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'8') (Obsoleted by RFC 8945) -- No information found for draft-ietf-dhc-authentication- - is the name correct? -- Possible downref: Normative reference to a draft: ref. '9' -- No information found for draft-ietf-dnsext-dhcid-rr- - is the name correct? -- Possible downref: Normative reference to a draft: ref. '11' ** Downref: Normative reference to an Informational RFC: RFC 1321 (ref. '12') -- No information found for draft-ietf-dhc-fqdn-option- - is the name correct? -- Possible downref: Normative reference to a draft: ref. '13' Summary: 8 errors (**), 0 flaws (~~), 7 warnings (==), 9 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DHC Working Group M. Stapp 3 Internet-Draft Cisco Systems, Inc. 4 Expires: August 31, 2001 March 2, 2001 6 Resolution of DNS Name Conflicts Among DHCP Clients 7 9 Status of this Memo 11 This document is an Internet-Draft and is in full conformance with 12 all provisions of Section 10 of RFC2026. 14 Internet-Drafts are working documents of the Internet Engineering 15 Task Force (IETF), its areas, and its working groups. Note that 16 other groups may also distribute working documents as 17 Internet-Drafts. 19 Internet-Drafts are draft documents valid for a maximum of six 20 months and may be updated, replaced, or obsoleted by other documents 21 at any time. It is inappropriate to use Internet-Drafts as reference 22 material or to cite them other than as "work in progress." 24 The list of current Internet-Drafts can be accessed at 25 http://www.ietf.org/ietf/1id-abstracts.txt. 27 The list of Internet-Draft Shadow Directories can be accessed at 28 http://www.ietf.org/shadow.html. 30 This Internet-Draft will expire on August 31, 2001. 32 Copyright Notice 34 Copyright (C) The Internet Society (2001). All Rights Reserved. 36 Abstract 38 DHCP provides a powerful mechanism for IP host configuration. 39 However, the configuration capability provided by DHCP does not 40 include updating DNS(RFC1034[1], RFC1035[2]), and specifically 41 updating the name to address and address to name mappings maintained 42 in the DNS. 44 The "Client FQDN Option"[13] specifies the client FQDN option, 45 through which DHCP clients and servers can exchange information 46 about client FQDNs. This document describes techniques for the 47 resolution of DNS name conflicts among DHCP clients. 49 Table of Contents 51 1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3 52 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 53 3. Issues with DDNS in DHCP Environments . . . . . . . . . . . 3 54 3.1 Name Conflicts . . . . . . . . . . . . . . . . . . . . . . . 4 55 3.2 Multiple DHCP servers . . . . . . . . . . . . . . . . . . . 5 56 3.3 Use of the DHCID RR . . . . . . . . . . . . . . . . . . . . 5 57 3.3.1 Format of the DHCID RRDATA . . . . . . . . . . . . . . . . . 5 58 3.4 DNS RR TTLs . . . . . . . . . . . . . . . . . . . . . . . . 7 59 4. Procedures for performing DNS updates . . . . . . . . . . . 7 60 4.1 Adding A RRs to DNS . . . . . . . . . . . . . . . . . . . . 7 61 4.2 Adding PTR RR Entries to DNS . . . . . . . . . . . . . . . . 8 62 4.3 Removing Entries from DNS . . . . . . . . . . . . . . . . . 9 63 4.4 Updating other RRs . . . . . . . . . . . . . . . . . . . . . 9 64 5. Security Considerations . . . . . . . . . . . . . . . . . . 9 65 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 66 References . . . . . . . . . . . . . . . . . . . . . . . . . 11 67 Author's Address . . . . . . . . . . . . . . . . . . . . . . 12 68 Full Copyright Statement . . . . . . . . . . . . . . . . . . 13 70 1. Terminology 72 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 73 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 74 document are to be interpreted as described in RFC 2119[6]. 76 2. Introduction 78 "The Client FQDN Option"[13] includes a description of the operation 79 of DHCP[3] clients and servers that use the client FQDN option. 80 Through the use of the client FQDN option, DHCP clients and servers 81 can negotiate the client's FQDN and the allocation of responsibility 82 for updating the DHCP client's A RR. This document identifies 83 situations in which conflicts in the use of FQDNs may arise among 84 DHCP clients, and describes a strategy for the use of the DHCID DNS 85 resource record[11] in resolving those conflicts. 