fastcgi_cache_key with cookie

[zenw]

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.

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,220510,220510#msg-220510