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hosts(4)                         File Formats                         hosts(4)

       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 file.

       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

       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

       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, inet(3SOCKET).

       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 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 upper and lower case.  The first character
       must be an alpha character.  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.

       RFC 952 has been modified by RFC 1123 to relax the restriction on the
       first character being a digit.

       Example 1: Example of a typical line from the hosts file.

       Here is a typical line from the hosts file:        gaia                        # John Smith

       in.named(1M), gethostbyname(3NSL), inet(3SOCKET), nsswitch.conf(4),

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

SunOS 5.9                         21 Mar 1995                         hosts(4)


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