87 In any case, whether a site permits all, some, or no DHCP servers 88 and clients to perform DNS updates into the zones which it controls 89 is entirely a matter of local administrative policy. This document 90 does not require any specific administrative policy, and does not 91 propose one. The range of possible policies is very broad, from 92 sites where only the DHCP servers have been given credentials that 93 the DNS servers will accept, to sites where each individual DHCP 94 client has been configured with credentials which allow the client 95 to modify its own domain name. Compliant implementations MAY support 96 some or all of these possibilities. Furthermore, this specification 97 applies only to DHCP client and server processes: it does not apply 98 to other processes which initiate DNS updates. 100 3. Issues with DDNS in DHCP Environments 102 There are two DNS update situations that require special 103 consideration in DHCP environments: cases where more than one DHCP 104 client has been configured with the same FQDN, and cases where more 105 than one DHCP server has been given authority to perform DNS updates 106 in a zone. In these cases, it is possible for DNS records to be 107 modified in inconsistent ways unless the updaters have a mechanism 108 that allows them to detect anomolous situations. If DNS updaters can 109 detect these situations, site administrators can configure the 110 updaters' behavior so that the site's policies can be enforced. We 111 use the term "Name Conflict" to refer to cases where more than one 112 DHCP client has been associated with a single FQDN. This 113 specification describes a mechanism designed to allow updaters to 114 detect these situations, and requires that DHCP implementations use 115 this mechanism by default. 117 3.1 Name Conflicts 119 How can the entity updating an A RR (either the DHCP client or DHCP 120 server) detect that a domain name has an A RR which is already in 121 use by a different DHCP client? Similarly, should a DHCP client or 122 server update a domain name which has an A RR that has been 123 configured by an administrator? In either of these cases, the 124 domain name in question would either have an additional A RR, or 125 would have its original A RR replaced by the new record. Either of 126 these effects may be considered undesirable by some sites. Different 127 authority and credential models have different levels of exposure to 128 name conflicts. 130 1. Client updates A RR, uses Secure DNS Update with credentials 131 that are associated with the client's FQDN, and exclusive to the 132 client. Name conflicts in this scenario are unlikely (though not 133 impossible), since the client has received credentials specific 134 to the name it desires to use. This implies that the name has 135 already been allocated (through some implementation- or 136 organization-specific procedure) to that client. 138 2. Client updates A RR, uses Secure DNS Update with credentials 139 that are valid for any name in the zone. Name conflicts in this 140 scenario are possible, since the credentials necessary for the 141 client to update DNS are not necessarily name-specific. Thus, 142 for the client to be attempting to update a unique name requires 143 the existence of some administrative procedure to ensure client 144 configuration with unique names. 146 3. Server updates the A RR, uses a name for the client which is 147 known to the server. Name conflicts in this scenario are likely 148 unless prevented by the server's name configuration procedures. 149 See Section 5 for security issues with this form of deployment. 151 4. Server updates the A RR, uses a name supplied by the client. 152 Name conflicts in this scenario are highly likely, even with 153 administrative procedures designed to prevent them. (This 154 scenario is a popular one in real-world deployments in many 155 types of organizations.) See Section 5 for security issues with 156 this type of deployment. 158 Scenarios 2, 3, and 4 rely on administrative procedures to ensure 159 name uniqueness for DNS updates, and these procedures may break 160 down. Experience has shown that, in fact, these procedures will 161 break down at least occasionally. The question is what to do when 162 these procedures break down or, for example in scenario #4, may not 163 even exist. 