The Linux-PAM Application Developers' Guide

Andrew G. Morgan

   <morgan@kernel.org>

Thorsten Kukuk

   <kukuk@thkukuk.de>

   Version 1.7.2, January 2026

   Abstract

   This manual documents what an application developer needs to know about
   the Linux-PAM library. It describes how an application might use the
   Linux-PAM library to authenticate users. In addition it contains a
   description of the functions to be found in libpam_misc library, that
   can be used in general applications. Finally, it contains some comments
   on PAM related security issues for the application developer.
     __________________________________________________________________

   1. Introduction

        1.1. Description
        1.2. Synopsis

   2. Overview
   3. The public interface to Linux-PAM

        3.1. What can be expected by the application

              3.1.1. Initialization of PAM transaction
              3.1.2. Termination of PAM transaction
              3.1.3. Setting PAM items
              3.1.4. Getting PAM items
              3.1.5. Strings describing PAM error codes
              3.1.6. Request a delay on failure
              3.1.7. Authenticating the user
              3.1.8. Setting user credentials
              3.1.9. Account validation management
              3.1.10. Updating authentication tokens
              3.1.11. Start PAM session management
              3.1.12. terminating PAM session management
              3.1.13. Set or change PAM environment variable
              3.1.14. Get a PAM environment variable
              3.1.15. Getting the PAM environment

        3.2. What is expected of an application

              3.2.1. The conversation function

        3.3. Programming notes

   4. Security issues of Linux-PAM

        4.1. Care about standard library calls
        4.2. Choice of a service name
        4.3. The conversation function
        4.4. The identity of the user
        4.5. Sufficient resources

   5. A library of miscellaneous helper functions

        5.1. Functions supplied

              5.1.1. Text based conversation function
              5.1.2. Transcribing an environment to that of PAM
              5.1.3. Liberating a locally saved environment
              5.1.4. BSD like PAM environment variable setting

   6. Porting legacy applications
   7. Glossary of PAM related terms
   8. An example application
   9. Files
   10. See also
   11. Author/acknowledgments
   12. Copyright information for this document

Chapter 1. Introduction

1.1. Description

   Linux-PAM (Pluggable Authentication Modules for Linux) is a library
   that enables the local system administrator to choose how individual
   applications authenticate users. For an overview of the Linux-PAM
   library see the Linux-PAM System Administrators' Guide.

   It is the purpose of the Linux-PAM project to liberate the development
   of privilege granting software from the development of secure and
   appropriate authentication schemes. This is accomplished by providing a
   documented library of functions that an application may use for all
   forms of user authentication management. This library dynamically loads
   locally configured authentication modules that actually perform the
   authentication tasks.

   From the perspective of an application developer the information
   contained in the local configuration of the PAM library should not be
   important. Indeed it is intended that an application treat the
   functions documented here as a 'black box' that will deal with all
   aspects of user authentication. 'All aspects' includes user
   verification, account management, session initialization/termination
   and also the resetting of passwords (authentication tokens).

1.2. Synopsis

   For general applications that wish to use the services provided by
   Linux-PAM the following is a summary of the relevant linking
   information:
#include <security/pam_appl.h>

cc -o application .... -lpam

   In addition to libpam, there is a library of miscellaneous functions
   that make the job of writing PAM-aware applications easier (this
   library is not covered in the DCE-RFC for PAM and is specific to the
   Linux-PAM distribution):
#include <security/pam_appl.h>
#include <security/pam_misc.h>

cc -o application .... -lpam -lpam_misc

Chapter 2. Overview

   Most service-giving applications are restricted. In other words, their
   service is not available to all and every prospective client. Instead,
   the applying client must jump through a number of hoops to convince the
   serving application that they are authorized to obtain service.

   The process of authenticating a client is what PAM is designed to
   manage. In addition to authentication, PAM provides account management,
   credential management, session management and authentication-token
   (password changing) management services. It is important to realize
   when writing a PAM based application that these services are provided
   in a manner that is transparent to the application. That is to say,
   when the application is written, no assumptions can be made about how
   the client will be authenticated.

   The process of authentication is performed by the PAM library via a
   call to pam_authenticate(). The return value of this function will
   indicate whether a named client (the user) has been authenticated. If
   the PAM library needs to prompt the user for any information, such as
   their name or a password then it will do so. If the PAM library is
   configured to authenticate the user using some silent protocol, it will
   do this too. (This latter case might be via some hardware interface for
   example.)

   It is important to note that the application must leave all decisions
   about when to prompt the user at the discretion of the PAM library.

   The PAM library, however, must work equally well for different styles
   of application. Some applications, like the familiar login and passwd
   are terminal based applications, exchanges of information with the
   client in these cases is as plain text messages. Graphically based
   applications, however, have a more sophisticated interface. They
   generally interact with the user via specially constructed dialogue
   boxes. Additionally, network based services require that text messages
   exchanged with the client are specially formatted for automated
   processing: one such example is ftpd which prefixes each exchanged
   message with a numeric identifier.

   The presentation of simple requests to a client is thus something very
   dependent on the protocol that the serving application will use. In
   spite of the fact that PAM demands that it drives the whole
   authentication process, it is not possible to leave such protocol
   subtleties up to the PAM library. To overcome this potential problem,
   the application provides the PAM library with a conversation function.
   This function is called from within the PAM library and enables the PAM
   to directly interact with the client. The sorts of things that this
   conversation function must be able to do are prompt the user with text
   and/or obtain textual input from the user for processing by the PAM
   library. The details of this function are provided in a later section.

   For example, the conversation function may be called by the PAM library
   with a request to prompt the user for a password. Its job is to
   reformat the prompt request into a form that the client will
   understand. In the case of ftpd, this might involve prefixing the
   string with the number 331 and sending the request over the network to
   a connected client. The conversation function will then obtain any
   reply and, after extracting the typed password, will return this string
   of text to the PAM library. Similar concerns need to be addressed in
   the case of an X-based graphical server.

   There are a number of issues that need to be addressed when one is
   porting an existing application to become PAM compliant. A section
   below has been devoted to this: Porting legacy applications.

   Besides authentication, PAM provides other forms of management. Session
   management is provided with calls to pam_open_session() and
   pam_close_session(). What these functions actually do is up to the
   local administrator. But typically, they could be used to log entry and
   exit from the system or for mounting and unmounting the user's home
   directory. If an application provides continuous service for a period
   of time, it should probably call these functions, first open after the
   user is authenticated and then close when the service is terminated.

   Account management is another area that an application developer should
   include with a call to pam_acct_mgmt(). This call will perform checks
   on the good health of the user's account (has it expired etc.). One of
   the things this function may check is whether the user's authentication
   token has expired - in such a case the application may choose to
   attempt to update it with a call to pam_chauthtok(), although some
   applications are not suited to this task (ftp for example) and in this
   case the application should deny access to the user.

