How is wired network set up by default?

[turboscrew]

Some history to clarify the situation:
I had Zyxel ADSL-box with 4-port router. Then I changed ISP and got an A-Link RR 24AP(N) router.
Zyxel seemed to work better (after firmware update) and I switched back to Zyxel.
With both routers, I set the ADSL-box DHCP-server to give a certain IP to my desktop (I have 5 machines).
Then the Zyxel broke, and I couldn't find the RR power, so I bought another ADSL-box - Asus DSL-N12U.
Then the MoBo of my desktop went ka-boom, and I had to get a new MoBo, and then I installed
Kubuntu 14.04 on the desktop.

Some time passed (a year or so).

Now I found the RR power and decided to switch back to it, because A-sucks haven't been working too well.
All other machines worked fine except that Kubuntu desktop.
It only has wired network, but suddenly I doesn't get an IP from the DHCP-server in the RR.
(The network cable is checked.)

There seems to be no dhcpd nor dhclient running (ps aux).
What the h*** happened? How did it work before?
(Of course the DHCP IP ranges changed.)
Also ifup doesn't recognize eth0,("Ignoring unknown interface eth0=eth0") but ifconfig does.
There are no networks in the NetworkManager - only wifi (disabled), mobile broadband (disabled), and Airplane mode.
Not a sign of wired.

What should I change to make as small changes as possible, but to get the wired network to work?
(I'm writing this on a laptop connected by wire to the same ADSL-box.)

[turboscrew]

Plugged the Kubuntu-desktop back to A-sucks, and here I am writing this with the Kubuntu-desktop.
Some more info:
/etc/network/interfaces.d is empty

/etc/network/interfaces:

# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback

/etc/NetworkManager/NetworkManager.conf:

[main]
plugins=ifupdown,keyfile,ofono
dns=dnsmasq

no-auto-default=90:2B:34:9F:3E:81,

[ifupdown]
managed=false

Also:

$ ps aux | grep dhclient
root      1202  0.0  0.0  10228  3692 ?        S    20:04   0:00 /sbin/dhclient -d -sf /usr/lib/NetworkManager/nm-dhcp-client.action -pf /run/sendsigs.omit.d/network-manager.dhclient-eth0.pid -lf /var/lib/NetworkManager/dhclient-f3bb04c3-d60a-48ba-a845-40f68f07a378-eth0.lease -cf /var/lib/NetworkManager/dhclient-eth0.conf eth0
jaa       2843  0.0  0.0  12964   956 pts/1    S+   20:23   0:00 grep --color=auto dhclient

$ ifup eth0
ifup: failed to open lockfile /run/network/.ifstate.lock: Permission denied

The network management applet looks like this:

[SecretCode]

I don't think I have used a wired connection on Linux for years (except for my Raspberry Pi for a bit).

I don't have the no-auto-default line in NetworkManager.conf. I'll wager a korvapuusti that's somehow connected to your issue.

[turboscrew]

Argh - it was a broken router port. Funny that dhclient disappears if the cable is disconnected.
Even the NetworkManager didn't indicate disconnection, but acted as if the wired network was never configured.