/lib/udev/rules.d/50-rbd.rules is in librbd1 0.41-1ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1
KERNEL=="rbd[0-9]*", PROGRAM="/usr/bin/ceph-rbdnamer %n", SYMLINK+="rbd/%c{1}/%c{2}"