Not waiting for dynamic interfaces to come up during boot / 2016-07-22

I had to shutdown the homeserver greenblatt to allow for work on our electricity meter and I noticed during boot-up it complained:

waiting for network configuration

waiting an additional 60 seconds for network configuration

According to networking - "waiting for network configuration" Problem - Ask Ubuntu this is caused by some error in /etc/network/interfaces but I could not figure out which one until I read about the difference between the auto and allow-hotplug settings for an interface. What I had was:

auto pppdray
iface pppdray inet ppp
        provider dray-vdsl

Which caused the boot process to wait until the PPPoE over VDSL link was completely up and running (which was not going to happen at that time).

Solution, change it to:

# set to allow-hotplug: not needed to boot
allow-hotplug pppdray
iface pppdray inet ppp
        provider dray-vdsl

So the startup continues.

The problem noted at the previous boot Boot-time IPv6 on the homeserver not working was still happening and I had to reconfigure interfaces and restart services to get everything stable, so disabling duplicate address detection did not help.

Update 2016-10-26: Found out using allow-hotplug means I have to ifup that interface some other way: Ok, allow-hotplug means some other process has to start the interface. One learns in slow iterations if you don't want to reboot constantly.