1PAM_CRACKLIB(8)                Linux-PAM Manual                PAM_CRACKLIB(8)
2
3
4

NAME

6       pam_cracklib - PAM module to check the password against dictionary
7       words
8

SYNOPSIS

10       pam_cracklib.so [...]
11

DESCRIPTION

13       This module can be plugged into the password stack of a given
14       application to provide some plug-in strength-checking for passwords.
15
16       The action of this module is to prompt the user for a password and
17       check its strength against a system dictionary and a set of rules for
18       identifying poor choices.
19
20       The first action is to prompt for a single password, check its strength
21       and then, if it is considered strong, prompt for the password a second
22       time (to verify that it was typed correctly on the first occasion). All
23       being well, the password is passed on to subsequent modules to be
24       installed as the new authentication token.
25
26       The strength checks works in the following manner: at first the
27       Cracklib routine is called to check if the password is part of a
28       dictionary; if this is not the case an additional set of strength
29       checks is done. These checks are:
30
31       Palindrome
32           Is the new password a palindrome?
33
34       Case Change Only
35           Is the new password the the old one with only a change of case?
36
37       Similar
38           Is the new password too much like the old one? This is primarily
39           controlled by one argument, difok which is a number of character
40           changes (inserts, removals, or replacements) between the old and
41           new password that are enough to accept the new password. This
42           defaults to 5 changes.
43
44       Simple
45           Is the new password too small? This is controlled by 6 arguments
46           minlen, maxclassrepeat, dcredit, ucredit, lcredit, and ocredit. See
47           the section on the arguments for the details of how these work and
48           there defaults.
49
50       Rotated
51           Is the new password a rotated version of the old password?
52
53       Same consecutive characters
54           Optional check for same consecutive characters.
55
56       Too long monotonic character sequence
57           Optional check for too long monotonic character sequence.
58
59       Contains user name
60           Optional check whether the password contains the user´s name in
61           some form.
62
63       This module with no arguments will work well for standard unix password
64       encryption. With md5 encryption, passwords can be longer than 8
65       characters and the default settings for this module can make it hard
66       for the user to choose a satisfactory new password. Notably, the
67       requirement that the new password contain no more than 1/2 of the
68       characters in the old password becomes a non-trivial constraint. For
69       example, an old password of the form "the quick brown fox jumped over
70       the lazy dogs" would be difficult to change... In addition, the default
71       action is to allow passwords as small as 5 characters in length. For a
72       md5 systems it can be a good idea to increase the required minimum size
73       of a password. One can then allow more credit for different kinds of
74       characters but accept that the new password may share most of these
75       characters with the old password.
76

