Two or three times now libvirt (0.4.4-3ubuntu3.1, Ubuntu Intrepid 8.10) has automatically generated overlapping MAC addresses on me. I can’t find the source for this MAC address generation in 0.4.4, but in 0.6.1 which is included in Ubuntu Jaunty 9.04 it’s virGenerateMacAddr in src/util.c. This leads me to believe it’s been rewritten, and I’m hoping it’s better. It looks perfectly fine.