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]
Writing hosts and networks Files
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 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]
# /etc/networks for the Virtual Brewery
brew-net 172.16.1.0
wine-net 172.16.2.0
Interface Configuration for IP
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.
The Loopback Interface
The very first interface to be activated is the loopback interface:
# ifconfig lo 127.0.0.1
Occasionally, you will see the dummy hostname localhost being used instead of the IP address. ifconfig will look up the name in the hosts file, where an entry should declare it as the hostname for 127.0.0.1:
# Sample /etc/hosts entry for localhost
localhost 127.0.0.1
To view the configuration of an interface, you invoke ifconfig, giving it only the interface name as argument:
$ ifconfig lo
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:3924 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
Collisions:0
As you can see, the loopback interface has been assigned a netmask of 255.0.0.0, since 127.0.0.1 is a class A address.
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.
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.
30
Note that names in networks must not collide with hostnames from the hosts file, or else some programs may produce strange results.