I'm trying to cache per user Drupal using this config :

fastcgi_ignore_headers Cache-Control Expires; #tried '__utma' 'Set-Cookie'
fastcgi_cache drupal;
fastcgi_cache_key $http_cookie$request_method$scheme$host$request_uri;
fastcgi_cache_methods GET HEAD;
fastcgi_cache_valid any 1s;
fastcgi_cache_use_stale error timeout invalid_header updating http_500;
fastcgi_max_temp_file_size 2M;
add_header X-Micro-Cache $upstream_cache_status;

I can get this working without google analytics, but when I enable google analytics it adds several cookies __utma __utmb __utmc __utmz. Nginx looks at $http_cookie and sees that its different so it passes it to the backend. Preferably I would like to just use the cache key as $cookie_SESS which is the drupal session cookie, however the full cookie name is encoded to something like this

SSESSae42ac488e03c2e2xxxxxda6ce2f5ee7=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

I've omitted some letters and numbers. So my question is how do I just extract the Drupal Session cookie? Or failing that just strip all cookies? I have tried fastcgi_ignore header '__utma' 'Set-Cookie' but it doesn't seem to work.