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 *addr, socklen_t addrlen,
                       char *host, socklen_t hostlen,
                       char *serv, socklen_t servlen, int flags);

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

           Since glibc 2.22: _POSIX_C_SOURCE >= 201112L
           Glibc 2.21 and earlier: _POSIX_C_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, getname-
       info() is reentrant and allows programs to  eliminate  IPv4-versus-IPv6

       The  addr  argument  is a pointer to a generic socket address structure
       (of type sockaddr_in or sockaddr_in6) of size addrlen  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 getnameinfo() 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 nonzero error
       codes is returned:

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

              The flags argument has an invalid value.

              A nonrecoverable 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.

       For   an   explanation   of   the  terms  used  in  this  section,  see

       |Interface     | Attribute     | Value              |
       |getnameinfo() | Thread safety | MT-Safe env locale |

       POSIX.1-2001, POSIX.1-2008, RFC 2553.

       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

       Since glibc 2.8, these definitions are exposed only if suitable feature
       test macros are defined, namely:  _GNU_SOURCE,  _DEFAULT_SOURCE  (since
       glibc   2.19),  or  (in  glibc  versions  up  to  and  including  2.19)

       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.

       Before glibc version 2.2, the hostlen and servlen arguments were  typed
       as size_t.

       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 *addr;     /* input */
           socklen_t addrlen;         /* input */
           char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];

           if (getnameinfo(addr, addrlen, 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 *addr;     /* input */
           socklen_t addrlen;         /* input */
           char hbuf[NI_MAXHOST];

           if (getnameinfo(addr, addrlen, 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 4.15 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

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