Rather than "roll your own", you could use one of the purpose-built firewall/ router Linux distributions. (Such may also exist for (Free|Net|Open)BSD.) I've used both IpCop and Smoothwall; both support various kinds of WAN links, including USB modems:
http://www.ipcop.org/ http://smoothwall.org/ HTH, David -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]