[Libguestfs] virt-builder fail with "bridge ‘virbr0’ not found" (fixed by LIBGUESTFS_BACKEND=direct ?!)

I'm trying to build fedora-27 image for testing uploads: $ virt-builder fedora-27 -o /var/tmp/fedora-27.img ... [ 25.2] Opening the new disk virt-builder: error: libguestfs error: bridge ‘virbr0’ not found. Try running: brctl show to get a list of bridges on the host, and then selecting the bridge you wish the appliance network to connect to using: export _SETTINGS=network_bridge=<bridge name> I don't want to add bridges to my new fresh oVirt test host, and remember that this used to work previously without the bridge. So I tried the usual "fix" (I know it should not be used in production): $ export LIBGUESTFS_BACKEND=direct And for some reason this works now: $ virt-builder fedora-27 -o /var/tmp/fedora-27.img ... [ 41.2] Finishing off Output file: /var/tmp/fedora-27.img Output size: 6.0G Output format: raw Total usable space: 5.3G Free space: 4.4G (81%) How LIBGUESTFS_BACKEND is related to the bridge? why do we need the bridge for building an image? Nir