/etc/rsyslog.d/21-cloudinit.conf is in cloud-init 0.7.7~bzr1212-0ubuntu1.
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 | # Log cloudinit generated log messages to file
:syslogtag, isequal, "[CLOUDINIT]" /var/log/cloud-init.log
# comment out the following line to allow CLOUDINIT messages through.
# Doing so means you'll also get CLOUDINIT messages in /var/log/syslog
& ~
|