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 are a number of supported options, which can be used to modify the behavior of pam_passwdqc (defaults are given in square brackets):
Load the specified configuration FILE, which must be in the passwdqc.conf format (described in the passwdqc.conf(5) manual page). This file may define any options described in here, including load of yet another configuration file, but loops are not allowed.
The minimum allowed password lengths for different kinds of passwords and passphrases. The keyword "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 is also a special class for non-ASCII characters, which could not be classified, but are assumed to be non-digits.
N1 is used for passwords consisting of characters from two character classes that do not meet the requirements for a passphrase.
N2 is used for passphrases. Note that besides meeting this length requirement, a passphrase must also consist of a sufficient number of 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 have been checked against.
The maximum allowed password length. This can be used to prevent users from setting passwords that may be too long for some system services.
The value 8 is treated specially: with max=8, 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 DES-based password hashes, which truncate the password at 8 characters.
It is important that you do set max=8 if you are using the traditional hashes, or some weak passwords will pass the checks.
The number of words required for a passphrase, or 0 to disable the support for user-chosen passphrases.
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; it will instead be subjected to the usual strength requirements with the weak substring partially discounted.
The substring search is case-insensitive and is able to detect and remove a common substring spelled backwards.
Whether a new password is allowed to be similar to the old one. The passwords are considered to be similar when there is a sufficiently long common substring and the new password with the substring partially discounted would be weak.
Deny passwords that are based on lines of a tiny external text file, which can reasonably be e.g. a list of a few thousand common passwords. Common dictionary words may also reasonably be included, especially in a local language other than English, or longer yet common English words. (passwdqc includes a list of a few thousand common English words of lengths from 3 to 6 built in. Any word list possibly specified with this option is used in addition to the built-in word list.)
Substring matching and discounting will be used if the "match" setting above is non-zero. Please note that this is very inefficient, and isn't to be used with large wordlists.
Deny passwords or passphrases directly appearing in a tiny external text file. That file can reasonably be e.g. a list of common passwords if only a relaxed policy is desired and stricter checks are thus disabled (using their separate options). Such policy would only be somewhat effective against online/remote attacks, but not against offline attacks on hashed passwords.
Deny passwords or passphrases directly appearing in a maybe huge binary filter file created with pwqfilter. This is very efficient, needing at most two random disk reads per query. A filter created from millions of leaked passwords can reasonably be used on top of passwdqc's other checks to further reduce the number of passing yet weak passwords without causing unreasonable inconvenience (as e.g. higher minimum lengths and character set requirements could).
The size of randomly-generated passphrases in bits (24 to 136), or 0 to disable this feature. Any passphrase that contains the offered randomly-generated string will be allowed regardless of other possible restrictions.
The "only" modifier can be used to disallow user-chosen passwords.
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 invocations by non-root users only.
Normally, 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 the "non-unix" option.
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 for the old password as well. Normally, pam_passwdqc leaves this task for subsequent modules. With no argument, the "ask_oldauthtok" option 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.
This tells pam_passwdqc to validate the old password before giving a new password prompt. Normally, this task is left for subsequent modules.
The primary use for this option is when "ask_oldauthtok=update" is also specified, in which case no other module gets a chance to ask for and validate the password. 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".
If audit is enabled at build time, the PAM module logs audit events once user tries to change their credentials. This option disables that audit logging.