NAME

getnameinfo — socket address structure to hostname and service name

SYNOPSIS

#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>

int
getnameinfo(const struct sockaddr *sa, socklen_t salen, char *host, socklen_t hostlen, char *serv, socklen_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.

If a link-layer address is passed to getnameinfo(), its ASCII representation will be stored in host. The string pointed to by serv will be set to the empty string if non-NULL; flags will always be ignored. This is intended as a replacement for the legacy link_ntoa(3) function.

The sockaddr structure sa should point to either a sockaddr_in, sockaddr_in6 or sockaddr_dl structure (for IPv4, IPv6 or link-layer 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 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.

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);

STANDARDS

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”.

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);
}