add async option for dns handler

handler/dns/handler.go

@@ -142,7 +142,7 @@ func (h *dnsHandler) Handle(ctx context.Context, conn net.Conn, opts ...handler.
 		return err
 	}
 
-	reply, err := h.exchange(ctx, (*b)[:n], log)
+	reply, err := h.request(ctx, (*b)[:n], log)
 	if err != nil {
 		return err
 	}
@@ -167,7 +167,7 @@ func (h *dnsHandler) checkRateLimit(addr net.Addr) bool {
 	return true
 }
 
-func (h *dnsHandler) exchange(ctx context.Context, msg []byte, log logger.Logger) ([]byte, error) {
+func (h *dnsHandler) request(ctx context.Context, msg []byte, log logger.Logger) ([]byte, error) {
 	mq := dns.Msg{}
 	if err := mq.Unpack(msg); err != nil {
 		log.Error(err)
@@ -210,50 +210,63 @@ func (h *dnsHandler) exchange(ctx context.Context, msg []byte, log logger.Logger
 
 	// only cache for single question message.
 	if len(mq.Question) == 1 {
-		key := resolver_util.NewCacheKey(&mq.Question[0])
-		mr = h.cache.Load(key)
+		var ttl time.Duration
+		mr, ttl = h.cache.Load(resolver_util.NewCacheKey(&mq.Question[0]))
 		if mr != nil {
-			log.Debugf("exchange message %d (cached): %s", mq.Id, mq.Question[0].String())
 			mr.Id = mq.Id
-
-			b := bufpool.Get(h.md.bufferSize)
-			return mr.PackBuffer(*b)
-		}
-
-		defer func() {
-			if mr != nil {
-				h.cache.Store(key, mr, h.md.ttl)
+			if int32(ttl.Seconds()) > 0 {
+				log.Debugf("message %d (cached): %s", mq.Id, mq.Question[0].String())
+				b := bufpool.Get(h.md.bufferSize)
+				return mr.PackBuffer(*b)
 			}
-		}()
+		}
 	}
 
+	if mr != nil && h.md.async {
+		b := bufpool.Get(h.md.bufferSize)
+		reply, err := mr.PackBuffer(*b)
+		if err != nil {
+			return nil, err
+		}
+		h.cache.RefreshTTL(resolver_util.NewCacheKey(&mq.Question[0]))
+
+		log.Debugf("exchange message %d (async): %s", mq.Id, mq.Question[0].String())
+		go h.exchange(ctx, &mq)
+		return reply, nil
+	}
+
+	log.Debugf("exchange message %d: %s", mq.Id, mq.Question[0].String())
+	return h.exchange(ctx, &mq)
+}
+
+func (h *dnsHandler) exchange(ctx context.Context, mq *dns.Msg) ([]byte, error) {
 	b := bufpool.Get(h.md.bufferSize)
 	defer bufpool.Put(b)
 
 	query, err := mq.PackBuffer(*b)
 	if err != nil {
-		log.Error(err)
 		return nil, err
 	}
 
 	ex := h.selectExchanger(ctx, strings.Trim(mq.Question[0].Name, "."))
 	if ex == nil {
-		err := fmt.Errorf("exchange not found for %s", mq.Question[0].Name)
-		log.Error(err)
+		err = fmt.Errorf("exchange not found for %s", mq.Question[0].Name)
 		return nil, err
 	}
 
 	reply, err := ex.Exchange(ctx, query)
 	if err != nil {
-		log.Error(err)
 		return nil, err
 	}
 
