conditional proxy ?

[Martin Minka]

Please advice how to achieve something like:

if ($query_string ~* "/?advert=") {
	proxy_pass http://banner.server.com;		
	proxy_buffering off;			
	proxy_redirect          off;
	proxy_set_header        Host $http_host;
	proxy_set_header        X-Real-IP $remote_addr;
	proxy_set_header        X-Forwarded-For $proxy_add_x_forwarded_for;
}

I am using Nginx 0.6.0 on production server now.

sincerely,
Martin Minka