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/cache_test.go

@@ -0,0 +1,32 @@
+package dnssec
+
+import (
+	"testing"
+	"time"
+
+	"github.com/miekg/coredns/middleware"
+	"github.com/miekg/coredns/middleware/test"
+)
+
+func TestCacheSet(t *testing.T) {
+	fPriv, rmPriv, _ := test.TempFile(t, ".", privKey)
+	fPub, rmPub, _ := test.TempFile(t, ".", pubKey)
+	defer rmPriv()
+	defer rmPub()
+
+	dnskey, err := ParseKeyFile(fPub, fPriv)
+	if err != nil {
+		t.Fatalf("failed to parse key: %v\n", err)
+	}
+
+	m := testMsg()
+	state := middleware.State{Req: m}
+	k := key(m.Answer) // calculate *before* we add the sig
+	d := NewDnssec([]string{"miek.nl."}, []*DNSKEY{dnskey}, nil)
+	m = d.Sign(state, "miek.nl.", time.Now().UTC())
+
+	_, ok := d.get(k)
+	if !ok {
+		t.Errorf("signature was not added to the cache")
+	}
+}