Comment 1 for bug 1560331

provider/maas/environ.go setupJujuNetworking() picks what interface to bridge using:

juju_ipv4_interface_to_bridge=$(ip -4 route list exact default | head -n1 | cut -d' ' -f5)

Clearly if there is no default route this won't work. There are plans to bridge everything so this issue will go away in 2.0, but for 1.25 we need to do something. I suggest that if there is only one interface then bridge it. We also need better error reporting.