Force absolute links in feed and xml sitemap page

Hi,
I have an issue with a primary domain hosted on GoDaddy.com – in the feed and in the xml sitemap the links are relative, not absolute, and this creates problems with validating the feed and indexing the pages with Google. This issue appears just with the primary domain, at others the links show up with their absolute path. How can I force, maybe with a code added to functions.php or with a change in settings, the links to be written only with their absolute path in both feed and xml files? Any other solution is welcome!
Thanks!