Runlevels; Tcp Wrappers - Red Hat LINUX 7.2 - OFFICIAL LINUX CUSTOMIZATION GUIDE Manual

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it will then configure a simple firewall for you. Refer to Chapter 7, Basic Firewall Configuration for
more information.

8.1 Runlevels

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:
id:3:initdefault:
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.

8.2 TCP Wrappers

Many UNIX system administrators are accustomed to using TCP wrappers to manage access to
certain network services. Any network services managed by xinetd (as well as any program
with built-in support for libwrap) can use TCP wrappers to manage access. xinetd can use the
/etc/hosts.allow and /etc/hosts.deny files to configure access to system services. As
the names imply, hosts.allow contains a list of rules clients allowed to access the network ser-
vices controlled by xinetd, and hosts.deny contains rules to deny access. The hosts.allow
Chapter 8:Controlling Access to Services