  The Linux-PAM Application Developers' Guide
  Andrew G. Morgan, morgan@kernel.org
  DRAFT v0.76 2001/12/08

  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 funtions to be found in libpam_misc library, that
  can be used in general applications.  Finally, it contains some com-
  ments on PAM related security issues for the application developer.
  ______________________________________________________________________

  Table of Contents





















































  1. Introduction

     1.1 Synopsis
     1.2 Description

  2. Overview

  3. The public interface to

     3.1 What can be expected by the application
        3.1.1 Initialization of Linux-PAM
        3.1.2 Termination of the library
        3.1.3 Setting PAM items
        3.1.4 Getting PAM items
        3.1.5 Understanding errors
        3.1.6 Planning for delays
        3.1.7 Authenticating the user
        3.1.8 Setting user credentials
        3.1.9 Account management
        3.1.10 Updating authentication tokens
        3.1.11 Session initialization
        3.1.12 Terminating sessions
        3.1.13 Setting PAM environment variables
        3.1.14 Getting 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

     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 Requirements
     5.2 Macros supplied
        5.2.1 Safe duplication of strings
     5.3 Functions supplied
        5.3.1 A text based conversation function
        5.3.2 Transcribing an environment to that of Linux-PAM
        5.3.3 Liberating a locally saved environment
        5.3.4 BSD like Linux-PAM environment variable setting

  6. Porting legacy applications

  7. Glossary of PAM related terms

  8. An example application

  9. Files

  10. See also

  11. Notes

  12. Author/acknowledgments

  13. Bugs/omissions

  14. Copyright information for this document

  ______________________________________________________________________

  1.  Introduction

  1.1.  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 -ldl





  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_misc.h>

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





  1.2.  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).


  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 users 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.


  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

  Below we document those functions in the Linux-PAM library that may be
  called from an application.


  3.1.1.  Initialization of Linux-PAM



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





  This is the first of the Linux-PAM functions that must be called by an
  application. It initializes the interface and reads the system
  configuration file, /etc/pam.conf (see the Linux-PAM System
  Administrators' Guide).  Following a successful return (PAM_SUCCESS)
  the contents of *pamh is a handle that provides continuity for
  successive calls to the Linux-PAM library.  The arguments expected by
  pam_start are as follows: the service_name of the program, the
  username of the individual to be authenticated, a pointer to an
  application-supplied pam_conv structure and a pointer to a
  pam_handle_t pointer.
  The pam_conv structure is discussed more fully in the section
  ``below''.  The pam_handle_t is a blind structure and the application
  should not attempt to probe it directly for information.  Instead the
  Linux-PAM library provides the functions pam_set_item and
  pam_get_item.  These functions are documented below.


  3.1.2.  Termination of the library



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





  This function is the last function an application should call in the
  Linux-PAM library.  Upon return the handle pamh is no longer valid and
  all memory associated with it will be invalid (likely to cause a
  segmentation fault if accessed).


  Under normal conditions the argument pam_status has the value
  PAM_SUCCESS, but in the event of an unsuccessful application for
  service the appropriate Linux-PAM error-return value should be used
  here. Note, pam_end() unconditionally shuts down the authentication
  stack associated with the pamh handle. The value taken by pam_status
  is used as an argument to the module specific callback functions,
  cleanup() (see the Linux-PAM Module Developers' Guide). 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.


  3.1.3.  Setting PAM items



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





  This function is used to (re)set the value of one of the following
  item_types:



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


     PAM_USER
        The username of the entity under who's 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 ``Please enter username: ''.


     PAM_TTY
        The terminal name: prefixed by /dev/ if it is a device file; for
        graphical, X-based, applications the value for this item should
        be the $DISPLAY variable.


     PAM_RUSER
        The requesting entity: user's username for a locally requesting
        user or 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. It should generally
        match the current PAM_RHOST value. That is,
        "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.
        "luser@localhost" or "evil@evilcom.com" are valid
        "PAM_RUSER@PAM_RHOST" examples. In some applications, PAM_RHOST
        may be NULL. In such situations, it is unclear where the
        authentication request is originating from.


     PAM_CONV
        The conversation structure (see section ``below'').


     PAM_FAIL_DELAY
        A function pointer to redirect centrally managed failure delays
        (see section ``below'').



