draft-ietf-krb-wg-kerberos-referrals-03.txt   [plain text]

Kerberos Working Group                                          Karthik 
Internet Draft                                                Larry Zhu 
Document: draft-ietf-krb-wg-kerberos-referrals-03.txt       John Brezak 
Category: Standards Track                                     Microsoft 
                                                             Mike Swift 
                                                          University of  
                                                       Jonathan Trostle 
                                                          Cisco Systems 
                                                        Expires: August 
           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 
   Task Force (IETF), its areas, and its working groups. Note that 
   other groups may also distribute working documents as Internet-
   Drafts. Internet-Drafts are draft documents valid for a maximum of 
   six months and may be updated, replaced, or obsoleted by other 
   documents at any time. It is inappropriate to use Internet- Drafts 
   as reference material or to cite them other than as "work in 
   The list of current Internet-Drafts can be accessed at 
   The list of Internet-Draft Shadow Directories can be accessed at 
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", 
   this document are to be interpreted as described in RFC-2119 [2]. 

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3. Introduction 
   Current implementations of the Kerberos AS and TGS protocols, as 
   defined in [3], use principal names constructed from a known user or 
   service name and realm. A service name is typically 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 
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   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 
   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 
   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: 
                /        \ 
               /          \ 
   In this configuration, all users in the MS.COM enterprise could have 
   a principal name such as alice@MS.COM, with the same realm portion. 
   In addition, servers at MS.COM should be able to have DNS host names 

