Windows Domain Authentication with Apache+Mongrel

Hi All,

I am running a rails application in my laptop which is connected to the
corporate intranet. I am using Apache 2.2.4 with Mongrel proxying. I
want to authenticate the corporate intranet users using Windows
Authentication. This is working fine with mod_sspi but I cant retrieve
the username in rails. Its not available in REMOTE_USER or
X_FORWARDED_USER. I have used the rewrite rule to add this information
in the request header but with no success. When I set the request header
like this for testing purpose,
           RequestHeader add X-FORWARDED-User "testuser"

its working fine and I can see the username but when I use this rewrite
    RewriteEngine On
    RewriteCond %{LA-U:REMOTE_USER} (.+)
    RewriteRule . -[E=RU:%1]
    RequestHeader add X-FORWARDED-User %{RU}e
all I get in HTTP_X_FORWARDED_USER is (null).
I know this issue has been discussed a lot and I can see people saying
its working for them but I cant get this working. My Apache
configuration for proxying is:-