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GETADDRINFO(3)           BSD 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>

     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 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 consist-ing consisting
     ing 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_ADDRCONFIG   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

                    AI_ALL          If the AI_ALL bit is set with the AI_V4MAPPED bit, then getaddrinfo()
                                    shall return all matching IPv6 and IPv4 addresses.  The AI_ALL bit with-out without
                                    out the AI_V4MAPPED bit is ignored.

                    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 hostname in the ai_canonname element of the first addrinfo
                                    structure returned.

                    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, then a non-null servname string sup-plied supplied
                                    plied 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.

                    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.

                    AI_V4MAPPED     If the AI_V4MAPPED flag is specified along with an ai_family of
                                    PF_INET6, then getaddrinfo() 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.
                    AI_V4MAPPED_CFG The AI_V4MAPPED_CFG flag behaves exactly like the AI_V4MAPPED flag if the kernel supports IPv4-mapped IPv6 addresses. Otherwise it is ignored.
		    AI_DEFAULT	   AI_DEFAULT is defined as ( AI_V4MAPPED_CFG
                                    | AI_ADDRCONFIG ).
     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 PF_UNSPEC 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 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 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 a 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, ``'' ) and ai_family is set to PF_UNSPEC or PF_INET6, getaddrinfo() will synthesize the appropriate IPv6 address(es) (for example, ``64:ff9b::'' ) 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.

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

     The following code tries to connect to ``'' 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 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 s;
           const char *cause = NULL;

           memset(&hints, 0, sizeof(hints));
           hints.ai_family = PF_UNSPEC;
           hints.ai_socktype = SOCK_STREAM;
           error = getaddrinfo("", "http", &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,
                   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);

     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));
           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);

     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,
     internet draft, draft-ietf-ipv6-scoping-arch-02.txt, work in progress material.

     Craig Metz, "Protocol Independence Using the Sockets API", Proceedings of the freenix track: 2___
     USENIX annual technical conference, June 2000.

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

BSD                              July 1, 2008                              BSD

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