1 2 3 4Kerberos Working Group K. Raeburn 5Internet-Draft MIT 6Updates: 4120 (if approved) L. Zhu 7Intended status: Standards Track Microsoft Corporation 8Expires: August 28, 2008 February 25, 2008 9 10 11 Generating KDC Referrals to Locate Kerberos Realms 12 draft-ietf-krb-wg-kerberos-referrals-10 13 14Status of this Memo 15 16 By submitting this Internet-Draft, each author represents that any 17 applicable patent or other IPR claims of which he or she is aware 18 have been or will be disclosed, and any of which he or she becomes 19 aware will be disclosed, in accordance with Section 6 of BCP 79. 20 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF), its areas, and its working groups. Note that 23 other groups may also distribute working documents as Internet- 24 Drafts. 25 26 Internet-Drafts are draft documents valid for a maximum of six months 27 and may be updated, replaced, or obsoleted by other documents at any 28 time. It is inappropriate to use Internet-Drafts as reference 29 material or to cite them other than as "work in progress." 30 31 The list of current Internet-Drafts can be accessed at 32 http://www.ietf.org/ietf/1id-abstracts.txt. 33 34 The list of Internet-Draft Shadow Directories can be accessed at 35 http://www.ietf.org/shadow.html. 36 37 This Internet-Draft will expire on August 28, 2008. 38 39Copyright Notice 40 41 Copyright (C) The IETF Trust (2008). 42 43Abstract 44 45 The memo documents a method for a Kerberos Key Distribution Center 46 (KDC) to respond to client requests for Kerberos tickets when the 47 client does not have detailed configuration information on the realms 48 of users or services. The KDC will handle requests for principals in 49 other realms by returning either a referral error or a cross-realm 50 TGT to another realm on the referral path. The clients will use this 51 referral information to reach the realm of the target principal and 52 53 54 55Raeburn & Zhu Expires August 28, 2008 [Page 1] 56 57Internet-Draft KDC Referrals February 2008 58 59 60 then receive the ticket. 61 62 63Table of Contents 64 65 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 66 2. Conventions Used in This Document . . . . . . . . . . . . . . 4 67 3. Requesting a Referral . . . . . . . . . . . . . . . . . . . . 4 68 4. Realm Organization Model . . . . . . . . . . . . . . . . . . . 5 69 5. Enterprise Principal Name Type . . . . . . . . . . . . . . . . 5 70 6. Name Canonicalization . . . . . . . . . . . . . . . . . . . . 5 71 7. Client Referrals . . . . . . . . . . . . . . . . . . . . . . . 7 72 8. Server Referrals . . . . . . . . . . . . . . . . . . . . . . . 9 73 9. Cross Realm Routing . . . . . . . . . . . . . . . . . . . . . 11 74 10. Caching Information . . . . . . . . . . . . . . . . . . . . . 11 75 11. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 12 76 12. Number Assignments . . . . . . . . . . . . . . . . . . . . . . 12 77 13. Security Considerations . . . . . . . . . . . . . . . . . . . 13 78 13.1. Shared-password case . . . . . . . . . . . . . . . . . . 13 79 14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14 80 15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14 81 15.1. Normative References . . . . . . . . . . . . . . . . . . 14 82 15.2. Informative References . . . . . . . . . . . . . . . . . 14 83 Appendix A. Compatibility with Earlier Implementations of 84 Name Canonicalization . . . . . . . . . . . . . . . . 14 85 Appendix B. Document history [REMOVE BEFORE PUBLICATION] . . . . 16 86 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16 87 Intellectual Property and Copyright Statements . . . . . . . . . . 18 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111Raeburn & Zhu Expires August 28, 2008 [Page 2] 112 113Internet-Draft KDC Referrals February 2008 114 115 1161. Introduction 117 118 Current implementations of the Kerberos AS and TGS protocols, as 119 defined in [RFC4120], use principal names constructed from a known 120 user or service name and realm. A service name is typically 121 constructed from a name of the service and the DNS host name of the 122 computer that is providing the service. Many existing deployments of 123 Kerberos use a single Kerberos realm where all users and services 124 would be using the same realm. However in an environment where there 125 are multiple trusted Kerberos realms, the client needs to be able to 126 determine what realm a particular user or service is in before making 127 an AS or TGS request. Traditionally this requires client 128 configuration to make this possible. 