shadow/man/passwd.1

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.\"	$Id: passwd.1,v 1.5 2000/08/26 18:27:17 marekm Exp $
.\"
.TH PASSWD 1
.SH NAME
passwd \- change user password
.SH SYNOPSIS
\fBpasswd\fR [\fB-f\fR|\fB-s\fR] [\fIname\fR]
.br
\fBpasswd\fR [\fB-g\fR] [\fB-r\fR|\fBR\fR] \fIgroup\fR
.br
\fBpasswd\fR [\fB-x\fR \fImax\fR] [\fB-n\fR \fImin\fR]
[\fB-w\fR \fIwarn\fR] [\fB-i\fR \fIinact\fR] \fIname\fR
.br
\fBpasswd\fR {\fB-l\fR|\fB-u\fR|\fB-d\fR|\fB-S\fR} \fIname\fR
.SH DESCRIPTION
\fBpasswd\fR 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.
\fBpasswd\fR also changes account information, such as the full name
of the user, their login shell, or password expiry dates and intervals.
.SS 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 permitted to bypass this step so that forgotten
passwords may be changed.
.PP
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, \fBpasswd\fR refuses to change the password and exits.
.PP
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:
.IP "" .5i
Lower case alphabetics
.IP "" .5i
Upper case alphabetics
.IP "" .5i
Digits 0 thru 9
.IP "" .5i
Punctuation marks
.PP
Care must be taken not to include the system default erase
or kill characters.
\fBpasswd\fR will reject any password which is not suitably
complex.
.PP
If the password is accepted,
\fBpasswd\fR 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.
.SS Group passwords
When the \fB-g\fR 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 \fB-r\fR option is used with the \fB-g\fR option to remove
the current password from the named group.
This allows group access to all members.
The \fB-R\fR option is used with the \fB-g\fR option to restrict
the named group for all users.
.SS Password expiry information
The password aging information may be changed by the super
user with the \fB-x\fR, \fB-n\fR, \fB-w\fR, and \fB-i\fR options.
The \fB-x\fR option is used to set the maximum number of days
a password remains valid.
After \fImax\fR days, the password is required to be changed.
The \fB-n\fR 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
\fImin\fR days have elapsed.
The \fB-w\fR option is used to set the number of days of warning
the user will receive before their password will expire.
The warning occurs \fIwarn\fR days before the expiration, telling
the user how many days until the password is set to expire.
The \fB-i\fR 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 \fIinact\fR
days, the user may no longer sign on to the account.
.SS Account maintenance
User accounts may be locked and unlocked with the \fB-l\fR and
\fB-u\fR flags.
The \fB-l\fR option disables an account by changing the password to a
value which matches no possible encrypted value.
The \fB-u\fR option re-enables an account by changing the password
back to its previous value.
.PP
The account status may be given with the \fB-S\fR 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.
.SS 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 \fB\s-2UNIX\s+2\fR 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.
.PP
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.
.PP
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.
.PP
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
.IP "" .5i
Ask not for whom the bell tolls.
.PP
which produces
.IP "" .5i
An4wtbt.
.PP
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.
.SS 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-operation
between different users.
.SH 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.
.SH FILES
/etc/passwd \- user account information
.br
/etc/shadow \- encrypted user passwords
.SH SEE ALSO
.BR passwd (3),
.\" .BR shadow (3),
.BR group (5),
.BR passwd (5)
.SH AUTHOR
Julianne Frances Haugh (jfh@austin.ibm.com)