Password Changes

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 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

*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.  

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 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.

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

*An4wtbt

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 here.  

OPTIONS

The options which apply to the passwd command are:

-a, --all

This option can be used only with -S and causes show status for all users.

-d, --delete

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 passwordless.

-e, --expire

Immediately expire an account's password. This in effect can force a user to change his/her password at the user's next login.

-h, --help

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.

-k, --keep-tokens

Indicate password change should be performed only for expired authentication tokens (passwords). The user wishes to keep their non-expired tokens as before.

-l, --lock

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.

-q, --quiet

Quiet mode.

-r, --repository REPOSITORY

change password in REPOSITORY repository

-S, --status

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.

-u, --unlock

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.

CAVEATS

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 NIS server.  

FILES

/etc/passwd

User account information.

/etc/shadow

Secure user account information.

EXIT VALUES

The passwd command exits with the following values:

0

success

1

permission denied

2

invalid combination of options

3

unexpected failure, nothing done

4

unexpected failure, passwd file missing

5

passwd file busy, try again

6

invalid argument to option

SEE ALSO

group(5), passwd(5), shadow(5).

Index

NAME

SYNOPSIS

DESCRIPTION

Password Changes

Hints for user passwords

OPTIONS

CAVEATS

FILES

EXIT VALUES

SEE ALSO

2. passwd.5.man

Manpage of PASSWD

PASSWD

NAME

DESCRIPTION

In the good old days there was no great problem with this general read permission. Everybody could read the encrypted passwords, but the hardware was too slow to crack a well-chosen password, and moreover, the basic assumption used to be that of a friendly user-community. These days many people run some version of the shadow password suite, where /etc/passwd has asterisks (*) instead of encrypted passwords, and the encrypted passwords are in /etc/shadow which is readable by the superuser only.

Regardless of whether shadow passwords are used, many sysadmins use an asterisk in the encrypted password field to make sure that this user can not authenticate him- or herself using a password. (But see the Notes below.)

If you create a new login, first put an asterisk in the password field, then use passwd(1) to set it.

There is one entry per line, and each line has the format:

account:password:UID:GID:GECOS:directory:shell

The field descriptions are:

account

the name of the user on the system. It should not contain capital letters.

password

the encrypted user password, an asterisk (*), or the letter 'x'. (See pwconv(8) for an explanation of 'x'.)

UID

the numerical user ID.

GID

the numerical primary group ID for this user.

GECOS

This field is optional and only used for informational purposes. Usually, it contains the full user name. GECOS means General Electric Comprehensive Operating System, which has been renamed to GCOS when GE's large systems division was sold to Honeywell. Dennis Ritchie has reported: "Sometimes we sent printer output or batch jobs to the GCOS machine. The gcos field in the password file was a place to stash the information for the $IDENTcard. Not elegant."

directory

the user's $HOME directory.

shell

the program to run at login (if empty, use /bin/sh). If set to a non-existing executable, the user will be unable to login through login(1).

FILES

NOTES

If the encrypted password is set to an asterisk, the user will be unable to login using login(1), but may still login using rlogin(1), run existing processes and initiate new ones through rsh(1), cron(8), at(1), or mail filters, etc. Trying to lock an account by simply changing the shell field yields the same result and additionally allows the use of su(1).  

SEE ALSO

COLOPHON

Index

NAME

DESCRIPTION

FILES

NOTES

SEE ALSO

COLOPHON