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GETNAMEINFO(3)             Library Functions Manual             GETNAMEINFO(3)

     getnameinfo - socket address structure to hostname and service name

     #include <netdb.h>

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

     The getnameinfo() function is used to convert a sockaddr structure to a
     pair of host name and service strings.  It is a replacement for and
     provides more flexibility than the gethostbyaddr(3) and getservbyport(3)
     functions and is the converse of the getaddrinfo(3) function.

     The sockaddr structure sa should point to a sockaddr_in (for IPv4),
     sockaddr_in6 (for IPv6), sockaddr_atalk (for AppleTalk), sockaddr_link
     (for link layer), or sockaddr_local (for local/unix) structures that are
     salen bytes long.

     The host and service names associated with sa are stored in host and serv
     which have length parameters hostlen and servlen.  The maximum value for
     hostlen is NI_MAXHOST and the maximum value for servlen is NI_MAXSERV, as
     defined by <netdb.h>.  If a length parameter is zero, no string will be
     stored.  Otherwise, enough space must be provided to store the host name
     or service string plus a byte for the NUL terminator.

     The flags argument is formed by OR'ing the following values:

     NI_NOFQDN         A fully qualified domain name is not required for local
                       hosts.  The local part of the fully qualified domain
                       name is returned instead.

     NI_NUMERICHOST    Return the address in numeric form, as if calling
                       inet_ntop(3), instead of a host name.

     NI_NAMEREQD       A name is required.  If the host name cannot be found
                       in DNS and this flag is set, a non-zero error code is
                       returned.  If the host name is not found and the flag
                       is not set, the address is returned in numeric form.

     NI_NUMERICSERV    The service name is returned as a digit string
                       representing the port number.

     NI_DGRAM          Specifies that the service being looked up is a
                       datagram service, and causes getservbyport(3) 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.

     This implementation allows numeric IPv6 address notation with scope
     identifier, as documented in chapter 11 of draft-ietf-ipv6-scoping-
     arch-02.txt.  IPv6 link-local address will appear as a string like
     "fe80::1%ne0".  Refer to getaddrinfo(3) for more information.

     getnameinfo() returns zero on success or one of the error codes listed in
     gai_strerror(3) if an error occurs.

     The following code tries to get a numeric host name, and service name,
     for a given socket address.  Observe that there is no hardcoded reference
     to a 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");
           printf("host=%s, serv=%s\n", hbuf, sbuf);

     The following version checks if the socket address has a reverse address

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

     gai_strerror(3), getaddrinfo(3), gethostbyaddr(3), getservbyport(3),
     inet_ntop(3), resolver(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, RFC 2553, March 1999.

     S. Deering, B. Haberman, T. Jinmei, E. Nordmark, and B. Zill, IPv6 Scoped
     Address Architecture, internet draft, draft-ietf-ipv6-scoping-
     arch-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.

     The getnameinfo() function is defined by the IEEE Std 1003.1g-2000
     ("POSIX.1g") draft specification and documented in RFC 2553, "Basic
     Socket Interface Extensions for IPv6".

     getnameinfo() can return both numeric and FQDN forms of the address
     specified in sa.  There is no return value that indicates whether the
     string returned in host is a result of binary to numeric-text translation
     (like inet_ntop(3)), or is the result of a DNS reverse lookup.  Because
     of this, malicious parties could set up a PTR record as follows:


     and trick the caller of getnameinfo() into believing that sa is
     when it is actually

     To prevent such attacks, the use of NI_NAMEREQD is recommended when the
     result of getnameinfo() is used for access control purposes:

           struct sockaddr *sa;
           socklen_t salen;
           char addr[NI_MAXHOST];
           struct addrinfo hints, *res;
           int error;

           error = getnameinfo(sa, salen, addr, sizeof(addr),
               NULL, 0, NI_NAMEREQD);
           if (error == 0) {
                   memset(&hints, 0, sizeof(hints));
                   hints.ai_socktype = SOCK_DGRAM; /*dummy*/
                   hints.ai_flags = AI_NUMERICHOST;
                   if (getaddrinfo(addr, "0", &hints, &res) == 0) {
                           /* malicious PTR record */
                           printf("bogus PTR record\n");
                           return -1;
                   /* addr is FQDN as a result of PTR lookup */
           } else {
                   /* addr is numeric string */
                   error = getnameinfo(sa, salen, addr, sizeof(addr),
                       NULL, 0, NI_NUMERICHOST);

NetBSD 10.99                    August 18, 2013                   NetBSD 10.99