HOSTS(5)

hosts(5)			   File Formats Manual				hosts(5)

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 ad‐
       dress.  For each host a single line should be present with the following informa‐
       tion:

	      IP_address canonical_hostname [aliases...]

       The IP address can conform to either IPv4 or IPv6.  Fields of the entry are sepa‐
       rated 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.  Op‐
       tional 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  com‐
       plete.

       In  modern  systems, even though the host table has been superseded by DNS, it is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  in‐
	      formation	 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 in‐
	      stead 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.example.org   thishost
       192.168.1.10    foo.example.org	      foo
       192.168.1.13    bar.example.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

SEE ALSO
       hostname(1), resolver(3), host.conf(5), resolv.conf(5), resolver(5), hostname(7),
       named(8)

       Internet RFC 952

Linux man-pages 6.7		       2023-10-31				hosts(5)

Generated 2026-11-03 from hosts.5 in the official Ubuntu 24.04 package manpages 6.7-2. Extracted without installing the package. Original copyright and license terms apply.