Debian – Why I’m giving message `A start job is running for raise network interfaces` at boot after disabling predictable network interfaces names

Remove the /etc/network/interfaces.d/setup file then edit your /etc/network/interfaces as follows :

auto lo
iface lo inet loopback

Save and reboot

The man interfaces:

INCLUDING OTHER FILES

  Lines  beginning  with  "source" are used to include stanzas from other

   files, so configuration can be split into many files. The word "source"
   is  followed  by the path of file to be sourced. Shell wildcards can be
   used.  (See wordexp(3) for details.)

In your case you are using the /etc/network/interfaces.d/setup to configure the network instead of /etc/network/interfaces

Lines beginning with "allow-" are used to identify interfaces that should be brought up automatically by various subsytems. This may be done using a command such as "ifup --allow=hotplug eth0 eth1", which will only bring up eth0 or eth1 if it is listed in an "allow-hotplug" line. Note that "allow-auto" and "auto" are synonyms. (Interfaces marked "allow-hotplug" are brought up when udev detects them. This can either be during boot if the interface is already present, or at a later time, for example when plugging in a USB network card. Please note that this does not have anything to do with detecting a network cable being plugged in.)

Why did it stop working? What did I do wrong?

It's hard to tell what happened "suddenly". Review your /var/log/syslog or /var/log/messages for DHCP logs around the time it stopped working and you may find out. Also check the modification time in stat /etc/network/interfaces.

Why was it ever eth0 in my /etc/network/interfaces, when Debian uses systemd and Predictable Network Interfaces?

This is the new default since Debian stretch. If you installed your system earlier and your /etc/network/interfaces originates from jessie (or wheezy...) then it was created by the installer (netcfg) using the old naming scheme.

What is the correct way to fix this?

Whatever suits you best.

• If you expect to ever have multiple Ethernet interfaces in this machine, then think about how you'd like to tell them apart. Remember that their ethX order would not be stable/predictable. PredictableNetworkInterfaceNames gives you several options, choose according to your use case.

• If this machine will only ever have a single Ethernet interface, then you can either

  • fall back to using the kernel name, which will always be eth0 (the above page gives you several options, my favorite is ln -s /dev/null /etc/systemd/network/99-default.link, just don't forget to regenerate your initramfs afterwards with update-initramfs -u);
  • or don't care about the name of your interface, ditch /etc/network/interfaces entirely and switch to systemd-networkd with a match-all config like

[Network]
DHCP=ipv4

in /etc/systemd/network/dhcp.network (choose an arbitrary file name). Sorry, I can't get the indentation right...