ipn, cmd/tailscale/cli: add LocalAPI IPN bus watch, Start, convert CLI

Updates #6417
Updates tailscale/corp#8051

Change-Id: I1ca360730c45ffaa0261d8422877304277fc5625
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

client/tailscale/localclient.go

@@ -555,6 +555,20 @@ func (lc *LocalClient) EditPrefs(ctx context.Context, mp *ipn.MaskedPrefs) (*ipn
 	return decodeJSON[*ipn.Prefs](body)
 }
 
+// StartLoginInteractive starts an interactive login.
+func (lc *LocalClient) StartLoginInteractive(ctx context.Context) error {
+	_, err := lc.send(ctx, "POST", "/localapi/v0/login-interactive", http.StatusNoContent, nil)
+	return err
+}
+
+// Start applies the configuration specified in opts, and starts the
+// state machine.
+func (lc *LocalClient) Start(ctx context.Context, opts ipn.Options) error {
+	_, err := lc.send(ctx, "POST", "/localapi/v0/start", http.StatusNoContent, jsonBody(opts))
+	return err
+}
+
+// Logout logs out the current node.
 func (lc *LocalClient) Logout(ctx context.Context) error {
 	_, err := lc.send(ctx, "POST", "/localapi/v0/logout", http.StatusNoContent, nil)
 	return err
@@ -966,3 +980,78 @@ func (lc *LocalClient) DeleteProfile(ctx context.Context, profile ipn.ProfileID)
 	_, err := lc.send(ctx, "DELETE", "/localapi/v0/profiles"+url.PathEscape(string(profile)), http.StatusNoContent, nil)
 	return err
 }
+
+// WatchIPNMask are filtering options for LocalClient.WatchIPNBus.
+//
+// The zero value is a valid WatchOpt that means to watch everything.
+//
+// TODO(bradfitz): flesh out.
+type WatchIPNMask uint64
+
+// WatchIPNBus subscribes to the IPN notification bus. It returns a watcher
+// once the bus is connected successfully.
+//
+// The context is used for the life of the watch, not just the call to
+// WatchIPNBus.
+//
+// The returned IPNBusWatcher's Close method must be called when done to release
+// resources.
+func (lc *LocalClient) WatchIPNBus(ctx context.Context, mask WatchIPNMask) (*IPNBusWatcher, error) {
+	req, err := http.NewRequestWithContext(ctx, "GET",
+		"http://"+apitype.LocalAPIHost+"/localapi/v0/watch-ipn-bus?mask="+fmt.Sprint(mask),
+		nil)
+	if err != nil {
+		return nil, err
+	}
+	res, err := lc.doLocalRequestNiceError(req)
+	if err != nil {
+		return nil, err
+	}
+	if res.StatusCode != 200 {
+		res.Body.Close()
+		return nil, errors.New(res.Status)
+	}
+	dec := json.NewDecoder(res.Body)
+	return &IPNBusWatcher{
+		ctx:     ctx,
+		httpRes: res,
+		dec:     dec,
+	}, nil
+}
+
+// IPNBusWatcher is an active subscription (watch) of the local tailscaled IPN bus.
+// It's returned by LocalClient.WatchIPNBus.
+//
+// It must be closed when done.
+type IPNBusWatcher struct {
+	ctx     context.Context // from original WatchIPNBus call
+	httpRes *http.Response
+	dec     *json.Decoder
+
+	mu     sync.Mutex
+	closed bool
+}
+
+// Close stops the watcher and releases its resources.
+func (w *IPNBusWatcher) Close() error {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+	if w.closed {
+		return nil
+	}
+	w.closed = true
+	return w.httpRes.Body.Close()
+}
+
+// Next returns the next ipn.Notify from the stream.
+// If the context from LocalClient.WatchIPNBus is done, that error is returned.
+func (w *IPNBusWatcher) Next() (ipn.Notify, error) {
+	var n ipn.Notify
+	if err := w.dec.Decode(&n); err != nil {
+		if cerr := w.ctx.Err(); cerr != nil {
+			err = cerr
+		}
+		return ipn.Notify{}, err
+	}
+	return n, nil
+}

cmd/tailscale/cli/cli.go

@@ -13,23 +13,18 @@ import (
 	"fmt"
 	"io"
 	"log"
-	"net"
 	"os"
-	"os/signal"
 	"runtime"
 	"strconv"
 	"strings"
 	"sync"
-	"syscall"
 	"text/tabwriter"
 
