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NetBSD 6.1.5 - man page for getnameinfo (netbsd section 3)

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

NAME
     getnameinfo -- socket address structure to hostname and service name

SYNOPSIS
     #include <netdb.h>

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

DESCRIPTION
     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 either a sockaddr_in or sockaddr_in6 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 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 num-
		       ber.

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

EXAMPLES
     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");
		   /*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[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);

SEE ALSO
     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.

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

CAVEATS
     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:

	   1.0.0.127.in-addr.arpa. IN PTR  10.1.1.1

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

     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 */
			   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, NI_NUMERICHOST);
	   }

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

BSD					  March 21, 2005				      BSD
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