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.
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).
# 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 11/05/2016 PAM_UNIX(8)
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