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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 INTERNET-DRAFT Donald Eastlake 2 Intended Status: Proposed Standard Huawei 3 Mark Andrews 4 ISC 5 Expires: January 31, 2016 August 1, 2015 7 Domain Name System (DNS) Cookies 8 10 Abstract 12 DNS cookies are a lightweight DNS transaction security mechanism that 13 provides limited protection to DNS servers and clients against a 14 variety of increasingly common denial-of-service and amplification / 15 forgery or cache poisoning attacks by off-path attackers. DNS Cookies 16 are tolerant of NAT, NAT-PT, and anycast and can be incrementally 17 deployed. 19 Status of This Document 21 This Internet-Draft is submitted to IETF in full conformance with the 22 provisions of BCP 78 and BCP 79. 24 Distribution of this document is unlimited. Comments should be sent 25 to the author or the DNSEXT mailing list . 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF), its areas, and its working groups. Note that 29 other groups may also distribute working documents as Internet- 30 Drafts. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 The list of current Internet-Drafts can be accessed at 38 http://www.ietf.org/1id-abstracts.html. The list of Internet-Draft 39 Shadow Directories can be accessed at 40 http://www.ietf.org/shadow.html. 42 Table of Contents 44 1. Introduction............................................4 45 1.1 Contents of This Document..............................4 46 1.2 Definitions............................................5 48 2. Threats Considered......................................6 49 2.1 Denial-of-Service Attacks..............................6 50 2.1.1 DNS Amplification Attacks............................6 51 2.1.2 DNS Server Denial-of-Service.........................6 52 2.2 Cache Poisoning and Answer Forgery Attacks.............7 54 3. Comments on Existing DNS Security.......................8 55 3.1 Existing DNS Data Security.............................8 56 3.2 DNS Message/Transaction Security.......................8 57 3.3 Conclusions on Existing DNS Security...................8 59 4. DNS Cookie Option......................................10 60 4.1 Client Cookie.........................................11 61 4.2 Server Cookie.........................................11 63 5. DNS Cookies Protocol Specification.....................12 64 5.1 Originating Requests..................................12 65 5.2 Responding to Request.................................12 66 5.2.1 No Opt RR or No COOKIE OPT option...................13 67 5.2.2 Malformed COOKIE OPT option.........................13 68 5.2.3 Only a Client Cookie................................13 69 5.2.4 A Client Cookie and Server Cookie...................14 70 5.2.4.1 A Client Cookie and Invalid Server Cookie.........14 71 5.2.4.2 A Client Cookie and Valid Server Cookie...........14 72 5.3 Processing Responses..................................14 73 5.4 QUERYing for a Server Cookie..........................15 74 5.5 Client and Server Secret Rollover.....................16 76 6. NAT Considerations and AnyCast Server Considerations...17 77 7. Deployment.............................................19 79 8. IANA Considerations....................................20 81 9. Security Considerations................................21 82 9.1 Cookie Algorithm Considerations.......................21 84 10. Implementation Considerations.........................23 86 Normative References......................................24 87 Informative References....................................24 88 Acknowledgements..........................................26 90 Appendix A: Example Client Cookie Algorithms..............27 91 A.1 A Simple Algorithm....................................27 92 A.2 A More Complex Algorithm..............................27 94 Table of Contents (continued) 96 Appendix B: Example Server Cookie Algorithms..............28 97 B.1 A Simple Algorithm....................................28 98 B.2 A More Complex Algorithm..............................28 100 Author's Address..........................................30 102 1. Introduction 104 As with many core Internet protocols, the Domain Name System (DNS) 105 was originally designed at a time when the Internet had only a small 106 pool of trusted users. As the Internet has grown exponentially to a 107 global information utility, the DNS has increasingly been subject to 108 abuse. 110 This document describes DNS cookies, a lightweight DNS transaction 111 security mechanism specified as an OPT [RFC6891] option. The DNS 112 cookies mechanism provides limited protection to DNS servers and 113 clients against a variety of increasingly common abuses by off-path 114 attackers. It is compatible with and can be used in conjunction with 115 other DNS transaction forgery resistance measures such as those in 116 [RFC5452]. 118 The protection provided by DNS cookies is similar to that provided by 119 using TCP for DNS transactions. To bypass the weak protection 120 provided by using TCP requires, among other things, that an off-path 121 attacker guessing the 32-bit TCP sequence number in use. To bypass 122 the weak protection provided by DNS Cookies requires such an attacker 123 to guess a 64-bit pseudo-random "cookie" quantity. Where DNS Cookies 124 are not available but TCP is, falling back to using TCP is 125 reasonable. 