   PAM is also capable of setting and deleting the user's credentials with
   the call pam_setcred(). This function should always be called after the
   user is authenticated and before service is offered to the user. By
   convention, this should be the last call to the PAM library before the
   PAM session is opened. What exactly a credential is, is not well
   defined. However, some examples are given in the glossary below.

Chapter 3.  The public interface to Linux-PAM

   Firstly, the relevant include file for the Linux-PAM library is
   <security/pam_appl.h>. It contains the definitions for a number of
   functions. After listing these functions, we collect some guiding
   remarks for programmers.

3.1. What can be expected by the application

3.1.1. Initialization of PAM transaction

#include <security/pam_appl.h>

   int pam_start( service_name,
                  user,
                  pam_conversation,
                  pamh);
   const char *service_name;
   const char *user;
   const struct pam_conv *pam_conversation;
   pam_handle_t **pamh;

   int pam_start_confdir( service_name,
                          user,
                          pam_conversation,
                          confdir,
                          pamh);
   const char *service_name;
   const char *user;
   const struct pam_conv *pam_conversation;
   const char *confdir;
   pam_handle_t **pamh;

3.1.1.1. DESCRIPTION

   The pam_start function creates the PAM context and initiates the PAM
   transaction. It is the first of the PAM functions that needs to be
   called by an application. The transaction state is contained entirely
   within the structure identified by this handle, so it is possible to
   have multiple transactions in parallel. But it is not possible to use
   the same handle for different transactions, a new one is needed for
   every new context.

   The service_name argument specifies the name of the service to apply
   and will be stored as PAM_SERVICE item in the new context. The policy
   for the service will be read from the file /etc/pam.d/service_name or,
   if that file does not exist, from /etc/pam.conf.

   The user argument can specify the name of the target user and will be
   stored as PAM_USER item. If the argument is NULL, the module has to ask
   for this item if necessary.

   The pam_conversation argument points to a struct pam_conv describing
   the conversation function to use. An application must provide this for
   direct communication between a loaded module and the application.

   Following a successful return (PAM_SUCCESS) the contents of pamh is a
   handle that contains the PAM context for successive calls to the PAM
   functions. In an error case is the content of pamh undefined.

   The pam_handle_t is a blind structure and the application should not
   attempt to probe it directly for information. Instead the PAM library
   provides the functions pam_set_item(3) and pam_get_item(3). The PAM
   handle cannot be used for multiple authentications at the same time as
   long as pam_end was not called on it before.

   The pam_start_confdir function behaves like the pam_start function but
   it also allows setting confdir argument with a path to a directory to
   override the default (/etc/pam.d) path for service policy files. If the
   confdir is NULL, the function works exactly the same as pam_start.

3.1.1.2. RETURN VALUES

   PAM_ABORT
          General failure.

   PAM_BUF_ERR
          Memory buffer error.

   PAM_SUCCESS
          Transaction was successfully started.

   PAM_SYSTEM_ERR
          System error, for example a NULL pointer was submitted instead
          of a pointer to data.

3.1.2. Termination of PAM transaction

#include <security/pam_appl.h>

   int pam_end( pamh,
                pam_status);
   pam_handle_t *pamh;
   int pam_status;

3.1.2.1. DESCRIPTION

   The pam_end function terminates the PAM transaction and is the last
   function an application should call in the PAM context. Upon return the
   handle pamh is no longer valid and all memory associated with it will
   be invalid.

   The pam_status argument should be set to the value returned to the
   application by the last PAM library call.

   The value taken by pam_status is used as an argument to the module
   specific callback function, cleanup() (See pam_set_data(3) and
   pam_get_data(3)). In this way the module can be given notification of
   the pass/fail nature of the tear-down process, and perform any last
   minute tasks that are appropriate to the module before it is unlinked.
   This argument can be logically OR'd with PAM_DATA_SILENT to indicate
   that the module should not treat the call too seriously. It is
   generally used to indicate that the current closing of the library is
   in a fork(2)ed process, and that the parent will take care of cleaning
   up things that exist outside of the current process space (files etc.).

   This function free's all memory for items associated with the
   pam_set_item(3) and pam_get_item(3) functions. Pointers associated with
   such objects are not valid anymore after pam_end was called.

3.1.2.2. RETURN VALUES

   PAM_SUCCESS
          Transaction was successful terminated.

   PAM_SYSTEM_ERR
          System error, for example a NULL pointer was submitted as PAM
          handle or the function was called by a module.

3.1.3. Setting PAM items

#include <security/pam_modules.h>

   int pam_set_item( pamh,
                     item_type,
                     item);
   pam_handle_t *pamh;
   int item_type;
   const void *item;

3.1.3.1. DESCRIPTION

   The pam_set_item function allows applications and PAM service modules
   to access and to update PAM information of item_type. For this a copy
   of the object pointed to by the item argument is created. The following
   item_types are supported:

   PAM_SERVICE
          The service name (which identifies that PAM stack that the PAM
          functions will use to authenticate the program).

   PAM_USER
          The username of the entity under whose identity service will be
          given. That is, following authentication, PAM_USER identifies
          the local entity that gets to use the service. Note, this value
          can be mapped from something (eg., "anonymous") to something
          else (eg. "guest119") by any module in the PAM stack. As such an
          application should consult the value of PAM_USER after each call
          to a PAM function.

   PAM_USER_PROMPT
          The string used when prompting for a user's name. The default
          value for this string is a localized version of "login: ".

   PAM_TTY
          The terminal name prefixed by /dev/ for device files. In the
          past, graphical X-based applications used to store the $DISPLAY
          variable here, but with the introduction of PAM_XDISPLAY this
          usage is deprecated.

   PAM_RUSER
          The requesting user name: local name for a locally requesting
          user or a remote user name for a remote requesting user.

          Generally an application or module will attempt to supply the
          value that is most strongly authenticated (a local account
          before a remote one. The level of trust in this value is
          embodied in the actual authentication stack associated with the
          application, so it is ultimately at the discretion of the system
          administrator.

          PAM_RUSER@PAM_RHOST should always identify the requesting user.
          In some cases, PAM_RUSER may be NULL. In such situations, it is
          unclear who the requesting entity is.

   PAM_RHOST
          The requesting hostname (the hostname of the machine from which
          the PAM_RUSER entity is requesting service). That is
          PAM_RUSER@PAM_RHOST does identify the requesting user. In some
          applications, PAM_RHOST may be NULL. In such situations, it is
          unclear where the authentication request is originating from.

   PAM_AUTHTOK
          The authentication token (often a password). This token should
          be ignored by all module functions besides
          pam_sm_authenticate(3) and pam_sm_chauthtok(3). In the former
          function it is used to pass the most recent authentication token
          from one stacked module to another. In the latter function the
          token is used for another purpose. It contains the currently
          active authentication token.

