/lib/udev/rules.d/50-rbd.rules is in ceph-common 10.1.2-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | KERNEL=="rbd[0-9]*", ENV{DEVTYPE}=="disk", PROGRAM="/usr/bin/ceph-rbdnamer %k", SYMLINK+="rbd/%c{1}/%c{2}"
KERNEL=="rbd[0-9]*", ENV{DEVTYPE}=="partition", PROGRAM="/usr/bin/ceph-rbdnamer %k", SYMLINK+="rbd/%c{1}/%c{2}-part%n"
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