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 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In modern systems, even though the host table has been superseded by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 ad-
dress information for important hosts on the local network.
This is useful when DNS is not running, for example during sys-
tem 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 cre-
ated from the official host data base maintained at the Network Infor-
mation 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 6.03 2022-10-30 hosts(5)
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