pam_nologin.so  [
        file=/path/nologin
      ] [
        successok
      ]
      pam_nologin is a PAM module that prevents users from logging into
      the system when /etc/nologin exists. The contents
      of the /etc/nologin file are displayed to the
      user. The pam_nologin module has no effect on the root user's ability
      to log in.
    
file=/path/nologin
        
            Use this file instead the default
            /etc/nologin.
          
successok
        Return PAM_SUCCESS if no file exists, the default is PAM_IGNORE.
            The user is not root and /etc/nologin
            exists, so the user is not permitted to log in.
          
Memory buffer error.
This is the default return value.
            Success:  either  the user is root or the
            /etc/nologin file does not exist.
          
User not known to the underlying authentication module.