pam-krb5 4.11
(PAM module for Kerberos authentication)
Maintained by Russ Allbery <eagle@eyrie.org>
Copyright 2005-2010, 2014-2015, 2017, 2020-2021 Russ Allbery
<eagle@eyrie.org>. Copyright 2009-2011 The Board of Trustees of the
Leland Stanford Junior University. Copyright 2005 Andres Salomon
<dilinger@debian.org>. Copyright 1999-2000 Frank Cusack
<fcusack@fcusack.com>. This software is distributed under a BSD-style
license. Please see the section LICENSE below for more information.
BLURB
pam-krb5 is a Kerberos PAM module for either MIT Kerberos or Heimdal.
It supports ticket refreshing by screen savers, configurable
authorization handling, authentication of non-local accounts for network
services, password changing, and password expiration, as well as all the
standard expected PAM features. It works correctly with OpenSSH, even
with ChallengeResponseAuthentication and PrivilegeSeparation enabled,
and supports extensive configuration either by PAM options or in
krb5.conf or both. PKINIT is supported with recent versions of both MIT
Kerberos and Heimdal and FAST is supported with recent MIT Kerberos.
DESCRIPTION
pam-krb5 provides a Kerberos PAM module that supports authentication,
user ticket cache handling, simple authorization (via .k5login or
checking Kerberos principals against local usernames), and password
changing. It can be configured through either options in the PAM
configuration itself or through entries in the system krb5.conf file,
and it tries to work around PAM implementation flaws in commonly-used
PAM-enabled applications such as OpenSSH and xdm. It supports both
PKINIT and FAST to the extent that the underlying Kerberos libraries
support these features.
This is not the Kerberos PAM module maintained on Sourceforge and used
on Red Hat systems. It is an independent implementation that, if it
ever shared any common code, diverged long ago. It supports some
features that the Sourceforge module does not (particularly around
authorization), and does not support some options (particularly ones not
directly related to Kerberos) that it does. This module will never
support Kerberos v4 or AFS. For an AFS session module that works with
this module (or any other Kerberos PAM module), see pam-afs-session [1].
[1] https://www.eyrie.org/~eagle/software/pam-afs-session/
If there are other options besides AFS and Kerberos v4 support from the
Sourceforge PAM module that you're missing in this module, please let me
know.
REQUIREMENTS
Either MIT Kerberos (or Kerberos implementations based on it) or Heimdal
are supported. MIT Keberos 1.3 or later may be required; this module
has not been tested with earlier versions.
For PKINIT support, Heimdal 0.8rc1 or later or MIT Kerberos 1.6.3 or
later are required. Earlier MIT Kerberos 1.6 releases have a bug in
their handling of PKINIT options. MIT Kerberos 1.12 or later is
required to use the use_pkinit PAM option.
For FAST (Flexible Authentication Secure Tunneling) support, MIT
Kerberos 1.7 or higher is required. For anonymous FAST support,
anonymous authentication (generally anonymous PKINIT) support is
required in both the Kerberos libraries and in the local KDC.
This module should work on Linux and build with gcc or clang. It may
still work on Solaris and build with the Sun C compiler, but I have only
tested it on Linux recently. There is beta-quality support for the AIX
NAS Kerberos implementation that has not been tested in years. Other
PAM implementations will probably require some porting, although
untested build system support is present for FreeBSD, Mac OS X, and
HP-UX. I personally can only test on Linux and rely on others to report
problems on other operating systems.
Old versions of OpenSSH are known to call pam_authenticate followed by
pam_setcred(PAM_REINITIALIZE_CRED) without first calling
pam_open_session, thereby requesting that an existing ticket cache be
renewed (similar to what a screensaver would want) rather than
requesting a new ticket cache be created. Since this behavior is
indistinguishable at the PAM level from a screensaver, pam-krb5 when
used with these old versions of OpenSSH will refresh the ticket cache of
the OpenSSH daemon rather than setting up a new ticket cache for the
user. The resulting ticket cache will have the correct permissions
(this is not a security concern), but will not be named correctly or
referenced in the user's environment and will be overwritten by the next
user login. The best solution to this problem is to upgrade OpenSSH.
