Apache - Reverse Proxy and HTTP 302 status message

The AskApache article is quite helpful, but in practice I found a combination of Rewrite rules and ProxyPassReverse to be more flexible. So in your case I'd do something like this:

    <VirtualHost example>
       ServerName www.example.com

       ProxyPassReverse /app1/some-path/ http://internal1.example.com/some-path/
       RewriteEngine On
       RewriteRule /app1/(.*)   http://internal1.example.com/some-path$1 [P]

       ...
    </VirtualHost>

I like this better because it gives you finer-grained control over the paths you're proxying for the internal server. In our case we wanted to expose only part of third-party application. Note that this doesn't address hard-coded links in HTML, which the AskApache article covers.

Also, note that you can have multiple ProxyPassReverse lines:

    ProxyPassReverse / http://internal1.example.com/some-path
    ProxyPassReverse / http://internal2.example.com/some-path

I mention this only because another third-party app we were proxying was sending out redirects that didn't include their internal host name, just a different port.

As a final note, keep in mind that Firebug is extremely useful when debugging the redirects.

Basically, ProxyPassReverse should take care of rewriting the Location header for you, as Kevin Hakanson pointed out.

One pitfall I have encountered is missing the trailing slash in the url argument. Make sure to use:

ProxyPassReverse / http://internal1.example.com/some-path/

(note the trailing slash!)

There is an article titled Running a Reverse Proxy in Apache that seems to address your problem. It even uses the same example.com and /app1 that you have in your example. Go to the "Configuring the Proxy" section for examples on how to use ProxyPassReverse.