Simple redirect handles more than I want

[Søren Sprogø]

I need a little help as I simply can't wrap my head around what I'm doing wrong.

What I want to accomplish is to do a permanent redirect of www.mydomain.com/portal/ to just www.mydomain.com.

Here's the rule I'm playing around with:

<add name="oldPortal01"
  virtualUrl="~/portal/"
  destinationUrl="~/"
  redirect="Application" redirectMode="Permanent" rewriteUrlParameter="IncludeQueryStringForRewrite" ignoreCase="true" />

Which actually works fine. But it also catches requests to fx. /portal/mypage.php and redirects it to //mypage.php, which I don't want it to. I just want it to catch "/portal/" and nothing else.

What am I doing wrong here?

Best regards,
Soeren S.