Linux Network Administrator Guide, Second Edition

system is booted. This is usually done by invoking them from the system /etc/rc* scripts.

Commonly, the network-specific part of this procedure is contained in a script. The name of this script varies in different Linux distributions. In many older Linux distributions, it is known as rc.net or rc.inet. Sometimes you will also see two scripts named rc.inet1 and rc.inet2; the former initializes the kernel part of networking and the latter starts basic networking services and applications. In modern distributions, the rc files are structured in a more sophisticated arrangement; here you may find scripts in the /etc/init.d/ (or /etc/rc.d/init.d/) directory that create the network devices and other rc files that run the network application programs. This book's examples are based on the latter arrangement.

This chapter discusses parts of the script that configure your network interfaces, while applications will be covered in later chapters. After finishing this chapter, you should have established a sequence of commands that properly configure TCP/IP networking on your computer. You should then replace any sample commands in your configuration scripts with your commands, make sure the script is executed from the basic rc script at startup time, and reboot your machine. The networking rc scripts that come along with your favorite Linux distribution should provide a solid example from which to work.

Mounting the /proc Filesystem

Some of the configuration tools of the Linux NET-2 and NET-3 release rely on the /proc filesystem for communicating with the kernel. This interface permits access to kernel runtime information through a filesystem-like mechanism. When mounted, you can list its files like any other filesystem, or display their contents. Typical items include the loadavg file, which contains the system load average, and meminfo, which shows current core memory and swap usage.

To this, the networking code adds the net directory. It contains a number of files that show things like the kernel ARP tables, the state of TCP connections, and the routing tables. Most network administration tools get their information from these files.

The proc filesystem (or procfs, as it is also known) is usually mounted on /proc at system boot time. The best method is to add the following line to /etc/fstab:

# procfs mount point:
none /proc proc defaults

Then execute mount /proc from your /etc/rc script.

The procfs is now configured into most kernels by default. If the procfs is not in your kernel, you will get a message such as: mount: fs type procfs not supported by kernel. You will then have to recompile the kernel and answer 'yes' when asked for procfs support.

Installing the Binaries

If you are using one of the prepackaged Linux distributions, it will contain the major networking applications and utilities along with a coherent set of sample files. The only case in which you might have to obtain and install new utilities is when you install a new kernel release. As they occasionally involve changes in the kernel networking layer, you will need to update the basic configuration tools. This update at least involves recompiling, but sometimes you may also be required to obtain the latest set of binaries. These binaries are available at their official home site at ftp.inka.de/pub/comp/Linux/networking/NetTools/, packaged in an archive called net-tools-XXX.tar.gz, where XXX is the version number. The release matching Linux 2.0 is net-tools-1.45.

If you want to compile and install the standard TCP/IP network applications yourself, you can obtain the sources from most Linux FTP servers. All modern Linux distributions include a fairly comprehensive range of TCP/IP network applications, such as World Wide Web browsers, telnet and ftp programs, and other network applications, such as talk. If you do find something that you do need to compile yourself, the chances are good that it will compile under Linux from source quite simply if you follow the instructions included in the source package.

Setting the Hostname

Most, if not all, network applications rely on you to set the local host's name to some reasonable value. This setting is usually made during the boot procedure by executing the hostname command. To set the hostname to name, enter:

# hostname name

It is common practice to use the unqualified hostname without specifying the domain name. For instance, hosts at the Virtual Brewery (described in Appendix A, Example Network: The Virtual Brewery) might be called vale.vbrew.com or vlager.vbrew.com. These are their official fully qualified domain names (FQDNs). Their local hostnames would be the first component of the name, such as vale. However, as the local hostname is frequently used to look up the host's IP address, you have to make sure that the resolver library is able to look up the host's IP address. This usually means that you have to enter the name in /etc/hosts.

Some people suggest using the domainname command to set the kernel's idea of a domain name to the remaining part of the FQDN. This way you could combine the output from hostname and domainname to get the FQDN again. However, this is at best only half correct. domainname is generally used to set the host's NIS domain, which may be entirely different from the DNS domain to which your host belongs. Instead, to ensure that the short form of your hostname is resolvable with all recent versions of the hostname command, either add it as an entry in your local Domain Name Server or place the fully qualified domain name in the /etc/hosts file. You may then use the -fqdn argument to the hostname command, and it will print the fully qualifed domain name.

Assigning IP Addresses

If you configure the networking software on your host for standalone operation (for instance, to be able to run the INN Netnews software), you can safely skip this section, because the only IP address you will need is for the loopback interface, which is always 127.0.0.1.

Things are a little more complicated with real networks like Ethernets. If you want to connect your host to an existing network, you have to ask its administrators to give you an IP address on this network. When setting up a network all by yourself, you have to assign IP addresses yourself.

Hosts within a local network should usually share addresses from the same logical IP network. Hence, you have to assign an IP network address. If you have several physical networks, you have to either assign them different network numbers, or use subnetting to split your IP address range into several subnetworks. Subnetting