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HOSTS(5) Linux Programmer's Manual HOSTS(5)
NAME
hosts - The static table lookup for hostnames
SYNOPSIS
/etc/hosts
DESCRIPTION
This manual page describes the format of the /etc/hosts file. This file is a sim-
ple 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. 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 hostname 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In modern systems, even though the host table has been superseded by DNS, it is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 infor-
mation 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 ta-
ble 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.
FILES
/etc/hosts
NOTES
Modifications to this file normally take effect immediately, except in cases where
the file is cached by applications.
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.
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
SEE ALSO
hostname(1), resolver(3), resolver(5), hostname(7), named(8), Internet RFC 952
COLOPHON
This page is part of release 3.22 of the Linux man-pages project. A description of
the project, and information about reporting bugs, can be found at http://www.ker-
nel.org/doc/man-pages/.
Linux 2002-06-16 HOSTS(5)
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