Return file when it's in cache/check if file exists in cache

[ixos]

I'm using ngnix as proxy server. There is a situation when backend server is
down and I know about it. Thus I'm adding 'Check-Cache' header to request
and what I want to do is get the file when it is in cache, and when it is
not just return error page. I don't want to pass the request to the
backend.

Scenario

$ curl -s -o /dev/null -w "%{http_code}" 0/images/2.jpg -H'Host:
example.com'
200
$ pkill -9 backend_server
$ curl -s -o /dev/null -w "%{http_code}" 0/images/2.jpg -H'Host:
example.com' -H'Check-Cache: true'
404 <-- I want to 200 here

nginx.conf

server {
    listen       80;

    underscores_in_headers on;
    proxy_cache_purge on;

    location / {
        error_page 599 = @jail;
        recursive_error_pages on;

        if ($http_check_cache = "true") {
            return 599;
        }

        proxy_pass http://localhost:8080;
        proxy_set_header Host $host;
        proxy_cache my-cache;
        proxy_cache_valid  200 302  10m;
        proxy_cache_valid  404      1m;
        proxy_cache_key    $uri$is_args$args;
    }

    location @jail {
        # try_files ??
        # proxy_cache_key (If i can get it i can use try_files)
        # what other solution...
        return 404;
    }
}

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,242488,242488#msg-242488