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GETADDRINFO(3)	       FreeBSD Library Functions Manual		GETADDRINFO(3)

     getaddrinfo, freeaddrinfo -- socket address structure to host and service

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

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

     freeaddrinfo(struct addrinfo *ai);

     The getaddrinfo() function	is used	to get a list of 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, an IPv6 address, or a UNIX-domain 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

     struct addrinfo {
	     int     ai_flags;	     /*	AI_PASSIVE, AI_CANONNAME, .. */
	     int     ai_family;	     /*	AF_xxx */
	     int     ai_socktype;    /*	SOCK_xxx */
	     int     ai_protocol;    /*	0 or IPPROTO_xxx for IPv4 and IPv6 */
	     socklen_t ai_addrlen;   /*	length of ai_addr */
	     char    *ai_canonname;  /*	canonical name for hostname */
	     struct  sockaddr *ai_addr;	     /*	binary address */
	     struct  addrinfo *ai_next;	     /*	next structure in linked 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 ad-
		    dress family supported by the operating system.

     ai_socktype    Denotes the	type of	socket that is wanted: SOCK_STREAM,
		    SOCK_DGRAM,	SOCK_SEQPACKET,	or SOCK_RAW.  When ai_socktype
		    is zero the	caller will accept any socket type.

     ai_protocol    Indicates which transport protocol is desired,
		    IPPROTO_UDPLITE.  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_V4MAPPED.  For a	UNIX-domain address, ai_flags is ig-

		    AI_ADDRCONFIG   If the AI_ADDRCONFIG bit is	set, IPv4 ad-
				    dresses 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.

		    AI_ALL	    If the AI_ALL flag is used with the
				    AI_V4MAPPED	flag, then getaddrinfo() shall
				    return all matching	IPv6 and IPv4 ad-

				    For	example, when using the	DNS, queries
				    are	made for both AAAA records and A
				    records, and getaddrinfo() returns the
				    combined results of	both queries.  Any
				    IPv4 addresses found are returned as
				    IPv4-mapped	IPv6 addresses.

				    The	AI_ALL flag without the	AI_V4MAPPED
				    flag is ignored.

		    AI_CANONNAME    If the AI_CANONNAME	bit is set, a success-
				    ful	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.

		    AI_NUMERICHOST  If the AI_NUMERICHOST bit is set, it indi-
				    cates that hostname	should be treated as a
				    numeric string defining an IPv4 or IPv6
				    address and	no name	resolution should be

		    AI_NUMERICSERV  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 resolu-
				    tion service (for example, NIS+) from be-
				    ing	invoked.

		    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 por-
				    tion 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 re-
				    turned 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 ad-
				    dress if hostname is the null pointer and
				    AI_PASSIVE is not set.

		    AI_V4MAPPED	    If the AI_V4MAPPED flag is specified along
				    with an ai_family of AF_INET6, then
				    getaddrinfo() shall	return IPv4-mapped
				    IPv6 addresses on finding no matching IPv6
				    addresses (	ai_addrlen shall be 16).

				    For	example, when using the	DNS, if	no
				    AAAA records are found then	a query	is
				    made for A records and any found are re-
				    turned as IPv4-mapped IPv6 addresses.

				    The	AI_V4MAPPED flag shall be ignored un-
				    less ai_family equals AF_INET6.

     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 pro-
     vided a struct addrinfo with ai_family set	to AF_UNSPEC and all other el-
     ements 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.  Each returned addrinfo structure contains	three
     members that are suitable for a call to socket(2):	ai_family,
     ai_socktype, and ai_protocol.  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 chapter 11	of RFC 4007.  By ap-
     pending 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 ad-

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

     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 allo-
     cated: the	addrinfo structures themselves as well as the socket address
     structures	and the	canonical host name strings included in	the addrinfo

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

     The behavior of freeadrinfo(NULL) is left unspecified by both  and	RFC
     3493.  The	current	implementation ignores a NULL argument for compatibil-
     ity with programs that rely on the	implementation details of other	oper-
     ating systems.

     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 "" service "http" via
     a stream socket.  It loops	through	all the	addresses available, regard-
     less 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()	re-
     turns 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 = AF_UNSPEC;
	   hints.ai_socktype = SOCK_STREAM;
	   error = getaddrinfo("", "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,
		   if (s < 0) {
			   cause = "socket";

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

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

     The following example tries to open a wildcard listening socket onto ser-
     vice "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 = AF_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,
		   if (s[nsock]	< 0) {
			   cause = "socket";

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

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

     bind(2), connect(2), send(2), socket(2), gai_strerror(3),
     gethostbyname(3), getnameinfo(3), getservbyname(3), resolver(3), inet(4),
     inet6(4), unix(4),	hosts(5), resolv.conf(5), services(5), hostname(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.

     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".

FreeBSD	13.0			 June 27, 2022			  FreeBSD 13.0


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