Comment 5 for bug 1777094

So it is not possible to have an interface as optional (for other reasons beyond that example) and still have a dnsmasq running for it?

In former versions of ubuntu I had my bridges (some containing usb ethernet interfaces to bridge virtual with physical machines) configured in /etc/network/if-up.d, and started separate dnsmasq instances for every single one of these bridges.

Now there is no regular way to start a daemon like dnsmasq through ifup-scripts, since netplan does not support them. Some people try to workaround that problem with udev clauses, starting systemd services, but then it's overcomplicated and error prone.

So what is the offical way to have services like dnsmasq run for optional interfaces under 18.04?

"Don't use optional" is a workaround, not a solution to the problem. If there is no ifup-script anymore, then how are services to be started, once the interface is up (late or just sometimes)?