[gentoo-user] Mystically disabled Ethernet connection

[gevisz]

I am currently installing a new Gentoo system
on a computer that is run by a Gentoo system
that has not been updated since January 2019.

Initially, I did so by chrooting to a newly formatted
disk partition. My intention was to build everything
in the chrooted environment and only then start
to work in the new Gentoo system.

However, when after installing Grub I rebooted into
the new Gentoo system to find out if it works and
rebooted again into the old Gentoo system to continue
compilation in the chroot environment, I found out
that my computer does not have a working ethernet
connection to my home router. The "ip addr" command
showed that my ethernet interface is down.

I tried "ip link set <eth0> up" and "ifconfig <eth0> up"
commands, where <eth0> substitutes the correct,
"predictable," name of my ethernet interface but it
had not helped.

Initially, I thought that something was wrong with my
hardware, but when I rebooted into my new Gentoo
system, the ethernet interface started to work again.
Then, I rebooted back to the old system, and
the ethernet interface stopped working again.

It is very strange because I am sure that I have not
changed anything on my old Gentoo system except
for installing a new Grub onto the system disk.

So, I have tried to reinstall (the old) Grub from
my old system, but it has not helped either.

Finally, I decided that it would be easier to complete
installation of the new Gentoo system than to find out
what is wrong with the old Gentoo system ethernet
connection. So, I compiled xorg-x11, awesome, gvim,
xfce4-terminal and are currently compiling firefox.

Hopefully, tomorrow I will already have my new Gentoo
system working.

However, I am still wondering why the ethernet interface
stopped working on my old Gentoo system.

As, I have already said, I am pretty sure that I have not
changed anything in its settings. Moreover, I have looked
through the entire /etc directory of my old Gentoo system
for config files that were modified during the installation
of the new Gentoo system (suspecting that instead of the
chrooted environment I put an installation command into
the su terminal of my old Gentoo system or mistakenly
edited a configuration file of my old Gentoo system) but
found no config file that was modified during this period of time.

So, I am not in a desperate need for help, but just interested
to find out the reason for such a strange behaviour of the
ethernet interface.

P.S. I have read in the Gentoo Network management wiki
that the connman, netifrc, dhcpcd, networkmanager, and
openrc should not be run at the same time because it could
lead to "unpredictable results" and checked if it is not
the case on my old Gentoo system. No, on my old Gentoo
system installed is only the openrc with the netifrc use flag
and netifrc (which, as I guess, leads to only netifrc being used).
But, anyway, it has been worked so for years and only
now suddenly stopped.