diff --git a/cmd/tailscaled/depaware.txt b/cmd/tailscaled/depaware.txt
index 58f61e60e..7e51976fc 100644
--- a/cmd/tailscaled/depaware.txt
+++ b/cmd/tailscaled/depaware.txt
@@ -282,6 +282,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
   LD    tailscale.com/tempfork/gliderlabs/ssh                        from tailscale.com/ssh/tailssh
         tailscale.com/tka                                            from tailscale.com/ipn/ipnlocal+
    W    tailscale.com/tsconst                                        from tailscale.com/net/interfaces
+        tailscale.com/tsd                                            from tailscale.com/cmd/tailscaled+
         tailscale.com/tstime                                         from tailscale.com/wgengine/magicsock
      💣 tailscale.com/tstime/mono                                    from tailscale.com/net/tstun+
         tailscale.com/tstime/rate                                    from tailscale.com/wgengine/filter+
diff --git a/cmd/tailscaled/tailscaled.go b/cmd/tailscaled/tailscaled.go
index d2c28de99..0d46cdb0f 100644
--- a/cmd/tailscaled/tailscaled.go
+++ b/cmd/tailscaled/tailscaled.go
@@ -50,6 +50,7 @@ import (
 	"tailscale.com/safesocket"
 	"tailscale.com/smallzstd"
 	"tailscale.com/syncs"
+	"tailscale.com/tsd"
 	"tailscale.com/tsweb/varz"
 	"tailscale.com/types/flagtype"
 	"tailscale.com/types/logger"
@@ -330,12 +331,16 @@ var debugMux *http.ServeMux
 
 func run() error {
 	var logf logger.Logf = log.Printf
+
+	sys := new(tsd.System)
+
 	netMon, err := netmon.New(func(format string, args ...any) {
 		logf(format, args...)
 	})
 	if err != nil {
 		return fmt.Errorf("netmon.New: %w", err)
 	}
+	sys.Set(netMon)
 
 	pol := logpolicy.New(logtail.CollectionNode, netMon)
 	pol.SetVerbosityLevel(args.verbose)
@@ -386,10 +391,10 @@ func run() error {
 		debugMux = newDebugMux()
 	}
 
-	return startIPNServer(context.Background(), logf, pol.PublicID, netMon)
+	return startIPNServer(context.Background(), logf, pol.PublicID, sys)
 }
 
-func startIPNServer(ctx context.Context, logf logger.Logf, logID logid.PublicID, netMon *netmon.Monitor) error {
+func startIPNServer(ctx context.Context, logf logger.Logf, logID logid.PublicID, sys *tsd.System) error {
 	ln, err := safesocket.Listen(args.socketpath)
 	if err != nil {
 		return fmt.Errorf("safesocket.Listen: %v", err)
@@ -415,7 +420,7 @@ func startIPNServer(ctx context.Context, logf logger.Logf, logID logid.PublicID,
 		}
 	}()
 
-	srv := ipnserver.New(logf, logID, netMon)
+	srv := ipnserver.New(logf, logID, sys.NetMon.Get())
 	if debugMux != nil {
 		debugMux.HandleFunc("/debug/ipn", srv.ServeHTMLStatus)
 	}
@@ -433,7 +438,7 @@ func startIPNServer(ctx context.Context, logf logger.Logf, logID logid.PublicID,
 				return
 			}
 		}
-		lb, err := getLocalBackend(ctx, logf, logID, netMon)
+		lb, err := getLocalBackend(ctx, logf, logID, sys)
 		if err == nil {
 			logf("got LocalBackend in %v", time.Since(t0).Round(time.Millisecond))
 			srv.SetLocalBackend(lb)
@@ -457,31 +462,28 @@ func startIPNServer(ctx context.Context, logf logger.Logf, logID logid.PublicID,
 	return nil
 }
 
-func getLocalBackend(ctx context.Context, logf logger.Logf, logID logid.PublicID, netMon *netmon.Monitor) (_ *ipnlocal.LocalBackend, retErr error) {
+func getLocalBackend(ctx context.Context, logf logger.Logf, logID logid.PublicID, sys *tsd.System) (_ *ipnlocal.LocalBackend, retErr error) {
 	if logPol != nil {
-		logPol.Logtail.SetNetMon(netMon)
+		logPol.Logtail.SetNetMon(sys.NetMon.Get())
 	}
 
 	socksListener, httpProxyListener := mustStartProxyListeners(args.socksAddr, args.httpProxyAddr)
 
 	dialer := &tsdial.Dialer{Logf: logf} // mutated below (before used)
-	e, onlyNetstack, err := createEngine(logf, netMon, dialer)
+	sys.Set(dialer)
+
+	onlyNetstack, err := createEngine(logf, sys)
 	if err != nil {
 		return nil, fmt.Errorf("createEngine: %w", err)
 	}
-	if _, ok := e.(wgengine.ResolvingEngine).GetResolver(); !ok {
-		panic("internal error: exit node resolver not wired up")
-	}
 	if debugMux != nil {
-		if ig, ok := e.(wgengine.InternalsGetter); ok {
-			if _, mc, _, ok := ig.GetInternals(); ok {
-				debugMux.HandleFunc("/debug/magicsock", mc.ServeHTTPDebug)
-			}
+		if ms, ok := sys.MagicSock.GetOK(); ok {
+			debugMux.HandleFunc("/debug/magicsock", ms.ServeHTTPDebug)
 		}
 		go runDebugServer(debugMux, args.debug)
 	}
 
-	ns, err := newNetstack(logf, dialer, e)
+	ns, err := newNetstack(logf, sys)
 	if err != nil {
 		return nil, fmt.Errorf("newNetstack: %w", err)
 	}
@@ -489,6 +491,7 @@ func getLocalBackend(ctx context.Context, logf logger.Logf, logID logid.PublicID
 	ns.ProcessSubnets = onlyNetstack || handleSubnetsInNetstack()
 
 	if onlyNetstack {
+		e := sys.Engine.Get()
 		dialer.UseNetstackForIP = func(ip netip.Addr) bool {
 			_, ok := e.PeerForIP(ip)
 			return ok
@@ -519,16 +522,15 @@ func getLocalBackend(ctx context.Context, logf logger.Logf, logID logid.PublicID
 		tshttpproxy.SetSelfProxy(addrs...)
 	}
 
-	e = wgengine.NewWatchdog(e)
-
 	opts := ipnServerOpts()
 
 	store, err := store.New(logf, statePathOrDefault())
 	if err != nil {
 		return nil, fmt.Errorf("store.New: %w", err)
 	}
+	sys.Set(store)
 
-	lb, err := ipnlocal.NewLocalBackend(logf, logID, store, dialer, e, opts.LoginFlags)
+	lb, err := ipnlocal.NewLocalBackend(logf, logID, sys, opts.LoginFlags)
 	if err != nil {
 		return nil, fmt.Errorf("ipnlocal.NewLocalBackend: %w", err)
 	}
@@ -554,21 +556,21 @@ func getLocalBackend(ctx context.Context, logf logger.Logf, logID logid.PublicID
 //
 // onlyNetstack is true if the user has explicitly requested that we use netstack
 // for all networking.
-func createEngine(logf logger.Logf, netMon *netmon.Monitor, dialer *tsdial.Dialer) (e wgengine.Engine, onlyNetstack bool, err error) {
+func createEngine(logf logger.Logf, sys *tsd.System) (onlyNetstack bool, err error) {
 	if args.tunname == "" {
-		return nil, false, errors.New("no --tun value specified")
+		return false, errors.New("no --tun value specified")
 	}
 	var errs []error
 	for _, name := range strings.Split(args.tunname, ",") {
 		logf("wgengine.NewUserspaceEngine(tun %q) ...", name)
-		e, onlyNetstack, err = tryEngine(logf, netMon, dialer, name)
+		onlyNetstack, err = tryEngine(logf, sys, name)
 		if err == nil {
-			return e, onlyNetstack, nil
+			return onlyNetstack, nil
 		}
 		logf("wgengine.NewUserspaceEngine(tun %q) error: %v", name, err)
 		errs = append(errs, err)
 	}
-	return nil, false, multierr.New(errs...)
+	return false, multierr.New(errs...)
 }
 
