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In short, the method described here should work for many Linux systems configured for an existing LAN—but certainly not all. For further details, we direct you to a book on TCP/IP network administration, such as those mentioned at the beginning of this chapter.

First of all, we assume that your Linux system has the necessary TCP/IP software installed. This includes basic clients such as ssh and FTP, system-administration commands such as ifconfig and route (usually found in /etc or /sbin), and networking configuration files (such as /etc/hosts). The other Linux-related networking documents described earlier explain how to go about installing the Linux networking software if you do not have it already.

A new system administration interface has been developed that unifies the various networking tasks (configurating, routing, etc.) into a single command named ip, provided by the IPROUTE2 package. We won't cover it here, because its value lies mainly in advanced features that most administrators don't use, but once you understand the concepts in this chapter you can figure out how to use those commands if you want to use that package.

We also assume that your kernel has been configured and compiled with TCP/IP support enabled. See "Building a New Kernel" in Chapter 18 for information on compiling your kernel. To enable networking, you must answer yes to the appropriate questions during the make config or make menuconfig step, rebuild the kernel, and boot from it.

Once this has been done, you must modify a number of configuration files. For the most part this is a simple procedure. Unfortunately, however, there is wide disagreement between Linux distributions as to where the various TCP/IP configuration files and support programs should go. Much of the time, they can be found in /etc or /etc/sysconfig, but in other cases they may be found in /usr/etc, /usr/etc/inet, or other bizarre locations. In the worst case, you'll have to use the find command to locate the files on your system. Also note that not all distributions keep the network configuration files and software in the same location; they may be spread across several directories.

Here we cover how to set up and configure networking on a Linux box manually. This should help you get some insight into what goes on behind the scenes and enable you to help yourself if something goes wrong with automatic setup tools provided by your distribution. It can be a good idea, though, to first try setting up your network with the configuration programs that your distribution provides; many of these are quite advanced these days and detect many of the necessary settings automatically. They are usually available from the menus on your desktop. If you understand the concepts in this chapter, it is not hard to figure out how to use them, but we do not cover them here because they tend to change, and what you mainly need to know is the effects you're aiming to achieve.

This section also assumes use of one Ethernet device on the system. These instructions should be fairly easy to extrapolate if your system has more than one network connection (and hence acts as a gateway).

Here, we also discuss configuration for loopback-only systems (systems with no Ethernet or PPP connection). If you have no network access, you may wish to configure your system for loopback-only TCP/IP so that you can use applications that require it.

Your network configuration

Before you can configure TCP/IP , you need to determine the following information about your network setup. In most cases, your local network administrator or ISP can provide you with this information. If your network is using DHCP, things are going to be a bit different—for example, you do not need to know your IP address, since it will be assigned to you automatically. But it's often easier to test things one step at a time and start with a static IP address. Just make sure with your network administrator or Internet service provider that you are not using