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.
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
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.
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 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.
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
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.
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