own NAT/FW rules

[Perun]

Hi

it is possible to define own NAT/FW Rules? I have configured a IPsec connectiion to my VPN Provider outside the WebUI (becaues of ms-eap auth) and this creates an interface tun0. I dont 'see' this interface in the WebUI but need some NAT/FW Rules for it.
Is there somewhere a file/dir where I can define own rules?

Greetz