Thanks !! This clarifies completely what is happening. I'll look into running virsh as root/attaching to qemu:///system. Or, perhaps I can 'statically' create tun devices, to which the domains attach when started (although I have no idea weather this is possible). 

Best,
Govert

2017-01-17 20:46 GMT+01:00 Laine Stump <laine@laine.org>:
On 01/14/2017 06:30 AM, Govert wrote:
Hi,

I'm trying to control the name of the 'tap0' device that gets created as I start a domain that uses bridged networking. The XML specification of the domain contains the following configuration

    <interface type='bridge'>
      <source bridge='br0'/>
    </interface>

The libvirt documentation (http://libvirt.org/formatdomain.html#elementsNICSBridge) and other discussions online tell me that I just need to include the <target dev='desired_dev_name'/> tag in the XML specification of the domain under the <interface> tag. Unfortunately doing so appears to have no effect; the tun device created and 'enslaved' in the bridge is still called 'tap0'. Interestingly, I never get a tun device with a name prefixed by 'vnet' or 'vif' which, according to the documentation, is the default behaviour (?). The host is running CentOS 7, and virsh is used to start the domain.

 This is because you're running virsh as  a non-privileged user (rather than root) and so are connecting to that user's personal non-privileged libvirtd (aka qemu:///session) rather than the system's privileged libvirtd (qemu:///system). When using qemu:///session, libvirtd is unable to create tap devices itself (because it doesn't have sufficient privilege for it), so it executes qemu-bridge-helper (from the qemu package) requesting that a tap device be created and attached to the bridge device specified on its commandline. Unfortunately, qemu-bridge-helper doesn't provide any way to specify the tap device name, so you get what it decides to give you (which happens to be "tap%d").

If you want more control over the name of the tap device (and many other things), you should look into using qemu:///system. That may seem less secure, but libvirt actually does a very good job of confining the qemu process - after setting up all the resources that will be needed (which are labeled with an selinux context unique to this particular guest) and setting up cgroups to limit use of system resources, it switches to a different (non-privileged) uid and drops all capabilities before exec'ing qemu.

(Note that, even if you're using qemu:///system, any manually-specified tap device name that starts with "vnet" will be discarded (it assumes that it is an auto-generated name left over from a previous run, or from plugging a domain's status XML into virsh define)).