pam_unix [...]

       This is the standard Unix authentication module. It uses standard calls
       from the system's libraries to retrieve and set account information as
       well as authentication. Usually this is obtained from the /etc/passwd
       and the /etc/shadow file as well if shadow is enabled.

       The account component performs the task of establishing the status of
       the user's account and password based on the following shadow elements:
       expire, last_change, max_change, min_change, warn_change. In the case
       of the latter, it may offer advice to the user on changing their
       password or, through the PAM_AUTHTOKEN_REQD return, delay giving
       service to the user until they have established a new password. The
       entries listed above are documented in the shadow(5) manual page.
       Should the user's record not contain one or more of these entries, the
       corresponding shadow check is not performed.

       The authentication component performs the task of checking the users
       credentials (password). The default action of this module is to not
       permit the user access to a service if their official password is

       A helper binary, unix_chkpwd(8), is provided to check the user's
       password when it is stored in a read protected database. This binary is
       very simple and will only check the password of the user invoking it.
       It is called transparently on behalf of the user by the authenticating
       component of this module. In this way it is possible for applications
       like xlock(1) to work without being setuid-root. The module, by
       default, will temporarily turn off SIGCHLD handling for the duration of
       execution of the helper binary. This is generally the right thing to
       do, as many applications are not prepared to handle this signal from a
       child they didn't know was fork()d. The noreap module argument can be
       used to suppress this temporary shielding and may be needed for use
       with certain applications.

       The password component of this module performs the task of updating the
       user's password. The default encryption hash is taken from the
       ENCRYPT_METHOD variable from /etc/login.defs

       The session component of this module logs when a user logins or leave
       the system.

       Remaining arguments, supported by others functions of this module, are
       silently ignored. Other arguments are logged as errors through

           Turns on debugging via syslog(3).

           A little more extreme than debug.

           Before prompting the user for their password, the module first
           tries the previous stacked module's password in case that satisfies
           this module as well.

           The argument use_first_pass forces the module to use a previous
           stacked modules password and will never prompt the user - if no
           password is available or the password is not appropriate, the user
           will be denied access.

           This argument can be used to discourage the authentication
           component from requesting a delay should the authentication as a
           whole fail. The default action is for the module to request a
           delay-on-failure of the order of two second.

           When password changing enforce the module to set the new password
           to the one provided by a previously stacked password module (this
           is used in the example of the stacking of the pam_cracklib module
           documented below).

           This argument is used to inform the module that it is not to pay
           attention to/make available the old or new passwords from/to other
           (stacked) password modules.

           NIS RPC is used for setting new passwords.

           The last n passwords for each user are saved in
           /etc/security/opasswd in order to force password change history and
           keep the user from alternating between the same password too
           frequently. Instead of this option the pam_pwhistory module should
           be used.

           Try to maintain a shadow based system.

           When a user changes their password next, encrypt it with the MD5

           When a user changes their password next, encrypt it with the DEC C2

           When a user changes their password next, encrypt it with the SHA256
           algorithm. If the SHA256 algorithm is not known to the crypt(3)
           function, fall back to MD5.
           Set the optional number of rounds of the SHA256, SHA512 and
           blowfish password hashing algorithms to n.

           Ignore errors reading shadow information for users in the account
           management module.

           Set a minimum password length of n characters. The default value is
           6. The maximum for DES crypt-based passwords is 8 characters.

           Enable some extra checks on password strength. These checks are
           based on the "obscure" checks in the original shadow package. The
           behavior is similar to the pam_cracklib module, but for
           non-dictionary-based checks. The following checks are implemented:

               Verifies that the new password is not a palindrome of (i.e.,
               the reverse of) the previous one.

           Case Change Only
               Verifies that the new password isn't the same as the old one
               with a change of case.

               Verifies that the new password isn't too much like the previous

               Is the new password too simple? This is based on the length of
               the password and the number of different types of characters
               (alpha, numeric, etc.) used.

               Is the new password a rotated version of the old password?
               (E.g., "billy" and "illyb")

       Invalid arguments are logged with syslog(3).

       All module types (account, auth, password and session) are provided.

           Ignore this module.

       An example usage for /etc/pam.d/login would be:

           # Authenticate the user
           auth       required
           # Ensure users account and password are still active
           account    required

Linux-PAM Manual                  09/19/2013                       PAM_UNIX(8)
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