129 130 When having to deal with multiple trusted realms, users are forced to 131 know what realm they are in before they can obtain a ticket granting 132 ticket (TGT) with an AS request. However, in many cases the user 133 would like to use a more familiar name that is not directly related 134 to the realm of their Kerberos principal name. A good example of 135 this is an RFC 822 style email name. This document describes a 136 mechanism that would allow a user to specify a user principal name 137 that is an alias for the user's Kerberos principal name. In practice 138 this would be the name that the user specifies to obtain a TGT from a 139 Kerberos KDC. The user principal name no longer has a direct 140 relationship with the Kerberos principal or realm. Thus the 141 administrator is able to move the user's principal to other realms 142 without the user having to know that it happened. 143 144 Once a user has a TGT, they would like to be able to access services 145 in any trusted Kerberos realm. To do this requires that the client 146 be able to determine what realm the target service principal is in 147 before making the TGS request. Current implementations of Kerberos 148 typically have a table that maps DNS host names to corresponding 149 Kerberos realms. The user-supplied host name or its domain component 150 is looked up in this table (often using the result of some form of 151 host name lookup performed with insecure DNS queries, in violation of 152 [RFC4120]). The corresponding realm is then used to complete the 153 target service principal name. 154 155 This traditional mechanism requires that each client have very 156 detailed configuration information about the hosts that are providing 157 services and their corresponding realms. Having client side 158 configuration information can be very costly from an administration 159 point of view - especially if there are many realms and computers in 160 the environment. 161 162 There are also cases where specific DNS aliases (local names) have 163 been setup in an organization to refer to a server in another 164 165 166 167Raeburn & Zhu Expires August 28, 2008 [Page 3] 168 169Internet-Draft KDC Referrals February 2008 170 171 172 organization (remote server). The server has different DNS names in 173 each organization and each organization has a Kerberos realm that is 174 configured to service DNS names within that organization. Ideally 175 users are able to authenticate to the server in the other 176 organization using the local server name. This would mean that the 177 local realm be able to produce a ticket to the remote server under 178 its name. The administrator in the local realm could give that 179 remote server an identity in the local realm and then have that 180 remote server maintain a separate secret for each alias it is known 181 as. Alternatively the administrator could arrange to have the local 182 realm issue a referral to the remote realm and notify the requesting 183 client of the server's remote name that should be used in order to 184 request a ticket. 185 186 This memo proposes a solution for these problems and simplifies 187 administration by minimizing the configuration information needed on 188 each computer using Kerberos. Specifically it describes a mechanism 189 to allow the KDC to handle canonicalization of names, provide for 190 principal aliases for users and services and allow the KDC to 191 determine the trusted realm authentication path by being able to 192 generate referrals to other realms in order to locate principals. 193 194 Two kinds of KDC referrals are introduced in this memo: 195 196 1. Client referrals, in which the client doesn't know which realm 197 contains a user account. 198 2. Server referrals, in which the client doesn't know which realm 199 contains a server account. 200 201 2022. Conventions Used in This Document 203 204 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 205 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 206 document are to be interpreted as described in [RFC2119]. 207 208 2093. Requesting a Referral 210 211 In order to request referrals defined in section 5, 6, and 7, the 212 Kerberos client MUST explicitly request the canonicalize KDC option 213 (bit 15) [RFC4120] for the AS-REQ or TGS-REQ. This flag indicates to 214 the KDC that the client is prepared to receive a reply that contains 215 a principal name other than the one requested. 216 217 218 KDCOptions ::= KerberosFlags 219 -- canonicalize (15) 220 221 222 223Raeburn & Zhu Expires August 28, 2008 [Page 4] 224 225Internet-Draft KDC Referrals February 2008 226 227 228 -- other KDCOptions values omitted 229 230 The client should expect, when sending names with the "canonicalize" 231 KDC option, that names in the KDC's reply MAY be different than the 232 name in the request. A referral TGT is a cross realm TGT that is 233 returned with the server name of the ticket being different from the 234 server name in the request [RFC4120]. 