pam_cracklib - PAM module to check the password against dictionary words at Linux.org

Back to main site | Back to man page index

PAM_CRACKLIB(8) Linux-PAM Manual PAM_CRACKLIB(8)

NAME
pam_cracklib - PAM module to check the password against dictionary words

SYNOPSIS
pam_cracklib.so [...]

DESCRIPTION
This module can be plugged into the password stack of a given application to provide some plug-in
strength-checking for passwords.

The action of this module is to prompt the user for a password and check its strength against a system
dictionary and a set of rules for identifying poor choices.

The first action is to prompt for a single password, check its strength and then, if it is considered strong,
prompt for the password a second time (to verify that it was typed correctly on the first occasion). All being
well, the password is passed on to subsequent modules to be installed as the new authentication token.

The strength checks works in the following manner: at first the Cracklib routine is called to check if the
password is part of a dictionary; if this is not the case an additional set of strength checks is done. These
checks are:

Palindrome
Is the new password a palindrome?

Case Change Only
Is the new password the the old one with only a change of case?

Similar
Is the new password too much like the old one? This is primarily controlled by one argument, difok which
is a number of character changes (inserts, removals, or replacements) between the old and new password
that are enough to accept the new password. This defaults to 5 changes.

Simple
Is the new password too small? This is controlled by 6 arguments minlen, maxclassrepeat, dcredit, ucredit,
lcredit, and ocredit. See the section on the arguments for the details of how these work and there
defaults.

Rotated
Is the new password a rotated version of the old password?

Same consecutive characters
Optional check for same consecutive characters.

Too long monotonic character sequence
Optional check for too long monotonic character sequence.

Contains user name
Optional check whether the password contains the user's name in some form.

This module with no arguments will work well for standard unix password encryption. With md5 encryption,
passwords can be longer than 8 characters and the default settings for this module can make it hard for the
user to choose a satisfactory new password. Notably, the requirement that the new password contain no more
than 1/2 of the characters in the old password becomes a non-trivial constraint. For example, an old password
of the form "the quick brown fox jumped over the lazy dogs" would be difficult to change... In addition, the
default action is to allow passwords as small as 5 characters in length. For a md5 systems it can be a good
idea to increase the required minimum size of a password. One can then allow more credit for different kinds
retry=N
Prompt user at most N times before returning with error. The default is 1.

difok=N
This argument will change the default of 5 for the number of character changes in the new password that
differentiate it from the old password.

minlen=N
The minimum acceptable size for the new password (plus one if credits are not disabled which is the
default). In addition to the number of characters in the new password, credit (of +1 in length) is given
for each different kind of character (other, upper, lower and digit). The default for this parameter is 9
which is good for a old style UNIX password all of the same type of character but may be too low to
exploit the added security of a md5 system. Note that there is a pair of length limits in Cracklib itself,
a "way too short" limit of 4 which is hard coded in and a defined limit (6) that will be checked without
reference to minlen. If you want to allow passwords as short as 5 characters you should not use this
module.

dcredit=N
(N >= 0) This is the maximum credit for having digits in the new password. If you have less than or N
digits, each digit will count +1 towards meeting the current minlen value. The default for dcredit is 1
which is the recommended value for minlen less than 10.

(N < 0) This is the minimum number of digits that must be met for a new password.

ucredit=N
(N >= 0) This is the maximum credit for having upper case letters in the new password. If you have less
than or N upper case letters each letter will count +1 towards meeting the current minlen value. The
default for ucredit is 1 which is the recommended value for minlen less than 10.

(N < 0) This is the minimum number of upper case letters that must be met for a new password.

lcredit=N
(N >= 0) This is the maximum credit for having lower case letters in the new password. If you have less
than or N lower case letters, each letter will count +1 towards meeting the current minlen value. The
default for lcredit is 1 which is the recommended value for minlen less than 10.

(N < 0) This is the minimum number of lower case letters that must be met for a new password.

ocredit=N
(N >= 0) This is the maximum credit for having other characters in the new password. If you have less than
or N other characters, each character will count +1 towards meeting the current minlen value. The default
for ocredit is 1 which is the recommended value for minlen less than 10.

(N < 0) This is the minimum number of other characters that must be met for a new password.

minclass=N
The minimum number of required classes of characters for the new password. The default number is zero. The
four classes are digits, upper and lower letters and other characters. The difference to the credit check
is that a specific class if of characters is not required. Instead N out of four of the classes are
required.

maxrepeat=N
Reject passwords which contain more than N same consecutive characters. The default is 0 which means that
this check is disabled.
found the new password is rejected.

gecoscheck
Check whether the words from the GECOS field (usualy full name of the user) longer than 3 characters in
straight or reversed form are contained in the new password. If any such word is found the new password is
rejected.

enforce_for_root
The module will return error on failed check also if the user changing the password is root. This option
is off by default which means that just the message about the failed check is printed but root can change
the password anyway. Note that root is not asked for an old password so the checks that compare the old
and new password are not performed.

use_authtok
This argument is used to force the module to not prompt the user for a new password but use the one
provided by the previously stacked password module.

dictpath=/path/to/dict
Path to the cracklib dictionaries.

MODULE TYPES PROVIDED
Only the password module type is provided.

RETURN VALUES
PAM_SUCCESS
The new password passes all checks.

PAM_AUTHTOK_ERR
No new password was entered, the username could not be determined or the new password fails the strength
checks.

PAM_AUTHTOK_RECOVERY_ERR
The old password was not supplied by a previous stacked module or got not requested from the user. The
first error can happen if use_authtok is specified.

PAM_SERVICE_ERR
A internal error occurred.

EXAMPLES
For an example of the use of this module, we show how it may be stacked with the password component of
pam_unix(8)

#
# These lines stack two password type modules. In this example the
# user is given 3 opportunities to enter a strong password. The
# "use_authtok" argument ensures that the pam_unix module does not
# prompt for a password, but instead uses the one provided by
# pam_cracklib.
#
passwd password required pam_cracklib.so retry=3
passwd password required pam_unix.so use_authtok

Another example (in the /etc/pam.d/passwd format) is for the case that you want to use md5 password

And here is another example in case you don't want to use credits:

#%PAM-1.0
#
# These lines require the user to select a password with a minimum
# length of 8 and with at least 1 digit number, 1 upper case letter,
# and 1 other character
#
password required pam_cracklib.so \
dcredit=-1 ucredit=-1 ocredit=-1 lcredit=0 minlen=8
password required pam_unix.so use_authtok nullok md5