   PAM_OLDAUTHTOK
          The old authentication token. This token should be ignored by
          all module functions except pam_sm_chauthtok(3).

   PAM_CONV
          The pam_conv structure. See pam_conv(3).

   The following additional items are specific to Linux-PAM and should not
   be used in portable applications:

   PAM_FAIL_DELAY
          A function pointer to redirect centrally managed failure delays.
          See pam_fail_delay(3).

   PAM_XDISPLAY
          The name of the X display. For graphical, X-based applications
          the value for this item should be the $DISPLAY variable. This
          value may be used independently of PAM_TTY for passing the name
          of the display.

   PAM_XAUTHDATA
          A pointer to a structure containing the X authentication data
          required to make a connection to the display specified by
          PAM_XDISPLAY, if such information is necessary. See
          pam_xauth_data(3).

   PAM_AUTHTOK_TYPE
          The default action is for the module to use the following
          prompts when requesting passwords: "New UNIX password: " and
          "Retype UNIX password: ". The example word UNIX can be replaced
          with this item, by default it is empty. This item is used by
          pam_get_authtok(3).

   For all item_types, other than PAM_CONV and PAM_FAIL_DELAY, item is a
   pointer to a <NUL> terminated character string. In the case of
   PAM_CONV, item points to an initialized pam_conv structure. In the case
   of PAM_FAIL_DELAY, item is a function pointer: void (*delay_fn)(int
   retval, unsigned usec_delay, void *appdata_ptr)

   Both, PAM_AUTHTOK and PAM_OLDAUTHTOK, will be reset before returning to
   the application. Which means an application is not able to access the
   authentication tokens.

3.1.3.2. RETURN VALUES

   PAM_BAD_ITEM
          The application attempted to set an undefined or inaccessible
          item.

   PAM_BUF_ERR
          Memory buffer error.

   PAM_SUCCESS
          Data was successful updated.

   PAM_SYSTEM_ERR
          The pam_handle_t passed as first argument was invalid.

3.1.4. Getting PAM items

#include <security/pam_modules.h>

   int pam_get_item( pamh,
                     item_type,
                     item);
   const pam_handle_t *pamh;
   int item_type;
   const void **item;

3.1.4.1. DESCRIPTION

   The pam_get_item function allows applications and PAM service modules
   to access and retrieve PAM information of item_type. Upon successful
   return, item contains a pointer to the value of the corresponding item.
   Note, this is a pointer to the actual data and should not be free()'ed
   or over-written! The following values are supported for item_type:

   PAM_SERVICE
          The service name (which identifies that PAM stack that the PAM
          functions will use to authenticate the program).

   PAM_USER
          The username of the entity under whose identity service will be
          given. That is, following authentication, PAM_USER identifies
          the local entity that gets to use the service. Note, this value
          can be mapped from something (eg., "anonymous") to something
          else (eg. "guest119") by any module in the PAM stack. As such an
          application should consult the value of PAM_USER after each call
          to a PAM function.

   PAM_USER_PROMPT
          The string used when prompting for a user's name. The default
          value for this string is a localized version of "login: ".

   PAM_TTY
          The terminal name prefixed by /dev/ for device files. In the
          past, graphical X-based applications used to store the $DISPLAY
          variable here, but with the introduction of PAM_XDISPLAY this
          usage is deprecated.

   PAM_RUSER
          The requesting user name: local name for a locally requesting
          user or a remote user name for a remote requesting user.

          Generally an application or module will attempt to supply the
          value that is most strongly authenticated (a local account
          before a remote one. The level of trust in this value is
          embodied in the actual authentication stack associated with the
          application, so it is ultimately at the discretion of the system
          administrator.

          PAM_RUSER@PAM_RHOST should always identify the requesting user.
          In some cases, PAM_RUSER may be NULL. In such situations, it is
          unclear who the requesting entity is.

   PAM_RHOST
          The requesting hostname (the hostname of the machine from which
          the PAM_RUSER entity is requesting service). That is
          PAM_RUSER@PAM_RHOST does identify the requesting user. In some
          applications, PAM_RHOST may be NULL. In such situations, it is
          unclear where the authentication request is originating from.

   PAM_AUTHTOK
          The authentication token (often a password). This token should
          be ignored by all module functions besides
          pam_sm_authenticate(3) and pam_sm_chauthtok(3). In the former
          function it is used to pass the most recent authentication token
          from one stacked module to another. In the latter function the
          token is used for another purpose. It contains the currently
          active authentication token.

   PAM_OLDAUTHTOK
          The old authentication token. This token should be ignored by
          all module functions except pam_sm_chauthtok(3).

   PAM_CONV
          The pam_conv structure. See pam_conv(3).

   The following additional items are specific to Linux-PAM and should not
   be used in portable applications:

   PAM_FAIL_DELAY
          A function pointer to redirect centrally managed failure delays.
          See pam_fail_delay(3).

   PAM_XDISPLAY
          The name of the X display. For graphical, X-based applications
          the value for this item should be the $DISPLAY variable. This
          value may be used independently of PAM_TTY for passing the name
          of the display.

   PAM_XAUTHDATA
          A pointer to a structure containing the X authentication data
          required to make a connection to the display specified by
          PAM_XDISPLAY, if such information is necessary. See
          pam_xauth_data(3).

   PAM_AUTHTOK_TYPE
          The default action is for the module to use the following
          prompts when requesting passwords: "New UNIX password: " and
          "Retype UNIX password: ". The example word UNIX can be replaced
          with this item, by default it is empty. This item is used by
          pam_get_authtok(3).

   If a service module wishes to obtain the name of the user, it should
   not use this function, but instead perform a call to pam_get_user(3).

   Only a service module is privileged to read the authentication tokens,
   PAM_AUTHTOK and PAM_OLDAUTHTOK.

3.1.4.2. RETURN VALUES

   PAM_BAD_ITEM
          The application attempted to set an undefined or inaccessible
          item.

   PAM_BUF_ERR
          Memory buffer error.

   PAM_PERM_DENIED
          The value of item was NULL.

   PAM_SUCCESS
          Data was successful updated.

   PAM_SYSTEM_ERR
          The pam_handle_t passed as first argument was invalid.

3.1.5. Strings describing PAM error codes

#include <security/pam_appl.h>

   const char *pam_strerror( pamh,
                             errnum);
   pam_handle_t *pamh;
   int errnum;

3.1.5.1. DESCRIPTION

   The pam_strerror function returns a pointer to a string describing the
   error code passed in the argument errnum, possibly using the
   LC_MESSAGES part of the current locale to select the appropriate
   language. This string must not be modified by the application. No
   library function will modify this string.

3.1.5.2. RETURN VALUES

   This function returns always a pointer to a string.

