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<H2><A NAME="s3">3. The public interface to <B>Linux-PAM</B></A></H2>
<P>Firstly, the relevant include file for the <B>Linux-PAM</B> library
is <CODE><security/pam_appl.h></CODE>. It contains the definitions
for a number of functions. After listing these functions, we collect
some guiding remarks for programmers.
<P>
<H2><A NAME="ss3.1">3.1 What can be expected by the application</A>
</H2>
<P>Below we document those functions in the <B>Linux-PAM</B> library that
may be called from an application.
<P>
<H3><A NAME="pam-start-section"></A> Initialization of Linux-PAM</H3>
<P>
<BLOCKQUOTE><CODE>
<PRE>
extern int pam_start(const char *service_name, const char *user,
const struct pam_conv *pam_conversation,
pam_handle_t **pamh);
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P>This is the first of the <B>Linux-PAM</B> functions that must be
called by an application. It initializes the interface and reads the
system configuration file, <CODE>/etc/pam.conf</CODE> (see the
<B>Linux-PAM</B> System Administrators' Guide). Following a successful
return (<CODE>PAM_SUCCESS</CODE>) the contents of <CODE>*pamh</CODE> is a handle that
provides continuity for successive calls to the <B>Linux-PAM</B>
library. The arguments expected by <CODE>pam_start</CODE> are as follows: the
<CODE>service_name</CODE> of the program, the <CODE>user</CODE>name of the individual
to be authenticated, a pointer to an application-supplied
<CODE>pam_conv</CODE> structure and a pointer to a <CODE>pam_handle_t</CODE>
<EM>pointer</EM>.
<P>
<P>The <CODE>pam_conv</CODE> structure is discussed more fully in the section
<A HREF="#the-conversation-function">below</A>. The
<CODE>pam_handle_t</CODE> is a <EM>blind</EM> structure and the
application should not attempt to probe it directly for information.
Instead the <B>Linux-PAM</B> library provides the functions
<CODE>pam_set_item</CODE> and <CODE>pam_get_item</CODE>. These functions are
documented below.
<P>
<H3><A NAME="pam-end-section"></A> Termination of the library</H3>
<P>
<BLOCKQUOTE><CODE>
<PRE>
extern int pam_end(pam_handle_t *pamh, int pam_status);
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P>This function is the last function an application should call in the
<B>Linux-PAM</B> library. Upon return the handle <CODE>pamh</CODE> is no
longer valid and all memory associated with it will be invalid (likely
to cause a segmentation fault if accessed).
<P>
<P>Under normal conditions the argument <CODE>pam_status</CODE> has the value
PAM_SUCCESS, but in the event of an unsuccessful application for
service the appropriate <B>Linux-PAM</B> error-return value should be
used here. Note, <CODE>pam_end()</CODE> unconditionally shuts down the
authentication stack associated with the <CODE>pamh</CODE> handle. The value
taken by <CODE>pam_status</CODE> is used as an argument to the module specific
callback functions, <CODE>cleanup()</CODE> (see the <B>Linux-PAM</B>
<A HREF="pam_modules.html">Module Developers' Guide</A>). 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.
<P>
<H3><A NAME="pam-set-item-section"></A> Setting PAM items</H3>
<P>
<BLOCKQUOTE><CODE>
<PRE>
extern int pam_set_item(pam_handle_t *pamh, int item_type,
const void *item);
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P>This function is used to (re)set the value of one of the following
<B>item_type</B>s:
<P>
<P>
<DL>
<DT><B><CODE>PAM_SERVICE</CODE></B><DD><P>The service name
<P>
<DT><B><CODE>PAM_USER</CODE></B><DD><P>The user name
<P>
<DT><B><CODE>PAM_USER_PROMPT</CODE></B><DD><P>The string used when prompting for a user's name. The default
value for this string is ``Please enter username: ''.
<P>
<DT><B><CODE>PAM_TTY</CODE></B><DD><P>The terminal name: prefixed by <CODE>/dev/</CODE> if it is a
device file; for graphical, X-based, applications the value for this
item should be the <CODE>$DISPLAY</CODE> variable.
