Devuan... Reduce time to wait while internet interfaces being configured during boot

Question

I am using devuan linux for my laptop after leaving debian back due to the first's better sequrity and its simplicity compared to debian.

I am really happy with it but I have the next problem:

When the OS starts, if my ethernet cable is not plunged in or even if there is a problem with my connection, it waits for about 20 seconds waiting while:

Configuring internet interfaces

... and after that it says:

ifup already configured. done

or something similar, even if it really configured the interfaces (that is really rare if the cable was not really attached from the beginning s supposed to be attached -I am possibly have a problem with the cable because it is too long or may be because the port has a problem-) even if the interface is not really configured and I will not be able to access internet.

What I want is: 1) A way to save the configuration and not need to wait for these 20 seconds for ifup to be reconfigured at every boot, but the ability to use

service networking restart

if my Internet is not really configure during the boot.

PS: 20 seconds are really many to wait at every boot and many times I just want to work offline without a cable plunged in.

/etc/network/interfaces:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
allow-hotplug eth0
iface eth0 inet dhcp
# This is an autoconfigured IPv6 interface
iface eth0 inet6 auto

Answer 1

Maybe the udev equivalent (eudev or mdev according to Devuan) isn't behaving 100% the same with regard to hotplug events. It's also possible if you're using a switch that the switch has a very long forwarding delay before actually letting traffic work both ways (in such case the 2nd solution below won't work).

Anyway, if you only want to have the interface unconfigured, this should be fairly easy. Here are two or three suggestions from easiest to more complex:

1. exclude the interface from configuration

   1.1. just change default network settings in /etc/default/networking:

   either

   CONFIGURE_INTERFACES=no
   

   this is a bit extreme, as even lo won't be brought up, probably preventing even some non-networked applications to work properly (gdm...).

   1.2. or

   EXCLUDE_INTERFACES=eth0
   

   to exclude just the problematic interface.

   You can then later use ifup eth0 to bring it up.

2. Or instead you can add a condition in the configuration to check if the link is available, and "fail" the interface if not. As I'm not sure if the dhcp hook is run before or after user-supplied up commands, I cheat and put it in pre-up which requires first bringing manually the interface up. You can check if using the up command is good enough without having to bring it up first.

   Edit the relevant /etc/network/interfaces part to look like this:

   allow-hotplug eth0
   iface eth0 inet dhcp
       pre-up ip link set eth0 up && sleep 1
       pre-up [ $(cat /sys/class/net/eth0/carrier) -eq 1 ]
   iface eth0 inet6 auto
       pre-up ip link set eth0 up && sleep 1
       pre-up [ $(cat /sys/class/net/eth0/carrier) -eq 1 ]
   

   (probably not needed for the ipv6 settings, this probably saves 1s. You could even remove the whole inet6 section if not interested)

   The return result of the test will make ifup consider the configuration failed and leave the logical (ifupdown's point of view) state of the card down.

You can then issue ifup eth0 or service networking restart later after having plugged the cable.