165 In all cases of name conflicts, the desire is to offer two modes of 166 operation to the administrator of the combined DHCP-DNS capability: 167 first-update-wins (i.e., the first updating entity gets the name) or 168 most-recent-update-wins (i.e., the last updating entity for a name 169 gets the name). 171 3.2 Multiple DHCP servers 173 If multiple DHCP servers are able to update the same DNS zones, or 174 if DHCP servers are performing A RR updates on behalf of DHCP 175 clients, and more than one DHCP server may be able to serve 176 addresses to the same DHCP clients, the DHCP servers should be able 177 to provide reasonable and consistent DNS name update behavior for 178 DHCP clients. 180 3.3 Use of the DHCID RR 182 A solution to both of these problems is for the updating entities 183 (both DHCP clients and DHCP servers) to be able to detect that 184 another entity has been associated with a DNS name, and to offer 185 administrators the opportunity to configure update behavior. 187 Specifically, a DHCID RR, described in DHCID RR[11] is used to 188 associate client identification information with a DNS name and the 189 A RR associated with that name. When either a client or server adds 190 an A RR for a client, it also adds a DHCID RR which specifies a 191 unique client identity (based on a "client specifier" created from 192 the client's client-id or MAC address). In this model, only one A 193 RR is associated with a given DNS name at a time. 195 By associating this ownership information with each A RR, 196 cooperating DNS updating entities may determine whether their client 197 is the first or last updater of the name (and implement the 198 appropriately configured administrative policy), and DHCP clients 199 which currently have domain names may move from one DHCP server to 200 another without losing their DNS names. 202 The specific algorithms utilizing the DHCID RR to signal client 203 ownership are explained below. The algorithms only work in the case 204 where the updating entities all cooperate -- this approach is 205 advisory only and is not substitute for DNS security, nor is it 206 replaced by DNS security. 208 3.3.1 Format of the DHCID RRDATA 210 The DHCID RR used to hold the DHCP client's identity is formatted as 211 follows: 213 The name of the DHCID RR is the name of the A or PTR RR which refers 214 to the DHCP client. 216 The RDATA section of a DHCID RR in transmission contains RDLENGTH 217 bytes of binary data. From the perspective of DHCP clients and 218 servers, the DHC resource record consists of a 16-bit identifier 219 type, followed by one or more bytes representing the actual 220 identifier. There are two possible forms for a DHCID RR - one that 221 is used when the client's link-layer address is being used to 222 identify it, and one that is used when some DHCP option that the 223 DHCP client has sent is being used to identify it. 225 DISCUSSION: 226 Implementors should note that the actual identifying data is 227 never placed into the DNS directly. Instead, the client-identity 228 data is used as the input into a one-way hash algorithm, and the 229 output of that hash is then used as DNS RRDATA. This has been 230 specified in order to avoid placing data about DHCP clients that 231 some sites might consider sensitive into the DNS. 233 When the updater is using the client's link-layer address, the first 234 two bytes of the DHCID RRDATA MUST be zero. To generate the rest of 235 the resource record, the updater MUST compute a one-way hash using 236 the MD5[12] algorithm across a buffer containing the client's 237 network hardware type and link-layer address. Specifically, the 238 first byte of the buffer contains the network hardware type as it 239 appears in the DHCP htype field of the client's DHCPREQUEST message. 240 All of the significant bytes of the chaddr field in the client's 241 DHCPREQUEST message follow, in the same order in which the bytes 242 appear in the DHCPREQUEST message. The number of significant bytes 243 in the chaddr field is specified in the hlen field of the 244 DHCPREQUEST message. 246 When the updater is using a DHCP option sent by the client in its 247 DHCPREQUEST message, the first two bytes of the DHCID RR MUST be the 248 option code of that option, in network byte order. For example, if 249 the DHCP client identifier option is being used, the first byte of 250 the DHCID RR should be zero, and the second byte should be 61 251 decimal. The rest of the DHCID RR MUST contain the results of 252 computing a one-way hash across the payload of the option being 253 used, using the MD5 algorithm. The payload of a DHCP option consists 254 of the bytes of the option following the option code and length. 