disallow requests naming principal as a service

[Douglas E. Engert]

I would look at this a little differently. The KDC_ERR_MUST_USE_USER2USER
flag's name is misleading. What the KDC is saying, is it won't issue a
normal service ticket for this principal, but it could issue a u2u ticket.
This is advising the client that if it wants a service ticket, this is the only
way to get one. Maybe the flag should be called 
KDC_ERR_NO_NORMAL_SRV_TICKET_BUT_USER2USER_MIGHT_WORK :-)
 

Sam Hartman wrote:
> 
> >>>>> "John" == John Brezak <jbrezak at windows.microsoft.com> writes:
> 
>     John> Since the response is not authenticated, the client should
>     John> not wholely depend on the KDC to guide its action.
> 
>     John> Ultimately, the client's policy should determine what action
>     John> to take when the KDC is not able to provide a ticket for the
>     John> requested service.  However, it would become very
>     John> inefficient for the client to always try user2user if the
>     John> KDC failed to return a service ticket.
> 
> My argument is that you shouldn't design a protocol that requires the
> client to depend on the KDC.  By the time the client asks for a
> Kerberos ticket it should already be committed to the non-u2u or U2U
> protocol.
> 
> In the case of SASL or GSSAPI applications, the server should offer
> the normal krb5 mechanism only when it has a service key, and a U2U
> mechanism only when it has a TGT.
> 
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-- 

 Douglas E. Engert  <DEEngert at anl.gov>
 Argonne National Laboratory
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