While this isn't an OpenVPN only problem, it is still the worst design
   problem in m0n0wall. After removing an optional interface, the optional
   interface numbering is reassigned. For example:

   Having three optional interface opt1 .. opt3. Deleting interface opt1
   will renumber interface opt2 to opt1 and opt3 to opt2.

   opt1 --> delete
   opt2 --> opt1
   opt3 --> opt2

   While this will help to avoid holes in the numbering scheme for optional
   interfaces, it will produce much problems in combination with filtering
   rules, nat configuration and anywhere where rules were added for an
   optional interface.