235 236 2374. Realm Organization Model 238 239 This memo assumes that the world of principals is arranged on 240 multiple levels: the realm, the enterprise, and the world. A KDC may 241 issue tickets for any principal in its realm or cross-realm tickets 242 for realms with which it has a direct trust relationship. The KDC 243 also has access to a trusted name service that can resolve any name 244 from within its enterprise into a realm. This trusted name service 245 removes the need to use an un-trusted DNS lookup for name resolution. 246 247 For example, consider the following configuration, where lines 248 indicate trust relationships: 249 250 EXAMPLE.COM 251 / \ 252 / \ 253 ADMIN.EXAMPLE.COM DEV.EXAMPLE.COM 254 255 In this configuration, all users in the EXAMPLE.COM enterprise could 256 have principal names such as alice@EXAMPLE.COM, with the same realm 257 portion. In addition, servers at EXAMPLE.COM should be able to have 258 DNS host names from any DNS domain independent of what Kerberos realm 259 their principals reside in. 260 261 2625. Enterprise Principal Name Type 263 264 The NT-ENTERPRISE type principal name contains one component, a 265 string of realm-defined content, which is intended to be used as an 266 alias for another principal name in some realm in the enterprise. It 267 is used for conveying the alias name, not for the real principal 268 names within the realms, and thus is only useful when name 269 canonicalization is requested. 270 271 2726. Name Canonicalization 273 274 A service or account may have multiple principal names. More useful, 275 though, is a globally unique name that allows unification of email 276 277 278 279Raeburn & Zhu Expires August 28, 2008 [Page 5] 280 281Internet-Draft KDC Referrals February 2008 282 283 284 and security principal names. For example, all users at EXAMPLE.COM 285 may have a client principal name of the form "joe@EXAMPLE.COM" even 286 though the principals are contained in multiple realms. This global 287 name is again an alias for the true client principal name, which 288 indicates what realm contains the principal. Thus, accounts "alice" 289 in the realm DEV.EXAMPLE.COM and "bob" in ADMIN.EXAMPLE.COM may log 290 on as "alice@EXAMPLE.COM" and "bob@EXAMPLE.COM". 291 292 This utilizes a new client principal name type, as the AS-REQ message 293 only contains a single realm field, and the realm portion of this 294 name corresponds to the Kerberos realm with which the request is 295 made. Thus, the entire name "alice@EXAMPLE.COM" is transmitted as a 296 single component in the client name field of the AS-REQ message, with 297 a name type of NT-ENTERPRISE [RFC4120] (and the local realm name). 298 The KDC will recognize this name type and then transform the 299 requested name into the true principal name if the client account 300 resides in the local realm. The true principal name can have a name 301 type different from the requested name type. Typically the true 302 principal name will be a NT-PRINCIPAL [RFC4120]. 303 304 If the "canonicalize" KDC option is set, then the KDC MAY change the 305 client principal name and type in the AS response and ticket returned 306 from the name type of the client name in the request, and include a 307 mandatory PA-DATA object authenticating the client name mapping: 308 309 ReferralInfo ::= SEQUENCE { 310 requested-name [0] PrincipalName, 311 mapped-name [1] PrincipalName, 312 ... 313 } 314 PA-CLIENT-CANONICALIZED ::= SEQUENCE { 315 names [0] ReferralInfo, 316 canon-checksum [1] Checksum 317 } 318 319 The canon-checksum field is computed over the DER encoding of the 320 names sequences, using the AS reply key and a key usage value of 321 (TBD). 322 323 If the client name is unchanged, the PA-CLIENT-CANONICALIZED data is 324 not included. If the client name is changed, and the PA-CLIENT- 325 CANONICALIZED field does not exist, or the checksum cannot be 326 verified, or the requested-name field doesn't match the client name 327 in the originally-transmitted request, the client should discard the 328 response. 329 330 For example the AS request may specify a client name of "bob@ 331 EXAMPLE.COM" as an NT-ENTERPRISE name with the "canonicalize" KDC 332 333 334 335Raeburn & Zhu Expires August 28, 2008 [Page 6] 336 337Internet-Draft KDC Referrals February 2008 338 339 340 option set and the KDC will return with a client name of "104567" as 341 a NT-UID, and a PA-CLIENT-CANONICALIZED field listing the NT- 342 ENTERPRISE "bob@EXAMPLE.COM" principal as the requested-name and the 343 NT-UID "104567" principal as the mapped-name. 344 345 (It is assumed that the client discovers whether the KDC supports the 346 NT-ENTERPRISE name type via out of band mechanisms.) 