pam_unix

PAM_UNIX(8)                    Linux-PAM Manual                    PAM_UNIX(8)

NAME
       pam_unix - Module for traditional password authentication

SYNOPSIS
       pam_unix.so [...]

DESCRIPTION
       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
       blank.

       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 maximum length of a password supported by the pam_unix module via
       the helper binary is PAM_MAX_RESP_SIZE - currently 512 bytes. The rest
       of the password provided by the conversation function to the module
       will be ignored.

       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
       syslog(3).

OPTIONS
       debug
           Turns on debugging via syslog(3).

       audit
           A little more extreme than debug.

       quiet
           Turns off informational messages namely messages about session open
           and close via syslog(3).

       nullok
           The default action of this module is to not permit the user access
           to a service if their official password is blank. The nullok
           argument overrides this default.

       try_first_pass
           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.

       use_first_pass
           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.

       nodelay
           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.

       use_authtok
           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).

       authtok_type=type
           This argument can be used to modify the password prompt when
           changing passwords to include the type of the password. Empty by
           default.

       nis
           NIS RPC is used for setting new passwords.

       remember=n
           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. The MD5 password hash algorithm is used for storing the
           old passwords. Instead of this option the pam_pwhistory module
           should be used.

       shadow
           Try to maintain a shadow based system.

       md5
           When a user changes their password next, encrypt it with the MD5
           algorithm.

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

       sha256
           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.

       sha512
           When a user changes their password next, encrypt it with the SHA512
           algorithm. If the SHA512 algorithm is not known to the crypt(3)
           function, fall back to MD5.

       blowfish
           When a user changes their password next, encrypt it with the
           blowfish algorithm. If the blowfish algorithm is not known to the
           crypt(3) function, fall back to MD5.

       rounds=n
           Set the optional number of rounds of the SHA256, SHA512 and
           blowfish password hashing algorithms to n.

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

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

       obscure
           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:

           Palindrome
               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.

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

           Simple
               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.

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

       no_pass_expiry
           When set ignore password expiration as defined by the shadow entry
           of the user. The option has an effect only in case pam_unix was not
           used for the authentication or it returned authentication failure
           meaning that other authentication source or method succeeded. The
           example can be public key authentication in sshd. The module will
           return PAM_SUCCESS instead of eventual PAM_NEW_AUTHTOK_REQD or
           PAM_AUTHTOK_EXPIRED.

       Invalid arguments are logged with syslog(3).

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

RETURN VALUES
       PAM_IGNORE
           Ignore this module.

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

           # Authenticate the user
           auth       required   pam_unix.so
           # Ensure users account and password are still active
           account    required   pam_unix.so
           # Change the user's password, but at first check the strength
           # with pam_cracklib(8)
           password   required   pam_cracklib.so retry=3 minlen=6 difok=3
           password   required   pam_unix.so use_authtok nullok md5
           session    required   pam_unix.so

SEE ALSO
       login.defs(5), pam.conf(5), pam.d(5), pam(7)

AUTHOR
       pam_unix was written by various people.

Linux-PAM Manual                  05/18/2017                       PAM_UNIX(8)