I'm not sure exactly when this problem was fixed, but at the very least
OpenSSH 4.3 and later do not exhibit it.
To bootstrap from a Git checkout, or if you change the Automake files
and need to regenerate Makefile.in, you will need Automake 1.11 or
later. For bootstrap or if you change configure.ac or any of the m4
files it includes and need to regenerate configure or config.h.in, you
will need Autoconf 2.64 or later. Perl is also required to generate
manual pages from a fresh Git checkout.
BUILDING AND INSTALLATION
You can build and install pam-krb5 with the standard commands:
./configure
make
make install
If you are building from a Git clone, first run ./bootstrap in the
source directory to generate the build files. make install will
probably have to be done as root. Building outside of the source
directory is also supported, if you wish, by creating an empty directory
and then running configure with the correct relative path.
The module will be installed in /usr/local/lib/security by default, but
expect to have to override this using --libdir. The correct
installation path for PAM modules varies considerably between systems.
The module will always be installed in a subdirectory named security
under the specified value of --libdir. On Red Hat Linux, for example,
--libdir=/usr/lib64 is appropriate to install the module into the system
PAM directory. On Debian's amd64 architecture,
--libdir=/usr/lib/x86_64-linux-gnu would be correct.
Normally, configure will use krb5-config to determine the flags to use
to compile with your Kerberos libraries. To specify a particular
krb5-config script to use, either set the PATH_KRB5_CONFIG environment
variable or pass it to configure like:
./configure PATH_KRB5_CONFIG=/path/to/krb5-config
If krb5-config isn't found, configure will look for the standard
Kerberos libraries in locations already searched by your compiler. If
the the krb5-config script first in your path is not the one
corresponding to the Kerberos libraries you want to use, or if your
Kerberos libraries and includes aren't in a location searched by default
by your compiler, you need to specify a different Kerberos installation
root via --with-krb5=PATH. For example:
./configure --with-krb5=/usr/pubsw
You can also individually set the paths to the include directory and the
library directory with --with-krb5-include and --with-krb5-lib. You may
need to do this if Autoconf can't figure out whether to use lib, lib32,
or lib64 on your platform.
To not use krb5-config and force library probing even if there is a
krb5-config script on your path, set PATH_KRB5_CONFIG to a nonexistent
path:
./configure PATH_KRB5_CONFIG=/nonexistent
krb5-config is not used and library probing is always done if either
--with-krb5-include or --with-krb5-lib are given.
Pass --enable-silent-rules to configure for a quieter build (similar to
the Linux kernel). Use make warnings instead of make to build with full
compiler warnings (requires either GCC or Clang and may require a
relatively current version of the compiler).
You can pass the --enable-reduced-depends flag to configure to try to
minimize the shared library dependencies encoded in the binaries. This
omits from the link line all the libraries included solely because other
libraries depend on them and instead links the programs only against
libraries whose APIs are called directly. This will only work with
shared libraries and will only work on platforms where shared libraries
properly encode their own dependencies (this includes most modern
platforms such as all Linux). It is intended primarily for building
packages for Linux distributions to avoid encoding unnecessary shared
library dependencies that make shared library migrations more difficult.
If none of the above made any sense to you, don't bother with this flag.
TESTING
pam-krb5 comes with a comprehensive test suite, but it requires some
configuration in order to test anything other than low-level utility
functions. For the full test suite, you will need to have a running KDC
in which you can create two test accounts, one with admin access to the
other. Using a test KDC environment, if you have one, is recommended.
Follow the instructions in tests/config/README to configure the test
suite.
Now, you can run the test suite with:
make check
If a test fails, you can run a single test with verbose output via:
tests/runtests -o <name-of-test>
Do this instead of running the test program directly since it will
ensure that necessary environment variables are set up.
The default libkadm5clnt library on the system must match the
implementation of your KDC for the module/expired test to work, since
the two kadmin protocols are not compatible. If you use the MIT library
against a Heimdal server, the test will be skipped; if you use the
Heimdal library against an MIT server, the test suite may hang.