<P>
<DT><B><CODE>PAM_RUSER</CODE></B><DD><P>The requesting user's username
<P>
<DT><B><CODE>PAM_RHOST</CODE></B><DD><P>The requesting hostname (the hostname of the machine from which
the <CODE>PAM_RUSER</CODE> is requesting service)
<P>
<DT><B><CODE>PAM_CONV</CODE></B><DD><P>The conversation structure (see section
<A HREF="#the-conversation-function">below</A>)
<P>
<DT><B><CODE>PAM_FAIL_DELAY</CODE></B><DD><P>A function pointer to redirect
centrally managed failure delays (see section
<A HREF="#the-failure-delay-function">below</A>).
<P>
</DL>
<P>
<P>For all <CODE>item_type</CODE>s, other than <CODE>PAM_CONV</CODE> and
<CODE>PAM_FAIL_DELAY</CODE>, <CODE>item</CODE> is a pointer to a <CODE><NUL></CODE>
terminated character string. In the case of <CODE>PAM_CONV</CODE>, <CODE>item</CODE>
points to an initialized <CODE>pam_conv</CODE> structure (see section
<A HREF="#the-conversation-function">below</A>). In the case of
<CODE>PAM_FAIL_DELAY</CODE>, <CODE>item</CODE> is a function pointer: <CODE>void
(*delay_fn)(int retval, unsigned usec_delay, void *appdata_ptr)</CODE> (see
section
<A HREF="#the-failure-delay-function">below</A>).
<P>
<P>A successful call to this function returns <CODE>PAM_SUCCESS</CODE>. However,
the application should expect one of the following errors:
<P>
<P>
<DL>
<DT><B><CODE>PAM_SYSTEM_ERR</CODE></B><DD><P>The <CODE>pam_handle_t</CODE> passed as a first argument to this
function was invalid.
<DT><B><CODE>PAM_PERM_DENIED</CODE></B><DD><P>An attempt was made to replace the conversation structure with
a <CODE>NULL</CODE> value.
<DT><B><CODE>PAM_BUF_ERR</CODE></B><DD><P>The function ran out of memory making a copy of the item.
<DT><B><CODE>PAM_BAD_ITEM</CODE></B><DD><P>The application attempted to set an undefined or inaccessible
item.
</DL>
<P>
<H3><A NAME="pam-get-item-section"></A> Getting PAM items</H3>
<P>
<BLOCKQUOTE><CODE>
<PRE>
extern int pam_get_item(const pam_handle_t *pamh, int item_type,
const void **item);
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P>This function is used to obtain the value of the indicated
<CODE>item_type</CODE>. Upon successful return, <CODE>*item</CODE> contains a pointer
to the value of the corresponding item. Note, this is a pointer to
the <EM>actual</EM> data and should <EM>not</EM> be <CODE>free()</CODE>'ed or
over-written!
<P>
<P>A successful call is signaled by a return value of <CODE>PAM_SUCCESS</CODE>.
However, the application should expect one of the following errors:
<P>
<P>
<DL>
<DT><B><CODE>PAM_SYSTEM_ERR</CODE></B><DD><P>The <CODE>pam_handle_t</CODE> passed as a first argument to this
function was invalid.
<DT><B><CODE>PAM_PERM_DENIED</CODE></B><DD><P>The value of <CODE>item</CODE> was <CODE>NULL</CODE>.
<DT><B><CODE>PAM_BAD_ITEM</CODE></B><DD><P>The application attempted to set an undefined or inaccessible
item.
</DL>
<P>
<P>Note, in the case of an error, the contents of <CODE>item</CODE> is not
modified - that is, it retains its pre-call value. One should take
care to initialize this value prior to calling
<CODE>pam_get_item()</CODE>. Since, if its value - despite the
<CODE>pam_get_item()</CODE> function failing - is to be used the consequences
are undefined.