347 348 In order to enable one party in a user-to-user exchange to confirm 349 the identity of another when only the alias is known, the KDC MAY 350 include the following authorization data element, wrapped in AD-KDC- 351 ISSUED, in the initial credentials and copy it from a ticket-granting 352 ticket into additional credentials: 353 354 AD-LOGIN-ALIAS ::= SEQUENCE { -- ad-type number TBD -- 355 login-aliases [0] SEQUENCE(1..MAX) OF PrincipalName, 356 } 357 358 The login-aliases field lists one or more of the aliases the 359 principal may have used in the initial ticket request. 360 361 The recipient of this authenticator must check the AD-LOGIN-ALIAS 362 names, if present, in addition to the normal client name field, 363 against the identity of the party with which it wishes to 364 authenticate; either should be allowed to match. (Note that this is 365 not backwards compatible with [RFC4120]; if the server side of the 366 user-to-user exchange does not support this extension, and does not 367 know the true principal name, authentication may fail if the alias is 368 sought in the client name field.) 369 370 The use of AD-KDC-ISSUED authorization data elements in cross-realm 371 cases has not been well explored at this writing; hence we will only 372 specify the inclusion of this data in the one-realm case. The alias 373 information should be dropped in the general cross-realm case. 374 However, a realm may implement a policy of accepting and re-signing 375 (wrapping in a new AD-KDC-ISSUED element) alias information provided 376 by certain other realms in the cross-realm ticket-granting service. 377 378 3797. Client Referrals 380 381 The simplest form of ticket referral is for a user requesting a 382 ticket using an AS-REQ. In this case, the client machine will send 383 the AS-REQ to a convenient trusted realm, for example the realm of 384 the client machine. In the case of the name alice@EXAMPLE.COM, the 385 client MAY optimistically choose to send the request to EXAMPLE.COM. 386 The realm in the AS-REQ is always the name of the realm that the 387 request is for as specified in [RFC4120]. 388 389 390 391Raeburn & Zhu Expires August 28, 2008 [Page 7] 392 393Internet-Draft KDC Referrals February 2008 394 395 396 The KDC will try to lookup the name in its local account database. 397 If the account is present in the realm of the request, it SHOULD 398 return a KDC reply structure with the appropriate ticket. 399 400 If the account is not present in the realm specified in the request 401 and the "canonicalize" KDC option is set, the KDC will try to lookup 402 the entire name, alice@EXAMPLE.COM, using a name service. If this 403 lookup is unsuccessful, it MUST return the error 404 KDC_ERR_C_PRINCIPAL_UNKNOWN [RFC4120]. If the lookup is successful, 405 it MUST return an error KDC_ERR_WRONG_REALM [RFC4120] and in the 406 error message the crealm field will contain either the true realm of 407 the client or another realm that MAY have better information about 408 the client's true realm. The client SHALL NOT use a cname returned 409 from a Kerberos error until that name is validated. 410 411 If the client receives a KDC_ERR_WRONG_REALM error, it will issue a 412 new AS request with the same client principal name used to generate 413 the first referral to the realm specified by the realm field of the 414 Kerberos error message corresponding to the first request. (The 415 client realm name will be updated in the new request to refer to this 416 new realm.) The client SHOULD repeat these steps until it finds the 417 true realm of the client. To avoid infinite referral loops, an 418 implementation should limit the number of referrals. A suggested 419 limit is 5 referrals before giving up. 420 421 Since the same client name is sent to the referring and referred-to 422 realms, both realms must recognize the same client names. In 423 particular, the referring realm cannot (usefully) define principal 424 name aliases that the referred-to realm will not know. 425 426 The true principal name of the client, returned in AS-REQ, can be 427 validated in a subsequent TGS message exchange where its value is 428 communicated back to the KDC via the authenticator in the PA-TGS-REQ 429 padata [RFC4120]. However, this requires trusting the referred-to 430 realm's KDCs. Clients should limit the referral mappings they will 431 accept to realms trusted via some local policy. Some possible 432 factors that might be taken into consideration for such a policy 433 might include: 434 435 o Any realm indicated by the local KDC, if the returned KRB-ERROR 436 message is protected, for example using a public key known to be 437 associated with the KDC 438 o A list of realms configured by an administrator 439 o Any realm accepted by the user when explicitly prompted 440 441 442 443 444 445 446 447Raeburn & Zhu Expires August 28, 2008 [Page 8] 448 449Internet-Draft KDC Referrals February 2008 450 451 4528. Server Referrals 453 454 The primary difference in server referrals is that the KDC MUST 455 return a referral TGT rather than an error message as is done in the 456 client referrals. There needs to be a place to include in the reply 457 information about what realm contains the server. This is done by 458 returning information about the server name in the pre-authentication 459 data field of the KDC reply [RFC4120], as specified later in this 460 section. 