Several module/expired tests are expected to fail with Heimdal 1.5 due
to a bug in Heimdal with reauthenticating immediately after a
library-mediated password change of an expired password. This is fixed
in later releases of Heimdal.
To run the full test suite, Perl 5.10 or later is required. The
following additional Perl modules will be used if present:
* Test::Pod
* Test::Spelling
All are available on CPAN. Those tests will be skipped if the modules
are not available.
To enable tests that don't detect functionality problems but are used to
sanity-check the release, set the environment variable RELEASE_TESTING
to a true value. To enable tests that may be sensitive to the local
environment or that produce a lot of false positives without uncovering
many problems, set the environment variable AUTHOR_TESTING to a true
value.
CONFIGURING
Just installing the module does not enable it or change anything about
your system authentication configuration. To use the module for all
system authentication on Debian systems, put something like:
auth sufficient pam_krb5.so minimum_uid=1000
auth required pam_unix.so try_first_pass nullok_secure
in /etc/pam.d/common-auth, something like:
session optional pam_krb5.so minimum_uid=1000
session required pam_unix.so
in /etc/pam.d/common-session, and something like:
account required pam_krb5.so minimum_uid=1000
account required pam_unix.so
in /etc/pam.d/common-account. The minimum_uid setting tells the PAM
module to pass on any users with a UID lower than 1000, thereby
bypassing Kerberos authentication for the root account and any system
accounts. You normally want to do this since otherwise, if the network
is down, the Kerberos authentication can time out and make it difficult
to log in as root and fix matters. This also avoids problems with
Kerberos principals that happen to match system accounts accidentally
getting access to those accounts.
Be sure to include the module in the session group as well as the auth
group. Without the session entry, the user's ticket cache will not be
created properly for ssh logins (among possibly others).
If your users should normally all use Kerberos passwords exclusively,
putting something like:
password sufficient pam_krb5.so minimum_uid=1000
password required pam_unix.so try_first_pass obscure md5
in /etc/pam.d/common-password will change users' passwords in Kerberos
by default and then only fall back on Unix if that doesn't work. (You
can make this tighter by using the more complex new-style PAM
configuration.) If you instead want to synchronize local and Kerberos
passwords and change them both at the same time, you can do something
like:
password required pam_unix.so obscure sha512
password required pam_krb5.so use_authtok minimum_uid=1000
If you have multiple environments that you want to synchronize and you
don't want password changes to continue if the Kerberos password change
fails, use the clear_on_fail option. For example:
password required pam_krb5.so clear_on_fail minimum_uid=1000
password required pam_unix.so use_authtok obscure sha512
password required pam_smbpass.so use_authtok
In this case, if pam_krb5 cannot change the password (due to password
strength rules on the KDC, for example), it will clear the stored
password (because of the clear_on_fail option), and since pam_unix and
pam_smbpass are both configured with use_authtok, they will both fail.
clear_on_fail is not the default because it would interfere with the
more common pattern of falling back to local passwords if the user
doesn't exist in Kerberos.
If you use a more complex configuration with the Linux PAM [] syntax for
the session and account groups, note that pam_krb5 returns a status of
ignore, not success, if the user didn't log on with Kerberos. You may
need to handle that explicitly with ignore=ignore in your action list.
There are many, many other possibilities. See the Linux PAM
documentation for all the configuration options.
On Red Hat systems, modify /etc/pam.d/system-auth instead, which
contains all of the configuration for the different stacks.
You can also use pam-krb5 only for specific services. In that case,
modify the files in /etc/pam.d for that particular service to use
pam_krb5.so for authentication. For services that are using passwords
over TLS to authenticate users, you may want to use the ignore_k5login
and no_ccache options to the authenticate module. .k5login
authorization is only meaningful for local accounts and ticket caches
are usually (although not always) only useful for interactive sessions.