<P>
<H3><A NAME="pam-strerror-section"></A> Understanding errors</H3>
<P>
<BLOCKQUOTE><CODE>
<PRE>
extern const char *pam_strerror(pam_handle_t *pamh, int errnum);
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P>This function returns some text describing the <B>Linux-PAM</B>
error associated with the argument <CODE>errnum</CODE>. If the error is not
recognized ``<CODE>Unknown Linux-PAM error</CODE>'' is returned.
<P>
<H3><A NAME="the-failure-delay-function"></A> Planning for delays</H3>
<P>
<BLOCKQUOTE><CODE>
<PRE>
extern int pam_fail_delay(pam_handle_t *pamh, unsigned int micro_sec);
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P>This function is offered by <B>Linux-PAM</B> to facilitate time delays
following a failed call to <CODE>pam_authenticate()</CODE> and before control
is returned to the application. When using this function the
application programmer should check if it is available with,
<BLOCKQUOTE><CODE>
<PRE>
#ifdef PAM_FAIL_DELAY
....
#endif /* PAM_FAIL_DELAY */
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P>
<P>Generally, an application requests that a user is authenticated by
<B>Linux-PAM</B> through a call to <CODE>pam_authenticate()</CODE> or
<CODE>pam_chauthtok()</CODE>. These functions call each of the <EM>stacked</EM>
authentication modules listed in the relevant <B>Linux-PAM</B>
configuration file. As directed by this file, one of more of the
modules may fail causing the <CODE>pam_...()</CODE> 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 <EM>brute force</EM> dictionary attacks primarily, but
also helps hinder <EM>timed</EM> (covert channel) attacks.
<P>
<P>The <CODE>pam_fail_delay()</CODE> function provides the mechanism by which an
application or module can suggest a minimum delay (of <CODE>micro_sec</CODE>
<EM>micro-seconds</EM>). <B>Linux-PAM</B> keeps a record of the longest time
requested with this function. Should <CODE>pam_authenticate()</CODE> fail,
the failing return to the application is delayed by an amount of time
randomly distributed (by up to 25%) about this longest value.
<P>
<P>Independent of success, the delay time is reset to its zero default
value when <B>Linux-PAM</B> returns control to the application.
<P>
<P>For applications written with a single thread that are event driven in
nature, <CODE>libpam</CODE> 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, <B>Linux-PAM</B> supplies
the <CODE>PAM_FAIL_DELAY</CODE> item. It can be queried and set with
<CODE>pam_get_item()</CODE> and <CODE>pam_set_item()</CODE> respectively. The value
used to set it should be a function pointer of the following
prototype:
<P>
<BLOCKQUOTE><CODE>
<PRE>
void (*delay_fn)(int retval, unsigned usec_delay, void *appdata_ptr);
</PRE>
</CODE></BLOCKQUOTE>
<P>The arguments being the <CODE>retval</CODE> return code of the module stack,
the <CODE>usec_delay</CODE> micro-second delay that libpam is requesting and
the <CODE>appdata_ptr</CODE> that the application has associated with the
current <CODE>pamh</CODE> (<CODE>pam_handle_t</CODE>). This last value was set by the
application when it called <CODE>pam_start</CODE> or explicitly with
<CODE>pam_set_item(... , PAM_CONV, ...)</CODE>. Note, if <CODE>PAM_FAIL_DELAY</CODE>
is unset (or set to <CODE>NULL</CODE>), then <CODE>libpam</CODE> will perform any
delay.
<P>
<H3>Authenticating the user</H3>
<P>
<BLOCKQUOTE><CODE>
<PRE>
extern int pam_authenticate(pam_handle_t *pamh, int flags);
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P>This function serves as an interface to the authentication mechanisms
of the loaded modules. The single <EM>optional</EM> flag, which may be
logically OR'd with <CODE>PAM_SILENT</CODE>, takes the following value,
<P>
<P>
<DL>
<P>
<DT><B><CODE>PAM_DISALLOW_NULL_AUTHTOK</CODE></B><DD><P>Instruct the authentication modules to return
<CODE>PAM_AUTH_ERR</CODE> if the user does not have a registered
authorization token---it is set to <CODE>NULL</CODE> in the system database.
