Maintaining security on your Red Hat Linux system is extremely important. One way to manage security on your system is to carefully manage access to system services. Your system may need to provide open access to particular services (for example, httpd if you are running a Web server). However, if you do not need to provide a service, you should turn it off — this will minimize your exposure to possible bug exploits.
There are several different methods for managing access to system services. You must decide which method you would like to use based on the service, your system's configuration, and your level of Linux expertise.
The easiest way to deny access to a service is to simply turn it off. Both the services managed by xinetd (which we will talk about more later in this section) and the services in the /etc/rc.d hierarchy can be configured to start or stop using three different applications:
serviceconf — a graphical application that displays a description of each service, displays whether each service is started at boot time (for runlevels 3, 4, and 5), and allows you to start, stop, and restart each service.
ntsysv — a text-based application that allows you to configure which services are started at boot time for each runlevel. Changes do not take effect immediately. Services can not be started, stopped, or restarted using this program.
chkconfig — a command-line utility that allows you to turn services on and off for the different runlevels. Changes do not take effect immediately. Services can not be started, stopped, or restarted using this utility.
You may find that these tools are easier to use than the alternatives — editing the numerous symbolic links located in the directories below /etc/rc.d by hand or editing the xinetd configuration files in /etc/xinetd.d.
Another way to manage access to system services is by using iptables to configure an IP firewall. If you are a new Linux user, please realize that iptables may not be the best solution for you. Setting up iptables can be complicated and is best tackled by experienced UNIX/Linux system administrators.
On the other hand, the benefit of using iptables is flexibility. For example, if you need a customized solution which provides certain hosts access to certain services, ipchains can provide it for you. See the Official Red Hat Linux Reference Guide for more information about iptables.
Alternatively, if you are looking for a utility which will set general access rules for your home machine, and/or if you are new to Linux, you should try the GNOME Lokkit utility. GNOME Lokkit is a GUI utility which will ask you questions about how you want to use your machine. Based on your answers, it will then configure a simple firewall for you. Refer to Chapter 7 for more information.
Before you can configure access to services, you must understand Linux runlevels. A runlevel is a state, or mode, that is defined by the services listed in the directory /etc/rc.d/rc<x>.d, where <x> is the number of the runlevel.
Red Hat Linux uses the following runlevels:
0 — Halt
1 — Single-user mode
2 — Not used (user-definable)
3 — Full multi-user mode
4 — Not used (user-definable)
5 — Full multi-user mode (with an X-based login screen)
6 — Reboot
If you configured the X Window System during the Red Hat Linux installation program, you had the option of choosing a graphical or text login screen. If you chose a text login screen, you are operating in runlevel 3. If you chose a graphical login screen, you are operating in runlevel 5.
The default runlevel can be changed by modifying the /etc/inittab file, which contains a line near the top of the file similar to the following:
Change the number in this line to the desired runlevel. The change will not take effect until you reboot the system.
To change the runlevel immediately, use the command telinit followed by the runlevel number. You must be root to use this command.