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       getnameinfo   -  address-to-name  translation  in  protocol-independent


       #include <sys/socket.h>
       #include <netdb.h>

       int getnameinfo(const struct sockaddr *sa, socklen_t salen,
                       char *host, size_t hostlen,
                       char *serv, size_t servlen, int flags);


       The getnameinfo() function is defined for protocol-independent address-
       to-nodename    translation.    It   combines   the   functionality   of
       gethostbyaddr(3)  and  getservbyport(3)   and   is   the   inverse   of
       getaddrinfo(3).   The  sa  argument  is  a  pointer to a generic socket
       address structure (of type sockaddr_in or sockaddr_in6) of  size  salen
       that  holds  the  input IP address and port number.  The arguments host
       and  serv  are  pointers  to  buffers  (of  size  hostlen  and  servlen
       respectively) to hold the return values.

       The  caller  can  specify  that  no  hostname  (or  no service name) is
       required by providing a NULL host (or serv) argument or a zero  hostlen
       (or  servlen)  parameter.  However, at least one of hostname or service
       name must be requested.

       The flags argument modifies the behavior of getnameinfo() as follows:

              If set, return only the hostname part  of  the  FQDN  for  local

              If  set,  then  the  numeric  form  of the hostname is returned.
              (When not set, this will still happen in case  the  node’s  name
              cannot be looked up.)

              If  set,  then  an  error  is returned if the hostname cannot be
              looked up.

              If set, then the service address is returned  in  numeric  form,
              for example by its port number.

              If  set,  then  the  service is datagram (UDP) based rather than
              stream  (TCP)  based.   This  is  required  for  the  few  ports
              (512-514) that have different services for UDP and TCP.

   Extensions to getaddrinfo() for Internationalized Domain Names
       Starting   with   glibc  2.3.4,  getnameinfo()  has  been  extended  to
       selectively allow host names to be transparently converted to and  from
       the   Internationalized   Domain  Name  (IDN)  format  (see  RFC  3490,
       Internationalizing Domain Names in  Applications  (IDNA)).   Three  new
       flags are defined:

       NI_IDN If  this flag is used, then the name found in the lookup process
              is converted  from  IDN  format  to  the  locale’s  encoding  if
              necessary.  ASCII-only names are not affected by the conversion,
              which  makes  this  flag  usable  in   existing   programs   and

              Setting these flags will enable the IDNA_ALLOW_UNASSIGNED (allow
              unassigned Unicode code  points)  and  IDNA_USE_STD3_ASCII_RULES
              (check  output  to  make sure it is a STD3 conforming host name)
              flags respectively to be used in the IDNA handling.


       On success 0 is returned, and node and service names, if requested, are
       filled  with  null-terminated  strings,  possibly  truncated to fit the
       specified buffer lengths.  On error one of the following nonzero  error
       codes is returned:

              The name could not be resolved at this time.  Try again later.

              The flags parameter has an invalid value.

              A non-recoverable error occurred.

              The address family was not recognized, or the address length was
              invalid for the specified family.

              Out of memory.

              The  name  does  not  resolve  for  the   supplied   parameters.
              NI_NAMEREQD  is  set  and  the host’s name cannot be located, or
              neither hostname nor service name were requested.

              The buffer pointed to by host or serv was too small.

              A system error occurred.  The error code can be found in  errno.

       The  gai_strerror(3)  function  translates these error codes to a human
       readable string, suitable for error reporting.




       RFC 2553, POSIX.1-2001.


       In order to assist the programmer in choosing reasonable sizes for  the
       supplied buffers, <netdb.h> defines the constants

           #define NI_MAXHOST      1025
           #define NI_MAXSERV      32

       The  former  is  the  constant  MAXDNAME  in  recent versions of BIND’s
       <arpa/nameser.h> header file.  The latter  is  a  guess  based  on  the
       services listed in the current Assigned Numbers RFC.


       The  following code tries to get the numeric hostname and service name,
       for a given socket address.  Note that there is no hardcoded  reference
       to a particular address family.

           struct sockaddr *sa;    /* input */
           char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];

           if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), sbuf,
                       sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV) == 0)
               printf("host=%s, serv=%s\n", hbuf, sbuf);

       The  following  version  checks  if  the  socket  address has a reverse
       address mapping.

           struct sockaddr *sa;    /* input */
           char hbuf[NI_MAXHOST];

           if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf),
                       NULL, 0, NI_NAMEREQD))
               printf("could not resolve hostname");
               printf("host=%s\n", hbuf);

       An example program using getnameinfo() can be found in  getaddrinfo(3).


       socket(2),    getaddrinfo(3),    gethostbyaddr(3),    getservbyname(3),
       getservbyport(3),  inet_ntop(3),  hosts(5),  services(5),  hostname(7),

       R.  Gilligan,  S.  Thomson,  J.  Bound  and  W.  Stevens,  Basic Socket
       Interface Extensions for IPv6, RFC 2553, March 1999.

       Tatsuya Jinmei and Atsushi Onoe, An Extension of Format for IPv6 Scoped
       Addresses,       internet       draft,      work      in      progress.

       Craig Metz, Protocol Independence Using the Sockets API, Proceedings of
       the freenix track: 2000 USENIX annual technical conference, June  2000.


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