FUCK YOU POETTERLING
root@pi03:~# ip link set eth0 mtu 1496
RTNETLINK answers: Device or resource busy
#### FUUUU ALERT ####
root@pi03:~# ps aux|grep -i "network"
root 580 0.0 2.3 54908 11696 ? Ssl 18:45 0:00 /usr/sbin/NetworkManager --no-daemon
root@pi03:~# ls /etc/NetworkManager/system-connections/
'Wired connection 1.nmconnection' # <= eth0.conf was way too complicated
root@pi03:~# vi /etc/NetworkManager/system-connections/Wired\ connection\ 1.nmconnection
root@pi03:~# grep -E '^\[|mtu' /etc/NetworkManager/system-connections/*
[connection]
[ethernet]
mtu=1496
[ipv4]
[ipv6]
[proxy]
root@pi03:~# nmcli connection reload
root@pi03:~# nmcli device reapply eth0
Connection successfully reapplied to device 'eth0'.
root@pi03:~# ip -br link show eth0
eth0 UP 02:81:74:bb:c7:d1 <BROADCAST,MULTICAST,UP,LOWER_UP>
root@pi03:~# nmcli device show eth0 | grep MTU
GENERAL.MTU: 1500
root@pi03:~# cat > /etc/NetworkManager/dispatcher.d/99-force-mtu-eth0
#!/bin/bash
if [ "$1" = "eth0" ] && [ "$2" = "up" ]; then
ip link set eth0 mtu 1496
fi
^C
root@pi03:~# chmod /etc/NetworkManager/dispatcher.d/99-force-mtu-eth0
root@pi03:~# systemctl restart NetworkManager
root@pi03:~# ifconfig|grep mtu
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
root@pi03:~# reboot
...
root@pi03:~# ifconfig|grep mtu
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
root@pi03:~# grep -i pretty /etc/os-release
PRETTY_NAME="Armbian 22.11.1 Bullseye"
# ~~~~~~
root@u3o1:~# ip link set eth0 mtu 1496
root@u3o1:~# grep -i pretty /etc/os-release
PRETTY_NAME="Debian GNU/Linux 13 (trixie)"
root@u3o1:~# ps aux|grep -iE "network"
root 1706 33.3 0.0 6980 1788 pts/1 S+ 17:15 0:00 grep --color=auto -i network