DRY approach to views for password protected parts of a site


I'm working on a system that has a protected part. Users have to login in
order to be able to go there.
Now I was wondering how I should handle the layout in a DRY way. All pages
have the same layout with a header that contains either a few options and a
login part or the full stack of options.
How should I code that layout in order to have a nice DRY layout? I want to
avoid to do checks on being authenticated in every layout. I prefer to have
a single template (with a fixed header and footer) for the entire site that
would be "automagically" included and rendered for users that are
authenticated and not authenticated at once. I guess this is a quite normal
question but I can't find an article that also discusses the layout
organisation (ie which file should I place where).