Cache Plugin: return non-fixed TTL for cached entries

dnscrypt-proxy/dnsutils.go

@@ -83,20 +83,24 @@ func getMinTTL(msg *dns.Msg, minTTL uint32, maxTTL uint32, negCacheMinTTL uint32
 	return time.Duration(ttl) * time.Second
 }
 
-func setMaxTTL(msg *dns.Msg, ttl uint32) {
+func setMaxTTL(msg *dns.Msg, ttl uint32, force bool) {
 	for _, rr := range msg.Answer {
-		if ttl < rr.Header().Ttl {
+		if (ttl < rr.Header().Ttl) || force {
 			rr.Header().Ttl = ttl
 		}
 	}
 	for _, rr := range msg.Ns {
-		if ttl < rr.Header().Ttl {
+    if (ttl < rr.Header().Ttl) || force {
 			rr.Header().Ttl = ttl
 		}
 	}
 	for _, rr := range msg.Extra {
-		if ttl < rr.Header().Ttl {
+    if (ttl < rr.Header().Ttl) || force {
 			rr.Header().Ttl = ttl
 		}
 	}
 }
+
+func updateTTL(msg *dns.Msg, expiration time.Time) {
+  setMaxTTL(msg, uint32(time.Until(expiration) / time.Second), true)
+}

dnscrypt-proxy/plugin_cache.go

@@ -70,6 +70,8 @@ func (plugin *PluginCacheResponse) Eval(pluginsState *PluginsState, msg *dns.Msg
 		}
 	}
 	plugin.cachedResponses.cache.Add(cacheKey, cachedResponse)
+
+  updateTTL(msg, cachedResponse.expiration)
 	return nil
 }
 
@@ -117,6 +119,9 @@ func (plugin *PluginCache) Eval(pluginsState *PluginsState, msg *dns.Msg) error
 	if time.Now().After(cached.expiration) {
 		return nil
 	}
+
+  updateTTL(&cached.msg, cached.expiration)
+
 	synth := cached.msg
 	synth.Id = msg.Id
 	synth.Response = true

dnscrypt-proxy/plugins.go

@@ -178,7 +178,7 @@ func (pluginsState *PluginsState) ApplyResponsePlugins(pluginsGlobals *PluginsGl
 	}
 	pluginsGlobals.RUnlock()
 	if ttl != nil {
-		setMaxTTL(&msg, *ttl)
+		setMaxTTL(&msg, *ttl, false)
 	}
 	packet2, err := msg.PackBuffer(packet)
 	if err != nil {