Have you tried using one of the single floppy linux routers.  Most will
allow you to open specific ports to specific machines running servers
and such.  I have used freesco with a cable modem and a dialup line. 
You can get freesco from www.freesco.org 

Joseph Key
 
Brian wrote:
> 
> I just got my cable modem up and running.  My plan is to plug a linux box
> into the cable modem as a router and have multiple boxen behind it.  I
> also want to run Apache, sendmail, IRC, SSH, and a few other services on
> it.  The problem is firewalling.  I like to write TIGHT scripts (after
> being comprimised once I'm a little over-paranoid) by opening up just the
> service I need and DENYing any other packet from any source that's not on
> my specific guest list.
> 
> The problem here is that everything inside the router gets blocked.  ICQ,
> Napster, and a plethora of other oddball IP apps stop working because I've
> firewalled them out, but I don't want people breaking into my router.  Is
> there a good way to run this setup?
> 
> -Brian
> 
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