Set NDP hop limit in accordance with RFC 4861

...and do not populate link address cache at dispatch. This partially
reverts 313c767b0001bf6271405f1b765b60a334d6e911, which caused malformed
packets (e.g. NDP Neighbor Adverts with incorrect hop limit values) to
populate the address cache. In particular, this masked a bug that was
introduced to the Neighbor Advert generation code in
7c1587e3401a010d1865df61dbaf117c77dd062e.

PiperOrigin-RevId: 274865182

3 files changed, 23 insertions, 11 deletions

diff --git a/pkg/tcpip/network/arp/arp.go b/pkg/tcpip/network/arp/arp.go
index 922181ac0..6b1e854dc 100644
--- a/pkg/tcpip/network/arp/arp.go
+++ b/pkg/tcpip/network/arp/arp.go
@@ -109,7 +109,11 @@ func (e *endpoint) HandlePacket(r *stack.Route, vv buffer.VectorisedView) {
 		copy(pkt.HardwareAddressTarget(), h.HardwareAddressSender())
 		copy(pkt.ProtocolAddressTarget(), h.ProtocolAddressSender())
 		e.linkEP.WritePacket(r, nil /* gso */, hdr, buffer.VectorisedView{}, ProtocolNumber)
+		fallthrough // also fill the cache from requests
 	case header.ARPReply:
+		addr := tcpip.Address(h.ProtocolAddressSender())
+		linkAddr := tcpip.LinkAddress(h.HardwareAddressSender())
+		e.linkAddrCache.AddLinkAddress(e.nicid, addr, linkAddr)
 	}
 }
 
diff --git a/pkg/tcpip/network/ipv6/icmp.go b/pkg/tcpip/network/ipv6/icmp.go
index b5df85455..f543ceb92 100644
--- a/pkg/tcpip/network/ipv6/icmp.go
+++ b/pkg/tcpip/network/ipv6/icmp.go
@@ -121,7 +121,6 @@ func (e *endpoint) handleICMP(r *stack.Route, netHeader buffer.View, vv buffer.V
 
 	case header.ICMPv6NeighborSolicit:
 		received.NeighborSolicit.Increment()
-
 		if len(v) < header.ICMPv6NeighborSolicitMinimumSize {
 			received.Invalid.Increment()
 			return
@@ -131,7 +130,6 @@ func (e *endpoint) handleICMP(r *stack.Route, netHeader buffer.View, vv buffer.V
 			// We don't have a useful answer; the best we can do is ignore the request.
 			return
 		}
-
 		hdr := buffer.NewPrependable(int(r.MaxHeaderLength()) + header.ICMPv6NeighborAdvertSize)
 		pkt := header.ICMPv6(hdr.Prepend(header.ICMPv6NeighborAdvertSize))
 		pkt.SetType(header.ICMPv6NeighborAdvert)
@@ -154,7 +152,22 @@ func (e *endpoint) handleICMP(r *stack.Route, netHeader buffer.View, vv buffer.V
 		r.LocalAddress = targetAddr
 		pkt.SetChecksum(header.ICMPv6Checksum(pkt, r.LocalAddress, r.RemoteAddress, buffer.VectorisedView{}))
 
-		if err := r.WritePacket(nil /* gso */, hdr, buffer.VectorisedView{}, stack.NetworkHeaderParams{Protocol: header.ICMPv6ProtocolNumber, TTL: r.DefaultTTL(), TOS: stack.DefaultTOS}); err != nil {
+		// TODO(tamird/ghanan): there exists an explicit NDP option that is
+		// used to update the neighbor table with link addresses for a
+		// neighbor from an NS (see the Source Link Layer option RFC
+		// 4861 section 4.6.1 and section 7.2.3).
+		//
+		// Furthermore, the entirety of NDP handling here seems to be
+		// contradicted by RFC 4861.
+		e.linkAddrCache.AddLinkAddress(e.nicid, r.RemoteAddress, r.RemoteLinkAddress)
+
+		// RFC 4861 Neighbor Discovery for IP version 6 (IPv6)
+		//
+		// 7.1.2. Validation of Neighbor Advertisements
+		//
+		// The IP Hop Limit field has a value of 255, i.e., the packet
+		// could not possibly have been forwarded by a router.
+		if err := r.WritePacket(nil /* gso */, hdr, buffer.VectorisedView{}, stack.NetworkHeaderParams{Protocol: header.ICMPv6ProtocolNumber, TTL: ndpHopLimit, TOS: stack.DefaultTOS}); err != nil {
 			sent.Dropped.Increment()
 			return
 		}
@@ -178,7 +191,6 @@ func (e *endpoint) handleICMP(r *stack.Route, netHeader buffer.View, vv buffer.V
 			received.Invalid.Increment()
 			return
 		}
-
 		vv.TrimFront(header.ICMPv6EchoMinimumSize)
 		hdr := buffer.NewPrependable(int(r.MaxHeaderLength()) + header.ICMPv6EchoMinimumSize)
 		pkt := header.ICMPv6(hdr.Prepend(header.ICMPv6EchoMinimumSize))
diff --git a/pkg/tcpip/network/ipv6/icmp_test.go b/pkg/tcpip/network/ipv6/icmp_test.go
index 501be208e..dd3c4d7c4 100644
--- a/pkg/tcpip/network/ipv6/icmp_test.go
+++ b/pkg/tcpip/network/ipv6/icmp_test.go
@@ -15,7 +15,6 @@
 package ipv6
 
 import (
-	"fmt"
 	"reflect"
 	"strings"
 	"testing"
@@ -179,13 +178,10 @@ func visitStats(v reflect.Value, f func(string, *tcpip.StatCounter)) {
 	t := v.Type()
 	for i := 0; i < v.NumField(); i++ {
 		v := v.Field(i)
-		switch v.Kind() {
-		case reflect.Ptr:
-			f(t.Field(i).Name, v.Interface().(*tcpip.StatCounter))
-		case reflect.Struct:
+		if s, ok := v.Interface().(*tcpip.StatCounter); ok {
+			f(t.Field(i).Name, s)
+		} else {
 			visitStats(v, f)
-		default:
-			panic(fmt.Sprintf("unexpected type %s", v.Type()))
 		}
 	}
 }