|
NAME | SYNOPSIS | DESCRIPTION | OPTIONS | MODULE TYPES PROVIDED | RETURN VALUES | EXAMPLES | SEE ALSO | AUTHOR | COLOPHON |
|
|
|
PAM_CRACKLIB(8) Linux-PAM Manual PAM_CRACKLIB(8)
pam_cracklib - PAM module to check the password against
dictionary words
pam_cracklib.so [...]
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.
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.
Only the password module type is provided.
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.
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
pam.conf(5), pam.d(5), pam(8)
pam_cracklib was written by Cristian Gafton <gafton@redhat.com>
This page is part of the linux-pam (Pluggable Authentication
Modules for Linux) project. Information about the project can be
found at ⟨http://www.linux-pam.org/⟩. If you have a bug report
for this manual page, see ⟨//www.linux-pam.org/⟩. This page was
obtained from the tarball Linux-PAM-1.3.0.tar.bz2 fetched from
⟨http://www.linux-pam.org/library/⟩ on 2021-08-27. 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)
|
HTML rendering created 2021-08-27 by Michael Kerrisk, author of The Linux Programming Interface, maintainer of the Linux man-pages project. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|