3 companies and 1 i-net gateway

[Slava <sawati@yandex.ru>]

Hi all,

There are 4 companies at our building and only one way to get internet
connection - use common gateway (it's server with Debian Sarge installed).
Three of them want to use this server as firewall, mailserver (with spam
filters) and proxy server. And they are ready to pay me for setting up
this server. And fourth company asks me to route all it's traffic
transparently through this server.

First I need to use iptables to set up Firewall.
Then Squid (would you advice smth. else?) will be our proxy.
NAT is the way for the 4th company.
MTA will be the last step and I left it afterwards.

The problem is to count traffic (both incoming and outcoming) for each
of four companies taken separatly.

Is it possible at all? 
If yes could you let me know where should I dig.


Best regards, 
             Slava                sawati AT yandex ru