Linux-PAM

 +----------------+
  | application: X |
  +----------------+       /  +----------+     +================+
  | authentication-[---->--\--] Linux-   |--<--| PAM config file|
  |       +        [----<--/--]   PAM    |     |================|
  |[conversation()][--+    \  |          |     | X auth .. a.so |
  +----------------+  |    /  +-n--n-----+     | X auth .. b.so |
  |                |  |       __|  |           |           _____/
  |  service user  |  A      |     |           |____,-----
  |                |  |      V     A
  +----------------+  +------|-----|---------+ -----+------+
                         +---u-----u----+    |      |      |
                         |   auth....   |--[ a ]--[ b ]--[ c ]
                         +--------------+
                         |   acct....   |--[ b ]--[ d ]
                         +--------------+
                         |   password   |--[ b ]--[ c ]
                         +--------------+
                         |   session    |--[ e ]--[ c ]
                         +--------------+

Simply put, these groups take care of different aspects of a typical
user´s request for a restricted service:

account - provide account verification types of service: has the user´s
password expired?; is this user permitted access to the requested
service?

authentication - authenticate a user and set up user credentials.
Typically this is via some challenge-response request that theuser must
satisfy: if you are who you claim to be please enter your password. Not
all authentications are of this type, there exist,hardware based
authentication schemes (such as the use of smart-cards and biometric
devices), with suitable modules, these may be substituted seamlessly for
more standard approaches to authentication - such is the flexibility of
Linux-PAM.

password - this group´s responsibility is the task of updating
authentication mechanisms. Typically, such services are strongly coupled
to those of the auth group. Some authentication mechanisms lend
themselves well to being updated with such a function. Standard UN\*X
password-based access is the obvious example: please enter a replacement
password.

session - this group of tasks cover things that should be done prior to
a service being given and after it is withdrawn. Such tasks include the
maintenance of audit trails and the mounting of the user´s home
directory. The session management group is important as it provides both
an opening and closing hook for modules to affect the services available
to a user.

service-name   module-type           control-flag   module-path   arguments
服务名称           模块类型（管理组）控制标志      模块路径       模块参数