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documents: getaddrinfo · freeaddrinfo
GETADDRINFO(3) Library Functions Manual GETADDRINFO(3)

getaddrinfo, freeaddrinfo — socket address structure to host and service name

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

int
getaddrinfo(const char *hostname, const char *servname, const struct addrinfo *hints, struct addrinfo **res);

void
freeaddrinfo(struct addrinfo *ai);

The () function is used to get a list of IP addresses and port numbers for host hostname and service servname. It is a replacement for and provides more flexibility than the gethostbyname(3) and getservbyname(3) functions.

The hostname and servname arguments are either pointers to NUL-terminated strings or the null pointer. An acceptable value for hostname is either a valid host name or a numeric host address string consisting of a dotted decimal IPv4 address or an IPv6 address. The servname is either a decimal port number or a service name listed in services(5). At least one of hostname and servname must be non-null.

hints is an optional pointer to a struct addrinfo, as defined by ⟨netdb.h⟩:

struct addrinfo {
	int ai_flags;		/* input flags */
	int ai_family;		/* protocol family for socket */
	int ai_socktype;	/* socket type */
	int ai_protocol;	/* protocol for socket */
	socklen_t ai_addrlen;	/* length of socket-address */
	struct sockaddr *ai_addr; /* socket-address for socket */
	char *ai_canonname;	/* canonical name for service location */
	struct addrinfo *ai_next; /* pointer to next in list */
};

This structure can be used to provide hints concerning the type of socket that the caller supports or wishes to use. The caller can supply the following structure elements in hints:

ai_family
The protocol family that should be used. When ai_family is set to PF_UNSPEC, it means the caller will accept any protocol family supported by the operating system.
ai_socktype
Denotes the type of socket that is wanted: SOCK_STREAM, SOCK_DGRAM, or SOCK_RAW. When ai_socktype is zero the caller will accept any socket type.
ai_protocol
Indicates which transport protocol is desired, IPPROTO_UDP or IPPROTO_TCP. If ai_protocol is zero the caller will accept any protocol.
ai_flags
The ai_flags field to which the hints parameter points shall be set to zero or be the bitwise-inclusive OR of one or more of the values AI_ADDRCONFIG, AI_ALL, AI_CANONNAME, AI_NUMERICHOST, AI_NUMERICSERV, AI_PASSIVE, AI_V4MAPPED, AI_V4MAPPED_CFG, and AI_DEFAULT.
If the AI_ADDRCONFIG bit is set, IPv4 addresses shall be returned only if an IPv4 address is configured on the local system, and IPv6 addresses shall be returned only if an IPv6 address is configured on the local system.
If the AI_ALL bit is set with the AI_V4MAPPED bit, then () shall return all matching IPv6 and IPv4 addresses. The AI_ALL bit without the AI_V4MAPPED bit is ignored.
If the AI_CANONNAME bit is set, a successful call to getaddrinfo() will return a NUL-terminated string containing the canonical name of the specified hostname in the ai_canonname element of the first addrinfo structure returned.
If the AI_NUMERICHOST bit is set, it indicates that hostname should be treated as a numeric string defining an IPv4 or IPv6 address and no name resolution should be attempted.
If the AI_NUMERICSERV bit is set, then a non-null servname string supplied shall be a numeric port string. Otherwise, an EAI_NONAME error shall be returned. This bit shall prevent any type of name resolution service (for example, NIS+) from being invoked.
If the AI_PASSIVE bit is set it indicates that the returned socket address structure is intended for use in a call to bind(2). In this case, if the hostname argument is the null pointer, then the IP address portion of the socket address structure will be set to INADDR_ANY for an IPv4 address or IN6ADDR_ANY_INIT for an IPv6 address.

If the AI_PASSIVE bit is not set, the returned socket address structure will be ready for use in a call to connect(2) for a connection-oriented protocol or connect(2), sendto(2), or sendmsg(2) if a connectionless protocol was chosen. The IP address portion of the socket address structure will be set to the loopback address if hostname is the null pointer and AI_PASSIVE is not set.