 // handleSubnetsInNetstack reports whether netstack should handle subnet routers
@@ -593,21 +595,23 @@ func handleSubnetsInNetstack() bool {
 
 var tstunNew = tstun.New
 
-func tryEngine(logf logger.Logf, netMon *netmon.Monitor, dialer *tsdial.Dialer, name string) (e wgengine.Engine, onlyNetstack bool, err error) {
+func tryEngine(logf logger.Logf, sys *tsd.System, name string) (onlyNetstack bool, err error) {
 	conf := wgengine.Config{
-		ListenPort: args.port,
-		NetMon:     netMon,
-		Dialer:     dialer,
+		ListenPort:   args.port,
+		NetMon:       sys.NetMon.Get(),
+		Dialer:       sys.Dialer.Get(),
+		SetSubsystem: sys.Set,
 	}
 
 	onlyNetstack = name == "userspace-networking"
+	netstackSubnetRouter := onlyNetstack // but mutated later on some platforms
 	netns.SetEnabled(!onlyNetstack)
 
 	if args.birdSocketPath != "" && createBIRDClient != nil {
 		log.Printf("Connecting to BIRD at %s ...", args.birdSocketPath)
 		conf.BIRDClient, err = createBIRDClient(args.birdSocketPath)
 		if err != nil {
-			return nil, false, fmt.Errorf("createBIRDClient: %w", err)
+			return false, fmt.Errorf("createBIRDClient: %w", err)
 		}
 	}
 	if onlyNetstack {
@@ -620,44 +624,55 @@ func tryEngine(logf logger.Logf, netMon *netmon.Monitor, dialer *tsdial.Dialer,
 			// TODO(bradfitz): add a Synology-specific DNS manager.
 			conf.DNS, err = dns.NewOSConfigurator(logf, "") // empty interface name
 			if err != nil {
-				return nil, false, fmt.Errorf("dns.NewOSConfigurator: %w", err)
+				return false, fmt.Errorf("dns.NewOSConfigurator: %w", err)
 			}
 		}
 	} else {
 		dev, devName, err := tstunNew(logf, name)
 		if err != nil {
 			tstun.Diagnose(logf, name, err)
-			return nil, false, fmt.Errorf("tstun.New(%q): %w", name, err)
+			return false, fmt.Errorf("tstun.New(%q): %w", name, err)
 		}
 		conf.Tun = dev
 		if strings.HasPrefix(name, "tap:") {
 			conf.IsTAP = true
 			e, err := wgengine.NewUserspaceEngine(logf, conf)
-			return e, false, err
+			if err != nil {
+				return false, err
+			}
+			sys.Set(e)
+			return false, err
 		}
 
-		r, err := router.New(logf, dev, netMon)
+		r, err := router.New(logf, dev, sys.NetMon.Get())
 		if err != nil {
 			dev.Close()
-			return nil, false, fmt.Errorf("creating router: %w", err)
+			return false, fmt.Errorf("creating router: %w", err)
 		}
+		sys.Set(r)
+
 		d, err := dns.NewOSConfigurator(logf, devName)
 		if err != nil {
 			dev.Close()
 			r.Close()
-			return nil, false, fmt.Errorf("dns.NewOSConfigurator: %w", err)
+			return false, fmt.Errorf("dns.NewOSConfigurator: %w", err)
 		}
 		conf.DNS = d
 		conf.Router = r
 		if handleSubnetsInNetstack() {
 			conf.Router = netstack.NewSubnetRouterWrapper(conf.Router)
+			netstackSubnetRouter = true
 		}
 	}
-	e, err = wgengine.NewUserspaceEngine(logf, conf)
+	e, err := wgengine.NewUserspaceEngine(logf, conf)
 	if err != nil {
-		return nil, onlyNetstack, err
+		return onlyNetstack, err
 	}
-	return e, onlyNetstack, nil
+	e = wgengine.NewWatchdog(e)
+	sys.Set(e)
+	sys.NetstackRouter.Set(netstackSubnetRouter)
+
+	return onlyNetstack, nil
 }
 
 func newDebugMux() *http.ServeMux {
@@ -687,12 +702,8 @@ func runDebugServer(mux *http.ServeMux, addr string) {
 	}
 }
 
-func newNetstack(logf logger.Logf, dialer *tsdial.Dialer, e wgengine.Engine) (*netstack.Impl, error) {
-	tunDev, magicConn, dns, ok := e.(wgengine.InternalsGetter).GetInternals()
-	if !ok {
-		return nil, fmt.Errorf("%T is not a wgengine.InternalsGetter", e)
-	}
-	return netstack.Create(logf, tunDev, e, magicConn, dialer, dns)
+func newNetstack(logf logger.Logf, sys *tsd.System) (*netstack.Impl, error) {
+	return netstack.Create(logf, sys.Tun.Get(), sys.Engine.Get(), sys.MagicSock.Get(), sys.Dialer.Get(), sys.DNSManager.Get())
 }
 
 // mustStartProxyListeners creates listeners for local SOCKS and HTTP
diff --git a/cmd/tailscaled/tailscaled_windows.go b/cmd/tailscaled/tailscaled_windows.go
index 077d3f99f..ae538bed6 100644
--- a/cmd/tailscaled/tailscaled_windows.go
+++ b/cmd/tailscaled/tailscaled_windows.go
@@ -47,6 +47,7 @@ import (
 	"tailscale.com/net/dns"
 	"tailscale.com/net/netmon"
 	"tailscale.com/net/tstun"
+	"tailscale.com/tsd"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/logid"
 	"tailscale.com/util/winutil"
@@ -292,13 +293,15 @@ func beWindowsSubprocess() bool {
 		}
 	}()
 
+	sys := new(tsd.System)
 	netMon, err := netmon.New(log.Printf)
 	if err != nil {
-		log.Printf("Could not create netMon: %v", err)
-		netMon = nil
+		log.Fatalf("Could not create netMon: %v", err)
 	}
+	sys.Set(netMon)
+
 	publicLogID, _ := logid.ParsePublicID(logID)
-	err = startIPNServer(ctx, log.Printf, publicLogID, netMon)
+	err = startIPNServer(ctx, log.Printf, publicLogID, sys)
 	if err != nil {
 		log.Fatalf("ipnserver: %v", err)
 	}
diff --git a/cmd/tsconnect/wasm/wasm_js.go b/cmd/tsconnect/wasm/wasm_js.go
index a1b0fd95e..153941b29 100644
--- a/cmd/tsconnect/wasm/wasm_js.go
+++ b/cmd/tsconnect/wasm/wasm_js.go
@@ -37,6 +37,7 @@ import (
 	"tailscale.com/safesocket"
 	"tailscale.com/smallzstd"
 	"tailscale.com/tailcfg"
+	"tailscale.com/tsd"
 	"tailscale.com/wgengine"
 	"tailscale.com/wgengine/netstack"
 	"tailscale.com/words"
@@ -96,6 +97,8 @@ func newIPN(jsConfig js.Value) map[string]any {
 	logtail := logtail.NewLogger(c, log.Printf)
 	logf := logtail.Logf
 
+	sys := new(tsd.System)
+	sys.Set(store)
 	dialer := &tsdial.Dialer{Logf: logf}
 	eng, err := wgengine.NewUserspaceEngine(logf, wgengine.Config{
 		Dialer: dialer,
@@ -103,12 +106,9 @@ func newIPN(jsConfig js.Value) map[string]any {
 	if err != nil {
 		log.Fatal(err)
 	}
+	sys.Set(eng)
 
