pam_unix(8) — Linux manual page


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

NAME         top

       pam_unix - Module for traditional password authentication

SYNOPSIS         top [...]

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

           Turns on debugging via syslog(3).

           A little more extreme than debug.

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

           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.

           Allow users to authenticate with blank password if password
           reset is enforced even if nullok is not set. If password
           reset is not required and nullok is not set the
           authentication with blank password will be denied.

           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_passwdqc module documented below).

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

           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. The MD5 password hash algorithm is
           used for storing the old passwords. 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 algorithm.

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

           When a user changes their password next, encrypt it with the
           SHA256 algorithm. The SHA256 algorithm must be supported by
           the crypt(3) function.

           When a user changes their password next, encrypt it with the
           SHA512 algorithm. The SHA512 algorithm must be supported by
           the crypt(3) function.

           When a user changes their password next, encrypt it with the
           blowfish algorithm. The blowfish algorithm must be supported
           by the crypt(3) function.

           When a user changes their password next, encrypt it with the
           gost-yescrypt algorithm. The gost-yescrypt algorithm must be
           supported by the crypt(3) function.

           When a user changes their password next, encrypt it with the
           yescrypt algorithm. The yescrypt algorithm must be supported
           by the crypt(3) function.

           Set the optional number of rounds of the SHA256, SHA512,
           blowfish, gost-yescrypt, and yescrypt 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 max. for
           DES crypt based passwords are 8 characters.

           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

       Invalid arguments are logged with syslog(3).


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

RETURN VALUES         top

           Ignore this module.

EXAMPLES         top

       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
           # Change the user's password, but at first check the strength
           # with pam_passwdqc(8)
           password   required config=/etc/passwdqc.conf
           password   required use_authtok nullok yescrypt
           session    required

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 ⟨⟩.  If you have a bug report
       for this manual page, see ⟨//⟩.  This page was
       obtained from the project's upstream Git repository
       ⟨⟩ on 2023-12-22.  (At
       that time, the date of the most recent commit that was found in
       the repository was 2023-12-18.)  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

Linux-PAM Manual               12/22/2023                    PAM_UNIX(8)

Pages that refer to this page: passwd(5)unix_chkpwd(8)unix_update(8)