Subject: Updated Kerberos Referrals I-D

From: "John Brezak" <[email-masked]>
Date: Mon, 23 Jul 2001 17:08:03 -0700

This didn't make the I-D submissions cutoff, but it contains the changes
discussed at the last meeting. We need to align the I-D and the 1510bis
changes asap.

Kerberos Working Group                                         M. Swift 
Internet Draft                                         University of WA 
Document: draft-ietf-krb-wg-kerberos-referrals-01.txt         J. Brezak 
Category: Standards Track                                     Microsoft 
                                                             J. Trostle 
                                                          Cisco Systems 
                                                              July 2001 

           Generating KDC Referrals to locate Kerberos realms 

Status of this Memo 
 
   This document is an Internet-Draft and is in full conformance with 
   all provisions of Section 10 of RFC2026 [1].  
    
   Internet-Drafts are working documents of the Internet Engineering 
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1. Abstract 
    
   The draft documents a new method for a Kerberos Key Distribution 
   Center (KDC) to respond to client requests for kerberos tickets when 
   the client does not have detailed configuration information on the 
   realms of users or services. The KDC will handle requests for 
   principals in other realms by returning either a referral error or a 
   cross-realm TGT to another realm on the referral path. The clients 
   will use this referral information to reach the realm of the target 
   principal and then receive the ticket. 
    
2. Conventions used in this document 
    
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in 
   this document are to be interpreted as described in RFC-2119 [2]. 
    
3. Introduction 
    
   Current implementations of the Kerberos AS and TGS protocols, as 
   defined in RFC 1510 [3], use principal names constructed from a 
   known user or service name and realm. A service name is typically 

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   constructed from a name of the service and the DNS host name of the 
   computer that is providing the service. Many existing deployments of 
   Kerberos use a single Kerberos realm where all users and services 
   would be using the same realm. However in an environment where there 
   are multiple trusted Kerberos realms, the client needs to be able to 
   determine what realm a particular user or service is in before 
   making an AS or TGS request. Traditionally this requires client 
   configuration to make this possible. 
    
   When having to deal with multiple trusted realms, users are forced 
   to know what realm they are in before they can obtain a ticket 
   granting ticket (TGT) with an AS request. However, in many cases the 
   user would like to use a more familiar name that is not directly 
   related to the realm of their Kerberos principal name. A good 
   example of this is an RFC-822 style email name. This document 
   describes a mechanism that would allow a user to specify a user 
   principal name that is an alias for the user's Kerberos principal 
   name. In practice this would be the name that the user specifies to 
   obtain a TGT from a Kerberos KDC. The user principal name no longer 
   has a direct relationship with the Kerberos principal or realm. Thus 
   the administrator is able to move the user's principal to other 
   realms without the user having to know that it happened. 
    
   Once a user has a TGT, they would like to be able to access services 
   in any trusted Kerberos realm. To do this requires that the client 
   be able to determine what realm the target service's host is in 
   before making the TGS request. Current implementations of Kerberos 
   typically have a table that maps DNS host names to corresponding 
   Kerberos realms. In order for this to work on the client, each 
   application canonicalizes the host name of the service by doing a 
   DNS lookup followed by a reverse lookup using the returned IP 
   address. The returned primary host name is then used in the 
   construction of the principal name for the target service. In order 
   for the correct realm to be added for the target host, the mapping 
   table [domain_to_realm] is consulted for the realm corresponding to 
   the DNS host name. The corresponding realm is then used to complete 
   the target service principal name. 
    
   This traditional mechanism requires that each client have very 
   detailed configuration information about the hosts that are 
   providing services and their corresponding realms. Having client 
   side configuration information can be very costly from an 
   administration point of view - especially if there are many realms 
   and computers in the environment. 
    
   There are also cases where specific DNS aliases (local names) have 
   been setup in an organization to refer to a server in another 
   organization (remote server). The server has different DNS names in 
   each organization and each organization has a Kerberos realm that is 
   configured to service DNS names within that organization. Ideally 
   users are able to authenticate to the server in the other 
   organization using the local server name. This would mean that the 
   local realm be able to produce a ticket to the remote server under 
  
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   its name. You could give that remote server an identity in the local 
   realm and then have that remote server maintain a separate secret 
   for each alias it is known as. Alternatively you could arrange to 
   have the local realm issue a referral to the remote realm and notify 
   the requesting client of the server's remote name that should be 
   used in order to request a ticket. 
    
   This draft proposes a solution for these problems and simplifies 
   administration by minimizing the configuration information needed on 
   each computer using Kerberos. Specifically it describes a mechanism 
   to allow the KDC to handle Canonicalization of names, provide for 
   principal aliases for users and services and provide a mechanism for 
   the KDC to determine the trusted realm authentication path by being 
   able to generate referrals to other realms in order to locate 
   principals. 
    
