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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  ta-
       ble. 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 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  alterna-
       tive  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 nick-
       names 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" nota-
       tion 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 dis-
       tinction	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 max-
       imum 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 sym-
       bolic link  /etc/hosts exists for  BSD compatibility.

SunOS 5.9			  21 Mar 1995			      hosts(4)


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