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

       gethostbyname, gethostbyaddr, sethostent, endhostent, herror, hstrerror
       - 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 char *addr,
         int len, int type);

       void sethostent(int stayopen);

       void endhostent(void);

       void herror(const char *s);

       const char *hstrerror(int err);

       /* GNU extensions */
       struct hostent *gethostbyname2(const char *name, int af);

       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);

       The gethostbyname() function returns a structure of  type  hostent  for
       the  given  host  name.   Here  name  is either a host name, or an IPv4
       address in standard dot notation, or an IPv6 address in colon (and pos-
       sibly  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

       The  gethostbyaddr()  function  returns a structure of type hostent for
       the given host address addr of length len and address type  type.   The
       only valid address type is currently 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.   (See  resolv+(8)).   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.

              A zero-terminated array of alternative names for the host.

              The type of address; always AF_INET at present.

              The length of the address in bytes.

              A zero-terminated array of network addresses  for  the  host  in
              network byte order.

       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.

       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

       BSD 4.3.

       The SUS-v2 standard is buggy and declares the len parameter of gethost-
       byaddr()  to  be  of type size_t.  (That is wrong, because it has to be
       int, and size_t is not. POSIX 1003.1-2001 makes it socklen_t, which  is

       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.

       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  gethostbyname_r()  and  gethostby-
       name2_r().   These return 0 on success and nonzero on error. The result
       of the call is now stored in the struct with address  ret.   After  the
       call,  *result will be NULL on error or point to the result on success.
       Auxiliary data is stored in the buffer buf of length buflen.   (If  the
       buffer  is  too  small, these functions will return ERANGE.)  No global
       variable h_errno is modified, but the address of a variable in which to
       store error numbers is passed in h_errnop.

       POSIX 1003.1-2001 marks gethostbyaddr() and gethostbyname() legacy, and

       struct hostent *getipnodebyaddr (const void *restrict addr,
         socklen_t len, int type, int *restrict error_num);

       struct hostent *getipnodebyname (const char *name,
         int type, int flags, int *error_num);

       resolver(3), hosts(5), hostname(7), resolv+(8), named(8)

BSD                               2000-08-12                  GETHOSTBYNAME(3)