2 (Kerberos password strength checking plugin)
4 Maintained by Russ Allbery <rra@stanford.edu>
6 Copyright 2006, 2007, 2009, 2010, 2012, 2013 The Board of Trustees of
7 the Leland Stanford Junior University. Portions copyright 1993 Alec
8 Muffett. Developed by Derrick Brashear and Ken Hornstein of Sine Nomine
9 Associates, on behalf of Stanford University.
11 This software is distributed under a BSD-style license and under the
12 Artistic License. Please see the section LICENSE for more information.
16 krb5-strength provides mechanisms for checking the strength of Kerberos
17 passwords against an external dictionary when a user changes passwords
18 in a Kerberos KDC. It is roughly equivalent to checking password
19 strength via CrackLib, except that it embeds a copy of Alec Muffett's
20 CrackLib that has been modified to perform slightly more strenuous
21 tests. It supports both Heimdal and MIT Kerberos (1.9 or later).
25 Heimdal includes a capability to plug in external password quality
26 checks and comes with an example that checks passwords against CrackLib.
27 However, in testing at Stanford, we found that CrackLib with its default
28 transform rules does not catch passwords that can be guessed using the
29 same dictionary with other tools, such as Jack the Ripper.
31 This plugin provides the ability to check password quality against the
32 standard version of CrackLib, or against a modified version of CrackLib
33 that only passes passwords that resist attacks from both Crack and Jack
34 the Ripper using the same rule sets. For Heimdal, it includes both a
35 program usable as an external password quality check and a plugin that
36 implements the dynamic module API. For MIT Kerberos (1.9 or later), it
37 includes a plugin for the password quality (pwqual) plugin API.
39 krb5-strength can be built with either the system CrackLib or with the
40 modified version of CrackLib included in this package. Note, however,
41 that if you're building against the system CrackLib, Heimdal includes in
42 the distribution a strength-checking plugin and an external password
43 check program that use the system CrackLib. With Heimdal, it would
44 probably be easier to use that plugin or program than build this package
45 unless you want the modified CrackLib.
47 For information about the changes to the CrackLib included in this
48 toolkit, see cracklib/HISTORY. The primary changes are tighter rules,
49 which are more aggressive at finding dictionary words with characters
50 appended and prepended, which tighten the requirements for password
51 entropy, and which add stricter rules for longer passwords. They are
52 also minor changes to fix portability issues and remove some code that
53 doesn't make sense in the kadmind context.
55 Ideally, the changes to CrackLib should be added to the standard
56 CrackLib distribution by adding an additional interface to configure its
57 behavior, at which point this package can likely wither away in favor of
58 much simpler plugins that link to the standard CrackLib library.
62 For Heimdal, you may use either the external password quality check
63 tool, installed as heimdal-strength, or the plugin as you choose. It
64 has been tested with Heimdal 1.2.1 and later, but has not recently been
65 tested with versions prior to 1.5.
67 For MIT Kerberos, version 1.9 or higher is required for the password
68 quality plugin interface. MIT Kerberos does not support an external
69 password quality check tool directly, so you will need to install the
72 You can optionally build against the system CrackLib library. Any
73 version should be supported, but note that some versions, particularly
74 older versions close to the original code, do things like printing
75 diagnostics to stderr, calling exit, and otherwise not being
76 well-behaved for use inside plugins or libraries. If using a system
77 CrackLib library, use version 2.8.22 or later to avoid these problems.
79 For this module to be effective for either Heimdal or MIT Kerberos, you
80 will also need to construct a dictionary. The mkdict and packer
81 utilities to build a CrackLib dictionary from a word list are included
82 in this toolkit but not installed by default. You can run them out of
83 the cracklib directory after building. You can also use the utilities
84 that come with the stock CrackLib package (often already packaged in a
85 Linux distribution); the database format is compatible.
87 For a word list to use as source for the dictionary, you can use
88 /usr/share/dict/words if it's available on your system, but it would be
89 better to find a more comprehensive word list. Since word lists are
90 bulky, often covered by murky copyrights, and easily locatable on the
91 Internet with a modicum of searching, none are included in this toolkit.
93 To bootstrap from a Git checkout, or If you change the Automake files
94 and need to regenerate Makefile.in, you will need Automake 1.11 or
95 later. For bootstrap or if you change configure.ac or any of the m4
96 files it includes and need to regenerate configure or config.h.in, you
97 will need Autoconf 2.64 or later.
99 COMPILING AND INSTALLING
101 You can build and install the plugin with the standard commands:
107 Pass --enable-silent-rules to configure for a quieter build (similar to
108 the Linux kernel). Use make warnings instead of make to build with full
109 GCC compiler warnings (requires a relatively current version of GCC).
111 The last step will probably have to be done as root. By default, the
112 plugin is installed as /usr/local/lib/kadmind/passwd_strength.so and the
113 Heimdal external password check function is installed as
114 /usr/local/bin/heimdal-strength. You can change these paths with the
115 --prefix, --libdir, and --bindir options to configure.
