pam_passwdqc is a simple password strength checking module for
PAM-aware password changing programs, such as passwd(1).  In addition
to checking regular passwords, it offers support for passphrases and
can provide randomly generated passwords.  All features are optional
and can be (re-)configured without rebuilding.

This module should be stacked before your usual password changing
module (such as pam_unix or pam_pwdb) in the password management group
(the "password" lines in /etc/pam.d/passwd or /etc/pam.conf).  The
password changing module should then be told to use the provided new
authentication token (new password) rather than request it from the
user.  There's usually the "use_authtok" option to do that.  If your
password changing module lacks the "use_authtok" option or its prompts
are inconsistent with pam_passwdqc's, you may tell pam_passwdqc to ask
for the old password as well, with "ask_oldauthtok".  In that case the
option to use with the password changing module is "use_first_pass".

There's a number of supported options which can be used to modify the
behavior of pam_passwdqc (defaults are given in square brackets):

	min=N0,N1,N2,N3,N4		[min=disabled,24,12,8,7]

The minimum allowed password lengths, separately for different kinds
of passwords/passphrases.  The special word "disabled" can be used to
disallow passwords of a given kind regardless of their length.  Each
subsequent number is required to be no larger than the preceding one.

N0 is used for passwords consisting of characters from one character
class only.  (The character classes are: digits, lower-case letters,
upper-case letters, and other characters.  There's also the special
class for non-ASCII characters which couldn't be classified, but are
assumed to be non-digits.)

N1 is used for passwords consisting of characters from two character
classes, which don't meet the requirements for a passphrase.

N2 is used for passphrases.  A passphrase must consist of sufficient
words (see the "passphrase" option, below).

N3 and N4 are used for passwords consisting of characters from three
and four character classes, respectively.

When calculating the number of character classes, upper-case letters
used as the first character and digits used as the last character of a
password are not counted.

In addition to being sufficiently long, passwords are required to
contain enough different characters for the character classes and
the minimum length they've been checked against.

	max=N				[max=40]

The maximum allowed password length.  This can be used to prevent
users from setting passwords which may be too long for some system
services.

The value 8 is treated specially.  Passwords longer than 8 characters
will not be rejected, but will be truncated to 8 characters for the
strength checks and the user will be warned.  This is to be used with
the traditional crypt(3) password hashes.

It is important that you do set max=8 if you're using the traditional
hashes, or some weak passwords will pass the checks.

	passphrase=N			[passphrase=3]

The number of words required for a passphrase, or 0 to disable the
support for passphrases.

	match=N				[match=4]

The length of common substring required to conclude that a password is
at least partially based on information found in a character string,
or 0 to disable the substring search.  Note that the password will not
be rejected once a weak substring is found.  Instead, the password
will be subjected to the usual strength requirements with the weak
substring removed.

The substring search is case-insensitive and is able to detect and
remove a common substring spelled backwards.

	similar=permit|deny		[similar=deny]

Whether a new password is allowed to be similar to the old one.  The
passwords are considered to be similar when there's a sufficiently
long common substring and the new password with the substring removed
would be weak.

	random=N[,only]			[random=42]

The size of randomly-generated passwords in bits, or 0 to disable this
feature.  Passwords that contain the offered randomly-generated string
will be allowed regardless of other possible restrictions.

The "only" modifier can be used to disallow user-chosen passwords.

	enforce=none|users|everyone	[enforce=everyone]

The module can be configured to warn of weak passwords only, but not
actually enforce strong passwords.  The "users" setting will enforce
strong passwords for non-root users only.

	non-unix			[]

By default, the module uses getpwnam(3) to obtain the user's personal
login information and use that during the password strength checks.
This behavior can be disabled with "non-unix".

	retry=N				[retry=3]

The number of times the module will ask for a new password if the user
fails to provide a sufficiently strong password and enter it twice the
first time.

	ask_oldauthtok[=update]		[]

Ask for the old password as well.  Normally, pam_passwdqc leaves this
task for the password changing module.  A simple "ask_oldauthtok" will
cause pam_passwdqc to ask for the old password during the preliminary
check phase.  With "ask_oldauthtok=update", pam_passwdqc will do that
during the update phase.

	check_oldauthtok		[]

This tells pam_passwdqc to validate the old password before giving a
new password prompt.  Normally, this task is left for the password
changing module.

The primary use for this option is with "ask_oldauthtok=update" in
which case no other modules have a chance to run and validate the
password between the prompts.  Of course, this will only work with
Unix passwords.

	use_first_pass			[]
	use_authtok			[]

Use the new password obtained by modules stacked before pam_passwdqc.
This disables user interaction within pam_passwdqc.  With this module,
the only difference between "use_first_pass" and "use_authtok" is that
the former is incompatible with "ask_oldauthtok".

--
Solar Designer <solar@openwall.com>