3.1.6. Request a delay on failure

#include <security/pam_appl.h>

   int pam_fail_delay( pamh,
                       usec);
   pam_handle_t *pamh;
   unsigned int usec;

3.1.6.1. DESCRIPTION

   The pam_fail_delay function provides a mechanism by which an
   application or module can suggest a minimum delay of usec
   micro-seconds. The function keeps a record of the longest time
   requested with this function. Should pam_authenticate(3) fail, the
   failing return to the application is delayed by an amount of time
   randomly distributed (by up to 50%) about this longest value.

   Independent of success, the delay time is reset to its zero default
   value when the PAM service module returns control to the application.
   The delay occurs after all authentication modules have been called, but
   before control is returned to the service application.

   When using this function the programmer should check if it is available
   with:
#ifdef HAVE_PAM_FAIL_DELAY
    ....
#endif /* HAVE_PAM_FAIL_DELAY */

   For applications written with a single thread that are event driven in
   nature, generating this delay may be undesirable. Instead, the
   application may want to register the delay in some other way. For
   example, in a single threaded server that serves multiple
   authentication requests from a single event loop, the application might
   want to simply mark a given connection as blocked until an application
   timer expires. For this reason the delay function can be changed with
   the PAM_FAIL_DELAY item. It can be queried and set with pam_get_item(3)
   and pam_set_item(3) respectively. The value used to set it should be a
   function pointer of the following prototype:
void (*delay_fn)(int retval, unsigned usec_delay, void *appdata_ptr);

   The arguments being the retval return code of the module stack, the
   usec_delay micro-second delay that libpam is requesting and the
   appdata_ptr that the application has associated with the current pamh.
   This last value was set by the application when it called pam_start(3)
   or explicitly with pam_set_item(3).

   Note that the PAM_FAIL_DELAY item is set to NULL by default. This
   indicates that PAM should perform a random delay as described above
   when authentication fails and a delay has been suggested. If an
   application does not want the PAM library to perform any delay on
   authentication failure, then the application must define a custom delay
   function that executes no statements and set the PAM_FAIL_DELAY item to
   point to this function.

3.1.6.2. RETURN VALUES

   PAM_SUCCESS
          Delay was successful adjusted.

   PAM_SYSTEM_ERR
          A NULL pointer was submitted as PAM handle.

3.1.7. Authenticating the user

#include <security/pam_appl.h>

   int pam_authenticate( pamh,
                         flags);
   pam_handle_t *pamh;
   int flags;

3.1.7.1. DESCRIPTION

   The pam_authenticate function is used to authenticate the user. The
   user is required to provide an authentication token depending upon the
   authentication service, usually this is a password, but could also be a
   finger print.

   The PAM service module may request that the user enter their username
   via the conversation mechanism (see pam_start(3) and pam_conv(3)). The
   name of the authenticated user will be present in the PAM item
   PAM_USER. This item may be recovered with a call to pam_get_item(3).

   The pamh argument is an authentication handle obtained by a prior call
   to pam_start(). The flags argument is the binary or of zero or more of
   the following values:

   PAM_SILENT
          Do not emit any messages.

   PAM_DISALLOW_NULL_AUTHTOK
          The PAM module service should return PAM_AUTH_ERR if the user
          does not have a registered authentication token.

3.1.7.2. RETURN VALUES

   PAM_ABORT
          The application should exit immediately after calling pam_end(3)
          first.

   PAM_AUTH_ERR
          The user was not authenticated.

   PAM_CRED_INSUFFICIENT
          For some reason the application does not have sufficient
          credentials to authenticate the user.

   PAM_AUTHINFO_UNAVAIL
          The modules were not able to access the authentication
          information. This might be due to a network or hardware failure
          etc.

   PAM_MAXTRIES
          One or more of the authentication modules has reached its limit
          of tries authenticating the user. Do not try again.

   PAM_SUCCESS
          The user was successfully authenticated.

   PAM_USER_UNKNOWN
          User unknown to authentication service.

3.1.8. Setting user credentials

#include <security/pam_appl.h>

   int pam_setcred( pamh,
                    flags);
   pam_handle_t *pamh;
   int flags;

3.1.8.1. DESCRIPTION

   The pam_setcred function is used to establish, maintain and delete the
   credentials of a user. It should be called to set the credentials after
   a user has been authenticated and before a session is opened for the
   user (with pam_open_session(3)). The credentials should be deleted
   after the session has been closed (with pam_close_session(3)).

   A credential is something that the user possesses. It is some property,
   such as a Kerberos ticket, or a supplementary group membership that
   make up the uniqueness of a given user. On a Linux system the user's
   UID and GID's are credentials too. However, it has been decided that
   these properties (along with the default supplementary groups of which
   the user is a member) are credentials that should be set directly by
   the application and not by PAM. Such credentials should be established,
   by the application, prior to a call to this function. For example,
   initgroups(2) (or equivalent) should have been performed.

   Valid flags, any one of which, may be logically OR'd with PAM_SILENT,
   are:

   PAM_ESTABLISH_CRED
          Initialize the credentials for the user.

   PAM_DELETE_CRED
          Delete the user's credentials.

   PAM_REINITIALIZE_CRED
          Fully reinitialize the user's credentials.

   PAM_REFRESH_CRED
          Extend the lifetime of the existing credentials.

3.1.8.2. RETURN VALUES

   PAM_BUF_ERR
          Memory buffer error.

   PAM_CRED_ERR
          Failed to set user credentials.

   PAM_CRED_EXPIRED
          User credentials are expired.

   PAM_CRED_UNAVAIL
          Failed to retrieve user credentials.

   PAM_SUCCESS
          Data was successful stored.

   PAM_SYSTEM_ERR
          A NULL pointer was submitted as PAM handle, the function was
          called by a module or another system error occurred.

   PAM_USER_UNKNOWN
          User is not known to an authentication module.

3.1.9. Account validation management

#include <security/pam_appl.h>

   int pam_acct_mgmt( pamh,
                      flags);
   pam_handle_t *pamh;
   int flags;

3.1.9.1. DESCRIPTION

   The pam_acct_mgmt function is used to determine if the user's account
   is valid. It checks for authentication token and account expiration and
   verifies access restrictions. It is typically called after the user has
   been authenticated.

   The pamh argument is an authentication handle obtained by a prior call
   to pam_start(). The flags argument is the binary or of zero or more of
   the following values:

   PAM_SILENT
          Do not emit any messages.

   PAM_DISALLOW_NULL_AUTHTOK
          The PAM module service should return PAM_NEW_AUTHTOK_REQD if the
          user has a null authentication token.

3.1.9.2. RETURN VALUES

   PAM_ACCT_EXPIRED
          User account has expired.

   PAM_AUTH_ERR
          Authentication failure.

   PAM_NEW_AUTHTOK_REQD
          The user account is valid but their authentication token is
          expired. The correct response to this return-value is to require
          that the user satisfies the pam_chauthtok() function before
          obtaining service. It may not be possible for some applications
          to do this. In such cases, the user should be denied access
          until such time as they can update their password.