127 If only one party to a DNS transaction supports DNS cookies, the 128 mechanism does not provide a benefit or significantly interfere; but, 129 if both support it, the additional security provided is automatically 130 available. 132 The DNS cookies mechanism is designed to work in the presence of NAT 133 and NAT-PT boxes and guidance is provided herein on supporting the 134 DNS cookies mechanism in anycast servers. 136 1.1 Contents of This Document 138 In Section 2, we discuss the threats against which the DNS cookie 139 mechanism provides some protection. 141 Section 3 describes existing DNS security mechanisms and why they are 142 not adequate substitutes for DNS cookies. 144 Section 4 describes the COOKIE OPT option. 146 Section 5 provides a protocol description. 148 Section 6 discusses some NAT and anycast related DNS Cookies design 149 considerations. 151 Section 7 discusses incremental deployment considerations. 153 Sections 8 and 9 describe IANA and Security Considerations. 155 1.2 Definitions 157 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 158 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 159 document are to be interpreted as described in [RFC2119]. 161 "Off-path attacker", for a particular DNS client and server, is 162 defined as an attacker who cannot observe the DNS request and 163 response messages between that client and server. 165 "Soft state" indicates information learned or derived by a host which 166 may be discarded when indicated by the policies of that host 167 but can be later re-instantiated if needed. For example, it 168 could be discarded after a period of time or when storage for 169 caching such data becomes full. If operations requiring that 170 soft state continue after it has been discarded, it will be 171 automatically re-generated, albeit at some cost. 173 "Silently discarded" indicates that there are no DNS protocol message 174 consequences; however, it is RECOMMENDED that appropriate 175 network management facilities be included in implementations, 176 such as a counter of the occurrences of each such event type. 178 "IP address" is used herein as a length independent term and includes 179 both IPv4 and IPv6 addresses. 181 2. Threats Considered 183 DNS cookies are intended to provide significant but limited 184 protection against certain attacks by off-path attackers as described 185 below. These attacks include denial-of-service, cache poisoning and 186 answer forgery. 188 2.1 Denial-of-Service Attacks 190 The typical form of the denial-of-service attacks considered herein 191 is to send DNS requests with forged source IP addresses to a server. 192 The intent can be to attack that server or some other selected host 193 as described below. 195 2.1.1 DNS Amplification Attacks 197 A request with a forged IP address generally causes a response to be 198 sent to that forged IP address. Thus the forging of many such 199 requests with a particular source IP address can result in enough 200 traffic being sent to the forged IP address to interfere with service 201 to the host at the IP address. Furthermore, it is generally easy in 202 the DNS to create short requests that produce much longer responses, 203 thus amplifying the attack. 205 The DNS Cookies mechanism can severely limit the traffic 206 amplification obtained by attackers off path for the server and the 207 attacked host. Enforced DNS cookies would make it hard for an off 208 path attacker to cause any more than rate-limited short error 209 responses to be sent to a forged IP address so the attack would be 210 attenuated rather than amplified. DNS cookies make it more effective 211 to implement a rate limiting scheme for error responses from the 212 server. Such a scheme would further restrict selected host denial- 213 of-service traffic from that server. 215 2.1.2 DNS Server Denial-of-Service 217 DNS requests that are accepted cause work on the part of DNS servers. 218 This is particularly true for recursive servers that may issue one or 219 more requests and process the responses thereto, in order to 220 determine their response to the initial request. And the situation 221 can be even worse for recursive servers implementing DNSSEC 222 ([RFC4033] [RFC4034] [RFC4035]) because they may be induced to 223 perform burdensome cryptographic computations in attempts to verify 224 the authenticity of data they retrieve in trying to answer the 225 request. 227 The computational or communications burden caused by such requests 228 may not depend on a forged IP source address, but the use of such 229 addresses makes 230 + the source of the requests causing the denial-of-service attack 231 harder to find and 232 + restriction of the IP addresses from which such requests should 233 be honored hard or impossible to specify or verify. 235 Use of DNS cookies should enable a server to reject forged requests 236 from an off path attacker with relative ease and before any recursive 237 queries or public key cryptographic operations are performed. 239 2.2 Cache Poisoning and Answer Forgery Attacks 241 The form of the cache poisoning attacks considered is to send forged 242 replies to a resolver. Modern network speeds for well-connected hosts 243 are such that, by forging replies from the IP addresses of a DNS 244 server to a resolver for names that resolver has been induced to 245 resolve or for common names whose resource records have short time- 246 to-live values, there can be an unacceptably high probability of 247 randomly coming up with a reply that will be accepted and cause false 248 DNS information to be cached by that resolver (the Dan Kaminsky 249 attack [Kaminsky]). This can be used to facilitate phishing attacks 250 and other diversion of legitimate traffic to a compromised or 251 malicious host such as a web server. 