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

NAME

       gethostbyname,  gethostbyaddr,  sethostent, gethostent, endhostent, h_errno, herror, hstrerror, gethostbyaddr_r, gethostbyname2, gethostby-
       name2_r, gethostbyname_r, gethostent_r - 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 void *addr,
				     socklen_t len, int type);

       void sethostent(int stayopen);

       void endhostent(void);

       void herror(const char *s);

       const char *hstrerror(int err);

       /* System V/POSIX extension */
       struct hostent *gethostent(void);

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

       int gethostent_r(
	       struct hostent *ret, char *buf, size_t buflen,
	       struct hostent **result, int *h_errnop);

       int gethostbyaddr_r(const void *addr, socklen_t len, int type,
	       struct hostent *ret, char *buf, size_t buflen,
	       struct hostent **result, int *h_errnop);

       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);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       gethostbyname2(), gethostent_r(), gethostbyaddr_r(), gethostbyname_r(), gethostbyname2_r():
	   _BSD_SOURCE || _SVID_SOURCE

       herror(), hstrerror()
	   (since glibc 2.8): _BSD_SOURCE || _SVID_SOURCE || _GNU_SOURCE

DESCRIPTION

       The gethostbyname*() and gethostbyaddr*() functions are obsolete.  Applications should use getaddrinfo(3) and getnameinfo(3) instead.

       The gethostbyname() function returns a structure of type hostent for the given host name.  Here name is	either	a  hostname,  or  an  IPv4
       address	in  standard  dot  notation (as for inet_addr(3)), or an IPv6 address in colon (and possibly 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  host-
       name(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.
       Valid address types are AF_INET and AF_INET6.  The host address argument is a pointer to a struct of a type depending on the address  type,
       for example a struct in_addr * (probably obtained via a call to inet_addr(3)) for address type 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.  Otherwise, 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 gethostbyaddr() 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.	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
	      An array of alternative names for the host, terminated by a NULL pointer.

       h_addrtype
	      The type of address; always AF_INET or AF_INET6 at present.

       h_length
	      The length of the address in bytes.

       h_addr_list
	      An array of pointers to network addresses for the host (in network byte order), terminated by a NULL pointer.

       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.	When non-NULL, the return value may point at static data, see the notes below.

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 nonrecoverable 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

       /etc/nsswitch.conf
	      name service switch configuration

CONFORMING TO

       POSIX.1-2001  specifies	gethostbyname(), gethostbyaddr(), sethostent(), endhostent(), gethostent(), and h_errno; gethostbyname(), gethost-
       byaddr(), and h_errno are marked obsolescent in that standard.	POSIX.1-2008  removes  the  specifications  of	gethostbyname(),  gethost-
       byaddr(), and h_errno, recommending the use of getaddrinfo(3) and getnameinfo(3) instead.

NOTES

       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.

       In the original BSD implementation the len argument of gethostbyname() was an int.  The SUSv2 standard is buggy and declares the len  argu-
       ment  of  gethostbyaddr()  to  be  of  type  size_t.   (That  is wrong, because it has to be int, and size_t is not.  POSIX.1-2001 makes it
       socklen_t, which is OK.)  See also accept(2).

       The BSD prototype for gethostbyaddr() uses const char * for the first argument.

   System V/POSIX Extension
       POSIX requires the gethostent() call, that should return the next entry in the host data base.  When using DNS/BIND this does not make much
       sense,  but  it	may  be  reasonable if the host data base is a file that can be read line by line.  On many systems a routine of this name
       reads from the file /etc/hosts.	It may be available only when the library was built without DNS support.  The glibc  version  will  ignore
       ipv6 entries.  This function is not reentrant, and glibc adds a reentrant version gethostent_r().

   GNU Extensions
       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 gethostent_r(), gethostbyaddr_r(), gethostbyname_r() and gethostbyname2_r().	The caller supplies a hos-
       tent  structure	ret which will be filled in on success, and a temporary work buffer buf of size buflen.  After the call, result will point
       to the result on success.  In case of an error or if no entry is found result will be NULL.  The  functions  return  0  on  success  and  a
       nonzero	error number on failure.  In addition to the errors returned by the nonreentrant versions of these functions, if buf is too small,
       the functions will return ERANGE, and the call should be retried with a larger buffer.  The global variable h_errno is  not  modified,  but
       the address of a variable in which to store error numbers is passed in h_errnop.

BUGS

       gethostbyname() does not recognize components of a dotted IPv4 address string that are expressed in hexadecimal.

SEE ALSO

       getaddrinfo(3), getnameinfo(3), inet(3), inet_ntop(3), inet_pton(3), resolver(3), hosts(5), nsswitch.conf(5), hostname(7), named(8)

COLOPHON

       This  page is part of release 3.27 of the Linux man-pages project.  A description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

								    2009-12-03							  GETHOSTBYNAME(3)