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$Id: pam_cracklib.sgml,v 1.4 2002/09/15 20:44:05 hartmans Exp $
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This file was written by Andrew G. Morgan <morgan@kernel.org>
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long password amendments are from Philip W. Dalrymple III <pwd@mdtsoft.com>
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<sect1>Cracklib pluggable password strength-checker
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<tag><bf>Module Name:</bf></tag>
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<tag><bf>Author:</bf></tag>
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Cristian Gafton <gafton@redhat.com>
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<tag><bf>Maintainer:</bf></tag>
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<tag><bf>Management groups provided:</bf></tag>
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<tag><bf>Cryptographically sensitive:</bf></tag>
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<tag><bf>Security rating:</bf></tag>
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<tag><bf>Clean code base:</bf></tag>
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<tag><bf>System dependencies:</bf></tag>
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Requires the system library <tt/libcrack/ and a system dictionary:
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<tt>/var/cache/cracklib/cracklib_dict</tt>.
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<tag><bf>Network aware:</bf></tag>
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<sect2>Overview of module
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This module can be plugged into the <tt/password/ stack of a given
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application to provide some plug-in strength-checking for passwords.
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This module works in the following manner: it first calls the
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<em>Cracklib</em> routine to check the strength of the password; if
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crack likes the password, the module does an additional set of
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strength checks. These checks are:
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<item> <bf/Palindrome/ -
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Is the new password a palindrome of the old one?
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<item> <bf/Case Change Only/ -
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Is the new password the the old one with only a change of case?
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Is the new password too much like the old one? This is primarily
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controlled by one argument, <tt/difok/ which is a number of characters
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that if different between the old and new are enough to accept the new
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password, this defaults to 10 or 1/2 the size of the new password
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To avoid the lockup associated with trying to change a long and
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complicated password, <tt/difignore/ is available. This argument can
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be used to specify the minimum length a new password needs to be
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before the <tt/difok/ value is ignored. The default value for
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Is the new password too small? This is controlled by 5 arguments
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<tt/minlen/, <tt/dcredit/, <tt/ucredit/, <tt/lcredit/, and
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<tt/ocredit/. See the section on the arguments for the details of how
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these work and there defaults.
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Is the new password a rotated version of the old password?
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<item> <bf/Already used/ -
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Was the password used in the past? Previously used passwords are to
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be found in /etc/security/opasswd.
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This module with no arguments will work well for standard unix
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password encryption. With md5 encryption, passwords can be longer
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than 8 characters and the default settings for this module can make it
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hard for the user to choose a satisfactory new password. Notably, the
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requirement that the new password contain no more than 1/2 of the
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characters in the old password becomes a non-trivial constraint. For
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example, an old password of the form "the quick brown fox jumped over
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the lazy dogs" would be difficult to change... In addition, the
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default action is to allow passwords as small as 5 characters in
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length. For a md5 systems it can be a good idea to increase the
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required minimum size of a password. One can then allow more credit
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for different kinds of characters but accept that the new password may
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share most of these characters with the old password.
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<sect2>Password component
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<tag><bf>Recognized arguments:</bf></tag>
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<tt/debug/; <tt/type=XXX/; <tt/retry=N/; <tt/difok=N/; <tt/minlen=N/;
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<tt/dcredit=N/; <tt/ucredit=N/; <tt/lcredit=N/; <tt/ocredit=N/;
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<tag><bf>Description:</bf></tag>
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The action of this module is to prompt the user for a password and
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check its strength against a system dictionary and a set of rules for
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identifying poor choices.
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The default action is to prompt for a single password, check its
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strength and then, if it is considered strong, prompt for the password
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a second time (to verify that it was typed correctly on the first
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occasion). All being well, the password is passed on to subsequent
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modules to be installed as the new authentication token.
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The default action may be modified in a number of ways using the
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arguments recognized by the module:
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this option makes the module write information to syslog(3) indicating
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the behavior of the module (this option does <bf/not/ write password
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information to the log file).
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<item> <tt/type=XXX/ -
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the default action is for the module to use the following prompts when
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requesting passwords: ``New UNIX password: '' and ``Retype UNIX
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password: ''. Using this option you can replace the word UNIX with
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<item> <tt/retry=N/ -
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the default number of times this module will request a new password
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(for strength-checking) from the user is 1. Using this argument this
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can be increased to <tt/N/.
