passwd - change user password
passwd [-f|-s] [name]
passwd [-g] [-r|R] group
passwd [-x max] [-n min] [-w warn] [-i inact] name
passwd {-l|-u|-d|-S|-e} name
passwd changes passwords for user and group accounts. A normal user
may only change the password for their own account, the super user may
change the password for any account. The administrator of a group may
change the password for the group. passwd also changes account information,
such as the full name of the user, their login shell, or password
expiry dates and intervals.
Password Changes [Toc] [Back]
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
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 their password at
this time. If not, passwd refuses to change the password and exits.
The user is then prompted for a replacement password. This password is
tested for complexity. As a general guideline, passwords should consist
of 6 to 8 characters including one or more from each of following
sets:
Lower case alphabetics
Upper 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 complex.
If the password is accepted, passwd will prompt again and compare the
second entry against the first. Both entries are require to match in
order for the password to be changed.
Group passwords [Toc] [Back]
When the -g option is used, the password for the named group is
changed. The user must either be the super user, or a group administrator
for the named group. The current group password is not prompted
for. The -r option is used with the -g option to remove the current
password from the named group. This allows group access to all members.
The -R option is used with the -g option to restrict the named
group for all users.
Password expiry information [Toc] [Back]
The password aging information may be changed by the super user with
the -x, -n, -w, and -i options. The -x option is used to set the maximum
number of days a password remains valid. After max days, the password
is required to be changed. The -n option is used to set the minimum
number of days before a password may be changed. The user will not
be permitted to change the password until min days have elapsed. The
-w option is used to set the number of days of warning the user will
receive before their password will expire. The warning occurs warn
days before the expiration, telling the user how many days until the
password is set to expire. The -i 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 inact days, the user may no
longer sign on to the account.
Account maintenance [Toc] [Back]
User accounts may be locked and unlocked with the -l and -u flags. The
-l option disables an account by changing the password to a value which
matches no possible encrypted value. The -u option re-enables an
account by changing the password back to its previous value.
If you wish to immediately expire an accounts password, you can use the
-e option. This in affect can force a user to change their password at
their next login. You can also use the -d option to delete a users
password (make it empty). Use caution with this option since it can
make an account not require a password at all to login, leaving your
system open to intruders.
The account status may be given with the -S option. The status information
consists of 6 parts. The first part indicates if the user
account is locked (L), has no password (NP), or has a usable password
(P). The second part gives the date of the last password change. The
next four parts are the minimum age, maximum age, warning period, and
inactivity period for the password.
Hints for user passwords [Toc] [Back]
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
selected.
The -s option makes passwd call chsh to change the users shell. The -f
option makes passwd call chfn to change the users gecos information.
These two options are only meant for compatiblity, since the other programs
can be called directly.
Compromises in password security normally result from careless password
selection or handling. For this reason, you should select a password
which does not appear 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 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. An example of this is
Ask not for whom the bell tolls.
which produces
An4wtbt.
You may be reasonably sure few crackers will have included this in
their dictionary. You should, however, select your own methods for
constructing passwords and not rely exclusively on the methods given
here.
Notes about group passwords [Toc] [Back]
Group passwords are an inherent security problem since more than one
person is permitted to know the password. However, groups are a useful
tool for permitting co-operation between different users.
Not all options may be supported. Password complexity checking may
vary from site to site. The user is urged to select as complex a password
as they feel comfortable with. User's may not be able to change
their password on a system if NIS is enabled and they are not logged
into the NIS server.
/etc/passwd - user account information
/etc/shadow - encrypted user passwords
group(5), passwd(5)
Julianne Frances Haugh ([email protected])
PASSWD(1)
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