Configuring the module for Solaris is both simpler and less flexible,
since Solaris (at least Solaris 8 and 9, which are the last versions of
Solaris with which this module was extensively tested) use a single
/etc/pam.conf file that contains configuration for all programs. For
console login on Solaris, try something like:
login auth sufficient /usr/local/lib/security/pam_krb5.so minimum_uid=100
login auth required /usr/lib/security/pam_unix_auth.so.1 use_first_pass
login account required /usr/local/lib/security/pam_krb5.so minimum_uid=100
login account required /usr/lib/security/pam_unix_account.so.1
login session required /usr/local/lib/security/pam_krb5.so retain_after_close minimum_uid=100
login session required /usr/lib/security/pam_unix_session.so.1
A similar configuration could be used for other services, such as ssh.
See the pam.conf(5) man page for more information. When using this
module with Solaris login (at least on Solaris 8 and 9), you will
probably also need to add retain_after_close to the PAM configuration to
avoid having the user's credentials deleted before they are logged in.
The Solaris Kerberos library reportedly does not support prompting for a
password change of an expired account during authentication. Supporting
password change for expired accounts on Solaris with native Kerberos may
therefore require setting the defer_pwchange or force_pwchange option
for selected login applications. See the description and warnings about
that option in the pam_krb5(5) man page.
Some configuration options may be put in the krb5.conf file used by your
Kerberos libraries (usually /etc/krb5.conf or /usr/local/etc/krb5.conf)
instead or in addition to the PAM configuration. See the man page for
more details.
The Kerberos library, via pam-krb5, will prompt the user to change their
password if their password is expired, but when using OpenSSH, this will
only work when ChallengeResponseAuthentication is enabled. Unless this
option is enabled, OpenSSH doesn't pass PAM messages to the user and can
only respond to a simple password prompt.
If you are using MIT Kerberos, be aware that users whose passwords are
expired will not be prompted to change their password unless the KDC
configuration for your realm in [realms] in krb5.conf contains a
master_kdc setting or, if using DNS SRV records, you have a DNS entry
for _kerberos-master as well as _kerberos.
DEBUGGING
The first step when debugging any problems with this module is to add
debug to the PAM options for the module (either in the PAM configuration
or in krb5.conf). This will significantly increase the logging from the
module and should provide a trace of exactly what failed and any
available error information.
Many Kerberos authentication problems are due to configuration issues in
krb5.conf. If pam-krb5 doesn't work, first check that kinit works on
the same system. That will test your basic Kerberos configuration. If
the system has a keytab file installed that's readable by the process
doing authentication via PAM, make sure that the keytab is current and
contains a key for host/<system> where <system> is the fully-qualified
hostname. pam-krb5 prevents KDC spoofing by checking the user's
credentials when possible, but this means that if a keytab is present it
must be correct or authentication will fail. You can check the keytab
with klist -k and kinit -k.
Be sure that all libraries and modules, including PAM modules, loaded by
a program use the same Kerberos libraries. Sometimes programs that use
PAM, such as current versions of OpenSSH, also link against Kerberos
directly. If your sshd is linked against one set of Kerberos libraries
and pam-krb5 is linked against a different set of Kerberos libraries,
this will often cause problems (such as segmentation faults, bus errors,
assertions, or other strange behavior). Similar issues apply to the
com_err library or any other library used by both modules and shared
libraries and by the application that loads them. If your OS ships
Kerberos libraries, it's usually best if possible to build all Kerberos
software on the system against those libraries.
IMPLEMENTATION NOTES
The normal sequence of actions taken for a user login is:
pam_authenticate
pam_setcred(PAM_ESTABLISH_CRED)
pam_open_session
pam_acct_mgmt
and then at logout:
pam_close_session
followed by closing the open PAM session. The corresponding pam_sm_*
functions in this module are called when an application calls those
public interface functions. Not all applications call all of those
functions, or in particularly that order, although pam_authenticate is
always first and has to be.
