hosts(5)                                                File Formats Manual                                                hosts(5)

       hosts - static table lookup for hostnames


       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 host‐
       names (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 re‐
       places 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       In modern systems, even though the host table has been superseded by DNS, it is still widely used for:

              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.


       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.

       # The following lines are desirable for IPv4 capable hosts       localhost

       # is often used for the FQDN of the machine   thishost        foo        bar      master

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

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

       Internet RFC 952

Linux man-pages 6.03                                         2022-10-30                                                    hosts(5)