253 With the use of DNS cookies, a resolver can generally reject such 254 forged replies. 256 3. Comments on Existing DNS Security 258 Two forms of security have been added to DNS, data security and 259 message/transaction security. 261 3.1 Existing DNS Data Security 263 DNS data security is one part of DNSSEC and is described in 264 [RFC4033], [RFC4034], [RFC4035], and updates thereto. It provides 265 data origin authentication and authenticated denial of existence. 266 DNSSEC is being deployed and can provide strong protection against 267 forged data; however, it has the unintended effect of making some 268 denial-of-service attacks worse because of the cryptographic 269 computational load it can require and the increased size in DNS 270 response packets that it tends to produce. 272 3.2 DNS Message/Transaction Security 274 The second form of security that has been added to DNS provides 275 "transaction" security through TSIG [RFC2845] or SIG(0) [RFC2931]. 276 TSIG could provide strong protection against the attacks for which 277 the DNS Cookies mechanism provides weak protection; however, TSIG is 278 non-trivial to deploy in the general Internet because of the burdens 279 it imposes. Among these burdens are pre-agreement and key 280 distribution between client and server, keeping track of server side 281 key state, and required time synchronization between client and 282 server. 284 TKEY [RFC2930] can solve the problem of key distribution for TSIG but 285 some modes of TKEY impose a substantial cryptographic computation 286 load and can be dependent on the deployment of DNS data security (see 287 Section 3.1). 289 SIG(0) [RFC2931] provides less denial of service protection than TSIG 290 or, in one way, even DNS cookies, because it does not authenticate 291 requests, only complete transactions. In any case, it also depends 292 on the deployment of DNS data security and requires computationally 293 burdensome public key cryptographic operations. 295 3.3 Conclusions on Existing DNS Security 297 The existing DNS security mechanisms do not provide the services 298 provided by the DNS Cookies mechanism: lightweight message 299 authentication of DNS requests and responses with no requirement for 300 pre-configuration or per client server side state. 302 4. DNS Cookie Option 304 The DNS Cookie Option is an OPT RR [RFC6891] option that can be 305 included in the RDATA portion of an OPT RR in DNS requests and 306 responses. The option length varies depending on the circumstances 307 in which it is being used. There are two cases as described below. 308 Both use the same OPTION-CODE; they are distinguished by their 309 length. 311 In a request sent by a client to a server when the client does not 312 know the server's cookie, its length is 8, consisting of an 8 byte 313 Client Cookie as shown in Figure 1. 315 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 316 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 317 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 318 | OPTION-CODE = 10 | OPTION-LENGTH = 8 | 319 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 320 | | 321 +-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+ 322 | | 323 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 325 Figure 1. COOKIE Option, Unknown Server Cookie 327 In a request sent by a client when a server cookie is known and in 328 all responses, the length is variable from 16 to 40 bytes, consisting 329 of an 8 bytes Client Cookie followed by the variable 8 to 32 bytes 330 Server Cookie as shown in Figure 2. The variability of the option 331 length stems from the variable length Server Cookie. The Server 332 Cookie is an integer number of bytes with a minimum size of 8 bytes 333 for security and a maximum size of 32 bytes for implementation 334 convenience. 336 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 337 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 338 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 339 | OPTION-CODE = 10 | OPTION-LENGTH >= 16, <= 40 | 340 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 341 | | 342 +-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+ 343 | | 344 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 345 | | 346 / Server Cookie (variable size, 8 to 32 bytes) / 347 / / 348 +-+-+-+-... 350 Figure 2. COOKIE Option, Known Server Cookie 352 4.1 Client Cookie 354 The Client Cookie SHOULD be a pseudo-random function of the server IP 355 address and a secret quantity known only to the client. This client 356 secret SHOULD have at least 64 bits of entropy [RFC4086] and be 357 changed periodically (see Section 5.5). The selection of the pseudo- 358 random function is a matter private to the client as only the client 359 needs to recognize its own DNS cookies. 361 For further discussion of the Client Cookie field, see Section 5.1. 362 For example methods of determining a Client Cookie, see Appendix A. 364 In order to maintain the security properties of this protocol, a 365 client MUST NOT use the same Client Cookie value for requests to all 366 servers. 