plugin/cache: allow cache TTLs above default 3600s (#8134)

* plugin/cache: allow cache TTLs above default 3600s

This change allows the cache plugin to honor configured maximum TTL values above the default 3600s limit. Default behavior remains unchanged
This PR fixes 7846

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Keep MinimalTTL

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

---------

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

plugin/cache/cache.go

@@ -221,14 +221,15 @@ func (w *ResponseWriter) WriteMsg(res *dns.Msg) error {
 	// key returns empty string for anything we don't want to cache.
 	hasKey, key := key(w.state.Name(), res, mt, w.do, w.cd)
 
-	msgTTL := dnsutil.MinimalTTL(res, mt)
 	var duration time.Duration
 	switch mt {
 	case response.NameError, response.NoData:
+		msgTTL := dnsutil.MinimalTTLWithMaximum(res, mt, w.nttl)
 		duration = computeTTL(msgTTL, w.minnttl, w.nttl)
 	case response.ServerError:
 		duration = w.failttl
 	default:
+		msgTTL := dnsutil.MinimalTTLWithMaximum(res, mt, w.pttl)
 		duration = computeTTL(msgTTL, w.minpttl, w.pttl)
 	}
 

plugin/cache/cache_test.go

@@ -362,6 +362,27 @@ func TestCacheZeroTTL(t *testing.T) {
 	}
 }
 
+func TestCacheHonorsConfiguredPositiveMaxTTLAboveDefault(t *testing.T) {
+	c := New()
+	c.pttl = 2 * time.Hour
+	c.minpttl = 0
+	c.Next = ttlBackend(24 * 60 * 60)
+
+	req := new(dns.Msg)
+	req.SetQuestion("example.org.", dns.TypeA)
+
+	rec := dnstest.NewRecorder(&test.ResponseWriter{})
+	c.ServeDNS(context.TODO(), rec, req)
+
+	if rec.Msg == nil || len(rec.Msg.Answer) == 0 {
+		t.Fatalf("expected answer, got %+v", rec.Msg)
+	}
+
+	if got, want := rec.Msg.Answer[0].Header().Ttl, uint32(7200); got != want {
+		t.Fatalf("expected TTL %d, got %d", want, got)
+	}
+}
+
 func TestCacheServfailTTL0(t *testing.T) {
 	c := New()
 	c.minpttl = minTTL