File Formats hosts(4)
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, the NIS] hosts table, or information from an LDAP
server. Programs use library interfaces to access informa-
tion 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 char-
acters. 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 differ-
ing 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
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File Formats hosts(4)
from each line containing a matching official name or nick-
name. 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 inter-
preted using the inetaddr routine from the Inter-
net address manipulation library, inet(3SOCKET).
o The IP Version 6 protocol [IPV6], defined in RFC
1884 and interpreted using the inetpton() routine
from the Internet address manipulation library. See
inet(3SOCKET).
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 com-
ponents 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 dis-
tinction 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.
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File Formats hosts(4)
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
SEE ALSO
gethostbyname(3NSL), getipnodebyname(3SOCKET),
inet(3SOCKET), nsswitch.conf(4), resolv.conf(4)
Braden, B., editor, RFC 1123, Requirements for Internet
Hosts - Application and Support, Network Working Group,
October, 1989.
Harrenstien, K., Stahl, M., and Feinler, E., RFC 952, DOD
Internet Host Table Specification, Network Working Group,
October 1985.
Hinden, R., and Deering, S., editors, RFC 1884, IP Version 6
Addressing Architecture, Network Working Group, December,
1995.
Postel, Jon, RFC 921, Domain Name System Implementation
Schedule (Revised), Network Working Group, October 1984.
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File Formats hosts(4)
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 com-
patibility with previous Solaris releases.
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