I have a TeamCity server which is actually installed on SBS 2003 Premium with ISA Server (firewall/proxy) installed. My ADSL connection has multiple IP addresses, which all resolve directly to my SBS external NIC. The NIC is therefore multi-homed and I have allocated one of the IP addresses specifically to TeamCity. In ISA, I've created an access rule to allow the traffic in. I can access my TeamCity server externally and view the web interface, that all works fine.

I want to use the Amazon EC2 integration in TeamCity to launch build agents 'in the cloud'. The problem I am having is that when the agent starts, it sees the server and registers, then just sits there waiting. On the server side, the agent appears as 'disconnected'.

Examining the settings, the agent's IP address appears to be that of the external NIC. What I think might be happening is that the traffic is undergoing Network Address Translation (NAT) so that TeamCity always thinks the agent is locally installed and therefore can't communicate with the actual remote agent. This seems to happen even though I have a permanent static IP address dedicated to TeamCity.

So, the question is this. How can I make traffic to a specific IP address pass through the ISA server un-NATted?