  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 (see
  section ``below''). In the case of PAM_FAIL_DELAY, item is a function
  pointer: void (*delay_fn)(int retval, unsigned usec_delay, void
  *appdata_ptr) (see section ``below'').


  A successful call to this function returns PAM_SUCCESS.  However, the
  application should expect at least one the following errors:



     PAM_SYSTEM_ERR
        The pam_handle_t passed as a first argument to this function was
        invalid.

     PAM_PERM_DENIED
        An attempt was made to replace the conversation structure with a
        NULL value.

     PAM_BUF_ERR
        The function ran out of memory making a copy of the item.

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


  3.1.4.  Getting PAM items



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





  This function is used to obtain the value of the indicated 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!


  A successful call is signaled by a return value of PAM_SUCCESS.
  However, the application should expect one of the following errors:



     PAM_SYSTEM_ERR
        The pam_handle_t passed as a first argument to this function was
        invalid.

     PAM_PERM_DENIED
        The value of item was NULL.

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


  Note, in the case of an error, the contents of item is not modified -
  that is, it retains its pre-call value. One should take care to
  initialize this value prior to calling pam_get_item(). Since, if its
  value - despite the pam_get_item() function failing - is to be used
  the consequences are undefined.


  3.1.5.  Understanding errors



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





  This function returns some text describing the Linux-PAM error
  associated with the argument errnum.  If the error is not recognized
  ``Unknown Linux-PAM error'' is returned.




  3.1.6.  Planning for delays



       extern int pam_fail_delay(pam_handle_t *pamh, unsigned int micro_sec);





  This function is offered by Linux-PAM to facilitate time delays
  following a failed call to pam_authenticate() and before control is
  returned to the application. When using this function the application
  programmer should check if it is available with,


       #ifdef PAM_FAIL_DELAY
           ....
       #endif /* PAM_FAIL_DELAY */






  Generally, an application requests that a user is authenticated by
  Linux-PAM through a call to pam_authenticate() or pam_chauthtok().
  These functions call each of the stacked authentication modules listed
  in the relevant Linux-PAM configuration file.  As directed by this
  file, one of more of the modules may fail causing the pam_...() call
  to return an error.  It is desirable for there to also be a pause
  before the application continues. The principal reason for such a
  delay is security: a delay acts to discourage brute force dictionary
  attacks primarily, but also helps hinder timed (covert channel)
  attacks.


  The pam_fail_delay() function provides the mechanism by which an
  application or module can suggest a minimum delay (of micro_sec micro-
  seconds). Linux-PAM keeps a record of the longest time requested with
  this function. Should pam_authenticate() fail, the failing return to
  the application is delayed by an amount of time randomly distributed
  (by up to 25%) about this longest value.


  Independent of success, the delay time is reset to its zero default
  value when Linux-PAM returns control to the application.


  For applications written with a single thread that are event driven in
  nature, libpam 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, Linux-PAM supplies the
  PAM_FAIL_DELAY item. It can be queried and set with pam_get_item() and
  pam_set_item() 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
  (pam_handle_t). This last value was set by the application when it
  called pam_start or explicitly with pam_set_item(... , PAM_CONV, ...).
  Note, if PAM_FAIL_DELAY is unset (or set to NULL), then libpam will
  perform any delay.


  3.1.7.  Authenticating the user



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





  This function serves as an interface to the authentication mechanisms
  of the loaded modules.  The single optional flag, which may be
  logically OR'd with PAM_SILENT, takes the following value,




     PAM_DISALLOW_NULL_AUTHTOK
        Instruct the authentication modules to return PAM_AUTH_ERR if
        the user does not have a registered authorization token---it is
        set to NULL in the system database.


  The value returned by this function is one of the following:




     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_USER_UNKNOWN
        The supplied username is not known to the authentication service

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



  If one or more of the authentication modules fails to load, for
  whatever reason, this function will return PAM_ABORT.


  3.1.8.  Setting user credentials



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





  This function is used to set the module-specific credentials of the
  user.  It is usually called after the user has been authenticated,
  after the account management function has been called but before a
  session has been opened for the user.


  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
  (or UN*X 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.


