Install under reverse proxy does not work

Hello,

Description
********
Drupal is installed at the following address :

http://server1.domain.com/dir/

Reverse Proxy configuration
********************

http://domain.com/dir/ => http://server2.domain.com/dir/
http://domain.com/www2/dir/ => http://server1.domain.com/dir/

When trying to install (GET http://domain.com/www2/dir/ ), Drupal redirects to http://domain.com/dir/install.php instead of
http://domain.com/www2/dir/install.php

I tried to use directly http://domain.com/www2/dir/install.php
but the problem remains for the next step.

I configured manually $base_url, but still does not work

This problem was not existing on the previous Drupal release when configuring $base_url.
I suppose that $base_url is not used properly

Thanks

Bertrand