-	tunDev, magicConn, dnsManager, ok := eng.(wgengine.InternalsGetter).GetInternals()
-	if !ok {
-		log.Fatalf("%T is not a wgengine.InternalsGetter", eng)
-	}
-	ns, err := netstack.Create(logf, tunDev, eng, magicConn, dialer, dnsManager)
+	ns, err := netstack.Create(logf, sys.Tun.Get(), eng, sys.MagicSock.Get(), dialer, sys.DNSManager.Get())
 	if err != nil {
 		log.Fatalf("netstack.Create: %v", err)
 	}
@@ -121,10 +121,11 @@ func newIPN(jsConfig js.Value) map[string]any {
 	dialer.NetstackDialTCP = func(ctx context.Context, dst netip.AddrPort) (net.Conn, error) {
 		return ns.DialContextTCP(ctx, dst)
 	}
+	sys.NetstackRouter.Set(true)
 
 	logid := lpc.PublicID
 	srv := ipnserver.New(logf, logid, nil /* no netMon */)
-	lb, err := ipnlocal.NewLocalBackend(logf, logid, store, dialer, eng, controlclient.LoginEphemeral)
+	lb, err := ipnlocal.NewLocalBackend(logf, logid, sys, controlclient.LoginEphemeral)
 	if err != nil {
 		log.Fatalf("ipnlocal.NewLocalBackend: %v", err)
 	}
diff --git a/ipn/ipnlocal/local.go b/ipn/ipnlocal/local.go
index e2894e663..15202b0fd 100644
--- a/ipn/ipnlocal/local.go
+++ b/ipn/ipnlocal/local.go
@@ -60,6 +60,7 @@ import (
 	"tailscale.com/syncs"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tka"
+	"tailscale.com/tsd"
 	"tailscale.com/types/dnstype"
 	"tailscale.com/types/empty"
 	"tailscale.com/types/key"
@@ -137,10 +138,11 @@ type LocalBackend struct {
 	logf                  logger.Logf        // general logging
 	keyLogf               logger.Logf        // for printing list of peers on change
 	statsLogf             logger.Logf        // for printing peers stats on change
-	e                     wgengine.Engine
+	sys                   *tsd.System
+	e                     wgengine.Engine // non-nil; TODO(bradfitz): remove; use sys
 	pm                    *profileManager
-	store                 ipn.StateStore
-	dialer                *tsdial.Dialer // non-nil
+	store                 ipn.StateStore // non-nil; TODO(bradfitz): remove; use sys
+	dialer                *tsdial.Dialer // non-nil; TODO(bradfitz): remove; use sys
 	backendLogID          logid.PublicID
 	unregisterNetMon      func()
 	unregisterHealthWatch func()
@@ -267,10 +269,10 @@ type clientGen func(controlclient.Options) (controlclient.Client, error)
 // but is not actually running.
 //
 // If dialer is nil, a new one is made.
-func NewLocalBackend(logf logger.Logf, logID logid.PublicID, store ipn.StateStore, dialer *tsdial.Dialer, e wgengine.Engine, loginFlags controlclient.LoginFlags) (*LocalBackend, error) {
-	if e == nil {
-		panic("ipn.NewLocalBackend: engine must not be nil")
-	}
+func NewLocalBackend(logf logger.Logf, logID logid.PublicID, sys *tsd.System, loginFlags controlclient.LoginFlags) (*LocalBackend, error) {
+	e := sys.Engine.Get()
+	store := sys.StateStore.Get()
+	dialer := sys.Dialer.Get()
 
 	pm, err := newProfileManager(store, logf)
 	if err != nil {
@@ -301,10 +303,11 @@ func NewLocalBackend(logf logger.Logf, logID logid.PublicID, store ipn.StateStor
 		logf:           logf,
 		keyLogf:        logger.LogOnChange(logf, 5*time.Minute, time.Now),
 		statsLogf:      logger.LogOnChange(logf, 5*time.Minute, time.Now),
+		sys:            sys,
 		e:              e,
-		pm:             pm,
-		store:          store,
 		dialer:         dialer,
+		store:          store,
+		pm:             pm,
 		backendLogID:   logID,
 		state:          ipn.NoState,
 		portpoll:       portpoll,
@@ -313,7 +316,8 @@ func NewLocalBackend(logf logger.Logf, logID logid.PublicID, store ipn.StateStor
 		loginFlags:     loginFlags,
 	}
 
-	b.sockstatLogger, err = sockstatlog.NewLogger(logpolicy.LogsDir(logf), logf, logID, e.GetNetMon())
+	netMon := sys.NetMon.Get()
+	b.sockstatLogger, err = sockstatlog.NewLogger(logpolicy.LogsDir(logf), logf, logID, netMon)
 	if err != nil {
 		log.Printf("error setting up sockstat logger: %v", err)
 	}
@@ -330,7 +334,6 @@ func NewLocalBackend(logf logger.Logf, logID logid.PublicID, store ipn.StateStor
 	b.statusChanged = sync.NewCond(&b.statusLock)
 	b.e.SetStatusCallback(b.setWgengineStatus)
 
-	netMon := e.GetNetMon()
 	b.prevIfState = netMon.InterfaceState()
 	// Call our linkChange code once with the current state, and
 	// then also whenever it changes:
@@ -339,14 +342,9 @@ func NewLocalBackend(logf logger.Logf, logID logid.PublicID, store ipn.StateStor
 
 	b.unregisterHealthWatch = health.RegisterWatcher(b.onHealthChange)
 
-	wiredPeerAPIPort := false
-	if ig, ok := e.(wgengine.InternalsGetter); ok {
-		if tunWrap, _, _, ok := ig.GetInternals(); ok {
-			tunWrap.PeerAPIPort = b.GetPeerAPIPort
-			wiredPeerAPIPort = true
-		}
-	}
-	if !wiredPeerAPIPort {
+	if tunWrap, ok := b.sys.Tun.GetOK(); ok {
+		tunWrap.PeerAPIPort = b.GetPeerAPIPort
+	} else {
 		b.logf("[unexpected] failed to wire up PeerAPI port for engine %T", e)
 	}
 
@@ -464,6 +462,7 @@ func (b *LocalBackend) GetComponentDebugLogging(component string) time.Time {
 }
 
 // Dialer returns the backend's dialer.
+// It is always non-nil.
 func (b *LocalBackend) Dialer() *tsdial.Dialer {
 	return b.dialer
 }
@@ -644,7 +643,7 @@ func (b *LocalBackend) updateStatus(sb *ipnstate.StatusBuilder, extraLocked func
 	defer b.mu.Unlock()
 	sb.MutateStatus(func(s *ipnstate.Status) {
 		s.Version = version.Long()
-		s.TUN = !wgengine.IsNetstack(b.e)
+		s.TUN = !b.sys.IsNetstack()
 		s.BackendState = b.state.String()
 		s.AuthURL = b.authURLSticky
 		if err := health.OverallError(); err != nil {
@@ -1315,8 +1314,8 @@ func (b *LocalBackend) Start(opts ipn.Options) error {
 	hostinfo := hostinfo.New()
 	hostinfo.BackendLogID = b.backendLogID.String()
 	hostinfo.FrontendLogID = opts.FrontendLogID
-	hostinfo.Userspace.Set(wgengine.IsNetstack(b.e))
-	hostinfo.UserspaceRouter.Set(wgengine.IsNetstackRouter(b.e))
+	hostinfo.Userspace.Set(b.sys.IsNetstack())
+	hostinfo.UserspaceRouter.Set(b.sys.IsNetstackRouter())
 
 	if b.cc != nil {
 		// TODO(apenwarr): avoid the need to reinit controlclient.
@@ -1401,7 +1400,7 @@ func (b *LocalBackend) Start(opts ipn.Options) error {
 
 	var err error
 
-	isNetstack := wgengine.IsNetstackRouter(b.e)
+	isNetstack := b.sys.IsNetstackRouter()
 	debugFlags := controlDebugFlags
 	if isNetstack {
 		debugFlags = append([]string{"netstack"}, debugFlags...)
@@ -1423,7 +1422,7 @@ func (b *LocalBackend) Start(opts ipn.Options) error {
 		HTTPTestClient:       httpTestClient,
 		DiscoPublicKey:       discoPublic,
 		DebugFlags:           debugFlags,
-		NetMon:               b.e.GetNetMon(),
+		NetMon:               b.sys.NetMon.Get(),
 		Pinger:               b,
 		PopBrowserURL:        b.tellClientToBrowseToURL,
 		OnClientVersion:      b.onClientVersion,
@@ -3317,14 +3316,12 @@ func (b *LocalBackend) initPeerAPIListener() {
 		directFileMode:          b.directFileRoot != "",
 		directFileDoFinalRename: b.directFileDoFinalRename,
 	}
-	if re, ok := b.e.(wgengine.ResolvingEngine); ok {
-		if r, ok := re.GetResolver(); ok {
-			ps.resolver = r
-		}
+	if dm, ok := b.sys.DNSManager.GetOK(); ok {
+		ps.resolver = dm.Resolver()
 	}
 	b.peerAPIServer = ps
 
