/lib/udev/rules.d/39-usbmuxd.rules is in usbmuxd 1.0.8+git20140527.e72f2f7-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 | # usbmuxd ("Apple Mobile Device" muxer listening on /var/run/usbmuxd)
# Check for systemd running as pid 1
TEST=="/run/systemd/system/", ACTION=="add", SUBSYSTEM=="usb", ATTR{idVendor}=="05ac", ATTR{idProduct}=="12[9a][0-9a-f]", ENV{USBMUX_SUPPORTED}="1", ATTR{bConfigurationValue}="0", OWNER="usbmux", TAG+="systemd", ENV{SYSTEMD_WANTS}="usbmuxd.service", RUN+="/usr/bin/pkill -x -SIGUSR2 usbmux" GOTO="usbmuxd_systemd_end"
# Legacy udev
ACTION=="add", SUBSYSTEM=="usb", ATTR{idVendor}=="05ac", ATTR{idProduct}=="12[9a][0-9a-f]", ENV{USBMUX_SUPPORTED}="1", ATTR{bConfigurationValue}="0", OWNER="usbmux", RUN+="/usr/sbin/usbmuxd -u -U usbmux"
LABEL="usbmuxd_systemd_end"
# Exit usbmuxd when the last device is removed
ACTION=="remove", SUBSYSTEM=="usb", ENV{PRODUCT}=="5ac/12[9a][0-9a-f]/*", ENV{INTERFACE}=="255/*", RUN+="/usr/sbin/usbmuxd -x"
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