Add ability to send unicast ARP requests and Neighbor Solicitations

The previous implementation of LinkAddressRequest only supported sending
broadcast ARP requests and multicast Neighbor Solicitations. The ability to
send these packets as unicast is required for Neighbor Unreachability
Detection.

Tests:
 pkg/tcpip/network/arp:arp_test
 - TestLinkAddressRequest

 pkg/tcpip/network/ipv6:ipv6_test
 - TestLinkAddressRequest

Updates #1889
Updates #1894
Updates #1895
Updates #1947
Updates #1948
Updates #1949
Updates #1950

PiperOrigin-RevId: 323451569

2 files changed, 56 insertions, 9 deletions

diff --git a/pkg/tcpip/network/arp/arp.go b/pkg/tcpip/network/arp/arp.go
index b0f57040c..31a242482 100644
--- a/pkg/tcpip/network/arp/arp.go
+++ b/pkg/tcpip/network/arp/arp.go
@@ -160,9 +160,12 @@ func (*protocol) LinkAddressProtocol() tcpip.NetworkProtocolNumber {
 }
 
 // LinkAddressRequest implements stack.LinkAddressResolver.LinkAddressRequest.
-func (*protocol) LinkAddressRequest(addr, localAddr tcpip.Address, linkEP stack.LinkEndpoint) *tcpip.Error {
+func (*protocol) LinkAddressRequest(addr, localAddr tcpip.Address, remoteLinkAddr tcpip.LinkAddress, linkEP stack.LinkEndpoint) *tcpip.Error {
 	r := &stack.Route{
-		RemoteLinkAddress: header.EthernetBroadcastAddress,
+		RemoteLinkAddress: remoteLinkAddr,
+	}
+	if len(r.RemoteLinkAddress) == 0 {
+		r.RemoteLinkAddress = header.EthernetBroadcastAddress
 	}
 
 	hdr := buffer.NewPrependable(int(linkEP.MaxHeaderLength()) + header.ARPSize)
diff --git a/pkg/tcpip/network/arp/arp_test.go b/pkg/tcpip/network/arp/arp_test.go
index 66e67429c..a35a64a0f 100644
--- a/pkg/tcpip/network/arp/arp_test.go
+++ b/pkg/tcpip/network/arp/arp_test.go
@@ -32,10 +32,14 @@ import (
 )
 
 const (
-	stackLinkAddr = tcpip.LinkAddress("\x0a\x0a\x0b\x0b\x0c\x0c")
-	stackAddr1    = tcpip.Address("\x0a\x00\x00\x01")
-	stackAddr2    = tcpip.Address("\x0a\x00\x00\x02")
-	stackAddrBad  = tcpip.Address("\x0a\x00\x00\x03")
+	stackLinkAddr1 = tcpip.LinkAddress("\x0a\x0a\x0b\x0b\x0c\x0c")
+	stackLinkAddr2 = tcpip.LinkAddress("\x0b\x0b\x0c\x0c\x0d\x0d")
+	stackAddr1     = tcpip.Address("\x0a\x00\x00\x01")
+	stackAddr2     = tcpip.Address("\x0a\x00\x00\x02")
+	stackAddrBad   = tcpip.Address("\x0a\x00\x00\x03")
+
+	defaultChannelSize = 1
+	defaultMTU         = 65536
 )
 
 type testContext struct {
@@ -50,8 +54,7 @@ func newTestContext(t *testing.T) *testContext {
 		TransportProtocols: []stack.TransportProtocol{icmp.NewProtocol4()},
 	})
 
-	const defaultMTU = 65536
-	ep := channel.New(256, defaultMTU, stackLinkAddr)
+	ep := channel.New(defaultChannelSize, defaultMTU, stackLinkAddr1)
 	wep := stack.LinkEndpoint(ep)
 
 	if testing.Verbose() {
@@ -119,7 +122,7 @@ func TestDirectRequest(t *testing.T) {
 			if !rep.IsValid() {
 				t.Fatalf("invalid ARP response pi.Pkt.Header.UsedLength()=%d", pi.Pkt.Header.UsedLength())
 			}
-			if got, want := tcpip.LinkAddress(rep.HardwareAddressSender()), stackLinkAddr; got != want {
+			if got, want := tcpip.LinkAddress(rep.HardwareAddressSender()), stackLinkAddr1; got != want {
 				t.Errorf("got HardwareAddressSender = %s, want = %s", got, want)
 			}
 			if got, want := tcpip.Address(rep.ProtocolAddressSender()), tcpip.Address(h.ProtocolAddressTarget()); got != want {
@@ -144,3 +147,44 @@ func TestDirectRequest(t *testing.T) {
 		t.Errorf("stackAddrBad: unexpected packet sent, Proto=%v", pkt.Proto)
 	}
 }
+
+func TestLinkAddressRequest(t *testing.T) {
+	tests := []struct {
+		name           string
+		remoteLinkAddr tcpip.LinkAddress
+		expectLinkAddr tcpip.LinkAddress
+	}{
+		{
+			name:           "Unicast",
+			remoteLinkAddr: stackLinkAddr2,
+			expectLinkAddr: stackLinkAddr2,
+		},
+		{
+			name:           "Multicast",
+			remoteLinkAddr: "",
+			expectLinkAddr: header.EthernetBroadcastAddress,
+		},
+	}
+
+	for _, test := range tests {
+		p := arp.NewProtocol()
+		linkRes, ok := p.(stack.LinkAddressResolver)
+		if !ok {
+			t.Fatal("expected ARP protocol to implement stack.LinkAddressResolver")
+		}
+
+		linkEP := channel.New(defaultChannelSize, defaultMTU, stackLinkAddr1)
+		if err := linkRes.LinkAddressRequest(stackAddr1, stackAddr2, test.remoteLinkAddr, linkEP); err != nil {
+			t.Errorf("got p.LinkAddressRequest(%s, %s, %s, _) = %s", stackAddr1, stackAddr2, test.remoteLinkAddr, err)
+		}
+
+		pkt, ok := linkEP.Read()
+		if !ok {
+			t.Fatal("expected to send a link address request")
+		}
+
+		if got, want := pkt.Route.RemoteLinkAddress, test.expectLinkAddr; got != want {
+			t.Errorf("got pkt.Route.RemoteLinkAddress = %s, want = %s", got, want)
+		}
+	}
+}