\documentclass{article}
\usepackage{hyperref}

\begin{document}

\section{Apache Redirections}

Apache provides many ways of redirecting requests to different
websites.  The two main ones are probably mod\_proxy and
mod\_rewrite, which will be briefly covered here.  These modules
allow us to access data that may not have been accessible
otherwise, for example behind a firewall.

\section{mod\_rewrite}

Mod\_rewrite is an incredibly powerful module, and can do much
much more than is covered here.  This example will show how
to redirect requests to another URL, perhaps on another port
and/or host, perhaps running under a java servlet engine.

\begin{verbatim}
<VirtualHost 192.168.1.1>
    ServerName demo.example.com
    DocumentRoot /var/www
    RewriteEngine on
    RewriteRule   ^/demo$ \ 
       http://java.example.com:8081/servlet/ [P]
    RewriteRule   ^/demo/$ \
       http://java.example.com:8081/servlet/ [P]
</VirtualHost>
\end{verbatim}

\section{mod\_proxy}

Mod\_proxy can be used in two main ways, either as a proxy
server similar to squid, or as a reverse proxy to forward http
requests to another host.  This example is forwarding requests
for /demo off to another host, using the reverse proxy mode.

\begin{verbatim}
<VirtualHost 192.168.2.1>
    ServerName proxy.example.com
    ProxyPass        /demo/ http://host.example.com:8081/demo/
    ProxyPassReverse /demo/ http://host.example.com:8081/demo/
</VirtualHost>
\end{verbatim}

\section{Combinations}

This example shows how it is possible to use both mod\_rewrite
and mod\_proxy to access a host behind a firewall without
having to worry too much about having ports in the URL.

\begin{verbatim}
<VirtualHost 192.168.1.2>
    ServerName virtual.example.com
    ProxyPass        /internal/ http://firewall.example.com/
    ProxyPassReverse /internal/ http://firewall.example.com/
    
    RewriteEngine on
    RewriteRule   ^/$ http://virtual.example.com/internal/ [P]
</VirtualHost>
\end{verbatim}

On the firewall box, you need something like the following
rules.  This assumes you are running Linux 2.4, and hence
using iptables.

\begin{verbatim}
# port forward for web from external webserver to internal
/sbin/iptables -t nat -A PREROUTING -i eth1 -p tcp \
       --dport 80 -s web.example.com -j DNAT \
       --to 192.168.1.2
/sbin/iptables -t nat -A PREROUTING -i eth1 -p tcp \
      --dport 8081 -s web.example.com -j DNAT \
      --to 192.168.1.2
\end{verbatim}

\section{Conclusion}

As you can see, the combination of mod\_proxy and mod\_rewrite
allow you to a wealth of things that would otherwise be
difficult with standard Apache.  The ability to share data that
is protected behind a firewall is often very useful, as you can
share data without exposing it absolutely more than is required.

\end{document}