256 The two byte identifier code 0xffff is reserved for future 257 assignment. 259 In order for independent DHCP implementations to be able to use the 260 DHCID RR as a prerequisite in dynamic DNS updates, each updater must 261 be able to reliably choose the same identifier that any other would 262 choose. To make this possible, we specify a prioritization which 263 will ensure that for any given DHCP client request, any updater will 264 select the same client-identity data. All updaters MUST use this 265 order of prioritization by default, but all implementations SHOULD 266 be configurable to use a different prioritization if so desired by 267 the site administrators. Because of the possibility of future 268 changes in the DHCP protocol, implementors SHOULD check for updated 269 versions of this draft when implementing new DHCP clients and 270 servers which can perform DDNS updates, and also when releasing new 271 versions of existing clients and servers. 273 DHCP clients and servers should use the following forms of client 274 identification, starting with the most preferable, and finishing 275 with the least preferable. If the client does not send any of these 276 forms of identification, the DHCP/DDNS interaction is not defined by 277 this specification. The most preferable form of identification is 278 the Globally Unique Identifier Option [TBD]. Next is the DHCP 279 Client Identifier option. Last is the client's link-layer address, 280 as conveyed in its DHCPREQUEST message. Implementors should note 281 that the link-layer address cannot be used if there are no 282 significant bytes in the chaddr field of the DHCP client's request, 283 because this does not constitute a unique identifier. 285 3.4 DNS RR TTLs 287 RRs associated with DHCP clients may be more volatile than 288 statically configured RRs. DHCP clients and servers which perform 289 dynamic updates should attempt to specify resource record TTLs which 290 reflect this volatility, in order to minimize the possibility that 291 there will be stale records in resolvers' caches. A reasonable basis 292 for RR TTLs is the lease duration itself: TTLs of 1/2 or 1/3 the 293 expected lease duration might be reasonable defaults. Because 294 configured DHCP lease times vary widely from site to site, it may 295 also be desirable to establish a fixed TTL ceiling. DHCP clients and 296 servers MAY allow administrators to configure the TTLs they will 297 supply, possibly as a fraction of the actual lease time, or as a 298 fixed value. 300 4. Procedures for performing DNS updates 302 4.1 Adding A RRs to DNS 304 When a DHCP client or server intends to update an A RR, it first 305 prepares a DNS UPDATE query which includes as a prerequisite the 306 assertion that the name does not exist. The update section of the 307 query attempts to add the new name and its IP address mapping (an A 308 RR), and the DHCID RR with its unique client-identity. 310 If this update operation succeeds, the updater can conclude that it 311 has added a new name whose only RRs are the A and DHCID RR records. 312 The A RR update is now complete (and a client updater is finished, 313 while a server might proceed to perform a PTR RR update). 315 If the first update operation fails with YXDOMAIN, the updater can 316 conclude that the intended name is in use. The updater then 317 attempts to confirm that the DNS name is not being used by some 318 other host. The updater prepares a second UPDATE query in which the 319 prerequisite is that the desired name has attached to it a DHCID RR 320 whose contents match the client identity. The update section of 321 this query deletes the existing A records on the name, and adds the 322 A record that matches the DHCP binding and the DHCID RR with the 323 client identity. 325 If this query succeeds, the updater can conclude that the current 326 client was the last client associated with the domain name, and that 327 the name now contains the updated A RR. The A RR update is now 328 complete (and a client updater is finished, while a server would 329 then proceed to perform a PTR RR update). 