1PAM_AFS_SESSION(5)              pam-afs-session             PAM_AFS_SESSION(5)
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NAME

6       pam_afs_session - AFS PAG and token PAM module
7

SYNOPSIS

9         auth          optional        pam_afs_session.so
10         session       required        pam_afs_session.so
11

DESCRIPTION

13       The AFS session service module for PAM, typically installed at
14       /lib/security/pam_afs_session.so, establishes new AFS sessions and
15       obtains AFS tokens when a new session is opened for a user.  It is a
16       shared object that is dynamically loaded by the PAM subsystem as
17       necessary, based on the system PAM configuration.  PAM is a system for
18       plugging in external authentication and session management modules so
19       that each application doesn't have to know the best way to check user
20       authentication or create a user session on that system.  For details on
21       how to configure PAM on your system, see the PAM man page, often
22       pam(7).
23
24       This module provides pam_setcred, pam_open_session, and
25       pam_close_session implementations for AFS.  Because pam_setcred is part
26       of the auth PAM group, it also implements a dummy pam_authenticate that
27       always succeeds (otherwise, it can't provide a pam_setcred).
28
29       Make sure that this module is NEVER listed as "sufficient" or as the
30       only "required" module in the auth group.  Doing so will potentially
31       allow users to log on without any password.  There unfortunately isn't
32       a way to work around this and still provide pam_setcred without running
33       afoul of a bug in (at least) Linux PAM 0.99.7.1 and probably earlier
34       that causes authentication to fail when the final module in the auth
35       group returns PAM_IGNORE and "[default=done]" was given as the action.
36
37       Here are the actions of this module:
38
39       pam_open_session
40           When a new session is opened, this module will first check to see
41           if AFS is running on the system.  If not, it will log a message and
42           exit successfully.  If AFS is running, it will place the user's
43           session in a new PAG (Process Authentication Group, often
44           implemented as supplemental groups, which limits user tokens to
45           only processes in that PAG).  It will then attempt to obtain
46           tokens, either directly if built with the Heimdal libkafs library
47           and Kerberos support or by running an external aklog program.  If
48           PAG creation fails, the module will fail; if obtaining tokens
49           fails, the module will log a warning but still return success.
50
51           The module will only attempt to obtain tokens if the environment
52           variable KRB5CCNAME is set in the environment, unless otherwise
53           configured (see the always_aklog option).  It will always create a
54           new PAG, however.
55
56       pam_close_session
57           If and only if pam_open_session successfully obtained AFS tokens
58           and AFS is still running on the system, pam_close_session will
59           delete the tokens in the current PAG (equivalent to running unlog).
60           To leave the tokens after session close, set the retain_after_close
61           option.
62
63       pam_setcred
64           When pam_setcred is called with the PAM_ESTABLISH_CRED flag, it
65           will do the same as if pam_open_session was called.  When
66           pam_setcred is called with the PAM_DELETE_CRED flag, it will do the
67           same as if pam_close_session was called.  When called with the
68           PAM_REINITIALIZE_CRED flag or the PAM_REFRESH_CRED flag, it won't
69           create a new PAG but instead will only attempt to get new tokens
70           (still skipping this unless KRB5CCNAME is set in the environment or
71           always_aklog is set).
72
73       This module is primarily intended for use with a Kerberos v5
74       authentication module.  It does not itself do any user authentication;
75       it cannot, for instance, be used to authenticate users to a kaserver.
76       While it is intended for use with an aklog that uses Kerberos v5 ticket
77       caches to obtain tokens, it can be used with any aklog implementation
78       (always_aklog may have to be set if no Kerberos v5 ticket cache will be
79       present).
80
81       This module performs no authorization checks and does not hook into
82       password changes; it only implements the session functions and
83       pam_setcred.
84

