host:                    "giant.",
			expectResolutionFailure: dns.RcodeSuccess,
			expected:                giantResponse,
			modifyReq: func(msg *dns.Msg) {
				msg.SetEdns0(dns.MaxMsgSize, false)
			},
		},
		{
			name:                    "large request truncated",
			host:                    "giant-tc.",
			expectResolutionFailure: dns.RcodeSuccess,
			expected:                giantResponse[:29],
		},
		{

	// which requires more than 512 bytes.
	{"8.8.8.8:53", mustQuestion("com.", dnsmessage.TypeALL, dnsmessage.ClassINET), 2, dnsmessage.RCodeSuccess},
	{"8.8.4.4:53", mustQuestion("com.", dnsmessage.TypeALL, dnsmessage.ClassINET), 4, dnsmessage.RCodeSuccess},
}

func TestDNSTransportFallback(t *testing.T) {
	fake := fakeDNSServer{
		rh: func(n, _ string, q dnsmessage.Message, _ time.Time) (dnsmessage.Message, error) {

		a.h.Question(h, q)
	}
	if err != nil && err != dnsmessage.ErrSectionDone {
		return 0, err
	}

	resh := h
	resh.Response = true
	resh.Authoritative = true
	if hadQ {
		resh.RCode = dnsmessage.RCodeSuccess
	} else {
		resh.RCode = dnsmessage.RCodeNotImplemented
	}
	a.rbuf.Grow(514)
	a.rbuf.WriteByte('X') // reserved header for beu16 length
	a.rbuf.WriteByte('Y') // reserved header for beu16 length

	// an invalid referral response. See golang.org/issue/15434.
	if rcode == dnsmessage.RCodeSuccess && !h.Authoritative && !h.RecursionAvailable && err == dnsmessage.ErrSectionDone && !hasAdd {
		return errLameReferral
	}

	if rcode != dnsmessage.RCodeSuccess && rcode != dnsmessage.RCodeNameError {
		// None of the error codes make sense
		// for the query we sent. If we didn't get

			log.Infof("upstream dns failure: %v: %v: %v", upstream, getReqNames(req), err)
			continue
		}
		response = cResponse
		if rcode == dns.RcodeSuccess {
			break
		}
		codeString := dns.RcodeToString[rcode]
		log.Debugf("upstream dns error: %v: %v: %v", upstream, getReqNames(req), codeString)
	}
	if response == nil {
		response = new(dns.Msg)

// Header.RCode values.
const (
	RCodeSuccess        RCode = 0 // NoError
	RCodeFormatError    RCode = 1 // FormErr
	RCodeServerFailure  RCode = 2 // ServFail
	RCodeNameError      RCode = 3 // NXDomain
	RCodeNotImplemented RCode = 4 // NotImp
	RCodeRefused        RCode = 5 // Refused
)

var rCodeNames = map[RCode]string{
	RCodeSuccess:        "RCodeSuccess",
	RCodeFormatError:    "RCodeFormatError",

		return nil
	}
	return lt.(*dnsProto.NameTable)
}

// Inspired by https://github.com/coredns/coredns/blob/master/plugin/loadbalance/loadbalance.go
func roundRobinResponse(res *dns.Msg) {
	if res.Rcode != dns.RcodeSuccess {
		return
	}

	if res.Question[0].Qtype == dns.TypeAXFR || res.Question[0].Qtype == dns.TypeIXFR {
		return
	}

	res.Answer = roundRobin(res.Answer)
	res.Ns = roundRobin(res.Ns)