Before we continue on, I need you to do something important (no doubt others will attempt to answer your question below me, but try to do it anyway). I (we) need to know how much networking knowledge you have, is your mobile on the LAN with your box or is it via a VPN or is it purely in the tubes.
What function does your linux box perform, is it a router, server, client, host, bridge, or redundant machine.
There are a lot of tools to do MiTM, but it depends on your infrastructure and what you are actually trying to achieve.
If there is a language barrier (reading your post seems like English is not your first language), it may be worth your time to draw a simple picture of what your setup looks like, include any firewalls you might have, and the "internet" if you are connecting via that at any time.