   To rectify these problems, this draft introduces three new kinds of

   KDC referrals: 
        
   1. AS ticket referrals, in which the client doesn't know which realm 
      contains a user account.  
   2. TGS ticket referrals, in which the client doesn't know which 
      realm contains a server account.  
   3. Cross realm shortcut referrals, in which the KDC chooses the next 
      path on a referral chain 
    
4. Realm Organization Model 
    
   This draft assumes that the world of principals is arranged on 
   multiple levels: the realm, the enterprise, and the world. A KDC may 
   issue tickets for any principal in its realm or cross-realm tickets 
   for realms with which it has a direct trust relationship. The KDC 
   also has access to a trusted name service that can resolve any name 
   from within its enterprise into a realm. This trusted name service 
   removes the need to use an untrusted DNS lookup for name resolution. 
    
   For example, consider the following configuration, where lines 
   indicate trust relationships: 
    
                  MS.COM  
                /        \ 
               /          \ 
        OFFICE.MS.COM    NT.MS.COM 
    
   In this configuration, all users in the MS.COM enterprise could have 
   a principal name such as [email-masked], with the same realm portion. 
   In addition, servers at MS.COM should be able to have DNS host names 
   from any DNS domain independent of what Kerberos realm their 
   principal resides in. 
    
5. Client Principal Names 

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   A client account may have multiple principal names. More useful, 
   though, is a globally unique name that allows unification of email 
   and security principal names. For example, all users at MS may have 
   a client principal name of the form "[email-masked]" even though the 
   principals are contained in multiple realms. This global name is 
   again an alias for the true client principal name, which indicates 
   what realm contains the principal. Thus, accounts "alice" in the 
   realm ntdev.MS.COM and "bob" in office.MS.COM may logon as 
   "[email-masked]" and "[email-masked]". 
    
   This utilizes a new client principal name type, as the AS-REQ 
   message only contains a single realm field, and the realm portion of 
   this name doesn't correspond to any Kerberos realm. Thus, the entire 
   name "[email-masked]" is transmitted in the client name field of the 
   AS-REQ message, with a name type of KRB-NT-ENTERPRISE-PRINCIPAL. 
    
        KRB-NT-ENTERPRISE-PRINCIPAL     10 
    
   The KDC will recognize this name type and then transform the 
   requested name into the true principal name. The true principal name 
   can be using a name type different from the requested name type. 
     
6. Requesting a referral 
    
   In order to request referrals, the Kerberos client must explicitly 
   request the canonicalize KDC option (bit 15) in the KDC options for 
   the TGS-REQ. This flag indicates to the KDC that the client is 
   prepared to receive a reply that is a cross-realm TGT. Thus, the 
   KDCOptions types is redefined as: 
    
        KDCOptions ::=   BIT STRING { 
                          reserved(0), 
                          forwardable(1), 
                          forwarded(2), 
                          proxiable(3), 
                          proxy(4), 
                          allow-postdate(5), 
                          postdated(6), 
                          unused7(7), 
                          renewable(8), 
                          unused9(9), 
                          unused10(10), 
                          unused11(11), 
                          canonicalize(15), 
                          renewable-ok(27), 
                          enc-tkt-in-skey(28), 
                          renew(30), 
                          validate(31) 
         } 
    
6.1 Client Referrals 

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   The simplest form of ticket referral is for a user requesting a 
   ticket using an AS-REQ. In this case, the client machine will send 
   the AS request to a convenient trusted realm, either the realm of 
   the client machine or the realm of the client name. In the case of 
   the name [email-masked], the client may optimistically choose to send 
   the request to MS.COM. 
    
   The client will send the string "[email-masked]" in the client 
   principal name field using the KRB-NT-ENTERPRISE-PRINCIPAL name type 
   with the crealm set to MS.COM. The KDC will try to lookup the name 
   in its local account database. If the account is present in the 
   realm of the request, it MUST return a KDC reply structure with the 
   appropriate ticket. If the account is not present in the realm 
   specified in the request and the canonicalize flag in the KDCoptions 
   is set, the KDC will try to lookup the entire name, [email-masked], 
   using a name service. If this lookup is unsuccessful, it MUST return 
   the error KDC_ERR_C_PRINCIPAL_UNKNOWN. If the lookup is successful, 
   it MUST return an error KDC_ERR_WRONG_REALM (0x44) and in the error 
   message the cname and crealm field will contain the client name and 
   the true realm of the client. If the KDC contains the account 
   locally, it MUST return a normal ticket. The client name and realm 
   portions of the ticket and KDC reply message MUST be the client's 
   true name in the realm, not the globally unique name. 
    
