illumos: manual page: hosts.5

HOSTS(5) File Formats and Configurations HOSTS(5)

NAME

hosts - host name database

SYNOPSIS

/etc/inet/hosts

/etc/hosts

/etc/inet/ipnodes

DESCRIPTION

The hosts file is a local database that associates the names of hosts
with their Internet Protocol (IP) addresses. An IP address can be in
either IPv4 or IPv6 format. The hosts file can be used in conjunction
with, or instead of, other hosts databases, including the Domain Name
System (DNS), the NIS hosts map, or information from an LDAP server.
Programs use library interfaces to access information in the hosts file.

Note that /etc/hosts and /etc/inet/ipnodes are symbolic links to
/etc/inet/hosts.

The hosts file has one entry for each IP address of each host. If a host
has more than one IP address, it will have one entry for each, on
consecutive lines. The format of each line is:

IP-address official-host-name nicknames...

Items are separated by any number of SPACE and/or TAB characters. The
first item on a line is the host's IP address. The second entry is the
host's official name. Subsequent entries on the same line are alternative
names for the same machine, or "nicknames." Nicknames are optional.

For a host with more than one IP address, consecutive entries for these
addresses may contain the same or differing nicknames. Different
nicknames are useful for assigning distinct names to different addresses.

A call to gethostbyname(3NSL) returns a hostent structure containing the
union of all IPv4 addresses and nicknames from each line containing a
matching official name or nickname. A call to getipnodebyname(3SOCKET) is
similar, but is capable of returning hostent structures containing IPv4
and IPv6 addresses. Applications might prefer to use the address-family
independent getaddrinfo(3SOCKET) API for name-to-address lookups.

A `#' indicates the beginning of a comment; characters up to the end of
the line are not interpreted by routines that search the file.

Network addresses are written in one of two ways:

o The conventional "decimal dot" notation and interpreted using
the inet_addr routine from the Internet address manipulation
library, inet(3C).

o The IP Version 6 protocol [IPV6], defined in RFC 1884 and
interpreted using the inet_pton() routine from the Internet
address manipulation library. See inet(3C).

This interface supports node names as defined in Internet RFC 952, which
states:

A "name" (Net, Host, Gateway, or Domain name) is a text string up to 24
characters drawn from the alphabet (A-Z), digits (0-9), minus sign (-),
and period (.). Note that periods are only allowed when they serve to
delimit components of "domain style names". (See RFC 921, "Domain Name
System Implementation Schedule," for background). No blank or space
characters are permitted as part of a name. No distinction is made
between uppercase and lowercase. The first character must be an alpha
character [or a digit. (RFC 1123 relaxed RFC 952's limitation of the
first character to only alpha characters.)] The last character must not
be a minus sign or period.

Host names must not consist of numbers only. A host name must contain at
least one alphabetical or special character.

Although the interface accepts host names longer than 24 characters for
the host portion (exclusive of the domain component), choosing names for
hosts that adhere to the 24 character restriction will insure maximum
interoperability on the Internet.

A host which serves as a GATEWAY should have "-GATEWAY" or "-GW" as part
of its name. Hosts which do not serve as Internet gateways should not use
"-GATEWAY" and "-GW" as part of their names. A host which is a TAC should
have "-TAC" as the last part of its host name, if it is a DoD host.
Single character names or nicknames are not allowed.

EXAMPLES

Example 1: Example hosts File Entry

The following is a typical line from the hosts file:

192.9.1.20 gaia # John Smith

Example 2: Example IPv6 Address Entry

The following is an example of an IPv6 hosts entry:

2001:0db8:3c4d:55:a00:20ff:fe8e:f3ad myhost # John Smith

NOTES

/etc/inet/hosts is the official SVR4 name of the hosts file. The symbolic
link /etc/hosts exists for BSD compatibility.

The symbolic link /etc/net/ipnodes exists for backwards compatibility
with previous Solaris releases.

March 30, 2022 HOSTS(5)