|Red Hat Enterprise Linux 4: System Administration Guide
|Chapter 33. User and Group Configuration
If you prefer command line tools or do not have the X Window System installed, use this section to configure users and groups.
To add a user to the system:
Issue the useradd command to create a locked user account:
Unlock the account by issuing the passwd command to assign a password and set password aging guidelines:
Command line options for useradd are detailed in Table 33-1.
|Comment for the user
|Home directory to be used instead of default /home/username/
|Date for the account to be disabled in the format YYYY-MM-DD
|Number of days after the password expires until the account is disabled. (If 0 is specified, the account is disabled immediately after the password expires. If -1 is specified, the account is not be disabled after the password expires.)
|Group name or group number for the user's default group (The group must exist prior to being specified here.)
|List of additional (other than default) group names or group numbers, separated by commas, of which the user is a member. (The groups must exist prior to being specified here.)
|Create the home directory if it does not exist
|Do not create the home directory
|Do not create a user private group for the user
|Create a system account with a UID less than 500 and without a home directory
|The password encrypted with crypt
|User's login shell, which defaults to /bin/bash
|User ID for the user, which must be unique and greater than 499
Table 33-1. useradd Command Line Options
To add a group to the system, use the command groupadd:
Command line options for groupadd are detailed in Table 33-2.
|Group ID for the group, which must be unique and greater than 499
|Create a system group with a GID less than 500
|Exit with an error if the group already exists (The group is not altered.) If -g and -f are specified, but the group already exists, the -g option is ignored
Table 33-2. groupadd Command Line Options
For security reasons, it is good practice to require users to change their passwords periodically. This can be done when adding or editing a user on the Password Info tab of the User Manager.
To configure password expiration for a user from a shell prompt, use the chage command, followed by an option from Table 33-3, followed by the username of the user.
Shadow passwords must be enabled to use the chage command.
|Specify the minimum number of days between which the user must change passwords. If the value is 0, the password does not expire.
|Specify the maximum number of days for which the password is valid. When the number of days specified by this option plus the number of days specified with the -d option is less than the current day, the user must change passwords before using the account.
|Specify the number of days since January 1, 1970 the password was changed.
|Specify the number of inactive days after the password expiration before locking the account. If the value is 0, the account is not locked after the password expires.
|Specify the date on which the account is locked, in the format YYYY-MM-DD. Instead of the date, the number of days since January 1, 1970 can also be used.
|Specify the number of days before the password expiration date to warn the user.
Table 33-3. chage Command Line Options
If the chage command is followed directly by a username (with no options), it displays the current password aging values and allows them to be changed.
If a system administrator wants a user to set a password the first time the user log in, the user's initial or null password can be set to expire immediately, forcing the user to change it immediately after logging in for the first time.
To force a user to configure a password the first time the user logs in at the console, follow these steps. Note, this process does not work if the user logs in using the SSH protocol.
Lock the user's password — If the user does not exist, use the useradd command to create the user account, but do not give it a password so that it remains locked.
If the password is already enabled, lock it with the command:
usermod -L username
Force immediate password expiration — Type the following command:
chage -d 0 username
This command sets the value for the date the password was last changed to the epoch (January 1, 1970). This value forces immediate password expiration no matter what password aging policy, if any, is in place.
Unlock the account — There are two common approaches to this step. The administrator can assign an initial password or assign a null password.
Do not use the passwd command to set the password as it disables the immediate password expiration just configured.
To assign an initial password, use the following steps:
Start the command line Python interpreter with the python command. It displays the following:
Python 2.2.2 (#1, Dec 10 2002, 09:57:09) [GCC 3.2.1 20021207 (Red Hat Enterprise Linux 4 3.2.1-2)] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>>
At the prompt, type the following (replacing password with the password to encrypt and salt with a combination of exactly 2 upper or lower case alphabetic characters, digits, the dot (.) character, or the slash (/) character such as ab or 12):
import crypt; print crypt.crypt("password","salt")
The output is the encrypted password, similar to 12CsGd8FRcMSM.
Cut and paste the exact encrypted password output, without a leading or trailing blank space, into the following command:
usermod -p "encrypted-password" username
Instead of assigning an initial password, a null password can be assigned using the following command:
usermod -p "" username
While using a null password is convenient for both the user and the administrator, there is a slight risk that a third party can log in first and access the system. To minimize this threat, it is recommended that the administrator verifies that the user is ready to log in when the account is unlocked.
In either case, upon initial log in, the user is prompted for a new password.