   PAM_PERM_DENIED
          Permission denied.

   PAM_SUCCESS
          The authentication token was successfully updated.

   PAM_USER_UNKNOWN
          User unknown to password service.

3.1.10. Updating authentication tokens

#include <security/pam_appl.h>

   int pam_chauthtok( pamh,
                      flags);
   pam_handle_t *pamh;
   int flags;

3.1.10.1. DESCRIPTION

   The pam_chauthtok function is used to change the authentication token
   for a given user (as indicated by the state associated with the handle
   pamh).

   The pamh argument is an authentication handle obtained by a prior call
   to pam_start(). The flags argument is the binary or of zero or more of
   the following values:

   PAM_SILENT
          Do not emit any messages.

   PAM_CHANGE_EXPIRED_AUTHTOK
          This argument indicates to the modules that the user's
          authentication token (password) should only be changed if it has
          expired. If this argument is not passed, the application
          requires that all authentication tokens are to be changed.

3.1.10.2. RETURN VALUES

   PAM_AUTHTOK_ERR
          A module was unable to obtain the new authentication token.

   PAM_AUTHTOK_RECOVERY_ERR
          A module was unable to obtain the old authentication token.

   PAM_AUTHTOK_LOCK_BUSY
          One or more of the modules was unable to change the
          authentication token since it is currently locked.

   PAM_AUTHTOK_DISABLE_AGING
          Authentication token aging has been disabled for at least one of
          the modules.

   PAM_PERM_DENIED
          Permission denied.

   PAM_SUCCESS
          The authentication token was successfully updated.

   PAM_TRY_AGAIN
          Not all of the modules were in a position to update the
          authentication token(s). In such a case none of the user's
          authentication tokens are updated.

   PAM_USER_UNKNOWN
          User unknown to password service.

3.1.11. Start PAM session management

#include <security/pam_appl.h>

   int pam_open_session( pamh,
                         flags);
   pam_handle_t *pamh;
   int flags;

3.1.11.1. DESCRIPTION

   The pam_open_session function sets up a user session for a previously
   successful authenticated user. The session should later be terminated
   with a call to pam_close_session(3).

   It should be noted that the effective uid, geteuid(2), of the
   application should be of sufficient privilege to perform such tasks as
   creating or mounting the user's home directory for example.

   The flags argument is the binary or of zero or more of the following
   values:

   PAM_SILENT
          Do not emit any messages.

3.1.11.2. RETURN VALUES

   PAM_ABORT
          General failure.

   PAM_BUF_ERR
          Memory buffer error.

   PAM_SESSION_ERR
          Session failure.

   PAM_SUCCESS
          Session was successful created.

3.1.12. terminating PAM session management

#include <security/pam_appl.h>

   int pam_close_session( pamh,
                          flags);
   pam_handle_t *pamh;
   int flags;

3.1.12.1. DESCRIPTION

   The pam_close_session function is used to indicate that an
   authenticated session has ended. The session should have been created
   with a call to pam_open_session(3).

   It should be noted that the effective uid, geteuid(2). of the
   application should be of sufficient privilege to perform such tasks as
   unmounting the user's home directory for example.

   The flags argument is the binary or of zero or more of the following
   values:

   PAM_SILENT
          Do not emit any messages.

3.1.12.2. RETURN VALUES

   PAM_ABORT
          General failure.

   PAM_BUF_ERR
          Memory buffer error.

   PAM_SESSION_ERR
          Session failure.

   PAM_SUCCESS
          Session was successful terminated.

3.1.13. Set or change PAM environment variable

#include <security/pam_appl.h>

   int pam_putenv( pamh,
                   name_value);
   pam_handle_t *pamh;
   const char *name_value;

3.1.13.1. DESCRIPTION

   The pam_putenv function is used to add or change the value of PAM
   environment variables as associated with the pamh handle.

   The pamh argument is an authentication handle obtained by a prior call
   to pam_start(). The name_value argument is a single NUL terminated
   string of one of the following forms:

   NAME=value of variable
          In this case the environment variable of the given NAME is set
          to the indicated value: value of variable. If this variable is
          already known, it is overwritten. Otherwise it is added to the
          PAM environment.

   NAME=
          This function sets the variable to an empty value. It is listed
          separately to indicate that this is the correct way to achieve
          such a setting.

   NAME
          Without an '=' the pam_putenv() function will delete the
          corresponding variable from the PAM environment.

   pam_putenv() operates on a copy of name_value, which means in contrast
   to putenv(3), the application is responsible for freeing the data.

3.1.13.2. RETURN VALUES

   PAM_PERM_DENIED
          Argument name_value given is a NULL pointer.

   PAM_BAD_ITEM
          Variable requested (for deletion) is not currently set.

   PAM_ABORT
          The pamh handle is corrupt.

   PAM_BUF_ERR
          Memory buffer error.

   PAM_SUCCESS
          The environment variable was successfully updated.

3.1.14. Get a PAM environment variable

#include <security/pam_appl.h>

   const char *pam_getenv( pamh,
                           name);
   pam_handle_t *pamh;
   const char *name;

3.1.14.1. DESCRIPTION

   The pam_getenv function searches the PAM environment list as associated
   with the handle pamh for an item that matches the string pointed to by
   name and returns a pointer to the value of the environment variable.
   The application is not allowed to free the data.

3.1.14.2. RETURN VALUES

   The pam_getenv function returns NULL on failure.

3.1.15. Getting the PAM environment

#include <security/pam_appl.h>

   char **pam_getenvlist( pamh);
   pam_handle_t *pamh;

3.1.15.1. DESCRIPTION

   The pam_getenvlist function returns a complete copy of the PAM
   environment as associated with the handle pamh. The PAM environment
   variables represent the contents of the regular environment variables
   of the authenticated user when service is granted.

   The format of the memory is a malloc()'d array of char pointers, the
   last element of which is set to NULL. Each of the non-NULL entries in
   this array point to a NUL terminated and malloc()'d char string of the
   form: "name=value".

   It should be noted that this memory will never be free()'d by libpam.
   Once obtained by a call to pam_getenvlist, it is the responsibility of
   the calling application to free() this memory.

   It is by design, and not a coincidence, that the format and contents of
   the returned array matches that required for the third argument of the
   execle(3) function call.

3.1.15.2. RETURN VALUES

   The pam_getenvlist function returns NULL on failure.

3.2. What is expected of an application

3.2.1. The conversation function

#include <security/pam_appl.h>

struct pam_message {
    int msg_style;
    const char *msg;
};

struct pam_response {
    char *resp;
    int resp_retcode;
};

struct pam_conv {
    int (*conv)(int num_msg, const struct pam_message **msg,
                struct pam_response **resp, void *appdata_ptr);
    void *appdata_ptr;
};

3.2.1.1. DESCRIPTION

   The PAM library uses an application-defined callback to allow a direct
   communication between a loaded module and the application. This
   callback is specified by the struct pam_conv passed to pam_start(3) at
   the start of the transaction.

