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pam_cracklib — PAM module to check the password against dictionary words
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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 of the old one?
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 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, difignore is available. This argument can be used to specify the
minimum length a new password needs to be before the difok value is
ignored. The default value for difignore is 23.
Simple
Is the new password too small? This is controlled by 5 arguments minlen,
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?
Already used
Was the password used in the past? Previously used passwords are to be
found in /etc/security/opasswd.
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).
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 default word UNIX can be replaced with this option.
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 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.
difignore=N
How many characters should the password have before difok will be ignored.
The default is 23.
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.
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.
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
AUTHOR
pam_cracklib was written by Cristian Gafton <gafton@redhat.com>
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