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Here we list the interface that the conventions that all Linux-PAM modules must adhere to.
PAM_ITEMs and data
First, we cover what the module should expect from the Linux-PAM
library and a Linux-PAM aware application. Essesntially this
is the libpam.* library.
Synopsis:
extern int pam_set_data(pam_handle_t *pamh,
const char *module_data_name,
void *data,
void (*cleanup)(pam_handle_t *pamh,
void *data, int error_status) );
The modules may be dynamically loadable objects. In general such files
should not contain static variables. This and the subsequent
function provide a mechanism for a module to associate some data with
the handle pamh. Typically a module will call the
pam_set_data() function to register some data under a (hopefully)
unique module_data_name. The data is available for use by other
modules too but not by an application.
The function cleanup() is associated with the data and, if
non-NULL, it is called when this data is over-written or
following a call to pam_end() (see the Linux-PAM Application
Developers' Guide).
The error_status argument is used to indicate to the module the
sort of action it is to take in cleaning this data item. As an
example, Kerberos creates a ticket file during the authentication
phase, this file might be associated with a data item. When
pam_end() is called by the module, the error_status
carries the return value of the pam_authenticate() or other
libpam function as appropriate. Based on this value the Kerberos
module may choose to delete the ticket file (authentication
failure) or leave it in place.
The error_status may have been logically OR'd with either of the
following two values:
PAM_DATA_REPLACEWhen a data item is being replaced (through a second call to
pam_set_data()) this mask is used. Otherwise, the call is assumed
to be from pam_end().
PAM_DATA_SILENTWhich indicates that the process would prefer to perform the
cleanup() quietly. That is, discourages logging/messages to the
user.
Synopsis:
extern int pam_get_data(const pam_handle_t *pamh,
const char *module_data_name,
const void **data);
This function together with the previous one provides a method of
associating module-specific data with the handle pamh. A
successful call to pam_get_data will result in *data
pointing to the data associated with the module_data_name. Note,
this data is not a copy and should be treated as constant
by the module.
Note, if there is an entry but it has the value NULL, then this
call returns PAM_NO_MODULE_DATA.
Synopsis:
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
item_types. The reader is urged to read the entry for this
function in the Linux-PAM application developers' manual.
In addition to the items listed there, the module can set the
following two item_types:
PAM_AUTHTOKThe authentication token (often a password). This token should be
ignored by all module functions besides pam_sm_authenticate() and
pam_sm_chauthtok(). 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_OLDAUTHTOKThe old authentication token. This token should be ignored by all
module functions except pam_sm_chauthtok().
Both of these items are reset before returning to the application.
When resetting these items, the Linux-PAM library first writes
0's to the current tokens and then free()'s the associated
memory.
The return values for this function are listed in the Linux-PAM Application Developers' Guide.
Synopsis:
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 specified
item_type. It is better documented in the Linux-PAM
Application Developers' Guide. However, there are three things worth
stressing here:
pam_get_user() (see section
below).
PAM_AUTHTOK and PAM_OLDAUTHTOK (see the section
above on pam_set_data()).
free() or alter the data pointed to by
*item after a successful return from pam_get_item(). This
pointer points directly at the data contained within the *pamh
structure. Should a module require that a change is made to the this
ITEM it should make the appropriate call to pam_set_item().
Following the call pam_get_item(pamh,PAM_CONV,&item), the
pointer item points to a structure containing an a pointer to a
conversation-function that provides limited but direct access to
the application. The purpose of this function is to allow the module
to prompt the user for their password and pass other information in a
manner consistent with the application. For example, an X-windows
based program might pop up a dialog box to report a login
failure. Just as the application should not be concerned with the
method of authentication, so the module should not dictate the manner
in which input (output) is obtained from (presented to) to the user.
The reader is strongly urged to read the more complete description of
the pam_conv structure, written from the perspective of the
application developer, in the Linux-PAM Application Developers'
Guide.
The return values for this function are listed in the Linux-PAM Application Developers' Guide.
