net/tstun: return the real interface name at device creation.

This is usually the same as the requested interface, but on some
unixes can vary based on device number allocation, and on Windows
it's the GUID instead of the pretty name, since everything relating
to configuration wants the GUID.

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

cmd/tailscaled/tailscaled.go

@@ -338,7 +338,7 @@ func tryEngine(logf logger.Logf, linkMon *monitor.Mon, name string) (e wgengine.
 	}
 	useNetstack = name == "userspace-networking"
 	if !useNetstack {
-		dev, err := tstun.New(logf, name)
+		dev, devName, err := tstun.New(logf, name)
 		if err != nil {
 			tstun.Diagnose(logf, name)
 			return nil, false, err
@@ -350,13 +350,7 @@ func tryEngine(logf logger.Logf, linkMon *monitor.Mon, name string) (e wgengine.
 			return nil, false, err
 		}
 		conf.Router = r
-		tunname, err := dev.Name()
-		if err != nil {
-			r.Close()
-			dev.Close()
-			return nil, false, err
-		}
-		conf.DNS = dns.NewOSConfigurator(logf, tunname)
+		conf.DNS = dns.NewOSConfigurator(logf, devName)
 	}
 	e, err = wgengine.NewUserspaceEngine(logf, conf)
 	if err != nil {

cmd/tailscaled/tailscaled_windows.go

@@ -30,6 +30,7 @@
 	"golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
 	"tailscale.com/ipn/ipnserver"
 	"tailscale.com/logpolicy"
+	"tailscale.com/net/dns"
 	"tailscale.com/net/tstun"
 	"tailscale.com/tempfork/wireguard-windows/firewall"
 	"tailscale.com/types/logger"
@@ -161,7 +162,7 @@ func startIPNServer(ctx context.Context, logid string) error {
 	var err error
 
 	getEngine := func() (wgengine.Engine, error) {
-		dev, err := tstun.New(logf, "Tailscale")
+		dev, devName, err := tstun.New(logf, "Tailscale")
 		if err != nil {
 			return nil, err
 		}
@@ -173,6 +174,7 @@ func startIPNServer(ctx context.Context, logid string) error {
 		eng, err := wgengine.NewUserspaceEngine(logf, wgengine.Config{
 			Tun:        dev,
 			Router:     r,
+			DNS:        dns.NewOSConfigurator(logf, devName),
 			ListenPort: 41641,
 		})
 		if err != nil {

net/tstun/tun.go

@@ -28,16 +28,23 @@
 // discovery.
 const minimalMTU = 1280
 
-// New returns a tun.Device for the requested device name.
-func New(logf logger.Logf, tunName string) (tun.Device, error) {
+// New returns a tun.Device for the requested device name, along with
+// the OS-dependent name that was allocated to the device.
+func New(logf logger.Logf, tunName string) (tun.Device, string, error) {
 	dev, err := tun.CreateTUN(tunName, minimalMTU)
 	if err != nil {
-		return nil, err
+		return nil, "", err
 	}
 	if err := waitInterfaceUp(dev, 90*time.Second, logf); err != nil {
-		return nil, err
+		dev.Close()
+		return nil, "", err
 	}
-	return dev, nil
+	name, err := interfaceName(dev)
+	if err != nil {
+		dev.Close()
+		return nil, "", err
+	}
+	return dev, name, nil
 }
 
 // Diagnose tries to explain a tuntap device creation failure.

net/tstun/tun_notwindows.go

@@ -0,0 +1,13 @@
+// Copyright (c) 2021 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.
+
+// +build !windows
+
+package tstun
+
+import "github.com/tailscale/wireguard-go/tun"
+
+func interfaceName(dev tun.Device) (string, error) {
+	return dev.Name()
+}

net/tstun/tun_windows.go

@@ -8,6 +8,7 @@
 	"github.com/tailscale/wireguard-go/tun"
 	"github.com/tailscale/wireguard-go/tun/wintun"
 	"golang.org/x/sys/windows"
+	"golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
 )
 
 func init() {
@@ -22,3 +23,11 @@ func init() {
 	}
 	tun.WintunStaticRequestedGUID = &guid
 }
+
+func interfaceName(dev tun.Device) (string, error) {
+	guid, err := winipcfg.LUID(dev.(*tun.NativeTun).LUID()).GUID()
+	if err != nil {
+		return "", err
+	}
+	return guid.String(), nil
+}