PASSWD

Password Changes

If the account has a non-empty password, the user is first prompted to enter their current password. The entered password is encrypted and compared to the stored value. The user has only one attempt to enter the correct password. The superuser can bypass this step to allow changing forgotten passwords.

After the password has been entered, password aging information is checked to determine 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 for the password to be changed.

Then, the password is tested for complexity. passwd rejects passwords that do not meet the complexity requirements. Do not include the system default erase or kill characters.  

Hints for user passwords

The security of a password depends on 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 on the randomness of the selected password.

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 one that 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 https://en.wikipedia.org/wiki/Password_strength  

OPTIONS

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

Deletes a user's password, making it empty. This command sets the account to be 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 sets the account's expire date to 1970-01-02).

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. No SELINUX support.

-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. No PAM support. 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 password expiration during which the user is 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.

CAVEATS

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.  

CONFIGURATION

The following configuration variables in /etc/login.defs change the behavior of this tool:

ENCRYPT_METHOD (string)

This defines the system default encryption algorithm for encrypting passwords (if no algorithm is specified on the command line).

It can take one of these values: DES (default), MD5, SHA256, SHA512. MD5 and DES should not be used for new hashes, see crypt(5) for recommendations.

Note: this parameter overrides the MD5_CRYPT_ENAB variable.

MD5_CRYPT_ENAB (boolean)

Indicate if passwords must be encrypted using the MD5-based algorithm. If set to yes, new passwords will be encrypted using the MD5-based algorithm compatible with the one used by recent releases of FreeBSD. It supports passwords of unlimited length and longer salt strings. Set to no if you need to copy encrypted passwords to other systems which don't understand the new algorithm. Default is no.

This variable is superseded by the ENCRYPT_METHOD variable or by any command line option used to configure the encryption algorithm.

This variable is deprecated. You should use ENCRYPT_METHOD.

OBSCURE_CHECKS_ENAB (boolean)

Enable additional checks upon password changes.

PASS_ALWAYS_WARN (boolean)

Warn about weak passwords (but still allow them) if you are root.

PASS_CHANGE_TRIES (number)

Maximum number of attempts to change password if rejected (too easy).

PASS_MAX_LEN (number), PASS_MIN_LEN (number)

Number of significant characters in the password for crypt().

PASS_MAX_LEN is 8 by default. Don't change unless your crypt() is better.

PASS_MAX_LEN is only used for DES. It is ignored if an encryption algorithm other than DES is used, either because MD5_CRYPT_ENAB set to yes or ENCRYPT_METHOD set to an algorithm other than DES.

In addition, PASS_MAX_LEN is only used if OBSCURE_CHECKS_ENAB set to yes.

Note: This only affects the generation of user passwords. Group password lengths are not checked.

SHA_CRYPT_MIN_ROUNDS (number), SHA_CRYPT_MAX_ROUNDS (number)

When ENCRYPT_METHOD is set to SHA256 or SHA512, this defines the number of SHA rounds used by the encryption algorithm by default (when the number of rounds is not specified on the command line).

With a lot of rounds, it is more difficult to brute force the password. But note also that more CPU resources will be needed to authenticate users.

If not specified, the libc will choose the default number of rounds (5000), which is orders of magnitude too low for modern hardware.

The values must be inside the 1000-999,999,999 range.

If only one of the SHA_CRYPT_MIN_ROUNDS or SHA_CRYPT_MAX_ROUNDS values is set, then this value will be used.

If SHA_CRYPT_MIN_ROUNDS > SHA_CRYPT_MAX_ROUNDS, the highest value will be used.

FILES

/etc/passwd

User account information.

/etc/shadow

Secure user account information.

/etc/login.defs

Shadow password suite configuration.

EXIT VALUES

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

10

an error was returned by pam(3)

SEE ALSO

chpasswd(8), makepasswd(1), passwd(5), shadow(5), login.defs(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/"

Index

NAME

SYNOPSIS

DESCRIPTION

Password Changes

Hints for user passwords

OPTIONS

CAVEATS

CONFIGURATION

FILES

EXIT VALUES

SEE ALSO