117 To build with the system version of CrackLib, pass --with-cracklib to
118 configure. You can optionally add a directory, giving the root
119 directory where CrackLib was installed, or separately set the include
120 and library path with --with-cracklib-include and --with-cracklib-lib.
122 Normally, configure will use krb5-config to determine the flags to use
123 to compile with your Kerberos libraries. If krb5-config isn't found, it
124 will look for the standard Kerberos libraries in locations already
125 searched by your compiler. If the the krb5-config script first in your
126 path is not the one corresponding to the Kerberos libraries you want to
127 use or if your Kerberos libraries and includes aren't in a location
128 searched by default by your compiler, you need to specify a different
129 Kerberos installation root via --with-krb5=PATH. For example:
131 ./configure --with-krb5=/usr/pubsw
133 You can also individually set the paths to the include directory and the
134 library directory with --with-krb5-include and --with-krb5-lib. You may
135 need to do this if Autoconf can't figure out whether to use lib, lib32,
136 or lib64 on your platform.
138 To specify a particular krb5-config script to use, either set the
139 PATH_KRB5_CONFIG environment variable or pass it to configure like:
141 ./configure PATH_KRB5_CONFIG=/path/to/krb5-config
143 To not use krb5-config and force library probing even if there is a
144 krb5-config script on your path, set PATH_KRB5_CONFIG to a nonexistent
147 ./configure PATH_KRB5_CONFIG=/nonexistent
149 krb5-config is not used and library probing is always done if either
150 --with-krb5-include or --with-krb5-lib are given.
152 You can pass the --enable-reduced-depends flag to configure to try to
153 minimize the shared library dependencies encoded in the binaries. This
154 omits from the link line all the libraries included solely because the
155 Kerberos libraries depend on them and instead links the programs only
156 against libraries whose APIs are called directly. This will only work
157 with shared Kerberos libraries and will only work on platforms where
158 shared libraries properly encode their own dependencies (such as Linux).
159 It is intended primarily for building packages for Linux distributions
160 to avoid encoding unnecessary shared library dependencies that make
161 shared library migrations more difficult. If none of the above made any
162 sense to you, don't bother with this flag.
166 First, build and install a CrackLib dictionary as described above. This
167 dictionary will consist of three files, one each ending in *.hwm, *.pwd,
168 and *.pwi. Install those files somewhere on your system. Then, follow
169 the relevant instructions below for either Heimdal or MIT Kerberos.
173 There are two options: using an external password check program, or
174 using the plugin. I recommend the external password check program
175 unless you encounter speed problems with that approach that cause
178 For either approach, first add a stanza like the following to the
179 [appdefaults] section of your /etc/krb5.conf (or wherever your krb5.conf
183 password_dictionary = /usr/local/lib/kadmind/dictionary
186 The provided path should be the full path to the dictionary files,
187 omitting the trailing *.hwm, *.pwd, and *.pwi extensions.
189 Then, for the external password checking program, add a new section (or
190 modify the existing [password_quality] section) to look like the
194 policies = external-check
195 external_program = /usr/local/bin/heimdal-strength
197 You can, of course, combine this policy with others. Replace the path
198 with the full path to wherever you have installed heimdal-strength. You
199 can put this section in your kdc.conf instead of krb5.conf if you
202 If you want to instead use the module, use the following section
206 policies = krb5-strength
207 policy_libraries = /usr/local/lib/kadmind/passwd_strength.so
209 in either krb5.conf or kdc.conf. Note that some older versions of
210 Heimdal have a bug in the support for loading modules when
211 policy_libraries is set. If you get an error like:
213 didn't find `kadm5_password_verifier' symbol in `(null)'
215 you may have to omit policy_libraries in your configuration and instead
216 pass the --check-library argument to kpasswdd specifying the library to
221 To add this module to the list of password quality checks, add a section
222 to krb5.conf (or to a separate kdc.conf if you use that) like:
226 module = strength:/usr/local/lib/kadmind/passwd_strength.so
229 to register the plugin.
231 There are two ways to tell where the dictionary is. One option is to
232 use krb5.conf (and in this case you must use krb5.conf, even if you use
233 a separate kdc.conf file). For this approach, add the following to the
234 [appdefaults] section:
237 password_dictionary = /path/to/cracklib/dictionary
240 The second option is to use the normal dict_path setting. In the
241 [realms] section of your krb5.conf kdc.conf, under the appropriate realm
242 or realms, specify the path to the dictionary:
244 dict_file = /path/to/cracklib/dictionary
246 The provided path should be the full path to the dictionary files,
247 omitting the trailing *.hwm, *.pwd, or *.pwi extension. However, be
248 aware that, if you use this approach, you will probably want to disable
249 the built-in standard dict pwqual plugin by adding the line:
253 to the pwqual block of the [plugins] section as shown above. Otherwise,
254 it will also try to load a dictionary at the same path to do simple