Resolve static link addresses in GetLinkAddress

If a network address has a static mapping to a link address, calculate
it in GetLinkAddress.

Test: stack_test.TestStaticGetLinkAddress
PiperOrigin-RevId: 353179616

pkg/tcpip/stack/stack.go

@@ -1533,7 +1533,7 @@ type LinkResolutionResult struct {
 	Success     bool
 }
 
-// GetLinkAddress finds the link address corresponding to a neighbor's address.
+// GetLinkAddress finds the link address corresponding to a network address.
 //
 // Returns ErrNotSupported if the stack is not configured with a link address
 // resolver for the specified network protocol.
@@ -1562,6 +1562,11 @@ func (s *Stack) GetLinkAddress(nicID tcpip.NICID, addr, localAddr tcpip.Address,
 		return tcpip.ErrNotSupported
 	}
 
+	if linkAddr, ok := linkRes.ResolveStaticAddress(addr); ok {
+		onResolve(LinkResolutionResult{LinkAddress: linkAddr, Success: true})
+		return nil
+	}
+
 	_, _, err := nic.getNeighborLinkAddress(addr, localAddr, linkRes, onResolve)
 	return err
 }

pkg/tcpip/stack/stack_test.go

@@ -4391,3 +4391,51 @@ func TestGetLinkAddressErrors(t *testing.T) {
 		t.Errorf("got s.GetLinkAddress(%d, '', '', %d, nil) = %s, want = %s", unknownNICID, ipv4.ProtocolNumber, err, tcpip.ErrNotSupported)
 	}
 }
+
+func TestStaticGetLinkAddress(t *testing.T) {
+	const (
+		nicID = 1
+	)
+
+	s := stack.New(stack.Options{
+		NetworkProtocols: []stack.NetworkProtocolFactory{arp.NewProtocol, ipv4.NewProtocol, ipv6.NewProtocol},
+	})
+	if err := s.CreateNIC(nicID, channel.New(0, 0, "")); err != nil {
+		t.Fatalf("CreateNIC(%d, _) = %s", nicID, err)
+	}
+
+	tests := []struct {
+		name             string
+		proto            tcpip.NetworkProtocolNumber
+		addr             tcpip.Address
+		expectedLinkAddr tcpip.LinkAddress
+	}{
+		{
+			name:             "IPv4",
+			proto:            ipv4.ProtocolNumber,
+			addr:             header.IPv4Broadcast,
+			expectedLinkAddr: header.EthernetBroadcastAddress,
+		},
+		{
+			name:             "IPv6",
+			proto:            ipv6.ProtocolNumber,
+			addr:             header.IPv6AllNodesMulticastAddress,
+			expectedLinkAddr: header.EthernetAddressFromMulticastIPv6Address(header.IPv6AllNodesMulticastAddress),
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			ch := make(chan stack.LinkResolutionResult, 1)
+			if err := s.GetLinkAddress(nicID, test.addr, "", test.proto, func(r stack.LinkResolutionResult) {
+				ch <- r
+			}); err != nil {
+				t.Fatalf("s.GetLinkAddress(%d, %s, '', %d, _): %s", nicID, test.addr, test.proto, err)
+			}
+
+			if diff := cmp.Diff(stack.LinkResolutionResult{LinkAddress: test.expectedLinkAddr, Success: true}, <-ch); diff != "" {
+				t.Fatalf("link resolution result mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}