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

       gethostbyname,  gethostbyaddr, sethostent, gethostent, endhostent, h_errno, herror, hstrerror, gethostbyaddr_r,
       gethostbyname2, gethostbyname2_r, gethostbyname_r, gethostent_r - get network host entry

       #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 ||

       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 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

       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), fol-
       lowed 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.

              An array of alternative names for the host, terminated by a NULL pointer.

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

              The length of the address in bytes.

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

       h_addr The first address in h_addr_list for backward compatibility.

       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.

       The variable h_errno can have the following values:

              The specified host is unknown.

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

              A non-recoverable name server error occurred.

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

              resolver configuration file

              host database file

              name service switch configuration

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

       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

       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  func-
       tion 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 hostent 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.

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

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

       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at

                                  2009-03-15                  GETHOSTBYNAME(3)