Running Linux, 5th Edition - Matthias Kalle Dalheimer [249]
Note that there are many ways to carry out the network configuration described here. Every Linux distribution uses a slightly different mechanism to help automate the process. What we describe here is a generic method that allows you to create two network configuration files that will run the appropriate commands to get your machine talking to the network. Most distributions have their own scripts that accomplish more or less the same thing. If in doubt, first attempt to configure networking as suggested by the documentation for your distribution and, as a last resort, use the methods described here. (As an example, the Red Hat distribution uses the script /etc/rc.d/init.d/network, which obtains network information from files in /etc/sysconfig. The control-panel system administration program provided with Red Hat configures networking automatically without editing any of these files. The SUSE distribution uses /etc/init.d/network, and lets you configure most networking aspects via the tool YaST2. And of course, any new version of either distribution could change this again.)
Here, we're going to describe the network configuration files used to configure TCP/IP in some of the better-known distributions:
Red Hat
Networking is scattered among files for each init level that includes networking. For instance, the /etc/rc.d/rc1.d directory controls a level 1 (single-user) boot, so it doesn't have any networking commands, but the /etc/rc.d/rc3.d controlling a level 3 boot has files specifically to start networking.
SUSE
All the startup files for all system services, including networking, are grouped together in the /etc/init.d directory. They are quite generic and get their actual values from the system-wide configuration file /etc/sysconfig. The most important files here are /etc/init.d/network, which starts and halts network interfaces and configures routing, and /etc/init.d/setserial, which configures serial ports. If you have ISDN hardware, the file /etc/init.d/isdn is applicable, too. Note that in general, you do not need to (and should not) edit those files; edit the files /etc/sysconfig instead (that's what YaST does).
Debian
The network configuration (Ethernet cards, IP addresses, and routing) and the base networking daemons (portman and inetd) are set up in the file /etc/init.d/networking.
We'll use two files here for illustrative purposes: /etc/init.d/rc.inet1 and /etc/init.d/rc.inet2. The former will set up the hardware and the basic networking, and the latter will configure the networking services. A number of distributions follow such a separation, even though the files might have other names.
init uses the file /etc/inittab to determine what processes to run at boot time. To run the files /etc/init.d/rc.inet1 and /etc/init.d/rc.inet2 from init, /etc/inittab might include entries such as:
n1:34:wait:/etc/init.d/rc.inet1
n2:34:wait:/etc/init.d/rc.inet2
The inittab file is described in "init, inittab, and rc Files" in Chapter 17. The first field gives a unique two-character identifier for each entry. The second field lists the run levels in which the scripts are run; on this system, we initialize networking in run levels 3 and 4. The word wait in the third field tells init to wait until the script has finished execution before continuing. The last field gives the name of the script to run.
While you are first setting up your network configuration, you may wish to run rc.inet1 and rc.inet2 by hand (as root) in order to debug any problems. Later you can include entries for them in another rc file or in /etc/inittab.
As mentioned earlier, rc.inet1 configures the basic network interface. This includes your IP and network address and the routing table information for your system. Two programs are used to configure these parameters: ifconfig and route. Both of these are usually found in /sbin.
ifconfig is used for configuring