More flexible than the single configuration file is it to
      configure libpam via the contents of
      pam.d directories. In this case the
      directories are filled with files each of which has a filename
      equal to a service-name (in lower-case): it is the personal
      configuration file for the named service.
    
      Vendor-supplied PAM configuration files might be installed in
      the system directory /usr/lib/pam.d/ or
      a configurable vendor specific directory instead
      of the machine configuration directory /etc/pam.d/.
      If no machine configuration file is found, the vendor-supplied file
      is used. All files in /etc/pam.d/ override
      files with the same name in other directories.
    
      The syntax of each file in pam.d is similar to that of the
      /etc/pam.conf file and is made up of lines
      of the following form:
    
type  control  module-path  module-arguments
    
      The only difference being that the service-name is not present. The
      service-name is of course the name of the given configuration file.
      For example, /etc/pam.d/login contains the
      configuration for the login service.