cmd/tailscale/cli: add ts2021 debug flag to set a dial plan

Updates tailscale/corp#32534

Change-Id: Ief4ee0a263ea1edbf652b74d8c335c1e5ee209d7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/debug.go

@@ -289,6 +289,7 @@ func debugCmd() *ffcli.Command {
 					fs.IntVar(&ts2021Args.version, "version", int(tailcfg.CurrentCapabilityVersion), "protocol version")
 					fs.BoolVar(&ts2021Args.verbose, "verbose", false, "be extra verbose")
 					fs.StringVar(&ts2021Args.aceHost, "ace", "", "if non-empty, use this ACE server IP/hostname as a candidate path")
+					fs.StringVar(&ts2021Args.dialPlanJSONFile, "dial-plan", "", "if non-empty, use this JSON file to configure the dial plan")
 					return fs
 				})(),
 			},
@@ -967,6 +968,8 @@ var ts2021Args struct {
 	version int    // 27 or whatever
 	verbose bool
 	aceHost string // if non-empty, FQDN of https ACE server to use ("ace.example.com")
+
+	dialPlanJSONFile string // if non-empty, path to JSON file [tailcfg.ControlDialPlan] JSON
 }
 
 func runTS2021(ctx context.Context, args []string) error {
@@ -1051,6 +1054,18 @@ func runTS2021(ctx context.Context, args []string) error {
 		return fmt.Errorf("creating netmon: %w", err)
 	}
 
+	var dialPlan *tailcfg.ControlDialPlan
+	if ts2021Args.dialPlanJSONFile != "" {
+		b, err := os.ReadFile(ts2021Args.dialPlanJSONFile)
+		if err != nil {
+			return fmt.Errorf("reading dial plan JSON file: %w", err)
+		}
+		dialPlan = new(tailcfg.ControlDialPlan)
+		if err := json.Unmarshal(b, dialPlan); err != nil {
+			return fmt.Errorf("unmarshaling dial plan JSON file: %w", err)
+		}
+	}
+
 	noiseDialer := &controlhttp.Dialer{
 		Hostname:        ts2021Args.host,
 		HTTPPort:        "80",
@@ -1058,6 +1073,7 @@ func runTS2021(ctx context.Context, args []string) error {
 		MachineKey:      machinePrivate,
 		ControlKey:      keys.PublicKey,
 		ProtocolVersion: uint16(ts2021Args.version),
+		DialPlan:        dialPlan,
 		Dialer:          dialFunc,
 		Logf:            logf,
 		NetMon:          netMon,