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():
	   Since glibc 2.19:
	       _DEFAULT_SOURCE
	   Glibc versions up to and including 2.19:
	       _BSD_SOURCE || _SVID_SOURCE

       herror(), hstrerror():
	   Since glibc 2.19:
	       _DEFAULT_SOURCE
	   Glibc 2.8 to 2.19:
	       _BSD_SOURCE || _SVID_SOURCE
	   Before glibc 2.8:
	       none

       h_errno:
	   Since glibc 2.19
	       _DEFAULT_SOURCE || _POSIX_C_SOURCE < 200809L
	   Glibc 2.12 to 2.19:
	       _BSD_SOURCE || _SVID_SOURCE || _POSIX_C_SOURCE < 200809L
	   Before glibc 2.12:
	       none

DESCRIPTION
       The gethostbyname*(), gethostbyaddr*(), herror(), and hstrerror() func-
       tions  are  obsolete.  Applications should use getaddrinfo(3), getname-
       info(3), and gai_strerror(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)).	If  name  is  an  IPv4
       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()	 rely on the Name Service Switch (nsswitch.conf(5)) configured
       sources or a local name server (named(8)).  The default	action	is  to
       query  the  Name	 Service Switch (nsswitch.conf(5)) configured sources,
       failing that, a local name server (named(8)).

   Historical
       The nsswitch.conf(5) file is the modern way of controlling the order of
       host lookups.

       In  glibc  2.4  and  earlier, the order keyword was used to control the
       order of host lookups as defined in /etc/host.conf (host.conf(5)).


       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_DATA
	      The  requested  name  is	valid but does not have an IP address.
	      Another type of request to the name server for this  domain  may
	      return  an  answer.   The	 constant  NO_ADDRESS is a synonym for
	      NO_DATA.

       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

ATTRIBUTES
       For   an	  explanation	of   the  terms	 used  in  this	 section,  see
       attributes(7).

       +-------------------+---------------+-------------------------------+
       |Interface	   | Attribute	   | Value			   |
       +-------------------+---------------+-------------------------------+
       |gethostbyname()	   | Thread safety | MT-Unsafe race:hostbyname env |
       |		   |		   | locale			   |
       +-------------------+---------------+-------------------------------+
       |gethostbyaddr()	   | Thread safety | MT-Unsafe race:hostbyaddr env |
       |		   |		   | locale			   |
       +-------------------+---------------+-------------------------------+
       |sethostent(),	   | Thread safety | MT-Unsafe race:hostent env	   |
       |endhostent(),	   |		   | locale			   |
       |gethostent_r()	   |		   |				   |
       +-------------------+---------------+-------------------------------+
       |herror(),	   | Thread safety | MT-Safe			   |
       |hstrerror()	   |		   |				   |
       +-------------------+---------------+-------------------------------+
       |gethostent()	   | Thread safety | MT-Unsafe race:hostent	   |
       |		   |		   | race:hostentbuf env locale	   |
       +-------------------+---------------+-------------------------------+
       |gethostbyname2()   | Thread safety | MT-Unsafe race:hostbyname2	   |
       |		   |		   | env locale			   |
       +-------------------+---------------+-------------------------------+
       |gethostbyaddr_r(), | Thread safety | MT-Safe env locale		   |
       |gethostbyname_r(), |		   |				   |
       |gethostbyname2_r() |		   |				   |
       +-------------------+---------------+-------------------------------+
       In the above table, hostent in race:hostent signifies that  if  any  of
       the  functions  sethostent(),  gethostent(), gethostent_r(), or endhos-
       tent() are used in parallel in different threads	 of  a	program,  then
       data races could occur.

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 get-
       nameinfo(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  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,  which  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	 reen-
       trant 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 4.10 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       https://www.kernel.org/doc/man-pages/.



				  2016-03-15		      GETHOSTBYNAME(3)