461 462 If the KDC resolves the server principal name into a principal in the 463 realm specified by the service realm name, it will return a normal 464 ticket. 465 466 If the "canonicalize" flag in the KDC options is not set, the KDC 467 MUST only look up the name as a normal principal name in the 468 specified server realm. If the "canonicalize" flag in the KDC 469 options is set and the KDC doesn't find the principal locally, the 470 KDC MAY return a cross-realm ticket granting ticket to the next hop 471 on the trust path towards a realm that may be able to resolve the 472 principal name. The true principal name of the server SHALL be 473 returned in the padata of the reply if it is different from what is 474 specified the request. 475 476 When a referral TGT is returned, the KDC MUST return the target realm 477 for the referral TGT as an KDC supplied pre-authentication data 478 element in the response. This referral information in pre- 479 authentication data MUST be encrypted using the session key from the 480 reply ticket. The key usage value for the encryption operation used 481 by PA-SERVER-REFERRAL is 26. 482 483 The pre-authentication data returned by the KDC, which contains the 484 referred realm and the true principal name of server, is encoded in 485 DER as follows. 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503Raeburn & Zhu Expires August 28, 2008 [Page 9] 504 505Internet-Draft KDC Referrals February 2008 506 507 508 PA-SERVER-REFERRAL 25 509 510 PA-SERVER-REFERRAL-DATA ::= EncryptedData 511 -- ServerReferralData -- 512 513 ServerReferralData ::= SEQUENCE { 514 referred-realm [0] Realm OPTIONAL, 515 -- target realm of the referral TGT 516 true-principal-name [1] PrincipalName OPTIONAL, 517 -- true server principal name 518 requested-principal-name [2] PrincipalName OPTIONAL, 519 -- requested server name 520 referral-valid-until [3] KerberosTime OPTIONAL, 521 ... 522 } 523 524 Clients SHALL NOT accept a reply ticket in which the server principal 525 name is different from that of the request, if the KDC response does 526 not contain a PA-SERVER-REFERRAL padata entry. 527 528 The requested-principal-name MUST be included by the KDC, and MUST be 529 verified by the client, if the client sent an AS-REQ, as protection 530 against a man-in-the-middle modification to the AS-REQ message. 531 532 The referred-realm field is present if and only if the returned 533 ticket is a referral TGT, not a service ticket for the requested 534 server principal. 535 536 When a referral TGT is returned and the true-principal-name field is 537 present, the client MUST use that name in the subsequent requests to 538 the server realm when following the referral. 539 540 Client SHALL NOT accept a true server principal name for a service 541 ticket if the true-principal-name field is not present in the PA- 542 SERVER-REFERRAL data. 543 544 The client will use this referral information to request a chain of 545 cross-realm ticket granting tickets until it reaches the realm of the 546 server, and can then expect to receive a valid service ticket. 547 548 However an implementation should limit the number of referrals that 549 it processes to avoid infinite referral loops. A suggested limit is 550 5 referrals before giving up. 551 552 The client may cache the mapping of the requested name to the name of 553 the next realm to use and the principal name to ask for. (See 554 Section 10.) The referral-valid-until field, if present, conveys how 555 long this information is valid for. 556 557 558 559Raeburn & Zhu Expires August 28, 2008 [Page 10] 560 561Internet-Draft KDC Referrals February 2008 562 563 564 Here is an example of a client requesting a service ticket for a 565 service in realm DEV.EXAMPLE.COM where the client is in 566 ADMIN.EXAMPLE.COM. 567 568 +NC = Canonicalize KDCOption set 569 +PA-REFERRAL = returned PA-SERVER-REFERRAL 570 C: TGS-REQ sname=http/foo.dev.example.com +NC to ADMIN.EXAMPLE.COM 571 S: TGS-REP sname=krbtgt/EXAMPLE.COM@ADMIN.EXAMPLE.COM +PA-REFERRAL 572 containing EXAMPLE.COM as the referred realm with no 573 true-principal-name 574 C: TGS-REQ sname=http/foo.dev.example.com +NC to EXAMPLE.COM 575 S: TGS-REP sname=krbtgt/DEV.EXAMPLE.COM@EXAMPLE.COM +PA-REFERRAL 576 containing DEV.EXAMPLE.COM as the referred realm with no 577 true-principal-name 578 C: TGS-REQ sname=http/foo.dev.example.com +NC to DEV.EXAMPLE.COM 579 S: TGS-REP sname=http/foo.dev.example.com@DEV.EXAMPLE.COM 580 581 Note that any referral or alias processing of the server name in 582 user-to-user authentication should use the same data as client name 583 canonicalization or referral. Otherwise, the name used by one user 584 to log in may not be useable by another for user-to-user 585 authentication to the first. 