Comment 1 for bug 1682871

It seems that this only reproduces on kernel 4.8, but does on 4.4.
The change put in to ignore certain nics was done based on 'addr_assign_type'
(/sys/class/net/<name>/addr_assign_type).
On 4.4 for a vlan, that shows as '0', (permanent address).
On 4.8, it (correctly) shows as '2' (stolen from another device)

To recreate this you can run the attached script.

$ MODE=vlan ./btest-launch.sh yakkety-server-cloudimg-amd64-proposed.img
...log in as ubuntu:password ...

Note, I can see this fail on xenial with 0.7.9-90-g61eb03fe-0ubuntu1~16.04.1
but cannot see it fail in yakkety I

## xenial
$ uname -r
4.4.0-72-generic
$ python3 -c 'from cloudinit.net import get_interfaces_by_mac; print(get_interfaces_by_mac())'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python3/dist-packages/cloudinit/net/__init__.py", line 403, in get_interfaces_by_mac
    (name, ret[mac], mac))
RuntimeError: duplicate mac found! both 'interface1.108' and 'interface1' have mac '52:54:00:12:34:02'

$ grep -r . /sys/class/net/*/ 2>/dev/null | grep addr_assign_type
/sys/class/net/interface0/addr_assign_type:0
/sys/class/net/interface1/addr_assign_type:0
/sys/class/net/interface1.108/addr_assign_type:0
/sys/class/net/lo/addr_assign_type:0

### yakkety
$ python3 -c 'from cloudinit.net import get_interfaces_by_mac; print(get_interfaces_by_mac())'
{'00:00:00:00:00:00': 'lo', '52:54:00:12:34:00': 'interface0', '52:54:00:12:34:02': 'interface1'}

$ uname -r
4.8.0-46-generic

$ grep -r . /sys/class/net/*/ 2>/dev/null | grep addr_assign_type
/sys/class/net/interface0/addr_assign_type:0
/sys/class/net/interface1/addr_assign_type:0
/sys/class/net/interface1.108/addr_assign_type:2
/sys/class/net/lo/addr_assign_type:0