Added padding

Added plaintext padding and fixed default interface MTU

src/tun.go

@@ -1,5 +1,11 @@
 package main
 
+/*
+ * The default MTU of the new device must be 1420
+ */
+
+const DefaultMTU = 1420
+
 type TUNDevice interface {
 	Read([]byte) (int, error)  // read a packet from the device (without any additional headers)
 	Write([]byte) (int, error) // writes a packet to the device (without any additional headers)