I just upgraded an RPi configured as a wireless router to from 15.10 to 16.04. dnsmasq is failing during startup with an unknown interface br0. Before the upgrade, the log showed a wait for auto interfaces to complete, which is missing in the new boot log. When startup is complete, I can log in and manually start dnsmasq, which runs without problem.

br0 is the only interface declared as auto in /etc/network/interfaces.

I found the solution myself. Add the follow two lines in the [Unit] section of /lib/systemd/system/dnsmasq.service

After=network-online.target
Wants=network-online.target

See https://bugs.launchpad.net/ubuntu/+source/dnsmasq/+bug/1531184 for more detail.

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