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

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

       #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 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 sock-
       addr_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-termi-
       nated strings containing the host and service names respectively.

       The caller can specify that no hostname (or no service name) is required by provid-
       ing a NULL host (or serv) argument or a zero hostlen (or servlen)  argument.   How-
       ever, 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 determined.

              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  deter-

   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.

              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 services  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

           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),  ser-
       vices(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.

       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  http://www.ker-

GNU                               2009-02-23                    GETNAMEINFO(3)

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