Suppose you have something like:

        ProxyPreserveHost On
        ProxyRequests off
        ProxyPass /
        ProxyPassReverse /

But you want to restrict access to this site to a certain set of IP addresses.
Allow/deny statements aren't allowed in the base of the <virtualhost>
directive, and this isn't a <directory>... the way to apply allow/deny
statements to the above is:

<Proxy *>
Order deny,allow
Deny from all
Allow from