Comment 6 for bug 1463461

I reproduced such a VM. The reason is that /etc/network/interfaces specifies the interface as "manual", so ifup (and ifup@.service) never gets called on it as /lib/udev/net.agent only starts those on auto and allow-hotplug interfaces.

This worked "by accident" in vivid as we called ifup@.service on all interfaces (regardless whether they were actually configured in /e/n/interfaces and auto/allow-hotplug), but not any more.

I see that under upstart that wasn't hooked into ifupdown, but came from /etc/init/iscsi-network-interface.conf. This is essentially an udev rule, not tied at all to ifupdown, so we could just replace that with an udev rule to do the same under upstart and systemd.