-	isNetstack := wgengine.IsNetstack(b.e)
+	isNetstack := b.sys.IsNetstack()
 	for i, a := range b.netMap.Addresses {
 		var ln net.Listener
 		var err error
@@ -4040,7 +4037,7 @@ func (b *LocalBackend) setTCPPortsInterceptedFromNetmapAndPrefsLocked(prefs ipn.
 		b.setServeProxyHandlersLocked()
 
 		// don't listen on netmap addresses if we're in userspace mode
-		if !wgengine.IsNetstack(b.e) {
+		if !b.sys.IsNetstack() {
 			b.updateServeTCPPortNetMapAddrListenersLocked(servePorts)
 		}
 	}
@@ -4391,7 +4388,7 @@ func nodeIP(n *tailcfg.Node, pred func(netip.Addr) bool) netip.Addr {
 }
 
 func (b *LocalBackend) CheckIPForwarding() error {
-	if wgengine.IsNetstackRouter(b.e) {
+	if b.sys.IsNetstackRouter() {
 		return nil
 	}
 
@@ -4537,13 +4534,9 @@ func (b *LocalBackend) DebugReSTUN() error {
 }
 
 func (b *LocalBackend) magicConn() (*magicsock.Conn, error) {
-	ig, ok := b.e.(wgengine.InternalsGetter)
-	if !ok {
-		return nil, errors.New("engine isn't InternalsGetter")
-	}
-	_, mc, _, ok := ig.GetInternals()
+	mc, ok := b.sys.MagicSock.GetOK()
 	if !ok {
-		return nil, errors.New("failed to get internals")
+		return nil, errors.New("failed to get magicsock from sys")
 	}
 	return mc, nil
 }
diff --git a/ipn/ipnlocal/local_test.go b/ipn/ipnlocal/local_test.go
index 2a474e46d..9fd44def8 100644
--- a/ipn/ipnlocal/local_test.go
+++ b/ipn/ipnlocal/local_test.go
@@ -20,6 +20,7 @@ import (
 	"tailscale.com/net/interfaces"
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/tailcfg"
+	"tailscale.com/tsd"
 	"tailscale.com/tstest"
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
@@ -501,13 +502,16 @@ func TestLazyMachineKeyGeneration(t *testing.T) {
 	tstest.Replace(t, &panicOnMachineKeyGeneration, func() bool { return true })
 
 	var logf logger.Logf = logger.Discard
+	sys := new(tsd.System)
 	store := new(mem.Store)
-	eng, err := wgengine.NewFakeUserspaceEngine(logf, 0)
+	sys.Set(store)
+	eng, err := wgengine.NewFakeUserspaceEngine(logf, sys.Set)
 	if err != nil {
 		t.Fatalf("NewFakeUserspaceEngine: %v", err)
 	}
 	t.Cleanup(eng.Close)
-	lb, err := NewLocalBackend(logf, logid.PublicID{}, store, nil, eng, 0)
+	sys.Set(eng)
+	lb, err := NewLocalBackend(logf, logid.PublicID{}, sys, 0)
 	if err != nil {
 		t.Fatalf("NewLocalBackend: %v", err)
 	}
@@ -765,13 +769,16 @@ func TestPacketFilterPermitsUnlockedNodes(t *testing.T) {
 func TestStatusWithoutPeers(t *testing.T) {
 	logf := tstest.WhileTestRunningLogger(t)
 	store := new(testStateStorage)
-	e, err := wgengine.NewFakeUserspaceEngine(logf, 0)
+	sys := new(tsd.System)
+	sys.Set(store)
+	e, err := wgengine.NewFakeUserspaceEngine(logf, sys.Set)
 	if err != nil {
 		t.Fatalf("NewFakeUserspaceEngine: %v", err)
 	}
+	sys.Set(e)
 	t.Cleanup(e.Close)
 
-	b, err := NewLocalBackend(logf, logid.PublicID{}, store, nil, e, 0)
+	b, err := NewLocalBackend(logf, logid.PublicID{}, sys, 0)
 	if err != nil {
 		t.Fatalf("NewLocalBackend: %v", err)
 	}
diff --git a/ipn/ipnlocal/loglines_test.go b/ipn/ipnlocal/loglines_test.go
index cde3f9198..361791858 100644
--- a/ipn/ipnlocal/loglines_test.go
+++ b/ipn/ipnlocal/loglines_test.go
@@ -12,6 +12,7 @@ import (
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/ipn/store/mem"
 	"tailscale.com/tailcfg"
+	"tailscale.com/tsd"
 	"tailscale.com/tstest"
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
@@ -47,14 +48,17 @@ func TestLocalLogLines(t *testing.T) {
 	idA := logid(0xaa)
 
 	// set up a LocalBackend, super bare bones. No functional data.
+	sys := new(tsd.System)
 	store := new(mem.Store)
-	e, err := wgengine.NewFakeUserspaceEngine(logf, 0)
+	sys.Set(store)
+	e, err := wgengine.NewFakeUserspaceEngine(logf, sys.Set)
 	if err != nil {
 		t.Fatal(err)
 	}
 	t.Cleanup(e.Close)
+	sys.Set(e)
 
-	lb, err := NewLocalBackend(logf, idA, store, nil, e, 0)
+	lb, err := NewLocalBackend(logf, idA, sys, 0)
 	if err != nil {
 		t.Fatal(err)
 	}
diff --git a/ipn/ipnlocal/peerapi.go b/ipn/ipnlocal/peerapi.go
index e8e06b6ee..e4e69727f 100644
--- a/ipn/ipnlocal/peerapi.go
+++ b/ipn/ipnlocal/peerapi.go
@@ -50,7 +50,6 @@ import (
 	"tailscale.com/util/clientmetric"
 	"tailscale.com/util/multierr"
 	"tailscale.com/version/distro"
-	"tailscale.com/wgengine"
 	"tailscale.com/wgengine/filter"
 )
 
@@ -469,7 +468,7 @@ func (s *peerAPIServer) listen(ip netip.Addr, ifState *interfaces.State) (ln net
 		}
 	}
 
-	if wgengine.IsNetstack(s.b.e) {
+	if s.b.sys.IsNetstack() {
 		ipStr = ""
 	}
 
@@ -1239,12 +1238,9 @@ func (h *peerAPIHandler) handleServeMagicsock(w http.ResponseWriter, r *http.Req
 		http.Error(w, "denied; no debug access", http.StatusForbidden)
 		return
 	}
-	eng := h.ps.b.e
-	if ig, ok := eng.(wgengine.InternalsGetter); ok {
-		if _, mc, _, ok := ig.GetInternals(); ok {
-			mc.ServeHTTPDebug(w, r)
-			return
-		}
+	if mc, ok := h.ps.b.sys.MagicSock.GetOK(); ok {
+		mc.ServeHTTPDebug(w, r)
+		return
 	}
 	http.Error(w, "miswired", 500)
 }
diff --git a/ipn/ipnlocal/serve.go b/ipn/ipnlocal/serve.go
index e6d1d0104..7bf4604dd 100644
--- a/ipn/ipnlocal/serve.go
+++ b/ipn/ipnlocal/serve.go
@@ -143,7 +143,7 @@ func (s *serveListener) Run() {
 }
 
 func (s *serveListener) shouldWarnAboutListenError(err error) bool {
-	if !s.b.e.GetNetMon().InterfaceState().HasIP(s.ap.Addr()) {
+	if !s.b.sys.NetMon.Get().InterfaceState().HasIP(s.ap.Addr()) {
 		// Machine likely doesn't have IPv6 enabled (or the IP is still being
 		// assigned). No need to warn. Notably, WSL2 (Issue 6303).
 		return false
diff --git a/ipn/ipnlocal/state_test.go b/ipn/ipnlocal/state_test.go
index 7a2c7132e..22e4ada82 100644
--- a/ipn/ipnlocal/state_test.go
+++ b/ipn/ipnlocal/state_test.go
@@ -17,6 +17,7 @@ import (
 	"tailscale.com/ipn"
 	"tailscale.com/ipn/store/mem"
 	"tailscale.com/tailcfg"
+	"tailscale.com/tsd"
 	"tailscale.com/tstest"
 	"tailscale.com/types/empty"
 	"tailscale.com/types/key"
@@ -297,14 +298,17 @@ func TestStateMachine(t *testing.T) {
 	c := qt.New(t)
 
 	logf := tstest.WhileTestRunningLogger(t)
+	sys := new(tsd.System)
 	store := new(testStateStorage)
-	e, err := wgengine.NewFakeUserspaceEngine(logf, 0)
+	sys.Set(store)
+	e, err := wgengine.NewFakeUserspaceEngine(logf, sys.Set)
 	if err != nil {
 		t.Fatalf("NewFakeUserspaceEngine: %v", err)
 	}
 	t.Cleanup(e.Close)
+	sys.Set(e)
 