-	mr = &dns.Msg{}
+	mr := &dns.Msg{}
 	if err = mr.Unpack(reply); err != nil {
-		log.Error(err)
 		return nil, err
 	}
+	if len(mq.Question) == 1 {
+		key := resolver_util.NewCacheKey(&mq.Question[0])
+		h.cache.Store(key, mr, h.md.ttl)
+	}
 
 	return reply, nil
 }

handler/dns/metadata.go

@@ -21,6 +21,7 @@ type metadata struct {
 	// nameservers
 	dns        []string
 	bufferSize int
+	async      bool
 }
 
 func (h *dnsHandler) parseMetadata(md mdata.Metadata) (err error) {
@@ -31,6 +32,7 @@ func (h *dnsHandler) parseMetadata(md mdata.Metadata) (err error) {
 		clientIP    = "clientIP"
 		dns         = "dns"
 		bufferSize  = "bufferSize"
+		async       = "async"
 	)
 
 	h.md.readTimeout = mdutil.GetDuration(md, readTimeout)
@@ -48,6 +50,7 @@ func (h *dnsHandler) parseMetadata(md mdata.Metadata) (err error) {
 	if h.md.bufferSize <= 0 {
 		h.md.bufferSize = defaultBufferSize
 	}
+	h.md.async = mdutil.GetBool(md, async)
 
 	return
 }

internal/util/resolver/cache.go

@@ -9,6 +9,10 @@ import (
 	"github.com/miekg/dns"
 )
 
+const (
+	defaultTTL = 60 * time.Second
+)
+
 type CacheKey string
 
 // NewCacheKey generates resolver cache key from question of dns query.
@@ -40,25 +44,31 @@ func (c *Cache) WithLogger(logger logger.Logger) *Cache {
 	return c
 }
 
-func (c *Cache) Load(key CacheKey) *dns.Msg {
+func (c *Cache) Load(key CacheKey) (msg *dns.Msg, ttl time.Duration) {
 	v, ok := c.m.Load(key)
 	if !ok {
-		return nil
+		return
 	}
 
 	item, ok := v.(*cacheItem)
 	if !ok {
-		return nil
+		return
 	}
 
-	if time.Since(item.ts) > item.ttl {
-		c.m.Delete(key)
-		return nil
+	msg = item.msg.Copy()
+	for i := range msg.Answer {
+		d := uint32(time.Since(item.ts).Seconds())
+		if msg.Answer[i].Header().Ttl > d {
+			msg.Answer[i].Header().Ttl -= d
+		} else {
+			msg.Answer[i].Header().Ttl = 1
+		}
 	}
+	ttl = item.ttl - time.Since(item.ts)
 
-	c.logger.Debugf("hit resolver cache: %s", key)
+	c.logger.Debugf("hit resolver cache: %s, ttl: %v", key, ttl)
 
-	return item.msg.Copy()
+	return
 }
 
 func (c *Cache) Store(key CacheKey, mr *dns.Msg, ttl time.Duration) {
@@ -73,9 +83,13 @@ func (c *Cache) Store(key CacheKey, mr *dns.Msg, ttl time.Duration) {
 				ttl = v
 			}
 		}
-	}
-	if ttl == 0 {
-		ttl = 30 * time.Second
+		if ttl == 0 {
+			ttl = defaultTTL
+		}
+	} else {
+		for i := range mr.Answer {
+			mr.Answer[i].Header().Ttl = uint32(ttl.Seconds())
+		}
 	}
 
 	c.m.Store(key, &cacheItem{
@@ -86,3 +100,16 @@ func (c *Cache) Store(key CacheKey, mr *dns.Msg, ttl time.Duration) {
 
 	c.logger.Debugf("resolver cache store: %s, ttl: %v", key, ttl)
 }
+
+func (c *Cache) RefreshTTL(key CacheKey) {
+	v, ok := c.m.Load(key)
+	if !ok {
+		return
+	}
+
+	item, ok := v.(*cacheItem)
+	if !ok {
+		return
+	}
+	item.ts = time.Now()
+}

resolver/resolver.go

@@ -150,8 +150,8 @@ func (r *resolver) resolve6(ctx context.Context, server *NameServer, host string
 
 func (r *resolver) resolveIPs(ctx context.Context, server *NameServer, mq *dns.Msg) (ips []net.IP, err error) {
 	key := resolver_util.NewCacheKey(&mq.Question[0])
-	mr := r.cache.Load(key)
-	if mr == nil {
+	mr, ttl := r.cache.Load(key)
+	if ttl <= 0 {
 		resolver_util.AddSubnetOpt(mq, server.ClientIP)
 		mr, err = r.exchange(ctx, server.exchanger, mq)
 		if err != nil {