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GETNAMEINFO(3) Library Functions Manual GETNAMEINFO(3)

getnameinfosocket address structure to hostname and service name

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

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

The () 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:

A fully qualified domain name is not required for local hosts. The local part of the fully qualified domain name is returned instead.
Return the address in numeric form, as if calling inet_ntop(3), instead of a host name.
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.
The service name is returned as a digit string representing the port number.
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 RFC 4007. 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,
	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 mapping:

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

if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), NULL, 0,
	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)

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 3493.

R. Gilligan, S. Thomson, J. Bound, J. McCann, and W. Stevens, Basic Socket Interface Extensions for IPv6, RFC 3493, February 2003.

S. Deering, B. Haberman, T. Jinmei, E. Nordmark, and B. Zill, IPv6 Scoped Address Architecture, RFC 4007, March 2005.

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: IN PTR

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;
char addr[NI_MAXHOST];
struct addrinfo hints, *res;
int error;

error = getnameinfo(sa, sa->sa_len, addr, sizeof(addr),
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, sa->sa_len, addr, sizeof(addr),

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

OpenBSD intentionally uses a different NI_MAXHOST value from what RFC 2553 suggests, to avoid buffer length handling mistakes.

August 23, 2014 OpenBSD-5.7