lib/dns: remove the fallback name servers (NS) from server options

The original idea of fallback NS is to send the query to the one define
to original resolv.conf, instead of using the one defined by user in
ServerOptions NameServers, when an error occured.

But, most of error usually caused by network (disconnected, time out),
so re-sending query to fallback NS does not have any effect if the network
it self is not working.

This changes remove the unnecessary and complex fallback NS from
server.

2 files changed, 37 insertions, 148 deletions

diff --git a/lib/dns/server.go b/lib/dns/server.go
index cda9877d..43640e0e 100644
--- a/lib/dns/server.go
+++ b/lib/dns/server.go
@@ -87,9 +87,8 @@ type Server struct {
 	dot net.Listener
 	doh *http.Server
 
-	requestq  chan *request
-	primaryq  chan *request
-	fallbackq chan *request
+	requestq chan *request
+	primaryq chan *request
 
 	fwLocker   sync.Mutex
 	fwStoppers []chan bool
@@ -292,11 +291,10 @@ func (srv *Server) RemoveLocalCachesByNames(names []string) {
 // and protocol.
 // Empty nameservers means server will run without forwarding request.
 //
-func (srv *Server) RestartForwarders(nameServers, fallbackNS []string) {
-	fmt.Printf("dns: RestartForwarders: %s %s\n", nameServers, fallbackNS)
+func (srv *Server) RestartForwarders(nameServers []string) {
+	fmt.Printf("dns: RestartForwarders: %s\n", nameServers)
 
 	srv.opts.NameServers = nameServers
-	srv.opts.FallbackNS = fallbackNS
 
 	srv.opts.initNameServers()
 
@@ -685,8 +683,6 @@ func (srv *Server) processRequest() {
 			switch {
 			case srv.hasForwarders():
 				srv.primaryq <- req
-			case srv.fallbackq != nil:
-				srv.fallbackq <- req
 			default:
 				if debug.Value >= 1 {
 					fmt.Printf("dns: * %s %d:%s\n",
@@ -710,9 +706,6 @@ func (srv *Server) processRequest() {
 				}
 				srv.primaryq <- req
 
-			case srv.fallbackq != nil:
-				srv.fallbackq <- req
-
 			default:
 				if debug.Value >= 1 {
 					fmt.Printf("dns: * %s %d:%s\n",
@@ -781,65 +774,31 @@ func (srv *Server) processResponse(req *request, res *Message) {
 
 func (srv *Server) startAllForwarders() {
 	srv.fwStoppers = nil
-	asFallback := "fallback"
 	asPrimary := "primary"
 
-	if srv.opts.hasFallback() && srv.fallbackq == nil {
-		srv.fallbackq = make(chan *request, 512)
-	} else {
-		srv.fallbackq = nil
-	}
-
 	for x := 0; x < len(srv.opts.primaryUDP); x++ {
 		tag := fmt.Sprintf("UDP-%d-%s", x, asPrimary)
 		nameserver := srv.opts.primaryUDP[x].String()
-		go srv.runUDPForwarder(true, tag, nameserver, srv.primaryq, srv.fallbackq)
+		go srv.runUDPForwarder(tag, nameserver, srv.primaryq)
 	}
 	for x := 0; x < len(srv.opts.primaryTCP); x++ {
 		tag := fmt.Sprintf("TCP-%d-%s", x, asPrimary)
 		nameserver := srv.opts.primaryTCP[x].String()
-		go srv.runTCPForwarder(true, tag, nameserver, srv.primaryq, srv.fallbackq)
+		go srv.runTCPForwarder(tag, nameserver, srv.primaryq)
 	}
 	for x := 0; x < len(srv.opts.primaryDoh); x++ {
 		tag := fmt.Sprintf("DoH-%d-%s", x, asPrimary)
 		nameserver := srv.opts.primaryDoh[x]
-		go srv.runDohForwarder(true, tag, nameserver, srv.primaryq, srv.fallbackq)
+		go srv.runDohForwarder(tag, nameserver, srv.primaryq)
 	}
 	for x := 0; x < len(srv.opts.primaryDot); x++ {
 		tag := fmt.Sprintf("DoT-%d-%s", x, asPrimary)
 		nameserver := srv.opts.primaryDot[x]
-		go srv.runTLSForwarder(true, tag, nameserver, srv.primaryq, srv.fallbackq)
-	}
-
-	if !srv.opts.hasFallback() {
-		return
-	}
-
-	for x := 0; x < len(srv.opts.fallbackUDP); x++ {
-		tag := fmt.Sprintf("UDP-%d-%s", x, asFallback)
-		nameserver := srv.opts.fallbackUDP[x].String()
-		go srv.runUDPForwarder(false, tag, nameserver, srv.fallbackq, nil)
-	}
-	for x := 0; x < len(srv.opts.fallbackTCP); x++ {
-		tag := fmt.Sprintf("TCP-%d-%s", x, asFallback)
-		nameserver := srv.opts.fallbackTCP[x].String()
-		go srv.runTCPForwarder(false, tag, nameserver, srv.fallbackq, nil)
-	}
-	for x := 0; x < len(srv.opts.fallbackDoh); x++ {
-		tag := fmt.Sprintf("DoH-%d-%s", x, asFallback)
-		nameserver := srv.opts.fallbackDoh[x]
-		go srv.runDohForwarder(false, tag, nameserver, srv.fallbackq, nil)
-	}
-	for x := 0; x < len(srv.opts.fallbackDot); x++ {
-		tag := fmt.Sprintf("DoT-%d-%s", x, asFallback)
-		nameserver := srv.opts.fallbackDot[x]
-		go srv.runTLSForwarder(false, tag, nameserver, srv.fallbackq, nil)
+		go srv.runTLSForwarder(tag, nameserver, srv.primaryq)
 	}
 }
 
