Add middleware/dnssec (#133)

This adds an online dnssec middleware. The middleware will sign
responses on the fly. Negative responses are signed with NSEC black
lies.

middleware/dnssec/handler.go

@@ -0,0 +1,61 @@
+package dnssec
+
+import (
+	"github.com/miekg/coredns/middleware"
+
+	"github.com/miekg/dns"
+	"github.com/prometheus/client_golang/prometheus"
+	"golang.org/x/net/context"
+)
+
+// ServeDNS implements the middleware.Handler interface.
+func (d Dnssec) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	state := middleware.State{W: w, Req: r}
+
+	do := state.Do()
+	qname := state.Name()
+	qtype := state.QType()
+	zone := middleware.Zones(d.zones).Matches(qname)
+	if zone == "" {
+		return d.Next.ServeDNS(ctx, w, r)
+	}
+
+	// Intercept queries for DNSKEY, but only if one of the zones matches the qname, otherwise we let
+	// the query through.
+	if qtype == dns.TypeDNSKEY {
+		for _, z := range d.zones {
+			if qname == z {
+				resp := d.getDNSKEY(state, z, do)
+				state.SizeAndDo(resp)
+				w.WriteMsg(resp)
+				return dns.RcodeSuccess, nil
+			}
+		}
+	}
+
+	drr := NewDnssecResponseWriter(w, d)
+	return d.Next.ServeDNS(ctx, drr, r)
+}
+
+var (
+	cacheHitCount = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Namespace: middleware.Namespace,
+		Subsystem: subsystem,
+		Name:      "hit_count_total",
+		Help:      "Counter of signatures that were found in the cache.",
+	}, []string{"zone"})
+
+	cacheMissCount = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Namespace: middleware.Namespace,
+		Subsystem: subsystem,
+		Name:      "miss_count_total",
+		Help:      "Counter of signatures that were not found in the cache.",
+	}, []string{"zone"})
+)
+
+const subsystem = "dnssec"
+
+func init() {
+	prometheus.MustRegister(cacheHitCount)
+	prometheus.MustRegister(cacheMissCount)
+}