pam_unix — Module for traditional password authentication

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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 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).

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


AUTHOR

pam_unix was written by various people.