-func (srv *Server) runDohForwarder(isPrimary bool, tag, nameserver string,
-	primaryq <-chan *request, fallbackq chan<- *request,
-) {
+func (srv *Server) runDohForwarder(tag, nameserver string, primaryq <-chan *request) {
 	stopper := srv.newStopper()
 
 	defer func() {
@@ -853,7 +812,7 @@ func (srv *Server) runDohForwarder(isPrimary bool, tag, nameserver string,
 
 			select {
 			case <-stopper:
-				srv.stopForwarder(isPrimary, nil)
+				srv.stopForwarder(nil)
 				return
 			default:
 				time.Sleep(3 * time.Second)
@@ -863,9 +822,7 @@ func (srv *Server) runDohForwarder(isPrimary bool, tag, nameserver string,
 
 		log.Printf("dns: forwarder %s for %s has been connected ...", tag, nameserver)
 
-		if isPrimary {
-			srv.incForwarder()
-		}
+		srv.incForwarder()
 
 		isRunning := true
 		ticker := time.NewTicker(aliveInterval)
@@ -874,7 +831,7 @@ func (srv *Server) runDohForwarder(isPrimary bool, tag, nameserver string,
 			case req, ok := <-primaryq:
 				if !ok {
 					log.Println("dns: primary queue has been closed")
-					srv.stopForwarder(isPrimary, forwarder)
+					srv.stopForwarder(forwarder)
 					return
 				}
 				if debug.Value >= 1 {
@@ -887,9 +844,6 @@ func (srv *Server) runDohForwarder(isPrimary bool, tag, nameserver string,
 				res, err := forwarder.Query(req.message)
 				if err != nil {
 					log.Printf("dns: %s forward failed: %s: %s", tag, req.message.Question.Name, err)
-					if fallbackq != nil {
-						fallbackq <- req
-					}
 					isRunning = false
 					continue
 				}
@@ -899,19 +853,17 @@ func (srv *Server) runDohForwarder(isPrimary bool, tag, nameserver string,
 					log.Printf("dns: %s alive", tag)
 				}
 			case <-stopper:
-				srv.stopForwarder(isPrimary, forwarder)
+				srv.stopForwarder(forwarder)
 				return
 			}
 		}
 
 		log.Printf("dns: reconnect forwarder %s for %s", tag, nameserver)
-		srv.stopForwarder(isPrimary, forwarder)
+		srv.stopForwarder(forwarder)
 	}
 }
 
-func (srv *Server) runTLSForwarder(isPrimary bool, tag, nameserver string,
-	primaryq <-chan *request, fallbackq chan<- *request,
-) {
+func (srv *Server) runTLSForwarder(tag, nameserver string, primaryq <-chan *request) {
 	stopper := srv.newStopper()
 
 	defer func() {
@@ -925,7 +877,7 @@ func (srv *Server) runTLSForwarder(isPrimary bool, tag, nameserver string,
 
 			select {
 			case <-stopper:
-				srv.stopForwarder(isPrimary, nil)
+				srv.stopForwarder(nil)
 				return
 			default:
 				time.Sleep(3 * time.Second)
@@ -935,9 +887,7 @@ func (srv *Server) runTLSForwarder(isPrimary bool, tag, nameserver string,
 
 		log.Printf("dns: forwarder %s for %s has been connected ...", tag, nameserver)
 
