Allow replacing filters

Add a function FilterReplace, which mirrors the behaviour of
QdiscReplace, etc. This makes it possible to swap out filters
with a single netlink message.

filter_linux.go

@@ -151,8 +151,24 @@ func FilterAdd(filter Filter) error {
 // FilterAdd will add a filter to the system.
 // Equivalent to: `tc filter add $filter`
 func (h *Handle) FilterAdd(filter Filter) error {
+	return h.filterModify(filter, unix.NLM_F_CREATE|unix.NLM_F_EXCL)
+}
+
+// FilterReplace will replace a filter.
+// Equivalent to: `tc filter replace $filter`
+func FilterReplace(filter Filter) error {
+	return pkgHandle.FilterReplace(filter)
+}
+
+// FilterReplace will replace a filter.
+// Equivalent to: `tc filter replace $filter`
+func (h *Handle) FilterReplace(filter Filter) error {
+	return h.filterModify(filter, unix.NLM_F_CREATE)
+}
+
+func (h *Handle) filterModify(filter Filter, flags int) error {
 	native = nl.NativeEndian()
-	req := h.newNetlinkRequest(unix.RTM_NEWTFILTER, unix.NLM_F_CREATE|unix.NLM_F_EXCL|unix.NLM_F_ACK)
+	req := h.newNetlinkRequest(unix.RTM_NEWTFILTER, flags|unix.NLM_F_ACK)
 	base := filter.Attrs()
 	msg := &nl.TcMsg{
 		Family:  nl.FAMILY_ALL,

filter_test.go

@@ -103,6 +103,67 @@ func TestFilterAddDel(t *testing.T) {
 	}
 }
 
+func TestFilterReplace(t *testing.T) {
+	tearDown := setUpNetlinkTest(t)
+	defer tearDown()
+	if err := LinkAdd(&Ifb{LinkAttrs{Name: "foo"}}); err != nil {
+		t.Fatal(err)
+	}
+	if err := LinkAdd(&Ifb{LinkAttrs{Name: "bar"}}); err != nil {
+		t.Fatal(err)
+	}
+	link, err := LinkByName("foo")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := LinkSetUp(link); err != nil {
+		t.Fatal(err)
+	}
+	redir, err := LinkByName("bar")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := LinkSetUp(redir); err != nil {
+		t.Fatal(err)
+	}
+	qdisc := &Ingress{
+		QdiscAttrs: QdiscAttrs{
+			LinkIndex: link.Attrs().Index,
+			Handle:    MakeHandle(0xffff, 0),
+			Parent:    HANDLE_INGRESS,
+		},
+	}
+	if err := QdiscAdd(qdisc); err != nil {
+		t.Fatal(err)
+	}
+
+	filter := &U32{
+		FilterAttrs: FilterAttrs{
+			LinkIndex: link.Attrs().Index,
+			Parent:    MakeHandle(0xffff, 0),
+			Priority:  1,
+			Protocol:  unix.ETH_P_IP,
+		},
+		RedirIndex: redir.Attrs().Index,
+		ClassId:    MakeHandle(1, 1),
+	}
+
+	if err := FilterReplace(filter); err != nil {
+		t.Fatal(err)
+	}
+	filters, err := FilterList(link, MakeHandle(0xffff, 0))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(filters) != 1 {
+		t.Fatal("Failed replace filter")
+	}
+
+	if err := FilterReplace(filter); err != nil {
+		t.Fatal(err)
+	}
+}
+
 func TestAdvancedFilterAddDel(t *testing.T) {
 	tearDown := setUpNetlinkTest(t)
 	defer tearDown()