fix makeRouteTree allowMTU (#611)

With the previous implementation, we check if route.MTU is greater than zero,
but it will always be because we set it to the default MTU in
parseUnsafeRoutes. This change leaves it as zero in parseUnsafeRoutes so
it can be examined later.

overlay/route.go

@@ -7,6 +7,7 @@ import (
 	"runtime"
 	"strconv"
 
+	"github.com/sirupsen/logrus"
 	"github.com/slackhq/nebula/cidr"
 	"github.com/slackhq/nebula/config"
 	"github.com/slackhq/nebula/iputil"
@@ -19,11 +20,11 @@ type Route struct {
 	Via    *iputil.VpnIp
 }
 
-func makeRouteTree(routes []Route, allowMTU bool) (*cidr.Tree4, error) {
+func makeRouteTree(l *logrus.Logger, routes []Route, allowMTU bool) (*cidr.Tree4, error) {
 	routeTree := cidr.NewTree4()
 	for _, r := range routes {
 		if !allowMTU && r.MTU > 0 {
-			return nil, fmt.Errorf("route MTU is not supported in %s", runtime.GOOS)
+			l.WithField("route", r).Warnf("route MTU is not supported in %s", runtime.GOOS)
 		}
 
 		if r.Via != nil {
@@ -127,12 +128,9 @@ func parseUnsafeRoutes(c *config.C, network *net.IPNet) ([]Route, error) {
 			return nil, fmt.Errorf("entry %v in tun.unsafe_routes is invalid", i+1)
 		}
 
-		rMtu, ok := m["mtu"]
+		var mtu int
-		if !ok {
+		if rMtu, ok := m["mtu"]; ok {
-			rMtu = c.GetInt("tun.mtu", DefaultMTU)
+			mtu, ok = rMtu.(int)
-		}
-
-		mtu, ok := rMtu.(int)
 			if !ok {
 				mtu, err = strconv.Atoi(rMtu.(string))
 				if err != nil {
@@ -140,9 +138,10 @@ func parseUnsafeRoutes(c *config.C, network *net.IPNet) ([]Route, error) {
 				}
 			}
 
-		if mtu < 500 {
+			if mtu != 0 && mtu < 500 {
 				return nil, fmt.Errorf("entry %v.mtu in tun.unsafe_routes is below 500: %v", i+1, mtu)
 			}
+		}
 
 		rMetric, ok := m["metric"]
 		if !ok {

overlay/route_test.go

@@ -191,7 +191,7 @@ func Test_parseUnsafeRoutes(t *testing.T) {
 	c.Settings["tun"] = map[interface{}]interface{}{"unsafe_routes": []interface{}{map[interface{}]interface{}{"via": "127.0.0.1", "route": "1.0.0.0/8"}}}
 	routes, err = parseUnsafeRoutes(c, n)
 	assert.Len(t, routes, 1)
-	assert.Equal(t, DefaultMTU, routes[0].MTU)
+	assert.Equal(t, 0, routes[0].MTU)
 
 	// bad mtu
 	c.Settings["tun"] = map[interface{}]interface{}{"unsafe_routes": []interface{}{map[interface{}]interface{}{"via": "127.0.0.1", "mtu": "nope"}}}
@@ -249,7 +249,7 @@ func Test_makeRouteTree(t *testing.T) {
 	routes, err := parseUnsafeRoutes(c, n)
 	assert.NoError(t, err)
 	assert.Len(t, routes, 2)
-	routeTree, err := makeRouteTree(routes, true)
+	routeTree, err := makeRouteTree(l, routes, true)
 	assert.NoError(t, err)
 
 	ip := iputil.Ip2VpnIp(net.ParseIP("1.0.0.2"))

overlay/tun_darwin.go

@@ -77,7 +77,7 @@ type ifreqMTU struct {
 }
 
 func newTun(l *logrus.Logger, name string, cidr *net.IPNet, defaultMTU int, routes []Route, _ int, _ bool) (*tun, error) {
-	routeTree, err := makeRouteTree(routes, false)
+	routeTree, err := makeRouteTree(l, routes, false)
 	if err != nil {
 		return nil, err
 	}

overlay/tun_freebsd.go

@@ -43,7 +43,7 @@ func newTunFromFd(_ *logrus.Logger, _ int, _ *net.IPNet, _ int, _ []Route, _ int
 }
 
 func newTun(l *logrus.Logger, deviceName string, cidr *net.IPNet, defaultMTU int, routes []Route, _ int, _ bool) (*tun, error) {
-	routeTree, err := makeRouteTree(routes, false)
+	routeTree, err := makeRouteTree(l, routes, false)
 	if err != nil {
 		return nil, err
 	}