CONFIGURATION

86       The AFS session PAM module supports the following configuration
87       options, which may be set in the PAM configuration as arguments listed
88       after "pam_afs_session.so" or in the system krb5.conf.
89
90       Some of them take arguments, in which case the argument will be given
91       after "=".  The rest are boolean options.  To set a boolean option in
92       the PAM configuration, just give the name of the option in the
93       arguments.  To set an option that takes an argument, follow the option
94       name with an equal sign ("=") and the value, with no separating
95       whitespace.  Whitespace in option arguments is not supported in the PAM
96       configuration files of most PAM implementations.
97
98       To set an option for the PAM module in the system krb5.conf file, put
99       that option in the [appdefaults] section.  The AFS session PAM module
100       will look for options either at the top level of the [appdefaults]
101       section or in a subsection named "pam-afs-session" (currently, realm-
102       specific configuration is not checked).  For example, the following
103       fragment of a krb5.conf file would set aklog_homedir to true and
104       minimum_uid to 100.
105
106           [appdefaults]
107               aklog_homedir = true
108               pam-afs-session = {
109                   minimum_uid = 100
110               }
111
112       There is no difference to the PAM module whether options are specified
113       at the top level or in a "pam-afs-session" section; the
114       "pam-afs-session" section is supported in case there are options that
115       should be set for the PAM module but not for other applications.  For
116       more information on the syntax of krb5.conf, see krb5.conf(5).
117
118       If the same option is set in krb5.conf and in the PAM configuration,
119       the latter takes precedent.  Note, however, that due to the
120       configuration syntax, there's no way to turn off a boolean option in
121       the PAM configuration that was turned on in krb5.conf.
122
123       afs_cells=cell[,cell...]
124           Obtain tokens for the listed cells instead of the default local
125           cell.  If more than one cell is listed, try to obtain tokens for
126           each cell.  If specified in krb5.conf, the cells can be separated
127           by any combination of spaces and commas; if specified in the PAM
128           configuration, they must be separated by commas.
129
130           If the AFS session PAM module is running an external program, this
131           option is implemented by passing a -c flag with the cell as its
132           argument for each listed cell to that program.  If aklog_homedir is
133           also set, the -c flags and the -p flag will all be passed to the
134           external program.
135
136       aklog_homedir
137           Try to obtain the necessary tokens to access the user's home
138           directory.  If the libkafs token-obtaining API is used, setting
139           this will cause the AFS session PAM module to pass the user's home
140           directory into that API and request that the appropriate tokens be
141           obtained.  If running an external aklog program, aklog will be
142           called with -p home-directory where home-directory is the home
143           directory of the local user for which the session is being opened
144           or refreshed.  This generally will tell aklog to check that path,
145           find all AFS cells involved in access to that path, and attempt to
146           obtain tokens for each one.  Note that this means that if the
147           user's home directory is not in AFS, no tokens will be obtained.
148
149           In either case, the user's home directory is obtained via
150           getpwnam() based on the username PAM says we are authenticating.
151
152       always_aklog
153           Normally, the AFS session PAM module only tries to obtain tokens if
154           KRB5CCNAME is set in the PAM environment.  If this option is set,
155           it will always attempt to obtain tokens.  This is only useful if it
156           is configured to run an external aklog program.
157
158           This can be used if your environment doesn't correctly set
159           KRB5CCNAME in the environment for some reason, or if your aklog
160           doesn't rely on a Kerberos ticket cache to obtain tokens (or can
161           find the cache on its own via some other means).
162
163       debug
164           If this option is set, additional trace information will be logged
165           to syslog with priority LOG_DEBUG.
166
167       ignore_root
168           If this option is set, the AFS session PAM module won't take any
169           action (and will exit successfully) if the account for which the
170           session is being established is named "root".
171
172       kdestroy
173           If this option is set and the AFS session PAM module was built with
174           Kerberos support, the user's ticket cache will be destroyed after
175           tokens are obtained successfully.  If tokens are not obtained
176           successfully, the ticket cache will be left intact.  Please note
177           that this is not properly a security feature, since the ticket
178           cache will still be written to disk between the time the Kerberos
179           PAM module authenticates the user and the time the AFS session PAM
180           module is run.  It can, however, be used to reduce the window
181           during which Kerberos ticket caches are lying about if the only use
182           one has for ticket caches is to obtain AFS tokens.
183
184       minimum_uid=uid
185           If this option is set, the AFS session PAM module won't take any
186           action (and will exit successfully) if the account for which the
187           session is being established has a UID lower than uid.
188
189       nopag
190           If this option is set, no PAG will be created.  Be careful when
191           using this option, since it means that the user will inherit a PAG
192           from the process managing the login.  If sshd, for instance, is
193           started in a PAG, every user who logs in via ssh will be put in the
194           same PAG and will share tokens if this option is used.
195
196           This is the default on Mac OS X, where PAGs are not supported.
197
198       notokens
199           If this option is set, the AFS session PAM module will only create
200           a PAG and not attempt to obtain tokens.  Setting this option
201           overrides all other settings related to acquiring tokens, including
202           always_aklog.  If both nopag and notokens are set, the module
203           essentially does nothing.
204
205           Setting notokens also implies retain_after_close, meaning that the
206           AFS session PAM module will also not attempt to delete tokens when
207           the user's session ends.
208
209       program=path
210           The path to the aklog program to run.  Setting this option tells
211           the AFS session PAM module to always run an external program to
212           obtain tokens and never use the libkafs interface, even if the
213           latter is available.
214
215           You may pass options to this program by separating them with commas
216           (or spaces or tabs in krb5.conf or if the configuration syntax of
217           your PAM configuration allows this).  For example, the setting:
218
219               program=/usr/bin/aklog,-noprdb,-524
220
221           will run "/usr/bin/aklog -noprdb -524" as the program to obtain
222           tokens.  The arguments are passed directly, not parsed by the
223           shell.
224
225           If this option is not set, the default behavior is to call the
226           libkafs function to obtain tokens, if available, and otherwise to
227           use a default path to aklog determined at compile time (the first
228           aklog found on the compiler's path by default).  If no aklog could
229           be found at compile time and libkafs isn't used, this option must
230           be set.
231
232       retain_after_close
233           If this option is set, pam_close_session will do nothing
234           (successfully) rather than deleting tokens.  This will allow
235           programs started in the user's PAG that are still running when the
236           log out to continue to use the user's tokens until they expire.
237           Normally, the AFS kernel module will automatically clean up tokens
238           once every process in that PAG has terminated.
239

