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NAME

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

SYNOPSIS

       #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)):

       getnameinfo():
           Since glibc 2.22: _POSIX_C_SOURCE >= 201112L
           Glibc 2.21 and earlier: _POSIX_C_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 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:

       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.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────┬───────────────┬────────────────────┐
       │InterfaceAttributeValue              │
       ├──────────────┼───────────────┼────────────────────┤
       │getnameinfo() │ Thread safety │ MT-Safe env locale │
       └──────────────┴───────────────┴────────────────────┘

CONFORMING TO

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

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 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) _BSD_SOURCE or _SVID_SOURCE.

       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.

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

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 *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");
           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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