From the Apache HTTP Server Project web page (http://httpd.apache.org/)
The Apache HTTP Server Project is an effort to develop and maintain an open-source HTTP server for modern operating systems including UNIX and Windows NT. The goal of this project is to provide a secure, efficient and extensible server that provides HTTP services in sync with the current HTTP standards.
From the Apache Tomcat web page (http://jakarta.apache.org/tomcat/index.html):
Apache Tomcat is the servlet container that is used in the official Reference Implementation for the Java Servlet and JavaServer Pages technologies. The Java Servlet and JavaServer Pages specifications are developed by Sun under the Java Community Process.
This HOWTO is intended as a complete, step-by-step guide to setting up Apache Tomcat servlet container in conjuction with the Apache HTTP Server configured to pass requests directly through proxy.
When using Apache as the Proxy Server to send requests to Tomcat, all requests will be from the Proxy Server, so the REQUESTOR will always show as the Apache Server and not the original requestor. The module mod_ip_forwarding reports to solve this issue, but I have not tested it myself. It is available here:
http://dev.w3.org/cvsweb/apache-modules/mod_ip_forwarding/mod_ip_forwarding.html?rev=1.3