Fix: DNS server returns the correct TTL

common/cache/cache.go

@@ -44,6 +44,21 @@ func (c *cache) Get(key interface{}) interface{} {
 	return elm.Payload
 }
 
+// GetWithExpire element in Cache with Expire Time
+func (c *cache) GetWithExpire(key interface{}) (item interface{}, expired time.Time) {
+	item, exist := c.mapping.Load(key)
+	if !exist {
+		return
+	}
+	elm := item.(*element)
+	// expired
+	if time.Since(elm.Expired) > 0 {
+		c.mapping.Delete(key)
+		return
+	}
+	return elm.Payload, elm.Expired
+}
+
 func (c *cache) cleanup() {
 	c.mapping.Range(func(k, v interface{}) bool {
 		key := k.(string)

dns/client.go

@@ -52,9 +52,16 @@ func (r *Resolver) Exchange(m *D.Msg) (msg *D.Msg, err error) {
 	}
 
 	q := m.Question[0]
-	cache := r.cache.Get(q.String())
+	cache, expireTime := r.cache.GetWithExpire(q.String())
 	if cache != nil {
-		return cache.(*D.Msg).Copy(), nil
+		msg = cache.(*D.Msg).Copy()
+		if len(msg.Answer) > 0 {
+			ttl := uint32(expireTime.Sub(time.Now()).Seconds())
+			for _, answer := range msg.Answer {
+				answer.Header().Ttl = ttl
+			}
+		}
+		return
 	}
 	defer func() {
 		if msg != nil {