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   from any DNS domain independent of what Kerberos realm their 
   principal resides in. 
5. Client Name Canonicalization 
   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 "joe@MS.COM" 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 NT.MS.COM and "bob" in OFFICE.MS.COM may logon as 
   "alice@MS.COM" and "bob@MS.COM". 
   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 "alice@MS.COM" is transmitted in the client name field of the 
   AS-REQ message, with a name type of KRB-NT-ENTERPRISE-PRINCIPAL. 
   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. 
   Typically the returned principal name will be a KRB-NT-PRINCIPAL. 
   The returned name will be the same in the AS response and in the 
   ticket. The KDC will always return a different name type than KRB-
   NT-ENTERPRISE-PRINCIPAL. This is regardless of the presence of the 
   "canonicalize" KDC option. 
   If the "canonicalize" KDC option is set, then the KDC MAY change the 
   client principal name and type in the AS response and ticket 
   regardless of the name type of the client name in the request. For 
   example the AS request may specify a client name of "fred@MS.COM" as 
   an KRB-NT-PRINCIPAL with the "canonicalize" KDC option set and the 
   KDC will return with a client name of "104567" as a KRB-NT-UID. 
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 contains a principal name other 
   than the one requested. Thus, the KDCOptions types is redefined as: 
        KDCOptions ::=   BIT STRING { 
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   The client should expect, when sending names with the "canonicalize" 
   KDC option, that names in the KDC's reply will be different than the 
   name in the request. 
6.1 Client Referrals 
   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 Alice@MS.COM, the client may optimistically choose to send 
   the request to MS.COM. The realm in the AS request is always the 
   name of the realm that the request is for as specified in [3]. 
   The client will send the string "alice@MS.COM" 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" KDC option is set, the KDC will try to lookup 
   the entire name, Alice@MS.COM, 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 crealm field will contain the the true realm of the client or 
   another realm that has better information about the client's true 
   realm. The client MUST NOT use a cname returned from a referral. 
   If the KDC contains the account locally and "canonicalize" KDC 
   option is not set, 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 
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   kerberos error message from the first request. This request MUST 
   produce a valid AS response with a ticket for the canonical user 
   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. In MicrosoftÆs implementation the 
   default limit is 3 since through the use of the global catalog any 
   domain in one forest is reachable from any other domain in another 
   trusting forest with 3 or less referrals.  
6.2 Service Referrals 
   The primary problem is that the KDC must return a referral ticket 
   rather than an error message as is done in AS request referrals. 
   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 service name in the pre-auth data field of the 
   KDC reply. 
   If the KDC resolves the service principal name into a principal in 
   the realm specified by the service realm name, it will return a 
   normal ticket. When using canonicalization, the client can omit the 
   service realm name. If it is supplied, it is used as a hint by the 
   KDC, but the service principal lookup is not constrained to locating 
   the service principal name in that specified realm. If the 
   "canonicalize" flag in the KDC options is not set, then the KDC MUST 
   only look up the name as a normal principal name in the specified 
   service realm.  
   If the "canonicalize" flag in the KDC options is set and the KDC 
   doesn't find the principal locally, the KDC can return a cross-realm 
   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 service realm as KDC supplied pre-authentication data 
   element. The preauth data MUST be encrypted using the sub-session 
   key from the authenticator if present or the session key from the 
   The data itself is an ASN.1 encoded structure containing the 
   server's realm, and if known, the real principal name.  
                PA-SERVER-REFERRAL-INFO        25 
                                           -- PA-SERVER-REFERRAL-DATA 
                        referred-server-realm[0]  KERB-REALM 
                        referred-name[1]         PrincipalName OPTIONAL 
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   If applicable to the encryption type, the key derivation value will 
   for the PA-SERVER-REFERRAL is 22. 
   If the referred-name field is present, the client MUST use that name 
   in a subsequent TGS request to the service realm when following the 
   The client will use this information to request a chain of cross-
   realm ticket granting tickets until it reaches the realm of the 
   service, 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. 
   This is an example of a client requesting a service ticket for a 
   service in realm NT.MS.COM where the client is in OFFICE.MS.COM. 
        +NC = Canonicalize KDCOption set 
        C: TGS-REQ sname=server/foo.nt.ms.com srealm=NULL +NC to 
        S: TGS-REP sname=krbtgt/MS.COM@OFFICE.MS.COM +PA-REFERRAL 
        containing NT.MS.COM 
        C: TGS-REQ sname=krbtgt/NT.MS.COM@MS.COM +NC to MS.COM 
        S: TGS-REP sname=krbtgt/NT.MS.COM@MS.COM 
        C: TGS-REQ sname=server/foo.nt.ms.com srealm=NT.MS.COM +NC to 
        S: TGS-REP sname=server/foo.nt.ms.com@NT.MS.COM 
   Notice that the client only specifies the service name in the 
   initial and final TGS request. 
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). 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. 
   For compatibility, the client MUST use the "canonicalize" KDC option 
   if it is able to use cross-realm routing from the KDC. 
8. Compatibility with earlier implementations of name canonicalization 
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   The Microsoft Windows 2000 release included an earlier form of name-
   canonicalization [4]. It has these differences: 
   1) The TGS referral data was returned inside of the KDC message as 
      "encrypted pre auth data". 
                        session-key[0]   KERB-ENCRYPTION-KEY, 
                        last-request[1]  PKERB-LAST-REQUEST, 
                        nonce[2]         INTEGER, 
                        key-expiration[3] KERB-TIME OPTIONAL, 
                        flags[4]         KERB-TICKET-FLAGS, 
                        authtime[5]      KERB-TIME, 
                        starttime[6]     KERB-TIME OPTIONAL, 
                        endtime[7]       KERB-TIME, 
                        renew-until[8]   KERB-TIME OPTIONAL, 
                        server-realm[9]  KERB-REALM, 
                        server-name[10]  KERB-PRINCIPAL-NAME, 
                        client-addresses[11] PKERB-HOST-ADDRESSES 
                        encrypted-pa-data[12] SEQUENCE OF KERB-PA-DATA 
   2) The preauth data type definition in the encrypted preauth data is 
      as follows: 
                PA-SVR-REFERRAL-INFO        20 
                PA-SVR-REFERRAL-DATA ::= SEQUENCE { 
                        referred-server-name[1]  PrincipalName OPTIONAL 
                        referred-server-realm[0] KERB-REALM 
9. Security Considerations 
   In the case of TGS requests the client may be vulnerable to a denial 
   of service attack by an attacker that replays replies from previous 
   requests. The client 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. Clients MUST NOT process cross 
   realm referral TGTs if the KDC reply does not include the encrypted 
   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 

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   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. 
10. Acknowledgements 
   The authors wish to thank Ken Raeburn for his comments and 
11.1 Normative 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  Neuman, C., Kohl, J., Ts'o, T., Yu, T., Hartman, S., and K. 
      Raeburn, "The Kerberos Network Authentication Service (V5)", 
      draft-ietf-krb-wg-kerberos-clarifications-00.txt, February 22, 
      2002.  Work in progress. 
11.2 Informative References 
   4  J. Trostle, I. Kosinovsky, and M. Swift,"Implementation of 
      Crossrealm Referral Handling in the MIT Kerberos Client", In 
      Network and Distributed System Security Symposium, February 2001. 
12. Author's Addresses 
   Karthik Jaganathan 
   One Microsoft Way 
   Redmond, Washington 
   Email: karthikj@Microsoft.com 
   Larry Zhu 
   One Microsoft Way 
   Redmond, Washington 
   Email: lzhu@Microsoft.com 
   Michael Swift 
   University of Washington 
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   Seattle, Washington 
   Email: mikesw@cs.washington.edu 
   John Brezak 
   One Microsoft Way 
   Redmond, Washington 
   Email: jbrezak@Microsoft.com 
   Jonathan Trostle 
   Cisco Systems 
   170 W. Tasman Dr. 
   San Jose, CA 95134 
   Email: jtrostle@cisco.com 

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