The pam_response structure returned after a call to the
pam_conv function must be free()'d by the module. Since the
call to the conversation function originates from the module, it is
clear that this pam_response structure could be either statically
or dynamically (using malloc() etc.) allocated within the
application. Repeated calls to the conversation function would likely
overwrite static memory, so it is required that for a successful
return from the conversation function the memory for the response
structure is dynamically allocated by the application with one of the
malloc() family of commands and must be free()'d by the
module.
If the pam_conv mechanism is used to enter authentication tokens,
the module should either pass the result to the pam_set_item()
library function, or copy it itself. In such a case, once the token
has been stored (by one of these methods or another one), the memory
returned by the application should be overwritten with 0's, and
then free()'d.
There is a handy macro _pam_drop_reply() to be found in
<security/_pam_macros.h> that can be used to
conveniently cleanup a pam_response structure. (Note, this
include file is specific to the Linux-PAM sources, and whilst it will
work with Sun derived PAM implementations, it is not generally
distributed by Sun.)
Synopsis:
extern int pam_get_user(pam_handle_t *pamh,
const char **user,
const char *prompt);
This is a Linux-PAM library function that returns the (prospective) name of the user. To determine the username it does the following things, in this order:
pam_get_item(pamh, PAM_USER, ... ); would have
returned. If this is not NULL this is what it returns. Otherwise,
pam_conv
mechanism, it prompts the user with the first non-NULL string in
the following list:
prompt argument passed to the functionpam_get_item(pamh,PAM_USER_PROMPT, ... );
By whatever means the username is obtained, a pointer to it is
returned as the contents of *user. Note, this memory should
not be free()'d by the module. Instead, it will be liberated
on the next call to pam_get_user(), or by pam_end() when the
application ends its interaction with Linux-PAM.
Also, in addition, it should be noted that this function sets the
PAM_USER item that is associated with the pam_[gs]et_item()
function.
The return value of this function is one of the following:
PAM_SUCCESS - username obtained.
PAM_CONV_AGAIN - converstation did not complete and the
caller is required to return control to the application, until such
time as the application has completed the conversation process. A
module calling pam_get_user() that obtains this return code,
should return PAM_INCOMPLETE and be prepared (when invoked the
next time) to recall pam_get_user() to fill in the user's name,
and then pick up where it left off as if nothing had happened. This
procedure is needed to support an event-driven application programming
model.
PAM_CONV_ERR - the conversation method supplied by the
application failed to obtain the username.
Synopsis:
extern int pam_putenv(pam_handle_t *pamh, const char *name_value);
Linux-PAM comes equipped with a series of functions for
maintaining a set of environment variables. The environment is
initialized by the call to pam_start() and is erased with a
call to pam_end(). This environment is associated with the
pam_handle_t pointer returned by the former call.
The default environment is all but empty. It contains a single
NULL pointer, which is always required to terminate the
variable-list. The pam_putenv() function can be used to add a
new environment variable, replace an existing one, or delete an old
one.
name_value, should be of the following form:
name_value="VARIABLE=VALUE OF VARIABLE"
VARIABLE is the environment variable's name and what
follows the `=' is its (new) value. (Note, that "VARIABLE="
is a valid value for name_value, indicating that the variable is
set to "".)
name_value, should be of the following form:
name_value="VARIABLE"
VARIABLE is the environment variable's name and the absence
of an `=' indicates that the variable should be removed.
In all cases PAM_SUCCESS indicates success.
Synopsis:
extern const char *pam_getenv(pam_handle_t *pamh, const char *name);
This function can be used to return the value of the given
variable. If the returned value is NULL, the variable is not
known.
Synopsis:
extern char * const *pam_getenvlist(pam_handle_t *pamh);
This function returns a pointer to the entire Linux-PAM
environment array. At first sight the type of the returned data
may appear a little confusing. It is basically a read-only array
of character pointers, that lists the NULL terminated list of
environment variables set so far.
Although, this is not a concern for the module programmer, we mention
here that an application should be careful to copy this entire array
before executing pam_end() otherwise all the variable information
will be lost. (There are functions in libpam_misc for this
purpose: pam_misc_copy_env() and pam_misc_drop_env().)
libpam
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 module
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 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 (cf. 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.