586 587 5889. Cross Realm Routing 589 590 The current Kerberos protocol requires the client to explicitly 591 request a cross-realm TGT for each pair of realms on a referral 592 chain. As a result, the client need to be aware of the trust 593 hierarchy and of any short-cut trusts (those that aren't parent- 594 child trusts). 595 596 Instead, using the server referral routing mechanism as defined in 597 Section 8, The KDC will determine the best path for the client and 598 return a cross-realm TGT as the referral TGT, and the target realm 599 for this TGT in the PA-SERVER-REFERRAL of the KDC reply. 600 601 If the "canonicalize" KDC option is not set, the KDC SHALL NOT return 602 a referral TGT. Clients SHALL NOT process referral TGTs if the KDC 603 response does not contain the PA-SERVER-REFERRAL padata. 604 605 60610. Caching Information 607 608 It is possible that the client may wish to get additional credentials 609 for the same service principal, perhaps with different authorization- 610 data restrictions or other changed attributes. The return of a 611 server referral from a KDC can be taken as an indication that the 612 613 614 615Raeburn & Zhu Expires August 28, 2008 [Page 11] 616 617Internet-Draft KDC Referrals February 2008 618 619 620 requested principal does not currently exist in the local realm. 621 Clearly, it would reduce network traffic if the clients could cache 622 that information and use it when acquiring the second set of 623 credentials for a service, rather than always having to re-check with 624 the local KDC to see if the name has been created locally. 625 626 If the referral-valid-until field is provided in the PA-SERVER- 627 REFERRAL-DATA message, it indicates the expiration time of this data; 628 if it is not included, the expiration time of the TGT is used. When 629 the TGT expires, the previously returned referral from the local KDC 630 should be considered invalid, and the local KDC must be asked again 631 for information for the desired service principal name. (Note that 632 the client may get back multiple referral TGTs from the local KDC to 633 the same remote realm, with different lifetimes. The lifetime 634 information must be properly associated with the requested service 635 principal names. Simply having another TGT for the same remote realm 636 does not extend the validity of previously acquired information about 637 one service principal name.) If the client is still in contact with 638 the service and needs to reauthenticate to the same service 639 regardless of local service principal name assignments, it should use 640 the referred-realm and true-principal-name values when requesting new 641 credentials. 642 643 Accordingly, KDC authors and maintainers should consider what factors 644 (e.g., DNS alias lifetimes) they may or may not wish to incorporate 645 into credential expiration times in cases of referrals. 646 647 64811. Open Issues 649 650 Client referral info validation 651 652 When should client name aliases be included in credentials? Should 653 all known client name aliases be included, or only the one used at 654 initial ticket acquisition? 655 656 Should list the policies that need to be defined. 657 658 More examples: u2u, policy checks, maybe cross-realm. 659 660 Restore server name canonicalization from early drafts. 661 662 66312. Number Assignments 664 665 Most number registries in the Kerberos protocol have not been turned 666 over to IANA for management at the time of this writing, hence this 667 is not an "IANA Considerations" section. 668 669 670 671Raeburn & Zhu Expires August 28, 2008 [Page 12] 672 673Internet-Draft KDC Referrals February 2008 674 675 676 Various values do need assigning for this draft: 677 AD-LOGIN-ALIAS 678 PA-CLIENT-CANONICALIZED 679 key usage value for PA-CLIENT-CANONICALIZED field canon-checksum 680 681 68213. Security Considerations 683 684 For the AS exchange case, it is important that the logon mechanism 685 not trust a name that has not been used to authenticate the user. 686 For example, the name that the user enters as part of a logon 687 exchange may not be the name that the user authenticates as, given 688 that the KDC_ERR_WRONG_REALM error may have been returned. The 689 relevant Kerberos naming information for logon (if any), is the 690 client name and client realm in the service ticket targeted at the 691 workstation that was obtained using the user's initial TGT. 692 693 How the client name and client realm is mapped into a local account 694 for logon is a local matter, but the client logon mechanism MUST use 695 additional information such as the client realm and/or authorization 696 attributes from the service ticket presented to the workstation by 697 the user, when mapping the logon credentials to a local account on 698 the workstation. 