 	"github.com/peterbourgon/ff/v3/ffcli"
 	"golang.org/x/exp/slices"
 	"tailscale.com/client/tailscale"
 	"tailscale.com/envknob"
-	"tailscale.com/ipn"
 	"tailscale.com/paths"
-	"tailscale.com/safesocket"
 	"tailscale.com/version/distro"
 )
 
@@ -248,58 +243,6 @@ var rootArgs struct {
 	socket string
 }
 
-func connect(ctx context.Context) (net.Conn, *ipn.BackendClient, context.Context, context.CancelFunc) {
-	s := safesocket.DefaultConnectionStrategy(rootArgs.socket)
-	c, err := safesocket.Connect(s)
-	if err != nil {
-		if runtime.GOOS != "windows" && rootArgs.socket == "" {
-			fatalf("--socket cannot be empty")
-		}
-		fatalf("Failed to connect to tailscaled. (safesocket.Connect: %v)\n", err)
-	}
-	clientToServer := func(b []byte) {
-		ipn.WriteMsg(c, b)
-	}
-
-	ctx, cancel := context.WithCancel(ctx)
-
-	go func() {
-		interrupt := make(chan os.Signal, 1)
-		signal.Notify(interrupt, syscall.SIGINT, syscall.SIGTERM)
-		select {
-		case <-interrupt:
-		case <-ctx.Done():
-			// Context canceled elsewhere.
-			signal.Reset(syscall.SIGINT, syscall.SIGTERM)
-			return
-		}
-		c.Close()
-		cancel()
-	}()
-
-	bc := ipn.NewBackendClient(log.Printf, clientToServer)
-	return c, bc, ctx, cancel
-}
-
-// pump receives backend messages on conn and pushes them into bc.
-func pump(ctx context.Context, bc *ipn.BackendClient, conn net.Conn) error {
-	defer conn.Close()
-	for ctx.Err() == nil {
-		msg, err := ipn.ReadMsg(conn)
-		if err != nil {
-			if ctx.Err() != nil {
-				return ctx.Err()
-			}
-			if errors.Is(err, io.EOF) || errors.Is(err, net.ErrClosed) {
-				return fmt.Errorf("%w (tailscaled stopped running?)", err)
-			}
-			return err
-		}
-		bc.GotNotifyMsg(msg)
-	}
-	return ctx.Err()
-}
-
 // usageFuncNoDefaultValues is like usageFunc but doesn't print default values.
 func usageFuncNoDefaultValues(c *ffcli.Command) string {
 	return usageFuncOpt(c, false)

cmd/tailscale/cli/debug.go

@@ -285,19 +285,23 @@ var watchIPNArgs struct {
 }
 
 func runWatchIPN(ctx context.Context, args []string) error {
-	c, bc, ctx, cancel := connect(ctx)
+	watcher, err := localClient.WatchIPNBus(ctx, 0)
-	defer cancel()
+	if err != nil {
-
+		return err
-	bc.SetNotifyCallback(func(n ipn.Notify) {
+	}
+	defer watcher.Close()
+	printf("Connected.\n")
+	for {
+		n, err := watcher.Next()
+		if err != nil {
+			return err
+		}
 		if !watchIPNArgs.netmap {
 			n.NetMap = nil
 		}
 		j, _ := json.MarshalIndent(n, "", "\t")
 		printf("%s\n", j)
-	})
+	}
-	bc.RequestEngineStatus()
-	pump(ctx, bc, c)
-	return errors.New("exit")
 }
 
