Add L3 routing support

transport/wireguard/device_nat.go

@@ -0,0 +1,75 @@
+package wireguard
+
+import (
+	"github.com/sagernet/sing-tun"
+	"github.com/sagernet/sing/common/buf"
+)
+
+var _ Device = (*natDeviceWrapper)(nil)
+
+type natDeviceWrapper struct {
+	Device
+	outbound chan *buf.Buffer
+	mapping  *tun.NatMapping
+	writer   *tun.NatWriter
+}
+
+func NewNATDevice(upstream Device, ipRewrite bool) NatDevice {
+	wrapper := &natDeviceWrapper{
+		Device:   upstream,
+		outbound: make(chan *buf.Buffer, 256),
+		mapping:  tun.NewNatMapping(ipRewrite),
+	}
+	if ipRewrite {
+		wrapper.writer = tun.NewNatWriter(upstream.Inet4Address(), upstream.Inet6Address())
+	}
+	return wrapper
+}
+
+func (d *natDeviceWrapper) Read(p []byte, offset int) (int, error) {
+	select {
+	case packet := <-d.outbound:
+		defer packet.Release()
+		return copy(p[offset:], packet.Bytes()), nil
+	default:
+	}
+	return d.Device.Read(p, offset)
+}
+
+func (d *natDeviceWrapper) Write(p []byte, offset int) (int, error) {
+	packet := p[offset:]
+	handled, err := d.mapping.WritePacket(packet)
+	if handled {
+		return len(packet), err
+	}
+	return d.Device.Write(p, offset)
+}
+
+func (d *natDeviceWrapper) CreateDestination(session tun.RouteSession, conn tun.RouteContext) tun.DirectDestination {
+	d.mapping.CreateSession(session, conn)
+	return &natDestinationWrapper{d, session}
+}
+
+var _ tun.DirectDestination = (*natDestinationWrapper)(nil)
+
+type natDestinationWrapper struct {
+	device  *natDeviceWrapper
+	session tun.RouteSession
+}
+
+func (d *natDestinationWrapper) WritePacket(buffer *buf.Buffer) error {
+	if d.device.writer != nil {
+		d.device.writer.RewritePacket(buffer.Bytes())
+	}
+	d.device.outbound <- buffer
+	return nil
+}
+
+func (d *natDestinationWrapper) Close() error {
+	d.device.mapping.DeleteSession(d.session)
+	return nil
+}
+
+func (d *natDestinationWrapper) Timeout() bool {
+	return false
+}