Subject: Re: comments on krb-dns-locate draft

From: Jeffrey Altman <[email-masked]>
Date: Mon, 22 Nov 99 22:15:53 EST

> The domain names to realm mapping portion of this draft allows an attacker
> to spoof a TXT record which can result in server spoofing. Kerberos
> shouldn't rely on the general security of DNS to be secure; in particular,
> an attacker in one realm shouldn't be able to spoof servers in another
> realm by modifying DNS replies.
> 
> We propose the following as a more secure alternative to the domain name to
> realm mapping section of the krb-dns-locate draft. The basic idea is that
> the client sends the server name and sets a new kdc-options bit
> (name-canonicalize bit) in the TGS request if it needs help in determining
> the realm of the requested server name. The KDC will lookup the name (could
> be in some directory or DNS - this could be the subject of a separate
> draft, the KDC name lookup must be secure) and returns the realm of the
> target server with a checksum in padata fields, along with a crossrealm TGT
> to the next realm on the path towards the target realm.

I will once again ask the simple question.  How can the user as the
KDC to instruct it as to which realm should be used for a host if I
have not yet determined the realm?

It is only after I know what realm I want to request the KDC info for
that I can send a request to the KDC.

If I want to telnet to kermit.cc.columbia.edu how do I know what REALM I
am in?  At this point I do not have a TGT so I can't make a TGS
request.

It is only after I ask the DNS for the REALM associated listed in 

  _kerberos.cc.columbia.edu IN TXT

that I will know what REALM (CC.COLUMBIA.EDU) I am in.  At this point
I can query for

  _kerberos._udp.CC.COLUMBIA.EDU IN SRV

to find out that the KDC is pryvet.cc.columbia.edu.  At this point I
can now ask the KDC for a TGT or for a TGS.  

Is there a spoofing attack?  Yes.

Is there a spoofing attack if I read this info from a config file? Yes.

Hence, there is no new attack.  Is your method more secure?  Maybe.

But it does not solve the general case.

    Jeffrey Altman * Sr.Software Designer * Kermit-95 for Win32 and OS/2
                 The Kermit Project * Columbia University
              612 West 115th St #716 * New York, NY * 10025
  http://www.kermit-project.org/k95.html * [email-masked]

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From: Ken Raeburn <[email-masked]>
Date: 23 Nov 1999 12:12:35 -0500

> have not yet determined the realm?

As I interpreted it, you'd ask your own KDC.  It deals with the name
lookup in some unspecified but secure way, and gives you the answer.
The proposal only addresses TGS_REQs, so I guess it's assumed you've
figured out your own KDC's location already for the AS_REQ, and the
security isn't that big a deal in this case, since if it's spoofed,
the check against some local secret will still fail.

Ken

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From: Jonathan Trostle <[email-masked]>
Date: Tue, 23 Nov 1999 13:35:59 -0800

>have not yet determined the realm?

The user asks the KDC which it originally obtained its TGT from in the AS
exchange. The KDC then looks in the global directory to find the name. 

>am in?  At this point I do not have a TGT so I can't make a TGS

The user needs to know his own realm before making the AS request to get
the initial TGT.

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From: Jonathan Trostle <[email-masked]>
Date: Tue, 23 Nov 1999 13:38:07 -0800

>the check against some local secret will still fail.

That's right.
Jonathan