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 * the DNS servers associated with the same interface that we would use to
 * make a general request to anything else.
 *
 * But, Windows won't sort the DNS servers by the metrics associated with the
 * routes and interfaces _even_ though it obviously sends IP packets based on
 * those same routes and metrics.  So, we must do it ourselves.
 *
 * So, we sort the DNS servers by the same metric values used to determine how
 * an outgoing IP packet will go, thus effectively using the DNS servers
 * associated with the interface that the DNS requests themselves will
 * travel.  This gives us optimal routing and avoids issues where DNS servers
 * won't respond to requests that don't arrive via some specific subnetwork
 * (and thus some specific interface).
 *
 * This function computes the metric we use to sort.  On the interface
 * identified by \a luid, it determines the best route to \a dest and combines
 * that route's metric with \a interfaceMetric to compute a metric for the
 * destination address on that interface.  This metric can be used as a weight
 * to sort the DNS server addresses associated with each interface (lower is
 * better).
 *
 * Note that by restricting the route search to the specific interface with
 * which the DNS servers are associated, this function asks the question "What
 * is the metric for sending IP packets to this DNS server?" which allows us
 * to sort the DNS servers correctly.
 */
static ULONG getBestRouteMetric(IF_LUID * const luid, /* Can't be const :( */
                                const SOCKADDR_INET * const dest,
                                const ULONG interfaceMetric)
{
  /* On this interface, get the best route to that destination. */
  MIB_IPFORWARD_ROW2 row;
  SOCKADDR_INET ignored;
  if(!ares_fpGetBestRoute2 ||
     ares_fpGetBestRoute2(/* The interface to use.  The index is ignored since we are
                           * passing a LUID.
                           */
                           luid, 0,
                           /* No specific source address. */
                           NULL,
                           /* Our destination address. */
                           dest,
                           /* No options. */
                           0,
                           /* The route row. */
                           &row,
                           /* The best source address, which we don't need. */
                           &ignored) != NO_ERROR
     /* If the metric is "unused" (-1) or too large for us to add the two
      * metrics, use the worst possible, thus sorting this last.
      */
     || row.Metric == (ULONG)-1
     || row.Metric > ((ULONG)-1) - interfaceMetric) {
    /* Return the worst possible metric. */
    return (ULONG)-1;
  }

  /* Return the metric value from that row, plus the interface metric.
   *
   * See
   * http://msdn.microsoft.com/en-us/library/windows/desktop/aa814494(v=vs.85).aspx
   * which describes the combination as a "sum".
   */
  return row.Metric + interfaceMetric;
}

/*
 * get_DNS_AdaptersAddresses()
 *
 * Locates DNS info using GetAdaptersAddresses() function from the Internet
 * Protocol Helper (IP Helper) API. When located, this returns a pointer
 * in *outptr to a newly allocated memory area holding a null-terminated
 * string with a space or comma seperated list of DNS IP addresses.
 *
 * Returns 0 and nullifies *outptr upon inability to return DNSes string.
 *
 * Returns 1 and sets *outptr when returning a dynamically allocated string.
 *
 * Implementation supports Windows XP and newer.
 */
#define IPAA_INITIAL_BUF_SZ 15 * 1024
#define IPAA_MAX_TRIES 3
static int get_DNS_AdaptersAddresses(char **outptr)
{
  IP_ADAPTER_DNS_SERVER_ADDRESS *ipaDNSAddr;
  IP_ADAPTER_ADDRESSES *ipaa, *newipaa, *ipaaEntry;
  ULONG ReqBufsz = IPAA_INITIAL_BUF_SZ;
  ULONG Bufsz = IPAA_INITIAL_BUF_SZ;
  ULONG AddrFlags = 0;
  int trying = IPAA_MAX_TRIES;
  int res;

  /* The capacity of addresses, in elements. */
  size_t addressesSize;
  /* The number of elements in addresses. */
  size_t addressesIndex = 0;
  /* The addresses we will sort. */
  Address *addresses;

  union {
    struct sockaddr     *sa;
    struct sockaddr_in  *sa4;
    struct sockaddr_in6 *sa6;
  } namesrvr;

  *outptr = NULL;

  /* Verify run-time availability of GetAdaptersAddresses() */
  if (ares_fpGetAdaptersAddresses == ZERO_NULL)
    return 0;

  ipaa = ares_malloc(Bufsz);
  if (!ipaa)
    return 0;

  /* Start with enough room for a few DNS server addresses and we'll grow it
   * as we encounter more.
   */
  addressesSize = 4;
  addresses = (Address*)ares_malloc(sizeof(Address) * addressesSize);
  if(addresses == NULL) {