  This function simply calls the pam_sm_setcred functions of each of the
  loaded modules.  Valid flags, any one of which, may be logically OR'd
  with PAM_SILENT, are:



     PAM_ESTABLISH_CRED
        Set the credentials for the authentication service,

     PAM_DELETE_CRED
        Delete the credentials associated with the authentication
        service,

     PAM_REINITIALIZE_CRED
        Reinitialize the user credentials, and

     PAM_REFRESH_CRED
        Extend the lifetime of the user credentials.


  A successful return is signalled with PAM_SUCCESS. Errors that are
  especially relevant to this function are the following:



     PAM_CRED_UNAVAIL
        A module cannot retrieve the user's credentials.

     PAM_CRED_EXPIRED
        The user's credentials have expired.

     PAM_USER_UNKNOWN
        The user is not known to an authentication module.

     PAM_CRED_ERR
        A module was unable to set the credentials of the user.


  3.1.9.  Account management



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

  This function is typically called after the user has been
  authenticated.  It establishes whether the user's account is healthy.
  That is to say, whether the user's account is still active and whether
  the user is permitted to gain access to the system at this time.
  Valid flags, any one of which, may be logically OR'd with PAM_SILENT,
  and are the same as those applicable to the flags argument of
  pam_authenticate.


  This function simply calls the corresponding functions of each of the
  loaded modules, as instructed by the configuration file,
  /etc/pam.conf.


  The normal response from this function is PAM_SUCCESS, however,
  specific failures are indicated by the following error returns:


     PAM_AUTHTOKEN_REQD
        The user is valid but their authentication token has 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_ACCT_EXPIRED
        The user is no longer permitted to access the system.

     PAM_AUTH_ERR
        There was an authentication error.


     PAM_PERM_DENIED
        The user is not permitted to gain access at this time.

     PAM_USER_UNKNOWN
        The user is not known to a module's account management
        component.



  3.1.10.  Updating authentication tokens



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





  This function is used to change the authentication token for a given
  user (as indicated by the state associated with the handle, pamh). The
  following is a valid but optional flag which may be logically OR'd
  with PAM_SILENT,


     PAM_CHANGE_EXPIRED_AUTHTOK
        This argument indicates to the modules that the users
        authentication token (password) should only be changed if it has
        expired.



  Note, if this argument is not passed, the application requires that
  all authentication tokens are to be changed.


  PAM_SUCCESS is the only successful return value, valid error-returns
  are:


     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_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
        The user is not known to the authentication token changing
        service.



  3.1.11.  Session initialization



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





  This function is used to indicate that an authenticated session has
  begun.  It is used to inform the modules that the user is currently in
  a session.  It should be possible for the Linux-PAM library to open a
  session and close the same session (see section ``below'') from
  different applications.


  Currently, this function simply calls each of the corresponding
  functions of the loaded modules. The only valid flag is PAM_SILENT and
  this is, of course, optional.


  If any of the required loaded modules are unable to open a session for
  the user, this function will return PAM_SESSION_ERR.
  3.1.12.  Terminating sessions



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





  This function is used to indicate that an authenticated session has
  ended. It is used to inform the modules that the user is exiting a
  session. It should be possible for the Linux-PAM library to open a
  session and close the same session from different applications.


  This function simply calls each of the corresponding functions of the
  loaded modules in the same order that they were invoked with
  pam_open_session().  The only valid flag is PAM_SILENT and this is, of
  course, optional.


  If any of the required loaded modules are unable to close a session
  for the user, this function will return PAM_SESSION_ERR.


  3.1.13.  Setting PAM environment variables

  The libpam library associates with each PAM-handle (pamh), a set of
  PAM environment variables. These variables are intended to hold the
  session environment variables that the user will inherit when the
  session is granted and the authenticated user obtains access to the
  requested service. For example, when login has finally given the user
  a shell, the environment (as viewed with the command env) will be what
  libpam was maintaining as the PAM environment for that service
  application. Note, these variables are not the environment variables
  of the login application. This is principally for two reasons: login
  may want to have an environment that cannot be seen or manipulated by
  a user; and login (or whatever the serving application is) may be
  maintaining a number of parallel sessions, via different pamh values,
  at the same time and a single environment may not be appropriately
  shared between each of these. The PAM environment may contain
  variables seeded by the applicant user's client program, for example,
  and as such it is not appropriate for one applicant to interfere with
  the environment of another applicant.




