Olaf Kirch - Linux Network Administrator Guide, Second Edition

Throughout the remainder of the book, we will assume that the brewery's network manager uses a class B network number, say 172.16.0.0 . Of course, a class C network number would definitely suffice to accommodate both the Brewery's and the Winery's networks. We'll use a class B network here for the sake of simplicity; it will make the subnetting examples in the next section of this chapter a little more intuitive.

To operate several Ethernets (or other networks, once a driver is available), you have to split your network into subnets. Note that subnetting is required only if you have more than one broadcast network - point-to-point links don't count. For instance, if you have one Ethernet, and one or more SLIP links to the outside world, you don't need to subnet your network. This is explained in more detail in Chapter 7, Serial Line IP.

To accommodate the two Ethernets, the Brewery's network manager decides to use 8 bits of the host part as additional subnet bits. This leaves another 8 bits for the host part, allowing for 254 hosts on each of the subnets. She then assigns subnet number 1 to the brewery, and gives the winery number 2. Their respective network addresses are thus 172.16.1.0 and 172.16.2.0 . The subnet mask is 255.255.255.0 .

vlager , which is the gateway between the two networks, is assigned a host number of 1 on both of them, which gives it the IP addresses 172.16.1.1 and 172.16.2.1 , respectively.

Note that in this example we are using a class B network to keep things simple, but a class C network would be more realistic. With the new networking code, subnetting is not limited to byte boundaries, so even a class C network may be split into several subnets. For instance, you could use two bits of the host part for the netmask, giving you 4 possible subnets with 64 hosts on each. [28] The first number on each subnet is the subnetwork address, and the last number on each subnet is reserved as the broadcast address, so it's actually 62 hosts per subnet.

After you have subnetted your network, you should prepare for some simple sort of hostname resolution using the /etc/hosts file. If you are not going to use DNS or NIS for address resolution, you have to put all hosts in the hosts file.

Even if you want to run DNS or NIS during normal operation, you should have some subset of all hostnames in /etc/hosts . You should have some sort of name resolution, even when no network interfaces are running, for example, during boot time. This is not only a matter of convenience, but it allows you to use symbolic hostnames in your network rc scripts. Thus, when changing IP addresses, you only have to copy an updated hosts file to all machines and reboot, rather than edit a large number of rc files separately. Usually you put all local hostnames and addresses in hosts , adding those of any gateways and NIS servers used. [29] You need the address of an NIS server only if you use Peter Eriksson's NYS. Other NIS implementations locate their servers only at runtime by using ypbind.

You should make sure your resolver only uses information from the hosts file during initial testing. Sample files that come with your DNS or NIS software may produce strange results. To make all applications use /etc/hosts exclusively when looking up the IP address of a host, you have to edit the /etc/host.conf file. Comment out any lines that begin with the keyword order by preceding them with a hash sign, and insert the line:

order hosts

The configuration of the resolver library is covered in detail in Chapter 6, Name Service and Resolver Configuration.

The hosts file contains one entry per line, consisting of an IP address, a hostname, and an optional list of aliases for the hostname. The fields are separated by spaces or tabs, and the address field must begin in the first column. Anything following a hash sign (#) is regarded as a comment and is ignored.

Hostnames can be either fully qualified or relative to the local domain. For vale , you would usually enter the fully qualified name, vale.vbrew.com , and vale by itself in the hosts file, so that it is known by both its official name and the shorter local name.

This is an example how a hosts file at the Virtual Brewery might look. Two special names are included, vlager-if1 and vlager-if2 , which give the addresses for both interfaces used on vlager :

#

# Hosts file for Virtual Brewery/Virtual Winery

#

# IP FQDN aliases

#

127.0.0.1 localhost

#

172.16.1.1 vlager.vbrew.com vlager vlager-if1 1

72.16.1.2 vstout.vbrew.com vstout

172.16.1.3 vale.vbrew.com vale

#

172.16.2.1 vlager-if2

172.16.2.2 vbeaujolais.vbrew.com vbeaujolais

172.16.2.3 vbardolino.vbrew.com vbardolino

172.16.2.4 vchianti.vbrew.com vchianti

Just as with a host's IP address, you should sometimes use a symbolic name for network numbers, too. Therefore, the hosts file has a companion called /etc/networks that maps network names to network numbers, and vice versa. At the Virtual Brewery, we might install a networks file like this: [30] Note that names in networks must not collide with hostnames from the hosts file, or else some programs may produce strange results.

# /etc/networks for the Virtual Brewery

brew-net 172.16.1.0

wine-net 172.16.2.0

After setting up your hardware as explained in Chapter 4, Configuring the Serial Hardware, you have to make these devices known to the kernel networking software. A couple of commands are used to configure the network interfaces and initialize the routing table. These tasks are usually performed from the network initialization script each time you boot the system. The basic tools for this process are called ifconfig (where "if" stands for interface) and route.

ifconfig is used to make an interface accessible to the kernel networking layer. This involves the assignment of an IP address and other parameters, and activation of the interface, also known as "bringing up" the interface. Being active here means that the kernel will send and receive IP datagrams through the interface. The simplest way to invoke it is with:

ifconfig interface ip-address

This command assigns ip-address to interface and activates it. All other parameters are set to default values. For instance, the default network mask is derived from the network class of the IP address, such as 255.255.0.0 for a class B address. ifconfig is described in detail in the section "All About ifconfig".

route allows you to add or remove routes from the kernel routing table. It can be invoked as:

route [add|del] [-net|-host] target [ if ]

The add and del arguments determine whether to add or delete the route to target . The -net and -host arguments tell the route command whether the target is a network or a host (a host is assumed if you don't specify). The if argument is again optional, and allows you to specify to which network interface the route should be directed - the Linux kernel makes a sensible guess if you don't supply this information. This topic will be explained in more detail in succeeding sections.