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 of
characters but accept that the new password may share most of these
characters with the old password.
OPTIONS
debug
This option makes the module write information to syslog(3)
indicating the behavior of the module (this option does not write
password information to the log file).
authtok_type=XXX
The default action is for the module to use the following prompts
when requesting passwords: "New UNIX password: " and "Retype UNIX
password: ". The example word UNIX can be replaced with this
option, by default it is empty.
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.
maxsequence=N
Reject passwords which contain monotonic character sequences longer
than N. The default is 0 which means that this check is disabled.
Examples of such sequence are '12345' or 'fedcb'. Note that most
such passwords will not pass the simplicity check unless the
sequence is only a minor part of the password.
maxclassrepeat=N
Reject passwords which contain more than N consecutive characters
of the same class. The default is 0 which means that this check is
disabled.
reject_username
Check whether the name of the user in straight or reversed form is
contained in the new password. If it is 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 encryption:
#%PAM-1.0
#
# These lines allow a md5 systems to support passwords of at least 14
# bytes with extra credit of 2 for digits and 2 for others the new
# password must have at least three bytes that are not present in the
# old password
#
password required pam_cracklib.so \
difok=3 minlen=15 dcredit= 2 ocredit=2
password required pam_unix.so use_authtok nullok md5
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
SEE ALSO
pam.conf(5), pam.d(5), pam(8)
AUTHOR
pam_cracklib was written by Cristian Gafton <gafton AT redhat.com>
Linux-PAM Manual 05/18/2017 PAM_CRACKLIB(8)
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