 func runDERPMap(ctx context.Context, args []string) error {

cmd/tailscale/cli/up.go

@@ -15,11 +15,13 @@ import (
 	"log"
 	"net/netip"
 	"os"
+	"os/signal"
 	"reflect"
 	"runtime"
 	"sort"
 	"strings"
 	"sync"
+	"syscall"
 	"time"
 
 	shellquote "github.com/kballard/go-shellquote"
@@ -535,109 +537,109 @@ func runUp(ctx context.Context, cmd string, args []string, upArgs upArgsT) (retE
 		return err
 	}
 
-	// At this point we need to subscribe to the IPN bus to watch
+	watchCtx, cancelWatch := context.WithCancel(ctx)
-	// for state transitions and possible need to authenticate.
+	defer cancelWatch()
-	c, bc, pumpCtx, cancel := connect(ctx)
+	watcher, err := localClient.WatchIPNBus(watchCtx, 0)
-	defer cancel()
+	if err != nil {
+		return err
+	}
+	defer watcher.Close()
+
+	go func() {
+		interrupt := make(chan os.Signal, 1)
+		signal.Notify(interrupt, syscall.SIGINT, syscall.SIGTERM)
+		select {
+		case <-interrupt:
+			cancelWatch()
+		case <-watchCtx.Done():
+		}
+	}()
 
-	running := make(chan bool, 1)         // gets value once in state ipn.Running
+	running := make(chan bool, 1) // gets value once in state ipn.Running
-	gotEngineUpdate := make(chan bool, 1) // gets value upon an engine update
 	pumpErr := make(chan error, 1)
-	go func() { pumpErr <- pump(pumpCtx, bc, c) }()
 
 	var printed bool // whether we've yet printed anything to stdout or stderr
 	var loginOnce sync.Once
-	startLoginInteractive := func() { loginOnce.Do(func() { bc.StartLoginInteractive() }) }
+	startLoginInteractive := func() { loginOnce.Do(func() { localClient.StartLoginInteractive(ctx) }) }
 
