cmd/tailscale: make netcheck use active DERP map, delete static copy

After allowing for custom DERP maps, it's convenient to be able to see their latency in
netcheck. This adds a query to the local tailscaled for the current DERPMap.

Updates #1264

Signed-off-by: julianknodt <julianknodt@gmail.com>

ipn/localapi/localapi.go

@@ -102,6 +102,8 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		h.serveFileTargets(w, r)
 	case "/localapi/v0/set-dns":
 		h.serveSetDNS(w, r)
+	case "/localapi/v0/derpmap":
+		h.serveDERPMap(w, r)
 	case "/":
 		io.WriteString(w, "tailscaled\n")
 	default:
@@ -403,6 +405,17 @@ func (h *Handler) serveSetDNS(w http.ResponseWriter, r *http.Request) {
 	json.NewEncoder(w).Encode(struct{}{})
 }
 
+func (h *Handler) serveDERPMap(w http.ResponseWriter, r *http.Request) {
+	if r.Method != "GET" {
+		http.Error(w, "want GET", 400)
+		return
+	}
+	w.Header().Set("Content-Type", "application/json")
+	e := json.NewEncoder(w)
+	e.SetIndent("", "\t")
+	e.Encode(h.b.DERPMap())
+}
+
 var dialPeerTransportOnce struct {
 	sync.Once
 	v *http.Transport