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

NAME
     ggeettnnaammeeiinnffoo - socket address structure to hostname and service name

SYNOPSIS
     ##iinncclluuddee <>
     ##iinncclluuddee <>
     ##iinncclluuddee <>

     int
     ggeettnnaammeeiinnffoo(const struct sockaddr *sa, socklent salen, char *host,
         sizet hostlen, char *serv, sizet servlen, int flags);

DESCRIPTION
     The ggeettnnaammeeiinnffoo() function is used to convert a sockaddr structure to a
     pair of host name and service strings.  It is a replacement for and pro-
     vides 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 either a sockaddrin or
     sockaddrin6 structure (for IPv4 or IPv6 respectively) that is 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 NIMAXHOST and the maximum value for servlen is NIMAXSERV, as
     defined by .  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 OORR'ing the following values:

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

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

     NINAMEREQD       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.

     NINUMERICSERV    The service name is returned as a digit string repre-
                       senting the port number.

     NIDGRAM          Specifies that the service being looked up is a data-
                       gram 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 iden-
     tifier, 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.

RETURN VALUES
     ggeettnnaammeeiinnffoo() returns zero on success or one of the error codes listed in
     gaistrerror(3) if an error occurs.

EEXXAAMMPPLLEESS
     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[NIMAXHOST], sbuf[NIMAXSERV];

           if (getnameinfo(sa, sa->salen, hbuf, sizeof(hbuf), sbuf,
               sizeof(sbuf), NINUMERICHOST | NINUMERICSERV)) {
                   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 a reverse address
     mapping:

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

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

SEE ALSO
     gaistrerror(3), getaddrinfo(3), gethostbyaddr(3), getservbyport(3),
     inetntop(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.

STANDARDS
     The ggeettnnaammeeiinnffoo() function is defined by the IEEE Std 1003.1g-2000
     (``POSIX.1'') draft specification and documented in RRFFCC 22555533, ``Basic
     Socket Interface Extensions for IPv6''.

CCAAVVEEAATTSS
     ggeettnnaammeeiinnffoo() can return both numeric and FQDN forms of the address spec-
     ified 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
     inetntop(3)), or is the result of a DNS reverse lookup.  Because of
     this, malicious parties could set up a PTR record as follows:

           1.0.0.127.in-addr.arpa. IN PTR  10.1.1.1

     and trick the caller of ggeettnnaammeeiinnffoo() into believing that sa is 10.1.1.1
     when it is actually 127.0.0.1.

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

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

           error = getnameinfo(sa, salen, addr, sizeof(addr),
               NULL, 0, NINAMEREQD);
           if (error == 0) {
                   memset(&hints, 0, sizeof(hints));
                   hints.aisocktype = SOCKDGRAM; /*dummy*/
                   hints.aiflags = AINUMERICHOST;
                   if (getaddrinfo(addr, "0", &hints, &res) == 0) {
                           /* malicious PTR record */
                           freeaddrinfo(res);
                           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, NINUMERICHOST);
           }

BUGS
     The implementation of ggeettnnaammeeiinnffoo() is not thread-safe.

BSD                            December 20, 2004                           BSD