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

       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 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

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.

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

           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

           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. If the SHA256 algorithm is not known to the
           crypt(3) function, fall back to MD5.

           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.

           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.

           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 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

       Invalid arguments are logged with syslog(3).


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

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_cracklib(8)
           password   required retry=3 minlen=6 difok=3
           password   required use_authtok nullok md5
           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
       tarball Linux-PAM-1.3.0.tar.bz2 fetched from
       ⟨⟩ on 2020-09-18.  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

Linux-PAM Manual                 04/19/2016                      PAM_UNIX(8)

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