From bc4381447f0403b74c5459fb4af179a843c1b8d1 Mon Sep 17 00:00:00 2001
From: David Anderson <danderson@tailscale.com>
Date: Mon, 5 Apr 2021 21:45:56 -0700
Subject: [PATCH] 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         | 10 ++--------
 cmd/tailscaled/tailscaled_windows.go |  4 +++-
 net/tstun/tun.go                     | 17 ++++++++++++-----
 net/tstun/tun_notwindows.go          | 13 +++++++++++++
 net/tstun/tun_windows.go             |  9 +++++++++
 5 files changed, 39 insertions(+), 14 deletions(-)
 create mode 100644 net/tstun/tun_notwindows.go

diff --git a/cmd/tailscaled/tailscaled.go b/cmd/tailscaled/tailscaled.go
index 86669d383..c5b3980df 100644
--- a/cmd/tailscaled/tailscaled.go
+++ b/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 {
diff --git a/cmd/tailscaled/tailscaled_windows.go b/cmd/tailscaled/tailscaled_windows.go
index b57190b7e..6a4c94149 100644
--- a/cmd/tailscaled/tailscaled_windows.go
+++ b/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/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 {
diff --git a/net/tstun/tun.go b/net/tstun/tun.go
index d480d3244..4e0187ed4 100644
--- a/net/tstun/tun.go
+++ b/net/tstun/tun.go
@@ -28,16 +28,23 @@ import (
 // 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.
diff --git a/net/tstun/tun_notwindows.go b/net/tstun/tun_notwindows.go
new file mode 100644
index 000000000..078bccd47
--- /dev/null
+++ b/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()
+}
diff --git a/net/tstun/tun_windows.go b/net/tstun/tun_windows.go
index dc5fc2d79..786c09408 100644
--- a/net/tstun/tun_windows.go
+++ b/net/tstun/tun_windows.go
@@ -8,6 +8,7 @@ import (
 	"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
+}