mod_proxy and mod_vhost_alias are two very important extensions/modules for Apache web-server. When it comes to hosting multiple websites on same Web-Server using Apache or requirement to run Apache and IIS together then mod_proxy and mod_vhost_alias are key to succeed.
Here is summary of definition for both modules from Apache docs.
mod_proxy:
This module implements a proxy/gateway for Apache. It implements proxying capability for
FTP,CONNECT(for SSL),HTTP/0.9,HTTP/1.0, andHTTP/1.1. The module can be configured to connect to other proxy modules for these and other protocols.Apache's proxy features are divided into several modules in addition to
mod_proxy:mod_proxy_http,mod_proxy_ftpandmod_proxy_connect. Thus, if you want to use one or more of the particular proxy functions, loadmod_proxyand the appropriate module(s) into the server (either statically at compile-time or dynamically via theLoadModuledirective).In addition, extended features are provided by other modules. Caching is provided by
mod_cacheand related modules. The ability to contact remote servers using the SSL/TLS protocol is provided by theSSLProxy*directives ofmod_ssl. These additional modules will need to be loaded and configured to take advantage of these features.mode_vhost_alias:
This module creates dynamically configured virtual hosts, by allowing the IP address and/or the
Host:header of the HTTP request to be used as part of the pathname to determine what files to serve. This allows for easy use of a huge number of virtual hosts with similar configurations.