File checked by shutdown if called from init (ie. when ctrl-alt-delete is pressed). Unless one of the users specified in it (or root) is logged in on a virtual console, the system will not reboot. This is a Good Thing, as it prevents dumbasses rebooting your server.

The file is a list of usernames, one per line. Comments are #ed out. Example:

# Users who are allowed to perform console shutdowns: