Use multicast Ethernet address for multicast NDP

As per RFC 2464 section 7, an IPv6 packet with a multicast destination
address is transmitted to the mapped Ethernet multicast address.

Test:
- ipv6.TestLinkResolution
- stack_test.TestDADResolve
- stack_test.TestRouterSolicitation
PiperOrigin-RevId: 292610529

3 files changed, 35 insertions, 2 deletions

diff --git a/pkg/tcpip/stack/ndp.go b/pkg/tcpip/stack/ndp.go
index 31294345d..6123fda33 100644
--- a/pkg/tcpip/stack/ndp.go
+++ b/pkg/tcpip/stack/ndp.go
@@ -538,6 +538,14 @@ func (ndp *ndpState) sendDADPacket(addr tcpip.Address) *tcpip.Error {
 	r := makeRoute(header.IPv6ProtocolNumber, header.IPv6Any, snmc, ndp.nic.linkEP.LinkAddress(), ref, false, false)
 	defer r.Release()
 
+	// Route should resolve immediately since snmc is a multicast address so a
+	// remote link address can be calculated without a resolution process.
+	if c, err := r.Resolve(nil); err != nil {
+		log.Fatalf("ndp: error when resolving route to send NDP NS for DAD (%s -> %s on NIC(%d)): %s", header.IPv6Any, snmc, ndp.nic.ID(), err)
+	} else if c != nil {
+		log.Fatalf("ndp: route resolution not immediate for route to send NDP NS for DAD (%s -> %s on NIC(%d))", header.IPv6Any, snmc, ndp.nic.ID())
+	}
+
 	hdr := buffer.NewPrependable(int(r.MaxHeaderLength()) + header.ICMPv6NeighborSolicitMinimumSize)
 	pkt := header.ICMPv6(hdr.Prepend(header.ICMPv6NeighborSolicitMinimumSize))
 	pkt.SetType(header.ICMPv6NeighborSolicit)
@@ -1197,6 +1205,15 @@ func (ndp *ndpState) startSolicitingRouters() {
 		r := makeRoute(header.IPv6ProtocolNumber, header.IPv6Any, header.IPv6AllRoutersMulticastAddress, ndp.nic.linkEP.LinkAddress(), ref, false, false)
 		defer r.Release()
 
+		// Route should resolve immediately since
+		// header.IPv6AllRoutersMulticastAddress is a multicast address so a
+		// remote link address can be calculated without a resolution process.
+		if c, err := r.Resolve(nil); err != nil {
+			log.Fatalf("ndp: error when resolving route to send NDP RS (%s -> %s on NIC(%d)): %s", header.IPv6Any, header.IPv6AllRoutersMulticastAddress, ndp.nic.ID(), err)
+		} else if c != nil {
+			log.Fatalf("ndp: route resolution not immediate for route to send NDP RS (%s -> %s on NIC(%d))", header.IPv6Any, header.IPv6AllRoutersMulticastAddress, ndp.nic.ID())
+		}
+
 		payloadSize := header.ICMPv6HeaderSize + header.NDPRSMinimumSize
 		hdr := buffer.NewPrependable(header.IPv6MinimumSize + payloadSize)
 		pkt := header.ICMPv6(hdr.Prepend(payloadSize))
diff --git a/pkg/tcpip/stack/ndp_test.go b/pkg/tcpip/stack/ndp_test.go
index bc7cfbcb4..8af8565f7 100644
--- a/pkg/tcpip/stack/ndp_test.go
+++ b/pkg/tcpip/stack/ndp_test.go
@@ -336,6 +336,7 @@ func TestDADResolve(t *testing.T) {
 			opts.NDPConfigs.DupAddrDetectTransmits = test.dupAddrDetectTransmits
 
 			e := channel.New(int(test.dupAddrDetectTransmits), 1280, linkAddr1)
+			e.LinkEPCapabilities |= stack.CapabilityResolutionRequired
 			s := stack.New(opts)
 			if err := s.CreateNIC(nicID, e); err != nil {
 				t.Fatalf("CreateNIC(%d, _) = %s", nicID, err)
@@ -413,6 +414,12 @@ func TestDADResolve(t *testing.T) {
 					t.Fatalf("got Proto = %d, want = %d", p.Proto, header.IPv6ProtocolNumber)
 				}
 
+				// Make sure the right remote link address is used.
+				snmc := header.SolicitedNodeAddr(addr1)
+				if want := header.EthernetAddressFromMulticastIPv6Address(snmc); p.Route.RemoteLinkAddress != want {
+					t.Errorf("got remote link address = %s, want = %s", p.Route.RemoteLinkAddress, want)
+				}
+
 				// Check NDP NS packet.
 				//
 				// As per RFC 4861 section 4.3, a possible option is the Source Link
@@ -420,7 +427,7 @@ func TestDADResolve(t *testing.T) {
 				// address of the packet is the unspecified address.
 				checker.IPv6(t, p.Pkt.Header.View().ToVectorisedView().First(),
 					checker.SrcAddr(header.IPv6Any),
-					checker.DstAddr(header.SolicitedNodeAddr(addr1)),
+					checker.DstAddr(snmc),
 					checker.TTL(header.NDPHopLimit),
 					checker.NDPNS(
 						checker.NDPNSTargetAddress(addr1),
@@ -3292,6 +3299,7 @@ func TestRouterSolicitation(t *testing.T) {
 			t.Run(test.name, func(t *testing.T) {
 				t.Parallel()
 				e := channel.New(int(test.maxRtrSolicit), 1280, linkAddr1)
+				e.LinkEPCapabilities |= stack.CapabilityResolutionRequired
 				waitForPkt := func(timeout time.Duration) {
 					t.Helper()
 					ctx, _ := context.WithTimeout(context.Background(), timeout)
@@ -3304,6 +3312,12 @@ func TestRouterSolicitation(t *testing.T) {
 					if p.Proto != header.IPv6ProtocolNumber {
 						t.Fatalf("got Proto = %d, want = %d", p.Proto, header.IPv6ProtocolNumber)
 					}
+
+					// Make sure the right remote link address is used.
+					if want := header.EthernetAddressFromMulticastIPv6Address(header.IPv6AllRoutersMulticastAddress); p.Route.RemoteLinkAddress != want {
+						t.Errorf("got remote link address = %s, want = %s", p.Route.RemoteLinkAddress, want)
+					}
+
 					checker.IPv6(t,
 						p.Pkt.Header.View(),
 						checker.SrcAddr(header.IPv6Any),
diff --git a/pkg/tcpip/stack/route.go b/pkg/tcpip/stack/route.go
index 517f4b941..f565aafb2 100644
--- a/pkg/tcpip/stack/route.go
+++ b/pkg/tcpip/stack/route.go
@@ -225,7 +225,9 @@ func (r *Route) Release() {
 // Clone Clone a route such that the original one can be released and the new
 // one will remain valid.
 func (r *Route) Clone() Route {
-	r.ref.incRef()
+	if r.ref != nil {
+		r.ref.incRef()
+	}
 	return *r
 }