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

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

       #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);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       getnameinfo(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE

       The getnameinfo() function is the inverse of  getaddrinfo(3):  it  con‐
       verts a socket address to a corresponding host and service, in a proto‐
       col-independent manner.	It  combines  the  functionality  of  gethost‐
       byaddr(3)  and  getservbyport(3),  but  unlike  those functions, getad‐
       drinfo(3) is reentrant and allows programs  to  eliminate  IPv4-versus-
       IPv6 dependencies.

       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
       caller-allocated buffers (of size  hostlen  and	servlen	 respectively)
       into  which getnameinfo() places null-terminated strings containing the
       host and service names respectively.

       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) argument.  However, at least one of	 hostname  or  service
       name must be requested.

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

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

	      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.

	      If set, return only the hostname part  of	 the  fully  qualified
	      domain name for local hosts.

	      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 determined.)

	      If  set,	then  the  numeric  form  of  the  service  address is
	      returned.	 (When not set, this will still	 happen	 in  case  the
	      service's name cannot be determined.)

   Extensions to getaddrinfo() for Internationalized Domain Names
       Starting	 with  glibc  2.3.4, getnameinfo() has been extended to selec‐
       tively allow hostnames to be transparently converted to	and  from  the
       Internationalized  Domain Name (IDN) format (see RFC 3490, Internation‐
       alizing Domain Names in Applications  (IDNA)).	Three  new  flags  are

       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	neces‐
	      sary.   ASCII-only  names	 are  not  affected by the conversion,
	      which makes this flag usable in existing programs	 and  environ‐

	      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 hostname)
	      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 non-zero error
       codes is returned:

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

	      The flags argument 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   arguments.
	      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.


       getnameinfo() is provided in glibc since version 2.1.

       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 ser‐
       vices 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 */
	   socklen_t len;	  /* input */
	   char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];

	   if (getnameinfo(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 */
	   socklen_t len;	  /* input */
	   char hbuf[NI_MAXHOST];

	   if (getnameinfo(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).

       accept(2),   getpeername(2),  getsockname(2),  recvfrom(2),  socket(2),
       getaddrinfo(3), gethostbyaddr(3),  getservbyname(3),  getservbyport(3),
       inet_ntop(3), hosts(5), services(5), hostname(7), named(8)

       R.  Gilligan,  S. Thomson, J. Bound and W. Stevens, Basic Socket Inter‐
       face 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.‐

       This  page  is  part of release 3.22 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

GNU				  2009-02-23			GETNAMEINFO(3)

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