| GETNAMEINFO(3) | Library Functions Manual | GETNAMEINFO(3) | 
getnameinfo —
#include <netdb.h>
int
  
  getnameinfo(const
    struct sockaddr * restrict sa,
    socklen_t salen,
    char * restrict host,
    socklen_t hostlen,
    char * restrict serv,
    socklen_t servlen,
    int flags);
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_NOFQDNNI_NUMERICHOSTNI_NAMEREQDThis 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.
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);
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.
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:
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);
}
| August 18, 2013 | NetBSD 9.3 |