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PAM Module Programming
Sample PAM Application
The following is a minimal implementation of <citerefentry><refentrytitle>su</refentrytitle><manvolnum>1</manvolnum></citerefentry> using PAM. Note that it uses the OpenPAM-specific <citerefentry><refentrytitle>openpam_ttyconv</refentrytitle><manvolnum>3</manvolnum></citerefentry> conversation function, which is prototyped in <filename>security/openpam.h</filename>. If you wish build this application on a system with a different PAM library, you will have to provide your own conversation function. A robust conversation function is surprisingly difficult to implement; the one presented in <xref linkend="pam-sample-conv"/> is a good starting point, but should not be used in real-world applications.
/*-
* Copyright (c) 2002,2003 Networks Associates Technology, Inc.
* All rights reserved.
*
* This software was developed for the FreeBSD Project by ThinkSec AS and
* Network Associates Laboratories, the Security Research Division of
* Network Associates, Inc. under DARPA/SPAWAR contract N66001-01-C-8035
* ("CBOSS"), as part of the DARPA CHATS research program.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. The name of the author may not be used to endorse or promote
* products derived from this software without specific prior written
* permission.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``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 OR CONTRIBUTORS 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.
*
* $P4: //depot/projects/openpam/bin/su/su.c#10 $
* $FreeBSD: head/en_US.ISO8859-1/articles/pam/su.c 38826 2012-05-17 19:12:14Z hrs $
*/

#include &lt;sys/param.h&gt;
#include &lt;sys/wait.h&gt;

#include &lt;err.h&gt;
#include &lt;pwd.h&gt;
#include &lt;stdio.h&gt;
#include &lt;stdlib.h&gt;
#include &lt;string.h&gt;
#include &lt;syslog.h&gt;
#include &lt;unistd.h&gt;

#include &lt;security/pam_appl.h&gt;
#include &lt;security/openpam.h&gt; /* for openpam_ttyconv() */

extern char **environ;

static pam_handle_t *pamh;
static struct pam_conv pamc;

static void
usage(void)
{

fprintf(stderr, "Usage: su [login [args]]\n");
exit(1);
}

int
main(int argc, char *argv[])
{
char hostname[MAXHOSTNAMELEN];
const char *user, *tty;
char **args, **pam_envlist, **pam_env;
struct passwd *pwd;
int o, pam_err, status;
pid_t pid;

while ((o = getopt(argc, argv, "h")) != -1)
switch (o) {
case 'h':
default:
usage();
}

argc -= optind;
argv += optind;

if (argc &gt; 0) {
user = *argv;
--argc;
++argv;
} else {
user = "root";
}

/* initialize PAM */
pamc.conv = &amp;openpam_ttyconv;
pam_start("su", user, &amp;pamc, &amp;pamh);

/* set some items */
gethostname(hostname, sizeof(hostname));
if ((pam_err = pam_set_item(pamh, PAM_RHOST, hostname)) != PAM_SUCCESS)
goto pamerr;
user = getlogin();
if ((pam_err = pam_set_item(pamh, PAM_RUSER, user)) != PAM_SUCCESS)
goto pamerr;
tty = ttyname(STDERR_FILENO);
if ((pam_err = pam_set_item(pamh, PAM_TTY, tty)) != PAM_SUCCESS)
goto pamerr;

/* authenticate the applicant */
if ((pam_err = pam_authenticate(pamh, 0)) != PAM_SUCCESS)
goto pamerr;
if ((pam_err = pam_acct_mgmt(pamh, 0)) == PAM_NEW_AUTHTOK_REQD)
pam_err = pam_chauthtok(pamh, PAM_CHANGE_EXPIRED_AUTHTOK);
if (pam_err != PAM_SUCCESS)
goto pamerr;

/* establish the requested credentials */
if ((pam_err = pam_setcred(pamh, PAM_ESTABLISH_CRED)) != PAM_SUCCESS)
goto pamerr;

