Comment 6 for bug 1483457

Wrt. correctly handling multiple interfaces we should IMHO keep the default net.ifnames. After all, providing stable names for cloud instances and virtual machines with multiple interfaces was one of the reasons why we switched to net.ifnames in the first place -- before, the old udev rules had blacklists for all QEMU/HyperV/VMWare/EC-2 etc. interfaces, which meant that while being able to rely on "eth0" was convenient while you had only one interface, this would break down once you add more than one.

So ideally cloud-init (I assume that's what writes interfaces.d/eth0.cfg) would stop hardcoding "eth0" and instead write a configuration for the first (or all, if that's desired) /sys/class/net/en* or /sys/class/eth* that it finds. For multiple interfaces that's semantically equivalent to what we did before (whatever became eth0 got chosen as "the outwards interface we do DHCP on").

For a workaround until this, net.ifnames=0 or overriding 99-default.link is okay as well. But I don't think that's a good long-term strategy.

Please see /usr/share/doc/udev/README.Debian.gz for details.