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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: April 18, 2016 October 19, 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.........................7 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 an Invalid Server Cookie........14 70 5.2.5 A Client Cookie and a Valid Server Cookie...........14 71 5.3 Processing Responses..................................14 72 5.4 QUERYing for a Server Cookie..........................15 73 5.5 Client and Server Secret Rollover.....................16 75 6. NAT Considerations and AnyCast Server Considerations...17 76 7. Deployment.............................................19 77 8. IANA Considerations....................................20 79 9. Security Considerations................................21 80 9.1 Cookie Algorithm Considerations.......................21 82 10. Implementation Considerations.........................23 84 Normative References......................................24 85 Informative References....................................24 87 Acknowledgements..........................................26 89 Appendix A: Example Client Cookie Algorithms..............27 90 A.1 A Simple Algorithm....................................27 91 A.2 A More Complex Algorithm..............................27 93 Table of Contents (continued) 95 Appendix B: Example Server Cookie Algorithms..............28 96 B.1 A Simple Algorithm....................................28 97 B.2 A More Complex Algorithm..............................28 99 Author's Address..........................................30 101 1. Introduction 103 As with many core Internet protocols, the Domain Name System (DNS) 104 was originally designed at a time when the Internet had only a small 105 pool of trusted users. As the Internet has grown exponentially to a 106 global information utility, the DNS has increasingly been subject to 107 abuse. 109 This document describes DNS cookies, a lightweight DNS transaction 110 security mechanism specified as an OPT [RFC6891] option. The DNS 111 cookies mechanism provides limited protection to DNS servers and 112 clients against a variety of increasingly common abuses by off-path 113 attackers. It is compatible with and can be used in conjunction with 114 other DNS transaction forgery resistance measures such as those in 115 [RFC5452]. 117 The protection provided by DNS cookies is similar to that provided by 118 using TCP for DNS transactions. To bypass the weak protection 119 provided by using TCP requires, among other things, that an off-path 120 attacker guessing the 32-bit TCP sequence number in use. To bypass 121 the weak protection provided by DNS Cookies requires such an attacker 122 to guess a 64-bit pseudo-random "cookie" quantity. Where DNS Cookies 123 are not available but TCP is, falling back to using TCP is 124 reasonable. 126 If only one party to a DNS transaction supports DNS cookies, the 127 mechanism does not provide a benefit or significantly interfere; but, 128 if both support it, the additional security provided is automatically 129 available. 131 The DNS cookies mechanism is designed to work in the presence of NAT 132 and NAT-PT boxes and guidance is provided herein on supporting the 133 DNS cookies mechanism in anycast servers. 135 1.1 Contents of This Document 137 In Section 2, we discuss the threats against which the DNS cookie 138 mechanism provides some protection. 140 Section 3 describes existing DNS security mechanisms and why they are 141 not adequate substitutes for DNS cookies. 143 Section 4 describes the COOKIE OPT option. 145 Section 5 provides a protocol description. 147 Section 6 discusses some NAT and anycast related DNS Cookies design 148 considerations. 150 Section 7 discusses incremental deployment considerations. 152 Sections 8 and 9 describe IANA and Security Considerations. 154 1.2 Definitions 156 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 157 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 158 document are to be interpreted as described in [RFC2119]. 160 "Off-path attacker", for a particular DNS client and server, is 161 defined as an attacker who cannot observe the DNS request and 162 response messages between that client and server. 164 "Soft state" indicates information learned or derived by a host which 165 may be discarded when indicated by the policies of that host 166 but can be later re-instantiated if needed. For example, it 167 could be discarded after a period of time or when storage for 168 caching such data becomes full. If operations requiring that 169 soft state continue after it has been discarded, it will be 170 automatically re-generated, albeit at some cost. 172 "Silently discarded" indicates that there are no DNS protocol message 173 consequences; however, it is RECOMMENDED that appropriate 174 network management facilities be included in implementations, 175 such as a counter of the occurrences of each such event type. 177 "IP address" is used herein as a length independent term and includes 178 both IPv4 and IPv6 addresses. 180 2. Threats Considered 182 DNS cookies are intended to provide significant but limited 183 protection against certain attacks by off-path attackers as described 184 below. These attacks include denial-of-service, cache poisoning, and 185 answer forgery. 187 2.1 Denial-of-Service Attacks 189 The typical form of the denial-of-service attacks considered herein 190 is to send DNS requests with forged source IP addresses to a server. 191 The intent can be to attack that server or some other selected host 192 as described below. 194 There are also on-path denial of service attacks that attempt to 195 saturate a server with DNS requests having correct souce addresses. 196 Cookies do not protect against such attacks but successful cookie 197 validation improves the probablity that the correct source IP address 198 for the requests is known. This facilitates contacting the managers 199 of or taking other actions for the networks from which the requests 200 originate. 202 2.1.1 DNS Amplification Attacks 204 A request with a forged IP source address generally causes a response 205 to be sent to that forged IP address. Thus the forging of many such 206 requests with a particular source IP address can result in enough 207 traffic being sent to the forged IP address to interfere with service 208 to the host at the IP address. Furthermore, it is generally easy in 209 the DNS to create short requests that produce much longer responses, 210 thus amplifying the attack. 