-		if isPrimary {
-			srv.incForwarder()
-		}
+		srv.incForwarder()
 
 		isRunning := true
 		ticker := time.NewTicker(aliveInterval)
@@ -946,7 +896,7 @@ func (srv *Server) runTLSForwarder(isPrimary bool, tag, nameserver string,
 			case req, ok := <-primaryq:
 				if !ok {
 					log.Println("dns: primary queue has been closed")
-					srv.stopForwarder(isPrimary, forwarder)
+					srv.stopForwarder(forwarder)
 					return
 				}
 				if debug.Value >= 1 {
@@ -959,9 +909,6 @@ func (srv *Server) runTLSForwarder(isPrimary bool, tag, nameserver string,
 				res, err := forwarder.Query(req.message)
 				if err != nil {
 					log.Printf("dns: %s forward failed: %s: %s", tag, req.message.Question.Name, err)
-					if fallbackq != nil {
-						fallbackq <- req
-					}
 					isRunning = false
 					continue
 				}
@@ -972,31 +919,25 @@ func (srv *Server) runTLSForwarder(isPrimary bool, tag, nameserver string,
 					log.Printf("dns: %s alive", tag)
 				}
 			case <-stopper:
-				srv.stopForwarder(isPrimary, forwarder)
+				srv.stopForwarder(forwarder)
 				return
 			}
 		}
 
 		log.Printf("dns: reconnect forwarder %s for %s", tag, nameserver)
-		srv.stopForwarder(isPrimary, forwarder)
+		srv.stopForwarder(forwarder)
 	}
 }
 
-func (srv *Server) runTCPForwarder(isPrimary bool, tag, nameserver string,
-	primaryq <-chan *request, fallbackq chan<- *request,
-) {
+func (srv *Server) runTCPForwarder(tag, nameserver string, primaryq <-chan *request) {
 	stopper := srv.newStopper()
 
 	log.Printf("dns: starting forwarder %s for %s", tag, nameserver)
 
-	if isPrimary {
-		srv.incForwarder()
-	}
+	srv.incForwarder()
 
 	defer func() {
-		if isPrimary {
-			srv.decForwarder()
-		}
+		srv.decForwarder()
 		log.Printf("dns: forwarder %s for %s has been stopped", tag, nameserver)
 	}()
 
@@ -1025,9 +966,6 @@ func (srv *Server) runTCPForwarder(isPrimary bool, tag, nameserver string,
 			cl.Close()
 			if err != nil {
 				log.Printf("dns: %s forward failed: %s: %s", tag, req.message.Question.Name, err)
-				if fallbackq != nil {
-					fallbackq <- req
-				}
 				continue
 			}
 
@@ -1046,9 +984,7 @@ func (srv *Server) runTCPForwarder(isPrimary bool, tag, nameserver string,
 // runUDPForwarder create a UDP client that consume request from forward queue
 // and forward it to parent server at "nameserver".
 //
-func (srv *Server) runUDPForwarder(isPrimary bool, tag, nameserver string,
-	primaryq <-chan *request, fallbackq chan<- *request,
-) {
+func (srv *Server) runUDPForwarder(tag, nameserver string, primaryq <-chan *request) {
 	stopper := srv.newStopper()
 
 	defer func() {
@@ -1063,7 +999,7 @@ func (srv *Server) runUDPForwarder(isPrimary bool, tag, nameserver string,
 
 			select {
 			case <-stopper:
-				srv.stopForwarder(isPrimary, nil)
+				srv.stopForwarder(nil)
 				return
 			default:
 				time.Sleep(3 * time.Second)
@@ -1073,9 +1009,7 @@ func (srv *Server) runUDPForwarder(isPrimary bool, tag, nameserver string,
 
 		log.Printf("dns: forwarder %s for %s has been connected ...", tag, nameserver)
 
-		if isPrimary {
-			srv.incForwarder()
-		}
+		srv.incForwarder()
 
 		// The second loop consume the forward queue.
 		isRunning := true
@@ -1085,7 +1019,7 @@ func (srv *Server) runUDPForwarder(isPrimary bool, tag, nameserver string,
 			case req, ok := <-primaryq:
 				if !ok {
 					log.Println("dns: primary queue has been closed")
-					srv.stopForwarder(isPrimary, forwarder)
+					srv.stopForwarder(forwarder)
 					return
 				}
 				if debug.Value >= 1 {
@@ -1098,9 +1032,6 @@ func (srv *Server) runUDPForwarder(isPrimary bool, tag, nameserver string,
 				res, err := forwarder.Query(req.message)
 				if err != nil {
 					log.Printf("dns: %s forward failed: %s: %s", tag, req.message.Question.Name, err)
-					if fallbackq != nil {
-						fallbackq <- req
-					}
 					isRunning = false
 					continue
 				}
@@ -1110,23 +1041,21 @@ func (srv *Server) runUDPForwarder(isPrimary bool, tag, nameserver string,
 					log.Printf("dns: %s alive", tag)
 				}
 			case <-stopper:
-				srv.stopForwarder(isPrimary, forwarder)
+				srv.stopForwarder(forwarder)
 				return
 			}
 		}
 
 		log.Printf("dns: reconnect forwarder %s for %s", tag, nameserver)
-		srv.stopForwarder(isPrimary, forwarder)
+		srv.stopForwarder(forwarder)
 	}
 }
 
