From ae500b63a746c0c3e8f02f375da2b02721d9370d Mon Sep 17 00:00:00 2001
From: Maycon Santos <mlsmaycon@gmail.com>
Date: Tue, 29 Nov 2022 11:49:18 +0100
Subject: [PATCH] User custom loopback address (#589)

We will probe a set of addresses and port
to define the one available for our DNS service

if none is available, we return an error
---
 client/internal/dns/server.go      | 57 +++++++++++++++++++-----------
 client/internal/dns/server_test.go |  6 ++--
 client/internal/engine_test.go     |  8 +++++
 3 files changed, 47 insertions(+), 24 deletions(-)

diff --git a/client/internal/dns/server.go b/client/internal/dns/server.go
index 91a38cd4a..494472cff 100644
--- a/client/internal/dns/server.go
+++ b/client/internal/dns/server.go
@@ -15,9 +15,10 @@ import (
 )
 
 const (
-	port       = 53
-	customPort = 5053
-	defaultIP  = "127.0.0.1"
+	defaultPort = 53
+	customPort  = 5053
+	defaultIP   = "127.0.0.1"
+	customIP    = "127.0.0.153"
 )
 
 // Server is a dns server interface
@@ -54,13 +55,8 @@ type muxUpdate struct {
 // NewDefaultServer returns a new dns server
 func NewDefaultServer(ctx context.Context, wgInterface *iface.WGIface) (*DefaultServer, error) {
 	mux := dns.NewServeMux()
-	listenIP := defaultIP
-	if runtime.GOOS != "darwin" && wgInterface != nil {
-		listenIP = wgInterface.GetAddress().IP.String()
-	}
 
 	dnsServer := &dns.Server{
-		Addr:    fmt.Sprintf("%s:%d", listenIP, port),
 		Net:     "udp",
 		Handler: mux,
 		UDPSize: 65535,
@@ -78,8 +74,7 @@ func NewDefaultServer(ctx context.Context, wgInterface *iface.WGIface) (*Default
 			registeredMap: make(registrationMap),
 		},
 		wgInterface: wgInterface,
-		runtimePort: port,
-		runtimeIP:   listenIP,
+		runtimePort: defaultPort,
 	}
 
 	hostmanager, err := newHostManager(wgInterface)
@@ -92,19 +87,15 @@ func NewDefaultServer(ctx context.Context, wgInterface *iface.WGIface) (*Default
 
 // Start runs the listener in a go routine
 func (s *DefaultServer) Start() {
-	s.runtimePort = port
-	udpAddr := net.UDPAddrFromAddrPort(netip.MustParseAddrPort(s.server.Addr))
-	probeListener, err := net.ListenUDP("udp", udpAddr)
+
+	ip, port, err := s.getFirstListenerAvailable()
 	if err != nil {
-		log.Warnf("using a custom port for dns server")
-		s.runtimePort = customPort
-		s.server.Addr = fmt.Sprintf("%s:%d", s.runtimeIP, customPort)
-	} else {
-		err = probeListener.Close()
-		if err != nil {
-			log.Errorf("got an error closing the probe listener, error: %s", err)
-		}
+		log.Error(err)
+		return
 	}
+	s.runtimeIP = ip
+	s.runtimePort = port
+	s.server.Addr = fmt.Sprintf("%s:%d", s.runtimeIP, s.runtimePort)
 
 	log.Debugf("starting dns on %s", s.server.Addr)
 
@@ -119,6 +110,30 @@ func (s *DefaultServer) Start() {
 	}()
 }
 
+func (s *DefaultServer) getFirstListenerAvailable() (string, int, error) {
+	ips := []string{defaultIP, customIP}
+	if runtime.GOOS != "darwin" && s.wgInterface != nil {
+		ips = append([]string{s.wgInterface.GetAddress().IP.String()}, ips...)
+	}
+	ports := []int{defaultPort, customPort}
+	for _, port := range ports {
+		for _, ip := range ips {
+			addrString := fmt.Sprintf("%s:%d", ip, port)
+			udpAddr := net.UDPAddrFromAddrPort(netip.MustParseAddrPort(addrString))
+			probeListener, err := net.ListenUDP("udp", udpAddr)
+			if err == nil {
+				err = probeListener.Close()
+				if err != nil {
+					log.Errorf("got an error closing the probe listener, error: %s", err)
+				}
+				return ip, port, nil
+			}
+			log.Warnf("binding dns on %s is not available, error: %s", addrString, err)
+		}
+	}
+	return "", 0, fmt.Errorf("unable to find an unused ip and port combination. IPs tested: %v and ports %v", ips, ports)
+}
+
 func (s *DefaultServer) setListenerStatus(running bool) {
 	s.listenerIsRunning = running
 }
diff --git a/client/internal/dns/server_test.go b/client/internal/dns/server_test.go
index b0b8cd1ec..9b65f1ae5 100644
--- a/client/internal/dns/server_test.go
+++ b/client/internal/dns/server_test.go
@@ -257,7 +257,7 @@ func TestDNSServerStartStop(t *testing.T) {
 			d := net.Dialer{
 				Timeout: time.Second * 5,
 			}
-			addr := fmt.Sprintf("127.0.0.1:%d", port)
+			addr := fmt.Sprintf("%s:%d", dnsServer.runtimeIP, dnsServer.runtimePort)
 			conn, err := d.DialContext(ctx, network, addr)
 			if err != nil {
 				t.Log(err)
@@ -297,7 +297,7 @@ func getDefaultServerWithNoHostManager(ip string) *DefaultServer {
 	}
 
 	dnsServer := &dns.Server{
-		Addr:    fmt.Sprintf("%s:%d", ip, port),
+		Addr:    fmt.Sprintf("%s:%d", ip, defaultPort),
 		Net:     "udp",
 		Handler: mux,
 		UDPSize: 65535,
@@ -314,7 +314,7 @@ func getDefaultServerWithNoHostManager(ip string) *DefaultServer {
 		localResolver: &localResolver{
 			registeredMap: make(registrationMap),
 		},
-		runtimePort: port,
+		runtimePort: defaultPort,
 		runtimeIP:   listenIP,
 	}
 }
diff --git a/client/internal/engine_test.go b/client/internal/engine_test.go
index 9e80f144d..895bfb3ac 100644
--- a/client/internal/engine_test.go
+++ b/client/internal/engine_test.go
@@ -71,6 +71,10 @@ func TestEngine_SSH(t *testing.T) {
 		WgPort:       33100,
 	}, nbstatus.NewRecorder())
 
+	engine.dnsServer = &dns.MockServer{
+		UpdateDNSServerFunc: func(serial uint64, update nbdns.Config) error { return nil },
+	}
+
 	var sshKeysAdded []string
 	var sshPeersRemoved []string
 
@@ -385,6 +389,10 @@ func TestEngine_Sync(t *testing.T) {
 		WgPort:       33100,
 	}, nbstatus.NewRecorder())
 
+	engine.dnsServer = &dns.MockServer{
+		UpdateDNSServerFunc: func(serial uint64, update nbdns.Config) error { return nil },
+	}
+
 	defer func() {
 		err := engine.Stop()
 		if err != nil {