When pam_authenticate is called, pam-krb5 creates a temporary ticket
cache in /tmp and sets the PAM environment variable PAM_KRB5CCNAME to
point to it. This ticket cache will be automatically destroyed when the
PAM session is closed and is there only to pass the initial credentials
to the call to pam_setcred. The module would use a memory cache, but
memory caches will only work if the application preserves the PAM
environment between the calls to pam_authenticate and pam_setcred. Most
do, but OpenSSH notoriously does not and calls pam_authenticate in a
subprocess, so this method is used to pass the tickets to the
pam_setcred call in a different process.
pam_authenticate does a complete authentication, including checking the
resulting TGT by obtaining a service ticket for the local host if
possible, but this requires read access to the system keytab. If the
keytab doesn't exist, can't be read, or doesn't include the appropriate
credentials, the default is to accept the authentication. This can be
controlled by setting verify_ap_req_nofail to true in [libdefaults] in
/etc/krb5.conf. pam_authenticate also does a basic authorization check,
by default calling krb5_kuserok (which uses ~/.k5login if available and
falls back to checking that the principal corresponds to the account
name). This can be customized with several options documented in the
pam_krb5(5) man page.
pam-krb5 treats pam_open_session and pam_setcred(PAM_ESTABLISH_CRED) as
synonymous, as some applications call one and some call the other. Both
copy the initial credentials from the temporary cache into a permanent
cache for this session and set KRB5CCNAME in the environment. It will
remember when the credential cache has been established and then avoid
doing any duplicate work afterwards, since some applications call
pam_setcred or pam_open_session multiple times (most notably X.Org 7 and
earlier xdm, which also throws away the module settings the last time it
calls them).
pam_acct_mgmt finds the ticket cache, reads it in to obtain the
authenticated principal, and then does is another authorization check
against .k5login or the local account name as described above.
After the call to pam_setcred or pam_open_session, the ticket cache will
be destroyed whenever the calling application either destroys the PAM
environment or calls pam_close_session, which it should do on user
logout.
The normal sequence of events when refreshing a ticket cache (such as
inside a screensaver) is:
pam_authenticate
pam_setcred(PAM_REINITIALIZE_CRED)
pam_acct_mgmt
(PAM_REFRESH_CRED may be used instead.) Authentication proceeds as
above. At the pam_setcred stage, rather than creating a new ticket
cache, the module instead finds the current ticket cache (from the
KRB5CCNAME environment variable or the default ticket cache location
from the Kerberos library) and then reinitializes it with the
credentials from the temporary pam_authenticate ticket cache. When
refreshing a ticket cache, the application should not open a session.
Calling pam_acct_mgmt is optional; pam-krb5 doesn't do anything
different when it's called in this case.
If pam_authenticate apparently didn't succeed, or if an account was
configured to be ignored via ignore_root or minimum_uid, pam_setcred
(and therefore pam_open_session) and pam_acct_mgmt return PAM_IGNORE,
which tells the PAM library to proceed as if that module wasn't listed
in the PAM configuration at all. pam_authenticate, however, returns
failure in the ignored user case by default, since otherwise a
configuration using ignore_root with pam-krb5 as the only PAM module
would allow anyone to log in as root without a password. There doesn't
appear to be a case where returning PAM_IGNORE instead would improve the
module's behavior, but if you know of a case, please let me know.
By default, pam_authenticate intentionally does not follow the PAM
standard for handling expired accounts and instead returns failure from
pam_authenticate unless the Kerberos libraries are able to change the
account password during authentication. Too many applications either do
not call pam_acct_mgmt or ignore its exit status. The fully correct PAM
behavior (returning success from pam_authenticate and
PAM_NEW_AUTHTOK_REQD from pam_acct_mgmt) can be enabled with the
defer_pwchange option.
The defer_pwchange option is unfortunately somewhat tricky to implement.
In this case, the calling sequence is:
pam_authenticate
pam_acct_mgmt
pam_chauthtok
pam_setcred
pam_open_session
During the first pam_authenticate, we can't obtain credentials and
therefore a ticket cache since the password is expired. But
pam_authenticate isn't called again after pam_chauthtok, so
pam_chauthtok has to create a ticket cache. We however don't want it to
do this for the normal password change (passwd) case.
