Kerberos Working Group M. Swift Internet Draft University of WA Document: draft-ietf-krb-wg-kerberos-referrals-00.txt J. Brezak Category: Standards Track Microsoft J. Trostle Cisco Systems K. Raeburn MIT February 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 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 progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 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 Swift Category - Standards Track 1 KDC Referrals February 2001 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 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. Current implementations of Kerberos also have difficulty with services on hosts that can have multiple host names (multi-homed hosts). Traditionally, each host name would need to have a distinct principal and a corresponding key. An extreme example of this would be a Web server with multiple host names for each domain that it is Swift Category - Standards Track 2 KDC Referrals February 2001 supporting. Principal aliases allow multi-homed hosts to have a single Kerberos principal (with a single key) that can have identities for each distinct host name. This mechanism allows the Kerberos client to request a service ticket for the distinct hostname and allows the KDC to return a ticket for the single principal that the host is using. This canonical principal name allows the host to only have to manage a single key for all of the identities that it supports. In addition, the client only needs to know the realm of the canonical service name, not all of the identities. 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 alice@MS.COM, 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. Swift Category - Standards Track 3 KDC Referrals February 2001 5. Principal Names 5.1 Service Principal Names The standard Kerberos model in RFC 1510 [3] gives each Kerberos principal a single name. However, if a service is reachable by several addresses, it is useful for a principal to have multiple names. Consider a service running on a multi-homed machine. Rather than requiring a separate principal and password for each name it exports, a single account with multiple names could be used. Multiple names are also useful for services in that clients need not perform DNS lookups to resolve a host name into a full DNS address. Instead, the service may have a name for each of its supported host names, including its IP address. Nonetheless, it is still convenient for the service to not have to be aware of all these names. Thus a new name may be added to DNS for a service by updating DNS and the KDC database without having to notify the service. In addition, it implies that these aliases are globally unique: they do not include a specifier dictating what realm contains the principal. Thus, an alias for a server is of the form "class/instance/name" and may be transmitted as any name type. 5.2 Client Principal Names Similarly, a client account may also 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 is indicates what realm contains the principal. Thus, accounts "alice" in the realm ntdev.MS.COM and "bob" in office.MS.COM may logon as "alice@MS.COM" and "bob@MS.COM". This requires 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. KRB-NT-ENTERPRISE-PRINCIPAL 10 5.3 Name Canonicalization In order to support name aliases, the Kerberos client must explicitly request the name-canonicalization KDC option (bit 15) in the ticket flags for the TGS-REQ. This flag indicates to the KDC that the client is prepared to receive a reply with a different client or server principal name than the request. Thus, the KDCOptions types is redefined as: KDCOptions ::= BIT STRING { Swift Category - Standards Track 4 KDC Referrals February 2001 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), name-canonicalize(15), renewable-ok(27), enc-tkt-in-skey(28), renew(30), validate(31) } 6. 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 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 crealm of the request, it MUST return a KDC reply structure with the appropriate ticket. If the account is not present in the crealm specified in the request and the name-canonicalize flag in the KDCoptions 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 cname and crealm field MUST 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 crealm 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 Swift Category - Standards Track 5 KDC Referrals February 2001 the AS request. Any other error or referral will terminate the request and result in a failed AS request. 7. 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, so it will 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 may return a normal ticket. If the name-canonicalize flag in the KDCoptions is not set, then the KDC MUST only look up the name as a normal principal name. Otherwise, it MUST search all aliases as well. The server principal name in both the ticket and the KDC reply MUST be the true server principal name instead of one of the aliases. This frees the application server from needing to know about all its aliases. If the name-canonicalize flag in the KDCoptions 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 can 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. The data itself is an ASN.1 encoded structure containing the server's realm, and if known, canonical principal name and alias names. The first name in the sequence is the canonical principal name. TE-REFERRAL-INFO 20 TE-REFERRAL-DATA ::= SEQUENCE { referred-server-realm[0] KERB-REALM referred-names[1] SEQUENCE OF PrincipalNames OPTIONAL } 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. 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 Swift Category - Standards Track 6 KDC Referrals February 2001 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-names field to tell if it already has a ticket to the 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. 8. 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 machines 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. 9. 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. 10. Discussion Swift Category - Standards Track 7 KDC Referrals February 2001 This section contains issues and suggestions that need to be incorporated into this draft. From Ken Raeburn [raeburn@mit.edu]: 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. 2) Can't refer to another realm and specify a different service name to give to that realm's KDC. The local KDC can tell you a different service name or a different realm name, but not both. This comes up in the "gnuftp.raeburn.org CNAME ftp.gnu.org" type of case I've mentioned. Except ... the KDC-REP structure includes padata and ticket extensions fields that are extensible. We could add a required value to one of them -- perhaps only in the case where you return a TGT when not asked -- that contains signed information about the principal name to ask for in the other realm. (It would have to be required, otherwise a man-in-the-middle could make it go away.) Signing would be done using the session key for the TGS. 3) 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. 4) 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: Swift Category - Standards Track 8 KDC Referrals February 2001 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. 11. 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 12. Author's Addresses Michael Swift University of Washington Seattle, Washington Email: mikesw@cs.washington.edu John Brezak Swift Category - Standards Track 9 KDC Referrals February 2001 Microsoft 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 Kenneth Raeburn Massachusetts Institute of Technology 77 Massachusetts Avenue Cambridge, Massachusetts 02139 Email: raeburn@mit.edu Swift Category - Standards Track 10 KDC Referrals February 2001 Full Copyright Statement Copyright (C) The Internet Society (1999). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. 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