Implement redirect logic in UDP proxy

client/iface/wgproxy/udp/rawsocket.go

@@ -0,0 +1,139 @@
+package udp
+
+import (
+	"fmt"
+	"net"
+	"os"
+	"syscall"
+
+	"github.com/google/gopacket"
+	"github.com/google/gopacket/layers"
+	log "github.com/sirupsen/logrus"
+
+	nbnet "github.com/netbirdio/netbird/util/net"
+)
+
+var (
+	serializeOpts = gopacket.SerializeOptions{
+		ComputeChecksums: true,
+		FixLengths:       true,
+	}
+
+	localHostNetIPAddr = &net.IPAddr{
+		IP: net.ParseIP("127.0.0.1"),
+	}
+)
+
+type SrcFaker struct {
+	srcAddr *net.UDPAddr
+
+	rawSocket   net.PacketConn
+	ipH         gopacket.SerializableLayer
+	udpH        gopacket.SerializableLayer
+	layerBuffer gopacket.SerializeBuffer
+}
+
+func NewSrcFaker(dstPort int, srcAddr *net.UDPAddr) (*SrcFaker, error) {
+	rawSocket, err := prepareSenderRawSocket()
+	if err != nil {
+		return nil, err
+	}
+
+	ipH, udpH, err := prepareHeaders(dstPort, srcAddr)
+	if err != nil {
+		return nil, err
+	}
+
+	f := &SrcFaker{
+		srcAddr:     srcAddr,
+		rawSocket:   rawSocket,
+		ipH:         ipH,
+		udpH:        udpH,
+		layerBuffer: gopacket.NewSerializeBuffer(),
+	}
+
+	return f, nil
+}
+
+func (f *SrcFaker) Close() error {
+	return f.rawSocket.Close()
+}
+
+func (f *SrcFaker) SendPkg(data []byte) (int, error) {
+	defer func() {
+		if err := f.layerBuffer.Clear(); err != nil {
+			log.Errorf("failed to clear layer buffer: %s", err)
+		}
+	}()
+
+	payload := gopacket.Payload(data)
+
+	err := gopacket.SerializeLayers(f.layerBuffer, serializeOpts, f.ipH, f.udpH, payload)
+	if err != nil {
+		return 0, fmt.Errorf("serialize layers: %w", err)
+	}
+	n, err := f.rawSocket.WriteTo(f.layerBuffer.Bytes(), localHostNetIPAddr)
+	if err != nil {
+		return 0, fmt.Errorf("write to raw conn: %w", err)
+	}
+	return n, nil
+}
+
+func prepareHeaders(dstPort int, srcAddr *net.UDPAddr) (gopacket.SerializableLayer, gopacket.SerializableLayer, error) {
+	ipH := &layers.IPv4{
+		DstIP:    net.ParseIP("127.0.0.1"),
+		SrcIP:    srcAddr.IP,
+		Version:  4,
+		TTL:      64,
+		Protocol: layers.IPProtocolUDP,
+	}
+	udpH := &layers.UDP{
+		SrcPort: layers.UDPPort(srcAddr.Port),
+		DstPort: layers.UDPPort(dstPort), // dst is the localhost WireGuard port
+	}
+
+	err := udpH.SetNetworkLayerForChecksum(ipH)
+	if err != nil {
+		return nil, nil, fmt.Errorf("set network layer for checksum: %w", err)
+	}
+
+	return ipH, udpH, nil
+}
+
+func prepareSenderRawSocket() (net.PacketConn, error) {
+	// Create a raw socket.
+	fd, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_RAW, syscall.IPPROTO_RAW)
+	if err != nil {
+		return nil, fmt.Errorf("creating raw socket failed: %w", err)
+	}
+
+	// Set the IP_HDRINCL option on the socket to tell the kernel that headers are included in the packet.
+	err = syscall.SetsockoptInt(fd, syscall.IPPROTO_IP, syscall.IP_HDRINCL, 1)
+	if err != nil {
+		return nil, fmt.Errorf("setting IP_HDRINCL failed: %w", err)
+	}
+
+	// Bind the socket to the "lo" interface.
+	err = syscall.SetsockoptString(fd, syscall.SOL_SOCKET, syscall.SO_BINDTODEVICE, "lo")
+	if err != nil {
+		return nil, fmt.Errorf("binding to lo interface failed: %w", err)
+	}
+
+	// Set the fwmark on the socket.
+	err = nbnet.SetSocketOpt(fd)
+	if err != nil {
+		return nil, fmt.Errorf("setting fwmark failed: %w", err)
+	}
+
+	// Convert the file descriptor to a PacketConn.
+	file := os.NewFile(uintptr(fd), fmt.Sprintf("fd %d", fd))
+	if file == nil {
+		return nil, fmt.Errorf("converting fd to file failed")
+	}
+	packetConn, err := net.FilePacketConn(file)
+	if err != nil {
+		return nil, fmt.Errorf("converting file to packet conn failed: %w", err)
+	}
+
+	return packetConn, nil
+}