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NAME

       gethostbyname,   gethostbyaddr,   sethostent,  gethostent,  endhostent,
       herror, hstrerror - 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, int len, int type);

       void sethostent(int stayopen);

       void endhostent(void);

       void herror(const char *s);

       const char *hstrerror(int err);

       /* SYSV/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 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);

DESCRIPTION

       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
       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()) 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 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.

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

       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
              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.

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

       NO_RECOVERY
              A non-recoverable 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

CONFORMING TO

       4.3BSD.

SYSV/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   gethostbyname_r()   and
       gethostbyname2_r().  These return 0 on success and non-zero  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.

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.

       The  SUS-v2  standard  is  buggy  and  declares  the  len  parameter of
       gethostbyaddr() 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 OK.)

       The BSD prototype for gethostbyaddr() uses const char * for  the  first
       argument.

       POSIX    1003.1-2001    marks   gethostbyaddr()   and   gethostbyname()
       obsolescent. See getaddrinfo(3), getnameinfo(3), gai_strerror(3).

SEE ALSO

       getaddrinfo(3), getipnodebyaddr(3), getipnodebyname(3), getnameinfo(3),
       inet_ntop(3),  inet_pton(3),  resolver(3),  hosts(5), nsswitch.conf(5),
       hostname(7), named(8), resolv+(8)

                                  2004-10-31                  GETHOSTBYNAME(3)