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



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

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

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

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

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

DESCRIPTION
       The  getnameinfo()  function  is  the inverse of getaddrinfo(3): it converts a socket address to a corresponding host and
       service, in a protocol-independent manner.  It combines the functionality of gethostbyaddr(3) and  getservbyport(3),  but
       unlike those functions, getaddrinfo(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:

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

       NI_DGRAM
              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.

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

       NI_NUMERICHOST
              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.)

       NI_NUMERICSERV
              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 selectively allow hostnames 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 environments.

       NI_IDN_ALLOW_UNASSIGNED, NI_IDN_USE_STD3_ASCII_RULES
              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.

RETURN VALUE
       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:

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

       EAI_BADFLAGS
              The flags argument has an invalid value.

       EAI_FAIL
              A nonrecoverable error occurred.

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

       EAI_MEMORY
              Out of memory.

       EAI_NONAME
              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.

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

       EAI_SYSTEM
              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.

FILES
       /etc/hosts
       /etc/nsswitch.conf
       /etc/resolv.conf

VERSIONS
       getnameinfo() is provided in glibc since version 2.1.

CONFORMING TO
       RFC 2553, POSIX.1-2001.

NOTES
       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 one of the  feature  test  macros  _BSD_SOURCE,  _SVID_SOURCE,  or
       _GNU_SOURCE is defined.

       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.

EXAMPLE
       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");
           else
               printf("host=%s\n", hbuf);

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

SEE ALSO
       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 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.
       ftp://ftp.ietf.org/internet-drafts/draft-ietf-ipngwg-scopedaddr-format-02.txt

       Craig  Metz,  Protocol Independence Using the Sockets API, Proceedings of the freenix track: 2000 USENIX annual technical
       conference, June 2000.  http://www.usenix.org/publications/library/proceedings/usenix2000/freenix/metzprotocol.html

COLOPHON
       This page is part of release 3.25 of the Linux man-pages project.  A description of the project,  and  information  about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



GNU                                                        2009-12-03                                             GETNAMEINFO(3)