.. my longer story is something like the following.

Networking in chimaera is really the same as in all prior system versions; it's a kernel function to operate the interfaces that dynamically rely on their configurations, and there are some forms of user software available for the administration of those configurations.

The first and oldest unit of such software is nowadays called ifupdown in Devuan (Debian).

ifupdown implements an administration principle where the administrator documents their desired configurations using a special purpose scripting language written into one or more plain text files starting with /etc/network/interfaces. The software is modular with a core bit that offers the adminstration programs ifup, ifdown and ifstate as well as the hook-in to the boot process and the hotplug event handling. ifupdown also provides a well-documented API for extensions that allows for other software packages to be seamlessly incorporated into the networking operations as if belonging to ifupdown; e.g., handling DHCP, handling wireless connectivity, VPN setup and whatnot.

On the side of that we find a couple of network management software packages (package groups), where chimaera in particular includes the two named connman and network-manager. These implement their own ideas of the network configuration task, with a common aim to make networking configuration be administrated "more easily" than by using the special purpose scripting language and programs of ifupdown, and most importantly to provide a "security bypass" that empowers non-root users with network administration abilities.

A side effect of that is that the network management software "wants to rule the city". Then "traditional" administration via ifupdown might be in conflict with it, unless one knows enough about the management software to avoid such conflicts. The easiest way for a root administrator to avoid such conflicts  is to get rid of all and any such management software.

Noted. Thank you,

Job wrote:

The machine is a desktop with no wireless hardware or button.

Oops, sorry. I will learn to read one day...

Job wrote:

the trouble started after I woke up the machine from hibernation

Is that reproducible? If so what do the logs say?

I let the box hibernate to see if I can reproduce the same scenario but the box and the interface came back on right way.

Networking in chimaera is really the same as in all prior system versions; it's a kernel function to operate the interfaces that dynamically rely on their configurations, and there are some forms of user software available for the administration of those configurations.

The first and oldest unit of such software is nowadays called ifupdown in Devuan (Debian).

ifupdown implements an administration principle where the administrator documents their desired configurations using a special purpose scripting language written into one or more plain text files starting with /etc/network/interfaces. The software is modular with a core bit that offers the adminstration programs ifup, ifdown and ifstate as well as the hook-in to the boot process and the hotplug event handling. ifupdown also provides a well-documented API for extensions that allows for other software packages to be seamlessly incorporated into the networking operations as if belonging to ifupdown; e.g., handling DHCP, handling wireless connectivity, VPN setup and whatnot.

On the side of that we find a couple of network management software packages (package groups), where chimaera in particular includes the two named connman and network-manager. These implement their own ideas of the network configuration task, with a common aim to make networking configuration be administrated "more easily" than by using the special purpose scripting language and programs of ifupdown, and most importantly to provide a "security bypass" that empowers non-root users with network administration abilities.

A side effect of that is that the network management software "wants to rule the city". Then "traditional" administration via ifupdown might be in conflict with it, unless one knows enough about the management software to avoid such conflicts. The easiest way for a root administrator to avoid such conflicts  is to get rid of all and any such management software.

The machine is a desktop with no wireless hardware or button.

Oops, sorry. I will learn to read one day...

Is that reproducible? If so what do the logs say?

# apt install sysv-rc-conf
# sysv-rc-conf

^ The second command will list all services and their state.

I'm pretty sure that NetworkManager, connman & wicd all check /etc/network/interfaces{,.d/*} and will ignore any interfaces listed there but it doesn't hurt to check and disable the services.

Your networking problem could have been caused by rfkill (is there a wireless button on your machine?) or possibly an over-zealous power saving mode for the wireless driver. The logs should show exactly what happened.

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback
#the primary network interface
auto eth0
allow-hotplug eth0
iface eth0 inet dhcp

Anyway:

Ralph, do you mind expanding on what you meant by purging "all "network manager" software except ifupdown? Do you mean manually removing everything in /etc/network and leaving the ifupdown folder?

HoaS,

root@luna:~# ifup --verbose eth0
ifup: interface eth0 already configured

I am guessing because the device is already busy.

Since moving to chimaera, I only have problems with the network interface.

At some point I only had

iface eth0 inet dhcp

in /etc/network/interfaces. After about two days, the network died again. Since then no matter what I tried I can't get the internet going.

I will appreciate some pointers.