Installing phpBB3 behind a proxy poses a problem. The default setting of Validate Referrer makes it impossible to change ANY settings in the ACP (including itself).

This is where my php/mysql box is internal and is access through another box running apache with mod_proxy

I had phpmyadmin installed so I was able to manually change the setting in the database. Doing it this way I had to also set the "is dynamic" value to '1' as well in order for the change to take effect.

Another way to do it if you have access to the command line database it to execute the following query:

UPDATE phpbb_config SET config_value = '0' WHERE config_name = 'referer_validation';

In order for this to work though you must clear the cache.
(Delete everything except index.htm and .htaccess from the cache folder)

So in some situations phpBB3 is broken out of the box.