Add "Allow LAN access" checkbox in Exit Node menu

Updates tailscale/tailscale#2155

cmd/tailscale/main.go

@@ -189,6 +189,7 @@ type (
 	GoogleAuthEvent   struct{}
 	LogoutEvent       struct{}
 	BeExitNodeEvent   bool
+	ExitAllowLANEvent bool
 )
 
 // serverOAuthID is the OAuth ID of the tailscale-android server, used
@@ -420,6 +421,9 @@ func (a *App) runBackend() error {
 			case BeExitNodeEvent:
 				state.Prefs.SetAdvertiseExitNode(bool(e))
 				go b.backend.SetPrefs(state.Prefs)
+			case ExitAllowLANEvent:
+				state.Prefs.ExitNodeAllowLANAccess = bool(e)
+				go b.backend.SetPrefs(state.Prefs)
 			case WebAuthEvent:
 				if !signingIn {
 					go b.backend.StartLoginInteractive()
@@ -865,6 +869,7 @@ func (a *App) runUI() error {
 		case p := <-a.prefs:
 			ui.enabled.Value = p.WantRunning
 			ui.runningExit = p.AdvertisesExitNode()
+			ui.exitLAN.Value = p.ExitNodeAllowLANAccess
 			w.Invalidate()
 		case state.browseURL = <-a.browseURLs:
 			ui.signinType = noSignin
@@ -1077,6 +1082,8 @@ func (a *App) processUIEvents(w *app.Window, events []UIEvent, act jni.Object, s
 			w.WriteClipboard(logMarker)
 		case BeExitNodeEvent:
 			requestBackend(e)
+		case ExitAllowLANEvent:
+			requestBackend(e)
 		case WebAuthEvent:
 			a.store.WriteString(loginMethodPrefKey, loginMethodWeb)
 			requestBackend(e)

cmd/tailscale/ui.go

@@ -49,6 +49,8 @@ type UI struct {
 	enabled widget.Bool
 	search  widget.Editor
 
+	exitLAN widget.Bool
+
 	// webSigin is the button for the web-based sign-in flow.
 	webSignin widget.Clickable
 
@@ -303,6 +305,9 @@ func (ui *UI) layout(gtx layout.Context, sysIns system.Insets, state *clientStat
 	} else {
 		d.exits.Value = string(exitID)
 	}
+	if ui.exitLAN.Changed() {
+		events = append(events, ExitAllowLANEvent(ui.exitLAN.Value))
+	}
 
 	if ui.googleSignin.Clicked() {
 		ui.signinType = googleSignin
@@ -892,12 +897,20 @@ func (ui *UI) layoutExitNodeDialog(gtx layout.Context, sysIns system.Insets, exi
 					}),
 					layout.Flexed(1, func(gtx C) D {
 						gtx.Constraints.Min.Y = 0
-						// Add "none" exit node.
+						// Add "none" exit node, then "Allow LAN" checkbox, then the exit nodes.
-						n := len(exits) + 1
+						n := len(exits) + 2
 						return d.list.Layout(gtx, n, func(gtx C, idx int) D {
+							if idx == 0 {
+								btn := material.CheckBox(ui.theme, &ui.exitLAN, "Allow LAN access")
+								return layout.Inset{
+									Right:  unit.Dp(16),
+									Left:   unit.Dp(16),
+									Bottom: unit.Dp(16),
+								}.Layout(gtx, btn.Layout)
+							}
 							node := Peer{Label: "None", Online: true}
-							if idx >= 1 {
+							if idx >= 2 {
-								node = exits[idx-1]
+								node = exits[idx-2]
 							}
 							lbl := node.Label
 							if !node.Online {