[Zope] site access and authentication

Hello, I need to verify and set some REQUEST variables for each and every request to my application. I thought that SiteAccess maybe does the job. It works as I expected, only now I want to check some user rights and all I ever get is the user "Anonymous User" which is not what I expected. (The application shows the real name.) E.g. SiteAccessRule <dtml-call "REQUEST.set('sa_user', _.SecurityGetUser().getUseName())"> and my index_html <dtml-var sa_user><br><dtml-var "_.SecurityGetUser().getUseName()"> output will be Anonymous User fakeid If I can't access the real user in a SiteAccess rule. How can evaluate a DTML Method every time a request is made to my app? Thanks Ulrich