   When a module calls the referenced conv() function, the argument
   appdata_ptr is set to the second element of this structure.

   The other arguments of a call to conv() concern the information
   exchanged by module and application. That is to say, num_msg holds the
   length of the array of pointers, msg. After a successful return, the
   pointer resp points to an array of pam_response structures, holding the
   application supplied text. The resp_retcode member of this struct is
   unused and should be set to zero. It is the caller's responsibility to
   release both, this array and the responses themselves, using free(3).
   Note, *resp is a struct pam_response array and not an array of
   pointers.

   The number of responses is always equal to the num_msg conversation
   function argument. This does require that the response array is
   free(3)'d after every call to the conversation function. The index of
   the responses corresponds directly to the prompt index in the
   pam_message array.

   On failure, the conversation function should release any resources it
   has allocated, and return one of the predefined PAM error codes.

   Each message can have one of four types, specified by the msg_style
   member of struct pam_message:

   PAM_PROMPT_ECHO_OFF
          Obtain a string without echoing any text.

   PAM_PROMPT_ECHO_ON
          Obtain a string whilst echoing text.

   PAM_ERROR_MSG
          Display an error message.

   PAM_TEXT_INFO
          Display some text.

   The point of having an array of messages is that it becomes possible to
   pass a number of things to the application in a single call from the
   module. It can also be convenient for the application that related
   things come at once: a windows based application can then present a
   single form with many messages/prompts on at once.

   In passing, it is worth noting that there is a discrepancy between the
   way Linux-PAM handles the const struct pam_message **msg conversation
   function argument and the way that Solaris' PAM (and derivatives, known
   to include HP/UX, are there others?) does. Linux-PAM interprets the msg
   argument as entirely equivalent to the following prototype const struct
   pam_message *msg[] (which, in spirit, is consistent with the commonly
   used prototypes for argv argument to the familiar main() function: char
   **argv; and char *argv[]). Said another way Linux-PAM interprets the
   msg argument as a pointer to an array of num_msg read only 'struct
   pam_message' pointers. Solaris' PAM implementation interprets this
   argument as a pointer to a pointer to an array of num_msg pam_message
   structures. Fortunately, perhaps, for most module/application
   developers when num_msg has a value of one these two definitions are
   entirely equivalent. Unfortunately, casually raising this number to two
   has led to unanticipated compatibility problems.

   For what its worth the two known module writer work-arounds for trying
   to maintain source level compatibility with both PAM implementations
   are:
     * never call the conversation function with num_msg greater than one.
     * set up msg as doubly referenced so both types of conversation
       function can find the messages. That is, make
       msg[n] = & (( *msg )[n])

3.2.1.2. RETURN VALUES

   PAM_BUF_ERR
          Memory buffer error.

   PAM_CONV_ERR
          Conversation failure. The application should not set *resp.

   PAM_SUCCESS
          Success.

3.3. Programming notes

   Note, all of the authentication service function calls accept the token
   PAM_SILENT, which instructs the modules to not send messages to the
   application. This token can be logically OR'd with any one of the
   permitted tokens specific to the individual function calls. PAM_SILENT
   does not override the prompting of the user for passwords etc., it only
   stops informative messages from being generated.

Chapter 4.  Security issues of Linux-PAM

   PAM, from the perspective of an application, is a convenient API for
   authenticating users. PAM modules generally have no increased privilege
   over that possessed by the application that is making use of it. For
   this reason, the application must take ultimate responsibility for
   protecting the environment in which PAM operates.

   A poorly (or maliciously) written application can defeat any Linux-PAM
   module's authentication mechanisms by simply ignoring it's return
   values. It is the applications task and responsibility to grant
   privileges and access to services. The Linux-PAM library simply assumes
   the responsibility of authenticating the user; ascertaining that the
   user is who they say they are. Care should be taken to anticipate all
   of the documented behavior of the Linux-PAM library functions. A
   failure to do this will most certainly lead to a future security
   breach.

4.1. Care about standard library calls

   In general, writers of authorization-granting applications should
   assume that each module is likely to call any or all 'libc' functions.
   For 'libc' functions that return pointers to static/dynamically
   allocated structures (ie. the library allocates the memory and the user
   is not expected to 'free()' it) any module call to this function is
   likely to corrupt a pointer previously obtained by the application. The
   application programmer should either re-call such a 'libc' function
   after a call to the Linux-PAM library, or copy the structure contents
   to some safe area of memory before passing control to the Linux-PAM
   library.

   Two important function classes that fall into this category are
   getpwnam(3) and syslog(3).

4.2. Choice of a service name

   When picking the service-name that corresponds to the first entry in
   the Linux-PAM configuration file, the application programmer should
   avoid the temptation of choosing something related to argv[0]. It is a
   trivial matter for any user to invoke any application on a system under
   a different name and this should not be permitted to cause a security
   breach.

   In general, this is always the right advice if the program is setuid,
   or otherwise more privileged than the user that invokes it. In some
   cases, avoiding this advice is convenient, but as an author of such an
   application, you should consider well the ways in which your program
   will be installed and used. (Its often the case that programs are not
   intended to be setuid, but end up being installed that way for
   convenience. If your program falls into this category, don't fall into
   the trap of making this mistake.)

   To invoke some target application by another name, the user may
   symbolically link the target application with the desired name. To be
   precise all the user need do is, ln -s /target/application
   ./preferred_name and then run ./preferred_name.

   By studying the Linux-PAM configuration file(s), an attacker can choose
   the preferred_name to be that of a service enjoying minimal protection;
   for example a game which uses Linux-PAM to restrict access to certain
   hours of the day. If the service-name were to be linked to the filename
   under which the service was invoked, it is clear that the user is
   effectively in the position of dictating which authentication scheme
   the service uses. Needless to say, this is not a secure situation.

   The conclusion is that the application developer should carefully
   define the service-name of an application. The safest thing is to make
   it a single hard-wired name.

4.3. The conversation function

   Care should be taken to ensure that the conv() function is robust. Such
   a function is provided in the library libpam_misc (see below).

4.4. The identity of the user

   The Linux-PAM modules will need to determine the identity of the user
   who requests a service, and the identity of the user who grants the
   service. These two users will seldom be the same. Indeed there is
   generally a third user identity to be considered, the new (assumed)
   identity of the user once the service is granted.

   The need for keeping tabs on these identities is clearly an issue of
   security. One convention that is actively used by some modules is that
   the identity of the user requesting a service should be the current UID
   (user ID) of the running process; the identity of the privilege
   granting user is the EUID (effective user ID) of the running process;
   the identity of the user, under whose name the service will be
   executed, is given by the contents of the PAM_USER pam_get_item(3).
   Note, modules can change the values of PAM_USER and PAM_RUSER during
   any of the pam_*() library calls. For this reason, the application
   should take care to use the pam_get_item() every time it wishes to
   establish who the authenticated user is (or will currently be).

