new-bie Question regarding kerberos authorisation

[sravanthi]

Hi friends,
I need to develop an web application where in i need to authenticate and authorise the user to access his files located on the server, For this  i need to use kerberos and i am using java servlets to do this. I did authentication part but i haven't used any principals or subject- so do i need to use that, Secondly for authorisation .. as my application is just client-server type architecture.. so how can i proceed further to authorise the user to OS(Red hat linux) with the same username and password.
As mine is not a cross realm - do i need to use TGS, Could any one tell me how to go about this (I am using GSS-API) 
Any Idea or help from you is fine.
Thanks,
Sravanthi

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