External authentication through http, ldap or external sql

[Thommie_ES]

Hi,
Sorry, if this was asked before somewhere else (I couldn't find an appropriate subject line):

I want SMF to authenticate its users from an external source, either:
- a user which has been previously authenticated by the webserver (401 response header)
- authentication from an ldap server
- authentication from an external (separate) sql database/table.

Note that I only want to authenticate, the SMF-internal authorization mechanism (-> membergroups) shall not be touched. I need this to avoid double authentication in an environment which is already protected by basic http authentication.

I know that there is the SSI file, but where can I find some in-depth documentation or, much better, some practical "cookbook example" from people who have already done this?

Thanks for your help,
Thommie