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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

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 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.	 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 exam‐
       ple 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.  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.	 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 net‐
	      work 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 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

       /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.

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  argument
       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 non-zero
       error number on failure.	 In addition to the  errors  returned  by  the
       non-reentrant  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 3.15 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/.

				  2008-10-21		      GETHOSTBYNAME(3)
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