You can use the ProxyPass configuration directive to map remote hosts into the URL space of the local server; the local server does not act as a proxy in the conventional sense, but appears to be a mirror of the remote server.

Let's explore what this rule does:

ProxyPass   /perl/ http://backend.example.com/perl/

When a user initiates a request to http://www.example.com/perl/foo.pl, the request is picked up by mod_proxy. It issues a request for http://backend.example.com/perl/foo.pl and forwards the response to the client. This reverse proxy process is mostly transparent to the client, as long as the response data does not contain absolute URLs.

One such situation occurs when the backend server issues a redirect. The URL to redirect to is provided in a Location header in the response. The backend server will use its own ServerName and Port to build the URL to redirect to. For example, mod_dir will redirect a request for http://www.example.com/somedir/ to http://backend.example.com/somedir/ by issuing a redirect with the following header:

Location: http://backend.example.com/somedir/

Since ProxyPass forwards the response unchanged to the client, the user will see http://backend.example.com/somedir/ in her browser's location window, instead of http://www.example.com/somedir/.

You have probably noticed many examples of this from real-life web sites you've visited. Free email service providers and other similar heavy online services display the login or the main page from their main server, and then when you log in you see something like x11.example.com, then w59.example.com, etc. These are the backend servers that do the actual work.

Obviously this is not an ideal solution, but since users don't usually care about what they see in the location window, you can sometimes get away with this approach. In the following section we show a better solution that solves this issue and provides even more useful functionalities.