i'll make it more easy

i want a device thats in unknown devices in the known devices as adsl modem and i want to root lan traffic through it ... i was hoping it could be done editing some files and manually configuring things but since i'm pretty new to linux i'll need more time or some expert help/ tips, but i want to install routing and firewall as quick as possible... security reasons
Or when i'm totally mistaking: is there anyone who pulled this thing off out there?
ANY help would be appreciated, thanx