ipn/localapi, cmd/tailscale: add API to get prefs, CLI debug command to show

Updates #1436

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

ipn/ipnlocal/local.go

@@ -238,6 +238,19 @@ func (b *LocalBackend) Shutdown() {
 	b.e.Wait()
 }
 
+// Prefs returns a copy of b's current prefs, with any private keys removed.
+func (b *LocalBackend) Prefs() *ipn.Prefs {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	p := b.prefs.Clone()
+	if p != nil && p.Persist != nil {
+		p.Persist.LegacyFrontendPrivateMachineKey = wgkey.Private{}
+		p.Persist.PrivateNodeKey = wgkey.Private{}
+		p.Persist.OldPrivateNodeKey = wgkey.Private{}
+	}
+	return p
+}
+
 // Status returns the latest status of the backend and its
 // sub-components.
 func (b *LocalBackend) Status() *ipnstate.Status {

ipn/localapi/localapi.go

@@ -87,6 +87,8 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		h.serveGoroutines(w, r)
 	case "/localapi/v0/status":
 		h.serveStatus(w, r)
+	case "/localapi/v0/prefs":
+		h.servePrefs(w, r)
 	case "/localapi/v0/check-ip-forwarding":
 		h.serveCheckIPForwarding(w, r)
 	case "/localapi/v0/bugreport":
@@ -198,6 +200,17 @@ func (h *Handler) serveStatus(w http.ResponseWriter, r *http.Request) {
 	e.Encode(st)
 }
 
+func (h *Handler) servePrefs(w http.ResponseWriter, r *http.Request) {
+	if !h.PermitRead {
+		http.Error(w, "prefs access denied", http.StatusForbidden)
+		return
+	}
+	w.Header().Set("Content-Type", "application/json")
+	e := json.NewEncoder(w)
+	e.SetIndent("", "\t")
+	e.Encode(h.b.Prefs())
+}
+
 func (h *Handler) serveFiles(w http.ResponseWriter, r *http.Request) {
 	if !h.PermitWrite {
 		http.Error(w, "file access denied", http.StatusForbidden)