pam_unix(8) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | OPTIONS | MODULE TYPES PROVIDED | RETURN VALUES | EXAMPLES | SEE ALSO | AUTHOR | COLOPHON

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

NAME         top

       pam_unix - Module for traditional password authentication

SYNOPSIS         top

       pam_unix.so [...]

DESCRIPTION         top

       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         top

       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 max. for
           DES crypt based passwords are 8 characters.

       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         top

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

RETURN VALUES         top

       PAM_IGNORE
           Ignore this module.

EXAMPLES         top

       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         top

       login.defs(5), pam.conf(5), pam.d(5), pam(8)

AUTHOR         top

       pam_unix was written by various people.

COLOPHON         top

       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               04/19/2016                    PAM_UNIX(8)

Pages that refer to this page: pam_cracklib(8)unix_chkpwd(8)unix_update(8)