/* authentication succeeded; open a session */
if ((pam_err = pam_open_session(pamh, 0)) != PAM_SUCCESS)
goto pamerr;

/* get mapped user name; PAM may have changed it */
pam_err = pam_get_item(pamh, PAM_USER, (const void **)&amp;user);
if (pam_err != PAM_SUCCESS || (pwd = getpwnam(user)) == NULL)
goto pamerr;

/* export PAM environment */
if ((pam_envlist = pam_getenvlist(pamh)) != NULL) {
for (pam_env = pam_envlist; *pam_env != NULL; ++pam_env) {
putenv(*pam_env);
free(*pam_env);
}
free(pam_envlist);
}

/* build argument list */
if ((args = calloc(argc + 2, sizeof *args)) == NULL) {
warn("calloc()");
goto err;
}
*args = pwd-&gt;pw_shell;
memcpy(args + 1, argv, argc * sizeof *args);

/* fork and exec */
switch ((pid = fork())) {
case -1:
warn("fork()");
goto err;
case 0:
/* child: give up privs and start a shell */

/* set uid and groups */
if (initgroups(pwd-&gt;pw_name, pwd-&gt;pw_gid) == -1) {
warn("initgroups()");
_exit(1);
}
if (setgid(pwd-&gt;pw_gid) == -1) {
warn("setgid()");
_exit(1);
}
if (setuid(pwd-&gt;pw_uid) == -1) {
warn("setuid()");
_exit(1);
}
execve(*args, args, environ);
warn("execve()");
_exit(1);
default:
/* parent: wait for child to exit */
waitpid(pid, &amp;status, 0);

/* close the session and release PAM resources */
pam_err = pam_close_session(pamh, 0);
pam_end(pamh, pam_err);

exit(WEXITSTATUS(status));
}

pamerr:
fprintf(stderr, "Sorry\n");
err:
pam_end(pamh, pam_err);
exit(1);
}
Sample PAM Module
The following is a minimal implementation of <citerefentry><refentrytitle>pam_unix</refentrytitle><manvolnum>8</manvolnum></citerefentry>, offering only authentication services. It should build and run with most PAM implementations, but takes advantage of OpenPAM extensions if available: note the use of <citerefentry><refentrytitle>pam_get_authtok</refentrytitle><manvolnum>3</manvolnum></citerefentry>, which enormously simplifies prompting the user for a password.
/*-
* Copyright (c) 2002 Networks Associates Technology, Inc.
* All rights reserved.
*
* This software was developed for the FreeBSD Project by ThinkSec AS and
* Network Associates Laboratories, the Security Research Division of
* Network Associates, Inc. under DARPA/SPAWAR contract N66001-01-C-8035
* ("CBOSS"), as part of the DARPA CHATS research program.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. The name of the author may not be used to endorse or promote
* products derived from this software without specific prior written
* permission.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``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 OR CONTRIBUTORS 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.
*
* $P4: //depot/projects/openpam/modules/pam_unix/pam_unix.c#3 $
* $FreeBSD: head/en_US.ISO8859-1/articles/pam/pam_unix.c 38826 2012-05-17 19:12:14Z hrs $
*/

#include &lt;sys/param.h&gt;

#include &lt;pwd.h&gt;
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;
#include &lt;unistd.h&gt;

#include &lt;security/pam_modules.h&gt;
#include &lt;security/pam_appl.h&gt;

#ifndef _OPENPAM
static char password_prompt[] = "Password:";
#endif

#ifndef PAM_EXTERN
#define PAM_EXTERN
#endif

PAM_EXTERN int
pam_sm_authenticate(pam_handle_t *pamh, int flags,
int argc, const char *argv[])
{
#ifndef _OPENPAM
struct pam_conv *conv;
struct pam_message msg;
const struct pam_message *msgp;
struct pam_response *resp;
#endif
struct passwd *pwd;
const char *user;
char *crypt_password, *password;
int pam_err, retry;