-	b, err := NewLocalBackend(logf, logid.PublicID{}, store, nil, e, 0)
+	b, err := NewLocalBackend(logf, logid.PublicID{}, sys, 0)
 	if err != nil {
 		t.Fatalf("NewLocalBackend: %v", err)
 	}
@@ -941,13 +945,16 @@ func TestStateMachine(t *testing.T) {
 
 func TestEditPrefsHasNoKeys(t *testing.T) {
 	logf := tstest.WhileTestRunningLogger(t)
-	e, err := wgengine.NewFakeUserspaceEngine(logf, 0)
+	sys := new(tsd.System)
+	sys.Set(new(mem.Store))
+	e, err := wgengine.NewFakeUserspaceEngine(logf, sys.Set)
 	if err != nil {
 		t.Fatalf("NewFakeUserspaceEngine: %v", err)
 	}
 	t.Cleanup(e.Close)
+	sys.Set(e)
 
-	b, err := NewLocalBackend(logf, logid.PublicID{}, new(mem.Store), nil, e, 0)
+	b, err := NewLocalBackend(logf, logid.PublicID{}, sys, 0)
 	if err != nil {
 		t.Fatalf("NewLocalBackend: %v", err)
 	}
@@ -1023,10 +1030,14 @@ func TestWGEngineStatusRace(t *testing.T) {
 	t.Skip("test fails")
 	c := qt.New(t)
 	logf := tstest.WhileTestRunningLogger(t)
-	eng, err := wgengine.NewFakeUserspaceEngine(logf, 0)
+	sys := new(tsd.System)
+	sys.Set(new(mem.Store))
+
+	eng, err := wgengine.NewFakeUserspaceEngine(logf, sys.Set)
 	c.Assert(err, qt.IsNil)
 	t.Cleanup(eng.Close)
-	b, err := NewLocalBackend(logf, logid.PublicID{}, new(mem.Store), nil, eng, 0)
+	sys.Set(eng)
+	b, err := NewLocalBackend(logf, logid.PublicID{}, sys, 0)
 	c.Assert(err, qt.IsNil)
 
 	var cc *mockControl
diff --git a/ipn/ipnserver/server.go b/ipn/ipnserver/server.go
index 3abfa9364..705c01016 100644
--- a/ipn/ipnserver/server.go
+++ b/ipn/ipnserver/server.go
@@ -37,7 +37,7 @@ import (
 type Server struct {
 	lb           atomic.Pointer[ipnlocal.LocalBackend]
 	logf         logger.Logf
-	netMon       *netmon.Monitor // optional; nil means interfaces will be looked up on-demand
+	netMon       *netmon.Monitor // must be non-nil
 	backendLogID logid.PublicID
 	// resetOnZero is whether to call bs.Reset on transition from
 	// 1->0 active HTTP requests. That is, this is whether the backend is
@@ -410,14 +410,15 @@ func (s *Server) addActiveHTTPRequest(req *http.Request, ci *ipnauth.ConnIdentit
 }
 
 // New returns a new Server.
-// The netMon parameter is optional; if non-nil it's used to do faster interface
-// lookups.
 //
 // To start it, use the Server.Run method.
 //
 // At some point, either before or after Run, the Server's SetLocalBackend
 // method must also be called before Server can do anything useful.
 func New(logf logger.Logf, logID logid.PublicID, netMon *netmon.Monitor) *Server {
+	if netMon == nil {
+		panic("nil netMon")
+	}
 	return &Server{
 		backendLogID: logID,
 		logf:         logf,
diff --git a/net/tstun/fake.go b/net/tstun/fake.go
index 4bf29d093..d1ae467fa 100644
--- a/net/tstun/fake.go
+++ b/net/tstun/fake.go
@@ -47,8 +47,11 @@ func (t *fakeTUN) Write(b [][]byte, n int) (int, error) {
 	return 1, nil
 }
 
+// FakeTUNName is the name of the fake TUN device.
+const FakeTUNName = "FakeTUN"
+
 func (t *fakeTUN) Flush() error             { return nil }
 func (t *fakeTUN) MTU() (int, error)        { return 1500, nil }
-func (t *fakeTUN) Name() (string, error)    { return "FakeTUN", nil }
+func (t *fakeTUN) Name() (string, error)    { return FakeTUNName, nil }
 func (t *fakeTUN) Events() <-chan tun.Event { return t.evchan }
 func (t *fakeTUN) BatchSize() int           { return 1 }
diff --git a/ssh/tailssh/tailssh_test.go b/ssh/tailssh/tailssh_test.go
index c0935d24b..607b4eeaa 100644
--- a/ssh/tailssh/tailssh_test.go
+++ b/ssh/tailssh/tailssh_test.go
@@ -38,6 +38,7 @@ import (
 	"tailscale.com/net/tsdial"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tempfork/gliderlabs/ssh"
+	"tailscale.com/tsd"
 	"tailscale.com/tstest"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/logid"
@@ -815,14 +816,14 @@ func TestSSHAuthFlow(t *testing.T) {
 
