cmd/tailscale: add licenses subcommand

Signed-off-by: Will Norris <will@tailscale.com>

cmd/tailscale/cli/cli.go

@@ -170,6 +170,7 @@ change in the future.
 			bugReportCmd,
 			certCmd,
 			netlockCmd,
+			licensesCmd,
 		},
 		FlagSet:   rootfs,
 		Exec:      func(context.Context, []string) error { return flag.ErrHelp },

cmd/tailscale/cli/licenses.go

@@ -0,0 +1,29 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cli
+
+import (
+	"context"
+
+	"github.com/peterbourgon/ff/v3/ffcli"
+)
+
+var licensesCmd = &ffcli.Command{
+	Name:       "licenses",
+	ShortUsage: "licenses",
+	ShortHelp:  "Get open source license information",
+	LongHelp:   "Get open source license information",
+	Exec:       runLicenses,
+}
+
+func runLicenses(ctx context.Context, args []string) error {
+	outln(`
+Tailscale wouldn't be possible without the contributions of thousands of open
+source developers. To see the open source packages included in Tailscale and
+their respective license information, visit:
+
+    https://tailscale.com/licenses/tailscale`)
+	return nil
+}