OPTIONS

78       debug
79           This option makes the module write information to syslog(3)
80           indicating the behavior of the module (this option does not write
81           password information to the log file).
82
83       authtok_type=XXX
84           The default action is for the module to use the following prompts
85           when requesting passwords: "New UNIX password: " and "Retype UNIX
86           password: ". The example word UNIX can be replaced with this
87           option, by default it is empty.
88
89       type=XXX
90           Alias for authtok_type kept for backwards compatibility.
91
92       retry=N
93           Prompt user at most N times before returning with error. The
94           default is 1.
95
96       difok=N
97           This argument will change the default of 5 for the number of
98           character changes in the new password that differentiate it from
99           the old password.
100
101       minlen=N
102           The minimum acceptable size for the new password (plus one if
103           credits are not disabled which is the default). In addition to the
104           number of characters in the new password, credit (of +1 in length)
105           is given for each different kind of character (other, upper, lower
106           and digit). The default for this parameter is 9 which is good for a
107           old style UNIX password all of the same type of character but may
108           be too low to exploit the added security of a md5 system. Note that
109           there is a pair of length limits in Cracklib itself, a "way too
110           short" limit of 4 which is hard coded in and a defined limit (6)
111           that will be checked without reference to minlen. If you want to
112           allow passwords as short as 5 characters you should not use this
113           module.
114
115       dcredit=N
116           (N >= 0) This is the maximum credit for having digits in the new
117           password. If you have less than or N digits, each digit will count
118           +1 towards meeting the current minlen value. The default for
119           dcredit is 1 which is the recommended value for minlen less than
120           10.
121
122           (N < 0) This is the minimum number of digits that must be met for a
123           new password.
124
125       ucredit=N
126           (N >= 0) This is the maximum credit for having upper case letters
127           in the new password. If you have less than or N upper case letters
128           each letter will count +1 towards meeting the current minlen value.
129           The default for ucredit is 1 which is the recommended value for
130           minlen less than 10.
131
132           (N < 0) This is the minimum number of upper case letters that must
133           be met for a new password.
134
135       lcredit=N
136           (N >= 0) This is the maximum credit for having lower case letters
137           in the new password. If you have less than or N lower case letters,
138           each letter will count +1 towards meeting the current minlen value.
139           The default for lcredit is 1 which is the recommended value for
140           minlen less than 10.
141
142           (N < 0) This is the minimum number of lower case letters that must
143           be met for a new password.
144
145       ocredit=N
146           (N >= 0) This is the maximum credit for having other characters in
147           the new password. If you have less than or N other characters, each
148           character will count +1 towards meeting the current minlen value.
149           The default for ocredit is 1 which is the recommended value for
150           minlen less than 10.
151
152           (N < 0) This is the minimum number of other characters that must be
153           met for a new password.
154
155       minclass=N
156           The minimum number of required classes of characters for the new
157           password. The default number is zero. The four classes are digits,
158           upper and lower letters and other characters. The difference to the
159           credit check is that a specific class if of characters is not
160           required. Instead N out of four of the classes are required.
161
162       maxrepeat=N
163           Reject passwords which contain more than N same consecutive
164           characters. The default is 0 which means that this check is
165           disabled.
166
167       maxsequence=N
168           Reject passwords which contain monotonic character sequences longer
169           than N. The default is 0 which means that this check is disabled.
170           Examples of such sequence are ´12345´ or ´fedcb´. Note that most
171           such passwords will not pass the simplicity check unless the
172           sequence is only a minor part of the password.
173
174       maxclassrepeat=N
175           Reject passwords which contain more than N consecutive characters
176           of the same class. The default is 0 which means that this check is
177           disabled.
178
179       reject_username
180           Check whether the name of the user in straight or reversed form is
181           contained in the new password. If it is found the new password is
182           rejected.
183
184       gecoscheck
185           Check whether the words from the GECOS field (usualy full name of
186           the user) longer than 3 characters in straight or reversed form are
187           contained in the new password. If any such word is found the new
188           password is rejected.
189
190       enforce_for_root
191           The module will return error on failed check also if the user
192           changing the password is root. This option is off by default which
193           means that just the message about the failed check is printed but
194           root can change the password anyway. Note that root is not asked
195           for an old password so the checks that compare the old and new
196           password are not performed.
197
198       use_authtok
199           This argument is used to force the module to not prompt the user
200           for a new password but use the one provided by the previously
201           stacked password module.
202
203       dictpath=/path/to/dict
204           Path to the cracklib dictionaries.
205

MODULE TYPES PROVIDED

207       Only the password module type is provided.
208

RETURN VALUES

210       PAM_SUCCESS
211           The new password passes all checks.
212
213       PAM_AUTHTOK_ERR
214           No new password was entered, the username could not be determined
215           or the new password fails the strength checks.
216
217       PAM_AUTHTOK_RECOVERY_ERR
218           The old password was not supplied by a previous stacked module or
219           got not requested from the user. The first error can happen if
220           use_authtok is specified.
221
222       PAM_SERVICE_ERR
223           A internal error occurred.
224

EXAMPLES

226       For an example of the use of this module, we show how it may be stacked
227       with the password component of pam_unix(8)
228
229           #
230           # These lines stack two password type modules. In this example the
231           # user is given 3 opportunities to enter a strong password. The
232           # "use_authtok" argument ensures that the pam_unix module does not
233           # prompt for a password, but instead uses the one provided by
234           # pam_cracklib.
235           #
236           passwd  password required       pam_cracklib.so retry=3
237           passwd  password required       pam_unix.so use_authtok
238
239
240       Another example (in the /etc/pam.d/passwd format) is for the case that
241       you want to use md5 password encryption:
242
243           #%PAM-1.0
244           #
245           # These lines allow a md5 systems to support passwords of at least 14
246           # bytes with extra credit of 2 for digits and 2 for others the new
247           # password must have at least three bytes that are not present in the
248           # old password
249           #
250           password  required pam_cracklib.so \
251                          difok=3 minlen=15 dcredit= 2 ocredit=2
252           password  required pam_unix.so use_authtok nullok md5
253
254
255       And here is another example in case you don´t want to use credits:
256
257           #%PAM-1.0
258           #
259           # These lines require the user to select a password with a minimum
260           # length of 8 and with at least 1 digit number, 1 upper case letter,
261           # and 1 other character
262           #
263           password  required pam_cracklib.so \
264                          dcredit=-1 ucredit=-1 ocredit=-1 lcredit=0 minlen=8
265           password  required pam_unix.so use_authtok nullok md5
266
267
268

SEE ALSO

270       pam.conf(5), pam.d(5), pam(8)
271

AUTHOR

273       pam_cracklib was written by Cristian Gafton <gafton@redhat.com>
274
275
276
277Linux-PAM Manual                  03/22/2017                   PAM_CRACKLIB(8)
Impressum