In my company we have a relatively small network. Than we have 2 different WAN (via ADSL and SHDSL) connection behind 2 different hardware firewalls (Zyxel and SonicWall). Those are connected to our network switch. Common users are supposed to use ADSL connection, while helpdesk and some other guys have to use SHDSL connection ('cause they often upload stuff to external servers). Proxy and content filtering on both routers are available in trial version and then one has to buy it and this sucks. Also the NAT and other kind of network management is a nightmare, 'cause sometimes people just switch their gateway without any permission. The MAC-based routing, blacklists etc are good options, but we would have to create/edit those rules twice (in the worst case), because we have to routes/firewalls.
So I decided to build a new Debian box with DHCP\DNS\Squid\DansGuardian\IPTables services. This PC has 3 Gigabit NICs and theoretically can handle 2 WAN connections + one connection to our switch.
Everybody will have to set this box as gateway, so it will manage NAT, proxy and content filter for both groups of users. Then I would like to build some sort of MAC-based list (IPTables routes?) that will help me to forcibly route the users that are supposed to use ADSL conenction to ADSL router.
My question: is is this set up is kind of setup is possible? are the software solutions I am going to use are suitable for this setup? am I missing something?
OPTIONAL: I would like to set up a VPN server on this PC, is it possible with current hardware(3 network adapters)/software configuration or some incompatibilities may arise?
P.S. I will probably use IP for user identification with a MAC-IP binding enabled on DHCP.