tunnel: windows does not always add/remove routes with up/down interface

On Linux, we're used to routes being added after an interface is up, and
routes being removed as a consequence of an interface going down. On
Windows, this isn't always the case, at least not from the perspective
of the route notifiers. In order to work around this and make a
multi-interface model coherent, we search for a new default route not
only whenever the routing table changes but also whenever any interface
link parameters change, such as up/down.

The practical consequence is that now WireGuard connects properly when
wifi is disconnected and then reconnected.

Reported-by: Nenad Kozul <me@nenadkozul.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

1 files changed, 17 insertions, 5 deletions

diff --git a/tunnel/defaultroutemonitor.go b/tunnel/defaultroutemonitor.go
index c1722c45..f9c63e56 100644
--- a/tunnel/defaultroutemonitor.go
+++ b/tunnel/defaultroutemonitor.go
@@ -7,6 +7,7 @@ package tunnel
 
 import (
 	"log"
+	"sync"
 
 	"golang.org/x/sys/windows"
 	"golang.zx2c4.com/wireguard/device"
@@ -29,7 +30,6 @@ func bindSocketRoute(family winipcfg.AddressFamily, device *device.Device, ourLU
 		}
 		ifrow, err := r[i].InterfaceLUID.Interface()
 		if err != nil || ifrow.OperStatus != winipcfg.IfOperStatusUp {
-			log.Printf("Found default route for interface %d, but not up, so skipping", r[i].InterfaceIndex)
 			continue
 		}
 		if r[i].Metric < lowestMetric {
@@ -53,7 +53,7 @@ func bindSocketRoute(family winipcfg.AddressFamily, device *device.Device, ourLU
 	return nil
 }
 
-func monitorDefaultRoutes(family winipcfg.AddressFamily, device *device.Device, autoMTU bool, tun *tun.NativeTun) (*winipcfg.RouteChangeCallback, error) {
+func monitorDefaultRoutes(family winipcfg.AddressFamily, device *device.Device, autoMTU bool, tun *tun.NativeTun) ([]winipcfg.ChangeCallback, error) {
 	var minMTU uint32
 	if family == windows.AF_INET {
 		minMTU = 576
@@ -64,7 +64,10 @@ func monitorDefaultRoutes(family winipcfg.AddressFamily, device *device.Device,
 	lastLUID := winipcfg.LUID(0)
 	lastIndex := uint32(0)
 	lastMTU := uint32(0)
+	mutex := sync.Mutex{}
 	doIt := func() error {
+		mutex.Lock()
+		defer mutex.Unlock()
 		err := bindSocketRoute(family, device, ourLUID, &lastLUID, &lastIndex)
 		if err != nil {
 			return err
@@ -104,13 +107,22 @@ func monitorDefaultRoutes(family winipcfg.AddressFamily, device *device.Device,
 	if err != nil {
 		return nil, err
 	}
-	cb, err := winipcfg.RegisterRouteChangeCallback(func(notificationType winipcfg.MibNotificationType, route *winipcfg.MibIPforwardRow2) {
+	cbr, err := winipcfg.RegisterRouteChangeCallback(func(notificationType winipcfg.MibNotificationType, route *winipcfg.MibIPforwardRow2) {
 		if route != nil && route.DestinationPrefix.PrefixLength == 0 {
-			_ = doIt()
+			doIt()
 		}
 	})
 	if err != nil {
 		return nil, err
 	}
-	return cb, nil
+	cbi, err := winipcfg.RegisterInterfaceChangeCallback(func(notificationType winipcfg.MibNotificationType, iface *winipcfg.MibIPInterfaceRow) {
+		if notificationType == winipcfg.MibParameterNotification {
+			doIt()
+		}
+	})
+	if err != nil {
+		cbr.Unregister()
+		return nil, err
+	}
+	return []winipcfg.ChangeCallback{cbr, cbi}, nil
 }