 func TestSSH(t *testing.T) {
 	var logf logger.Logf = t.Logf
-	eng, err := wgengine.NewFakeUserspaceEngine(logf, 0)
+	sys := &tsd.System{}
+	eng, err := wgengine.NewFakeUserspaceEngine(logf, sys.Set)
 	if err != nil {
 		t.Fatal(err)
 	}
-	lb, err := ipnlocal.NewLocalBackend(logf, logid.PublicID{},
-		new(mem.Store),
-		new(tsdial.Dialer),
-		eng, 0)
+	sys.Set(eng)
+	sys.Set(new(mem.Store))
+	lb, err := ipnlocal.NewLocalBackend(logf, logid.PublicID{}, sys, 0)
 	if err != nil {
 		t.Fatal(err)
 	}
diff --git a/tsd/tsd.go b/tsd/tsd.go
new file mode 100644
index 000000000..ffa245bc9
--- /dev/null
+++ b/tsd/tsd.go
@@ -0,0 +1,135 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package tsd (short for "Tailscale Daemon") contains a System type that
+// containing all the subsystems a Tailscale node (tailscaled or platform
+// equivalent) uses.
+//
+// The goal of this package (as of 2023-05-03) is to eventually unify
+// initialization across tailscaled, tailscaled as a Windows services, the mac
+// GUI, tsnet, wasm, tests, and other places that wire up all the subsystems.
+// And doing so without weird optional interface accessors on some subsystems
+// that return other subsystems. It's all a work in progress.
+//
+// This package depends on nearly all parts of Tailscale, so it should not be
+// imported by (or thus passed to) any package that does not want to depend on
+// the world. In practice this means that only things like cmd/tailscaled,
+// ipn/ipnlocal, and ipn/ipnserver should import this package.
+package tsd
+
+import (
+	"fmt"
+	"reflect"
+
+	"tailscale.com/ipn"
+	"tailscale.com/net/dns"
+	"tailscale.com/net/netmon"
+	"tailscale.com/net/tsdial"
+	"tailscale.com/net/tstun"
+	"tailscale.com/wgengine"
+	"tailscale.com/wgengine/magicsock"
+	"tailscale.com/wgengine/router"
+)
+
+// System contains all the subsystems of a Tailscale node (tailscaled, etc.)
+type System struct {
+	Dialer         SubSystem[*tsdial.Dialer]
+	DNSManager     SubSystem[*dns.Manager] // can get its *resolver.Resolver from DNSManager.Resolver
+	Engine         SubSystem[wgengine.Engine]
+	NetMon         SubSystem[*netmon.Monitor]
+	MagicSock      SubSystem[*magicsock.Conn]
+	NetstackRouter SubSystem[bool] // using Netstack at all (either entirely or at least for subnets)
+	Router         SubSystem[router.Router]
+	Tun            SubSystem[*tstun.Wrapper]
+	StateStore     SubSystem[ipn.StateStore]
+}
+
+// Set is a convenience method to set a subsystem value.
+// It panics if the type is unknown or has that type
+// has already been set.
+func (s *System) Set(v any) {
+	switch v := v.(type) {
+	case *netmon.Monitor:
+		s.NetMon.Set(v)
+	case *dns.Manager:
+		s.DNSManager.Set(v)
+	case *tsdial.Dialer:
+		s.Dialer.Set(v)
+	case wgengine.Engine:
+		s.Engine.Set(v)
+	case router.Router:
+		s.Router.Set(v)
+	case *tstun.Wrapper:
+		s.Tun.Set(v)
+	case *magicsock.Conn:
+		s.MagicSock.Set(v)
+	case ipn.StateStore:
+		s.StateStore.Set(v)
+	default:
+		panic(fmt.Sprintf("unknown type %T", v))
+	}
+}
+
+// IsNetstackRouter reports whether Tailscale is either fully netstack based
+// (without TUN) or is at least using netstack for routing.
+func (s *System) IsNetstackRouter() bool {
+	if v, ok := s.NetstackRouter.GetOK(); ok && v {
+		return true
+	}
+	return s.IsNetstack()
+}
+
+// IsNetstack reports whether Tailscale is running as a netstack-based TUN-free engine.
+func (s *System) IsNetstack() bool {
+	name, _ := s.Tun.Get().Name()
+	return name == tstun.FakeTUNName
+}
+
+// SubSystem represents some subsystem of the Tailscale node daemon.
+//
+// A subsystem can be set to a value, and then later retrieved. A subsystem
+// value tracks whether it's been set and, once set, doesn't allow the value to
+// change.
+type SubSystem[T any] struct {
+	set bool
+	v   T
+}
+
+// Set sets p to v.
+//
+// It panics if p is already set to a different value.
+//
+// Set must not be called concurrently with other Sets or Gets.
+func (p *SubSystem[T]) Set(v T) {
+	if p.set {
+		var oldVal any = p.v
+		var newVal any = v
+		if oldVal == newVal {
+			// Allow setting to the same value.
+			// Note we had to box them through "any" to force them to be comparable.
+			// We can't set the type constraint T to be "comparable" because the interfaces
+			// aren't comparable. (See https://github.com/golang/go/issues/52531 and
+			// https://github.com/golang/go/issues/52614 for some background)
+			return
+		}
+
+		var z *T
+		panic(fmt.Sprintf("%v is already set", reflect.TypeOf(z).Elem().String()))
+	}
+	p.v = v
+	p.set = true
+}
+
+// Get returns the value of p, panicking if it hasn't been set.
+func (p *SubSystem[T]) Get() T {
+	if !p.set {
+		var z *T
+		panic(fmt.Sprintf("%v is not set", reflect.TypeOf(z).Elem().String()))
+	}
+	return p.v
+}
+
+// GetOK returns the value of p (if any) and whether it's been set.
+func (p *SubSystem[T]) GetOK() (_ T, ok bool) {
+	return p.v, p.set
+}
diff --git a/tsnet/tsnet.go b/tsnet/tsnet.go
index 2bea9026b..1b0ae7bb6 100644
--- a/tsnet/tsnet.go
+++ b/tsnet/tsnet.go
@@ -47,6 +47,7 @@ import (
 	"tailscale.com/net/socks5"
 	"tailscale.com/net/tsdial"
 	"tailscale.com/smallzstd"
+	"tailscale.com/tsd"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/logid"
 	"tailscale.com/types/nettype"
@@ -482,23 +483,21 @@ func (s *Server) start() (reterr error) {
 	}
 	closePool.add(s.netMon)
 
+	sys := new(tsd.System)
 	s.dialer = &tsdial.Dialer{Logf: logf} // mutated below (before used)
 	eng, err := wgengine.NewUserspaceEngine(logf, wgengine.Config{
-		ListenPort: 0,
-		NetMon:     s.netMon,
-		Dialer:     s.dialer,
+		ListenPort:   0,
+		NetMon:       s.netMon,
+		Dialer:       s.dialer,
+		SetSubsystem: sys.Set,
 	})
 	if err != nil {
 		return err
 	}
 	closePool.add(s.dialer)
+	sys.Set(eng)
 
-	tunDev, magicConn, dns, ok := eng.(wgengine.InternalsGetter).GetInternals()
-	if !ok {
-		return fmt.Errorf("%T is not a wgengine.InternalsGetter", eng)
-	}
-
-	ns, err := netstack.Create(logf, tunDev, eng, magicConn, s.dialer, dns)
+	ns, err := netstack.Create(logf, sys.Tun.Get(), eng, sys.MagicSock.Get(), s.dialer, sys.DNSManager.Get())
 	if err != nil {
 		return fmt.Errorf("netstack.Create: %w", err)
 	}
@@ -522,12 +521,13 @@ func (s *Server) start() (reterr error) {
 			return err
 		}
 	}
+	sys.Set(s.Store)
 
 	loginFlags := controlclient.LoginDefault
 	if s.Ephemeral {
 		loginFlags = controlclient.LoginEphemeral
 	}
-	lb, err := ipnlocal.NewLocalBackend(logf, s.logid, s.Store, s.dialer, eng, loginFlags)
+	lb, err := ipnlocal.NewLocalBackend(logf, s.logid, sys, loginFlags)
 	if err != nil {
 		return fmt.Errorf("NewLocalBackend: %v", err)
 	}
diff --git a/tstest/integration/tailscaled_deps_test_darwin.go b/tstest/integration/tailscaled_deps_test_darwin.go
index ed2e6e5ff..75ef83af0 100644
--- a/tstest/integration/tailscaled_deps_test_darwin.go
+++ b/tstest/integration/tailscaled_deps_test_darwin.go
@@ -37,6 +37,7 @@ import (
 	_ "tailscale.com/ssh/tailssh"
 	_ "tailscale.com/syncs"
 	_ "tailscale.com/tailcfg"
+	_ "tailscale.com/tsd"
 	_ "tailscale.com/tsweb/varz"
 	_ "tailscale.com/types/flagtype"
 	_ "tailscale.com/types/key"
diff --git a/tstest/integration/tailscaled_deps_test_freebsd.go b/tstest/integration/tailscaled_deps_test_freebsd.go
index ed2e6e5ff..75ef83af0 100644
--- a/tstest/integration/tailscaled_deps_test_freebsd.go
+++ b/tstest/integration/tailscaled_deps_test_freebsd.go
@@ -37,6 +37,7 @@ import (
 	_ "tailscale.com/ssh/tailssh"
 	_ "tailscale.com/syncs"
 	_ "tailscale.com/tailcfg"
+	_ "tailscale.com/tsd"
 	_ "tailscale.com/tsweb/varz"
 	_ "tailscale.com/types/flagtype"
 	_ "tailscale.com/types/key"
diff --git a/tstest/integration/tailscaled_deps_test_linux.go b/tstest/integration/tailscaled_deps_test_linux.go
index ed2e6e5ff..75ef83af0 100644
--- a/tstest/integration/tailscaled_deps_test_linux.go
+++ b/tstest/integration/tailscaled_deps_test_linux.go
@@ -37,6 +37,7 @@ import (
 	_ "tailscale.com/ssh/tailssh"
 	_ "tailscale.com/syncs"
 	_ "tailscale.com/tailcfg"
+	_ "tailscale.com/tsd"
 	_ "tailscale.com/tsweb/varz"
 	_ "tailscale.com/types/flagtype"
 	_ "tailscale.com/types/key"
diff --git a/tstest/integration/tailscaled_deps_test_openbsd.go b/tstest/integration/tailscaled_deps_test_openbsd.go
index ed2e6e5ff..75ef83af0 100644
--- a/tstest/integration/tailscaled_deps_test_openbsd.go
+++ b/tstest/integration/tailscaled_deps_test_openbsd.go
@@ -37,6 +37,7 @@ import (
 	_ "tailscale.com/ssh/tailssh"
 	_ "tailscale.com/syncs"
 	_ "tailscale.com/tailcfg"
+	_ "tailscale.com/tsd"
 	_ "tailscale.com/tsweb/varz"
 	_ "tailscale.com/types/flagtype"
 	_ "tailscale.com/types/key"
diff --git a/tstest/integration/tailscaled_deps_test_windows.go b/tstest/integration/tailscaled_deps_test_windows.go
index 33620c9d9..6eb00eff4 100644
--- a/tstest/integration/tailscaled_deps_test_windows.go
+++ b/tstest/integration/tailscaled_deps_test_windows.go
@@ -44,6 +44,7 @@ import (
 	_ "tailscale.com/smallzstd"
 	_ "tailscale.com/syncs"
 	_ "tailscale.com/tailcfg"
+	_ "tailscale.com/tsd"
 	_ "tailscale.com/tsweb/varz"
 	_ "tailscale.com/types/flagtype"
 	_ "tailscale.com/types/key"
diff --git a/wgengine/netstack/netstack_test.go b/wgengine/netstack/netstack_test.go
index 7a30fd84a..f08308e2d 100644
--- a/wgengine/netstack/netstack_test.go
+++ b/wgengine/netstack/netstack_test.go
@@ -16,6 +16,7 @@ import (
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/net/tsdial"
 	"tailscale.com/net/tstun"
+	"tailscale.com/tsd"
 	"tailscale.com/tstest"
 	"tailscale.com/types/ipproto"
 	"tailscale.com/types/logid"
@@ -33,29 +34,26 @@ func TestInjectInboundLeak(t *testing.T) {
 			t.Logf(format, args...)
 		}
 	}
+	sys := new(tsd.System)
 	eng, err := wgengine.NewUserspaceEngine(logf, wgengine.Config{
-		Tun:    tunDev,
-		Dialer: dialer,
+		Tun:          tunDev,
+		Dialer:       dialer,
+		SetSubsystem: sys.Set,
 	})
 	if err != nil {
 		t.Fatal(err)
 	}
 	defer eng.Close()
-	ig, ok := eng.(wgengine.InternalsGetter)
-	if !ok {
-		t.Fatal("not an InternalsGetter")
-	}
-	tunWrap, magicSock, dns, ok := ig.GetInternals()
-	if !ok {
-		t.Fatal("failed to get internals")
-	}
+	sys.Set(eng)
+	sys.Set(new(mem.Store))
 
