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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.
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