   If the client receives a KDC_ERR_WRONG_REALM error, it will issue a 
   new AS request with the same client principal name used to generate 
   the first referral to the realm specified by the realm field of the 
   kerberos error message from the first request. This request MUST 
   produce a valid AS response with a ticket for the canonical user 
   name. The ticket MUST also include the ticket extension containing 
   the TE-REFERRAL-DATA with the referred-names set to the name from 
   the AS request. Any other error or referral will terminate the 
   request and result in a failed AS request. 
    
   [Discussion: Can a PA-REFERRAL-DATA be used instead of a ticket 
   extension?] 
    
6.2 Server Referrals 
    
   The server referral mechanism is a bit more complex than the client 
   referral mechanism. The primary problem is that the KDC must return 
   a referral ticket rather than an error message. There needs to be a 
   place to include in the TGS response information about what realm 
   contains the service. This is done by returning information about 
   the server name in the pre-auth data field of the KDC reply. 
    
   If the KDC resolves the server principal name into a principal in 
   its realm, it will return a normal ticket. If the canonicalize flag 
   in the KDCoptions is not set, then the KDC MUST only look up the 
   name as a normal principal name.  
    
   If the canonicalize flag in the KDCoptions is set and the KDC 
   doesn't find the principal locally, the KDC can return a cross-realm 
  
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   ticket granting ticket to the next hop on the trust path towards a 
   realm that may be able to resolve the principal name. 
    
   If the KDC can determine the service principal's realm, it SHOULD 
   return the server realm as ticket extension data. The ticket 
   extension MUST be encrypted using the session key from the ticket, 
   and the same etype as is used to protect the TGS reply body. 
    
   [Discussion: Can preauth data be used instead of a ticket-extension 
   for this purpose?] 
    
   The data itself is an ASN.1 encoded structure containing the 
   server's realm, and if known, canonical principal name.  
    
                TE-REFERRAL-INFO        20 
                 
                TE-REFERRAL-DATA ::= SEQUENCE { 
                        referred-server-realm[0]  KERB-REALM 
                        referred-name[1]         PrincipalName OPTIONAL 
                } 

   In order to facilitate cross-realm interoperability, a client SHOULD 
   NOT send short names in TGS requests to the KDC. A short name is 
   defined as a Kerberos name that includes a DNS name that is not 
   fully qualified. The client MAY use forward DNS lookups to obtain 
   the long name that corresponds to the user entered short name (the 
   short name will be a prefix of the corresponding long name). 
    
   The client may use the referred-name field to tell if it already has 
   a ticket to the target server in its ticket cache. 
    
   The client can use this information to request a chain of cross-
   realm ticket granting tickets until it reaches the realm of the 
   server, and can then expect to receive a valid service ticket. 
   However an implementation should limit the number of referrals that 
   it processes to avoid infinite referral loops. A suggested limit is 
   5 referrals before giving up. 
    
7. Cross Realm Routing 
    
   The current Kerberos protocol requires the client to explicitly 
   request a cross-realm TGT for each pair of realms on a referral 
   chain. As a result, the client need to be aware of the trust 
   hierarchy and of any short-cut trusts (those that aren't parent-
   child trusts). This requires more configurations on the client. 
   Instead, the client should be able to request a TGT to the target 
   realm from each realm on the route. The KDC will determine the best 
   path for the client and return a cross-realm TGT. The client has to 
   be aware that a request for a cross-realm TGT may return a TGT for a 
   realm different from the one requested. 

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                            KDC Referrals               February 2001 

   For compatability, the client MUST use the canonicalize KDCoption if 
   it is able to use cross-realm routing from the KDC. 
    
8. Security Considerations 
 
   The original Kerberos specification stated that the server principal 
   name in the KDC reply was the same as the server name in the 
   request. These protocol changes break that assumption, so the client 
   may be vulnerable to a denial of service attack by an attacker that 
   replays replies from previous requests. It can verify that the 
   request was one of its own by checking the client-address field or 
   authtime field, though, so the damage is limited and detectable. 
    
   For the AS exchange case, it is important that the logon mechanism 
   not trust a name that has not been used to authenticate the user. 
   For example, the name that the user enters as part of a logon 
   exchange may not be the name that the user authenticates as, given 
   that the KDC_ERR_WRONG_REALM error may have been returned. The 
   relevant Kerberos naming information for logon (if any), is the 
   client name and client realm in the service ticket targeted at the 
   workstation that was obtained using the user's initial TGT. 
    
