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 
       ⟨https://fedorahosted.org/linux-pam/⟩.  If you have a bug report for
       this manual page, see ⟨https://fedorahosted.org/linux-pam/report⟩.
       This page was obtained from the tarball Linux-PAM-1.3.0.tar.gz
       fetched from ⟨http://www.linux-pam.org/library/⟩ on 2017-03-13.  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)