368 4.2 Server Cookie 370 The Server Cookie SHOULD consist of or include a 64-bit or larger 371 pseudo-random function of the request source IP address, the request 372 Client Cookie, and a secret quantity known only to the server. (See 373 Section 6 for a discussion of why the Client Cookie is used as input 374 to the Server Cookie but the Server Cookie is not used as an input to 375 the Client Cookie.) This server secret SHOULD have at least 64 bits 376 of entropy [RFC4086] and be changed periodically (see Section 5.5). 377 The selection of the pseudo-random function is a matter private to 378 the server as only the server needs to recognize its own DNS cookies. 380 For further discussion of the Server Cookie field see Section 5.2. 381 For example methods of determining a Server Cookie, see Appendix B. 383 In order to maintain the security properties of this protocol, a 384 server MUST NOT use the same Server Cookie value for responses to all 385 clients. 387 5. DNS Cookies Protocol Specification 389 This section discusses using DNS Cookies in the DNS Protocol. The 390 cycle of originating a request, responding to that request, and 391 processing the response are covered in Sections 5.1, 5.2, and 5.3. A 392 de facto extension to QUERY to allow pre-fetching a Server Cookie is 393 specified in Section 5.4. Rollover of the client and server secrets 394 and transient retention of the old cookie or secret is covered in 395 Section 5.5. 397 DNS clients and servers SHOULD implement DNS cookies to decrease 398 their vulnerability to the threats discussed in Section 2. 400 5.1 Originating Requests 402 A DNS client that implements DNS Cookies includes one DNS COOKIE OPT 403 option containing a Client Cookie in every DNS request it sends 404 unless DNS cookies are disabled. 406 If the client has a cached Server Cookie for the server against its 407 IP address it uses the longer cookie form and includes that Server 408 Cookie in the option along with the Client Cookie (Figure 2). 409 Otherwise it just sends the shorter form option with a Client Cookie 410 (Figure 1). 412 5.2 Responding to Request 414 The Server Cookie, when it occurs in a COOKIE OPT option in a 415 request, is intended to weakly assure the server that the request 416 came from a client that is both at the source IP address of the 417 request and using the Client Cookie included in the option. This weak 418 assurance is provided by the Server Cookie that server would send to 419 that client in an earlier response appearing as the Server Cookie 420 field in the request. 422 At a server where DNS Cookies are not implemented and enabled, 423 presence of a COOKIE OPT option is ignored and the server responds as 424 if no COOKIE OPT option had been included in the request. 426 When DNS Cookies are implemented and enabled, there are four 427 possibilities: (1) there is no OPT RR at all in the request; (2) 428 there is no valid Client Cookie in the request because the COOKIE OPT 429 option is absent from the request, or one is present but is not a 430 legal length; (3) there is a valid length cookie option in the 431 request with no Server Cookie or an incorrect Server Cookie; or (4) 432 there is a cookie option in the request with a correct Server Cookie. 434 The four possibilities are discussed in the subsections below. 436 In all cases of multiple COOKIE OPT options in a request, only the 437 first (the one closest to the DNS header) is considered. All others 438 are ignored. 440 5.2.1 No Opt RR or No COOKIE OPT option 442 If there is no OPT record or no COOKIE OPT option present in the 443 request then the server responds to the request as if the server 444 doesn't implement the COOKIE OPT. 446 5.2.2 Malformed COOKIE OPT option 448 If the COOKIE OPT is too short to contain a Client Cookie then 449 FORMERR is generated. If the COOKIE OPT is longer than that required 450 to hold a COOKIE OPT with just a Client Cookie (8) but is shorter 451 that the minimum COOKIE OPT with both a Client and Server Cookie (16) 452 then FORMERR is generated. If the COOKIE OPT is longer than the 453 maximum valid COOKIE OPT (40) then a FORMERR is generated. 455 In summary, valid cookie lengths are 8 and 16 to 40 inclusive. 457 5.2.3 Only a Client Cookie 459 Based on server policy, including rate limiting, the server chooses 460 one of the following: 462 (1) Silently discard the request. 464 (2) Send a BADCOOKIE error response. 466 (3) Process the request and provide a normal response. The RCODE is 467 NOERROR unless some non-cookie error occurs in processing the 468 request. 470 If the server responds, choosing 2 or 3 above, it SHALL generate its 471 own COOKIE OPT containing both the Client Cookie copied from the 472 request and a Server Cookie it has generated and adds this COOKIE OPT 473 to the response's OPT record. Servers MUST, at least occasionally, 474 respond to such requests to inform the client of the correct Server 475 Cookie. This is necessary so that such a client can bootstrap to the 476 weakly secure state where requests and responses have recognized 477 Server Cookies and Client Cookies. 479 If the request was received over TCP, the server SHOULD take the weak 480 authentication provided by the use of TCP into account and SHOULD 481 choose 3. In this case, if the server is not willing to accept the 482 weak security provided by TCP as a substitute for the weak security 483 provided by DNS Cookies but instead chooses 2, there is some danger 484 of an indefinite loop of retries (see Section 5.3). 