-	bc.SetNotifyCallback(func(n ipn.Notify) {
+	go func() {
-		if n.Engine != nil {
+		for {
-			select {
+			n, err := watcher.Next()
-			case gotEngineUpdate <- true:
+			if err != nil {
-			default:
+				pumpErr <- err
+				return
 			}
-		}
+			if n.ErrMessage != nil {
-		if n.ErrMessage != nil {
+				msg := *n.ErrMessage
-			msg := *n.ErrMessage
+				if msg == ipn.ErrMsgPermissionDenied {
-			if msg == ipn.ErrMsgPermissionDenied {
+					switch effectiveGOOS() {
-				switch effectiveGOOS() {
+					case "windows":
-				case "windows":
+						msg += " (Tailscale service in use by other user?)"
-					msg += " (Tailscale service in use by other user?)"
+					default:
-				default:
+						msg += " (try 'sudo tailscale up [...]')"
-					msg += " (try 'sudo tailscale up [...]')"
+					}
 				}
+				fatalf("backend error: %v\n", msg)
 			}
-			fatalf("backend error: %v\n", msg)
+			if s := n.State; s != nil {
-		}
+				switch *s {
-		if s := n.State; s != nil {
+				case ipn.NeedsLogin:
-			switch *s {
+					startLoginInteractive()
-			case ipn.NeedsLogin:
+				case ipn.NeedsMachineAuth:
-				startLoginInteractive()
+					printed = true
-			case ipn.NeedsMachineAuth:
+					if env.upArgs.json {
-				printed = true
+						printUpDoneJSON(ipn.NeedsMachineAuth, "")
-				if env.upArgs.json {
+					} else {
-					printUpDoneJSON(ipn.NeedsMachineAuth, "")
+						fmt.Fprintf(Stderr, "\nTo authorize your machine, visit (as admin):\n\n\t%s\n\n", prefs.AdminPageURL())
-				} else {
+					}
-					fmt.Fprintf(Stderr, "\nTo authorize your machine, visit (as admin):\n\n\t%s\n\n", prefs.AdminPageURL())
+				case ipn.Running:
-				}
+					// Done full authentication process
-			case ipn.Running:
+					if env.upArgs.json {
-				// Done full authentication process
+						printUpDoneJSON(ipn.Running, "")
-				if env.upArgs.json {
+					} else if printed {
-					printUpDoneJSON(ipn.Running, "")
+						// Only need to print an update if we printed the "please click" message earlier.
-				} else if printed {
+						fmt.Fprintf(Stderr, "Success.\n")
-					// Only need to print an update if we printed the "please click" message earlier.
+					}
-					fmt.Fprintf(Stderr, "Success.\n")
+					select {
-				}
+					case running <- true:
-				select {
+					default:
-				case running <- true:
+					}
-				default:
+					cancelWatch()
 				}
-				cancel()
 			}
-		}
+			if url := n.BrowseToURL; url != nil && printAuthURL(*url) {
-		if url := n.BrowseToURL; url != nil && printAuthURL(*url) {
+				printed = true
-			printed = true
+				if upArgs.json {
-			if upArgs.json {
+					js := &upOutputJSON{AuthURL: *url, BackendState: st.BackendState}
-				js := &upOutputJSON{AuthURL: *url, BackendState: st.BackendState}
+
-
+					q, err := qrcode.New(*url, qrcode.Medium)
-				q, err := qrcode.New(*url, qrcode.Medium)
-				if err == nil {
-					png, err := q.PNG(128)
 					if err == nil {
-						js.QR = "data:image/png;base64," + base64.StdEncoding.EncodeToString(png)
+						png, err := q.PNG(128)
+						if err == nil {
+							js.QR = "data:image/png;base64," + base64.StdEncoding.EncodeToString(png)
+						}
 					}
-				}
 
-				data, err := json.MarshalIndent(js, "", "\t")
+					data, err := json.MarshalIndent(js, "", "\t")
-				if err != nil {
-					printf("upOutputJSON marshalling error: %v", err)
-				} else {
-					outln(string(data))
-				}
-			} else {
-				fmt.Fprintf(Stderr, "\nTo authenticate, visit:\n\n\t%s\n\n", *url)
-				if upArgs.qr {
-					q, err := qrcode.New(*url, qrcode.Medium)
 					if err != nil {
-						log.Printf("QR code error: %v", err)
+						printf("upOutputJSON marshalling error: %v", err)
 					} else {
-						fmt.Fprintf(Stderr, "%s\n", q.ToString(false))
+						outln(string(data))
+					}
+				} else {
+					fmt.Fprintf(Stderr, "\nTo authenticate, visit:\n\n\t%s\n\n", *url)
+					if upArgs.qr {
+						q, err := qrcode.New(*url, qrcode.Medium)
+						if err != nil {
+							log.Printf("QR code error: %v", err)
+						} else {
+							fmt.Fprintf(Stderr, "%s\n", q.ToString(false))
+						}
 					}
 				}
 			}
 		}
-	})
+	}()
-	// Wait for backend client to be connected so we know
-	// we're subscribed to updates. Otherwise we can miss
-	// an update upon its transition to running. Do so by causing some traffic
-	// back to the bus that we then wait on.
-	bc.RequestEngineStatus()
-	select {
-	case <-gotEngineUpdate:
-	case <-pumpCtx.Done():
-		return pumpCtx.Err()
-	case err := <-pumpErr:
-		return err
-	}
 