699 70013.1. Shared-password case 701 702 A special case to examine is when the user is known (or correctly 703 suspected) to use the same password for multiple accounts. A man-in- 704 the-middle attacker can either alter the request on its way to the 705 KDC, changing the client principal name, or reply to the client with 706 a response previously send by the KDC in response to a request from 707 the attacker. The response received by the client can then be 708 decrypted by the user, though if the default "salt" generated from 709 the principal name is used to produce the user's key, a PA-ETYPE-INFO 710 or PA-ETYPE-INFO2 preauth record may need to be added or altered by 711 the attacker to cause the client software to generate the key needed 712 for the message it will receive. None of this requires the attacker 713 to know the user's password, and without further checking, could 714 cause the user to unknowingly use the wrong credentials. 715 716 In normal [RFC4120] operation, a generated AP-REQ message includes in 717 the Authenticator field a copy of the client's idea of its own 718 principal name. If this differs from the name in the KDC-generated 719 Ticket, the application server will reject the message. 720 721 With client name canonicalization as described in this document, the 722 client may get its principal name from the response from the KDC. 723 Requiring the PA-CLIENT-CANONICALIZED data lets the client securely 724 725 726 727Raeburn & Zhu Expires August 28, 2008 [Page 13] 728 729Internet-Draft KDC Referrals February 2008 730 731 732 check that the requested name was not altered in transit. If the PA- 733 CLIENT-CANONICALIZED data is absent, the client can use the principal 734 name it requested. 735 736 73714. Acknowledgments 738 739 Sam Hartman and authors came up with the idea of using the ticket key 740 to encrypt the referral data, which prevents cut and paste attack 741 using the referral data and referral TGTs. 742 743 John Brezak, Mike Swift, and Jonathan Trostle wrote the initial 744 version of this document. 745 746 Karthik Jaganathan contributed to earlier versions. 747 748 74915. References 750 75115.1. Normative References 752 753 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 754 Requirement Levels", BCP 14, RFC 2119, March 1997. 755 756 [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The 757 Kerberos Network Authentication Service (V5)", RFC 4120, 758 July 2005. 759 76015.2. Informative References 761 762 [RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet 763 X.509 Public Key Infrastructure Certificate and 764 Certificate Revocation List (CRL) Profile", RFC 3280, 765 April 2002. 766 767 [RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial 768 Authentication in Kerberos (PKINIT)", RFC 4556, June 2006. 769 770 [XPR] Trostle, J., Kosinovsky, I., and M. Swift, "Implementation 771 of Crossrealm Referral Handling in the MIT Kerberos 772 Client", Network and Distributed System Security 773 Symposium, February 2001. 774 775 776Appendix A. Compatibility with Earlier Implementations of Name 777 Canonicalization 778 779 The Microsoft Windows 2000 and Windows 2003 releases included an 780 781 782 783Raeburn & Zhu Expires August 28, 2008 [Page 14] 784 785Internet-Draft KDC Referrals February 2008 786 787 788 earlier form of name-canonicalization [XPR]. Here are the 789 differences: 790 791 1) The TGS referral data is returned inside of the KDC message as 792 "encrypted pre-authentication data". 793 794 795 796 EncKDCRepPart ::= SEQUENCE { 797 key [0] EncryptionKey, 798 last-req [1] LastReq, 799 nonce [2] UInt32, 800 key-expiration [3] KerberosTime OPTIONAL, 801 flags [4] TicketFlags, 802 authtime [5] KerberosTime, 803 starttime [6] KerberosTime OPTIONAL, 804 endtime [7] KerberosTime, 805 renew-till [8] KerberosTime OPTIONAL, 806 srealm [9] Realm, 807 sname [10] PrincipalName, 808 caddr [11] HostAddresses OPTIONAL, 809 encrypted-pa-data [12] SEQUENCE OF PA-DATA OPTIONAL 810 } 811 812 2) The preauth data type definition in the encrypted preauth data is 813 as follows: 814 815 816 817 PA-SVR-REFERRAL-INFO 20 818 819 PA-SVR-REFERRAL-DATA ::= SEQUENCE { 820 referred-name [1] PrincipalName OPTIONAL, 821 referred-realm [0] Realm 822 }} 823 824 3) When PKINIT ([RFC4556]) is used, the