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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.
Below we document those functions in the Linux-PAM library that may be called from an application.
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.
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 Upon return the handle pamh is no
longer valid and all memory associated with it will be invalid (likely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.
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_SERVICEThe service name (which identifies that PAM stack that
libpam will use to authenticate the program).
PAM_USERThe 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_PROMPTThe string used when prompting for a user's name. The default value for this string is ``Please enter username: ''.
PAM_TTYThe 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_RUSERThe 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_RHOSTThe 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_CONVThe conversation structure (see section below).
PAM_FAIL_DELAYA 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_ERRThe pam_handle_t passed as a first argument to this
function was invalid.
PAM_PERM_DENIEDAn attempt was made to replace the conversation structure with
a NULL value.
PAM_BUF_ERRThe function ran out of memory making a copy of the item.
PAM_BAD_ITEMThe application attempted to set an undefined or inaccessible item.
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_ERRThe pam_handle_t passed as a first argument to this
function was invalid.
PAM_PERM_DENIEDThe value of item was NULL.
PAM_BAD_ITEMThe 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.
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.
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.
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_AUTHTOKInstruct 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_ERRThe user was not authenticated
PAM_CRED_INSUFFICIENTFor some reason the application does not have sufficient credentials to authenticate the user.
PAM_AUTHINFO_UNAVAILThe modules were not able to access the authentication information. This might be due to a network or hardware failure etc.
PAM_USER_UNKNOWNThe supplied username is not known to the authentication service
PAM_MAXTRIESOne 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.
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_CREDSet the credentials for the authentication service,
PAM_DELETE_CREDDelete the credentials associated with the authentication service,
PAM_REINITIALIZE_CREDReinitialize the user credentials, and
PAM_REFRESH_CREDExtend 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_UNAVAILA module cannot retrieve the user's credentials.
PAM_CRED_EXPIREDThe user's credentials have expired.
PAM_USER_UNKNOWNThe user is not known to an authentication module.
PAM_CRED_ERRA module was unable to set the credentials of the user.
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_REQDThe 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_EXPIREDThe user is no longer permitted to access the system.
PAM_AUTH_ERRThere was an authentication error.
PAM_PERM_DENIEDThe user is not permitted to gain access at this time.
PAM_USER_UNKNOWNThe user is not known to a module's account management component.
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_AUTHTOKThis 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_ERRA module was unable to obtain the new authentication token.
PAM_AUTHTOK_RECOVERY_ERRA module was unable to obtain the old authentication token.
PAM_AUTHTOK_LOCK_BUSYOne or more of the modules was unable to change the authentication token since it is currently locked.
PAM_AUTHTOK_DISABLE_AGINGAuthentication token aging has been disabled for at least one of the modules.
PAM_PERM_DENIEDPermission denied.
PAM_TRY_AGAINNot 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_UNKNOWNThe user is not known to the authentication token changing service.
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.
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.
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_DENIEDname given is a NULL pointer
PAM_BAD_ITEMvariable requested (for deletion) is not currently set
PAM_ABORTthe Linux-PAM handle, pamh, is corrupt
PAM_BUF_ERRfailed to allocate memory when attempting update
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.
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.
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_OFFObtain a string without echoing any text
PAM_PROMPT_ECHO_ONObtain a string whilst echoing text
PAM_ERROR_MSGDisplay an error
PAM_TEXT_INFODisplay 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:
num_msg greater
than one.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.
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.