redirect to subsite front page the user logged in on

Hi,

How do we get the login redirect to redirect the user to the home page of the site they logged in on?

For instance, if they logged in on http://www.mysite.com/site3 then I want them to end up on the home page of /site3

It seems we can only enter a single url which is fine if you want to push them to a single point but we need them to stay on the site they logged in on.

Thanks in advance
Gavin