-	lb, err := ipnlocal.NewLocalBackend(logf, logid.PublicID{}, new(mem.Store), dialer, eng, 0)
+	tunWrap := sys.Tun.Get()
+	lb, err := ipnlocal.NewLocalBackend(logf, logid.PublicID{}, sys, 0)
 	if err != nil {
 		t.Fatal(err)
 	}
 
-	ns, err := Create(logf, tunWrap, eng, magicSock, dialer, dns)
+	ns, err := Create(logf, tunWrap, eng, sys.MagicSock.Get(), dialer, sys.DNSManager.Get())
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -89,32 +87,28 @@ func getMemStats() (ms runtime.MemStats) {
 
 func makeNetstack(t *testing.T, config func(*Impl)) *Impl {
 	tunDev := tstun.NewFake()
+	sys := &tsd.System{}
+	sys.Set(new(mem.Store))
 	dialer := new(tsdial.Dialer)
 	logf := tstest.WhileTestRunningLogger(t)
 	eng, err := wgengine.NewUserspaceEngine(logf, wgengine.Config{
-		Tun:    tunDev,
-		Dialer: dialer,
+		Tun:          tunDev,
+		Dialer:       dialer,
+		SetSubsystem: sys.Set,
 	})
 	if err != nil {
 		t.Fatal(err)
 	}
 	t.Cleanup(func() { eng.Close() })
-	ig, ok := eng.(wgengine.InternalsGetter)
-	if !ok {
-		t.Fatal("not an InternalsGetter")
-	}
-	tunWrap, magicSock, dns, ok := ig.GetInternals()
-	if !ok {
-		t.Fatal("failed to get internals")
-	}
+	sys.Set(eng)
 
-	ns, err := Create(logf, tunWrap, eng, magicSock, dialer, dns)
+	ns, err := Create(logf, sys.Tun.Get(), eng, sys.MagicSock.Get(), dialer, sys.DNSManager.Get())
 	if err != nil {
 		t.Fatal(err)
 	}
 	t.Cleanup(func() { ns.Close() })
 
-	lb, err := ipnlocal.NewLocalBackend(logf, logid.PublicID{}, new(mem.Store), dialer, eng, 0)
+	lb, err := ipnlocal.NewLocalBackend(logf, logid.PublicID{}, sys, 0)
 	if err != nil {
 		t.Fatalf("NewLocalBackend: %v", err)
 	}
diff --git a/wgengine/netstack/subnet_router_wrapper.go b/wgengine/netstack/subnet_router_wrapper.go
index dd9c96ec6..97eb6406f 100644
--- a/wgengine/netstack/subnet_router_wrapper.go
+++ b/wgengine/netstack/subnet_router_wrapper.go
@@ -4,16 +4,9 @@
 package netstack
 
 import (
-	"reflect"
-
-	"tailscale.com/wgengine"
 	"tailscale.com/wgengine/router"
 )
 
-func init() {
-	wgengine.NetstackRouterType = reflect.TypeOf(&subnetRouter{})
-}
-
 type subnetRouter struct {
 	router.Router
 }
diff --git a/wgengine/userspace.go b/wgengine/userspace.go
index 1356b36df..e29e20dae 100644
--- a/wgengine/userspace.go
+++ b/wgengine/userspace.go
@@ -10,8 +10,8 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"math"
 	"net/netip"
-	"reflect"
 	"runtime"
 	"strings"
 	"sync"
@@ -25,7 +25,6 @@ import (
 	"tailscale.com/health"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/net/dns"
-	"tailscale.com/net/dns/resolver"
 	"tailscale.com/net/flowtrack"
 	"tailscale.com/net/interfaces"
 	"tailscale.com/net/netmon"
@@ -150,29 +149,6 @@ type userspaceEngine struct {
 	// Lock ordering: magicsock.Conn.mu, wgLock, then mu.
 }
 
-// InternalsGetter is implemented by Engines that can export their internals.
-type InternalsGetter interface {
-	GetInternals() (_ *tstun.Wrapper, _ *magicsock.Conn, _ *dns.Manager, ok bool)
-}
-
-func (e *userspaceEngine) GetInternals() (_ *tstun.Wrapper, _ *magicsock.Conn, _ *dns.Manager, ok bool) {
-	return e.tundev, e.magicConn, e.dns, true
-}
-
-// ResolvingEngine is implemented by Engines that have DNS resolvers.
-type ResolvingEngine interface {
-	GetResolver() (_ *resolver.Resolver, ok bool)
-}
-
-var (
-	_ ResolvingEngine = (*userspaceEngine)(nil)
-	_ ResolvingEngine = (*watchdogEngine)(nil)
-)
-
-func (e *userspaceEngine) GetResolver() (r *resolver.Resolver, ok bool) {
-	return e.dns.Resolver(), true
-}
-
 // BIRDClient handles communication with the BIRD Internet Routing Daemon.
 type BIRDClient interface {
 	EnableProtocol(proto string) error
@@ -219,47 +195,37 @@ type Config struct {
 	// BIRDClient, if non-nil, will be used to configure BIRD whenever
 	// this node is a primary subnet router.
 	BIRDClient BIRDClient
-}
-
-func NewFakeUserspaceEngine(logf logger.Logf, listenPort uint16) (Engine, error) {
-	logf("Starting userspace WireGuard engine (with fake TUN device)")
-	return NewUserspaceEngine(logf, Config{
-		ListenPort:    listenPort,
-		RespondToPing: true,
-	})
-}
 
-// NetstackRouterType is a gross cross-package init-time registration
-// from netstack to here, informing this package of netstack's router
-// type.
-var NetstackRouterType reflect.Type
-
-// IsNetstackRouter reports whether e is either fully netstack based
-// (without TUN) or is at least using netstack for routing.
-func IsNetstackRouter(e Engine) bool {
-	switch e := e.(type) {
-	case *userspaceEngine:
-		if reflect.TypeOf(e.router) == NetstackRouterType {
-			return true
-		}
-	case *watchdogEngine:
-		return IsNetstackRouter(e.wrap)
-	}
-	return IsNetstack(e)
+	// SetSubsystem, if non-nil, is called for each new subsystem created, just before a successful return.
+	SetSubsystem func(any)
 }
 
