cmd/tailscale, ipn/ipnlocal: add 'debug dial-types' command

This command allows observing whether a given dialer ("SystemDial",
"UserDial", etc.) will successfully obtain a connection to a provided
host, from inside tailscaled itself. This is intended to help debug a
variety of issues from subnet routers to split DNS setups.

Updates #9619

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ie01ebb5469d3e287eac633ff656783960f697b84

cmd/tailscale/cli/debug.go

@@ -274,6 +274,16 @@ var debugCmd = &ffcli.Command{
 			Exec:      runPeerEndpointChanges,
 			ShortHelp: "prints debug information about a peer's endpoint changes",
 		},
+		{
+			Name:      "dial-types",
+			Exec:      runDebugDialTypes,
+			ShortHelp: "prints debug information about connecting to a given host or IP",
+			FlagSet: (func() *flag.FlagSet {
+				fs := newFlagSet("dial-types")
+				fs.StringVar(&debugDialTypesArgs.network, "network", "tcp", `network type to dial ("tcp", "udp", etc.)`)
+				return fs
+			})(),
+		},
 	},
 }
 
@@ -1015,3 +1025,61 @@ func debugControlKnobs(ctx context.Context, args []string) error {
 	e.Encode(v)
 	return nil
 }
+
+var debugDialTypesArgs struct {
+	network string
+}
+
+func runDebugDialTypes(ctx context.Context, args []string) error {
+	st, err := localClient.Status(ctx)
+	if err != nil {
+		return fixTailscaledConnectError(err)
+	}
+	description, ok := isRunningOrStarting(st)
+	if !ok {
+		printf("%s\n", description)
+		os.Exit(1)
+	}
+
+	if len(args) != 2 || args[0] == "" || args[1] == "" {
+		return errors.New("usage: dial-types <hostname-or-IP> <port>")
+	}
+
+	port, err := strconv.ParseUint(args[1], 10, 16)
+	if err != nil {
+		return fmt.Errorf("invalid port %q: %w", args[1], err)
+	}
+
+	hostOrIP := args[0]
+	ip, _, err := tailscaleIPFromArg(ctx, hostOrIP)
+	if err != nil {
+		return err
+	}
+	if ip != hostOrIP {
+		log.Printf("lookup %q => %q", hostOrIP, ip)
+	}
+
+	qparams := make(url.Values)
+	qparams.Set("ip", ip)
+	qparams.Set("port", strconv.FormatUint(port, 10))
+	qparams.Set("network", debugDialTypesArgs.network)
+
+	req, err := http.NewRequestWithContext(ctx, "POST", "http://local-tailscaled.sock/localapi/v0/debug-dial-types?"+qparams.Encode(), nil)
+	if err != nil {
+		return err
+	}
+
+	resp, err := localClient.DoLocalRequest(req)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return err
+	}
+
+	fmt.Printf("%s", body)
+	return nil
+}