||11-16-2007 12:30 AM
Output of man passwd using Kubuntu Feisty Fawn:
PASSWD(1) User Commands PASSWD(1)
passwd - change user password
passwd [options] [LOGIN]
passwd changes passwords for user accounts. A normal user may only
change the password for his/her own account, while the super user may
change the password for any account. passwd also changes account
information, such as the full name of the user, the user’s login shell,
or his/her password expiry date and interval.
The user is first prompted for his/her 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 super user 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
· punctuation marks
Care must be taken not to include the system default erase or kill
characters. passwd will reject any password which is not suitably
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 UNIX System encryption
method is based on the NBS DES algorithm and is very secure. The size
of the key space depends upon the randomness of the password which is
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.
Your password must be easily remembered so that you will not be forced
to write it on a piece of paper. This can be accomplished by appending
two small words together and separating each with a special character
or digit. For example, Pass%word.
Other methods of construction involve selecting an easily remembered
phrase from literature and selecting the first or last letter from each
word. An example of this is:
· Ask not for whom the bell tolls
· which produces
You may be reasonably sure few crackers will have included this in
their dictionaries. You should, however, select your own methods for
constructing passwords and not rely exclusively on the methods given
The options which apply to the passwd command are:
This option can be used only with -S and causes show status for all
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
Immediately expire an account’s password. This in effect can force a
user to change his/her password at the user’s 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 named account. This option disables an account by changing
the password to a value which matches no possible encrypted value.
-m, --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
his/her password at any time.
-r, --repository REPOSITORY
change password in REPOSITORY repository
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 is locked (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 named account. This option re-enables an account by
changing the password back to its previous value (to value before
using -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 his/her 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.
Not all options may be supported. 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
User account information.
Secure user account information.
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
group(5), passwd(5), shadow(5).
User Commands 12/19/2006 PASSWD(1)