net/interfaces: include DefaultRouteInterface in interfaces.State

And log it in wgengine on change. Changing bug in #643.

Updates #643

net/interfaces/interfaces.go

@@ -163,6 +163,10 @@ type State struct {
 	// considered "expensive", which currently means LTE/etc
 	// instead of Wifi. This field is not populated by GetState.
 	IsExpensive bool
+
+	// DefaultRouteInterface is the interface name for the machine's default route.
+	// It is not yet populated on all OSes.
+	DefaultRouteInterface string
 }
 
 func (s *State) Equal(s2 *State) bool {
@@ -198,6 +202,7 @@ func GetState() (*State, error) {
 	}); err != nil {
 		return nil, err
 	}
+	s.DefaultRouteInterface, _ = DefaultRouteInterface()
 	return s, nil
 }
 

net/interfaces/interfaces_defaultrouteif_todo.go

@@ -0,0 +1,15 @@
+// Copyright (c) 2020 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 !linux
+
+package interfaces
+
+import "errors"
+
+var errTODO = errors.New("TODO")
+
+func DefaultRouteInterface() (string, error) {
+	return "TODO", errTODO
+}

wgengine/userspace.go

@@ -212,6 +212,7 @@ func newUserspaceEngineAdvanced(conf EngineConfig) (_ Engine, reterr error) {
 	}
 	e.localAddrs.Store(map[packet.IP]bool{})
 	e.linkState, _ = getLinkState()
+	logf("link state: %+v", e.linkState)
 
 	// Respond to all pings only in fake mode.
 	if conf.Fake {
@@ -1105,7 +1106,11 @@ func (e *userspaceEngine) LinkChange(isExpensive bool) {
 	cur.IsExpensive = isExpensive
 	needRebind := e.setLinkState(cur)
 
-	e.logf("LinkChange(isExpensive=%v); needsRebind=%v", isExpensive, needRebind)
+	if needRebind {
+		e.logf("LinkChange: major, rebinding. New state: %+v", cur)
+	} else {
+		e.logf("LinkChange: minor")
+	}
 
 	why := "link-change-minor"
 	if needRebind {