Stop Waiting on booting Ubuntu

The real(!) solution to this problem is following command:

sudo sed -i.old-`date +%Y%m%d-%H%M%S` '/^auto lo$/!s/^auto /allow-hotplug /' /etc/network/interfaces

In /etc/network/interfaces this changes all interfaces (except lo) from auto to allow-hotplug. This way the boot is no more waiting for the interfaces to come up first.

    Warning: After this change a permanently connected interface might stay down after boot until systemd receives a real plug event. See Notes below.

Example before (look at auto eth0):

auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet dhcp

Example after (look at allow-hotplug eth0):

auto lo
iface lo inet loopback

# The primary network interface
allow-hotplug eth0
iface eth0 inet dhcp

Notes:

    If you mount network shares in /etc/fstab, use auto and not allow-hotplug for the interface to the network shares. Else you might see strange things happen on boot process, because network must be available prior to network share mounts. allow-hotplug does not ensure this.

    If the interfaces are in auto mode, you express: "These interfaces are crucial for boot, so we must wait for them to come up before we have booted." Hence, if they do not come up, Ubuntu delays the boot with failsafe, waiting for them to appear for up to 120 seconds. And this is the right thing to do.

    In contrast, Interfaces which are set to allow-hotplug tell Ubuntu, that they are optional. Hence they are not essential to boot.

    Ubuntu records which interfaces are available at install time, and assumes, that they are important for later operation. This is a conservative choice, in case the interface is later needed because some Service binds to it, as such services fail to start if they miss the interface being up.

    There also is a kernel setting which allows processes to bind to nonexistent IPs, so you can always use allow-hotplug if you like, without harming the stability of the boot process. However, this is a completely different story.

Notes (update 2018-01-04):

    At my side, allow-auto does the same as auto, so it does not help (tried with br0).

    After upgrading one of my systems to Debian Stretch and switching to SystemD, boot became unbearably delayed while waiting for the (permanently connected outside) interface br0 to come up. However with allow-hotplug the interface br0 stayed down after boot. Perhaps this is caused by SystemD not receiving any real or synthetic plug event on such an interface. I did not dig deeper into this, as some obscure crontab entry @reboot /sbin/ifup br0 for root fixed it for me. (This works, but probably is something, which better should not be recommended to others. I'd like to hear if somebody has some better idea.)<br>


The setting of the interface, which lives in /etc/network/interfaces, is blown up with the sleep in failsafe removed, and even if somebody sees the closed valve (auto), nobody spottet that it could be openend as well!