Add support for MTU Lock

When adding a route with "mtu lock <mtu>" path MTU discovery (PMTUD)
will not be tried and packets will be sent without DF bit set. Upon
receiving an ICMP needs frag due to PMTUD, the kernel will not install a
cached route and lower the MTU.

Signed-off-by: Tim Rozet <[email protected]>

Author: trozet

route.go

@@ -45,7 +45,7 @@ type Encap interface {
 	Equal(Encap) bool
 }
 
-//Protocol describe what was the originator of the route
+// Protocol describe what was the originator of the route
 type RouteProtocol int
 
 // Route represents a netlink route.
@@ -70,6 +70,7 @@ type Route struct {
 	Via              Destination
 	Realm            int
 	MTU              int
+	MTULock          bool
 	Window           int
 	Rtt              int
 	RttVar           int

route_linux.go

@@ -1072,6 +1072,10 @@ func (h *Handle) prepareRouteReq(route *Route, req *nl.NetlinkRequest, msg *nl.R
 	if route.MTU > 0 {
 		b := nl.Uint32Attr(uint32(route.MTU))
 		metrics = append(metrics, nl.NewRtAttr(unix.RTAX_MTU, b))
+		if route.MTULock {
+			b := nl.Uint32Attr(uint32(1 << unix.RTAX_MTU))
+			metrics = append(metrics, nl.NewRtAttr(unix.RTAX_LOCK, b))
+		}
 	}
 	if route.Window > 0 {
 		b := nl.Uint32Attr(uint32(route.Window))
@@ -1440,6 +1444,8 @@ func deserializeRoute(m []byte) (Route, error) {
 				switch metric.Attr.Type {
 				case unix.RTAX_MTU:
 					route.MTU = int(native.Uint32(metric.Value[0:4]))
+				case unix.RTAX_LOCK:
+					route.MTULock = native.Uint32(metric.Value[0:4]) == uint32(1<<unix.RTAX_MTU)
 				case unix.RTAX_WINDOW:
 					route.Window = int(native.Uint32(metric.Value[0:4]))
 				case unix.RTAX_RTT:

route_test.go

@@ -2260,6 +2260,64 @@ func TestMTURouteAddDel(t *testing.T) {
 	}
 }
 
+func TestMTULockRouteAddDel(t *testing.T) {
+	_, err := RouteList(nil, FAMILY_V4)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tearDown := setUpNetlinkTest(t)
+	defer tearDown()
+
+	// get loopback interface
+	link, err := LinkByName("lo")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// bring the interface up
+	if err := LinkSetUp(link); err != nil {
+		t.Fatal(err)
+	}
+
+	// add a gateway route
+	dst := &net.IPNet{
+		IP:   net.IPv4(192, 168, 0, 0),
+		Mask: net.CIDRMask(24, 32),
+	}
+
+	route := Route{LinkIndex: link.Attrs().Index, Dst: dst, MTU: 500, MTULock: true}
+	if err := RouteAdd(&route); err != nil {
+		t.Fatal(err)
+	}
+	routes, err := RouteList(link, FAMILY_V4)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(routes) != 1 {
+		t.Fatal("Route not added properly")
+	}
+
+	if route.MTU != routes[0].MTU {
+		t.Fatal("Route MTU not set properly")
+	}
+
+	if route.MTULock != routes[0].MTULock {
+		t.Fatal("Route MTU lock not set properly")
+	}
+
+	if err := RouteDel(&route); err != nil {
+		t.Fatal(err)
+	}
+	routes, err = RouteList(link, FAMILY_V4)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(routes) != 0 {
+		t.Fatal("Route not removed properly")
+	}
+}
+
 func TestRouteViaAddDel(t *testing.T) {
 	minKernelRequired(t, 5, 4)
 	tearDown := setUpNetlinkTest(t)