Find route before sending NA response

This change also brings back the stack.Route.ResolveWith method so that
we can immediately resolve a route when sending an NA in response to a
a NS with a source link layer address option.

Test: ipv6_test.TestNeighorSolicitationResponse
PiperOrigin-RevId: 337185461

2 files changed, 146 insertions, 53 deletions

diff --git a/pkg/tcpip/network/ipv6/icmp.go b/pkg/tcpip/network/ipv6/icmp.go
index a454f6c34..913b2140c 100644
--- a/pkg/tcpip/network/ipv6/icmp.go
+++ b/pkg/tcpip/network/ipv6/icmp.go
@@ -252,26 +252,29 @@ func (e *endpoint) handleICMP(r *stack.Route, pkt *stack.PacketBuffer, hasFragme
 			return
 		}
 
-		it, err := ns.Options().Iter(false /* check */)
-		if err != nil {
-			// Options are not valid as per the wire format, silently drop the packet.
-			received.Invalid.Increment()
-			return
-		}
+		var sourceLinkAddr tcpip.LinkAddress
+		{
+			it, err := ns.Options().Iter(false /* check */)
+			if err != nil {
+				// Options are not valid as per the wire format, silently drop the
+				// packet.
+				received.Invalid.Increment()
+				return
+			}
 
-		sourceLinkAddr, ok := getSourceLinkAddr(it)
-		if !ok {
-			received.Invalid.Increment()
-			return
+			sourceLinkAddr, ok = getSourceLinkAddr(it)
+			if !ok {
+				received.Invalid.Increment()
+				return
+			}
 		}
 
-		unspecifiedSource := r.RemoteAddress == header.IPv6Any
-
 		// As per RFC 4861 section 4.3, the Source Link-Layer Address Option MUST
 		// NOT be included when the source IP address is the unspecified address.
 		// Otherwise, on link layers that have addresses this option MUST be
 		// included in multicast solicitations and SHOULD be included in unicast
 		// solicitations.
+		unspecifiedSource := r.RemoteAddress == header.IPv6Any
 		if len(sourceLinkAddr) == 0 {
 			if header.IsV6MulticastAddress(r.LocalAddress) && !unspecifiedSource {
 				received.Invalid.Increment()
@@ -297,41 +300,51 @@ func (e *endpoint) handleICMP(r *stack.Route, pkt *stack.PacketBuffer, hasFragme
 			return
 		}
 
-		// ICMPv6 Neighbor Solicit messages are always sent to
-		// specially crafted IPv6 multicast addresses. As a result, the
-		// route we end up with here has as its LocalAddress such a
-		// multicast address. It would be nonsense to claim that our
-		// source address is a multicast address, so we manually set
-		// the source address to the target address requested in the
-		// solicit message. Since that requires mutating the route, we
-		// must first clone it.
-		r := r.Clone()
-		defer r.Release()
-		r.LocalAddress = targetAddr
-
-		// As per RFC 4861 section 7.2.4, if the the source of the solicitation is
-		// the unspecified address, the node MUST set the Solicited flag to zero and
-		// multicast the advertisement to the all-nodes address.
-		solicited := true
+		// As per RFC 4861 section 7.2.4:
+		//
+		//   If the source of the solicitation is the unspecified address, the node
+		//   MUST [...] and multicast the advertisement to the all-nodes address.
+		//
+		remoteAddr := r.RemoteAddress
 		if unspecifiedSource {
-			solicited = false
-			r.RemoteAddress = header.IPv6AllNodesMulticastAddress
+			remoteAddr = header.IPv6AllNodesMulticastAddress
+		}
+
+		// Even if we were able to receive a packet from some remote, we may not
+		// have a route to it - the remote may be blocked via routing rules. We must
+		// always consult our routing table and find a route to the remote before
+		// sending any packet.
+		r, err := e.protocol.stack.FindRoute(e.nic.ID(), targetAddr, remoteAddr, ProtocolNumber, false /* multicastLoop */)
+		if err != nil {
+			// If we cannot find a route to the destination, silently drop the packet.
+			return
 		}
+		defer r.Release()
 
-		// If the NS has a source link-layer option, use the link address it
-		// specifies as the remote link address for the response instead of the
-		// source link address of the packet.
+		// If the NS has a source link-layer option, resolve the route immediately
+		// to avoid querying the neighbor table when the neighbor entry was updated
+		// as probing the neighbor table for a link address will transition the
+		// entry's state from stale to delay.
+		//
+		// Note, if the source link address is unspecified and this is a unicast
+		// solicitation, we may need to perform neighbor discovery to send the
+		// neighbor advertisement response. This is expected as per RFC 4861 section
+		// 7.2.4:
+		//
+		//   Because unicast Neighbor Solicitations are not required to include a
+		//   Source Link-Layer Address, it is possible that a node sending a
+		//   solicited Neighbor Advertisement does not have a corresponding link-
+		//   layer address for its neighbor in its Neighbor Cache. In such
+		//   situations, a node will first have to use Neighbor Discovery to
+		//   determine the link-layer address of its neighbor (i.e., send out a
+		//   multicast Neighbor Solicitation).
 		//
-		// TODO(#2401): As per RFC 4861 section 7.2.4 we should consult our link
-		// address cache for the right destination link address instead of manually
-		// patching the route with the remote link address if one is specified in a
-		// Source Link-Layer Address option.
 		if len(sourceLinkAddr) != 0 {
-			r.RemoteLinkAddress = sourceLinkAddr
+			r.ResolveWith(sourceLinkAddr)
 		}
 
