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[linux-pam] / doc / modules / pam_cracklib.sgml

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<sect1>Cracklib pluggable password strength-checker

<sect2>Synopsis

<p>
<descrip>

<tag><bf>Module Name:</bf></tag>

pam_cracklib

<tag><bf>Author:</bf></tag>

Cristian Gafton &lt;gafton@redhat.com&gt;

<tag><bf>Maintainer:</bf></tag>

Author.

<tag><bf>Management groups provided:</bf></tag>

password

<tag><bf>Cryptographically sensitive:</bf></tag>
        
<tag><bf>Security rating:</bf></tag>

<tag><bf>Clean code base:</bf></tag>

<tag><bf>System dependencies:</bf></tag> 

Requires the system library <tt/libcrack/ and a system dictionary:
<tt>/usr/lib/cracklib_dict</tt>.

<tag><bf>Network aware:</bf></tag>

</descrip>

<sect2>Overview of module

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

<p>
This module works in the following manner: it first calls the
<em>Cracklib</em> routine to check the strength of the password; if
crack likes the password, the module does an additional set of
strength checks.  These checks are:
<itemize>

<item> <bf/Palindrome/ -

Is the new password a palindrome of the old one?

<item> <bf/Case Change Only/ -

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

<item> <bf/Similar/ -

Is the new password too much like the old one?  This is primarily
controlled by one argument, <tt/difok/ which is a number of characters
that if different between the old and new are enough to accept the new
password, this defaults to 10 or 1/2 the size of the new password
whichever is smaller.

To avoid the lockup associated with trying to change a long and
complicated password, <tt/difignore/ is available. This argument can
be used to specify the minimum length a new password needs to be
before the <tt/difok/ value is ignored. The default value for
<tt/difignore/ is 23.


<item> <bf/Simple/ -

Is the new password too small?  This is controlled by 5 arguments
<tt/minlen/, <tt/dcredit/, <tt/ucredit/, <tt/lcredit/, and
<tt/ocredit/. See the section on the arguments for the details of how
these work and there defaults.

<item> <bf/Rotated/ -

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

<item> <bf/Already used/ -

Was the password used in the past?  Previously used passwords are to
be found in /etc/security/opasswd.

</itemize>

<p>
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.

<sect2>Password component

<p>
<descrip>

<tag><bf>Recognized arguments:</bf></tag>

<tt/debug/; <tt/type=XXX/; <tt/retry=N/; <tt/difok=N/; <tt/minlen=N/;
<tt/dcredit=N/; <tt/ucredit=N/; <tt/lcredit=N/; <tt/ocredit=N/;
<tt/use_authtok/;

<tag><bf>Description:</bf></tag>

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.

<p>
The default 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.

<p>
The default action may be modified in a number of ways using the
arguments recognized by the module:
<itemize>

<item> <tt/debug/ -

this option makes the module write information to syslog(3) indicating
the behavior of the module (this option does <bf/not/ write password
information to the log file).

<item> <tt/type=XXX/ -

the default action is for the module to use the following prompts when
requesting passwords: ``New UNIX password: '' and ``Retype UNIX
password: ''. Using this option you can replace the word UNIX with
<tt/XXX/.

<item> <tt/retry=N/ -

the default number of times this module will request a new password
(for strength-checking) from the user is 1. Using this argument this
can be increased to <tt/N/.

<item> <tt/difok=N/ -

This argument will change the default of 10 for the number of
characters in the new password that must not be present in the old
password.  In addition, if 1/2 of the characters in the new password
are different then the new password will be accepted anyway.

<item> <tt/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 (<em>other,
upper, lower</em> and <em/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
<em>Cracklib</em> 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 <tt>minlen</tt>.  If you want to allow passwords as short
as 5 characters you should either not use this module or recompile
the crack library and then recompile this module.

<item> <tt/dcredit=N/ -

(N >= 0) This is the maximum credit for having digits in the new password. If
you have less than or <tt/N/ digits, each digit will count +1 towards
meeting the current <tt/minlen/ value.  The default for <tt/dcredit/
is 1 which is the recommended value for <tt/minlen/ less than 10.
(N < 0) This is the minimum number of digits that must be met for a new 
password.

<item> <tt/ucredit=N/ -

(N >= 0) This is the maximum credit for having upper case letters in the new
password.  If you have less than or <tt/N/ upper case letters each
letter will count +1 towards meeting the current <tt/minlen/ value.
The default for <tt/ucredit/ is 1 which is the recommended value for
<tt/minlen/ less than 10. (N < 0) This is the minimum number of upper 
case letters that must be met for a new password.

<item> <tt/lcredit=N/ -

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

<item> <tt/ocredit=N/ -

(N >= 0) This is the maximum credit for having other characters in the new
password.  If you have less than or <tt/N/ other characters, each
character will count +1 towards meeting the current <tt/minlen/ value.
The default for <tt/ocredit/ is 1 which is the recommended value for
<tt/minlen/ less than 10. (N < 0) This is the minimum number of other 
characters that must be met for a new password.

<item> <tt/use_authtok/ -

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

</itemize>

<tag><bf>Examples/suggested usage:</bf></tag>

<p>
For an example of the use of this module, we show how it may be
stacked with the password component of <tt/pam_unix/:
<tscreen>
<verb>
#
# 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
</verb>
</tscreen>

<p>
Another example (in the <tt>/etc/pam.d/passwd</tt> format) is for the
case that you want to use md5 password encryption:
<tscreen>
<verb>
#%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
</verb>
</tscreen>

<p>
And here is another example in case you don't want to use credits:
<tscreen>
<verb>
#%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
</verb>
</tscreen>

<p>
In this example we simply say that the password must have a minimum 
length of 8:
<tscreen>
<verb>
#%PAM-1.0
#
# These lines require the user to select a password with a mimimum
# length of 8. He gets no credits and he is not forced to use
# digit numbers, upper case letters etc.
#
password  required pam_cracklib.so \
               dcredit=0 ucredit=0 ocredit=0 lcredit=0 minlen=8
password  required pam_unix.so use_authtok nullok md5
</verb>
</tscreen>

</descrip>

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