331 If the second query fails with NXRRSET, the updater must conclude 332 that the client's desired name is in use by another host. At this 333 juncture, the updater can decide (based on some administrative 334 configuration outside of the scope of this document) whether to let 335 the existing owner of the name keep that name, and to (possibly) 336 perform some name disambiguation operation on behalf of the current 337 client, or to replace the RRs on the name with RRs that represent 338 the current client. If the configured policy allows replacement of 339 existing records, the updater submits a query that deletes the 340 existing A RR and the existing DHCID RR, adding A and DHCID RRs that 341 represent the IP address and client-identity of the new client. 343 DISCUSSION: 344 The updating entity may be configured to allow the existing DNS 345 records on the domain name to remain unchanged, and to perform 346 disambiguation on the name of the current client in order to 347 attempt to generate a similar but unique name for the current 348 client. In this case, once another candidate name has been 349 generated, the updater should restart the process of adding an A 350 RR as specified in this section. 352 4.2 Adding PTR RR Entries to DNS 354 The DHCP server submits a DNS query which deletes all of the PTR RRs 355 associated with the lease IP address, and adds a PTR RR whose data 356 is the client's (possibly disambiguated) host name. The server also 357 adds a DHCID RR specified in Section 3.3. 359 4.3 Removing Entries from DNS 361 The most important consideration in removing DNS entries is be sure 362 that an entity removing a DNS entry is only removing an entry that 363 it added, or for which an administrator has explicitly assigned it 364 responsibility. 366 When a lease expires or a DHCP client issues a DHCPRELEASE request, 367 the DHCP server SHOULD delete the PTR RR that matches the DHCP 368 binding, if one was successfully added. The server's update query 369 SHOULD assert that the name in the PTR record matches the name of 370 the client whose lease has expired or been released. 372 The entity chosen to handle the A record for this client (either the 373 client or the server) SHOULD delete the A record that was added when 374 the lease was made to the client. 376 In order to perform this delete, the updater prepares an UPDATE 377 query which contains two prerequisites. The first prerequisite 378 asserts that the DHCID RR exists whose data is the client identity 379 described in Section 3.3. The second prerequisite asserts that the 380 data in the A RR contains the IP address of the lease that has 381 expired or been released. 383 If the query fails, the updater MUST NOT delete the DNS name. It 384 may be that the host whose lease on the server has expired has moved 385 to another network and obtained a lease from a different server, 386 which has caused the client's A RR to be replaced. It may also be 387 that some other client has been configured with a name that matches 388 the name of the DHCP client, and the policy was that the last client 389 to specify the name would get the name. In this case, the DHCID RR 390 will no longer match the updater's notion of the client-identity of 391 the host pointed to by the DNS name. 393 4.4 Updating other RRs 395 The procedures described in this document only cover updates to the 396 A and PTR RRs. Updating other types of RRs is outside the scope of 397 this document. 399 5. Security Considerations 401 Unauthenticated updates to the DNS can lead to tremendous confusion, 402 through malicious attack or through inadvertent misconfiguration. 403 Administrators should be wary of permitting unsecured DNS updates to 404 zones which are exposed to the global Internet. Both DHCP clients 405 and servers SHOULD use some form of update request origin 406 authentication procedure (e.g., Secure DNS Dynamic Update[10]) when 407 performing DNS updates. 409 Whether a DHCP client may be responsible for updating an FQDN to IP 410 address mapping, or whether this is the responsibility of the DHCP 411 server is a site-local matter. The choice between the two 412 alternatives may be based on the security model that is used with 413 the Dynamic DNS Update protocol (e.g., only a client may have 414 sufficient credentials to perform updates to the FQDN to IP address 415 mapping for its FQDN). 