486 5.2.4 A Client Cookie and Server Cookie 488 The server examines the Server Cookie to determine if it is a valid 489 Server Cookie it has generated. This examination will result in a 490 determination of whether the Server Cookie is valid or not. These 491 cases are discussed below. 493 5.2.4.1 A Client Cookie and Invalid Server Cookie 495 This can occur due to a stale Server Cookie being returned, a 496 client's IP address or Client Cookie changing without the DNS server 497 being aware, an anycast server cluster that is not consistently 498 configured, or an attempt to spoof the client. 500 The server SHALL process the request as if the invalid Server Cookie 501 was not present as described in Section 5.2.3. 503 5.2.4.2 A Client Cookie and Valid Server Cookie 505 When this occurs the server can assume that the request is from a 506 client that it has talked to before and defensive measures for 507 spoofed UDP requests, if any, are no longer required. 509 The server SHALL process the request and include a COOKIE OPT in the 510 response by (a) copying the complete COOKIE OPT from the request or 511 (b) generating a new COOKIE OPT containing both the Client Cookie 512 copied from the request and a valid Server Cookie it has generated. 514 5.3 Processing Responses 516 The Client Cookie, when it occurs in a COOKIE OPT option in a DNS 517 reply, is intended to weakly assure the client that the reply came 518 from a server at the source IP address used in the response packet 519 because the Client Cookie value is the value that client would send 520 to that server in a request. In a DNS reply with multiple COOKIE OPT 521 options, all but the first (the one closest to the DNS Header) are 522 ignored. 524 A DNS client where DNS cookies are implemented and enabled examines 525 the response for DNS cookies and MUST discard the response if it 526 contains an illegal COOKIE OPT option length or an incorrect Client 527 Cookie value. If the COOKIE OPT option Client Cookie is correct, the 528 client caches the Server Cookie provided even if the response is an 529 error response (RCODE non-zero). 531 If the reply extended RCODE is BADCOOKIE and the Client Cookie 532 matches what was sent, it means that the server was unwilling to 533 process the request because it did not have the correct Server Cookie 534 in it. The client SHOULD retry the request using the new Server 535 Cookie from the response. Repeated BADCOOKIE responses to requests 536 that use the Server Cookie provided in the previous response may be 537 an indication that the shared secrets / secret generation method in 538 an anycast cluster of servers are inconsistent. If the reply to a 539 retried request with a fresh Server Cookie is BADCOOKIE, the client 540 SHOULD retry using TCP as the transport since the server will likely 541 process the request normally based on the weak security provided by 542 TCP (see Section 5.2.3). 544 If the RCODE is some value other than BADCOOKIE, including zero, the 545 further processing of the response proceeds normally. 547 5.4 QUERYing for a Server Cookie 549 In many cases a client will learn the Server Cookie for a server as 550 the side effect of another transaction; however, there may be times 551 when this is not desirable. Therefore a means is provided for 552 obtaining a Server Cookie through an extension to the QUERY opcode 553 for which opcode most existing implementations require that QDCOUNT 554 be one (see Section 4.1.2 of [RFC1035]). 556 For servers with DNS Cookies enabled, the QUERY opcode behavior is 557 extended to support queries with a empty question section (QDCOUNT 558 zero) provided that an OPT record is present with a COOKIE option. 559 Such servers will reply with an empty answer section and a COOKIE 560 option giving the Client Cookie provided in the query and a valid 561 Server Cookie. 563 If such a query provided just a Client Cookie and no Server Cookie, 564 the response SHALL have the RCODE NOERROR. 566 This mechanism can also be used to confirm/re-establish a existing 567 Server Cookie by sending a cached Server Cookie with the Client 568 Cookie. In this case the response SHALL have the RCODE BADCOOKIE if 569 the Server Cookie sent with the query was invalid and the RCODE 570 NOERROR if it was valid. 572 Servers which don't support the COOKIE option will normally send 573 FORMERR in response to such a query, though REFUSED, NOTIMP, and 574 NOERROR without a COOKIE option are also possible in such responses. 576 5.5 Client and Server Secret Rollover 578 Clients and servers MUST NOT continue to use the same secret in new 579 requests and responses for more than 36 days and SHOULD NOT continue 580 to do so for more than 26 hours. Many clients rolling over their 581 secret at the same time could briefly increase server traffic and 582 exactly predictable rollover times for clients or servers might 583 facilitate guessing attacks. For example, an attacker might increase 584 the priority of attacking secrets they believe will be in effect for 585 an extended period of time. To avoid rollover synchronization and 586 predictability, it is RECOMMENDED that pseudorandom jitter in the 587 range of plus zero to minus at least 40% be applied to the time until 588 a scheduled rollover of a DNS cookie secret. 