This seems to have been asked many times before, but never answered.
I've got an existing authentication/authorization scheme in place. I would like to be able to tie in WordPress to use that scheme. So basically, if someone accesses the login page, I'd like to be able to write a plugin that immediately checks my own authentication scheme, logs the person into WordPress automatically, and forwards them on to the admin page.
It doesn't appear that there are any hooks in the necessary spots for me to do this. Specifically, I need a hook at the top of the login page before any output, so I can redirect properly.
Has anyone done this before or have any ideas?