Different Services, Different Realms, but One Host

[Tim Alsop]

Ok,

The service address lookup is dependant on the actual ssh product you are using. The product I am familiar with guesses the name of the service principal by prefixing it with "host/" and then adding the target hostname. The realm is based on the default realm of the local system. This is not a very flexible approach since it will not work in multi-realm architecture like yours. You therefore need to talk to the supplier of your ssh product and see if they have a way to configure the SPN (service principal name) for a target system so that you can tell the ssh client which realm to use in the request.

Thanks, Tim. 

-----Original Message-----
From: ms419 at freezone.co.uk [mailto:ms419 at freezone.co.uk] 
Sent: 13 March 2004 12:15
To: Tim Alsop
Cc: kerberos at mit.edu
Subject: Re: Different Services, Different Realms, but One Host

Ahh ... I'm getting the picture ...

How about ssh, for starters? Most of host.blue's keys are in BLUE, though host/host.blue is in RED. So ssh need's to get host/host.blue at RED ...

Thanks again for all the help!

Jack

On Mar 11, 2004, at 2:59 AM, Tim Alsop wrote:

> Jack,
>
> The process works something like :
>
> 1. User enters logon details at workstation 2. Workstation kerberos 
> client requests initial tgt from REALM0
> (krbtgt/REALM0 at REALM0)
> 3. User has now authenticated against REALM0 4. You run a client 
> application which needs to send your principal name to service2 on 
> server1.domain.com. This service is using REALM2.
>
> 5. The client application needs to request a service ticket called
> service1/server1.domain.com at REALM2 from the KDC it knows about (i.e. 
> REALM0).
>
>  6. The KDC for REALM0 recognises that the service ticket request is 
> for REALM2 so it uses its trust relationship to send back to 
> workstation a new cross realm tgt (krbtgt/REALM2 at REALM0)
>
> 7. The workstation sends a fresh service ticket request to REALM2 
> using the cross realm tgt issued by REALM0 KDC.
> 8. The KDC for REALM2 responds with a service ticket 9. The service 
> ticket can be sent to the server by the client application.
> 10. The service on the server decrypts the service ticket to determine 
> the principal name of the user who authenticated at the workstation.
>
> The bit that may be causing you an issue is step 5. It is necessary 
> for the client application to be aware of the full service principal 
> name of the service it is trying to communicate with.
>
> Perhaps, if you can advise what the application is I can let you know 
> how to make it request the correct service ticket ?
>
> Thanks, Tim.
>
> -----Original Message-----
> From: ms419 at freezone.co.uk [mailto:ms419 at freezone.co.uk]
> Sent: 11 March 2004 10:48
> To: Tim Alsop
> Subject: Re: Different Services, Different Realms, but One Host
>
> Thanks everyone!
>
> I agree, Tim: Being able to store one host's keys in different realms 
> would be nice - in my simple opinion.
>
> I think you understand my question exactly - your example is just what 
> I'm trying to do. Unfortunately, I still don't know how to do it.
>
>  Truly, the confusing part is how the kerberos client obtains a cross 
> realm TGT for the correct realm.
>
> Since your example is precisely what I'm trying to do, suppose I'm 
> connecting to service2 on server1.domain.com - my principle resides in 
> REALM0, say. How does my kerberos client know whether to request a 
> cross realm TGT for REALM1, 2, or 3? It seems domain_realm will only 
> map server1.domain.com - and all of its instances - to one realm ...
>
> I'd be very happy to find a way around this!
>
> Jack
>
> On Mar 9, 2004, at 9:36 AM, Tim Alsop wrote:
>
> > Hi,
> >
> > Perhaps I missunderstand the question, but it is possible to use 
> > multiple realms on a specific host. The naming of the service 
> > principals (keys for which would be stored in a key table) would 
> > look something like :
> >
> > host/server1.domain.com at REALM1
> > service1/server1.domain.com at REALM1
> > service2/server1.domain.com at REALM2
> > service3/server1.domain.com at REALM3
> >
> > Essentially, the key in the key table which would be acquired by a 
> > specific application (a Kerberos service) maintains the 
> > shared-secret between the service and the realm.
> >
> > A user can be in one realm with one password. The services can be in 
> > multiple realms. When an application is run by a user and requests a 
> > service ticket it will get the ticket from the appropriate KDC for
> the
> > realm.
> >
> >  The part which is perhaps a little confusing is the approach used 
> >by  a kerberos client to determine the KDC that issues the service
> ticket.
> > Normally, the service ticket request is sent to the realm where the 
> > initial user's ticket (tgt) was issued, and then a cross realm tgt 
> > is returned for the service realm so that a service ticket can then 
> > be requested from the correct realm. The service ticket is then sent 
> > to the service and decrypted with the key in the key table in order 
> > to determine the principal name of the user.
> >
> > So, in summary - having multiple realms works well, but is more 
> > complex. The simplest approach is to use one realm for everthing, 
> > but this does not have the granularity that some implementations require.
> > It is nice to have the option to use either approach to protect 
> > multiple applications on a specific host !!
> >
> > Regards, Tim.
> >
> > -----Original Message-----
> > From: Sam Hartman [mailto:hartmans at MIT.EDU]
> > Sent: 09 March 2004 17:09
> > To: ms419 at freezone.co.uk
> > Cc: kerberos at MIT.EDU
> > Subject: Re: Different Services, Different Realms, but One Host
> >
> > >>>>> "ms419" == ms419  <ms419 at freezone.co.uk> writes:
> >
> >     ms419> Pardon this newbish question, but here's the setup: I 
> > want
> >     ms419> to distribute the keys for one host among two
> >     ms419> realms. Basically, I've got a sensitive service running 
> > on
> >     ms419> a couple of hosts, and a less secure service running on
> the
> >     ms419> same hosts. I want to store the keys for the sensitive
> >     ms419> service in one realm, and the keys for the others in
> >     ms419> another. Any problems with these premises?
> >
> > Yes.  Current Kerberos implementations assume a host belongs to one 
> > realm.  You'll find it difficult to actually do this.
> >
> > Also, users will end up having multiple passwords which will be 
> > annoying.
> >
> > I recommend having one KDC which is more secure than your most
>  > sensitive service.
> >
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