Enable RouteListFiltered to seach default routes

The current implementation of RouteListFiltered checks for null Dst in
filter and this prevents filtering for default routes.

route_linux.go

@@ -283,14 +283,20 @@ func (h *Handle) RouteListFiltered(family int, filter *Route, filterMask uint64)
 				continue
 			case filterMask&RT_FILTER_SRC != 0 && !route.Src.Equal(filter.Src):
 				continue
-			case filterMask&RT_FILTER_DST != 0 && filter.Dst != nil:
-				if route.Dst == nil {
-					continue
-				}
-				aMaskLen, aMaskBits := route.Dst.Mask.Size()
-				bMaskLen, bMaskBits := filter.Dst.Mask.Size()
-				if !(route.Dst.IP.Equal(filter.Dst.IP) && aMaskLen == bMaskLen && aMaskBits == bMaskBits) {
-					continue
+			case filterMask&RT_FILTER_DST != 0:
+				if filter.Dst == nil {
+					if route.Dst != nil {
+						continue
+					}
+				} else {
+					if route.Dst == nil {
+						continue
+					}
+					aMaskLen, aMaskBits := route.Dst.Mask.Size()
+					bMaskLen, bMaskBits := filter.Dst.Mask.Size()
+					if !(route.Dst.IP.Equal(filter.Dst.IP) && aMaskLen == bMaskLen && aMaskBits == bMaskBits) {
+						continue
+					}
 				}
 			}
 		}

route_test.go

@@ -303,3 +303,85 @@ func TestRouteMultiPath(t *testing.T) {
 		t.Fatal("MultiPath Route not added properly")
 	}
 }
+
+func TestFilterDefaultRoute(t *testing.T) {
+	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)
+	}
+
+	address := &Addr{
+		IPNet: &net.IPNet{
+			IP:   net.IPv4(127, 0, 0, 2),
+			Mask: net.CIDRMask(24, 32),
+		},
+	}
+	if err = AddrAdd(link, address); err != nil {
+		t.Fatal(err)
+	}
+
+	// Add default route
+	gw := net.IPv4(127, 0, 0, 2)
+
+	defaultRoute := Route{
+		Dst: nil,
+		Gw:  gw,
+	}
+
+	if err := RouteAdd(&defaultRoute); err != nil {
+		t.Fatal(err)
+	}
+
+	// add an extra route
+	dst := &net.IPNet{
+		IP:   net.IPv4(192, 168, 0, 0),
+		Mask: net.CIDRMask(24, 32),
+	}
+
+	extraRoute := Route{
+		Dst: dst,
+		Gw:  gw,
+	}
+
+	if err := RouteAdd(&extraRoute); err != nil {
+		t.Fatal(err)
+	}
+	var filterTests = []struct {
+		filter   *Route
+		mask     uint64
+		expected net.IP
+	}{
+		{
+			&Route{Dst: nil},
+			RT_FILTER_DST,
+			gw,
+		},
+		{
+			&Route{Dst: dst},
+			RT_FILTER_DST,
+			gw,
+		},
+	}
+
+	for _, f := range filterTests {
+		routes, err := RouteListFiltered(FAMILY_V4, f.filter, f.mask)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if len(routes) != 1 {
+			t.Fatal("Route not filtered properly")
+		}
+		if !routes[0].Gw.Equal(gw) {
+			t.Fatal("Unexpected Gateway")
+		}
+	}
+
+}