 	// Special case: bare "tailscale up" means to just start
 	// running, if there's ever been a login.
@@ -660,10 +662,12 @@ func runUp(ctx context.Context, cmd string, args []string, upArgs upArgsT) (retE
 		if err != nil {
 			return err
 		}
-		bc.Start(ipn.Options{
+		if err := localClient.Start(ctx, ipn.Options{
 			AuthKey:     authKey,
 			UpdatePrefs: prefs,
-		})
+		}); err != nil {
+			return err
+		}
 		if upArgs.forceReauth {
 			startLoginInteractive()
 		}
@@ -685,13 +689,13 @@ func runUp(ctx context.Context, cmd string, args []string, upArgs upArgsT) (retE
 	select {
 	case <-running:
 		return nil
-	case <-pumpCtx.Done():
+	case <-watchCtx.Done():
 		select {
 		case <-running:
 			return nil
 		default:
 		}
-		return pumpCtx.Err()
+		return watchCtx.Err()
 	case err := <-pumpErr:
 		select {
 		case <-running:

cmd/tailscale/cli/web.go

@@ -28,8 +28,8 @@ import (
 
 	"github.com/peterbourgon/ff/v3/ffcli"
 	"tailscale.com/ipn"
+	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/tailcfg"
-	"tailscale.com/types/preftype"
 	"tailscale.com/util/groupmember"
 	"tailscale.com/version/distro"
 )
@@ -317,6 +317,7 @@ req.send(null);
 `
 
 func webHandler(w http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
 	if authRedirect(w, r) {
 		return
 	}
@@ -327,7 +328,18 @@ func webHandler(w http.ResponseWriter, r *http.Request) {
 	}
 
 	if r.URL.Path == "/redirect" || r.URL.Path == "/redirect/" {
-		w.Write([]byte(authenticationRedirectHTML))
+		io.WriteString(w, authenticationRedirectHTML)
+		return
+	}
+
+	st, err := localClient.Status(ctx)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	prefs, err := localClient.GetPrefs(ctx)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
 	}
 
@@ -344,23 +356,31 @@ func webHandler(w http.ResponseWriter, r *http.Request) {
 			json.NewEncoder(w).Encode(mi{"error": err.Error()})
 			return
 		}
-		prefs, err := localClient.GetPrefs(r.Context())
+
-		if err != nil && !postData.Reauthenticate {
+		routes, err := calcAdvertiseRoutes(postData.AdvertiseRoutes, postData.AdvertiseExitNode)
+		if err != nil {
+			w.WriteHeader(http.StatusInternalServerError)
+			json.NewEncoder(w).Encode(mi{"error": err.Error()})
+			return
+		}
+		mp := &ipn.MaskedPrefs{
+			AdvertiseRoutesSet: true,
+			WantRunningSet:     true,
+		}
+		mp.Prefs.WantRunning = true
+		mp.Prefs.AdvertiseRoutes = routes
+		log.Printf("Doing edit: %v", mp.Pretty())
+
+		if _, err := localClient.EditPrefs(ctx, mp); err != nil {
 			w.WriteHeader(http.StatusInternalServerError)
 			json.NewEncoder(w).Encode(mi{"error": err.Error()})
 			return
-		} else {
-			routes, err := calcAdvertiseRoutes(postData.AdvertiseRoutes, postData.AdvertiseExitNode)
-			if err != nil {
-				w.WriteHeader(http.StatusInternalServerError)
-				json.NewEncoder(w).Encode(mi{"error": err.Error()})
-				return
-			}
-			prefs.AdvertiseRoutes = routes
 		}
 
