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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(), gethostby-
       name2_r(): _BSD_SOURCE || _SVID_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 vari-
       able 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 depend-
       ing  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

       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  cur-
       rent 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.

              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 pointer.

       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

       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

              resolver configuration file

              host database file

              name service switch configuration

       POSIX.1-2001  specifies  gethostbyname(),  gethostbyaddr(),  sethostent(),  endhos-
       tent(), 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  getad-
       drinfo(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 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 rea-
       sonable 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

   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(), gethostby-
       name_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 hexadecimal.

       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 information about reporting bugs, can be found at http://www.ker-

                                  2009-03-15                  GETHOSTBYNAME(3)

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