590 It is RECOMMENDED that a client keep the Client Cookie it is 591 expecting in a reply associated with the outstanding request to avoid 592 rejection of replies due to a bad Client Cookie right after a change 593 in the client secret. It is RECOMMENDED that a server retain its 594 previous secret for a period of time not less than 1 second or more 595 than 5 minutes, after a change in its secret, and consider requests 596 with Server Cookies based on its previous secret to have a correct 597 Server Cookie during that time. 599 When a server or client starts receiving an increased level of 600 requests with bad server cookies or replies with bad client cookies, 601 it would be reasonable for it to believe it is likely under attack 602 and it should consider a more frequent rollover of its secret. 604 6. NAT Considerations and AnyCast Server Considerations 606 In the Classic Internet, DNS Cookies could simply be a pseudo-random 607 function of the client IP address and a server secret or the server 608 IP address and a client secret. You would want to compute the Server 609 Cookie that way, so a client could cache its Server Cookie for a 610 particular server for an indefinitely amount of time and the server 611 could easily regenerate and check it. You could consider the Client 612 Cookie to be a weak client signature over the server IP address that 613 the client checks in replies and you could extend this weak signature 614 to cover the request ID, for example, or any other information that 615 is returned unchanged in the reply. 617 But we have this reality called NAT [RFC3022], Network Address 618 Translation (including, for the purposes of this document, NAT-PT, 619 Network Address and Protocol Translation, which has been declared 620 Historic [RFC4966]). There is no problem with DNS transactions 621 between clients and servers behind a NAT box using local IP 622 addresses. Nor is there a problem with NAT translation of internal 623 addresses to external addresses or translations between IPv4 and IPv6 624 addresses, as long as the address mapping is relatively stable. 625 Should the external IP address an internal client is being mapped to 626 change occasionally, the disruption is little more than when a client 627 rolls-over its DNS COOKIE secret. And normally external access to a 628 DNS server behind a NAT box is handled by a fixed mapping which 629 forwards externally received DNS requests to a specific host. 631 However, NAT devices sometimes also map ports. This can cause 632 multiple DNS requests and responses from multiple internal hosts to 633 be mapped to a smaller number of external IP addresses, such as one 634 address. Thus there could be many clients behind a NAT box that 635 appear to come from the same source IP address to a server outside 636 that NAT box. If one of these were an attacker (think Zombie or 637 Botnet), that behind-NAT attacker could get the Server Cookie for 638 some server for the outgoing IP address by just making some random 639 request to that server. It could then include that Server Cookie in 640 the COOKIE OPT of requests to the server with the forged local IP 641 address of some other host and/or client behind the NAT box. 642 (Attacker possession of this Server Cookie will not help in forging 643 responses to cause cache poisoning as such responses are protected by 644 the required Client Cookie.) 646 To fix this potential defect, it is necessary to distinguish 647 different clients behind a NAT box from the point of view of the 648 server. It is for this reason that the Server Cookie is specified as 649 a pseudo-random function of both the request source IP address and 650 the Client Cookie. From this inclusion of the Client Cookie in the 651 calculation of the Server Cookie, it follows that a stable Client 652 Cookie, for any particular server, is needed. If, for example, the 653 request ID was included in the calculation of the Client Cookie, it 654 would normally change with each request to a particular server. This 655 would mean that each request would have to be sent twice: first to 656 learn the new Server Cookie based on this new Client Cookie based on 657 the new ID and then again using this new Client Cookie to actually 658 get an answer. Thus the input to the Client Cookie computation must 659 be limited to the server IP address and one or more things that 660 change slowly such as the client secret. 662 In principle, there could be a similar problem for servers, not due 663 to NAT but due to mechanisms like anycast which may cause requests to 664 a DNS server at an IP address to be delivered to any one of several 665 machines. (External requests to a DNS server behind a NAT box usually 666 occur via port forwarding such that all such requests go to one 667 host.) However, it is impossible to solve this the way the similar 668 problem was solved for NATed clients; if the Server Cookie was 669 included in the calculation of the Client Cookie the same way the 670 Client Cookie is included in the Server Cookie, you would just get an 671 almost infinite series of errors as a request was repeatedly retried. 673 For servers accessed via anycast to successfully support DNS COOKIES, 674 the server clones must either all use the same server secret or the 675 mechanism that distributes requests to them must cause the requests 676 from a particular client to go to a particular server for a 677 sufficiently long period of time that extra requests due to changes 678 in Server Cookie resulting from accessing different server machines 679 are not unduly burdensome. (When such anycast-accessed servers act 680 as recursive servers or otherwise act as clients they normally use a 681 different unique address to source their requests to avoid confusion 682 in the delivery of responses.) 