The NoLogin authentication service module for PAM provides functionality for only one PAM category: authentication. In terms of the 
” feature. It also provides a null function for session management.
NoLogin Authentication Module
 The NoLogin authentication component (
pam_sm_authenticate()), always returns success for the superuser, and returns success for all other users if the file 
/etc/nologin does not exist. If 
/etc/nologin does exist, then its contents are echoed to non-superusers before failure is returned. If a "nologin" capability is specified in 
login.conf(5), then the file thus specified is used instead. This usually defaults to 
/etc/nologin.
The following options may be passed to the authentication module:
- 
debug
- 
syslog(3) debugging information at LOG_DEBUG level.
- 
no_warn
- 
suppress warning messages to the user. These messages include reasons why the user's authentication attempt was declined.