-// IsNetstack reports whether e is a netstack-based TUN-free engine.
-func IsNetstack(e Engine) bool {
-	ig, ok := e.(InternalsGetter)
-	if !ok {
-		return false
+// NewFakeUserspaceEngine returns a new userspace engine for testing.
+//
+// The opts may contain the following types:
+//
+//   - int or uint16: to set the ListenPort.
+func NewFakeUserspaceEngine(logf logger.Logf, opts ...any) (Engine, error) {
+	conf := Config{
+		RespondToPing: true,
 	}
-	tw, _, _, ok := ig.GetInternals()
-	if !ok {
-		return false
+	for _, o := range opts {
+		switch v := o.(type) {
+		case uint16:
+			conf.ListenPort = v
+		case int:
+			if v < 0 || v > math.MaxUint16 {
+				return nil, fmt.Errorf("invalid ListenPort: %d", v)
+			}
+			conf.ListenPort = uint16(v)
+		case func(any):
+			conf.SetSubsystem = v
+		default:
+			return nil, fmt.Errorf("unknown option type %T", v)
+		}
 	}
-	name, err := tw.Name()
-	return err == nil && name == "FakeTUN"
+	logf("Starting userspace WireGuard engine (with fake TUN device)")
+	return NewUserspaceEngine(logf, conf)
 }
 
 // NewUserspaceEngine creates the named tun device and returns a
@@ -458,6 +424,15 @@ func NewUserspaceEngine(logf logger.Logf, conf Config) (_ Engine, reterr error)
 	e.logf("Starting network monitor...")
 	e.netMon.Start()
 
+	if conf.SetSubsystem != nil {
+		conf.SetSubsystem(e.tundev)
+		conf.SetSubsystem(e.magicConn)
+		conf.SetSubsystem(e.dns)
+		conf.SetSubsystem(conf.Router)
+		conf.SetSubsystem(conf.Dialer)
+		conf.SetSubsystem(e.netMon)
+	}
+
 	e.logf("Engine created.")
 	return e, nil
 }
@@ -1119,10 +1094,6 @@ func (e *userspaceEngine) Wait() {
 	<-e.waitCh
 }
 
-func (e *userspaceEngine) GetNetMon() *netmon.Monitor {
-	return e.netMon
-}
-
 // LinkChange signals a network change event. It's currently
 // (2021-03-03) only called on Android. On other platforms, netMon
 // generates link change events for us.
diff --git a/wgengine/userspace_ext_test.go b/wgengine/userspace_ext_test.go
index b2235c9c9..67b337051 100644
--- a/wgengine/userspace_ext_test.go
+++ b/wgengine/userspace_ext_test.go
@@ -8,6 +8,7 @@ import (
 
 	"github.com/tailscale/wireguard-go/tun"
 	"tailscale.com/net/tstun"
+	"tailscale.com/tsd"
 	"tailscale.com/types/logger"
 	"tailscale.com/wgengine"
 	"tailscale.com/wgengine/netstack"
@@ -15,21 +16,23 @@ import (
 )
 
 func TestIsNetstack(t *testing.T) {
-	e, err := wgengine.NewUserspaceEngine(t.Logf, wgengine.Config{})
+	sys := new(tsd.System)
+	e, err := wgengine.NewUserspaceEngine(t.Logf, wgengine.Config{SetSubsystem: sys.Set})
 	if err != nil {
 		t.Fatal(err)
 	}
 	defer e.Close()
-	if !wgengine.IsNetstack(e) {
+	if !sys.IsNetstack() {
 		t.Errorf("IsNetstack = false; want true")
 	}
 }
 
 func TestIsNetstackRouter(t *testing.T) {
 	tests := []struct {
-		name string
-		conf wgengine.Config
-		want bool
+		name              string
+		conf              wgengine.Config
+		setNetstackRouter bool
+		want              bool
 	}{
 		{
 			name: "no_netstack",
@@ -50,23 +53,26 @@ func TestIsNetstackRouter(t *testing.T) {
 				Tun:    newFakeOSTUN(),
 				Router: netstack.NewSubnetRouterWrapper(newFakeOSRouter()),
 			},
-			want: true,
+			setNetstackRouter: true,
+			want:              true,
 		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			e, err := wgengine.NewUserspaceEngine(logger.Discard, tt.conf)
+			sys := &tsd.System{}
+			if tt.setNetstackRouter {
+				sys.NetstackRouter.Set(true)
+			}
+			conf := tt.conf
+			conf.SetSubsystem = sys.Set
+			e, err := wgengine.NewUserspaceEngine(logger.Discard, conf)
 			if err != nil {
 				t.Fatal(err)
 			}
 			defer e.Close()
-			if got := wgengine.IsNetstackRouter(e); got != tt.want {
+			if got := sys.IsNetstackRouter(); got != tt.want {
 				t.Errorf("IsNetstackRouter = %v; want %v", got, tt.want)
 			}
-
-			if got := wgengine.IsNetstackRouter(wgengine.NewWatchdog(e)); got != tt.want {
-				t.Errorf("IsNetstackRouter(watchdog-wrapped) = %v; want %v", got, tt.want)
-			}
 		})
 	}
 }
diff --git a/wgengine/watchdog.go b/wgengine/watchdog.go
index 76abfa8cc..19505be89 100644
--- a/wgengine/watchdog.go
+++ b/wgengine/watchdog.go
@@ -17,15 +17,11 @@ import (
 	"tailscale.com/envknob"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/net/dns"
-	"tailscale.com/net/dns/resolver"
-	"tailscale.com/net/netmon"
-	"tailscale.com/net/tstun"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
 	"tailscale.com/types/netmap"
 	"tailscale.com/wgengine/capture"
 	"tailscale.com/wgengine/filter"
-	"tailscale.com/wgengine/magicsock"
 	"tailscale.com/wgengine/router"
 	"tailscale.com/wgengine/wgcfg"
 )
@@ -126,9 +122,6 @@ func (e *watchdogEngine) watchdog(name string, fn func()) {
 func (e *watchdogEngine) Reconfig(cfg *wgcfg.Config, routerCfg *router.Config, dnsCfg *dns.Config, debug *tailcfg.Debug) error {
 	return e.watchdogErr("Reconfig", func() error { return e.wrap.Reconfig(cfg, routerCfg, dnsCfg, debug) })
 }
-func (e *watchdogEngine) GetNetMon() *netmon.Monitor {
-	return e.wrap.GetNetMon()
-}
 func (e *watchdogEngine) GetFilter() *filter.Filter {
 	return e.wrap.GetFilter()
 }
@@ -181,18 +174,6 @@ func (e *watchdogEngine) WhoIsIPPort(ipp netip.AddrPort) (tsIP netip.Addr, ok bo
 func (e *watchdogEngine) Close() {
 	e.watchdog("Close", e.wrap.Close)
 }
-func (e *watchdogEngine) GetInternals() (tw *tstun.Wrapper, c *magicsock.Conn, d *dns.Manager, ok bool) {
-	if ig, ok := e.wrap.(InternalsGetter); ok {
-		return ig.GetInternals()
-	}
-	return
-}
-func (e *watchdogEngine) GetResolver() (r *resolver.Resolver, ok bool) {
-	if re, ok := e.wrap.(ResolvingEngine); ok {
-		return re.GetResolver()
-	}
-	return nil, false
-}
 func (e *watchdogEngine) PeerForIP(ip netip.Addr) (ret PeerForIP, ok bool) {
 	e.watchdog("PeerForIP", func() { ret, ok = e.wrap.PeerForIP(ip) })
 	return ret, ok
diff --git a/wgengine/wgengine.go b/wgengine/wgengine.go
index 4d4e240c1..df591c9e0 100644
--- a/wgengine/wgengine.go
+++ b/wgengine/wgengine.go
@@ -10,7 +10,6 @@ import (
 
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/net/dns"
-	"tailscale.com/net/netmon"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
 	"tailscale.com/types/netmap"
@@ -92,9 +91,6 @@ type Engine interface {
 	// WireGuard status changes.
 	SetStatusCallback(StatusCallback)
 
-	// GetNetMon returns the network monitor.
-	GetNetMon() *netmon.Monitor
-
 	// RequestStatus requests a WireGuard status update right
 	// away, sent to the callback registered via SetStatusCallback.
 	RequestStatus()