[Typo3-dev] fe user login by script

[Nikolic Antonio]

Hello List,
 
I've been looking around this list and checked several documentations ("Authentication Services", "cc_svauthdemo", "cc_sv_auth" by René Fritz) but as I am new to typo3 extension development I find it really hard to find a point to start. 
 
What I need to implement quite quickly is a typo-independent login with a custom form via a frontend plugin. Logindata provided here by user will not contain typo-users.
The plugin in the
    1st step: passes the data from my form to socket connection to do authentification (already works) and in the
    2nd step: depending on the result decides which one of two or three existing typo3 fe_users should be logged in automatically and 
    finally: logs in this particular "mapped" user.
   
All approaches I made by using the auth-service method and typo3 login forms do not work for me, because the fe_user already has to be specified when my custom authentication would take place. It therefore cannot decide anymore which of my predefined fe_users to log in.
 
Is there a way to maybe call some auth-service methods from a frontend plugin to login a user by his username? It cannot be that complicated - I don't really want to write an auth_service since I do not yet understand the whole framework.. maybe sometime in future...
 
Thanx in advance
Nikolic, Antonio