   For network-serving databases and other applications that provide their
   own security model (independent of the OS kernel) the above scheme is
   insufficient to identify the requesting user.

   A more portable solution to storing the identity of the requesting user
   is to use the PAM_RUSER pam_get_item(3). The application should supply
   this value before attempting to authenticate the user with
   pam_authenticate(). How well this name can be trusted will ultimately
   be at the discretion of the local administrator (who configures PAM for
   your application) and a selected module may attempt to override the
   value where it can obtain more reliable data. If an application is
   unable to determine the identity of the requesting entity/user, it
   should not call pam_set_item(3) to set PAM_RUSER.

   In addition to the PAM_RUSER item, the application should supply the
   PAM_RHOST (requesting host) item. As a general rule, the following
   convention for its value can be assumed: NULL = unknown; localhost =
   invoked directly from the local system; other.place.xyz = some
   component of the user's connection originates from this
   remote/requesting host. At present, PAM has no established convention
   for indicating whether the application supports a trusted path to
   communication from this host.

4.5. Sufficient resources

   Care should be taken to ensure that the proper execution of an
   application is not compromised by a lack of system resources. If an
   application is unable to open sufficient files to perform its service,
   it should fail gracefully, or request additional resources.
   Specifically, the quantities manipulated by the setrlimit(2) family of
   commands should be taken into consideration.

   This is also true of conversation prompts. The application should not
   accept prompts of arbitrary length with out checking for resource
   allocation failure and dealing with such extreme conditions gracefully
   and in a manner that preserves the PAM API. Such tolerance may be
   especially important when attempting to track a malicious adversary.

Chapter 5. A library of miscellaneous helper functions

   To aid the work of the application developer a library of miscellaneous
   functions is provided. It is called libpam_misc, and contains a text
   based conversation function, and routines for enhancing the standard
   PAM-environment variable support.

   The functions, structures and macros, made available by this library
   can be defined by including <security/pam_misc.h>. It should be noted
   that this library is specific to Linux-PAM and is not referred to in
   the defining DCE-RFC (see See also) below.

5.1. Functions supplied

5.1.1. Text based conversation function

#include <security/pam_misc.h>

   int misc_conv( num_msg,
                  msgm,
                  response,
                  appdata_ptr);
   int num_msg;
   const struct pam_message **msgm;
   struct pam_response **response;
   void *appdata_ptr;

5.1.1.1. DESCRIPTION

   The misc_conv function is part of libpam_misc and not of the standard
   libpam library. This function will prompt the user with the appropriate
   comments and obtain the appropriate inputs as directed by
   authentication modules.

   In addition to simply slotting into the appropriate pam_conv(3), this
   function provides some time-out facilities. The function exports five
   variables that can be used by an application programmer to limit the
   amount of time this conversation function will spend waiting for the
   user to type something. The five variables are as follows:

   time_t pam_misc_conv_warn_time;
          This variable contains the time (as returned by time(2)) that
          the user should be first warned that the clock is ticking. By
          default it has the value 0, which indicates that no such warning
          will be given. The application may set its value to sometime in
          the future, but this should be done prior to passing control to
          the Linux-PAM library.

   const char *pam_misc_conv_warn_line;
          Used in conjunction with pam_misc_conv_warn_time, this variable
          is a pointer to the string that will be displayed when it
          becomes time to warn the user that the timeout is approaching.
          Its default value is a translated version of "...Time is running
          out...", but this can be changed by the application prior to
          passing control to Linux-PAM.

   time_t pam_misc_conv_die_time;
          This variable contains the time (as returned by time(2)) that
          the will time out. By default it has the value 0, which
          indicates that the conversation function will not timeout. The
          application may set its value to sometime in the future, but
          this should be done prior to passing control to the Linux-PAM
          library.

   const char *pam_misc_conv_die_line;
          Used in conjunction with pam_misc_conv_die_time, this variable
          is a pointer to the string that will be displayed when the
          conversation times out. Its default value is a translated
          version of "...Sorry, your time is up!", but this can be changed
          by the application prior to passing control to Linux-PAM.

   int pam_misc_conv_died;
          Following a return from the Linux-PAM library, the value of this
          variable indicates whether the conversation has timed out. A
          value of 1 indicates the time-out occurred.

   The following two function pointers are available for supporting binary
   prompts in the conversation function. They are optimized for the
   current incarnation of the libpamc library and are subject to change.

   int (*pam_binary_handler_fn)(void *appdata, pamc_bp_t *prompt_p);
          This function pointer is initialized to NULL but can be filled
          with a function that provides machine-machine (hidden) message
          exchange. It is intended for use with hidden authentication
          protocols such as RSA or Diffie-Hellman key exchanges. (This is
          still under development.)

   int (*pam_binary_handler_free)(void *appdata, pamc_bp_t *delete_me);
          This function pointer is initialized to PAM_BP_RENEW(delete_me,
          0, 0), but can be redefined as desired by the application.

5.1.2. Transcribing an environment to that of PAM

#include <security/pam_misc.h>

   int pam_misc_paste_env( pamh,
                           user);
   pam_handle_t *pamh;
   const char * const *user;

5.1.2.1. DESCRIPTION

   This function takes the supplied list of environment pointers and
   uploads its contents to the PAM environment. Success is indicated by
   PAM_SUCCESS.

5.1.3. Liberating a locally saved environment

#include <security/pam_misc.h>

   int pam_misc_drop_env( env);
   char **env;

5.1.3.1. DESCRIPTION

   This function is defined to complement the pam_getenvlist(3) function.
   It liberates the memory associated with env, overwriting with 0 all
   memory before free()ing it.

5.1.4. BSD like PAM environment variable setting

#include <security/pam_misc.h>

   int pam_misc_setenv( pamh,
                        name,
                        value,
                        readonly);
   pam_handle_t *pamh;
   const char *name;
   const char *value;
   int readonly;

5.1.4.1. DESCRIPTION

   This function performs a task equivalent to pam_putenv(3), its syntax
   is, however, more like the BSD style function; setenv(). The name and
   value are concatenated with an '=' to form a name=value and passed to
   pam_putenv(). If, however, the PAM variable is already set, the
   replacement will only be applied if the last argument, readonly, is
   zero.

Chapter 6. Porting legacy applications

   The point of PAM is that the application is not supposed to have any
   idea how the attached authentication modules will choose to
   authenticate the user. So all they can do is provide a conversation
   function that will talk directly to the user(client) on the modules'
   behalf.

   Consider the case that you plug a retinal scanner into the login
   program. In this situation the user would be prompted: "please look
   into the scanner". No username or password would be needed - all this
   information could be deduced from the scan and a database lookup. The
   point is that the retinal scanner is an ideal task for a "module".

