pam_unix - Module for traditional password authentication
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).
debug
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 and allows any user with a blank
password to access the service.
nullok_secure
The default action of this module is to not permit the
user access to a service if their official password is blank. The
nullok_secure argument overrides this default and allows any user with
a blank password to access the service as long as the value of PAM_TTY is set
to one of the values found in /etc/securetty.
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
default value is 6. The maximum for DES crypt-based passwords is 8
characters.
obscure
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:
Palindrome
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.
Similar
Verifies that the new password isn't too much like the
previous one.
Simple
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.
Rotated
Is the new password a rotated version of the old
password? (E.g., "billy" and "illyb")
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).
All module types (account, auth, password and
session) are provided.
PAM_IGNORE
Ignore this module.
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
pam_unix was written by various people.