From Linux Man Pages
hosts - The static table lookup for host names
/etc/hosts
DESCRIPTION
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 follow-
ing 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 char-
acter. Optional aliases provide for name changes, alternate spellings, shorter hostnames, or generic hostnames
(for example, localhost).
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In modern systems, even though the host table has been superseded by DNS, it is still widely used for: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 infor-
mation rarely changes, and the network is not connected to the Internet, DNS offers little advantage.
EXAMPLE
127.0.0.1 localhost
192.168.1.10 foo.mydomain.org foo
192.168.1.13 bar.mydomain.org bar
146.82.138.7 master.debian.org master
209.237.226.90 www.opensource.org
NOTE
Modifications to this file normally take effect immediately, except in cases where the file is cached by applica-
tions.
HISTORICAL NOTES
RFC 952 gave the original format for the host table, though it has since changed.
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.
FILES
/etc/hosts
RELATED
hostname(1), resolver(3), resolver(5), hostname(7), named(8), Internet RFC 952
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