/* identify user */
if ((pam_err = pam_get_user(pamh, &amp;user, NULL)) != PAM_SUCCESS)
return (pam_err);
if ((pwd = getpwnam(user)) == NULL)
return (PAM_USER_UNKNOWN);

/* get password */
#ifndef _OPENPAM
pam_err = pam_get_item(pamh, PAM_CONV, (const void **)&amp;conv);
if (pam_err != PAM_SUCCESS)
return (PAM_SYSTEM_ERR);
msg.msg_style = PAM_PROMPT_ECHO_OFF;
msg.msg = password_prompt;
msgp = &amp;msg;
#endif
for (retry = 0; retry &lt; 3; ++retry) {
#ifdef _OPENPAM
pam_err = pam_get_authtok(pamh, PAM_AUTHTOK,
(const char **)&amp;password, NULL);
#else
resp = NULL;
pam_err = (*conv-&gt;conv)(1, &amp;msgp, &amp;resp, conv-&gt;appdata_ptr);
if (resp != NULL) {
if (pam_err == PAM_SUCCESS)
password = resp-&gt;resp;
else
free(resp-&gt;resp);
free(resp);
}
#endif
if (pam_err == PAM_SUCCESS)
break;
}
if (pam_err == PAM_CONV_ERR)
return (pam_err);
if (pam_err != PAM_SUCCESS)
return (PAM_AUTH_ERR);

/* compare passwords */
if ((!pwd-&gt;pw_passwd[0] &amp;&amp; (flags &amp; PAM_DISALLOW_NULL_AUTHTOK)) ||
(crypt_password = crypt(password, pwd-&gt;pw_passwd)) == NULL ||
strcmp(crypt_password, pwd-&gt;pw_passwd) != 0)
pam_err = PAM_AUTH_ERR;
else
pam_err = PAM_SUCCESS;
#ifndef _OPENPAM
free(password);
#endif
return (pam_err);
}

PAM_EXTERN int
pam_sm_setcred(pam_handle_t *pamh, int flags,
int argc, const char *argv[])
{

return (PAM_SUCCESS);
}

PAM_EXTERN int
pam_sm_acct_mgmt(pam_handle_t *pamh, int flags,
int argc, const char *argv[])
{

return (PAM_SUCCESS);
}

PAM_EXTERN int
pam_sm_open_session(pam_handle_t *pamh, int flags,
int argc, const char *argv[])
{

return (PAM_SUCCESS);
}

PAM_EXTERN int
pam_sm_close_session(pam_handle_t *pamh, int flags,
int argc, const char *argv[])
{

return (PAM_SUCCESS);
}

PAM_EXTERN int
pam_sm_chauthtok(pam_handle_t *pamh, int flags,
int argc, const char *argv[])
{

return (PAM_SERVICE_ERR);
}

#ifdef PAM_MODULE_ENTRY
PAM_MODULE_ENTRY("pam_unix");
#endif
Sample PAM Conversation Function
The conversation function presented below is a greatly simplified version of OpenPAM's <citerefentry><refentrytitle>openpam_ttyconv</refentrytitle><manvolnum>3</manvolnum></citerefentry>. It is fully functional, and should give the reader a good idea of how a conversation function should behave, but it is far too simple for real-world use. Even if you are not using OpenPAM, feel free to download the source code and adapt <citerefentry><refentrytitle>openpam_ttyconv</refentrytitle><manvolnum>3</manvolnum></citerefentry> to your uses; we believe it to be as robust as a tty-oriented conversation function can reasonably get.
/*-
* Copyright (c) 2002 Networks Associates Technology, Inc.
* All rights reserved.
*
* This software was developed for the FreeBSD Project by ThinkSec AS and
* Network Associates Laboratories, the Security Research Division of
* Network Associates, Inc. under DARPA/SPAWAR contract N66001-01-C-8035
* ("CBOSS"), as part of the DARPA CHATS research program.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. The name of the author may not be used to endorse or promote
* products derived from this software without specific prior written
* permission.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``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 OR CONTRIBUTORS 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.
*
* $FreeBSD: head/en_US.ISO8859-1/articles/pam/converse.c 38826 2012-05-17 19:12:14Z hrs $
*/

