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GETHOSTBYNAME(3)	 BSD Library Functions Manual	      GETHOSTBYNAME(3)

     gethostbyname, gethostbyname2, gethostbyaddr, gethostent, sethostent,
     endhostent, herror, hstrerror -- get network host entry

     Standard C	Library	(libc, -lc)

     #include <netdb.h>
     extern int	h_errno;

     struct hostent *
     gethostbyname(const char *name);

     struct hostent *
     gethostbyname2(const char *name, int af);

     struct hostent *
     gethostbyaddr(const char *addr, socklen_t len, int	type);

     struct hostent *

     sethostent(int stayopen);


     herror(const char *string);

     const char	*
     hstrerror(int err);

     The gethostbyname(), gethostbyname2() and gethostbyaddr() functions each
     return a pointer to an object with	the following structure	describing an
     internet host.

     struct  hostent {
	     char    *h_name;	     /*	official name of host */
	     char    **h_aliases;    /*	alias list */
	     int     h_addrtype;     /*	host address type */
	     int     h_length;	     /*	length of address */
	     char    **h_addr_list;  /*	list of	addresses from name server */
     #define h_addr  h_addr_list[0]  /*	address, for backward compatibility */

     The members of this structure are:

     h_name	  Official name	of the host.

     h_aliases	  A NULL-terminated array of alternative names for the host.

     h_addrtype	  The type of address being returned; currently	always

     h_length	  The length, in bytes,	of the address.

     h_addr_list  A NULL-terminated array of network addresses for the host.
		  Host addresses are returned in network byte order.

     h_addr	  The first address in h_addr_list; this is for	backward com-

     In	the case of gethostbyname() and	gethostbyname2(), the host is speci-
     fied by name, or using a string representation of a numeric address.  In
     the case of gethostbyaddr(), the host is specified	using a	binary repre-
     sentation of an address.

     The returned struct hostent structure may contain the result of a simple
     string to binary conversion, information obtained from the	domain name
     resolver (see resolver(3)), broken-out fields from	a line in /etc/hosts,
     or	database entries supplied by the yp(8) system.	The order of the
     lookups is	controlled by the `hosts' entry	in nsswitch.conf(5).

     When using	the domain name	resolver, gethostbyname() and gethostbyname2()
     will search for the named host in the current domain and its parents un-
     less the name ends	in a dot.  If the name contains	no dot,	and if the en-
     vironment variable	"HOSTALIASES" contains the name	of an alias file, the
     alias file	will first be searched for an alias matching the input name.
     See hostname(7) for the domain search procedure and the alias file	for-

     The gethostbyname2() function is an evolution of gethostbyname() which is
     intended to allow lookups in address families other than AF_INET, for ex-
     ample AF_INET6.  Currently	the af argument	must be	specified as AF_INET
     or	AF_INET6, else the function will return	NULL after having set h_errno

     The gethostent() function reads the next line of the /etc/hosts file,
     opening the file if necessary.

     The sethostent() function may be used to request the use of a connected
     TCP socket	for queries.  If the stayopen flag is non-zero,	this sets the
     option to send all	queries	to the name server using TCP and to retain the
     connection	after each call	to gethostbyname(), gethostbyname2(), or
     gethostbyaddr().  Otherwise, queries are performed	using UDP datagrams.

     The endhostent() function closes the TCP connection.

     The herror() function writes a message to the diagnostic output consist-
     ing of the	string parameter s, the	constant string	": ", and a message
     corresponding to the value	of h_errno.

     The hstrerror() function returns a	string which is	the message text cor-
     responding	to the value of	the err	parameter.


     Error return status from gethostbyent(), gethostbyname(),
     gethostbyname2(), and gethostbyaddr() is indicated	by return of a null
     pointer.  The external integer h_errno may	then be	checked	to see whether
     this is a temporary failure or an invalid or unknown host.	 The routine
     herror() can be used to print an error message describing the failure.
     If	its argument string is non-NULL, it is printed,	followed by a colon
     and a space.  The error message is	printed	with a trailing	newline.

     The variable h_errno can have the following values:

     HOST_NOT_FOUND  No	such host is known.

     TRY_AGAIN	     This is usually a temporary error and means that the lo-
		     cal server	did not	receive	a response from	an authorita-
		     tive server.  A retry at some later time may succeed.

     NO_RECOVERY     Some unexpected server failure was	encountered.  This is
		     a non-recoverable error.

     NO_DATA	     The requested name	is valid but does not have an IP ad-
		     dress; this is not	a temporary error.  This means that
		     the name is known to the name server but there is no ad-
		     dress associated with this	name.  Another type of request
		     to	the name server	using this domain name will result in
		     an	answer;	for example, a mail-forwarder may be regis-
		     tered for this domain.

     resolver(3), hosts(5), nsswitch.conf(5), hostname(7), named(8)

     The herror() function appeared in 4.3BSD.	The endhostent(),
     gethostbyaddr(), gethostbyname(), gethostent(), and sethostent() func-
     tions appeared in 4.2BSD.	The gethostbyname2() function first appeared
     in	bind-4.9.4.  IPv6 support was implemented in WIDE Hydrangea IPv6 pro-
     tocol stack kit.

     If	the search routines specified in nsswitch.conf(5) decide to read the
     /etc/hosts	file, gethostbyname(), gethostbyname2(), and gethostbyaddr()
     will read the next	line of	the file, re-opening the file if necessary.

     The sethostent() function opens and/or rewinds the	file /etc/hosts.  If
     the stayopen argument is non-zero,	the file will not be closed after each
     call to gethostbyname(), gethostbyname2(),	gethostbyaddr(), or

     The endhostent() function closes the file.

     These functions use static	data storage; if the data is needed for	future
     use, it should be copied before any subsequent calls overwrite it.	 Only
     the Internet address format is currently understood.

     The gethostent() does not currently follow	the search order specified in
     nsswitch.conf(5) and only reads the /etc/hosts file.

BSD				October	7, 2006				   BSD


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