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 You can also optionally build against the TinyCDB library, which
80 provides support for simpler and faster password checking against a CDB
81 dictionary file. Building a CDB dictionary with cdbmake-wordlist
82 (included) requires Perl 5.006 or later and the CDB utility that comes
85 For this module to be effective for either Heimdal or MIT Kerberos, you
86 will also need to construct a dictionary. The mkdict and packer
87 utilities to build a CrackLib dictionary from a word list are included
88 in this toolkit but not installed by default. You can run them out of
89 the cracklib directory after building. You can also use the utilities
90 that come with the stock CrackLib package (often already packaged in a
91 Linux distribution); the database format is compatible. For building a
92 CDB dictionary, use the provided cdbmake-wordlist program. The CDB
93 utility must be on your PATH.
95 For a word list to use as source for the dictionary, you can use
96 /usr/share/dict/words if it's available on your system, but it would be
97 better to find a more comprehensive word list. Since word lists are
98 bulky, often covered by murky copyrights, and easily locatable on the
99 Internet with a modicum of searching, none are included in this toolkit.
101 To bootstrap from a Git checkout, or If you change the Automake files
102 and need to regenerate Makefile.in, you will need Automake 1.11 or
103 later. For bootstrap or if you change configure.ac or any of the m4
104 files it includes and need to regenerate configure or config.h.in, you
105 will need Autoconf 2.64 or later.
107 COMPILING AND INSTALLING
109 You can build and install the plugin with the standard commands:
115 Pass --enable-silent-rules to configure for a quieter build (similar to
116 the Linux kernel). Use make warnings instead of make to build with full
117 GCC compiler warnings (requires a relatively current version of GCC).
119 The last step will probably have to be done as root. By default, the
120 plugin is installed as /usr/local/lib/krb5/plugins/pwqual/strength.so
121 and the Heimdal external password check function is installed as
122 /usr/local/bin/heimdal-strength. You can change these paths with the
123 --prefix, --libdir, and --bindir options to configure.
125 To build with the system version of CrackLib, pass --with-cracklib to
126 configure. You can optionally add a directory, giving the root
127 directory where CrackLib was installed, or separately set the include
128 and library path with --with-cracklib-include and --with-cracklib-lib.
130 krb5-strength will automatically build with TinyCDB if it is found. To
131 specify the installation path of TinyCDB, use --with-tinycdb. You can
132 also separately set the include and library path with
133 --with-tinycdb-include and --with-tinycdb-lib.
135 Normally, configure will use krb5-config to determine the flags to use
136 to compile with your Kerberos libraries. If krb5-config isn't found, it
137 will look for the standard Kerberos libraries in locations already
138 searched by your compiler. If the the krb5-config script first in your
139 path is not the one corresponding to the Kerberos libraries you want to
140 use or if your Kerberos libraries and includes aren't in a location
141 searched by default by your compiler, you need to specify a different
142 Kerberos installation root via --with-krb5=PATH. For example:
144 ./configure --with-krb5=/usr/pubsw
146 You can also individually set the paths to the include directory and the
147 library directory with --with-krb5-include and --with-krb5-lib. You may
148 need to do this if Autoconf can't figure out whether to use lib, lib32,
149 or lib64 on your platform.
151 To specify a particular krb5-config script to use, either set the
152 PATH_KRB5_CONFIG environment variable or pass it to configure like:
154 ./configure PATH_KRB5_CONFIG=/path/to/krb5-config
156 To not use krb5-config and force library probing even if there is a
157 krb5-config script on your path, set PATH_KRB5_CONFIG to a nonexistent
160 ./configure PATH_KRB5_CONFIG=/nonexistent
162 krb5-config is not used and library probing is always done if either
163 --with-krb5-include or --with-krb5-lib are given.
165 You can pass the --enable-reduced-depends flag to configure to try to
166 minimize the shared library dependencies encoded in the binaries. This
167 omits from the link line all the libraries included solely because the
168 Kerberos libraries depend on them and instead links the programs only
169 against libraries whose APIs are called directly. This will only work
170 with shared Kerberos libraries and will only work on platforms where
171 shared libraries properly encode their own dependencies (such as Linux).
172 It is intended primarily for building packages for Linux distributions
173 to avoid encoding unnecessary shared library dependencies that make
174 shared library migrations more difficult. If none of the above made any
175 sense to you, don't bother with this flag.
179 First, build and install either a CrackLib dictionary as described in
180 REQUIREMENTS above, or build a CDB dictionary with cdbmake-wordlist.
181 (Or both.) The CrackLib dictionary will consist of three files, one
182 each ending in *.hwm, *.pwd, and *.pwi. The CDB dictionary will consist
183 of a single file ending in *.cdb. Install those files somewhere on your
184 system. Then, follow the relevant instructions below for either Heimdal
187 See "Other Settings" below for additional krb5.conf setting supported by
188 both Heimdal and MIT Kerberos.
