hosts - host name database
The hosts file is a local database that associates the names
of hosts with their Internet Protocol (IP) addresses. 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 and the NIS+ hosts table. Programs
use library interfaces to access information in the 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
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 addresses and nicknames from
each line containing a matching official name or nickname.
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 the conventional "decimal
dot" notation and interpreted using the inet_addr routine
from the Internet address manipulation library,
This interface supports host 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 del-
imit components of "domain style names". (See RFC 921,
"Domain Name System Implementation Schedule," for back-
ground). No blank or space characters are permitted as
part of a name. No distinction is made between upper-
case 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.
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.
Example 1: Example of a typical line from the hosts file.
Here is a typical line from the hosts file:
220.127.116.11 gaia # John Smith
in.named(1M), gethostbyname(3NSL), inet(3SOCKET),
/etc/inet/hosts is the official SVR4 name of the hosts file.
The symbolic link /etc/hosts exists for BSD compatibility.
Man(1) output converted with