</DL>
<P>
<P>The value returned by this function is one of the following:
<P>
<P>
<DL>
<P>
<DT><B><CODE>PAM_AUTH_ERR</CODE></B><DD><P>The user was not authenticated
<DT><B><CODE>PAM_CRED_INSUFFICIENT</CODE></B><DD><P>For some reason the application does not have sufficient
credentials to authenticate the user.
<DT><B><CODE>PAM_AUTHINFO_UNAVAIL</CODE></B><DD><P>The modules were not able to access the authentication
information. This might be due to a network or hardware failure etc.
<DT><B><CODE>PAM_USER_UNKNOWN</CODE></B><DD><P>The supplied username is not known to the authentication
service
<DT><B><CODE>PAM_MAXTRIES</CODE></B><DD><P>One or more of the authentication modules has reached its
limit of tries authenticating the user. Do not try again.
<P>
</DL>
<P>
<P>If one or more of the authentication modules fails to load, for
whatever reason, this function will return <CODE>PAM_ABORT</CODE>.
<P>
<H3><A NAME="pam-setcred-section"></A> Setting user credentials</H3>
<P>
<BLOCKQUOTE><CODE>
<PRE>
extern int pam_setcred(pam_handle_t *pamh, int flags);
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P>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.
<P>
<P>A credential is something that the user possesses. It is some
property, such as a <EM>Kerberos</EM> 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 <CODE>UID</CODE> and <CODE>GID</CODE>'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.
<P>
<P>This function simply calls the <CODE>pam_sm_setcred</CODE> functions of each
of the loaded modules. Valid <CODE>flags</CODE>, any one of which, may be
logically OR'd with <CODE>PAM_SILENT</CODE>, are:
<P>
<P>
<DL>
<DT><B><CODE>PAM_ESTABLISH_CRED</CODE></B><DD><P>Set the credentials for the authentication service,
<DT><B><CODE>PAM_DELETE_CRED</CODE></B><DD><P>Delete the credentials associated with the authentication service,
<DT><B><CODE>PAM_REINITIALIZE_CRED</CODE></B><DD><P>Reinitialize the user credentials, and
<DT><B><CODE>PAM_REFRESH_CRED</CODE></B><DD><P>Extend the lifetime of the user credentials.
</DL>
<P>
<P>A successful return is signalled with <CODE>PAM_SUCCESS</CODE>. Errors that
are especially relevant to this function are the following:
<P>
<P>
<DL>
<DT><B><CODE>PAM_CRED_UNAVAIL</CODE></B><DD><P>A module cannot retrieve the user's credentials.
<DT><B><CODE>PAM_CRED_EXPIRED</CODE></B><DD><P>The user's credentials have expired.
<DT><B><CODE>PAM_USER_UNKNOWN</CODE></B><DD><P>The user is not known to an authentication module.
<DT><B><CODE>PAM_CRED_ERR</CODE></B><DD><P>A module was unable to set the credentials of the user.
</DL>
<P>
<H3>Account management</H3>
<P>
<BLOCKQUOTE><CODE>
<PRE>
extern int pam_acct_mgmt(pam_handle_t *pamh, int flags);
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P>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
<CODE>PAM_SILENT</CODE>, and are the same as those applicable to the
<CODE>flags</CODE> argument of <CODE>pam_authenticate</CODE>.
<P>
<P>This function simply calls the corresponding functions of each of the
loaded modules, as instructed by the configuration file,
<CODE>/etc/pam.conf</CODE>.
<P>
<P>The normal response from this function is <CODE>PAM_SUCCESS</CODE>, however,
specific failures are indicated by the following error returns:
<P>
<DL>
<DT><B><CODE>PAM_AUTHTOKEN_REQD</CODE></B><DD><P>The user <B>is</B> valid but their authentication token has
<EM>expired</EM>. The correct response to this return-value is to require
that the user satisfies the <CODE>pam_chauthtok()</CODE> 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.
<P>
<DT><B><CODE>PAM_ACCT_EXPIRED</CODE></B><DD><P>The user is no longer permitted to access the system.
