PAM_UNIX(8) Linux-PAM Manual PAM_UNIX(8)
NAME
pam_unix - Module for traditional password authentication
SYNOPSIS
pam_unix.so [...]
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.
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).
not_set_pass
This argument is used to inform the module that it is not to pay
attention to/make available the old or new passwords from/to other
(stacked) password modules.
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. 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. The SHA256 algorithm must be supported by the crypt(3)
function.
sha512
When a user changes their password next, encrypt it with the SHA512
algorithm. The SHA512 algorithm must be supported by the crypt(3)
function.
blowfish
When a user changes their password next, encrypt it with the
blowfish algorithm. The blowfish algorithm must be supported by the
crypt(3) function.
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
All module types (account, auth, password and session) are provided.
RETURN VALUES
PAM_IGNORE
Ignore this module.
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 users 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
login.defs(5), pam.conf(5), pam.d(5), pam(8)
AUTHOR
pam_unix was written by various people.
Linux-PAM Manual 04/01/2020 PAM_UNIX(8)
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