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<item> <tt/difok=N/ -
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This argument will change the default of 10 for the number of
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characters in the new password that must not be present in the old
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password. In addition, if 1/2 of the characters in the new password
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are different then the new password will be accepted anyway.
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<item> <tt/minlen=N/ -
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The minimum acceptable size for the new password (plus one if credits
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are not disabled which is the default).
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In addition to the number of characters in the new password, credit (of
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+1 in length) is given for each different kind of character (<em>other,
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upper, lower</em> and <em/digit/). The default for this parameter is
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9 which is good for a old style UNIX password all of the same type of
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character but may be too low to exploit the added security of a md5
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system. Note that there is a pair of length limits in
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<em>Cracklib</em> itself, a "way too short" limit of 4 which is hard
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coded in and a defined limit (6) that will be checked without
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reference to <tt>minlen</tt>. If you want to allow passwords as short
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as 5 characters you should either not use this module or recompile
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the crack library and then recompile this module.
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<item> <tt/dcredit=N/ -
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(N >= 0) This is the maximum credit for having digits in the new password. If
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you have less than or <tt/N/ digits, each digit will count +1 towards
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meeting the current <tt/minlen/ value. The default for <tt/dcredit/
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is 1 which is the recommended value for <tt/minlen/ less than 10.
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(N < 0) This is the minimum number of digits that must be met for a new
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<item> <tt/ucredit=N/ -
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(N >= 0) This is the maximum credit for having upper case letters in the new
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password. If you have less than or <tt/N/ upper case letters each
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letter will count +1 towards meeting the current <tt/minlen/ value.
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The default for <tt/ucredit/ is 1 which is the recommended value for
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<tt/minlen/ less than 10. (N < 0) This is the minimum number of upper
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case letters that must be met for a new password.
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<item> <tt/lcredit=N/ -
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(N >= 0) This is the maximum credit for having lower case letters in the new
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password. If you have less than or <tt/N/ lower case letters, each
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letter will count +1 towards meeting the current <tt/minlen/ value.
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The default for <tt/lcredit/ is 1 which is the recommended value for
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<tt/minlen/ less than 10. (N < 0) This is the minimum number of lower
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case letters that must be met for a new password.
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<item> <tt/ocredit=N/ -
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(N >= 0) This is the maximum credit for having other characters in the new
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password. If you have less than or <tt/N/ other characters, each
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character will count +1 towards meeting the current <tt/minlen/ value.
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The default for <tt/ocredit/ is 1 which is the recommended value for
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<tt/minlen/ less than 10. (N < 0) This is the minimum number of other
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characters that must be met for a new password.
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<item> <tt/use_authtok/ -
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This argument is used to <em/force/ the module to not prompt the user
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for a new password but use the one provided by the previously stacked
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<tt/password/ module.
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<tag><bf>Examples/suggested usage:</bf></tag>
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For an example of the use of this module, we show how it may be
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stacked with the password component of <tt/pam_unix/:
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# These lines stack two password type modules. In this example the
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# user is given 3 opportunities to enter a strong password. The
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# "use_authtok" argument ensures that the pam_unix module does not
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# prompt for a password, but instead uses the one provided by
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passwd password required pam_cracklib.so retry=3
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passwd password required pam_unix.so use_authtok
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Another example (in the <tt>/etc/pam.d/passwd</tt> format) is for the
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case that you want to use md5 password encryption:
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# These lines allow a md5 systems to support passwords of at least 14
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# bytes with extra credit of 2 for digits and 2 for others the new
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# password must have at least three bytes that are not present in the
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password required pam_cracklib.so \
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difok=3 minlen=15 dcredit= 2 ocredit=2
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password required pam_unix.so use_authtok nullok md5
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And here is another example in case you don't want to use credits:
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# These lines require the user to select a password with a minimum
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# length of 8 and with at least 1 digit number, 1 upper case letter,
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# and 1 other character
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password required pam_cracklib.so \
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dcredit=-1 ucredit=-1 ocredit=-1 lcredit=0 minlen=8
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password required pam_pwdb.so use_authtok nullok md5
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In this example we simply say that the password must have a minimum
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# These lines require the user to select a password with a mimimum
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# length of 8. He gets no credits and he is not forced to use
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# digit numbers, upper case letters etc.
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password required pam_cracklib.so \
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dcredit=0 ucredit=0 ocredit=0 lcredit=0 minlen=8
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password required pam_pwdb.so use_authtok nullok md5
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End of sgml insert for this module.
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Linux-PAM/doc/modules/pam_cracklib.sgml