What we do is set a flag in our PAM data structure saying that we're
processing an expired password, and pam_chauthtok, if it sees that flag,
redoes the authentication with password prompting disabled after it
finishes changing the password.
Unfortunately, when handling password changes this way, pam_chauthtok
will always have to prompt the user for their current password again
even though they just typed it. This is because the saved
authentication tokens are cleared after pam_authenticate returns, for
security reasons. We could hack around this by saving the password in
our PAM data structure, but this would let the application gain access
to it (exactly what the clearing is intended to prevent) and breaks a
PAM library guarantee. We could also work around this by having
pam_authenticate get the kadmin/changepw authenticator in the expired
password case and store it for pam_chauthtok, but it doesn't seem worth
the hassle.
HISTORY AND ACKNOWLEDGEMENTS
Originally written by Frank Cusack <fcusack@fcusack.com>, with the
following acknowledgement:
Thanks to Naomaru Itoi <itoi@eecs.umich.edu>, Curtis King
<curtis.king@cul.ca>, and Derrick Brashear <shadow@dementia.org>, all
of whom have written and made available Kerberos 4/5 modules.
Although no code in this module is directly from these author's
modules, (except the get_user_info() routine in support.c; derived
from whichever of these authors originally wrote the first module the
other 2 copied from), it was extremely helpful to look over their code
which aided in my design.
The module was then patched for the FreeBSD ports collection with
additional modifications by unknown maintainers and then was modified by
Joel Kociolek <joko@logidee.com> to be usable with Debian GNU/Linux.
It was packaged by Sam Hartman as the Kerberos v5 PAM module for Debian
and improved and modified by him and later by Russ Allbery to fix bugs
and add additional features. It was then adopted by Andres Salomon, who
added support for refreshing credentials.
The current distribution is maintained by Russ Allbery, who also added
support for reading configuration from krb5.conf, added many features
for compatibility with the Sourceforge module, commented and
standardized the formatting of the code, and overhauled the
documentation.
Thanks to Douglas E. Engert for the initial implementation of PKINIT
support. I have since modified and reworked it extensively, so any bugs
or compilation problems are my fault.
Thanks to Markus Moeller for lots of debugging and multiple patches and
suggestions for improved portability.
Thanks to Booker Bense for the implementation of the alt_auth_map
option.
Thanks to Sam Hartman for the FAST support implementation.
SUPPORT
The pam-krb5 web page at:
https://www.eyrie.org/~eagle/software/pam-krb5/
will always have the current version of this package, the current
documentation, and pointers to any additional resources.
For bug tracking, use the issue tracker on GitHub:
https://github.com/rra/pam-krb5/issues
However, please be aware that I tend to be extremely busy and work
projects often take priority. I'll save your report and get to it as
soon as I can, but it may take me a couple of months.
SOURCE REPOSITORY
pam-krb5 is maintained using Git. You can access the current source on
GitHub at:
https://github.com/rra/pam-krb5
or by cloning the repository at:
https://git.eyrie.org/git/kerberos/pam-krb5.git
or view the repository via the web at:
https://git.eyrie.org/?p=kerberos/pam-krb5.git
The eyrie.org repository is the canonical one, maintained by the author,
but using GitHub is probably more convenient for most purposes. Pull
requests are gratefully reviewed and normally accepted.
LICENSE
The pam-krb5 package as a whole is covered by the following copyright
statement and license:
Copyright 2005-2010, 2014-2015, 2017, 2020-2021
Russ Allbery <eagle@eyrie.org>
Copyright 2009-2011
The Board of Trustees of the Leland Stanford Junior University
Copyright 2005 Andres Salomon <dilinger@debian.org>
Copyright 1999-2000 Frank Cusack <fcusack@fcusack.com>
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, and the entire permission notice in its entirety, including
the disclaimer of warranties.
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.
ALTERNATIVELY, this product may be distributed under the terms of the
GNU General Public License, in which case the provisions of the GPL
are required INSTEAD OF the above restrictions. (This clause is
necessary due to a potential bad interaction between the GPL and the
restrictions contained in a BSD-style copyright.)
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