If the AI_V4MAPPED flag is specified along with an ai_family of PF_INET6, then () shall return IPv4-mapped IPv6 addresses on finding no matching IPv6 addresses ( ai_addrlen shall be 16). The AI_V4MAPPED flag shall be ignored unless ai_family equals PF_INET6.
The AI_V4MAPPED_CFG flag behaves exactly like the AI_V4MAPPED flag if the kernel supports IPv4-mapped IPv6 addresses. Otherwise it is ignored.
is defined as ( AI_V4MAPPED_CFG | ).
To override the automatic AI_DEFAULT behavior that occurs when ai_flags is zero pass AI_UNUSABLE instead of zero. This suppresses the implicit setting of AI_V4MAPPED_CFG and AI_ADDRCONFIG, thereby causing unusable addresses to be included in the results.

If ai_flags is zero, () gives the AI_DEFAULT behavior ( AI_V4MAPPED_CFG | AI_ADDRCONFIG ). To override this default behavior, pass any nonzero value for ai_flags, by setting any desired flag values, or setting AI_UNUSABLE if no other flags are desired.

All other elements of the addrinfo structure passed via hints must be zero or the null pointer.

If hints is the null pointer, () behaves as if the caller provided a struct addrinfo with ai_family set to PF_UNSPEC and all other elements set to zero or NULL (which includes treating the ai_flags field as effectively zero, giving the automatic default AI_DEFAULT behavior).

After a successful call to (), *res is a pointer to a linked list of one or more addrinfo structures. The list can be traversed by following the ai_next pointer in each addrinfo structure until a null pointer is encountered. The three members ai_family, ai_socktype, and ai_protocol in each returned addrinfo structure are suitable for a call to socket(2). For each addrinfo structure in the list, the ai_addr member points to a filled-in socket address structure of length ai_addrlen.

This implementation of () allows numeric IPv6 address notation with scope identifier, as documented in section 11 of RFC 4007. By appending the percent character and scope identifier to addresses, one can fill the sin6_scope_id field for addresses. This would make management of scoped addresses easier and allows cut-and-paste input of scoped addresses.

At this moment the code supports only link-local addresses with the format. The scope identifier is hardcoded to the name of the hardware interface associated with the link (such as ne0). An example is “fe80::1%ne0”, which means “fe80::1 on the link associated with the ne0 interface”.

The current implementation assumes a one-to-one relationship between the interface and link, which is not necessarily true from the specification.

All of the information returned by () is dynamically allocated: the addrinfo structures themselves as well as the socket address structures and the canonical host name strings included in the addrinfo structures.

Memory allocated for the dynamically allocated structures created by a successful call to () is released by the () function. The ai pointer should be an addrinfo structure created by a call to getaddrinfo().

The current implementation supports synthesis of NAT64 mapped IPv6 addresses. If hostname is a numeric string defining an IPv4 address (for example, “192.0.2.1” ) and ai_family is set to PF_UNSPEC or PF_INET6, () will synthesize the appropriate IPv6 address(es) (for example, “64:ff9b::192.0.2.1” ) if the current interface supports IPv6, NAT64 and DNS64 and does not support IPv4. If the AI_ADDRCONFIG flag is set, the IPv4 address will be suppressed on those interfaces. On non-qualifying interfaces, getaddrinfo() is guaranteed to return immediately without attempting any resolution, and will return the IPv4 address if ai_family is PF_UNSPEC or PF_INET. NAT64 address synthesis can be disabled by setting the AI_NUMERICHOST flag. To best support NAT64 networks, it is recommended to resolve all IP address literals with ai_family set to PF_UNSPEC and ai_flags set to AI_DEFAULT.

