receive: implement flush semantics

tun/tun.go

@@ -21,6 +21,7 @@ type TUNDevice interface {
 	File() *os.File                 // returns the file descriptor of the device
 	Read([]byte, int) (int, error)  // read a packet from the device (without any additional headers)
 	Write([]byte, int) (int, error) // writes a packet to the device (without any additional headers)
+	Flush() error                   // flush all previous writes to the device
 	MTU() (int, error)              // returns the MTU of the device
 	Name() (string, error)          // fetches and returns the current name
 	Events() chan TUNEvent          // returns a constant channel of events related to the device