<DT><B><CODE>PAM_AUTH_ERR</CODE></B><DD><P>There was an authentication error.
<P>
<DT><B><CODE>PAM_PERM_DENIED</CODE></B><DD><P>The user is not permitted to gain access at this time.
<DT><B><CODE>PAM_USER_UNKNOWN</CODE></B><DD><P>The user is not known to a module's account management
component.
<P>
</DL>
<P>
<H3><A NAME="pam-chauthtok-section"></A> Updating authentication tokens</H3>
<P>
<BLOCKQUOTE><CODE>
<PRE>
extern int pam_chauthtok(pam_handle_t *pamh, const int flags);
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P>This function is used to change the authentication token for a given
user (as indicated by the state associated with the handle,
<CODE>pamh</CODE>). The following is a valid but optional flag which may be
logically OR'd with <CODE>PAM_SILENT</CODE>,
<P>
<DL>
<DT><B><CODE>PAM_CHANGE_EXPIRED_AUTHTOK</CODE></B><DD><P>This argument indicates to the modules that the users
authentication token (password) should only be changed if it has
expired.
</DL>
<P>
<P>Note, if this argument is not passed, the application requires that
<EM>all</EM> authentication tokens are to be changed.
<P>
<P><CODE>PAM_SUCCESS</CODE> is the only successful return value, valid
error-returns are:
<P>
<DL>
<DT><B><CODE>PAM_AUTHTOK_ERR</CODE></B><DD><P>A module was unable to obtain the new authentication token.
<P>
<DT><B><CODE>PAM_AUTHTOK_RECOVERY_ERR</CODE></B><DD><P>A module was unable to obtain the old authentication token.
<P>
<DT><B><CODE>PAM_AUTHTOK_LOCK_BUSY</CODE></B><DD><P>One or more of the modules was unable to change the
authentication token since it is currently locked.
<P>
<DT><B><CODE>PAM_AUTHTOK_DISABLE_AGING</CODE></B><DD><P>Authentication token aging has been disabled for at least one
of the modules.
<P>
<DT><B><CODE>PAM_PERM_DENIED</CODE></B><DD><P>Permission denied.
<P>
<DT><B><CODE>PAM_TRY_AGAIN</CODE></B><DD><P>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.
<P>
<DT><B><CODE>PAM_USER_UNKNOWN</CODE></B><DD><P>The user is not known to the authentication token changing
service.
<P>
</DL>
<P>
<H3><A NAME="pam-open-session-section"></A> Session initialization</H3>
<P>
<BLOCKQUOTE><CODE>
<PRE>
extern int pam_open_session(pam_handle_t *pamh, int flags);
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P>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 <B>Linux-PAM</B> library
to open a session and close the same session (see section
<A HREF="#pam-close-session-section">below</A>) from different
applications.
<P>
<P>Currently, this function simply calls each of the corresponding
functions of the loaded modules. The only valid flag is
<CODE>PAM_SILENT</CODE> and this is, of course, <EM>optional</EM>.
<P>
<P>If any of the <EM>required</EM> loaded modules are unable to open a
session for the user, this function will return <CODE>PAM_SESSION_ERR</CODE>.
<P>
<H3><A NAME="pam-close-session-section"></A> Terminating sessions</H3>
<P>
<BLOCKQUOTE><CODE>
<PRE>
extern int pam_close_session(pam_handle_t *pamh, int flags);
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P>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 <B>Linux-PAM</B> library to
open a session and close the same session from different applications.
<P>
<P>This function simply calls each of the corresponding functions of the
loaded modules in the same order that they were invoked with
<CODE>pam_open_session()</CODE>. The only valid flag is <CODE>PAM_SILENT</CODE> and
this is, of course, <EM>optional</EM>.
<P>
<P>If any of the <EM>required</EM> loaded modules are unable to close a
session for the user, this function will return <CODE>PAM_SESSION_ERR</CODE>.