192 There are two options: using an external password check program, or
193 using the plugin. I recommend the external password check program
194 unless you encounter speed problems with that approach that cause
197 For either approach, first add a stanza like the following to the
198 [appdefaults] section of your /etc/krb5.conf (or wherever your krb5.conf
202 password_dictionary = /path/to/cracklib/dictionary
203 password_dictionary_cdb = /path/to/cdb/dictionary.cdb
206 The first setting configures a CrackLib dictionary and the second a CDB
207 dictionary. The provided path should be the full path to the dictionary
208 files, omitting the trailing *.hwm, *.pwd, and *.pwi extensions for the
209 CrackLib dictionary. You can use either or both settings. If you use
210 both, CrackLib will be checked first, and then CDB.
212 Then, for the external password checking program, add a new section (or
213 modify the existing [password_quality] section) to look like the
217 policies = external-check
218 external_program = /usr/local/bin/heimdal-strength
220 You can, of course, combine this policy with others. Replace the path
221 with the full path to wherever you have installed heimdal-strength. You
222 can put this section in your kdc.conf instead of krb5.conf if you
225 If you want to instead use the module, use the following section
229 policies = krb5-strength
230 policy_libraries = /usr/local/lib/krb5/plugins/pwqual/strength.so
232 in either krb5.conf or kdc.conf. Note that some older versions of
233 Heimdal have a bug in the support for loading modules when
234 policy_libraries is set. If you get an error like:
236 didn't find `kadm5_password_verifier' symbol in `(null)'
238 you may have to omit policy_libraries in your configuration and instead
239 pass the --check-library argument to kpasswdd specifying the library to
244 To add this module to the list of password quality checks, add a section
245 to krb5.conf (or to a separate kdc.conf if you use that) like:
249 module = strength:/usr/local/lib/krb5/plugins/pwqual/strength.so
252 to register the plugin.
254 There are two ways to tell where the dictionary is. One option is to
255 use krb5.conf (and in this case you must use krb5.conf, even if you use
256 a separate kdc.conf file). For this approach, add the following to the
257 [appdefaults] section:
260 password_dictionary = /path/to/cracklib/dictionary
261 password_dictionary_cdb = /path/to/cdb/dictionary.cdb
264 The first setting configures a CrackLib dictionary and the second a CDB
265 dictionary. The provided path should be the full path to the dictionary
266 files, omitting the trailing *.hwm, *.pwd, and *.pwi extensions for the
267 CrackLib dictionary. You can use either or both settings. If you use
268 both, CrackLib will be checked first, and then CDB.
270 The second option is to use the normal dict_path setting. In the
271 [realms] section of your krb5.conf kdc.conf, under the appropriate realm
272 or realms, specify the path to the dictionary:
274 dict_file = /path/to/cracklib/dictionary
276 This will be taken as a CrackLib dictionary path, the same as the
277 setting for password_dictionary above. The provided path should be the
278 full path to the dictionary files, omitting the trailing *.hwm, *.pwd,
279 or *.pwi extension. However, be aware that, if you use this approach,
280 you will probably want to disable the built-in standard dict pwqual
281 plugin by adding the line:
285 to the pwqual block of the [plugins] section as shown above. Otherwise,
286 it will also try to load a dictionary at the same path to do simple
289 You can also mix and match these settings, by using dict_path for the
290 CrackLib dictionary path and krb5.conf for the CDB dictionary path. If
291 both settings are used, krb5.conf overrides the dict_path setting (so
292 that dict_path can be used for other password quality modules). There
293 is no way to specify a CDB dictionary via the dict_path setting.
297 The following additional settings are supported in the [appdefaults]
298 section of krb5.conf when running under either Heimdal or MIT Kerberos.
302 If set to a numeric value, passwords with fewer than that number of
303 characters will be rejected, independent of any length restrictions
304 in CrackLib. Note that this setting does not bypass the minimum
305 length requirements in CrackLib itself (which, for the version
306 embedded in this package, is eight characters).
308 require_ascii_printable
310 If set to a true boolean value, rejects any password that contains
311 non-ASCII characters or ASCII control characters. Spaces are
312 allowed; tabs are not (at least assuming the POSIX C locale). No
313 canonicalization or character set is defined for Kerberos passwords
314 in general, so you may want to reject non-ASCII characters to avoid
315 interoperability problems with computers with different default
316 character sets or Unicode normalization forms.
320 If set to a true boolean value, the password must contain at least
321 one character that is not a letter (uppercase or lowercase) or a
322 space. This may be helpful in combination with passphrases; users
323 may choose a stock English phrase, and this will force at least some
324 additional complexity.
326 You can omit any dictionary setting and only use the above settings, in
327 which case only the above checks and checks for passwords based on the
328 principal will be done, bypassing any dictionary check. (But for that
329 simple style of password strength checking, there are probably better
330 strength checking plugins already available.)