[ic] Allowing a particular domain access to content

[Paul Jordan]

Is there a way to allow only a particular external domain to access content 
from an Interchange website when one does not have control of the external 
domain?

Site1 grabs stuff from Site2, and Site2 doesn't want to show this content to 
anyone other than Site1. I have complete control over Site2, and limited 
(practically zero) control of Site1.

I've been controlling access using environment variables, but I'm pretty 
sure all environment variables can be faked.

Any ideas are appreciated.

Paul