417 Whether a DHCP server is always responsible for updating the FQDN to 418 IP address mapping (in addition to updating the IP to FQDN mapping), 419 regardless of the wishes of an individual DHCP client, is also a 420 site-local matter. The choice between the two alternatives may be 421 based on the security model that is being used with dynamic DNS 422 updates. In cases where a DHCP server is performing DNS updates on 423 behalf of a client, the DHCP server should be sure of the DNS name 424 to use for the client, and of the identity of the client. 426 Currently, it is difficult for DHCP servers to develop much 427 confidence in the identities of its clients, given the absence of 428 entity authentication from the DHCP protocol itself. There are many 429 ways for a DHCP server to develop a DNS name to use for a client, 430 but only in certain relatively unusual circumstances will the DHCP 431 server know for certain the identity of the client. If DHCP 432 Authentication[9] becomes widely deployed this may become more 433 customary. 435 One example of a situation which offers some extra assurances is one 436 where the DHCP client is connected to a network through an MCNS 437 cable modem, and the CMTS (head-end) of the cable modem ensures that 438 MAC address spoofing simply does not occur. Another example of a 439 configuration that might be trusted is one where clients obtain 440 network access via a network access server using PPP. The NAS itself 441 might be obtaining IP addresses via DHCP, encoding a client 442 identification into the DHCP client-id option. In this case, the 443 network access server as well as the DHCP server might be operating 444 within a trusted environment, in which case the DHCP server could be 445 configured to trust that the user authentication and authorization 446 procedure of the remote access server was sufficient, and would 447 therefore trust the client identification encoded within the DHCP 448 client-id. 450 6. Acknowledgements 452 Many thanks to Mark Beyer, Jim Bound, Ralph Droms, Robert Elz, Peter 453 Ford, Edie Gunter, Andreas Gustafsson, R. Barr Hibbs, Kim Kinnear, 454 Stuart Kwan, Ted Lemon, Ed Lewis, Michael Lewis, Josh Littlefield, 455 Michael Patton, and Glenn Stump for their review and comments. 457 References 459 [1] Mockapetris, P., "Domain names - Concepts and Facilities", RFC 460 1034, Nov 1987. 462 [2] Mockapetris, P., "Domain names - Implementation and 463 Specification", RFC 1035, Nov 1987. 465 [3] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, 466 March 1997. 468 [4] Marine, A., Reynolds, J. and G. Malkin, "FYI on Questions and 469 Answers to Commonly asked ``New Internet User'' Questions", 470 RFC 1594, March 1994. 472 [5] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic 473 Updates in the Domain Name System", RFC 2136, April 1997. 475 [6] Bradner, S., "Key words for use in RFCs to Indicate 476 Requirement Levels", RFC 2119, March 1997. 478 [7] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671, 479 August 1999. 481 [8] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington, 482 "Secret Key Transaction Authentication for DNS (TSIG)", RFC 483 2845, May 2000. 485 [9] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages 486 (draft-ietf-dhc-authentication-*)", June 1999. 488 [10] Wellington, B., "Secure DNS Dynamic Update", RFC 3007, 489 November 2000. 491 [11] Stapp, M., Gustafsson, A. and T. Lemon, "A DNS RR for Encoding 492 DHCP Information (draft-ietf-dnsext-dhcid-rr-*)", November 493 2000. 495 [12] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, 496 April 1992. 498 [13] Stapp, M. and Y. Rekhter, "The DHCP Client FQDN Option 499 (draft-ietf-dhc-fqdn-option-*.txt)", July 2000. 501 Author's Address 503 Mark Stapp 504 Cisco Systems, Inc. 505 250 Apollo Dr. 506 Chelmsford, MA 01824 507 USA 509 Phone: 978.244.8498 510 EMail: mjs@cisco.com 512 Full Copyright Statement 514 Copyright (C) The Internet Society (2001). All Rights Reserved. 516 This document and translations of it may be copied and furnished to 517 others, and derivative works that comment on or otherwise explain it 518 or assist in its implementation may be prepared, copied, published 519 and distributed, in whole or in part, without restriction of any 520 kind, provided that the above copyright notice and this paragraph 521 are included on all such copies and derivative works. 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