/lib/udev/rules.d/xen-backend.rules is in xen-utils-common 4.4.0-0ubuntu5.
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 | SUBSYSTEM=="xen-backend", KERNEL=="tap*", ENV{UDEV_CALL}="1", RUN+="/etc/xen/scripts/blktap $env{ACTION}"
SUBSYSTEM=="xen-backend", KERNEL=="vbd*", ENV{UDEV_CALL}="1", RUN+="/etc/xen/scripts/block $env{ACTION}"
SUBSYSTEM=="xen-backend", KERNEL=="vif2-*", RUN+="/etc/xen/scripts/vif2 $env{ACTION}"
SUBSYSTEM=="xen-backend", KERNEL=="vif-*", ENV{UDEV_CALL}="1", ACTION=="online", RUN+="/etc/xen/scripts/vif-setup online type_if=vif"
SUBSYSTEM=="xen-backend", KERNEL=="vif-*", ENV{UDEV_CALL}="1", ACTION=="offline", RUN+="/etc/xen/scripts/vif-setup offline type_if=vif"
SUBSYSTEM=="xen-backend", KERNEL=="vscsi*", RUN+="/etc/xen/scripts/vscsi $env{ACTION}"
SUBSYSTEM=="xen-backend", ACTION=="remove", ENV{UDEV_CALL}="1", RUN+="/etc/xen/scripts/xen-hotplug-cleanup"
SUBSYSTEM=="net", KERNEL=="vif*-emu", ACTION=="add", ENV{UDEV_CALL}="1", RUN+="/etc/xen/scripts/vif-setup $env{ACTION} type_if=tap"
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