 		optsSerializer := header.NDPOptionsSerializer{
-			header.NDPTargetLinkLayerAddressOption(r.LocalLinkAddress),
+			header.NDPTargetLinkLayerAddressOption(e.nic.LinkAddress()),
 		}
 		neighborAdvertSize := header.ICMPv6NeighborAdvertMinimumSize + optsSerializer.Length()
 		pkt := stack.NewPacketBuffer(stack.PacketBufferOptions{
@@ -341,7 +354,14 @@ func (e *endpoint) handleICMP(r *stack.Route, pkt *stack.PacketBuffer, hasFragme
 		packet := header.ICMPv6(pkt.TransportHeader().Push(neighborAdvertSize))
 		packet.SetType(header.ICMPv6NeighborAdvert)
 		na := header.NDPNeighborAdvert(packet.NDPPayload())
-		na.SetSolicitedFlag(solicited)
+
+		// As per RFC 4861 section 7.2.4:
+		//
+		//   If the source of the solicitation is the unspecified address, the node
+		//   MUST set the Solicited flag to zero and [..]. Otherwise, the node MUST
+		//   set the Solicited flag to one and [..].
+		//
+		na.SetSolicitedFlag(!unspecifiedSource)
 		na.SetOverrideFlag(true)
 		na.SetTargetAddress(targetAddr)
 		na.Options().Serialize(optsSerializer)
@@ -635,6 +655,7 @@ func (*protocol) LinkAddressRequest(addr, localAddr tcpip.Address, remoteLinkAdd
 	r := stack.Route{
 		LocalAddress:      localAddr,
 		RemoteAddress:     addr,
+		LocalLinkAddress:  linkEP.LinkAddress(),
 		RemoteLinkAddress: remoteLinkAddr,
 	}
 
diff --git a/pkg/tcpip/network/ipv6/ndp_test.go b/pkg/tcpip/network/ipv6/ndp_test.go
index 9033a9ed5..ac20f217e 100644
--- a/pkg/tcpip/network/ipv6/ndp_test.go
+++ b/pkg/tcpip/network/ipv6/ndp_test.go
@@ -15,6 +15,7 @@
 package ipv6
 
 import (
+	"context"
 	"strings"
 	"testing"
 	"time"
@@ -398,16 +399,17 @@ func TestNeighorSolicitationResponse(t *testing.T) {
 	}
 
 	tests := []struct {
-		name          string
-		nsOpts        header.NDPOptionsSerializer
-		nsSrcLinkAddr tcpip.LinkAddress
-		nsSrc         tcpip.Address
-		nsDst         tcpip.Address
-		nsInvalid     bool
-		naDstLinkAddr tcpip.LinkAddress
-		naSolicited   bool
-		naSrc         tcpip.Address
-		naDst         tcpip.Address
+		name                   string
+		nsOpts                 header.NDPOptionsSerializer
+		nsSrcLinkAddr          tcpip.LinkAddress
+		nsSrc                  tcpip.Address
+		nsDst                  tcpip.Address
+		nsInvalid              bool
+		naDstLinkAddr          tcpip.LinkAddress
+		naSolicited            bool
+		naSrc                  tcpip.Address
+		naDst                  tcpip.Address
+		performsLinkResolution bool
 	}{
 		{
 			name:          "Unspecified source to solicited-node multicast destination",
@@ -416,7 +418,7 @@ func TestNeighorSolicitationResponse(t *testing.T) {
 			nsSrc:         header.IPv6Any,
 			nsDst:         nicAddrSNMC,
 			nsInvalid:     false,
-			naDstLinkAddr: remoteLinkAddr0,
+			naDstLinkAddr: header.EthernetAddressFromMulticastIPv6Address(header.IPv6AllNodesMulticastAddress),
 			naSolicited:   false,
 			naSrc:         nicAddr,
 			naDst:         header.IPv6AllNodesMulticastAddress,
@@ -449,7 +451,6 @@ func TestNeighorSolicitationResponse(t *testing.T) {
 			nsDst:         nicAddr,
 			nsInvalid:     true,
 		},
-
 		{
 			name: "Specified source with 1 source ll to multicast destination",
 			nsOpts: header.NDPOptionsSerializer{
@@ -509,6 +510,10 @@ func TestNeighorSolicitationResponse(t *testing.T) {
 			naSolicited:   true,
 			naSrc:         nicAddr,
 			naDst:         remoteAddr,
+			// Since we send a unicast solicitations to a node without an entry for
+			// the remote, the node needs to perform neighbor discovery to get the
+			// remote's link address to send the advertisement response.
+			performsLinkResolution: true,
 		},
 		{
 			name: "Specified source with 1 source ll to unicast destination",
@@ -615,11 +620,78 @@ func TestNeighorSolicitationResponse(t *testing.T) {
 						t.Fatalf("got invalid = %d, want = 0", got)
 					}
 
