Ubuntu Manpage: hosts - static table lookup for hostnames

NAME

       hosts - static table lookup for hostnames

SYNOPSIS

       /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 following information:

IP_address canonical_hostname [aliases...]

The IP address can conform to either IPv4 or IPv6. 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). If required, a host may have two separate entries in this file; one for each
version of the Internet Protocol (IPv4 and IPv6).

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In modern systems, even though the host table has been superseded by DNS, it is still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 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.

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.

EXAMPLES

       # The following lines are desirable for IPv4 capable hosts
       127.0.0.1       localhost

       # 127.0.1.1 is often used for the FQDN of the machine
       127.0.1.1       thishost.mydomain.org  thishost
       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

       # The following lines are desirable for IPv6 capable hosts
       ::1             localhost ip6-localhost ip6-loopback
       ff02::1         ip6-allnodes
       ff02::2         ip6-allrouters

COLOPHON

       This page is part of release 5.13 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.