-func (srv *Server) stopForwarder(isPrimary bool, fw Client) {
+func (srv *Server) stopForwarder(fw Client) {
 	if fw != nil {
 		fw.Close()
 	}
-	if isPrimary {
-		srv.decForwarder()
-	}
+	srv.decForwarder()
 }
 
 //
diff --git a/lib/dns/server_options.go b/lib/dns/server_options.go
index d1ada8ce..b038acf5 100644
--- a/lib/dns/server_options.go
+++ b/lib/dns/server_options.go
@@ -62,14 +62,6 @@ type ServerOptions struct {
 	//
 	NameServers []string `ini:"dns:server:parent"`
 
-	//
-	// FallbackNS contains list of parent name servers that will be
-	// queried if the primary NameServers return an error.
-	//
-	// This field use the same format as NameServers.
-	//
-	FallbackNS []string
-
 	// TLSCertFile contains path to certificate for serving DNS over TLS
 	// and HTTPS.
 	// This field is optional, if its empty, server will listening on
@@ -124,11 +116,6 @@ type ServerOptions struct {
 	// primaryDot contains list of parent name server addresses using DoT
 	// protocol.
 	primaryDot []string
-
-	fallbackUDP []net.Addr
-	fallbackTCP []net.Addr
-	fallbackDoh []string
-	fallbackDot []string
 }
 
 //
@@ -195,11 +182,6 @@ func (opts *ServerOptions) getDoTAddress() *net.TCPAddr {
 	}
 }
 
-func (opts *ServerOptions) hasFallback() bool {
-	return len(opts.fallbackUDP) > 0 || len(opts.fallbackTCP) > 0 ||
-		len(opts.fallbackDot) > 0 || len(opts.fallbackDoh) > 0
-}
-
 //
 // parseNameServers parse each name server in NameServers list based on scheme
 // and store the result either in udpAddrs, tcpAddrs, dohAddrs, or dotAddrs.
@@ -207,7 +189,7 @@ func (opts *ServerOptions) hasFallback() bool {
 // If the name server format contains no scheme, it will be assumed to be
 // "udp".
 //
-func (opts *ServerOptions) parseNameServers(nameServers []string, isPrimary bool) {
+func (opts *ServerOptions) parseNameServers(nameServers []string) {
 	for _, ns := range nameServers {
 		dnsURL, err := url.Parse(ns)
 		if err != nil {
@@ -223,26 +205,14 @@ func (opts *ServerOptions) parseNameServers(nameServers []string, isPrimary bool
 				continue
 			}
 
-			if isPrimary {
-				opts.primaryTCP = append(opts.primaryTCP, tcpAddr)
-			} else {
-				opts.fallbackTCP = append(opts.fallbackTCP, tcpAddr)
-			}
+			opts.primaryTCP = append(opts.primaryTCP, tcpAddr)
 
 		case "https":
 			ip := net.ParseIP(dnsURL.Hostname())
 			if ip == nil {
-				if isPrimary {
-					opts.primaryDoh = append(opts.primaryDoh, ns)
-				} else {
-					opts.fallbackDoh = append(opts.fallbackDoh, ns)
-				}
+				opts.primaryDoh = append(opts.primaryDoh, ns)
 			} else {
-				if isPrimary {
-					opts.primaryDot = append(opts.primaryDot, dnsURL.Host)
-				} else {
-					opts.fallbackDot = append(opts.fallbackDot, dnsURL.Host)
-				}
+				opts.primaryDot = append(opts.primaryDot, dnsURL.Host)
 			}
 
 		default:
@@ -256,11 +226,7 @@ func (opts *ServerOptions) parseNameServers(nameServers []string, isPrimary bool
 				continue
 			}
 
-			if isPrimary {
-				opts.primaryUDP = append(opts.primaryUDP, udpAddr)
-			} else {
-				opts.fallbackUDP = append(opts.fallbackUDP, udpAddr)
-			}
+			opts.primaryUDP = append(opts.primaryUDP, udpAddr)
 		}
 	}
 }
@@ -270,11 +236,5 @@ func (opts *ServerOptions) initNameServers() {
 	opts.primaryTCP = nil
 	opts.primaryDoh = nil
 	opts.primaryDot = nil
-	opts.parseNameServers(opts.NameServers, true)
-
-	opts.fallbackUDP = nil
-	opts.fallbackTCP = nil
-	opts.fallbackDoh = nil
-	opts.fallbackDot = nil
-	opts.parseNameServers(opts.FallbackNS, false)
+	opts.parseNameServers(opts.NameServers)
 }