Note that NAT64 address synthesis is always disabled for IPv4 addresses in the following ranges: 0.0.0.0/8, 127.0.0.0/8, 169.254.0.0/16, 192.0.0.0/29, 192.88.99.0/24, 224.0.0.0/4, and 255.255.255.255/32. Additionally, NAT64 address synthesis is disabled when the network uses the well-known prefix (64:ff9b::/96) for IPv4 addresses in the following ranges: 10.0.0.0/8, 100.64.0.0/10, 172.16.0.0/12, and 192.168.0.0/16.

Historically, passing a host's own hostname to () has been a popular technique for determining that host's IP address(es), but this is fragile, and doesn't work reliably in all cases. The appropriate way for software to discover the IP address(es) of the host it is running on is to use getifaddrs(3).

The () implementations on all versions of OS X and iOS are now, and always have been, thread-safe. Previous versions of this man page incorrectly reported that getaddrinfo() was not thread-safe.

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

The following code tries to connect to “www.kame.net” service “http” via a stream socket. It loops through all the addresses available, regardless of address family. If the destination resolves to an IPv4 address, it will use an PF_INET socket. Similarly, if it resolves to IPv6, an PF_INET6 socket is used. Observe that there is no hardcoded reference to a particular address family. The code works even if getaddrinfo() returns addresses that are not IPv4/v6.

struct addrinfo hints, *res, *res0;
int error;
int s;
const char *cause = NULL;

memset(&hints, 0, sizeof(hints));
hints.ai_family = PF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
error = getaddrinfo("www.kame.net", "http", &hints, &res0);
if (error) {
	errx(1, "%s", gai_strerror(error));
	/*NOTREACHED*/
}
s = -1;
for (res = res0; res; res = res->ai_next) {
	s = socket(res->ai_family, res->ai_socktype,
	    res->ai_protocol);
	if (s < 0) {
		cause = "socket";
		continue;
	}

	if (connect(s, res->ai_addr, res->ai_addrlen) < 0) {
		cause = "connect";
		close(s);
		s = -1;
		continue;
	}

	break;	/* okay we got one */
}
if (s < 0) {
	err(1, "%s", cause);
	/*NOTREACHED*/
}
freeaddrinfo(res0);

The following example tries to open a wildcard listening socket onto service “http”, for all the address families available.

struct addrinfo hints, *res, *res0;
int error;
int s[MAXSOCK];
int nsock;
const char *cause = NULL;

memset(&hints, 0, sizeof(hints));
hints.ai_family = PF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE;
error = getaddrinfo(NULL, "http", &hints, &res0);
if (error) {
	errx(1, "%s", gai_strerror(error));
	/*NOTREACHED*/
}
nsock = 0;
for (res = res0; res && nsock < MAXSOCK; res = res->ai_next) {
	s[nsock] = socket(res->ai_family, res->ai_socktype,
	    res->ai_protocol);
	if (s[nsock] < 0) {
		cause = "socket";
		continue;
	}

	if (bind(s[nsock], res->ai_addr, res->ai_addrlen) < 0) {
		cause = "bind";
		close(s[nsock]);
		continue;
	}
	(void) listen(s[nsock], 5);

	nsock++;
}
if (nsock == 0) {
	err(1, "%s", cause);
	/*NOTREACHED*/
}
freeaddrinfo(res0);

bind(2), connect(2), send(2), socket(2), gai_strerror(3), gethostbyname(3), getnameinfo(3), getservbyname(3), resolver(3), hosts(5), resolv.conf(5), services(5), hostname(7), named(8)

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.

Craig Metz, Protocol Independence Using the Sockets API, Proceedings of the freenix track: 2000 USENIX annual technical conference, June 2000.

The getaddrinfo() function is defined by the IEEE Std 1003.1-2004 (“POSIX.1”) specification and documented in RFC 3493, “Basic Socket Interface Extensions for IPv6”.

July 1, 2008 macOS 26.4