Getting a forwarded URL from thousands of different domains in Google App Engine

I actually asked this question before, but I cannot get my account details back, so I'm asking again:

I have a series of different domain names that I would like to all point (via URL forwarding from my domain host) to a google app engine application that reads what the forwarding URL is. So if the domain typed in was original XYZ.com, then when I am forwarded to my application, I can return what that original domain name was. I'm using the python variant. How best can I do this without coding for each and every variant?

So for example I might have aaa.com and bbb.com and ccc.com that all should point to the same appspotdomain, and I wish to somehow determine what the r开发者_如何学编程eferring URL was. I have thousands of domains and I have URL forwarding set-up. So unless I put something in the header is there a smart way to pull out the referring URL. I have tried the os.environ["SERVER_NAME"] route but this just gives the app-engine domain.

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Try

os.environ['HTTP_REFERER'] 

or

self.request.headers['Referer']

Be careful though, it might not always be available.