Section 1: passwd
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PASSWD(1) PASSWD(1)
NAME
passwd - change user password
SYNOPSIS
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
DESCRIPTION
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
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 permit-
ted to bypass this step so that forgotten passwords may be
changed.
After the password has been entered, password aging infor-
mation 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.
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PASSWD(1) PASSWD(1)
Group passwords
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
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 num-
ber 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 dis-
able 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
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 pass-
word (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
The security of a password depends upon the strength of
the encryption algorithm and the size of the key space.
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PASSWD(1) PASSWD(1)
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, Passord.
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
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-opera-
tion between different users.
CAVEATS
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.
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PASSWD(1) PASSWD(1)
FILES
/etc/passwd - user account information
/etc/shadow - encrypted user passwords
SEE ALSO
passwd(3), group(5), passwd(5)
AUTHOR
Julianne Frances Haugh (jfh@bga.com)
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