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





  This function attempts to (re)set a Linux-PAM environment variable.
  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 Linux-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 Linux-PAM environment.



  Success is indicated with a return value of PAM_SUCCESS. Failure is
  indicated by one of the following returns:


     PAM_PERM_DENIED
        name given is a NULL pointer


     PAM_BAD_ITEM
        variable requested (for deletion) is not currently set


     PAM_ABORT
        the Linux-PAM handle, pamh, is corrupt


     PAM_BUF_ERR
        failed to allocate memory when attempting update



  3.1.14.  Getting a PAM environment variable



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





  Obtain the value of the indicated Linux-PAM environment variable. On
  error, internal failure or the unavailability of the given variable
  (unspecified), this function simply returns NULL.


  3.1.15.  Getting the PAM environment



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





  The PAM environment variables (see section ``above'') are a complete
  set of enviroment variables that are associated with a PAM-handle
  (pamh). They represent the contents of the regular environment
  variables of the authenticated user when service is granted.


  Th function, pam_getenvlist() returns a pointer to a complete,
  malloc()'d, copy of the PAM environment.  It is a pointer to a
  duplicated list of environment variables.  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.


  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 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.


  3.2.  What is expected of an application

  3.2.1.  The conversation function

  An application must provide a ``conversation function''. It is used
  for direct communication between a loaded module and the application
  and will typically provide a means for the module to prompt the user
  for a password etc. . The structure, pam_conv, is defined by including
  <security/pam_appl.h>; to be,




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





  It is initialized by the application before it is passed to the
  library.  The contents of this structure are attached to the *pamh
  handle.  The point of this argument is to provide a mechanism for any
  loaded module to interact directly with the application program. This
  is why it is called a conversation structure.


  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.  Note, *resp is an struct pam_response
  array and not an array of pointers.


  The message (from the module to the application) passing structure is
  defined by <security/pam_appl.h> as:



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





  Valid choices for msg_style are:



     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

     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 descrepency between the
  way Linux-PAM handles the const struct pam_message **msg conversation
  function argument from the way that Solaris' PAM (and derivitives,
  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_meg read
  only 'struct pam_message' pointers.  Solaris' PAM implementation
  interprets this argument as a pointer to a pointer to an array of
  num_meg 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:

  o  never call the conversation function with num_msg greater than one.

  o  set up msg as doubly referenced so both types of conversation
     function can find the messages. That is, make



       msg[n] = & (( *msg )[n])



  The response (from the application to the module) passing structure is
  defined by including <security/pam_appl.h> as:




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





  Currently, there are no definitions for resp_retcode values; the
  normal value is 0.


  Prior to the 0.59 release of Linux-PAM, the length of the returned
  pam_response array was equal to the number of prompts (types
  PAM_PROMPT_ECHO_OFF and PAM_PROMPT_ECHO_ON) in the pam_message array
  with which the conversation function was called.  This meant that it
  was not always necessary for the module to free(3) the responses if
  the conversation function was only used to display some text.


  Post Linux-PAM-0.59.  The number of responses is always equal to the
  num_msg conversation function argument.  This is slightly easier to
  program but 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.


  The maximum length of the pam_msg.msg and pam_response.resp character
  strings is PAM_MAX_MSG_SIZE.  (This is not enforced by Linux-PAM.)


  PAM_SUCCESS is the expected return value of this function.  However,
  should an error occur the application should not set *resp but simply
  return PAM_CONV_ERR.


  Note, if an application wishes to use two conversation functions, it
  should activate the second with a call to pam_set_item().


  Notes: New item types are being added to the conversation protocol.
  Currently Linux-PAM supports: PAM_BINARY_PROMPT and PAM_BINARY_MSG.
  These two are intended for server-client hidden information exchange
  and may be used as an interface for maching-machine authentication.


  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.





  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 (userid) of the running process; the identity of the
  privilege granting user is the euid (effective userid) 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: <unset> = 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 mannor that preserves the PAM API. Such tolerance may be
  especially important when attempting to track a malicious adversary.


  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 functions for allocating memory (securely), a text based
  conversation function, and routines for enhancing the standard PAM-
  environment variable support.


