cmd/tailscale/cli: redact private key in debug netmap output by default

This makes `tailscale debug watch-ipn` safe to use for troubleshooting
user issues, in addition to local debugging during development.

Signed-off-by: David Anderson <danderson@tailscale.com>

cmd/tailscale/cli/debug.go

@@ -134,6 +134,7 @@ var debugCmd = &ffcli.Command{
 				fs := newFlagSet("watch-ipn")
 				fs.BoolVar(&watchIPNArgs.netmap, "netmap", true, "include netmap in messages")
 				fs.BoolVar(&watchIPNArgs.initial, "initial", false, "include initial status")
+				fs.BoolVar(&watchIPNArgs.showPrivateKey, "show-private-key", false, "include node private key in printed netmap")
 				return fs
 			})(),
 		},
@@ -319,8 +320,9 @@ func runPrefs(ctx context.Context, args []string) error {
 }
 
 var watchIPNArgs struct {
-	netmap  bool
-	initial bool
+	netmap         bool
+	initial        bool
+	showPrivateKey bool
 }
 
 func runWatchIPN(ctx context.Context, args []string) error {
@@ -328,6 +330,9 @@ func runWatchIPN(ctx context.Context, args []string) error {
 	if watchIPNArgs.initial {
 		mask = ipn.NotifyInitialState | ipn.NotifyInitialPrefs | ipn.NotifyInitialNetMap
 	}
+	if !watchIPNArgs.showPrivateKey {
+		mask |= ipn.NotifyNoPrivateKeys
+	}
 	watcher, err := localClient.WatchIPNBus(ctx, mask)
 	if err != nil {
 		return err

ipn/backend.go

@@ -65,6 +65,8 @@ const (
 	NotifyInitialState  // if set, the first Notify message (sent immediately) will contain the current State + BrowseToURL
 	NotifyInitialPrefs  // if set, the first Notify message (sent immediately) will contain the current Prefs
 	NotifyInitialNetMap // if set, the first Notify message (sent immediately) will contain the current NetMap
+
+	NotifyNoPrivateKeys // if set, private keys that would normally be sent in updates are zeroed out
 )
 
 // Notify is a communication from a backend (e.g. tailscaled) to a frontend

ipn/ipnlocal/local.go

@@ -1742,6 +1742,24 @@ func (b *LocalBackend) readPoller() {
 func (b *LocalBackend) WatchNotifications(ctx context.Context, mask ipn.NotifyWatchOpt, fn func(roNotify *ipn.Notify) (keepGoing bool)) {
 	ch := make(chan *ipn.Notify, 128)
 
+	origFn := fn
+	if mask&ipn.NotifyNoPrivateKeys != 0 {
+		fn = func(n *ipn.Notify) bool {
+			if n.NetMap == nil || n.NetMap.PrivateKey.IsZero() {
+				return origFn(n)
+			}
+
+			// The netmap in n is shared across all watchers, so to mutate it for a
+			// single watcher we have to clone the notify and the netmap. We can
+			// make shallow clones, at least.
+			nm2 := *n.NetMap
+			n2 := *n
+			n2.NetMap = &nm2
+			n2.NetMap.PrivateKey = key.NodePrivate{}
+			return origFn(&n2)
+		}
+	}
+
 	var ini *ipn.Notify
 
 	b.mu.Lock()