 		w.Header().Set("Content-Type", "application/json")
-		url, err := tailscaleUp(r.Context(), prefs, postData.Reauthenticate)
+		log.Printf("tailscaleUp(reauth=%v) ...", postData.Reauthenticate)
+		url, err := tailscaleUp(r.Context(), st, postData.Reauthenticate)
+		log.Printf("tailscaleUp = (URL %v, %v)", url != "", err)
 		if err != nil {
 			w.WriteHeader(http.StatusInternalServerError)
 			json.NewEncoder(w).Encode(mi{"error": err.Error()})
@@ -374,17 +394,6 @@ func webHandler(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	st, err := localClient.Status(r.Context())
-	if err != nil {
-		http.Error(w, err.Error(), http.StatusInternalServerError)
-		return
-	}
-	prefs, err := localClient.GetPrefs(r.Context())
-	if err != nil {
-		http.Error(w, err.Error(), http.StatusInternalServerError)
-		return
-	}
-
 	profile := st.User[st.Self.UserID]
 	deviceName := strings.Split(st.Self.DNSName, ".")[0]
 	data := tmplData{
@@ -418,26 +427,18 @@ func webHandler(w http.ResponseWriter, r *http.Request) {
 	w.Write(buf.Bytes())
 }
 
-// TODO(crawshaw): some of this is very similar to the code in 'tailscale up', can we share anything?
+func tailscaleUp(ctx context.Context, st *ipnstate.Status, forceReauth bool) (authURL string, retErr error) {
-func tailscaleUp(ctx context.Context, prefs *ipn.Prefs, forceReauth bool) (authURL string, retErr error) {
+	origAuthURL := st.AuthURL
-	if prefs == nil {
+	isRunning := st.BackendState == ipn.Running.String()
-		prefs = ipn.NewPrefs()
-		prefs.ControlURL = ipn.DefaultControlURL
-		prefs.WantRunning = true
-		prefs.CorpDNS = true
-		prefs.AllowSingleHosts = true
-		prefs.ForceDaemon = (runtime.GOOS == "windows")
-	}
-
-	if distro.Get() == distro.Synology {
-		prefs.NetfilterMode = preftype.NetfilterOff
-	}
 
-	st, err := localClient.Status(ctx)
+	if !forceReauth {
-	if err != nil {
+		if origAuthURL != "" {
-		return "", fmt.Errorf("can't fetch status: %v", err)
+			return origAuthURL, nil
+		}
+		if isRunning {
+			return "", nil
+		}
 	}
-	origAuthURL := st.AuthURL
 
 	// printAuthURL reports whether we should print out the
 	// provided auth URL from an IPN notify.
@@ -445,18 +446,27 @@ func tailscaleUp(ctx context.Context, prefs *ipn.Prefs, forceReauth bool) (authU
 		return url != origAuthURL
 	}
 
-	c, bc, pumpCtx, cancel := connect(ctx)
+	watchCtx, cancelWatch := context.WithCancel(ctx)
-	defer cancel()
+	defer cancelWatch()
+	watcher, err := localClient.WatchIPNBus(watchCtx, 0)
+	if err != nil {
+		return "", err
+	}
+	defer watcher.Close()
 
-	gotEngineUpdate := make(chan bool, 1) // gets value upon an engine update
+	go func() {
-	go pump(pumpCtx, bc, c)
+		if !isRunning {
+			localClient.Start(ctx, ipn.Options{})
+		}
+		if forceReauth {
+			localClient.StartLoginInteractive(ctx)
+		}
+	}()
 