   How the client name and client realm is mapped into a local account 
   for logon is a local matter, but the client logon mechanism MUST use 
   additional information such as the client realm and/or authorization 
   attributes from the service ticket presented to the workstation by 
   the user, when mapping the logon credentials to a local account on 
   the workstation. 
    
9. Discussion 
 
   This section contains issues and suggestions that need to be 
   incorporated into this draft. From Ken Raeburn [[email-masked]]: 
    
   1) No means to do name canonicalization if you're not 
      authenticating. Is it okay to require credentials in order to do 
      canonicalization? If so, how about this: Send a TGS_REQ for the 
      service name you have.  If you get back a TGS_REP for a service, 
      great; pull out the name and throw out the credentials.  If you 
      get back a TGS_REP for a TGT service, ask again in the specified 
      realm.  If you get back a KRB_ERROR because policy prohibits you 
      from authenticating to that service, we can add to the 
      specification that the {realm,sname} in the KRB_ERROR must be the 
      canonical name, and the checksum must be used.  As long as the 
      checksum is present, it's still a secure exchange with the KDC.  
       
      If we have to be able to do name canonicalization without any 
      sort of credentials, either client-side (tickets) or server-side 
      (tickets automatically acquired via service key), I think we just 
      lose. But maybe GSSAPI should be changed if that's the case. 
       
      There are cases where GSSAPI is not able to know of a canonical 

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      name even without the client name canonicalization proposed in 
      this draft. 
    
   2) Secure canonicalization of service name in AS_REQ. If the 
      response is an AS_REP, we need a way to tell that the altered 
      server name wasn't a result of a MITM attack on the AS_REQ 
      message.  Again, the KDC-REP extensible fields could have a new 
      required value added when name canonicalization happens, 
      indicating what the original principal name (in the AS_REQ 
      message) was, and signed using the same key as protects the 
      AS_REP.  If it doesn't match what the client requested, the 
      messages were altered in transit. 
    
   3) Client name needs referral to another realm, and server name 
      needs canonicalization of some sort. The above fixes wouldn't 
      work for this case, and I'm not even sure which KDC should be 
      doing the canonicalization anyways. 

   The other-principal-name datum would probably look something like: 
    
       PrincipalAndNonce ::= SEQUENCE { 
                    name[0]     PrincipalName, 
                    nonce[1]    INTEGER         -- copied from KDC_REQ 
       } 
       SignedPrincipal ::= SEQUENCE { 
                    name-and-nonce[0]   PrincipalAndNonce, 
                    cksum[1]    Checksum 
       } 
       {PA,TE}-ORIGINAL-SERVER-PRINCIPAL ::= SignedPrincipal 
       {PA,TE}-REMOTE-SERVER-PRINCIPAL ::= SignedPrincipal 
    
   with the checksum computed over the encoding of the 'name-and-nonce' 
   field, and appropriate PA- or TE- numbers assigned.  I don't have a 
   strong opinion on whether it'd be a pa-data or ticket extension; 
   conceptually it seems like an abuse of either, but, well, I think 
   I'd rather abuse them than leave the facility both in and 
   inadequate. 
    
   The nonce is needed because multiple exchanges may be made with the 
   same key, and these extension fields aren't packed in with the other 
   encrypted data in the same response, so a MITM could pick apart 
   multiple messages and mix-and-match components.  (In a TGS_REQ 
   exchange, a subsession key would help, but it's not required.) 
    
   The extension field would be required to prevent a MITM from 
   discarding the field from a response; a flag bit in a protected part 
   of the message (probably in 'flags' in EncKDCRepPart) could also let 
   us know of a cases where the information can be omitted, namely, 
   when no name change is done.  Perhaps the bit should be set to 
   indicate that a name change *was* done, and clear if it wasn't, 
   making the no-change case more directly compatible with RFC1510. 

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                            KDC Referrals               February 2001 

10. References 

   1  Bradner, S., "The Internet Standards Process -- Revision 3", BCP 
      9, RFC 2026, October 1996. 
    
   2  Bradner, S., "Key words for use in RFCs to Indicate Requirement 
      Levels", BCP 14, RFC 2119, March 1997 
    
   3  Kohl, J., Neuman, C., "The Kerberos Network Authentication 
      Service (V5)", RFC 1510, September 1993 

11. Author's Addresses 
    
   Michael Swift 
   University of Washington 
   Seattle, Washington 
   Email: [email-masked] 
    
   John Brezak 
   Microsoft 
   One Microsoft Way 
   Redmond, Washington 
   Email: [email-masked] 
    
   Jonathan Trostle 
   Cisco Systems 
   170 W. Tasman Dr. 
   San Jose, CA 95134 
   Email: [email-masked] 

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                            KDC Referrals               February 2001 

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