interface with no bridge at the
source?

    <interface type='bridge'>
        <source/>
        <target dev='vnet1'/>
        <model type='virtio'/>
    </interface>

And how about the recommended config for <interface>s, which is
type='network'?  Would it make sense to predefine

    <network>
        <name>null-bridge-network</name>
        <forward mode='bridge'>
        <bridge/>
    </network>

so that

    <interface type='network'>
        <source network='null-bridge-network'/>
    </interface>

means "keep this interface dangling out there"?

Regards,
Dan.