Add roundrobin middleware

This middleware allows you to round robin a/aaaa records in a reply and
maybe more in the future (i.e.) sort a packet?

middleware/loadbalance/loadbalance.go

@@ -0,0 +1,68 @@
+package loadbalance
+
+import "github.com/miekg/dns"
+
+type RoundRobinResponseWriter struct {
+	dns.ResponseWriter
+}
+
+func NewRoundRobinResponseWriter(w dns.ResponseWriter) *RoundRobinResponseWriter {
+	return &RoundRobinResponseWriter{w}
+}
+
+func (r *RoundRobinResponseWriter) WriteMsg(res *dns.Msg) error {
+	if res.Rcode != dns.RcodeSuccess {
+		return r.ResponseWriter.WriteMsg(res)
+	}
+	if len(res.Answer) == 1 {
+		return r.ResponseWriter.WriteMsg(res)
+	}
+
+	// put CNAMEs first, randomize a/aaaa's and put packet back together.
+	// TODO(miek): check family and give v6 more prio?
+	cname := []dns.RR{}
+	address := []dns.RR{}
+	rest := []dns.RR{}
+	for _, r := range res.Answer {
+		switch r.Header().Rrtype {
+		case dns.TypeCNAME:
+			cname = append(cname, r)
+		case dns.TypeA, dns.TypeAAAA:
+			address = append(address, r)
+		default:
+			rest = append(rest, r)
+		}
+	}
+
+	switch l := len(address); l {
+	case 0, 1:
+		return r.ResponseWriter.WriteMsg(res)
+	case 2:
+		if dns.Id()%2 == 0 {
+			address[0], address[1] = address[1], address[0]
+		}
+	default:
+		for j := 0; j < l*(int(dns.Id())%4+1); j++ {
+			q := int(dns.Id()) % l
+			p := int(dns.Id()) % l
+			if q == p {
+				p = (p + 1) % l
+			}
+			address[q], address[p] = address[p], address[q]
+		}
+	}
+	res.Answer = append(cname, rest...)
+	res.Answer = append(res.Answer, address...)
+	return r.ResponseWriter.WriteMsg(res)
+}
+
+func (r *RoundRobinResponseWriter) Write(buf []byte) (int, error) {
+	// pack and unpack? Not likely
+	n, err := r.ResponseWriter.Write(buf)
+	return n, err
+}
+
+func (r *RoundRobinResponseWriter) Hijack() {
+	r.ResponseWriter.Hijack()
+	return
+}