Block all site users from logging in while your site is live and without having to put your site into maintenance mode.
This functionality can be useful for busy sites when administrators are applying maintenance updates or other configuration changes that require a maintenance window but do not necessarily wish to put the site into full maintenance mode.
When active, the module displays a warning message on the login screen alerting users that maintenance is underway and that they are temporarily blocked. User with uid=1 is allowed to login by default, but additional usernames can be allowed. Current logged-in users will automatically be logged out.