add subnet to StackAsync and add test for local arp resolving

x/xnet/xnet_arp_test.go

@@ -0,0 +1,49 @@
+package xnet
+
+import (
+	"bytes"
+	"net/netip"
+	"testing"
+)
+
+func TestARPLocal(t *testing.T) {
+	const mtu = 1500
+	const seed = 1
+	s1, s2, c1, c2 := newTCPStacks(t, seed, mtu)
+	routerHw := [6]byte{1, 2, 3, 4, 5, 6}
+	// Most common case: we have a router in between computers.
+	s1.SetGateway6(routerHw)
+	s2.SetGateway6(routerHw)
+	addr1 := netip.AddrPortFrom(s1.Addr(), 1024) // dialer, client.
+	addr2 := netip.AddrPortFrom(s2.Addr(), 80)   // listener, server.
+	err := s1.AssimilateDHCPResults(&DHCPResults{
+		Router:        netip.AddrFrom4([4]byte{10, 0, 0, 255}),
+		BroadcastAddr: netip.AddrFrom4([4]byte{255, 255, 255, 255}),
+		AssignedAddr:  s1.Addr(),
+		Subnet:        netip.PrefixFrom(s2.Addr(), 24), // Subnet containing s2 will force an ARP on s1.
+		TRenewal:      1000,
+		TRebind:       1000,
+		TLease:        1000,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	hw2 := s2.HardwareAddress()
+	err = s1.DialTCP(c1, addr1.Port(), addr2) // addr2 MAC address is unknown and must be resolved by stack.
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = s2.ListenTCP(c2, addr2.Port())
+	if err != nil {
+		t.Fatal(err)
+	}
+	tst := testerFrom(t, mtu)
+	_ = tst
+	tst.ARPExchangeOnly(s1, s2)
+	hwaddr, err := s1.arp.QueryResult(addr2.Addr().AsSlice())
+	if err != nil {
+		t.Fatal(err)
+	} else if !bytes.Equal(hwaddr[:], hw2[:]) {
+		t.Errorf("expected hardware address %x, got %x", hw2, hwaddr)
+	}
+}