overlay/tun_linux.go

@@ -64,7 +64,7 @@ type ifreqQLEN struct {
 }
 
 func newTunFromFd(l *logrus.Logger, deviceFd int, cidr *net.IPNet, defaultMTU int, routes []Route, txQueueLen int) (*tun, error) {
-	routeTree, err := makeRouteTree(routes, true)
+	routeTree, err := makeRouteTree(l, routes, true)
 	if err != nil {
 		return nil, err
 	}
@@ -105,12 +105,16 @@ func newTun(l *logrus.Logger, deviceName string, cidr *net.IPNet, defaultMTU int
 
 	maxMTU := defaultMTU
 	for _, r := range routes {
+		if r.MTU == 0 {
+			r.MTU = defaultMTU
+		}
+
 		if r.MTU > maxMTU {
 			maxMTU = r.MTU
 		}
 	}
 
-	routeTree, err := makeRouteTree(routes, true)
+	routeTree, err := makeRouteTree(l, routes, true)
 	if err != nil {
 		return nil, err
 	}

overlay/tun_tester.go

@@ -25,7 +25,7 @@ type TestTun struct {
 }
 
 func newTun(l *logrus.Logger, deviceName string, cidr *net.IPNet, _ int, routes []Route, _ int, _ bool) (*TestTun, error) {
-	routeTree, err := makeRouteTree(routes, false)
+	routeTree, err := makeRouteTree(l, routes, false)
 	if err != nil {
 		return nil, err
 	}

overlay/tun_water_windows.go

@@ -7,6 +7,7 @@ import (
 	"os/exec"
 	"strconv"
 
+	"github.com/sirupsen/logrus"
 	"github.com/slackhq/nebula/cidr"
 	"github.com/slackhq/nebula/iputil"
 	"github.com/songgao/water"
@@ -22,8 +23,8 @@ type waterTun struct {
 	*water.Interface
 }
 
-func newWaterTun(cidr *net.IPNet, defaultMTU int, routes []Route) (*waterTun, error) {
+func newWaterTun(l *logrus.Logger, cidr *net.IPNet, defaultMTU int, routes []Route) (*waterTun, error) {
-	routeTree, err := makeRouteTree(routes, false)
+	routeTree, err := makeRouteTree(l, routes, false)
 	if err != nil {
 		return nil, err
 	}

overlay/tun_windows.go

@@ -30,14 +30,14 @@ func newTun(l *logrus.Logger, deviceName string, cidr *net.IPNet, defaultMTU int
 	}
 
 	if useWintun {
-		device, err := newWinTun(deviceName, cidr, defaultMTU, routes)
+		device, err := newWinTun(l, deviceName, cidr, defaultMTU, routes)
 		if err != nil {
 			return nil, fmt.Errorf("create Wintun interface failed, %w", err)
 		}
 		return device, nil
 	}
 
-	device, err := newWaterTun(cidr, defaultMTU, routes)
+	device, err := newWaterTun(l, cidr, defaultMTU, routes)
 	if err != nil {
 		return nil, fmt.Errorf("create wintap driver failed, %w", err)
 	}

overlay/tun_wintun_windows.go

@@ -7,6 +7,7 @@ import (
 	"net"
 	"unsafe"
 
+	"github.com/sirupsen/logrus"
 	"github.com/slackhq/nebula/cidr"
 	"github.com/slackhq/nebula/iputil"
 	"github.com/slackhq/nebula/wintun"
@@ -45,7 +46,7 @@ func generateGUIDByDeviceName(name string) (*windows.GUID, error) {
 	return (*windows.GUID)(unsafe.Pointer(&sum[0])), nil
 }
 
-func newWinTun(deviceName string, cidr *net.IPNet, defaultMTU int, routes []Route) (*winTun, error) {
+func newWinTun(l *logrus.Logger, deviceName string, cidr *net.IPNet, defaultMTU int, routes []Route) (*winTun, error) {
 	guid, err := generateGUIDByDeviceName(deviceName)
 	if err != nil {
 		return nil, fmt.Errorf("generate GUID failed: %w", err)
@@ -56,7 +57,7 @@ func newWinTun(deviceName string, cidr *net.IPNet, defaultMTU int, routes []Rout
 		return nil, fmt.Errorf("create TUN device failed: %w", err)
 	}
 
-	routeTree, err := makeRouteTree(routes, false)
+	routeTree, err := makeRouteTree(l, routes, false)
 	if err != nil {
 		return nil, err
 	}