Add traffic plugin

This allows for advanced loadbalancing and maybe geoIP loadbalancing.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/traffic/traffic.go

@@ -0,0 +1,73 @@
+package traffic
+
+import (
+	"context"
+	"math/rand"
+	"sync"
+	"time"
+
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/pkg/response"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+// Traffic is a plugin that load balances according to assignments.
+type Traffic struct {
+	assignments map[string]assignment // zone -> assignment
+	mu          sync.RWMutex          // protects assignments
+	Next        plugin.Handler
+}
+
+// ServeDNS implements the plugin.Handler interface.
+func (t *Traffic) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	state := request.Request{W: w, Req: r}
+
+	tw := &ResponseWriter{ResponseWriter: w}
+	t.mu.RLock()
+	a, ok := t.assignments[state.Name()]
+	t.mu.RUnlock()
+	if ok {
+		tw.a = &a
+	}
+	return plugin.NextOrFailure(t.Name(), t.Next, ctx, tw, r)
+}
+
+// Name implements the plugin.Handler interface.
+func (t *Traffic) Name() string { return "traffic" }
+
+// ResponseWriter writes a traffic load balanced response.
+type ResponseWriter struct {
+	dns.ResponseWriter
+	a *assignment
+}
+
+// WriteMsg implements the dns.ResponseWriter interface.
+func (r *ResponseWriter) WriteMsg(res *dns.Msg) error {
+	// set all TTLs to 5, also negative TTL?
+	if res.Rcode != dns.RcodeSuccess {
+		return r.ResponseWriter.WriteMsg(res)
+	}
+
+	if res.Question[0].Qtype != dns.TypeA && res.Question[0].Qtype != dns.TypeAAAA {
+		return r.ResponseWriter.WriteMsg(res)
+	}
+
+	typ, _ := response.Typify(res, time.Now().UTC())
+	if typ != response.NoError {
+		return r.ResponseWriter.WriteMsg(res)
+	}
+
+	// ok, traffic-lb
+	if r.a != nil {
+
+	}
+	if len(res.Answer) > 1 {
+		res.Answer = []dns.RR{res.Answer[rand.Intn(len(res.Answer))]}
+		res.Answer[0].Header().Ttl = 5
+	}
+	res.Ns = []dns.RR{} // remove auth section, we don't care
+
+	return r.ResponseWriter.WriteMsg(res)
+}