gethostbyaddr(3)



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

NAME
       gethostbyname,  gethostbyaddr, sethostent, gethostent, endhostent, her-
       ror, 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);

       /* 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 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 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 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())
       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 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, POSIX.1-2001.

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  gethostbyname_r()  and  gethostby-
       name2_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 gethost-
       byaddr()  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.)

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

       POSIX.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)


                                  2004-10-31                  GETHOSTBYNAME(3)

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