-	bc.SetNotifyCallback(func(n ipn.Notify) {
+	for {
-		if n.Engine != nil {
+		n, err := watcher.Next()
-			select {
+		if err != nil {
-			case gotEngineUpdate <- true:
+			return "", err
-			default:
-			}
 		}
 		if n.ErrMessage != nil {
 			msg := *n.ErrMessage
@@ -468,48 +478,10 @@ func tailscaleUp(ctx context.Context, prefs *ipn.Prefs, forceReauth bool) (authU
 					msg += " (try 'sudo tailscale up [...]')"
 				}
 			}
-			retErr = fmt.Errorf("backend error: %v", msg)
+			return "", fmt.Errorf("backend error: %v", msg)
-			cancel()
-		} else if url := n.BrowseToURL; url != nil && printAuthURL(*url) {
-			authURL = *url
-			cancel()
 		}
-		if !forceReauth && n.Prefs != nil && n.Prefs.Valid() {
+		if url := n.BrowseToURL; url != nil && printAuthURL(*url) {
-			p1, p2 := n.Prefs.AsStruct(), *prefs
+			return *url, nil
-			p1.Persist = nil
-			p2.Persist = nil
-			if p1.Equals(&p2) {
-				cancel()
-			}
 		}
-	})
-	// Wait for backend client to be connected so we know
-	// we're subscribed to updates. Otherwise we can miss
-	// an update upon its transition to running. Do so by causing some traffic
-	// back to the bus that we then wait on.
-	bc.RequestEngineStatus()
-	select {
-	case <-gotEngineUpdate:
-	case <-pumpCtx.Done():
-		return authURL, pumpCtx.Err()
-	}
-
-	bc.SetPrefs(prefs)
-
-	bc.Start(ipn.Options{})
-	if forceReauth {
-		bc.StartLoginInteractive()
-	}
-
-	<-pumpCtx.Done() // wait for authURL or complete failure
-	if authURL == "" && retErr == nil {
-		if !forceReauth {
-			return "", nil // no auth URL is fine
-		}
-		retErr = pumpCtx.Err()
-	}
-	if authURL == "" && retErr == nil {
-		return "", fmt.Errorf("login failed with no backend error message")
 	}
-	return authURL, retErr
 }

ipn/ipnlocal/local.go

@@ -181,7 +181,8 @@ type LocalBackend struct {
 	loginFlags       controlclient.LoginFlags
 	incomingFiles    map[*incomingFile]bool
 	fileWaiters      map[*mapSetHandle]context.CancelFunc // handle => func to call on file received
-	lastStatusTime   time.Time                            // status.AsOf value of the last processed status update
+	notifyWatchers   map[*mapSetHandle]chan *ipn.Notify
+	lastStatusTime   time.Time // status.AsOf value of the last processed status update
 	// directFileRoot, if non-empty, means to write received files
 	// directly to this directory, without staging them in an
 	// intermediate buffered directory for "pick-up" later. If
@@ -1690,24 +1691,70 @@ func (b *LocalBackend) readPoller() {
 	}
 }
 
-// send delivers n to the connected frontend. If no frontend is
+// WatchNotifications subscribes to the ipn.Notify message bus notification
-// connected, the notification is dropped without being delivered.
+// messages.
-func (b *LocalBackend) send(n ipn.Notify) {
+//
+// WatchNotifications blocks until ctx is done.
+//
+// The provided fn will only be called with non-nil pointers. The caller must
+// not modify roNotify. If fn returns false, the watch also stops.
+//
+// Failure to consume many notifications in a row will result in dropped
+// notifications. There is currently (2022-11-22) no mechanism provided to
+// detect when a message has been dropped.
+func (b *LocalBackend) WatchNotifications(ctx context.Context, fn func(roNotify *ipn.Notify) (keepGoing bool)) {
+	handle := new(mapSetHandle)
+	ch := make(chan *ipn.Notify, 128)
+
 	b.mu.Lock()
-	notifyFunc := b.notify
+	mak.Set(&b.notifyWatchers, handle, ch)
-	apiSrv := b.peerAPIServer
 	b.mu.Unlock()
+	defer func() {
+		b.mu.Lock()
+		delete(b.notifyWatchers, handle)
+		b.mu.Unlock()
+	}()
 
-	if notifyFunc == nil {
+	for {
-		return
+		select {
+		case <-ctx.Done():
+			return
+		case n := <-ch:
+			if !fn(n) {
+				return
+			}
+		}
 	}
+}
+
+// send delivers n to the connected frontend and any API watchers from
+// LocalBackend.WatchNotifications (via the LocalAPI).
+//
+// If no frontend is connected or API watchers are backed up, the notification
+// is dropped without being delivered.
+func (b *LocalBackend) send(n ipn.Notify) {
+	n.Version = version.Long
 
