So let's see, what I need is that every request to a certain subdomain to be mapped to another server, with a private IP-Address. This is great because I only expose parts of my server to the Network that I want.

First things first: in the apache configuration, which is usually located at /etc/sysconfig/apache2 edit the APACHE_OPTS to contain -D REVERSE, -D PROXY and -D PROXY_HTML (actually the last one's not needed as we wont be adjusting any URLs, but let's keep it anyway).

APACHE2_OPTS="-D DEFAULT_VHOST -D PHP5 -D PERL -D PROXY_HTML -D REVERSE -D PROXY"

Now that we have enabled the proxy stuff, we can start configuring it. In the vhost directory (something like /etc/apache2/vhosts.d/) create a new file that will hold the configurationfor the subdomain:

<virtualhost your-ip-here>
        DocumentRoot /srv/www/subdomain/htdocs
        ServerName subdomain.domain.tld
        ServerAdmin webmaster@subdomain.domain.tld
        AccessFileName .htaccess
        ScriptAlias /cgi-bin/ /srv/www/subdomain/cgi-bin/
        ServerAlias subdomain.domain.tld sudomain
 
        <directory /srv/www/subdomain/htdocs>
                Order allow,deny
                Allow from all
        </directory>
 
        <directory /srv/www/subdomain/cgi-bin>
                AllowOverride None
                Order allow,deny
                Options ExecCGI
                Allow from all
        </directory>
</virtualhost>

Ok so by bow we have created the new subdomain, just to let apache shut up about missing folders we'll create the required folder:

mkdir -p /srv/www/subdomain/{htdocs,cgi-bin}

What's missing now? Right, the reverse Proxy so we have to add two new lines to the definition above:

       ProxyPass / http://internal-ip-address/
        ProxyPassReverse / http://internal-ip-address/

That's it! Just put the above before the Closing VirtualHost tag and it should work just fine.