plugin/cache: don't cache msg with TTL=0 in them (#1116)

Don't cache these - may be lead to weird side effects.

Fixes #1113

plugin/cache/cache.go

@@ -106,7 +106,7 @@ func (w *ResponseWriter) WriteMsg(res *dns.Msg) error {
 		duration = msgTTL
 	}
 
-	if key != -1 {
+	if key != -1 && duration > 0 {
 		w.set(res, key, mt, duration)
 
 		cacheSize.WithLabelValues(Success).Set(float64(w.pcache.Len()))
@@ -134,8 +134,7 @@ func (w *ResponseWriter) WriteMsg(res *dns.Msg) error {
 }
 
 func (w *ResponseWriter) set(m *dns.Msg, key int, mt response.Type, duration time.Duration) {
-	if key == -1 {
-		log.Printf("[ERROR] Caching called with empty cache key")
+	if key == -1 || duration == 0 {
 		return
 	}
 

plugin/cache/cache_test.go

@@ -208,6 +208,25 @@ func TestCache(t *testing.T) {
 	}
 }
 
+func TestCacheZeroTTL(t *testing.T) {
+	c := &Cache{Zones: []string{"."}, pcap: defaultCap, ncap: defaultCap, pttl: maxTTL, nttl: maxTTL}
+	c.pcache = cache.New(c.pcap)
+	c.ncache = cache.New(c.ncap)
+	c.Next = zeroTTLBackend()
+
+	req := new(dns.Msg)
+	req.SetQuestion("example.org.", dns.TypeA)
+	ctx := context.TODO()
+
+	c.ServeDNS(ctx, &test.ResponseWriter{}, req)
+	if c.pcache.Len() != 0 {
+		t.Errorf("Msg with 0 TTL should not have been cached")
+	}
+	if c.ncache.Len() != 0 {
+		t.Errorf("Msg with 0 TTL should not have been cached")
+	}
+}
+
 func BenchmarkCacheResponse(b *testing.B) {
 	c := &Cache{Zones: []string{"."}, pcap: defaultCap, ncap: defaultCap, pttl: maxTTL, nttl: maxTTL}
 	c.pcache = cache.New(c.pcap)
@@ -249,3 +268,15 @@ func BackendHandler() plugin.Handler {
 		return dns.RcodeSuccess, nil
 	})
 }
+
+func zeroTTLBackend() plugin.Handler {
+	return plugin.HandlerFunc(func(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+		m := new(dns.Msg)
+		m.SetReply(r)
+		m.Response, m.RecursionAvailable = true, true
+
+		m.Answer = []dns.RR{test.A("example.org. 0 IN A 127.0.0.53")}
+		w.WriteMsg(m)
+		return dns.RcodeSuccess, nil
+	})
+}