Feat fake dns address (#902)

Works only with userspace implementation:
1. Configure host to solve DNS requests via a fake DSN server address in the Netbird network.
2. Add to firewall catch rule for these DNS requests.
3. Resolve these DNS requests and respond by writing directly to wireguard device.

client/internal/dns/response_writer_test.go

@@ -0,0 +1,93 @@
+package dns
+
+import (
+	"net"
+	"testing"
+
+	"github.com/golang/mock/gomock"
+	"github.com/google/gopacket"
+	"github.com/google/gopacket/layers"
+	"github.com/miekg/dns"
+
+	"github.com/netbirdio/netbird/iface/mocks"
+)
+
+func TestResponseWriterLocalAddr(t *testing.T) {
+	ctrl := gomock.NewController(t)
+	defer ctrl.Finish()
+
+	device := mocks.NewMockDevice(ctrl)
+	device.EXPECT().Write(gomock.Any(), gomock.Any())
+
+	request := &dns.Msg{
+		Question: []dns.Question{{
+			Name:   "google.com.",
+			Qtype:  dns.TypeA,
+			Qclass: dns.TypeA,
+		}},
+	}
+
+	replyMessage := &dns.Msg{}
+	replyMessage.SetReply(request)
+	replyMessage.RecursionAvailable = true
+	replyMessage.Rcode = dns.RcodeSuccess
+	replyMessage.Answer = []dns.RR{
+		&dns.A{
+			A: net.IPv4(8, 8, 8, 8),
+		},
+	}
+
+	ipv4 := &layers.IPv4{
+		Protocol: layers.IPProtocolUDP,
+		SrcIP:    net.IPv4(127, 0, 0, 1),
+		DstIP:    net.IPv4(127, 0, 0, 2),
+	}
+	udp := &layers.UDP{
+		DstPort: 53,
+		SrcPort: 45223,
+	}
+	if err := udp.SetNetworkLayerForChecksum(ipv4); err != nil {
+		t.Error("failed to set network layer for checksum")
+		return
+	}
+
+	// Serialize the packet
+	buffer := gopacket.NewSerializeBuffer()
+	options := gopacket.SerializeOptions{
+		ComputeChecksums: true,
+		FixLengths:       true,
+	}
+
+	requestData, err := request.Pack()
+	if err != nil {
+		t.Errorf("got an error while packing the request message, error: %v", err)
+		return
+	}
+	payload := gopacket.Payload(requestData)
+
+	if err := gopacket.SerializeLayers(buffer, options, ipv4, udp, payload); err != nil {
+		t.Errorf("failed to serialize packet: %v", err)
+		return
+	}
+
+	rw := &responseWriter{
+		local: &net.UDPAddr{
+			IP:   net.IPv4(127, 0, 0, 1),
+			Port: 55223,
+		},
+		remote: &net.UDPAddr{
+			IP:   net.IPv4(127, 0, 0, 1),
+			Port: 53,
+		},
+		packet: gopacket.NewPacket(
+			buffer.Bytes(),
+			layers.LayerTypeIPv4,
+			gopacket.Default,
+		),
+		device: device,
+	}
+	if err := rw.WriteMsg(replyMessage); err != nil {
+		t.Errorf("got an error while writing the local resolver response, error: %v", err)
+		return
+	}
+}