NT-ENTERPRISE client name is 825 encoded as a Subject Alternative Name (SAN) extension [RFC3280] in 826 the client's X.509 certificate. The type of the otherName field 827 for this SAN extension is AnotherName [RFC3280]. The type-id 828 field of the type AnotherName is id-ms-sc-logon-upn 829 (1.3.6.1.4.1.311.20.2.3) and the value field of the type 830 AnotherName is a KerberosString [RFC4120]. The value of this 831 KerberosString type is the single component in the name-string 832 [RFC4120] sequence for the corresponding NT-ENTERPRISE name type. 833 834 In Microsoft's current implementation through the use of global 835 catalogs any domain in one forest is reachable from any other domain 836 837 838 839Raeburn & Zhu Expires August 28, 2008 [Page 15] 840 841Internet-Draft KDC Referrals February 2008 842 843 844 in the same forest or another trusted forest with 3 or less 845 referrals. A forest is a collection of realms with hierarchical 846 trust relationships: there can be multiple trust trees in a forest; 847 each child and parent realm pair and each root realm pair have 848 bidirectional transitive direct rusts between them. 849 850 While we might want to permit multiple aliases to exist and even be 851 reported in AD-LOGIN-ALIAS, the Microsoft implementation permits only 852 one NT-ENTERPRISE alias to exist, so this question had not previously 853 arisen. 854 855 856Appendix B. Document history [REMOVE BEFORE PUBLICATION] 857 858 10 TBD 859 09 Changed to EXAMPLE.COM instead of using Morgan Stanley's domain. 860 Rewrote description of existing practice. (Don't name the lookup 861 table consulted. Mention that DNS "canonicalization" is contrary 862 to [RFC4120].) Noted Microsoft behavior should be moved out into 863 a separate document. Changed some second-person references in the 864 introduction to identify the proper parties. Changed PA-CLIENT- 865 CANONICALIZED to use a separate type for the actual referral data, 866 add an extension marker to that type, and change the checksum key 867 from the "returned session key" to the "AS reply key". Changed 868 AD-LOGIN-ALIAS to contain a sequence of names, to be contained in 869 AD-KDC-ISSUED instead of AD-IF-RELEVANT, and to drop the no longer 870 needed separate checksum. Attempt to clarify the cache lifetime 871 of referral information. 872 08 Moved Microsoft implementation info to appendix. Clarify lack of 873 local server name canonicalization. Added optional authz-data for 874 login alias, to support user-to-user case. Added requested- 875 principal-name to ServerReferralData. Added discussion of caching 876 information, and referral TGT lifetime. 877 07 Re-issued with new editor. Fixed up some references. Started 878 history. 879 880 881Authors' Addresses 882 883 Kenneth Raeburn 884 Massachusetts Institute of Technology 885 77 Massachusetts Avenue 886 Cambridge, MA 02139 887 US 888 889 Email: raeburn@mit.edu 890 891 892 893 894 895Raeburn & Zhu Expires August 28, 2008 [Page 16] 896 897Internet-Draft KDC Referrals February 2008 898 899 900 Larry Zhu 901 Microsoft Corporation 902 One Microsoft Way 903 Redmond, WA 98052 904 US 905 906 Email: lzhu@microsoft.com 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951Raeburn & Zhu Expires August 28, 2008 [Page 17] 952 953Internet-Draft KDC Referrals February 2008 954 955 956Full Copyright Statement 957 958 Copyright (C) The IETF Trust (2008). 959 960 This document is subject to the rights, licenses and restrictions 961 contained in BCP 78, and except as set forth therein, the authors 962 retain all their rights. 963 964 This document and the information contained herein are provided on an 965 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 966 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 967 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 968 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 969 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 970 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 971 972 973Intellectual Property 974 975 The IETF takes no position regarding the validity or scope of any 976 Intellectual Property Rights or other rights that might be claimed to 977 pertain to the implementation or use of the technology described in 978 this document or the extent to which any license under such rights 979 might or might not be available; nor does it represent that it has 980 made any independent effort to identify any such rights. 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Please address the information to the IETF at 995 ietf-ipr@ietf.org. 996 997 998Acknowledgment 999 1000 Funding for the RFC Editor function is provided by the IETF 1001 Administrative Support Activity (IASA). 1002 1003 1004 1005 1006 1007Raeburn & Zhu Expires August 28, 2008 [Page 18] 1008 1009