twentyeleven theme in WordPress - Nginx as reverse proxy to Apache2

[Loggy]

There are many attractions of Nginx as a front-end to Apache for WordPress -
not least the retention of permalink facilities.  

I have been trying this for some time now and, when it works, the
performance is certainly excellent.  However there is one problem with all
the configurations I have been trying and that is the header image in the
default WordPress twentyeleven theme when not logged in.

This should load a random image on each refresh but, even though the image
name should change, the same image is always reloaded.

The platform is Ubuntu 10.04 LTS, nginx 1.2.3 stable, Apache/2.2.14
(Ubuntu), WordPress 3.4.2.

Here is the essential bit from nginx.conf in my current tests:

=============================================================================
proxy_cache_path  /var/lib/nginx/cache  levels=1:2  
keys_zone=staticfilecache:180m  max_size=100m;
proxy_temp_path /var/lib/nginx/proxy;
proxy_connect_timeout 30;
proxy_read_timeout 120;
proxy_send_timeout 120;

#IMPORTANT - this sets the basic cache key that's used in the static file
cache.
proxy_cache_key "$scheme://$host$request_uri";
=============================================================================

... and the essentail /etc/nginx/conf.d/wordpress.conf file which is
included in every server {} definition:


=============================================================================
        # Set the real IP.
        proxy_set_header X-Real-IP  $remote_addr;

        # Set the hostname
        proxy_set_header Host $host;

        #Set the forwarded-for header.
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

        # Let the Set-Cookie header through.
        proxy_pass_header Set-Cookie;

        location / {

                        # If logged in, don't cache.
                        if ($http_cookie ~*
"comment_author_|wordpress_(?!test_cookie)|wp-postpass_" ) {
                                set $do_not_cache 1;
                        }
                        proxy_cache_key "$scheme://$host$request_uri
$do_not_cache";
                        proxy_cache staticfilecache;
                        proxy_pass http://apache;
        }

        location ~* wp\-.*\.php|wp\-admin {
                        # Don't static file cache admin-looking things.
                        proxy_pass http://apache;
        }

        location ~*
\.(jpg|png|gif|jpeg|css|js|mp3|wav|swf|mov|doc|pdf|xls|ppt|docx|pptx|xlsx)$
{
                        # Cache static-looking files for 120 minutes,
setting a 10 day expiry time in the HTTP header,
                        # whether logged in or not (may be too
heavy-handed).
                        proxy_cache_valid 200 120m;
                        expires 864000;
                        proxy_pass http://apache;
                        proxy_cache staticfilecache;
        }

        location ~* \/[^\/]+\/(feed|\.xml)\/? {
                        # Cache RSS looking feeds for 45 minutes unless
logged in.
                        if ($http_cookie ~*
"comment_author_|wordpress_(?!test_cookie)|wp-postpass_" ) {
                                set $do_not_cache 1;
                        }
                        proxy_cache_key "$scheme://$host$request_uri
$do_not_cache";
                        proxy_cache_valid 200 45m;
                        proxy_cache staticfilecache;
                        proxy_pass http://apache;
        }

        location = /50x.html {
                root   /var/www/nginx-default;
        }

        # No access to .htaccess files.
        location ~ /\.ht {
                deny  all;
        }
=============================================================================

So clearly WordPress is not being asked to provide a new header.  Clearly I
can make it do so by setting expires 0 but that rather defeats the purpose
of having nginx up front.

Any ideas?

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,230916,230916#msg-230916