codeigniter www子域问题

I have strange issue with codeigniter and www subdomain.

When i load http://foobar.com the page opens normally but when i load http://www.foobar.com I get CodeIgniter's 404 message

Here is the content of .htaccess file

RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php/$1 [L]

What could be the problem ?

Some settings treat the subdomain as a folder inside the root. perhaps it tries to load the codeigniter url www? Troubleshoot with $route['404_override']

http://codeigniter.com/user_guide/general/routing.html

create a controller and point to that controller, and inside the controller's function, run

print_r($this->router) and echo $this->uri->uri_string()

Hopefully this may help you understand what's going on