<P>
<H3><A NAME="pam-putenv-section"></A> Setting PAM environment variables</H3>
<P>
<BLOCKQUOTE><CODE>
<PRE>
extern int pam_putenv(pam_handle_t *pamh, const char *name_value);
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P>This function attempts to (re)set a <B>Linux-PAM</B> environment
variable. The <CODE>name_value</CODE> argument is a single <CODE>NUL</CODE> terminated
string of one of the following forms:
<DL>
<DT><B>``<CODE>NAME=value of variable</CODE>''</B><DD><P>In this case the environment variable of the given <CODE>NAME</CODE> is set to
the indicated value: ``<CODE>value of variable</CODE>''. If this variable is
already known, it is overwritten. Otherwise it is added to the
<B>Linux-PAM</B> environment.
<P>
<DT><B>``<CODE>NAME=</CODE>''</B><DD><P>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.
<P>
<DT><B>``<CODE>NAME</CODE>''</B><DD><P>Without an `<CODE>=</CODE>' the <CODE>pam_putenv()</CODE> function will delete the
corresponding variable from the <B>Linux-PAM</B> environment.
<P>
</DL>
<P>
<P>Success is indicated with a return value of <CODE>PAM_SUCCESS</CODE>. Failure
is indicated by one of the following returns:
<P>
<DL>
<DT><B><CODE>PAM_PERM_DENIED</CODE></B><DD><P>name given is a <CODE>NULL</CODE> pointer
<P>
<DT><B><CODE>PAM_BAD_ITEM</CODE></B><DD><P>variable requested (for deletion) is not currently set
<P>
<DT><B><CODE>PAM_ABORT</CODE></B><DD><P>the <B>Linux-PAM</B> handle, <CODE>pamh</CODE>, is corrupt
<P>
<DT><B><CODE>PAM_BUF_ERR</CODE></B><DD><P>failed to allocate memory when attempting update
<P>
</DL>
<P>
<H3><A NAME="pam-getenv-section"></A> Getting a PAM environment variable</H3>
<P>
<BLOCKQUOTE><CODE>
<PRE>
extern const char *pam_getenv(pam_handle_t *pamh, const char *name);
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P><EM>Warning, the environment support in <B>Linux-PAM</B> is based solely
on a six-line email from the developers at Sun. Its interface is
likely to be generally correct, however, the details are likely to be
changed as more information becomes available.</EM>
<P>
<P>Obtain the value of the indicated <B>Linux-PAM</B> environment
variable. On error, internal failure or the unavailability of the
given variable (unspecified), this function simply returns <CODE>NULL</CODE>.
<P>
<H3><A NAME="pam-getenvlist-section"></A> Getting the PAM environment</H3>
<P>
<BLOCKQUOTE><CODE>
<PRE>
extern const char * const *pam_getenvlist(pam_handle_t *pamh);
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P><EM>Warning, the environment support in <B>Linux-PAM</B> is based solely
on a six line email from the developers at Sun. Its interface is
likely to be generally correct, however, the details are likely to be
changed as more information becomes available.</EM>
<P>
<P>This function returns a pointer to the complete <CODE>Linux-PAM</CODE>
environment. It is a pointer to a <EM>read-only</EM> list of
<EM>read-only</EM> environment variables. It should be noted that this
memory will become invalid after a call to <CODE>pam_end()</CODE> (see the
section
<A HREF="#pam-end-section">above</A>). If application
wishes to make use of this list after such a call, it should first
make a copy of all the set variables. (A function that performs such a
transcription is to be found in <CODE>libpam_misc</CODE>.)
<P>
<H2><A NAME="ss3.2">3.2 What is expected of an application</A>
</H2>
<H3><A NAME="the-conversation-function"></A> The conversation function</H3>
<P>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, <CODE>pam_conv</CODE>, is defined by
including <CODE><security/pam_appl.h></CODE>; to be,
<P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
struct pam_conv {
int (*conv)(int num_msg,
const struct pam_message **msg,
struct pam_response **resp,
void *appdata_ptr);
void *appdata_ptr;
};
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P>It is initialized by the application before it is passed to the
library. The <EM>contents</EM> of this structure are attached to the
<CODE>*pamh</CODE> 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 <EM>conversation</EM>
structure.
