User and Group Management Tools

Posted by M

3/18/2015

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3.3. User and Group Management Tools

Managing users and groups can be tiresome; this is why Fedora provides tools and conventions to make this task easier to manage.

The easiest way to manage users and groups is through the graphical application, User Manager(system-config-users). For more information on User Manager, refer to Section 3.2, “User Manager Tool”.

The following command line tools can also be used to manage users and groups:

useradd, usermod, and userdel — Industry-standard methods of adding, deleting and modifying user accounts

groupadd, groupmod, and groupdel — Industry-standard methods of adding, deleting, and modifying user groups

gpasswd — Industry-standard method of administering the /etc/group file

pwck, grpck — Tools used for the verification of the password, group, and associated shadow files

pwconv, pwunconv — Tools used for the conversion of passwords to shadow passwords and back to standard passwords

3.3.1. Command Line Configuration

If you prefer command line tools or do not have the X Window System installed, use following to configure users and groups.

Adding a User

To add a user to the system:

Issue the useradd command to create a locked user account:
```
useradd username
```

Unlock the account by issuing the passwd command to assign a password and set password aging guidelines:
```
passwd username
```

Command line options for useradd are detailed in Table 3.1, “useradd Command Line Options”.

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Table 3.1. `useradd` Command Line Options

Option	Description
`-c` '`comment`'	`comment` can be replaced with any string. This option is generally used to specify the full name of a user.
`-d` `home_directory`	Home directory to be used instead of default `/home/username/`.
`-e` `date`	Date for the account to be disabled in the format YYYY-MM-DD.
`-f` `days`	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.
`-g` `group_name`	Group name or group number for the user's default group. The group must exist prior to being specified here.
`-G` `group_list`	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.
`-m`	Create the home directory if it does not exist.
`-M`	Do not create the home directory.
`-N`	Do not create a user private group for the user.
`-p` `password`	The password encrypted with `crypt`.
`-r`	Create a system account with a UID less than 500 and without a home directory.
`-s`	User's login shell, which defaults to `/bin/bash`.
`-u` `uid`	User ID for the user, which must be unique and greater than 499.

Adding a Group

To add a group to the system, use the command groupadd:

groupadd group_name

Command line options for groupadd are detailed in Table 3.2, “groupadd Command Line Options”.

Table 3.2. `groupadd` Command Line Options

Option	Description
`-f`, `--force`	When used with `-g` `gid` and `gid` already exists, `groupadd` will choose another unique `gid` for the group.
`-g` `gid`	Group ID for the group, which must be unique and greater than 499.
`-K`, `--key` `key`=`value`	Override `/etc/login.defs` defaults.
`-o`, `--non-unique`	Allow to create groups with duplicate.
`-p`, `--password` `password`	Use this encrypted password for the new group.
`-r`	Create a system group with a GID less than 500.

Password Aging

For security reasons, it is advisable 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 with an option fromTable 3.3, “chage Command Line Options”, followed by the username.

Shadow passwords must be enabled to use chage

Shadow passwords must be enabled to use the chage command. For more information, see Section 3.7, “Shadow Passwords”.

Table 3.3. `chage` Command Line Options

Option	Description
`-d` `days`	Specifies the number of days since January 1, 1970 the password was changed.
`-E` `date`	Specifies 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.
`-I` `days`	Specifies 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.
`-l`	Lists current account aging settings.
`-m` `days`	Specify the minimum number of days after which the user must change passwords. If the value is `0`, the password does not expire.
`-M` `days`	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.
`-W` `days`	Specifies the number of days before the password expiration date to warn the user.

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chage interactivity

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 interactively.

You can configure a password to expire the first time a user logs in. This forces users to change passwords immediately.

Set up an initial password — There are two common approaches to this step: the administrator can assign a default password, or he can use a null password.

To assign a default password, type the following at a shell prompt:
```
passwd username
```
To assign a null password instead, use the following command:
```
passwd -d username
```
Avoid using null passwords whenever possible

Using a null password, while convenient, is a highly insecure practice, as any third party can log in first and access the system using the insecure username. Always make sure that the user is ready to log in before unlocking an account with a null password.

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.

Upon the initial log in, the user is now prompted for a new password.

3.3. User and Group Management Tools

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