684 For simplicity, it is RECOMMENDED that the same server secret be used 685 by each DNS server in a set of anycast servers. If there is limited 686 time skew in updating this secret in different anycast servers, this 687 can be handled by a server accepting requests containing a Server 688 Cookie based on either its old or new secret for the maximum likely 689 time period of such time skew (see also Section 5.5). 691 7. Deployment 693 The DNS cookies mechanism is designed for incremental deployment and 694 to complement the orthogonal techniques in [RFC5452]. Either or both 695 techniques can be deployed independently at each DNS server and 696 client. 698 In particular, a DNS server or client that implements the DNS COOKIE 699 mechanism can interoperate successfully with a DNS client or server 700 that does not implement this mechanism although, of course, in this 701 case it will not get the benefit of the mechanism and the server 702 involved might choose to severely rate limit responses. When such a 703 server or client interoperates with a client or server which also 704 implements the DNS cookies mechanism, they get the weak security 705 benefits of the DNS Cookies mechanism. 707 8. IANA Considerations 709 IANA has assigned the following OPT option value: 711 Value Name Status Reference 712 -------- ------ -------- --------------- 713 10 COOKIE Standard [this document] 715 IANA has assigned the following DNS error code as an early 716 allocation: 718 RCODE Name Description Reference 719 -------- --------- ------------------------- --------------- 720 23 BADCOOKIE Bad/missing server cookie [this document] 722 9. Security Considerations 724 DNS Cookies provide a weak form of authentication of DNS requests and 725 responses. In particular, they provide no protection against "on- 726 path" adversaries; that is, they provide no protection against any 727 adversary that can observe the plain text DNS traffic, such as an on- 728 path router, bridge, or any device on an on-path shared link (unless 729 the DNS traffic in question on that path is encrypted). 731 For example, if a host is connected via an unsecured IEEE Std 802.11 732 link (Wi-Fi), any device in the vicinity that could receive and 733 decode the 802.11 transmissions must be considered "on-path". On the 734 other hand, in a similar situation but one where 802.11 Robust 735 Security (WPAv2) is appropriately deployed on the Wi-Fi network 736 nodes, only the Access Point via which the host is connecting is "on- 737 path" as far as the 802.11 link is concerned. 739 Despite these limitations, deployment of DNS Cookies on the global 740 Internet is expected to provide a substantial reduction in the 741 available launch points for the traffic amplification and denial of 742 service forgery attacks described in Section 2 above. 744 Should stronger message/transaction security be desired, it is 745 suggested that TSIG or SIG(0) security be used (see Section 3.2); 746 however, it may be useful to use DNS Cookies in conjunction with 747 these features. In particular, DNS Cookies could screen out many DNS 748 messages before the cryptographic computations of TSIG or SIG(0) are 749 required and, if SIG(0) is in use, DNS Cookies could usefully screen 750 out many requests given that SIG(0) does not screen requests but only 751 authenticates the response of complete transactions. 753 9.1 Cookie Algorithm Considerations 755 The cookie computation algorithm for use in DNS Cookies SHOULD be 756 based on a pseudo-random function at least as strong as FNV (Fowler- 757 Noll-Vo [FNV]) because an excessively weak or trivial algorithm could 758 enable adversaries to guess cookies. However, in light of the weak 759 plain-text token security provided by DNS Cookies, a strong 760 cryptography hash algorithm may not be warranted in many cases, and 761 would cause an increased computational burden. Nevertheless there is 762 nothing wrong with using something stronger, for example, HMAC- 763 SHA256-64 [RFC6234], assuming a DNS processor has adequate 764 computational resources available. DNS processors that feel the need 765 for somewhat stronger security without a significant increase in 766 computational load should consider more frequent changes in their 767 client and/or server secret; however, this does require more frequent 768 generation of a cryptographically strong random number [RFC4086]. See 769 Appendices A and B for specific examples of cookie computation 770 algorithms. 772 10. Implementation Considerations 774 The DNS Cookie Option specified herein is implemented in BIND 9.10 775 using a experimental option code. 777 Normative References 779 [RFC1035] - Mockapetris, P., "Domain names - implementation and 780 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 781 November 1987, . 783 [RFC2119] - Bradner, S., "Key words for use in RFCs to Indicate 784 Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, 785 March 1997, . 787 [RFC4086] - Eastlake 3rd, D., Schiller, J., and S. Crocker, 788 "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 789 10.17487/RFC4086, June 2005, . 792 [RFC6891] - Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms 793 for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/RFC6891, 794 April 2013, . 796 Informative References 798 [FNV] - G. Fowler, L. C. Noll, K.-P. Vo, D. Eastlake, "The FNV Non- 799 Cryptographic Hash Algorithm", draft-eastlake-fnv, work in 800 progress. 