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

NAME
       gethostbyname,   gethostbyaddr,   sethostent,  gethostent,  endhostent,
       h_errno, herror, hstrerror, gethostbyaddr_r, gethostbyname2, gethostby-
       name2_r, gethostbyname_r, gethostent_r - 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,
                                     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 || _SVID_SOURCE

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

DESCRIPTION
       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 exam-
       ple 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.  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.   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
              An array of alternative names for the host, terminated by a NULL
              pointer.

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

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

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.

       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  hos-
       tent  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.

BUGS
       gethostbyname() does not recognize components of a dotted IPv4  address
       string that are expressed in hexadecimal.

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

COLOPHON
       This page is part of release 3.27 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

                                  2009-12-03                  GETHOSTBYNAME(3)

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