<P>
<P>When a module calls the referenced <CODE>conv()</CODE> function, the argument
<CODE>*appdata_ptr</CODE> is set to the second element of this structure.
<P>
<P>The other arguments of a call to <CODE>conv()</CODE> concern the information
exchanged by module and application. That is to say, <CODE>num_msg</CODE>
holds the length of the array of pointers, <CODE>msg</CODE>. After a
successful return, the pointer <CODE>*resp</CODE> points to an array of
<CODE>pam_response</CODE> structures, holding the application supplied text.
Note, <CODE>*resp</CODE> is an <CODE>struct pam_response</CODE> array and <EM>not</EM> an
array of pointers.
<P>
<P>The message (from the module to the application) passing structure is
defined by <CODE><security/pam_appl.h></CODE> as:
<P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
struct pam_message {
int msg_style;
const char *msg;
};
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P>Valid choices for <CODE>msg_style</CODE> are:
<P>
<P>
<DL>
<DT><B><CODE>PAM_PROMPT_ECHO_OFF</CODE></B><DD><P>Obtain a string without echoing any text
<DT><B><CODE>PAM_PROMPT_ECHO_ON</CODE></B><DD><P>Obtain a string whilst echoing text
<DT><B><CODE>PAM_ERROR_MSG</CODE></B><DD><P>Display an error
<DT><B><CODE>PAM_TEXT_INFO</CODE></B><DD><P>Display some text.
</DL>
<P>
<P>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.
<P>
<P>The response (from the application to the module) passing structure is
defined by including <CODE><security/pam_appl.h></CODE> as:
<P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
struct pam_response {
char *resp;
int resp_retcode;
};
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P>Currently, there are no definitions for <CODE>resp_retcode</CODE> values; the
normal value is <CODE>0</CODE>.
<P>
<P>Prior to the 0.59 release of Linux-PAM, the length of the returned
<CODE>pam_response</CODE> array was equal to the number of <EM>prompts</EM> (types
<CODE>PAM_PROMPT_ECHO_OFF</CODE> and <CODE>PAM_PROMPT_ECHO_ON</CODE>) in the
<CODE>pam_message</CODE> array with which the conversation function was
called. This meant that it was not always necessary for the module to
<CODE>free(3)</CODE> the responses if the conversation function was only used
to display some text.
<P>
<P>Post Linux-PAM-0.59 (and in the interests of compatibility with
Sunsoft). The number of responses is always equal to the <CODE>num_msg</CODE>
conversation function argument. This is slightly easier to program
but does require that the response array is <CODE>free(3)</CODE>'d after every
call to the conversation function. The index of the responses
corresponds directly to the prompt index in the <CODE>pam_message</CODE>
array.
<P>
<P>The maximum length of the <CODE>pam_msg.msg</CODE> and <CODE>pam_response.resp</CODE>
character strings is <CODE>PAM_MAX_MSG_SIZE</CODE>. (This is not enforced by
Linux-PAM.)
<P>
<P><CODE>PAM_SUCCESS</CODE> is the expected return value of this
function. However, should an error occur the application should not
set <CODE>*resp</CODE> but simply return <CODE>PAM_CONV_ERR</CODE>.
<P>
<P>Note, if an application wishes to use two conversation functions, it
should activate the second with a call to <CODE>pam_set_item()</CODE>.
<P>
<P><B>Notes:</B> New item types are being added to the conversation
protocol. Currently Linux-PAM supports: <CODE>PAM_BINARY_PROMPT</CODE>
and <CODE>PAM_BINARY_MSG</CODE>. These two are intended for server-client
hidden information exchange and may be used as an interface for
maching-machine authentication.
<P>
<H2><A NAME="ss3.3">3.3 Programming notes</A>
</H2>
<P>Note, all of the authentication service function calls accept the
token <CODE>PAM_SILENT</CODE>, 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.
<CODE>PAM_SILENT</CODE> does not override the prompting of the user for
passwords etc., it only stops informative messages from being
generated.
<P>
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