1PAM_CRACKLIB(8) Linux-PAM Manual PAM_CRACKLIB(8)
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6 pam_cracklib - PAM module to check the password against dictionary
7 words
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10 pam_cracklib.so [...]
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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 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 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 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 Palindrome
32 Is the new password a palindrome?
33 Case Change Only
35 Is the new password the the old one with only a change of case?
36 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 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 Rotated
51 Is the new password a rotated version of the old password?
52 Same consecutive characters
54 Optional check for same consecutive characters.
55 Too long monotonic character sequence
57 Optional check for too long monotonic character sequence.
58 Contains user name
60 Optional check whether the password contains the user's name in
61 some form.
62 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
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 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 retry=N
90 Prompt user at most N times before returning with error. The
91 default is 1.
92 difok=N
94 This argument will change the default of 5 for the number of
95 character changes in the new password that differentiate it from
96 the old password.
97 minlen=N
99 The minimum acceptable size for the new password (plus one if
100 credits are not disabled which is the default). In addition to the
101 number of characters in the new password, credit (of +1 in length)
102 is given for each different kind of character (other, upper, lower
103 and digit). The default for this parameter is 9 which is good for a
104 old style UNIX password all of the same type of character but may
105 be too low to exploit the added security of a md5 system. Note that
106 there is a pair of length limits in Cracklib itself, a "way too
107 short" limit of 4 which is hard coded in and a defined limit (6)
108 that will be checked without reference to minlen. If you want to
109 allow passwords as short as 5 characters you should not use this
110 module.
111 dcredit=N
113 (N >= 0) This is the maximum credit for having digits in the new
114 password. If you have less than or N digits, each digit will count
115 +1 towards meeting the current minlen value. The default for
116 dcredit is 1 which is the recommended value for minlen less than
117 10.
118 (N < 0) This is the minimum number of digits that must be met for a
120 new password.
121 ucredit=N
123 (N >= 0) This is the maximum credit for having upper case letters
124 in the new password. If you have less than or N upper case letters
125 each letter will count +1 towards meeting the current minlen value.
126 The default for ucredit is 1 which is the recommended value for
127 minlen less than 10.
128 (N < 0) This is the minimum number of upper case letters that must
130 be met for a new password.
131 lcredit=N
133 (N >= 0) This is the maximum credit for having lower case letters
134 in the new password. If you have less than or N lower case letters,
135 each letter will count +1 towards meeting the current minlen value.
136 The default for lcredit is 1 which is the recommended value for
137 minlen less than 10.
138 (N < 0) This is the minimum number of lower case letters that must
140 be met for a new password.
141 ocredit=N
143 (N >= 0) This is the maximum credit for having other characters in
144 the new password. If you have less than or N other characters, each
145 character will count +1 towards meeting the current minlen value.
146 The default for ocredit is 1 which is the recommended value for
147 minlen less than 10.
148 (N < 0) This is the minimum number of other characters that must be
150 met for a new password.
151 minclass=N
153 The minimum number of required classes of characters for the new
154 password. The default number is zero. The four classes are digits,
155 upper and lower letters and other characters. The difference to the
156 credit check is that a specific class if of characters is not
157 required. Instead N out of four of the classes are required.
158 maxrepeat=N
160 Reject passwords which contain more than N same consecutive
161 characters. The default is 0 which means that this check is
162 disabled.
163 maxsequence=N
165 Reject passwords which contain monotonic character sequences longer
166 than N. The default is 0 which means that this check is disabled.
167 Examples of such sequence are '12345' or 'fedcb'. Note that most
168 such passwords will not pass the simplicity check unless the
169 sequence is only a minor part of the password.
170 maxclassrepeat=N
172 Reject passwords which contain more than N consecutive characters
173 of the same class. The default is 0 which means that this check is
174 disabled.
175 reject_username
177 Check whether the name of the user in straight or reversed form is
178 contained in the new password. If it is found the new password is
179 rejected.
180 gecoscheck
182 Check whether the words from the GECOS field (usualy full name of
183 the user) longer than 3 characters in straight or reversed form are
184 contained in the new password. If any such word is found the new
185 password is rejected.
186 enforce_for_root
188 The module will return error on failed check also if the user
189 changing the password is root. This option is off by default which
190 means that just the message about the failed check is printed but
191 root can change the password anyway. Note that root is not asked
192 for an old password so the checks that compare the old and new
193 password are not performed.
194 use_authtok
196 This argument is used to force the module to not prompt the user
197 for a new password but use the one provided by the previously
198 stacked password module.
199 dictpath=/path/to/dict
201 Path to the cracklib dictionaries.
202
204 Only the password module type is provided.
205
207 PAM_SUCCESS
208 The new password passes all checks.
209 PAM_AUTHTOK_ERR
211 No new password was entered, the username could not be determined
212 or the new password fails the strength checks.
213 PAM_AUTHTOK_RECOVERY_ERR
215 The old password was not supplied by a previous stacked module or
216 got not requested from the user. The first error can happen if
217 use_authtok is specified.
218 PAM_SERVICE_ERR
220 A internal error occurred.
221
223 For an example of the use of this module, we show how it may be stacked
224 with the password component of pam_unix(8)
225 #
227 # These lines stack two password type modules. In this example the
228 # user is given 3 opportunities to enter a strong password. The
229 # "use_authtok" argument ensures that the pam_unix module does not
230 # prompt for a password, but instead uses the one provided by
231 # pam_cracklib.
232 #
233 passwd password required pam_cracklib.so retry=3
234 passwd password required pam_unix.so use_authtok
235 Another example (in the /etc/pam.d/passwd format) is for the case that
238 you want to use md5 password encryption:
239 #%PAM-1.0
241 #
242 # These lines allow a md5 systems to support passwords of at least 14
243 # bytes with extra credit of 2 for digits and 2 for others the new
244 # password must have at least three bytes that are not present in the
245 # old password
246 #
247 password required pam_cracklib.so \
248 difok=3 minlen=15 dcredit= 2 ocredit=2
249 password required pam_unix.so use_authtok nullok md5
250 And here is another example in case you don't want to use credits:
253 #%PAM-1.0
255 #
256 # These lines require the user to select a password with a minimum
257 # length of 8 and with at least 1 digit number, 1 upper case letter,
258 # and 1 other character
259 #
260 password required pam_cracklib.so \
261 dcredit=-1 ucredit=-1 ocredit=-1 lcredit=0 minlen=8
262 password required pam_unix.so use_authtok nullok md5
263
267 pam.conf(5), pam.d(5), pam(8)
268
270 pam_cracklib was written by Cristian Gafton <gafton@redhat.com>
271Linux-PAM Manual 05/18/2017 PAM_CRACKLIB(8)