   While it is true that a pop-daemon program is designed with the POP
   protocol in mind and no-one ever considered attaching a retinal scanner
   to it, it is also the case that the "clean" PAM'ification of such a
   daemon would allow for the possibility of a scanner module being be
   attached to it. The point being that the "standard" pop-authentication
   protocol(s) [which will be needed to satisfy inflexible/legacy clients]
   would be supported by inserting an appropriate pam_qpopper module(s).
   However, having rewritten popd once in this way any new protocols can
   be implemented in-situ.

   One simple test of a ported application would be to insert the
   pam_permit module and see if the application demands you type a
   password... In such a case, xlock would fail to lock the terminal - or
   would at best be a screen-saver, ftp would give password free access to
   all etc.. Neither of these is a very secure thing to do, but they do
   illustrate how much flexibility PAM puts in the hands of the local
   admin.

   The key issue, in doing things correctly, is identifying what is part
   of the authentication procedure (how many passwords etc..) the exchange
   protocol (prefixes to prompts etc., numbers like 331 in the case of
   ftpd) and what is part of the service that the application delivers.
   PAM really needs to have total control in the authentication
   "procedure", the conversation function should only deal with
   reformatting user prompts and extracting responses from raw input.

Chapter 7. Glossary of PAM related terms

   The following are a list of terms used within this document.

   Authentication token
          Generally, this is a password. However, users can authenticate
          themselves in a variety of ways. Updating the user's
          authentication token thus corresponds to refreshing the object
          they use to authenticate themselves with the system. The word
          password is avoided to keep open the possibility that the
          authentication involves a retinal scan or other non-textual mode
          of challenge/response.

   Credentials
          Having successfully authenticated the user, PAM is able to
          establish certain characteristics/attributes of the user. These
          are termed credentials. Examples of which are group memberships
          to perform privileged tasks with, and tickets in the form of
          environment variables etc. . Some user-credentials, such as the
          user's UID and GID (plus default group memberships) are not
          deemed to be PAM-credentials. It is the responsibility of the
          application to grant these directly.

Chapter 8. An example application

   To get a flavor of the way a Linux-PAM application is written we
   include the following example. It prompts the user for their password
   and indicates whether their account is valid on the standard output,
   its return code also indicates the success (0 for success; 1 for
   failure).
/*
  This program was contributed by Shane Watts
  [modifications by AGM and kukuk]

  You need to add the following (or equivalent) to the
  /etc/pam.d/check_user file:
  # check authorization
  auth       required     pam_unix.so
  account    required     pam_unix.so
 */

#include <security/pam_appl.h>
#include <security/pam_misc.h>
#include <stdio.h>

static struct pam_conv conv = {
    misc_conv,
    NULL
};

int main(int argc, char *argv[])
{
    pam_handle_t *pamh=NULL;
    int retval;
    const char *user="nobody";

    if(argc == 2) {
        user = argv[1];
    }

    if(argc > 2) {
        fprintf(stderr, "Usage: check_user [username]\n");
        exit(1);
    }

    retval = pam_start("check_user", user, &conv, &pamh);

    if (retval == PAM_SUCCESS)
        retval = pam_authenticate(pamh, 0);    /* is user really user? */

    if (retval == PAM_SUCCESS)
        retval = pam_acct_mgmt(pamh, 0);       /* permitted access? */

    /* This is where we have been authorized or not. */

    if (retval == PAM_SUCCESS) {
        fprintf(stdout, "Authenticated\n");
    } else {
        fprintf(stdout, "Not Authenticated\n");
    }

    if (pam_end(pamh,retval) != PAM_SUCCESS) {     /* close Linux-PAM */
        pamh = NULL;
        fprintf(stderr, "check_user: failed to release authenticator\n");
        exit(1);
    }

    return ( retval == PAM_SUCCESS ? 0:1 );       /* indicate success */
}


Chapter 9. Files

   /usr/include/security/pam_appl.h
          Header file with interfaces for Linux-PAM applications.

   /usr/include/security/pam_misc.h
          Header file for useful library functions for making applications
          easier to write.

Chapter 10. See also

     * The Linux-PAM System Administrators' Guide.
     * The Linux-PAM Module Writers' Guide.
     * The V. Samar and R. Schemers (SunSoft), ``UNIFIED LOGIN WITH
       PLUGGABLE AUTHENTICATION MODULES'', Open Software Foundation
       Request For Comments 86.0, October 1995.

Chapter 11. Author/acknowledgments

   This document was written by Andrew G. Morgan (morgan@kernel.org) with
   many contributions from Chris Adams, Peter Allgeyer, Tim Baverstock,
   Tim Berger, Craig S. Bell, Derrick J. Brashear, Ben Buxton, Seth
   Chaiklin, Oliver Crow, Chris Dent, Marc Ewing, Cristian Gafton,
   Emmanuel Galanos, Brad M. Garcia, Eric Hester, Roger Hu, Eric Jacksch,
   Michael K. Johnson, David Kinchlea, Olaf Kirch, Marcin Korzonek,
   Thorsten Kukuk, Stephen Langasek, Nicolai Langfeldt, Elliot Lee, Luke
   Kenneth Casson Leighton, Al Longyear, Ingo Luetkebohle, Marek
   Michalkiewicz, Robert Milkowski, Aleph One, Martin Pool, Sean
   Reifschneider, Jan Rekorajski, Erik Troan, Theodore Ts'o, Jeff Uphoff,
   Myles Uyema, Savochkin Andrey Vladimirovich, Ronald Wahl, David Wood,
   John Wilmes, Joseph S. D. Yao and Alex O. Yuriev.

   Thanks are also due to Sun Microsystems, especially to Vipin Samar and
   Charlie Lai for their advice. At an early stage in the development of
   Linux-PAM, Sun graciously made the documentation for their
   implementation of PAM available. This act greatly accelerated the
   development of Linux-PAM.

Chapter 12. Copyright information for this document

Copyright (c) 2006 Thorsten Kukuk <kukuk@thkukuk.de>
Copyright (c) 1996-2002 Andrew G. Morgan <morgan@kernel.org>

   Redistribution and use in source and binary forms, with or without
   modification, are permitted provided that the following conditions are
   met:
1. Redistributions of source code must retain the above copyright
   notice, and the entire permission notice in its entirety,
   including the disclaimer of warranties.

2. Redistributions in binary form must reproduce the above copyright
   notice, this list of conditions and the following disclaimer in the
   documentation and/or other materials provided with the distribution.

3. The name of the author may not be used to endorse or promote
   products derived from this software without specific prior
   written permission.

   Alternatively, this product may be distributed under the terms of the
   GNU General Public License (GPL), in which case the provisions of the
   GNU GPL are required instead of the above restrictions. (This clause is
   necessary due to a potential bad interaction between the GNU GPL and
   the restrictions contained in a BSD-style copyright.)
THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR
TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
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