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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>

     int h_errno;

     struct hostent *
     gethostbyname(const char *name);

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

     struct hostent *
     gethostbyaddr(const void *addr, socklen_t len, int	af);

     struct hostent *

     sethostent(int stayopen);


     herror(const char *string);

     const char	*
     hstrerror(int err);

     The getaddrinfo(3)	and getnameinfo(3) functions are preferred over	the
     gethostbyname(), gethostbyname2(),	and gethostbyaddr() functions.

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

     The name argument passed to gethostbyname() or gethostbyname2() should
     point to a	NUL-terminated hostname.  The addr argument passed to
     gethostbyaddr() should point to an	address	which is len bytes long, in
     binary form (i.e.,	not an IP address in human readable ASCII form).  The
     af	argument specifies the address family (e.g. AF_INET, AF_INET6, etc.)
     of	this address.

     The structure returned contains either the	information obtained from the
     name server, named(8), 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).

     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 alternate names for the host.

     h_addrtype	  The type of address being returned; usually AF_INET.

     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-

     When using	the nameserver,	gethostbyname()	and gethostbyname2() will
     search for	the named host in the current domain and its parents unless
     the name ends in a	dot.  If the name contains no dot, and if the environ-
     ment 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 format.

     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.

     The sethostent() function may be used to request the use of a connected
     TCP socket	for queries.  Queries will by default use UDP datagrams.  If
     the stayopen flag is non-zero, a TCP connection to	the name server	will
     be	used.  It will remain open after calls to gethostbyname(),
     gethostbyname2() or gethostbyaddr() have completed.

     The endhostent() function closes the TCP connection.

     The herror() function writes a message to the diagnostic output consist-
     ing of the	string argument	string,	the constant string ": ", and a	mes-
     sage 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	argument.


     Print out the hostname associated with a specific IP address:

	   const char *ipstr = "";
	   struct in_addr ip;
	   struct hostent *hp;

	   if (!inet_aton(ipstr, &ip))
		   errx(1, "can't parse	IP address %s",	ipstr);

	   if ((hp = gethostbyaddr((const void *)&ip,
	       sizeof ip, AF_INET)) == NULL)
		   errx(1, "no name associated with %s", ipstr);

	   printf("name	associated with	%s is %s\n", ipstr, hp->h_name);

     Error return status from gethostbyname(), gethostbyname2()	and
     gethostbyaddr() is	indicated by return of a NULL pointer.	The 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 mes-
     sage 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.

     getaddrinfo(3), getnameinfo(3), inet_aton(3), resolver(3),	hosts(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 version 4.9.4.

     The gethostent() function is defined, and sethostent() and	endhostent()
     are redefined, when Standard C Library (libc, -lc)	is built to use	only
     the routines to lookup in /etc/hosts and not the name server.

     The gethostent() function reads the next line of /etc/hosts, 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() or gethostbyaddr().

     The endhostent() function closes the file.

     These functions use a thread-specific data	storage; if the	data is	needed
     for future	use, it	should be copied before	any subsequent calls overwrite

     Though these functions are	thread-safe, still it is recommended to	use
     the getaddrinfo(3)	family of functions, instead.

     Only the Internet address format is currently understood.

BSD				October	4, 2017				   BSD


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