pam_sm - PAM Service Module APIs
#include <security/pam_appl.h> #include <security/pam_modules.h> cc [ flag ...] file ... –lpam [ library ...]
PAM gives system administrators the flexibility of choosing any authentication 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 multiple 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 modules, also called the Service Provider Interface (PAM-SPI).
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) respectively.
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 functionality 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.
A sequence of calls sharing a common set of state information is referred to as an authentication transaction. An authentication transaction begins with a call to pam_start(). pam_start() allocates space, performs various initialization activities, and assigns an authentication 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 recommended 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 modules 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 handle, 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.
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 structure 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.
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 smartcard 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 module 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 . Architecture 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 functionality of the module, along with the options it supports. The dependencies should be clearly identified to the system administrator. For example, it should be made clear whether this module is a standalone 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:
Syslog debugging information at LOG_DEBUG level. Be careful as to not log any sensitive information such as passwords.
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.
If there are any errors, the modules should log them using syslog(3C) at the LOG_ERR level.
The PAM service module functions may return any of the PAM error numbers 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 sufficient. This error number is normally returned when the module does not contribute to the decision being made by the PAM framework.
See attributes(7) for description of the following attributes:
syslog(3C), pam_setcred(3PAM), pam(3PAM), pam_authenticate(3PAM), pam_chauthtok(3PAM), pam_get_user(3PAM), pam_open_session(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), pam.conf(5), attributes(7), pam_authtok_check(7), pam_authtok_get(7), pam_authtok_store(7), pam_dhkeys(7), pam_passwd_auth(7), pam_unix_account(7), pam_unix_auth(7), pam_unix_session(7)
The interfaces in libpam are MT-Safe only if each thread within the multithreaded application uses its own PAM handle.