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# getnameinfo

    GETNAMEINFO(3)           BSD Library Functions Manual           GETNAMEINFO(3)

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

    SYNOPSIS
         #include <sys/types.h>
         #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);

    DESCRIPTION
         The getnameinfo() function is defined for protocol-independent address-
         to-nodename translation.  Its functionality is a reverse conversion of
         getaddrinfo(3), and implements similar functionality with
         gethostbyaddr(3) and getservbyport(3) in more sophisticated manner.

         This function looks up an IP address and port number provided by the
         caller in the DNS and system-specific database, and returns text strings
         for both in buffers provided by the caller.  The function indicates suc-
         cessful completion by a zero return value; a non-zero return value indi-
         cates failure.

         The first argument, sa, points to either a sockaddr_in structure (for
         IPv4) or a sockaddr_in6 structure (for IPv6) that holds the IP address
         and port number.  The salen argument gives the length of the sockaddr_in
         or sockaddr_in6 structure.

         The function returns the nodename associated with the IP address in the
         buffer pointed to by the host argument.  The caller provides the size of
         this buffer via the hostlen argument.  The service name associated with
         the port number is returned in the buffer pointed to by serv, and the
         servlen argument gives the length of this buffer.  The caller specifies
         not to return either string by providing a zero value for the hostlen or
         servlen arguments.  Otherwise, the caller must provide buffers large
         enough to hold the nodename and the service name, including the terminat-
         ing null characters.

         Unfortunately most systems do not provide constants that specify the max-
         imum size of either a fully-qualified domain name or a service name.
         Therefore to aid the application in allocating buffers for these two
         returned strings the following constants are defined in <netdb.h>:

         #define NI_MAXHOST    MAXHOSTNAMELEN
         #define NI_MAXSERV    32

         The first value is actually defined as the constant MAXDNAME in recent
         versions of BIND's <arpa/nameser.h> header (older versions of BIND define
         this constant to be 256) and the second is a guess based on the services
         listed in the current Assigned Numbers RFC.

         The final argument is a flag that changes the default actions of this
         function.  By default the fully-qualified domain name (FQDN) for the host
         is looked up in the DNS and returned.  If the flag bit NI_NOFQDN is set,
         only the nodename portion of the FQDN is returned for local hosts.

         If the flag bit NI_NUMERICHOST is set, or if the host's name cannot be
         located in the DNS, the numeric form of the host's address is returned
         instead of its name (e.g., by calling inet_ntop() instead of
         gethostbyaddr()).  If the flag bit NI_NAMEREQD is set, an error is
         returned if the host's name cannot be located in the DNS.

         If the flag bit NI_NUMERICSERV is set, the numeric form of the service
         address is returned (e.g., its port number) instead of its name.  The two
         NI_NUMERICxxx flags are required to support the -n flag that many com-
         mands provide.

         A fifth flag bit, NI_DGRAM, specifies that the service is a datagram ser-
         vice, and causes getservbyport() to be called with a second argument of
         "udp" instead of its default of "tcp".  This is required for the few
         ports (512-514) that have different services for UDP and TCP.

         These NI_xxx flags are defined in <netdb.h>.

       Extension for scoped IPv6 address
         The implementation allows experimental numeric IPv6 address notation with
         scope identifier.  IPv6 link-local address will appear as string like
         ``fe80::1%ne0'', if NI_WITHSCOPEID bit is enabled in flags argument.
         Refer to getaddrinfo(3) for the notation.

    EXAMPLES
         The following code tries to get numeric hostname, and service name, for
         given socket address.  Observe that there is no hardcoded reference to
         particular address family.

               struct sockaddr *sa;    /* input */
               char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];

               if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), sbuf,
                   sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV)) {
                       errx(1, "could not get numeric hostname");
                       /*NOTREACHED*/
               }
               printf("host=%s, serv=%s\n", hbuf, sbuf);

         The following version checks if the socket address has reverse address
         mapping.

               struct sockaddr *sa;    /* input */
               char hbuf[NI_MAXHOST];

               if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), NULL, 0,
                   NI_NAMEREQD)) {
                       errx(1, "could not resolve hostname");
                       /*NOTREACHED*/
               }
               printf("host=%s\n", hbuf);

    DIAGNOSTICS
         The function indicates successful completion by a zero return value; a
         non-zero return value indicates failure.  Error codes are as below:

         EAI_AGAIN          The name could not be resolved at this time.  Future
                            attempts may succeed.

         EAI_BADFLAGS       The flags had an invalid value.

         EAI_FAIL           A non-recoverable error occurred.

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

         EAI_MEMORY         There was a memory allocation failure.

         EAI_NONAME         The name does not resolve for the supplied parameters.
                            NI_NAMEREQD is set and the host's name cannot be
                            located, or both nodename and servname were null.

         EAI_SYSTEM         A system error occurred.  The error code can be found
                            in errno.

    SEE ALSO
         getaddrinfo(3), gethostbyaddr(3), getservbyport(3), hosts(5),
         resolv.conf(5), services(5), hostname(7), named(8)

         R. Gilligan, S. Thomson, J. Bound, and W. Stevens, Basic Socket Interface
         Extensions for IPv6, RFC2553, March 1999.

         Tatsuya Jinmei and Atsushi Onoe, An Extension of Format for IPv6 Scoped
         Addresses, internet draft, draft-ietf-ipngwg-scopedaddr-format-02.txt,
         work in progress material.

         Craig Metz, "Protocol Independence Using the Sockets API", Proceedings of
         the freenix track: 2000 USENIX annual technical conference, June 2000.

    HISTORY
         The implementation first appeared in WIDE Hydrangea IPv6 protocol stack
         kit.

    STANDARDS
         The getaddrinfo() function is defined IEEE POSIX 1003.1g draft specifica-
         tion, and documented in ``Basic Socket Interface Extensions for IPv6''
         (RFC2553).

    BUGS
         The current implementation is not thread-safe.

         The text was shamelessly copied from RFC2553.

         OpenBSD intentionally uses different NI_MAXHOST value from what RFC2553
         suggests, to avoid buffer length handling mistakes.

    BSD                              May 25, 1995                              BSD

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