pam_cracklib(8) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | OPTIONS | MODULE TYPES PROVIDED | RETURN VALUES | EXAMPLES | SEE ALSO | AUTHOR | COLOPHON

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

NAME         top

       pam_cracklib - PAM module to check the password against
       dictionary words

SYNOPSIS         top

       pam_cracklib.so [...]

DESCRIPTION         top

       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         top

       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         top

       Only the password module type is provided.

RETURN VALUES         top

       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         top

       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         top

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

AUTHOR         top

       pam_cracklib was written by Cristian Gafton <gafton@redhat.com>

COLOPHON         top

       This page is part of the linux-pam (Pluggable Authentication
       Modules for Linux) project.  Information about the project can be
       found at ⟨http://www.linux-pam.org/⟩.  If you have a bug report
       for this manual page, see ⟨//www.linux-pam.org/⟩.  This page was
       obtained from the tarball Linux-PAM-1.3.0.tar.bz2 fetched from
       ⟨http://www.linux-pam.org/library/⟩ on 2021-04-01.  If you
       discover any rendering problems in this HTML version of the page,
       or you believe there is a better or more up-to-date source for
       the page, or you have corrections or improvements to the
       information in this COLOPHON (which is not part of the original
       manual page), send a mail to man-pages@man7.org

Linux-PAM Manual               04/01/2016                PAM_CRACKLIB(8)