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gethostbyname(3) - endhostent, gethostbyaddr, gethostbyname, herror, hstrerror, sethostent, endhostent, gethostbyaddr, gethostbyname, gethostend, herror, hstrerror, sethostent - get network host entry - man 3 gethostbyname

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



NAME
       gethostbyname,  gethostbyaddr, sethostent, gethostend, endhostent, her-
       ror, hstrerror - get network host(1,5) 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(1,5)  name.   Here  name  is either a host(1,5) name, or an IPv4
       address in(1,8) standard dot notation, or an IPv6 address in(1,8) 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(1,8) a dot and the environment
       variable HOSTALIASES is set(7,n,1 builtins), the alias file(1,n) pointed to  by  HOSTALIASES
       will  first be searched for name (see hostname(7) for the file(1,n) format).
       The current domain and its parents are searched unless name ends  in(1,8)  a
       dot.

       The  gethostbyaddr()  function  returns a structure of type hostent for
       the given host(1,5) address addr of length len and address type type.  Valid
       address types are AF_INET and AF_INET6.  The host(1,5) 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())
       for address type AF_INET.

       The sethostent() function specifies, if(3,n) stayopen is true  (1),  that  a
       connected  TCP  socket(2,7,n)  should  be used for the name server queries and
       that the connection should remain open(2,3,n) 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(8,n)  message  associated
       with the current value of h_errno on stderr.

       The  (obsolete)  hstrerror()  function takes an error(8,n) 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(5,8)(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(1,8)
       /etc/host.conf.  (See resolv+(8)).  The  default  action  is  to  query
       named(5,8)(8), followed by /etc/hosts.

       The hostent structure is defined in(1,8) <netdb.h> as follows:

              struct hostent {
                      char    *h_name;        /* official name of host(1,5) */
                      char    **h_aliases;    /* alias list */
                      int     h_addrtype;     /* host(1,5) 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(1,8) bytes.

       h_addr_list
              A  zero-terminated  array  of  network addresses for the host(1,5) in(1,8)
              network byte order.

       h_addr The first address in(1,8) h_addr_list for backward compatibility.

RETURN VALUE
       The gethostbyname() and gethostbyaddr() functions  return  the  hostent
       structure  or a NULL pointer if(3,n) an error(8,n) occurs.  On error(8,n), the h_errno
       variable holds an error(8,n) 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(1,5) 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(8,n) occurred.

       TRY_AGAIN
              A temporary error(8,n) occurred on an authoritative name server.  Try
              again later.

FILES
       /etc/host.conf
              resolver(3,5) configuration file(1,n)

       /etc/hosts
              host(1,5) database file(1,n)

CONFORMING TO
       BSD 4.3.

SYSV/POSIX EXTENSION
       POSIX requires the gethostent() call, that should return the next entry
       in(1,8)  the  host(1,5)  data  base.  When using DNS/BIND this does not make much
       sense, but it may be reasonable if(3,n) the host(1,5) data base is  a  file(1,n)  that
       can  be read(2,n,1 builtins) line by line. On many systems a routine of this name reads
       from the file(1,n) /etc/hosts.  It may be available only  when  the  library
       was  built  without  DNS  support.   The glibc version(1,3,5) will ignore ipv6
       entries. This function is not reentrant, and  glibc  adds  a  reentrant
       version(1,3,5) 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 gethostby-
       name2_r().  These return 0 on success and nonzero on error. The  result
       of  the  call  is now stored in(1,8) the struct with address ret.  After the
       call, *result will be NULL on error(8,n) or point to the result on  success.
       Auxiliary  data  is stored in(1,8) 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(1,8) which to
       store error(8,n) numbers is passed in(1,8) 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 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
       OK.)

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

       POSIX  1003.1-2001  marks  gethostbyaddr() and gethostbyname() obsoles-
       cent. 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(3,5)(3),   hosts(5),   hostname(7),
       named(5,8)(8), resolv+(8)



                                  2004-10-31                  GETHOSTBYNAME(3)

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