Comment 20 for bug 1301015

"If you look at upstart's log, you'll notice the following message:
Stopping or restarting the networking job is not supported.
Use ifdown & ifup to reconfigure desired interface."
This error message should at least be visible in the standard output. "stop: Job failed while stopping" is really not very informative.

Anyway, ifdown & ifup do not work unless you configure the network manually:
ifdown: interface eth0 not configured

while ifconfig up & down do, so I would change the error message:
sudo ifconfig eth0 down && sudo ifconfig eth0 up

"This was never a supported way to bounce the network, starting with 14.04, we changed the init script to fail rather than possibly trash your machine."
This doesn't seem like a solution to me: it was not supported, but it was still working in many cases. Now it doesn't work, ever. Please at least provide a working alternative in a visible (i.e., not the log) error message.