  5.1.  Requirements

  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 ``the bibliography'') below.


  5.2.  Macros supplied

  5.2.1.  Safe duplication of strings



       x_strdup(const char *s)





  This macro is a replacement for the xstrdup() function that was
  present in earlier versions of the library and which clashed horribly
  with a number of applications. It returns a duplicate copy of the NUL
  terminated string, s. NULL is returned if there is insufficient memory
  available for the duplicate or if s is NULL to begin with.


  5.3.  Functions supplied

  5.3.1.  A text based conversation function



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





  This is a function that 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 struct pam_conv,
  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:

     extern time_t pam_misc_conv_warn_time;
        This variable contains the time (as returned by time()) 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.


     extern const char *pam_misc_conv_warn_line;
        Used in conjuction 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.Time is running out...\n'', but this
        can be changed by the application prior to passing control to
        Linux-PAM.


     extern time_t pam_misc_conv_die_time;
        This variable contains the time (as returned by time()) that the
        conversation 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, this
        should be done prior to passing control to the Linux-PAM
        library.


     extern const char *pam_misc_conv_die_line;
        Used in conjuction 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.Sorry, your
        time is up!\n'', but this can be changed by the application
        prior to passing control to Linux-PAM.


     extern int pam_misc_conv_died;
        Following a return from the Linux-PAM libraray, 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.

     extern 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.)


     extern 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.



  5.3.2.  Transcribing an environment to that of Linux-PAM



       extern int pam_misc_paste_env(pam_handle_t *pamh,
                                     const char * const * user_env);




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


  5.3.3.  Liberating a locally saved environment



       extern char **pam_misc_drop_env(char **env);




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


  5.3.4.  BSD like Linux-PAM environment variable setting



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




  This function performs a task equivalent to pam_putenv(), 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 Linux-PAM variable is already set,
  the replacement will only be applied if the last argument, readonly,
  is zero.




  6.  Porting legacy applications

  The following is extracted from an email.  I'll tidy it up later.


  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.


  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, a user can authenticate
        him/herself in a variety of ways.  Updating the user's
        authentication token thus corresponds to refreshing the object
        they use to authenticate themself 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.



  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]

    You need to add the following (or equivalent) to the /etc/pam.conf file.
    # check authorization
    check_user   auth       required     /usr/lib/security/pam_unix_auth.so
    check_user   account    required     /usr/lib/security/pam_unix_acct.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 */
  }








  9.  Files



     /usr/include/security/pam_appl.h
        header file for Linux-PAM applications interface


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


     /usr/lib/libpam.so.*
        the shared library providing applications with access to Linux-
        PAM.


     /etc/pam.conf
        the Linux-PAM configuration file.


     /usr/lib/security/pam_*.so
        the primary location for Linux-PAM dynamically loadable object
        files; the modules.



  10.  See also


  o  The Linux-PAM System Administrators' Guide.

  o  The Linux-PAM Module Writers' Guide.

  o  The V. Samar and R. Schemers (SunSoft), ``UNIFIED LOGIN WITH
     PLUGGABLE AUTHENTICATION MODULES'', Open Software Foundation
     Request For Comments 86.0, October 1995.


  11.  Notes

  I intend to put development comments here... like ``at the moment this
  isn't actually supported''. At release time what ever is in this
  section will be placed in the Bugs section below! :)



  o  pam_strerror() should be internationalized....

  o  Note, the resp_retcode of struct pam_message, has no purpose at the
     moment. Ideas/suggestions welcome!

  o  more security issues are required....


  12.  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,
  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.


  13.  Bugs/omissions

  This manual is hopelessly unfinished. Only a partial list of people is
  credited for all the good work they have done.


  14.  Copyright information for this document

  Copyright (c) Andrew G. Morgan 1996-9,2000-1.  All rights reserved.
  Email: <morgan@kernel.org>


  Redistribution and use in source and binary forms, with or without
  modification, are permitted provided that the following conditions are
  met:



  o  1. Redistributions of source code must retain the above copyright
     notice, and the entire permission notice in its entirety, including
     the disclaimer of warranties.

  o  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.

  o  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
  DAMAGE.



  $Id: pam_appl.sgml,v 1.9 2002/05/10 05:25:52 agmorgan Exp $

































































