[client] Android - Create the Android fake IP manager lazily on DNS flag enable (#6989)

The fake IP manager was only created at route manager construction, from the DNS feature flag fetched by the initial GetNetworkMap call. When the flag flipped to true mid-session, UpdateRoutes set useNewDNSRoute but never created the manager, so domain routes added after the flip got a DNS interceptor with a nil fake IP manager.

internalDnatFw only checked for a firewall and GOOS, so the interceptor took the DNAT path and called GetFakeIP/AllocateFakeIP on the nil *fakeip.Manager. These methods lock m.mu first, which is a nil pointer dereference: the first DNS answer for such a route panicked and crashed the VPN service. The fake IP blocks (240.0.0.0/8 and its v6 pair) also never reached the TUN, since only the constructor registered them.

Create the manager and its TUN routes from UpdateRoutes when the flag turns on, notify so the fake IP blocks get into the TUN without a client route change, and treat a nil manager as no internal DNAT.

This is groundwork for removing the initial GetNetworkMap fetch, after which every startup goes through the flag-off-to-on transition.

client/internal/routemanager/dnsinterceptor/handler.go

@@ -479,7 +479,7 @@ func (d *DnsInterceptor) removeDNATMappings(realPrefixes []netip.Prefix, logger
 
 // internalDnatFw checks if the firewall supports internal DNAT
 func (d *DnsInterceptor) internalDnatFw() (internalDNATer, bool) {
-	if d.firewall == nil || runtime.GOOS != "android" {
+	if d.firewall == nil || d.fakeIPManager == nil || runtime.GOOS != "android" {
 		return nil, false
 	}
 	fw, ok := d.firewall.(internalDNATer)

client/internal/routemanager/manager.go

@@ -165,31 +165,36 @@ func (m *DefaultManager) setupAndroidRoutes(config ManagerConfig) {
 	routesForComparison := slices.Clone(cr)
 
 	if config.DNSFeatureFlag {
-		m.fakeIPManager = fakeip.NewManager()
-
-		v4ID := uuid.NewString()
-		fakeIPRoute := &route.Route{
-			ID:          route.ID(v4ID),
-			Network:     m.fakeIPManager.GetFakeIPBlock(),
-			NetID:       route.NetID(v4ID),
-			Peer:        m.pubKey,
-			NetworkType: route.IPv4Network,
-		}
-		v6ID := uuid.NewString()
-		fakeIPv6Route := &route.Route{
-			ID:          route.ID(v6ID),
-			Network:     m.fakeIPManager.GetFakeIPv6Block(),
-			NetID:       route.NetID(v6ID),
-			Peer:        m.pubKey,
-			NetworkType: route.IPv6Network,
-		}
-		cr = append(cr, fakeIPRoute, fakeIPv6Route)
-		m.notifier.SetFakeIPRoutes([]*route.Route{fakeIPRoute, fakeIPv6Route})
+		cr = append(cr, m.enableFakeIPRoutes()...)
 	}
 
 	m.notifier.SetInitialClientRoutes(cr, routesForComparison)
 }
 
+func (m *DefaultManager) enableFakeIPRoutes() []*route.Route {
+	m.fakeIPManager = fakeip.NewManager()
+
+	v4ID := uuid.NewString()
+	fakeIPRoute := &route.Route{
+		ID:          route.ID(v4ID),
+		Network:     m.fakeIPManager.GetFakeIPBlock(),
+		NetID:       route.NetID(v4ID),
+		Peer:        m.pubKey,
+		NetworkType: route.IPv4Network,
+	}
+	v6ID := uuid.NewString()
+	fakeIPv6Route := &route.Route{
+		ID:          route.ID(v6ID),
+		Network:     m.fakeIPManager.GetFakeIPv6Block(),
+		NetID:       route.NetID(v6ID),
+		Peer:        m.pubKey,
+		NetworkType: route.IPv6Network,
+	}
+	fakeRoutes := []*route.Route{fakeIPRoute, fakeIPv6Route}
+	m.notifier.SetFakeIPRoutes(fakeRoutes)
+	return fakeRoutes
+}
+
 func (m *DefaultManager) setupRefCounters(useNoop bool) {
 	var once sync.Once
 	var wgIface *net.Interface
@@ -464,6 +469,9 @@ func (m *DefaultManager) UpdateRoutes(
 
 	var merr *multierror.Error
 	if !m.disableClientRoutes {
+		if runtime.GOOS == "android" && useNewDNSRoute && m.fakeIPManager == nil {
+			m.enableFakeIPRoutes()
+		}
 
 		// Update route selector based on management server's isSelected status
 		m.updateRouteSelectorFromManagement(clientRoutes)

client/internal/routemanager/notifier/notifier_android.go

@@ -41,6 +41,7 @@ func (n *Notifier) SetInitialClientRoutes(initialRoutes []*route.Route, routesFo
 // SetFakeIPRoutes stores the fake IP routes to be included in every TUN rebuild.
 func (n *Notifier) SetFakeIPRoutes(routes []*route.Route) {
 	n.fakeIPRoutes = routes
+	n.notify()
 }
 
 func (n *Notifier) OnNewRoutes(idMap route.HAMap) {