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
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 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
Remaining arguments, supported by others functions of this module, are
silently ignored. Other arguments are logged as errors through
Turns on debugging via syslog(3).
password to access the service as long as the value of PAM_TTY is
set to one of the values found in /etc/securetty.
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
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 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. Instead of this option the pam_pwhistory module should
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
When a user changes their password next, encrypt it with the SHA256
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
Set a minimum password length of n characters. The default value is
6. The maximum for DES crypt-based passwords is 8 characters.
Enable some extra checks on password strength. These checks are
based on the "obscure" checks in the original shadow package. The
behavior is similar to the pam_cracklib module, but for
non-dictionary-based checks. The following checks are implemented:
Verifies that the new password is not a palindrome of (i.e.,
the reverse of) the previous one.
Case Change Only
Verifies that the new password isn't the same as the old one
with a change of case.
Verifies that the new password isn't too much like the previous
Is the new password too simple? This is based on the length of
the password and the number of different types of characters
(alpha, numeric, etc.) used.
Is the new password a rotated version of the old password?
(E.g., "billy" and "illyb")
Invalid arguments are logged with syslog(3).
MODULE TYPES PROVIDED
All module types (account, auth, password and session) are provided.
Ignore this module.
An example usage for /etc/pam.d/login would be:
# Authenticate the user
login.defs(5), pam.conf(5), pam.d(5), pam(7)
pam_unix was written by various people.
Linux-PAM Manual 09/19/2013 PAM_UNIX(8)
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