the password file
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.
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. As a general guideline, passwords should consist of 6 to 8 characters including one or more characters from each of the following sets:
lower case alphabetics
digits 0 thru 9
Care must be taken not to include the system default erase or kill characters. passwd will reject any password which is not suitably complex.
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.
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:
This option can be used only with -S and causes show status for all users.
Delete a users password (make it empty). This is a quick way to disable a password for an account. It will set the named account passwordless.
Immediately expire an accounts password. This in effect can force a user to change their password at the users next login.
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.
Indicate password change should be performed only for expired authentication tokens (passwords). The user wishes to keep their non-expired tokens as before.
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 accounts 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.
-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.
Display account status information. The status information consists of 7 fields. The first field is the users 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.
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 passwords validity.
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.
The following configuration variables in /etc/login.defs change the behavior of this tool:
This defines the system default encryption algorithm for encrypting passwords (if no algorithm are specified on the command line).
It can take one of these values: DES (default), MD5, SHA256, SHA512.
Note: this parameter overrides the MD5_CRYPT_ENAB variable.
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 dont 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.
Enable additional checks upon password changes.
Warn about weak passwords (but still allow them) if you are root.
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. Dont change unless your crypt() is better. This is ignored if MD5_CRYPT_ENAB set to yes.
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 forcing 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).
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.
User account information.
Secure user account information.
Shadow password suite configuration.
The passwd command exits with the following values:
invalid combination of options
unexpected failure, nothing done
unexpected failure, passwd file missing
passwd file busy, try again
invalid argument to option