Hi,
UDT does'nt not work in 3-tier (http) mode. so http://externaldomain.com/ wont work in the system field.
For you to connect to the CMS thru client tools like UDT, the BO server will have to be exposed to the internet or they will have to give you a NAT'd ip which port forwards the request to CMS server.
In short, you cannot use HTTP address in system field of UDT.
-Nikhil Nair