Comment 12 for bug 1737640

On xenial, deployed with maas 2.3.0. Node with two network interfaces: the first one (ens3) set to "auto" in maas, and the second one (ens8) left as "unconfigured" in MAAS:
2: ens3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 52:54:00:58:04:57 brd ff:ff:ff:ff:ff:ff
    inet 10.0.5.10/24 brd 10.0.5.255 scope global ens3
       valid_lft forever preferred_lft forever
    inet6 fe80::5054:ff:fe58:457/64 scope link
       valid_lft forever preferred_lft forever
3: ens8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 52:54:00:66:6c:1c brd ff:ff:ff:ff:ff:ff
    inet6 fe80::5054:ff:fe66:6c1c/64 scope link
       valid_lft forever preferred_lft forever

With ubuntu-fan 0.12.8~16.04.1 installed:

root@04-57:~# ifdown -a; ifup -a
/usr/sbin/fanctl: 41: /usr/sbin/fanctl: arithmetic expression: expecting primary: " (32-)/4 "
run-parts: /etc/network/if-up.d/ubuntu-fan exited with return code 2
Failed to bring up ens3.

That also leaves the networking system in a bit of an inconsistent state:
root@04-57:~# ifdown ens3
ifdown: interface ens3 not configured

Bug confirmed.

I chose to reboot now, and then install the updated ubuntu-fan package from proposed:

SKIP - package not available yet - will continue sometime later