GNU Linux howto – direct LAN ethernet network connection between two PCs (can not bring up interface)

06.Dec.2023

in the past, this was no issue.

no matter what.

neither:

ifup enp0s25
# nor
ifconfig enp0s25 up
# nor
ip link set enp0s25 up
# worked

two notebook NICs were connected with a known-to-be-good LAN cable but both interfaces still reported to be “down”.

it has something to do with networking.service

on both machines perform this action:

connect them with a known-to-be-good LAN cable (the user can check if a cable is good to side-by-side test if all contact are good with the “go-through” test of a multimeter)

hostnamectl; # tested on
Operating System: Debian GNU/Linux 12 (bookworm) 
Kernel: Linux 6.1.0-13-amd64
Architecture: x86-64

systemctl stop NetworkManager.service
service networking stop

# check what is the user's LAN interface
ip -c a
vim /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 enp0s25
iface enp0s25 inet dhcp

# on machine A
while true; do ifconfig enp0s25 192.168.123.123 up; ip -c a;  ping -c 5 192.168.123.124; sleep 3; done;

# on machine B
while true; do ifconfig enp0s25 192.168.123.124 up; ip -c a;  ping -c 5 192.168.123.123; sleep 3; done;

UNPLUG THE CABLE

REPLUG THE CABLE

both machines should be able to ping each other and connect

BUT the speed might be 100MBit/s kind of shitty (not sure if it is possible (tried but failed) to set higher speeds via this as the two NICs SHOULD auto negotiate their speed)

machine A is lenovo t440 (Intel I218-LM), machine B is lenovo 14g (Intel I219-LM) #wtf intel

# check what nics are available
su - root
apt update
apt install lshw
lshw -class network

have phun X-D

PS: is this a gnome network manager or systemd issue or a intel issue?

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