systemd-stable/etc/udev/debian/hotplugd.rules

# do not call hotplug.d and dev.d for "drivers" and "module" events
SUBSYSTEM=="drivers",	OPTIONS="last_rule"
SUBSYSTEM=="module",	OPTIONS="last_rule"

# compatibility support for the obsolete hotplug.d and dev.d directories
# do not run the hotplug.d scripts when started from udevstart
ENV{UDEVD_EVENT}=="1",	RUN+="/sbin/udev_run_hotplugd"
RUN+="/sbin/udev_run_devd"

# run a command on remove events
ACTION=="remove", ENV{REMOVE_CMD}!="", RUN+="$env{REMOVE_CMD}"

# debugging monitor
RUN+="socket:/org/kernel/udev/monitor"