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...]

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In modern systems, even though the host table has been superseded by DNS, it is still widely used for: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

EXAMPLE

       # 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 4.15 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/.

Linux                                              2017-09-15                                           HOSTS(5)