gethostbyaddr(3)

NAME

   gethostbyname,   gethostbyaddr,   sethostent,  gethostent,  endhostent,
   h_errno,   herror,    hstrerror,    gethostbyaddr_r,    gethostbyname2,
   gethostbyname2_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():
       Since glibc 2.19:
           _DEFAULT_SOURCE
       Glibc versions up to and including 2.19:
           _BSD_SOURCE || _SVID_SOURCE

   herror(), hstrerror():
       Since glibc 2.19:
           _DEFAULT_SOURCE
       Glibc 2.8 to 2.19:
           _BSD_SOURCE || _SVID_SOURCE
       Before glibc 2.8:
           none

   h_errno:
       Since glibc 2.19
           _DEFAULT_SOURCE || _POSIX_C_SOURCE < 200809L
       Glibc 2.12 to 2.19:
           _BSD_SOURCE || _SVID_SOURCE || _POSIX_C_SOURCE < 200809L
       Before glibc 2.12:
           none

DESCRIPTION

   The   gethostbyname*(),  gethostbyaddr*(),  herror(),  and  hstrerror()
   functions  are  obsolete.   Applications  should  use   getaddrinfo(3),
   getnameinfo(3), and gai_strerror(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)).  If name is an IPv4
   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(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 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() rely on  the  Name  Service  Switch  (nsswitch.conf(5))
   configured  sources  or  a  local  name server (named(8)).  The default
   action  is  to  query  the  Name  Service   Switch   (nsswitch.conf(5))
   configured sources, failing that, a local name server (named(8)).

   Historical
   The nsswitch.conf(5) file is the modern way of controlling the order of
   host lookups.

   In glibc 2.4 and earlier, the order keyword was  used  to  control  the
   order of host lookups as defined in /etc/host.conf (host.conf(5)).

   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
          network 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_DATA
          The requested name is valid but does not  have  an  IP  address.
          Another  type  of request to the name server for this domain may
          return an answer.  The constant  NO_ADDRESS  is  a  synonym  for
          NO_DATA.

   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

ATTRIBUTES

   For  an  explanation  of  the  terms  used   in   this   section,   see
   attributes(7).

   ┌───────────────────┬───────────────┬───────────────────────────────┐
   │Interface          │ Attribute     │ Value                         │
   ├───────────────────┼───────────────┼───────────────────────────────┤
   │gethostbyname()    │ Thread safety │ MT-Unsafe race:hostbyname env │
   │                   │               │ locale                        │
   ├───────────────────┼───────────────┼───────────────────────────────┤
   │gethostbyaddr()    │ Thread safety │ MT-Unsafe race:hostbyaddr env │
   │                   │               │ locale                        │
   ├───────────────────┼───────────────┼───────────────────────────────┤
   │sethostent(),      │ Thread safety │ MT-Unsafe race:hostent env    │
   │endhostent(),      │               │ locale                        │
   │gethostent_r()     │               │                               │
   ├───────────────────┼───────────────┼───────────────────────────────┤
   │herror(),          │ Thread safety │ MT-Safe                       │
   │hstrerror()        │               │                               │
   ├───────────────────┼───────────────┼───────────────────────────────┤
   │gethostent()       │ Thread safety │ MT-Unsafe race:hostent        │
   │                   │               │ race:hostentbuf env locale    │
   ├───────────────────┼───────────────┼───────────────────────────────┤
   │gethostbyname2()   │ Thread safety │ MT-Unsafe race:hostbyname2    │
   │                   │               │ env locale                    │
   ├───────────────────┼───────────────┼───────────────────────────────┤
   │gethostbyaddr_r(), │ Thread safety │ MT-Safe env locale            │
   │gethostbyname_r(), │               │                               │
   │gethostbyname2_r() │               │                               │
   └───────────────────┴───────────────┴───────────────────────────────┘
   In  the  above  table, hostent in race:hostent signifies that if any of
   the  functions  sethostent(3),   gethostent(3),   gethostent_r(3),   or
   endhostent(3)  are  used in parallel in different threads of a program,
   then data races could occur.

CONFORMING TO

   POSIX.1-2001 specifies gethostbyname(), gethostbyaddr(),  sethostent(),
   endhostent(),     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  getaddrinfo(3)
   and getnameinfo(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,  which 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.

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 4.09 of the Linux  man-pages  project.   A
   description  of  the project, information about reporting bugs, and the
   latest    version    of    this    page,    can     be     found     at
   https://www.kernel.org/doc/man-pages/.

                              2016-03-15                  GETHOSTBYNAME(3)

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