-					p, got := e.Read()
+					if test.performsLinkResolution {
+						p, got := e.ReadContext(context.Background())
+						if !got {
+							t.Fatal("expected an NDP NS response")
+						}
+
+						if p.Route.LocalAddress != nicAddr {
+							t.Errorf("got p.Route.LocalAddress = %s, want = %s", p.Route.LocalAddress, nicAddr)
+						}
+						if p.Route.LocalLinkAddress != nicLinkAddr {
+							t.Errorf("p.Route.LocalLinkAddress = %s, want = %s", p.Route.LocalLinkAddress, nicLinkAddr)
+						}
+						respNSDst := header.SolicitedNodeAddr(test.nsSrc)
+						if p.Route.RemoteAddress != respNSDst {
+							t.Errorf("got p.Route.RemoteAddress = %s, want = %s", p.Route.RemoteAddress, respNSDst)
+						}
+						if want := header.EthernetAddressFromMulticastIPv6Address(respNSDst); p.Route.RemoteLinkAddress != want {
+							t.Errorf("got p.Route.RemoteLinkAddress = %s, want = %s", p.Route.RemoteLinkAddress, want)
+						}
+
+						checker.IPv6(t, stack.PayloadSince(p.Pkt.NetworkHeader()),
+							checker.SrcAddr(nicAddr),
+							checker.DstAddr(respNSDst),
+							checker.TTL(header.NDPHopLimit),
+							checker.NDPNS(
+								checker.NDPNSTargetAddress(test.nsSrc),
+								checker.NDPNSOptions([]header.NDPOption{
+									header.NDPSourceLinkLayerAddressOption(nicLinkAddr),
+								}),
+							))
+
+						ser := header.NDPOptionsSerializer{
+							header.NDPTargetLinkLayerAddressOption(linkAddr1),
+						}
+						ndpNASize := header.ICMPv6NeighborAdvertMinimumSize + ser.Length()
+						hdr := buffer.NewPrependable(header.IPv6MinimumSize + ndpNASize)
+						pkt := header.ICMPv6(hdr.Prepend(ndpNASize))
+						pkt.SetType(header.ICMPv6NeighborAdvert)
+						na := header.NDPNeighborAdvert(pkt.NDPPayload())
+						na.SetSolicitedFlag(true)
+						na.SetOverrideFlag(true)
+						na.SetTargetAddress(test.nsSrc)
+						na.Options().Serialize(ser)
+						pkt.SetChecksum(header.ICMPv6Checksum(pkt, test.nsSrc, nicAddr, buffer.VectorisedView{}))
+						payloadLength := hdr.UsedLength()
+						ip := header.IPv6(hdr.Prepend(header.IPv6MinimumSize))
+						ip.Encode(&header.IPv6Fields{
+							PayloadLength: uint16(payloadLength),
+							NextHeader:    uint8(header.ICMPv6ProtocolNumber),
+							HopLimit:      header.NDPHopLimit,
+							SrcAddr:       test.nsSrc,
+							DstAddr:       nicAddr,
+						})
+						e.InjectLinkAddr(ProtocolNumber, "", stack.NewPacketBuffer(stack.PacketBufferOptions{
+							Data: hdr.View().ToVectorisedView(),
+						}))
+					}
+
+					p, got := e.ReadContext(context.Background())
 					if !got {
 						t.Fatal("expected an NDP NA response")
 					}
 
+					if p.Route.LocalAddress != test.naSrc {
+						t.Errorf("got p.Route.LocalAddress = %s, want = %s", p.Route.LocalAddress, test.naSrc)
+					}
+					if p.Route.LocalLinkAddress != nicLinkAddr {
+						t.Errorf("p.Route.LocalLinkAddress = %s, want = %s", p.Route.LocalLinkAddress, nicLinkAddr)
+					}
+					if p.Route.RemoteAddress != test.naDst {
+						t.Errorf("got p.Route.RemoteAddress = %s, want = %s", p.Route.RemoteAddress, test.naDst)
+					}
 					if p.Route.RemoteLinkAddress != test.naDstLinkAddr {
 						t.Errorf("got p.Route.RemoteLinkAddress = %s, want = %s", p.Route.RemoteLinkAddress, test.naDstLinkAddr)
 					}