ENVIRONMENT

241       KRB5CCNAME
242           This module looks for KRB5CCNAME in the PAM environment and by
243           default does not run aklog if it is not set.
244
245       The entire PAM environment is passed to aklog as its environment
246       (rather than the environment of the process running the PAM functions).
247

WARNINGS

249       As mentioned above, this module implements a dummy pam_authenticate
250       function so that it can provide a pam_setcred function.  Never list
251       this module as "sufficient" or as the only "required" module or you may
252       allow users to log on without a password.
253
254       While spawning an external aklog program, the AFS session PAM module
255       resets the SIGCHLD signal handler to the default handler while the
256       program runs and then restores it afterward.  This is done to avoid
257       having aklog interfere with process handling in the calling
258       application, but it means that there's a race condition that can cause
259       children to be incorrectly handled if they exit while aklog is running.
260       There is unfortunately no good solution to this other than building
261       against Heimdal and using the libkafs interface to obtain tokens
262       instead of an external program.
263
264       To detect whether AFS is running on the system, the AFS session PAM
265       module temporarily sets a SIGSYS handler before attempting an AFS
266       system call.  That handler may also modify a static variable.  Neither
267       of these should ideally be done in a PAM module, but there is no other
268       good way of checking for the non-existence of a system call that
269       doesn't crash the application on some operating systems.  The PAM
270       module will attempt to restore the previous SIGSYS handler, if any,
271       after the test is done, and the static variable is used in such a way
272       that running it from multiple threads shouldn't be an issue, but be
273       aware that the PAM module is doing this behind the back of the
274       application and may interfere with unusual SIGSYS handlers or similar
275       application actions.
276

NOTES

278       When using the libkafs interface to obtain tokens, be sure that it is
279       configured properly for the type of AFS tokens expected at your site.
280       As of Heimdal 0.7, the default behavior is to contact the krb524
281       service to translate Kerberos v5 tickets into Kerberos v4 tickets to
282       use as tokens.  AFS cells running current server software no longer
283       need this, and if your site doesn't run the krb524 service, this may
284       break token acquisition.
285
286       Sites running AFS servers that understand Kerberos-v5-derived tokens
287       should add configuration like:
288
289           libkafs = {
290               EXAMPLE.ORG = {
291                   afs-use-524 = no
292               }
293           }
294
295       to the [appdefaults] section of their krb5.conf files to disable use of
296       the krb524 service.  See the Heimdal kafs(3) man page for more
297       information.
298

SEE ALSO

300       aklog(1), kafs(3), pam(7), syslog(3), unlog(1)
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3042.5                               2011-07-25                PAM_AFS_SESSION(5)
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