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GETHOSTBYNAME(3)	   Linux Programmer's Manual	      GETHOSTBYNAME(3)

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

SYNOPSIS
       #include	<netdb.h>
       extern int h_errno;

       struct hostent *gethostbyname(const char	*name);

       #include	<sys/socket.h>	      /* for AF_INET */
       struct hostent *gethostbyaddr(const char	*addr,
	 int len, int type);

       void sethostent(int stayopen);

       void endhostent(void);

       void herror(const char *s);

       const char *hstrerror(int err);

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

       int gethostbyname_r (const char *name,
	 struct	hostent	*ret, char *buf, size_t	buflen,
	 struct	hostent	**result, int *h_errnop);

       int gethostbyname2_r (const char	*name, int af,
	 struct	hostent	*ret, char *buf, size_t	buflen,
	 struct	hostent	**result, int *h_errnop);

DESCRIPTION
       The gethostbyname() function returns a structure	of  type  hostent  for
       the  given  host	 name.	 Here  name  is	either a host name, or an IPv4
       address in standard dot notation, or an IPv6 address in colon (and pos-
       sibly  dot)  notation.  (See  RFC  1884	for  the  description  of IPv6
       addresses.)  If name is an IPv4 or IPv6 address,	no lookup is performed
       and  gethostbyname()  simply  copies name into the h_name field and its
       struct in_addr equivalent into the h_addr_list[0] field of the returned
       hostent	structure.   If	 name doesn't end in a dot and the environment
       variable	HOSTALIASES is set, the	alias file pointed to  by  HOSTALIASES
       will  first be searched for name	(see hostname(7) for the file format).
       The current domain and its parents are searched unless name ends	 in  a
       dot.

       The  gethostbyaddr()  function  returns a structure of type hostent for
       the given host address addr of length len and address type  type.   The
       only valid address type is currently AF_INET.

       The  sethostent()  function  specifies, if stayopen is true (1),	that a
       connected TCP socket should be used for the  name  server  queries  and
       that the	connection should remain open during successive	queries.  Oth-
       erwise, name server queries will	use UDP	datagrams.

       The endhostent()	function ends the use of a  TCP	 connection  for  name
       server queries.

       The  (obsolete)	herror()  function prints the error message associated
       with the	current	value of h_errno on stderr.

       The (obsolete) hstrerror() function takes an  error  number  (typically
       h_errno)	and returns the	corresponding message string.

       The  domain  name  queries  carried out by gethostbyname() and gethost-
       byaddr()	use a combination of any or all	of the name server named(8), a
       broken  out  line  from /etc/hosts, and the Network Information Service
       (NIS or	YP),  depending	 upon  the  contents  of  the  order  line  in
       /etc/host.conf.	 (See  resolv+(8)).   The  default  action is to query
       named(8), followed by /etc/hosts.

       The hostent structure is	defined	in _netdb.h_ as	follows:

	      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 */
	      }
	      #define h_addr  h_addr_list[0]  /* for backward compatibility */

       The members of the hostent structure are:

       h_name The official name	of the host.

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

       h_addrtype
	      The type of address; always AF_INET at present.

       h_length
	      The length of the	address	in bytes.

       h_addr_list
	      A	zero-terminated	array of network addresses  for	 the  host  in
	      network byte order.

       h_addr The first	address	in h_addr_list for backward compatibility.

RETURN VALUE
       The  gethostbyname()  and  gethostbyaddr() functions return the hostent
       structure or a NULL pointer if an error occurs.	On error, the  h_errno
       variable	holds an error number.

ERRORS
       The variable h_errno can	have the following values:

       HOST_NOT_FOUND
	      The specified host is unknown.

       NO_ADDRESS or NO_DATA
	      The requested name is valid but does not have an IP address.

       NO_RECOVERY
	      A	non-recoverable	name server error occurred.

       TRY_AGAIN
	      A	temporary error	occurred on an authoritative name server.  Try
	      again later.

FILES
       /etc/host.conf
	      resolver configuration file

       /etc/hosts
	      host database file

CONFORMING TO
       BSD 4.3.

NOTES
       The SUS-v2 standard is buggy and	declares the len parameter of gethost-
       byaddr()	 to  be	 of type size_t.  (That	is wrong, because it has to be
       int, and	size_t is not. POSIX 1003.1-2001 makes it socklen_t, which  is
       OK.)

       The  functions  gethostbyname() and gethostbyaddr() may return pointers
       to static data, which may be overwritten	by later  calls.  Copying  the
       struct  hostent	does  not suffice, since it contains pointers -	a deep
       copy is required.

       Glibc2 also has a gethostbyname2() that works like gethostbyname(), but
       permits to specify the address family to	which the address must belong.

       Glibc2 also has reentrant  versions  gethostbyname_r()  and  gethostby-
       name2_r().   These return 0 on success and nonzero on error. The	result
       of the call is now stored in the	struct with address  ret.   After  the
       call,  *result will be NULL on error or point to	the result on success.
       Auxiliary data is stored	in the buffer buf of length buflen.   (If  the
       buffer  is  too	small, these functions will return ERANGE.)  No	global
       variable	h_errno	is modified, but the address of	a variable in which to
       store error numbers is passed in	h_errnop.

       POSIX 1003.1-2001 marks gethostbyaddr() and gethostbyname() legacy, and
       introduces

       struct hostent *getipnodebyaddr (const void *restrict addr,
	 socklen_t len,	int type, int *restrict	error_num);

       struct hostent *getipnodebyname (const char *name,
	 int type, int flags, int *error_num);

SEE ALSO
       resolver(3), hosts(5), hostname(7), resolv+(8), named(8)

BSD				  2000-08-12		      GETHOSTBYNAME(3)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | FILES | CONFORMING TO | NOTES | SEE ALSO

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