5.6. 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.[1]

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.

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 # localhost # vlager.vbrew.com vlager vlager-if1 vstout.vbrew.com vstout vale.vbrew.com vale # vlager-if2 vbeaujolais.vbrew.com vbeaujolais vbardolino.vbrew.com vbardolino 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:[2]
# /etc/networks for the Virtual Brewery brew-net wine-net



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.


Note that names in networks must not collide with hostnames from the hosts file, or else some programs may produce strange results.