Proxying header question

[Wayne E. Seguin]

Igor,

I'm trying to setup dynamic handling based on the subdomain asked for  
in an application that is proxied to.

Basically I'm using:

upstream http://domain.tld {
...
}

server {
   location / {
     proxy_set_header  X-Real-IP  $remote_addr;
     proxy_set_header  X-Forwarded-For $proxy_add_x_forwarded_for;
     proxy_set_header Host $http_host;
     proxy_redirect false;
     proxy_max_temp_file_size 0;
...
     if (!-f $request_filename) {
       proxy_pass http://domain.tld;
       break;
     }
   }
}

What I'm finding is that the requested url "subdomain.domain.tld"  
doesn't pass through, instead I only get "domain.tld".

Is there a way to tell Nginx to pass this through?
I've scanned the wiki but have not found this yet in there.

Thanks,
   ~Wayne