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

       gethostbyname,  gethostbyaddr,  sethostent, gethostent, endhostent, h_errno, herror, hstr-
       error, 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-
           _BSD_SOURCE || _SVID_SOURCE

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

       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 per-
       formed 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 dur-
       ing 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 combina-
       tion 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 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 nonrecoverable 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(), geth-
       ostent(), and h_errno; gethostbyname(), gethostbyaddr(), and h_errno are  marked  obsoles-
       cent  in that standard.  POSIX.1-2008 removes the specifications of gethostbyname(), geth-
       ostbyaddr(), and h_errno,  recommending  the  use  of  getaddrinfo(3)  and  getnameinfo(3)

       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 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 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.

       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 3.53 of the Linux man-pages project.  A  description  of  the
       project,     and    information    about    reporting    bugs,    can    be    found    at

                                            2010-10-04                           GETHOSTBYNAME(3)

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