This manual page describes the format of the
/etc/hosts file. This file is a simple text file that associates IP addresses
with hostnames, one line per IP address. For each host a single
line should be present with the following information:
IP_address canonical_hostname aliases
Fields of the entry are separated by any number of blanks and/or
tab characters. Text from a "#" character until the end of the line is
a comment, and is ignored. Host names may contain only alphanumeric
characters, minus signs ("-"), and periods ("."). They must begin with an
alphabetic character and end with an alphanumeric character.
Aliases provide for name changes, alternate spellings,
shorter hostnames, or generic hostnames (for example,
localhost ). The format of the host table is described in RFC 952.
The Berkeley Internet Name Domain (BIND) Server implements the
Internet name server for UNIX systems. It augments or replaces the
/etc/hosts file or host name lookup, and frees a host from relying on
/etc/hosts being up to date and complete.
In modern systems, even though the host table has been superseded by
DNS, it is still widely used for:
bootstrapping
Most systems have a small host table containing the name and address
information for important hosts on the local network. This is useful
when DNS is not running, for example during system bootup.
NIS
Sites that use NIS use the host table as input to the NIS host
database. Even though NIS can be used with DNS, most NIS sites still
use the host table with an entry for all local hosts as a backup.
isolated nodes
Very small sites that are isolated from the network use the host table
instead of DNS. If the local information rarely changes, and the
network is not connected to the Internet, DNS offers little
advantage.
Before the advent of DNS, the host table was the only way of resolving
hostnames on the fledgling Internet. Indeed, this file could be
created from the official host data base maintained at the Network
Information Control Center (NIC), though local changes were often
required to bring it up to date regarding unofficial aliases and/or
unknown hosts. The NIC no longer maintains the hosts.txt files,
though looking around at the time of writing (circa 2000), there are
historical hosts.txt files on the WWW. I just found three, from 92,
94, and 95.
