wgengine: remove Config.TUNName, require caller to create device.

Also factors out device creation and associated OS workarounds to
net/tun.

Signed-off-by: David Anderson <danderson@tailscale.com>

cmd/tailscaled/depaware.txt

@@ -105,6 +105,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/net/tlsdial                                    from tailscale.com/control/controlclient+
         tailscale.com/net/tsaddr                                     from tailscale.com/ipn/ipnlocal+
      💣 tailscale.com/net/tshttpproxy                                from tailscale.com/control/controlclient+
+        tailscale.com/net/tun                                        from tailscale.com/cmd/tailscaled
         tailscale.com/paths                                          from tailscale.com/cmd/tailscaled+
         tailscale.com/portlist                                       from tailscale.com/ipn/ipnlocal
         tailscale.com/safesocket                                     from tailscale.com/ipn/ipnserver

cmd/tailscaled/tailscaled.go

@@ -32,6 +32,7 @@ import (
 	"tailscale.com/ipn/ipnserver"
 	"tailscale.com/logpolicy"
 	"tailscale.com/net/socks5"
+	"tailscale.com/net/tun"
 	"tailscale.com/paths"
 	"tailscale.com/types/flagtype"
 	"tailscale.com/types/logger"
@@ -316,18 +317,7 @@ func createEngine(logf logger.Logf, linkMon *monitor.Mon) (e wgengine.Engine, is
 	var errs []error
 	for _, name := range strings.Split(args.tunname, ",") {
 		logf("wgengine.NewUserspaceEngine(tun %q) ...", name)
-		conf := wgengine.Config{
-			ListenPort:  args.port,
-			LinkMonitor: linkMon,
-		}
-		isUserspace = name == "userspace-networking"
-		if isUserspace {
-			conf.TUN = tstun.NewFakeTUN()
-			conf.RouterGen = router.NewFake
-		} else {
-			conf.TUNName = name
-		}
-		e, err := wgengine.NewUserspaceEngine(logf, conf)
+		e, isUserspace, err = tryEngine(logf, linkMon, name)
 		if err == nil {
 			return e, isUserspace, nil
 		}
@@ -337,6 +327,29 @@ func createEngine(logf logger.Logf, linkMon *monitor.Mon) (e wgengine.Engine, is
 	return nil, false, multierror.New(errs)
 }
 
+func tryEngine(logf logger.Logf, linkMon *monitor.Mon, name string) (e wgengine.Engine, isUserspace bool, err error) {
+	conf := wgengine.Config{
+		ListenPort:  args.port,
+		LinkMonitor: linkMon,
+	}
+	isUserspace = name == "userspace-networking"
+	if isUserspace {
+		conf.TUN = tstun.NewFakeTUN()
+		conf.RouterGen = router.NewFake
+	} else {
+		dev, err := tun.New(logf, name)
+		if err != nil {
+			return nil, false, err
+		}
+		conf.TUN = dev
+	}
+	e, err = wgengine.NewUserspaceEngine(logf, conf)
+	if err != nil {
+		return nil, isUserspace, err
+	}
+	return e, isUserspace, nil
+}
+
 func newDebugMux() *http.ServeMux {
 	mux := http.NewServeMux()
 	mux.HandleFunc("/debug/pprof/", pprof.Index)

cmd/tailscaled/tailscaled_windows.go

@@ -30,6 +30,7 @@ import (
 	"golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
 	"tailscale.com/ipn/ipnserver"
 	"tailscale.com/logpolicy"
+	"tailscale.com/net/tun"
 	"tailscale.com/tempfork/wireguard-windows/firewall"
 	"tailscale.com/types/logger"
 	"tailscale.com/version"
@@ -159,11 +160,16 @@ func startIPNServer(ctx context.Context, logid string) error {
 	var err error
 
 	getEngine := func() (wgengine.Engine, error) {
+		dev, err := tun.New(logf, "Tailscale")
+		if err != nil {
+			return nil, err
+		}
 		eng, err := wgengine.NewUserspaceEngine(logf, wgengine.Config{
-			TUNName:    "Tailscale",
+			TUN:        dev,
 			ListenPort: 41641,
 		})
 		if err != nil {
+			dev.Close()
 			return nil, err
 		}
 		return wgengine.NewWatchdog(eng), nil