setting up frontend backend firewalls

[alpha88]

I want to set up a frontend backend firewall setup. The frontend firewall will be a astaro and the backend will be isa server 2000. I have astaro in place running as a frontend. I than put isa server behind it but when I enter the isa servers ip address in as my proxy I can't get to the internet. Is there something that I must do on the frontend firewall for astaro to be able to gain internet access and proxy it to the internal lan clients. Also let me tell you the physical setup I have my cable modem connected to a switch the external interface of the astaro box and the isa server is also connected to that switch. The isa box is using the external interface is using the ip address of astaro's internal interface as its gateway.