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>

core/dnsserver/server_https.go

@@ -236,7 +236,7 @@ func (s *ServerHTTPS) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	buf, _ := dw.Msg.Pack()
 
 	mt, _ := response.Typify(dw.Msg, time.Now().UTC())
-	age := dnsutil.MinimalTTL(dw.Msg, mt)
+	age := dnsutil.MinimalTTLWithMaximum(dw.Msg, mt, dnsutil.MaximumDefaultTTL)
 
 	w.Header().Set("Content-Type", doh.MimeType)
 	w.Header().Set("Cache-Control", fmt.Sprintf("max-age=%d", uint32(age.Seconds())))

core/dnsserver/server_https3.go

@@ -216,7 +216,7 @@ func (s *ServerHTTPS3) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 	buf, _ := dw.Msg.Pack()
 	mt, _ := response.Typify(dw.Msg, time.Now().UTC())
-	age := dnsutil.MinimalTTL(dw.Msg, mt)
+	age := dnsutil.MinimalTTLWithMaximum(dw.Msg, mt, dnsutil.MaximumDefaultTTL)
 
 	w.Header().Set("Content-Type", doh.MimeType)
 	w.Header().Set("Cache-Control", fmt.Sprintf("max-age=%d", uint32(age.Seconds())))

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

plugin/pkg/dnsutil/ttl.go

@@ -10,6 +10,12 @@ import (
 
 // MinimalTTL scans the message returns the lowest TTL found taking into the response.Type of the message.
 func MinimalTTL(m *dns.Msg, mt response.Type) time.Duration {
+	return MinimalTTLWithMaximum(m, mt, MaximumDefaultTTL)
+}
+
+// MinimalTTLWithMaximum scans the DNS message and returns the lowest TTL found,
+// constrained by maximumTTL and the response type.
+func MinimalTTLWithMaximum(m *dns.Msg, mt response.Type, maximumTTL time.Duration) time.Duration {
 	if mt != response.NoError && mt != response.NameError && mt != response.NoData {
 		return MinimalDefaultTTL
 	}
@@ -20,7 +26,7 @@ func MinimalTTL(m *dns.Msg, mt response.Type) time.Duration {
 		return MinimalDefaultTTL
 	}
 
-	minTTL := MaximumDefaulTTL
+	minTTL := maximumTTL
 	for _, r := range m.Answer {
 		if r.Header().Ttl < uint32(minTTL.Seconds()) {
 			minTTL = time.Duration(r.Header().Ttl) * time.Second