NAME

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

SYNOPSIS

       #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

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, getnameinfo() 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 getnameinfo() 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

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