GETADDRINFO(3) | Library Functions Manual | GETADDRINFO(3)

# NAME

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

# SYNOPSIS

`#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);

# DESCRIPTION

The `getaddrinfo`() 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;		/* address 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 address family that should be used. When ai_family is set to `AF_UNSPEC`, it means the caller will accept any address 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
  ai_flags is formed by OR'ing the following values:
  `AI_ADDRCONFIG`
    If the `AI_ADDRCONFIG` bit is set, IPv4 addresses will be returned only if an IPv4 address is configured on an interface, and IPv6 addresses will be returned only if an IPv6 address is configured on an interface. Addresses on a loopback interface and link-local IPv6 addresses are not considered valid as configured addresses. This bit is only considered when determining whether a DNS query should be performed or not.
  `AI_CANONNAME`
    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 host name in the ai_canonname element of the first addrinfo structure returned.
  `AI_FQDN`
    If the `AI_FQDN` bit is set, a successful call to `getaddrinfo`() will return a NUL-terminated string containing the fully qualified domain name of the specified host name in the ai_canonname element of the first addrinfo structure returned.

    This is different from the `AI_CANONNAME` bit flag that returns the canonical name registered in DNS, which may be different from the fully qualified domain name that the host name resolved to. Only one of the `AI_FQDN` and `AI_CANONNAME` bits can be set.

  `AI_NUMERICHOST`
    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.
  `AI_NUMERICSERV`
    If the `AI_NUMERICSERV` bit is set, it indicates that servname should be treated as a numeric port string and no service name resolution should be attempted.
  `AI_PASSIVE`
    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.

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

If hints is the null pointer, `getaddrinfo`() behaves as if the caller provided a struct addrinfo with ai_family set to `AF_UNSPEC`, ai_flags set to `AI_ADDRCONFIG`, and all other elements set to zero or `NULL`.

After a successful call to `getaddrinfo`(), *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 `getaddrinfo`() allows numeric IPv6 address notation with scope identifier, as documented in 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 `getaddrinfo`() 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 `getaddrinfo`() is released by the `freeaddrinfo`() function. The ai pointer should be an addrinfo structure created by a call to `getaddrinfo`().

# RETURN VALUES

`getaddrinfo`() returns zero on success or one of the error codes listed in gai_strerror(3) if an error occurs. If an error occurs, no memory is allocated by `getaddrinfo`(), therefore it is not necessary to release the addrinfo structure(s).

# EXAMPLES

The following code tries to connect to “`www.kame.net`” service “`www`” 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 `AF_INET` socket. Similarly, if it resolves to IPv6, an `AF_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 save_errno;
    int s;
    const char *cause = NULL;

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

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

    	break;	/* okay we got one */
    }
    if (s == -1)
    	err(1, "%s", cause);
    freeaddrinfo(res0);

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

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

    memset(&hints, 0, sizeof(hints));
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = AI_PASSIVE;
    error = getaddrinfo(NULL, "www", &hints, &res0);
    if (error)
    	errx(1, "%s", gai_strerror(error));
    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] == -1) {
    		cause = "socket";
    		continue;
    	}

    	if (bind(s[nsock], res->ai_addr, res->ai_addrlen) == -1) {
    		cause = "bind";
    		save_errno = errno;
    		close(s[nsock]);
    		errno = save_errno;
    		continue;
    	}
    	(void) listen(s[nsock], 5);

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

# SEE ALSO

bind(2), connect(2), send(2), socket(2), gai_strerror(3), gethostbyname(3), getnameinfo(3), getservbyname(3), res_init(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.

# STANDARDS

The `getaddrinfo`() function is defined by the IEEE Std 1003.1g-2000 (“POSIX.1g”) draft specification and documented in RFC 3493.

The `AI_FQDN` flag bit first appeared in Windows 7.

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.

September 11, 2022 | Debian