Looking for a redirect code in htaccess
I need a redirect for my forums, so users, each time they access www.example.com (www.example.com/index.php) will be redirected to www.example.com/portal.php. I have a forum, and a webhost, since 2008, and was using the DirectoryIndex portal.php index.php command and was working just fine. But recently, i changed my webhost, and ever since, the DirectoryIndex portal.php index.php command is giving the 500 error (Internal Server Error). I contacted the webhosting company and explained my issue, and they said they never heard of a DirectoryIndex command, and sugested me the Redirect command (Redirect /index.php http://example.com/portal.php). Ok, i tried that also, BUT, wen you are on portal.php, if you click on ANY links or buttons, like index.php or whatever, it keeps you there, so, you are stuck there... i need my users to be able to browse away from /portal.php on clicking of links found there.
In a few words i need a working, equivalent command to DirectoryIndex command.
Thank you all!