gethostbyaddr(3)



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
           Before glibc 2.8:
               none

       h_errno:
           Since glibc 2.12:
               _BSD_SOURCE || _SVID_SOURCE ||
                   (_POSIX_C_SOURCE < 200809L && _XOPEN_SOURCE < 700)
           Before glibc 2.12:
               none

DESCRIPTION
       The gethostbyname*(), gethostbyaddr*(), herror(), and hstrerror() func-
       tions  are  obsolete.  Applications should use getaddrinfo(3), getname-
       info(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)),  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 3.74 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
       http://www.kernel.org/doc/man-pages/.

                                  2014-03-11                  GETHOSTBYNAME(3)

Man(1) output converted with man2html
list of all man pages