Firewall Recommendation - Basic but flexible with 1 to 1 public to private mapping

In the original question below, what feature is it called that I would be looking for that enables me to map public ip to private ip internally for certain ports?

If anyone has experience with WatchGuard and the most simple and basic for the feature I am looking for - what would you recommend?

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The router (pseudo firewall - really NAT) maps 3 PUBLIC IP / External Address (our mail, web site, and FTP) to 3 of the Internal Servers. It does a one to map mapping.

Server 1=Exchange 2003/Outlook Web Access(port 80,443) - (public ip

100.100.100.100 to private 192.168.1.10); Server 2=Sharepoint Portal 2003/Project Server 2003(port 80 and 443) - (public ip 100.100.100.101 to private 192.168.1.11); Server 3=FTP Site and MS PPTP VPN (port 21,1721) - (public ip 100.100.100.102 to private 192.168.1.12);

My GOALis to get a Linux firewall that is SIMPLE to use to place between the internal network and our Internet router. Also, it has to be able to route traffic destined on public ip xxx.xxx.xxx.xxx to private ip xxx.xxx.xxx.xxx- same as 1 to 1 NAT mapping but more locked down due to firewall features. Because multiple servers have port 80 and 443, I can't just do port forwarding. It must be intelligent enough to see the URL/URI to forward to the right box.

Hope this made sense.

What would you guys suggest in terms in the Linux distro with this capability, and how I should set it up?

Thank you!