Upon typing the su command, the user is prompted for the root password and, after authentication, is given a root shell prompt.

Once logged in via the su command, the user is the root user and has absolute administrative access to the system. In addition, once a user has become root, it is possible for them to use the su command to change to any other user on the system without being prompted for a password.

Because this program is so powerful, administrators within an organization may wish to limit who has access to the command.

One of the simplest ways to do this is to add users to the special administrative group called wheel. To do this, type the following command as root:

 usermod -G wheel <username>

In the previous command, replace <username> with the username you want to add to the wheel group.

To use the User Manager for this purpose, go to the Main Menu Button (on the Panel) => System Settings => Users & Groups or type the command system-config-users at a shell prompt. Select the Users tab, select the user from the user list, and click Properties from the button menu (or choose File => Properties from the pull-down menu).

Then select the Groups tab and click on the wheel group, as shown in Figure 4.2, “Groups Pane”.

Next, open the PAM configuration file for su (/etc/pam.d/su) in a text editor and remove the comment # from the following line:

auth  required /lib/security/$ISA/pam_wheel.so use_uid

Doing this permits only members of the administrative group wheel to use the program.