+	b.mu.Lock()
+	notifyFunc := b.notify
+	apiSrv := b.peerAPIServer
 	if apiSrv.hasFilesWaiting() {
 		n.FilesWaiting = &empty.Message{}
 	}
 
-	n.Version = version.Long
+	for _, ch := range b.notifyWatchers {
-	notifyFunc(n)
+		select {
+		case ch <- &n:
+		default:
+			// Drop the notification if the channel is full.
+		}
+	}
+
+	b.mu.Unlock()
+
+	if notifyFunc != nil {
+		notifyFunc(n)
+	}
 }
 
 func (b *LocalBackend) sendFileNotify() {

ipn/localapi/localapi.go

@@ -80,6 +80,7 @@ var handler = map[string]localAPIHandler{
 	"serve-config":            (*Handler).serveServeConfig,
 	"set-dns":                 (*Handler).serveSetDNS,
 	"set-expiry-sooner":       (*Handler).serveSetExpirySooner,
+	"start":                   (*Handler).serveStart,
 	"status":                  (*Handler).serveStatus,
 	"tka/init":                (*Handler).serveTKAInit,
 	"tka/log":                 (*Handler).serveTKALog,
@@ -88,6 +89,7 @@ var handler = map[string]localAPIHandler{
 	"tka/status":              (*Handler).serveTKAStatus,
 	"tka/disable":             (*Handler).serveTKADisable,
 	"upload-client-metrics":   (*Handler).serveUploadClientMetrics,
+	"watch-ipn-bus":           (*Handler).serveWatchIPNBus,
 	"whois":                   (*Handler).serveWhoIs,
 }
 
@@ -572,6 +574,34 @@ func (h *Handler) serveStatus(w http.ResponseWriter, r *http.Request) {
 	e.Encode(st)
 }
 
+func (h *Handler) serveWatchIPNBus(w http.ResponseWriter, r *http.Request) {
+	if !h.PermitWrite {
+		http.Error(w, "denied", http.StatusForbidden)
+		return
+	}
+	f, ok := w.(http.Flusher)
+	if !ok {
+		http.Error(w, "not a flusher", http.StatusInternalServerError)
+		return
+	}
+	w.Header().Set("Content-Type", "application/json")
+	f.Flush()
+
+	ctx := r.Context()
+	h.b.WatchNotifications(ctx, func(roNotify *ipn.Notify) (keepGoing bool) {
+		js, err := json.Marshal(roNotify)
+		if err != nil {
+			h.logf("json.Marshal: %v", err)
+			return false
+		}
+		if _, err := fmt.Fprintf(w, "%s\n", js); err != nil {
+			return false
+		}
+		f.Flush()
+		return true
+	})
+}
+
 func (h *Handler) serveLoginInteractive(w http.ResponseWriter, r *http.Request) {
 	if !h.PermitWrite {
 		http.Error(w, "login access denied", http.StatusForbidden)
@@ -586,6 +616,29 @@ func (h *Handler) serveLoginInteractive(w http.ResponseWriter, r *http.Request)
 	return
 }
 
+func (h *Handler) serveStart(w http.ResponseWriter, r *http.Request) {
+	if !h.PermitWrite {
+		http.Error(w, "access denied", http.StatusForbidden)
+		return
+	}
+	if r.Method != "POST" {
+		http.Error(w, "want POST", 400)
+		return
+	}
+	var o ipn.Options
+	if err := json.NewDecoder(r.Body).Decode(&o); err != nil {
+		http.Error(w, err.Error(), http.StatusBadRequest)
+		return
+	}
+	err := h.b.Start(o)
+	if err != nil {
+		// TODO(bradfitz): map error to a good HTTP error
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	w.WriteHeader(http.StatusNoContent)
+}
+
 func (h *Handler) serveLogout(w http.ResponseWriter, r *http.Request) {
 	if !h.PermitWrite {
 		http.Error(w, "logout access denied", http.StatusForbidden)