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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | CAVEATS | FILES | EXIT VALUES | SEE ALSO | COLOPHON |
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PASSWD(1) User Commands PASSWD(1)
passwd - change user password
passwd [options] [LOGIN]
The passwd command changes passwords for user accounts. A normal
user may only change the password for their own account, while
the superuser may change the password for any account. passwd
also changes the account or associated password validity period.
Password Changes
The user is first prompted for their old password, if one is
present. This password is then encrypted and compared against the
stored password. The user has only one chance to enter the
correct password. The superuser is permitted to bypass this step
so that forgotten passwords may be changed.
After the password has been entered, password aging information
is checked to see if the user is permitted to change the password
at this time. If not, passwd refuses to change the password and
exits.
The user is then prompted twice for a replacement password. The
second entry is compared against the first and both are required
to match in order for the password to be changed.
Then, the password is tested for complexity. passwd will reject
any password which is not suitably complex. Care must be taken
not to include the system default erase or kill characters.
Hints for user passwords
The security of a password depends upon the strength of the
encryption algorithm and the size of the key space. The legacy
UNIX System encryption method is based on the NBS DES algorithm.
More recent methods are now recommended (see ENCRYPT_METHOD). The
size of the key space depends upon the randomness of the password
which is selected.
Compromises in password security normally result from careless
password selection or handling. For this reason, you should not
select a password which appears in a dictionary or which must be
written down. The password should also not be a proper name, your
license number, birth date, or street address. Any of these may
be used as guesses to violate system security.
As a general guideline, passwords should be long and random. It's
fine to use simple character sets, such as passwords consisting
only of lowercase letters, if that helps memorizing longer
passwords. For a password consisting only of lowercase English
letters randomly chosen, and a length of 32, there are 26^32
(approximately 2^150) different possible combinations. Being an
exponential equation, it's apparent that the exponent (the
length) is more important than the base (the size of the
character set).
You can find advice on how to choose a strong password on
http://en.wikipedia.org/wiki/Password_strength
The options which apply to the passwd command are:
-a, --all
This option can be used only with -S and causes show status
for all users.
-d, --delete
Delete a user's password (make it empty). This is a quick way
to disable a password for an account. It will set the named
account passwordless.
-e, --expire
Immediately expire an account's password. This in effect can
force a user to change their password at the user's next
login.
-h, --help
Display help message and exit.
-i, --inactive INACTIVE
This option is used to disable an account after the password
has been expired for a number of days. After a user account
has had an expired password for INACTIVE days, the user may
no longer sign on to the account.
-k, --keep-tokens
Indicate password change should be performed only for expired
authentication tokens (passwords). The user wishes to keep
their non-expired tokens as before.
-l, --lock
Lock the password of the named account. This option disables
a password by changing it to a value which matches no
possible encrypted value (it adds a ´!´ at the beginning of
the password).
Note that this does not disable the account. The user may
still be able to login using another authentication token
(e.g. an SSH key). To disable the account, administrators
should use usermod --expiredate 1 (this set the account's
expire date to Jan 2, 1970).
Users with a locked password are not allowed to change their
password.
-n, --mindays MIN_DAYS
Set the minimum number of days between password changes to
MIN_DAYS. A value of zero for this field indicates that the
user may change their password at any time.
-q, --quiet
Quiet mode.
-r, --repository REPOSITORY
change password in REPOSITORY repository
-R, --root CHROOT_DIR
Apply changes in the CHROOT_DIR directory and use the
configuration files from the CHROOT_DIR directory. Only
absolute paths are supported.
-P, --prefix PREFIX_DIR
Apply changes to configuration files under the root
filesystem found under the directory PREFIX_DIR. This option
does not chroot and is intended for preparing a
cross-compilation target. Some limitations: NIS and LDAP
users/groups are not verified. PAM authentication is using
the host files. No SELINUX support.
-S, --status
Display account status information. The status information
consists of 7 fields. The first field is the user's login
name. The second field indicates if the user account has a
locked password (L), has no password (NP), or has a usable
password (P). The third field gives the date of the last
password change. The next four fields are the minimum age,
maximum age, warning period, and inactivity period for the
password. These ages are expressed in days.
-u, --unlock
Unlock the password of the named account. This option
re-enables a password by changing the password back to its
previous value (to the value before using the -l option).
-w, --warndays WARN_DAYS
Set the number of days of warning before a password change is
required. The WARN_DAYS option is the number of days prior to
the password expiring that a user will be warned that their
password is about to expire.
-x, --maxdays MAX_DAYS
Set the maximum number of days a password remains valid.
After MAX_DAYS, the password is required to be changed.
Passing the number -1 as MAX_DAYS will remove checking a
password's validity.
-s, --stdin
This option is used to indicate that passwd should read the
new password from standard input, which can be a pipe.
Password complexity checking may vary from site to site. The user
is urged to select a password as complex as he or she feels
comfortable with.
Users may not be able to change their password on a system if NIS
is enabled and they are not logged into the NIS server.
passwd uses PAM to authenticate users and to change their
passwords.
/etc/passwd
User account information.
/etc/shadow
Secure user account information.
/etc/pam.d/passwd
PAM configuration for passwd.
The passwd command exits with the following values:
0
success
1
permission denied
2
invalid combination of options
3
unexpected failure, nothing done
4
unexpected failure, passwd file missing
5
passwd file busy, try again
6
invalid argument to option
chpasswd(8), makepasswd(1), passwd(5), shadow(5), usermod(8).
The following web page comically (yet correctly) compares the
strength of two different methods for choosing a password:
"https://xkcd.com/936/"
This page is part of the shadow-utils (utilities for managing
accounts and shadow password files) project. Information about
the project can be found at
⟨https://github.com/shadow-maint/shadow⟩. If you have a bug
report for this manual page, send it to
[email protected]. This page was obtained
from the project's upstream Git repository
⟨https://github.com/shadow-maint/shadow⟩ on 2024-06-15. (At that
time, the date of the most recent commit that was found in the
repository was 2024-06-13.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
shadow-utils 4.14.0 06/15/2024 PASSWD(1)
Pages that refer to this page: ldappasswd(1), login(1), login(1@@shadow-utils), crypt(3), pts(4), login.defs(5), passwd(5), passwd(5@@shadow-utils), shadow(5), chpasswd(8), groupadd(8), groupdel(8), groupmems(8), groupmod(8), newusers(8), useradd(8), userdel(8), usermod(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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