802 [Kaminsky] - Olney, M., P. Mullen, K. Miklavicic, "Dan Kaminsky's 803 2008 DNS Vulnerability", 25 July 2008, 804 . 807 [RFC2845] - Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B. 808 Wellington, "Secret Key Transaction Authentication for DNS 809 (TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000, 810 . 812 [RFC2930] - Eastlake 3rd, D., "Secret Key Establishment for DNS (TKEY 813 RR)", RFC 2930, DOI 10.17487/RFC2930, September 2000, 814 . 816 [RFC2931] - Eastlake 3rd, D., "DNS Request and Transaction Signatures 817 ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September 2000, 818 . 820 [RFC3022] - Srisuresh, P. and K. Egevang, "Traditional IP Network 821 Address Translator (Traditional NAT)", RFC 3022, DOI 822 10.17487/RFC3022, January 2001, . 825 [RFC4033] - Arends, R., Austein, R., Larson, M., Massey, D., and S. 826 Rose, "DNS Security Introduction and Requirements", RFC 4033, 827 DOI 10.17487/RFC4033, March 2005, . 830 [RFC4034] - Arends, R., Austein, R., Larson, M., Massey, D., and S. 831 Rose, "Resource Records for the DNS Security Extensions", RFC 832 4034, DOI 10.17487/RFC4034, March 2005, . 835 [RFC4035] - Arends, R., Austein, R., Larson, M., Massey, D., and S. 836 Rose, "Protocol Modifications for the DNS Security Extensions", 837 RFC 4035, DOI 10.17487/RFC4035, March 2005, . 840 [RFC4966] - Aoun, C. and E. Davies, "Reasons to Move the Network 841 Address Translator - Protocol Translator (NAT-PT) to Historic 842 Status", RFC 4966, DOI 10.17487/RFC4966, July 2007, 843 . 845 [RFC5452] - Hubert, A. and R. van Mook, "Measures for Making DNS More 846 Resilient against Forged Answers", RFC 5452, DOI 847 10.17487/RFC5452, January 2009, . 850 [RFC6234] - Eastlake 3rd, D. and T. Hansen, "US Secure Hash 851 Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI 852 10.17487/RFC6234, May 2011, . 855 Acknowledgements 857 The suggestions and contributions of the following are gratefully 858 acknowledged: 860 Bob Harold, Paul Hoffman, Gayle Noble, Tim Wicinski 862 The document was prepared in raw nroff. All macros used were defined 863 within the source file. 865 Appendix A: Example Client Cookie Algorithms 867 A.1 A Simple Algorithm 869 An simple example method to compute Client Cookies is the FNV-64 870 [FNV] of the server IP address and the client secret. That is 872 Client Cookie = FNV-64 ( Client Secret | Server IP Address ) 874 where "|" indicates concatenation. 876 A.2 A More Complex Algorithm 878 A more complex algorithm to calculate Client Cookies is given below. 879 It uses more computational resources than the simpler algorithm shown 880 in A.1. 882 Client Cookie = HMAC-SHA256-64 ( Client Secret, 883 Server IP Address ) 885 Appendix B: Example Server Cookie Algorithms 887 B.1 A Simple Algorithm 889 An example of a simple method producing a 64-bit Server Cookie is the 890 FNV-64 [FNV] of the request IP address, the Client Cookie, and the 891 server secret. That is 893 Server Cookie = 894 FNV-64 ( Server Secret | Request IP Address | Client Cookie ) 896 where "|" represents concatenation. 898 B.2 A More Complex Algorithm 900 Since the Server Cookie has a variable size, the server can store 901 various information in that field as long as it is hard for an 902 adversary to guess the entire quantity used for weak authentication. 903 There should be 64 bits of entropy in the Server Cookie; for example 904 it could have a sub-field of 64-bits computed pseudo-randomly with 905 the server secret as one of the inputs to the pseudo-random function. 906 Types of additional information that could be stored include a time 907 stamp and/or a nonce. 909 The example below is one variation for the Server Cookie that has 910 been implemented in a beta release of BIND where the Server Cookie is 911 128 bits composed as follows: 913 Sub-field Size 914 --------- --------- 915 Nonce 32 bits 916 Time 32 bits 917 Hash 64 bits 919 With this algorithm, the server sends a new 128-bit cookie back with 920 every request. The Nonce field assures a low probability that there 921 would be a duplicate. 923 The Time field gives the server time and makes it easy to reject old 924 cookies. 926 The Hash part of the Server Cookie is the hard-to-guess part. In the 927 beta release of BIND, its computation can be configured to use AES, 928 HMAC-SHA1, or, as shown below, HMAC-SHA256: 930 hash = 931 HMAC-SHA256-64 ( Server Secret, 932 (Client Cookie | nonce | time | client IP Address) ) 934 where "|" represents concatenation. 936 Author's Address 938 Donald E. Eastlake 3rd 939 Huawei Technologies 940 155 Beaver Street 941 Milford, MA 01757 USA 943 Telephone: +1-508-333-2270 944 EMail: d3e3e3@gmail.com 946 Mark Andrews 947 Internet Systems Consortium 948 950 Charter Street 949 Redwood City, CA 94063 USA 951 Email: marka@isc.org 953 Copyright, Disclaimer, and Additional IPR Provisions 955 Copyright (c) 2015 IETF Trust and the persons identified as the 956 document authors. All rights reserved. 958 This document is subject to BCP 78 and the IETF Trust's Legal 959 Provisions Relating to IETF Documents 960 (http://trustee.ietf.org/license-info) in effect on the date of 961 publication of this document. Please review these documents 962 carefully, as they describe your rights and restrictions with respect 963 to this document. Code Components extracted from this document must 964 include Simplified BSD License text as described in Section 4.e of 965 the Trust Legal Provisions and are provided without warranty as 966 described in the Simplified BSD License.