#include &lt;stdio.h&gt;
#include &lt;stdlib.h&gt;
#include &lt;string.h&gt;
#include &lt;unistd.h&gt;

#include &lt;security/pam_appl.h&gt;

int
converse(int n, const struct pam_message **msg,
struct pam_response **resp, void *data)
{
struct pam_response *aresp;
char buf[PAM_MAX_RESP_SIZE];
int i;

data = data;
if (n &lt;= 0 || n &gt; PAM_MAX_NUM_MSG)
return (PAM_CONV_ERR);
if ((aresp = calloc(n, sizeof *aresp)) == NULL)
return (PAM_BUF_ERR);
for (i = 0; i &lt; n; ++i) {
aresp[i].resp_retcode = 0;
aresp[i].resp = NULL;
switch (msg[i]-&gt;msg_style) {
case PAM_PROMPT_ECHO_OFF:
aresp[i].resp = strdup(getpass(msg[i]-&gt;msg));
if (aresp[i].resp == NULL)
goto fail;
break;
case PAM_PROMPT_ECHO_ON:
fputs(msg[i]-&gt;msg, stderr);
if (fgets(buf, sizeof buf, stdin) == NULL)
goto fail;
aresp[i].resp = strdup(buf);
if (aresp[i].resp == NULL)
goto fail;
break;
case PAM_ERROR_MSG:
fputs(msg[i]-&gt;msg, stderr);
if (strlen(msg[i]-&gt;msg) &gt; 0 &amp;&amp;
msg[i]-&gt;msg[strlen(msg[i]-&gt;msg) - 1] != '\n')
fputc('\n', stderr);
break;
case PAM_TEXT_INFO:
fputs(msg[i]-&gt;msg, stdout);
if (strlen(msg[i]-&gt;msg) &gt; 0 &amp;&amp;
msg[i]-&gt;msg[strlen(msg[i]-&gt;msg) - 1] != '\n')
fputc('\n', stdout);
break;
default:
goto fail;
}
}
*resp = aresp;
return (PAM_SUCCESS);
fail:
for (i = 0; i &lt; n; ++i) {
if (aresp[i].resp != NULL) {
memset(aresp[i].resp, 0, strlen(aresp[i].resp));
free(aresp[i].resp);
}
}
memset(aresp, 0, n * sizeof *aresp);
*resp = NULL;
return (PAM_CONV_ERR);
}
Further Reading
This is a list of documents relevant to PAM and related issues. It is by no means complete.
Papers
Making Login Services Independent of Authentication Technologies
<personname> <surname>Samar</surname> <firstname>Vipin</firstname> </personname>
<personname> <surname>Lai</surname> <firstname>Charlie</firstname> </personname>
Sun Microsystems
<link xlink:href="https://pubs.opengroup.org/onlinepubs/8329799/toc.htm">X/Open Single Sign-on Preliminary Specification</link>
The Open Group
1-85912-144-6
June 1997
<link xlink:href="https://mirrors.kernel.org/pub/linux/libs/pam/pre/doc/draft-morgan-pam-07.txt"> Pluggable Authentication Modules</link>
<personname> <surname>Morgan</surname> <firstname>Andrew</firstname> <othername role="mi">G.</othername> </personname>
1999-10-06
User Manuals
<link xlink:href="https://docs.oracle.com/cd/E26505_01/html/E27224/pam-1.html"> PAM Administration</link>
Related Web Pages
<link xlink:href="https://www.openpam.org/">OpenPAM homepage</link>
<personname> <surname>Smørgrav</surname> <firstname>Dag-Erling</firstname> </personname>
ThinkSec AS

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Translation file
articles/pam.pot, string 236