Use the keyboard scroll lock LED as a network activity indicator.

Makefile

# A Makefile to turn the keyboard scroll lock LED into a network activity
# indicator LED through kernel instrumentation, as documented below.
#
# https://www.kernel.org/doc/html/latest/leds/index.html
# https://www.reddit.com/r/linux/comments/m2kkna/turn_your_keyboard_leds_into_network_indicators/gqjqmx0/

LEDTRIG_MODULE = /lib/modules/$$(uname -r)/kernel/drivers/leds/trigger/ledtrig-netdev.ko.xz
SCROLL_LOCK_LED = $(wildcard /sys/class/leds/input*::scrolllock)
NETWORK_INTERFACE = $(shell ip -br addr | awk '/UP/ { print $$1 }')

on:
	modprobe ledtrig-netdev
	echo "netdev" | tee $(SCROLL_LOCK_LED)/trigger >/dev/null
	echo "1" | tee $(SCROLL_LOCK_LED)/rx >/dev/null
	echo "1" | tee $(SCROLL_LOCK_LED)/tx >/dev/null
	echo $(NETWORK_INTERFACE) | tee $(SCROLL_LOCK_LED)/device_name >/dev/null

off:
	echo "kbd-scrolllock" | tee $(SCROLL_LOCK_LED)/trigger >/dev/null
	modprobe -r ledtrig-netdev