NAME¶
pam_cracklib - PAM module to check the password against dictionary
words
SYNOPSIS¶
pam_cracklib.so [...]
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?
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¶
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¶
Only the password module type is provided.
RETURN VALUES¶
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¶
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>