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GETADDRINFO(3)             Library Functions Manual             GETADDRINFO(3)

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

     #include <netdb.h>

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

     freeaddrinfo(struct addrinfo *ai);

     struct addrinfo *
     allocaddrinfo(socklen_t len);

     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

     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 */
             char *ai_canonname;     /* canonical name for service location */
             struct sockaddr *ai_addr; /* socket-address for socket */
             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 (and protocol) 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.  Note that while address families (AF_*) and
                    protocol families (PF_*) are theoretically distinct, in
                    practice the distinction has been lost.  RFC 3493 defines
                    getaddrinfo() in terms of the address family constants
                    AF_* even though ai_family is to be passed as a protocol
                    family to socket(2).

     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_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, it
                                    indicates that the servname string
                                    contains a numeric port number.  This is
                                    used to prevent service name resolution.

                    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

                                    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_SRV          Perform SRV (RFC 2782) record lookups
                                    first.  This is a non-portable extension.

     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 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

     By default IPv6 address entries are ordered before IPv4 ones, but the
     order of the entries in the list can be controlled using ip6addrctl(8).

     This implementation of getaddrinfo() allows numeric IPv6 address notation
     with scope identifier, as documented in chapter 11 of draft-ietf-
     ipv6-scoping-arch-02.txt.  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

     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() or allocaddrinfo().  The allocaddrinfo() function
     is intended primarily for authors of nsdispatch(3) plugins implementing
     getaddrinfo() backends.  allocaddrinfo() allocates a struct addrinfo in a
     way that is compatible with being returned from getaddrinfo() and being
     ultimately freed by freeaddrinfo().  The returned structure is zeroed,
     except for the ai_addr field, which will point to len bytes of memory for
     storage of a socket address.  It is safe to allocate memory separately
     for ai_canonname with malloc(3), or in any other way that is compatible
     with deallocation by free(3).

     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 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 = AF_UNSPEC;
           hints.ai_socktype = SOCK_STREAM;
           error = getaddrinfo("www.kame.net", "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 = 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));
           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), ip6addrctl(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: 2000 USENIX Annual Technical Conference, USENIX
     99-108, June 18-23, 2000.

     The getaddrinfo() function is defined by the IEEE Std 1003.1g-2000
     ("POSIX.1g") draft specification and documented in RFC 3493, "Basic
     Socket Interface Extensions for IPv6".

NetBSD 10.99                     June 4, 2020                     NetBSD 10.99