diff --git a/tun/tun.go b/tun/tun.go
index 4b651ca..7cdecad 100644
--- a/tun/tun.go
+++ b/tun/tun.go
@@ -52,7 +52,11 @@ func (t *SourceSink) Source() (proxy.Packet, error) {
 var good, bad float64
 
 func (t *SourceSink) Sink(packet proxy.Packet) error {
-	_, err := t.tun.Write(packet.Contents(), 4)
+	// make space for tun header
+	content := make([]byte, len(packet.Contents())+4)
+	copy(content[4:], packet.Contents())
+
+	_, err := t.tun.Write(content, 4)
 	if err != nil {
 		switch err.(type) {
 		case *os.PathError: