PAM_SM(3PAM)PAM_SM(3PAM)NAMEpam_sm - PAM Service Module APIs
SYNOPSIS
#include <security/pam_appl.h>
#include <security/pam_modules.h>
cc [ flag ...] file ... -lpam [ library ...]
DESCRIPTION
PAM gives system administrators the flexibility of choosing any authen‐
tication service available on the system to perform authentication. The
framework also allows new authentication service modules to be plugged
in and made available without modifying the applications.
The PAM framework, libpam, consists of an interface library and multi‐
ple authentication service modules. The PAM interface library is the
layer that implements the Application Programming Interface ( API ).
The authentication service modules are a set of dynamically loadable
objects invoked by the PAM API to provide a particular type of user
authentication.
This manual page gives an overview of the PAM APIs for the service mod‐
ules, also called the Service Provider Interface (PAM-SPI).
Interface Overview
The PAM service module interface consists of functions which can be
grouped into four categories. The names for all the authentication
library functions start with pam_sm. The only difference between the
pam_*() interfaces and their corresponding pam_sm_*() interfaces is
that all the pam_sm_*() interfaces require extra parameters to pass
service-specific options to the shared modules. They are otherwise
identical.
The first category contains functions to authenticate an individual
user, pam_sm_authenticate(3PAM), and to set the credentials of the
user, pam_sm_setcred(3PAM). These back-end functions implement the
functionality of pam_authenticate(3PAM) and pam_setcred(3PAM) respec‐
tively.
The second category contains the function to do account management:
pam_sm_acct_mgmt(3PAM). This includes checking for password aging and
access-hour restrictions. This back-end function implements the func‐
tionality of pam_acct_mgmt(3PAM).
The third category contains the functions pam_sm_open_session(3PAM) and
pam_sm_close_session(3PAM) to perform session management after access
to the system has been granted. These back-end functions implement the
functionality of pam_open_session(3PAM) and pam_close_session(3PAM),
respectively.
The fourth category consists a function to change authentication tokens
pam_sm_chauthtok(3PAM). This back-end function implements the function‐
ality of pam_chauthtok(3PAM).
Stateful Interface
A sequence of calls sharing a common set of state information is
referred to as an authentication transaction. An authentication trans‐
action begins with a call to pam_start(). pam_start() allocates space,
performs various initialization activities, and assigns an authentica‐
tion handle to be used for subsequent calls to the library. Note that
the service modules do not get called or initialized when pam_start()
is called. The modules are loaded and the symbols resolved upon first
use of that function.
The PAM handle keeps certain information about the transaction that can
be accessed through the pam_get_item() API. Though the modules can also
use pam_set_item() to change any of the item information, it is recom‐
mended that nothing be changed except PAM_AUTHTOK and PAM_OLDAUTHTOK.
If the modules want to store any module specific state information then
they can use the pam_set_data(3PAM) function to store that information
with the PAM handle. The data should be stored with a name which is
unique across all modules and module types. For example,
SUNW_PAM_UNIX_AUTH_userid can be used as a name by the UNIX module to
store information about the state of user's authentication. Some mod‐
ules use this technique to share data across two different module
types.
Also, during the call to pam_authenticate(), the UNIX module may store
the authentication status (success or reason for failure) in the han‐
dle, using a unique name such as SUNW_SECURE_RPC_DATA. This information
is intended for use by pam_setcred().
During the call to pam_acct_mgmt(), the account modules may store data
in the handle to indicate which passwords have aged. This information
is intended for use by pam_chauthtok().
The module can also store a cleanup function associated with the data.
The PAM framework calls this cleanup function, when the application
calls pam_end() to close the transaction.
Interaction with the User
The PAM service modules do not communicate directly with the user;
instead they rely on the application to perform all such interactions.
The application passes a pointer to the function, conv(), along with
any associated application data pointers, through the pam_conv struc‐
ture when it initiates an authentication transaction (by means of a
call to pam_start(). The service module will then use the function,
conv(), to prompt the user for data, output error messages, and display
text information. Refer to pam_start(3PAM) for more information. The
modules are responsible for the localization of all messages to the
user.
Conventions
By convention, applications that need to prompt for a user name should
call pam_set_item() and set the value of PAM_USER_PROMPT before calling
pam_authenticate(). The service module's pam_sm_authenticate() function
will then call pam_get_user() to prompt for the user name. Note that
certain PAM service modules (such as a smart card module) may override
the value of PAM_USER_PROMPT and pass in their own prompt.
Though the PAM framework enforces no rules about the module's names,
location, options and such, there are certain conventions that all mod‐
ule providers are expected to follow.
By convention, the modules should be located in the /usr/lib/security
directory. Additional modules may be located in /opt/<pkg>/lib. Archi‐
tecture specific libraries (for example, sparcv9 or amd64) are located
in their respective subdirectories.
For every such module, there should be a corresponding manual page in
section 5 which should describe the module_type it supports, the func‐
tionality of the module, along with the options it supports. The depen‐
dencies should be clearly identified to the system administrator. For
example, it should be made clear whether this module is a stand-alone
module or depends upon the presence of some other module. One should
also specify whether this module should come before or after some other
module in the stack.
By convention, the modules should support the following options:
debug
Syslog debugging information at LOG_DEBUG level. Be careful
as to not log any sensitive information such as passwords.
nowarn
Turn off warning messages such as "password is about to
expire."
If an unsupported option is passed to the modules, it should syslog the
error at LOG_ERR level.
The permission bits on the service module should be set such that it is
not writable by either "group" or "other." The service module should
also be owned by root. The PAM framework will not load the module if
the above permission rules are not followed.
ERRORS
If there are any errors, the modules should log them using syslog(3C)
at the LOG_ERR level.
RETURN VALUES
The PAM service module functions may return any of the PAM error num‐
bers specified in the specific man pages. It can also return a
PAM_IGNORE error number to mean that the PAM framework should ignore
this module regardless of whether it is required, optional or suffi‐
cient. This error number is normally returned when the module does not
contribute to the decision being made by the PAM framework.
ATTRIBUTES
See attributes(5) for description of the following attributes:
┌────────────────────┬─────────────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├────────────────────┼─────────────────────────┤
│Interface Stability │ Stable │
├────────────────────┼─────────────────────────┤
│MT-Level │ MT-Safe with exceptions │
└────────────────────┴─────────────────────────┘
SEE ALSOpam(3PAM), pam_authenticate(3PAM), pam_chauthtok(3PAM),
pam_get_user(3PAM), pam_open_session(3PAM), pam_setcred(3PAM),
pam_set_item(3PAM), pam_sm_authenticate(3PAM), pam_sm_chauthtok(3PAM),
pam_sm_open_session(3PAM), pam_sm_setcred(3PAM), pam_start(3PAM),
pam_strerror(3PAM), syslog(3C), pam.conf(4), attributes(5), pam_auth‐
tok_check(5), pam_authtok_get(5), pam_authtok_store(5), pam_dhkeys(5),
pam_passwd_auth(5), pam_unix_account(5), pam_unix_auth(5),
pam_unix_session(5)NOTES
The interfaces in libpam are MT-Safe only if each thread within the
multithreaded application uses its own PAM handle.
Mar 16, 2005 PAM_SM(3PAM)
