Process NAs without target link-layer addresses

RFC 4861 section 4.4 comments the Target link-layer address option is sometimes
optional in a Neighbor Advertisement packet:

  "When responding to a unicast Neighbor Solicitation this option SHOULD be
  included."

Tests:
 pkg/tcpip/stack:stack_test
 - TestEntryStaleToReachableWhenSolicitedConfirmationWithoutAddress
 - TestEntryDelayToReachableWhenSolicitedConfirmationWithoutAddress
 - TestEntryProbeToReachableWhenSolicitedConfirmationWithoutAddress
 pkg/tcpip/network/ipv6:ipv6_test
 - TestCallsToNeighborCache
PiperOrigin-RevId: 337396493

1 files changed, 9 insertions, 9 deletions

diff --git a/pkg/tcpip/network/ipv6/icmp.go b/pkg/tcpip/network/ipv6/icmp.go
index 913b2140c..ead6bedcb 100644
--- a/pkg/tcpip/network/ipv6/icmp.go
+++ b/pkg/tcpip/network/ipv6/icmp.go
@@ -439,19 +439,19 @@ func (e *endpoint) handleICMP(r *stack.Route, pkt *stack.PacketBuffer, hasFragme
 
 		// If the NA message has the target link layer option, update the link
 		// address cache with the link address for the target of the message.
-		if len(targetLinkAddr) != 0 {
-			if e.nud == nil {
+		if e.nud == nil {
+			if len(targetLinkAddr) != 0 {
 				e.linkAddrCache.AddLinkAddress(e.nic.ID(), targetAddr, targetLinkAddr)
-				return
 			}
-
-			e.nud.HandleConfirmation(targetAddr, targetLinkAddr, stack.ReachabilityConfirmationFlags{
-				Solicited: na.SolicitedFlag(),
-				Override:  na.OverrideFlag(),
-				IsRouter:  na.RouterFlag(),
-			})
+			return
 		}
 
+		e.nud.HandleConfirmation(targetAddr, targetLinkAddr, stack.ReachabilityConfirmationFlags{
+			Solicited: na.SolicitedFlag(),
+			Override:  na.OverrideFlag(),
+			IsRouter:  na.RouterFlag(),
+		})
+
 	case header.ICMPv6EchoRequest:
 		received.EchoRequest.Increment()
 		icmpHdr, ok := pkt.TransportHeader().Consume(header.ICMPv6EchoMinimumSize)