[ale] Next apache puzzle: RewriteCond on auth failure

[rekoil at semihuman.com (Chris Woodfield)]

Thanks for the response on my last question, I got that problem licked  
- the answer was to use <LocationMatch> instead of <FileMatch>, since  
the path did not actually link to an actual file, but a string being  
fed to the php script.

Next up, I'm attempting to configure apache to issue redirect requests  
for all "heavy" objects on my server in the /gallery/ directory to  
point to a CDN - requests for HTML will still hit the server, but  
requests for .jpg, .gif, etc should receive a 301 redirect to a  
different hostname, which is configured with the proper auth  
credentials to access the content from apache.

Below is my best guess on how to make it work, but so far no dice:

      <LocationMatch "/gallery/.*\.(gif|jpg|jpeg|png)$">
         Header set Cache-control "max-age=86400, stale-if-error=3600"
         <IfModule mod_rewrite.c>
           RewriteEngine on
           RewriteCond !%{HTTP_AUTH} .*
           RewriteRule ^(.*)$ http://wwwcache.semihuman.com/$1 [R=301,L]
         </IfModule>
      </LocationMatch>

     <Directory "/usr/share/gallery2/">

       AuthUserFile /usr/share/gallery2/.htpasswd
       AuthGroupFile /dev/null
       AuthName ByPassword
       AuthType Basic
       ...

     </Directory>

I've confirmed the RewriteRule issues the proper redirect if I comment  
out the RewriteCond line - the tough part is crafting a RewriteCond  
that will only be triggered if HTTP authentication fails.

Again, any advice?

-C