absolute url for PDF link does not work behind reverse proxy

Hello, I am running xwiki 0.9.840 in tomcat, with apache accepting outside connections and using mod_proxy to forward them to tomcat. I have seen the question in the FAQ about reverse proxies and absolute URLs. However I find that the 'PDF' link is absolute, and shows as 'localhost:8080' in the xwiki pages. How can I tell xwiki to use the correct (external) base url ? Thanks a lot, -Benoit