212 The DNS Cookies mechanism can severely limit the traffic 213 amplification obtained by attacker requests that are off the path 214 between the server and the request's source address. Enforced DNS 215 cookies would make it hard for an off path attacker to cause any more 216 than rate-limited short error responses to be sent to a forged IP 217 address so the attack would be attenuated rather than amplified. DNS 218 cookies make it more effective to implement a rate limiting scheme 219 for error responses from the server. Such a scheme would further 220 restrict selected host denial-of-service traffic from that server. 222 2.1.2 DNS Server Denial-of-Service 224 DNS requests that are accepted cause work on the part of DNS servers. 225 This is particularly true for recursive servers that may issue one or 226 more requests and process the responses thereto, in order to 227 determine their response to the initial request. And the situation 228 can be even worse for recursive servers implementing DNSSEC 229 ([RFC4033] [RFC4034] [RFC4035]) because they may be induced to 230 perform burdensome cryptographic computations in attempts to verify 231 the authenticity of data they retrieve in trying to answer the 232 request. 234 The computational or communications burden caused by such requests 235 may not depend on a forged IP source address, but the use of such 236 addresses makes 237 + the source of the requests causing the denial-of-service attack 238 harder to find and 239 + restriction of the IP addresses from which such requests should 240 be honored hard or impossible to specify or verify. 242 Use of DNS cookies should enable a server to reject forged requests 243 from an off path attacker with relative ease and before any recursive 244 queries or public key cryptographic operations are performed. 246 2.2 Cache Poisoning and Answer Forgery Attacks 248 The form of the cache poisoning attacks considered is to send forged 249 replies to a resolver. Modern network speeds for well-connected hosts 250 are such that, by forging replies from the IP addresses of a DNS 251 server to a resolver for names that resolver has been induced to 252 resolve or for common names whose resource records have short time- 253 to-live values, there can be an unacceptably high probability of 254 randomly coming up with a reply that will be accepted and cause false 255 DNS information to be cached by that resolver (the Dan Kaminsky 256 attack [Kaminsky]). This can be used to facilitate phishing attacks 257 and other diversion of legitimate traffic to a compromised or 258 malicious host such as a web server. 260 With the use of DNS cookies, a resolver can generally reject such 261 forged replies. 263 3. Comments on Existing DNS Security 265 Two forms of security have been added to DNS, data security and 266 message/transaction security. 268 3.1 Existing DNS Data Security 270 DNS data security is one part of DNSSEC and is described in 271 [RFC4033], [RFC4034], [RFC4035], and updates thereto. It provides 272 data origin authentication and authenticated denial of existence. 273 DNSSEC is being deployed and can provide strong protection against 274 forged data and cache poisoning; however, it has the unintended 275 effect of making some denial-of-service attacks worse because of the 276 cryptographic computational load it can require and the increased 277 size in DNS response packets that it tends to produce. 279 3.2 DNS Message/Transaction Security 281 The second form of security that has been added to DNS provides 282 "transaction" security through TSIG [RFC2845] or SIG(0) [RFC2931]. 283 TSIG could provide strong protection against the attacks for which 284 the DNS Cookies mechanism provides weak protection; however, TSIG is 285 non-trivial to deploy in the general Internet because of the burdens 286 it imposes. Among these burdens are pre-agreement and key 287 distribution between client and server, keeping track of server side 288 key state, and required time synchronization between client and 289 server. 291 TKEY [RFC2930] can solve the problem of key distribution for TSIG but 292 some modes of TKEY impose a substantial cryptographic computation 293 load and can be dependent on the deployment of DNS data security (see 294 Section 3.1). 296 SIG(0) [RFC2931] provides less denial of service protection than TSIG 297 or, in one way, even DNS cookies, because it does not authenticate 298 requests, only complete transactions. In any case, it also depends 299 on the deployment of DNS data security and requires computationally 300 burdensome public key cryptographic operations. 302 3.3 Conclusions on Existing DNS Security 304 The existing DNS security mechanisms do not provide the services 305 provided by the DNS Cookies mechanism: lightweight message 306 authentication of DNS requests and responses with no requirement for 307 pre-configuration or per client server side state. 309 4. DNS Cookie Option 311 The DNS Cookie Option is an OPT RR [RFC6891] option that can be 312 included in the RDATA portion of an OPT RR in DNS requests and 313 responses. The option length varies depending on the circumstances 314 in which it is being used. There are two cases as described below. 315 Both use the same OPTION-CODE; they are distinguished by their 316 length. 318 In a request sent by a client to a server when the client does not 319 know the server's cookie, its length is 8, consisting of an 8 byte 320 Client Cookie as shown in Figure 1. 322 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 323 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 1 324 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 325 | OPTION-CODE = 10 | OPTION-LENGTH = 8 | 326 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 327 | | 328 +-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+ 329 | | 330 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 332 Figure 1. COOKIE Option, Unknown Server Cookie 334 In a request sent by a client when a server cookie is known and in 335 all responses, the length is variable from 16 to 40 bytes, consisting 336 of an 8 bytes Client Cookie followed by the variable 8 to 32 bytes 337 Server Cookie as shown in Figure 2. The variability of the option 338 length stems from the variable length Server Cookie. The Server 339 Cookie is an integer number of bytes with a minimum size of 8 bytes 340 for security and a maximum size of 32 bytes for implementation 341 convenience. 343 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 344 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 1 345 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 346 | OPTION-CODE = 10 | OPTION-LENGTH >= 16, <= 40 | 347 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 348 | | 349 +-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+ 350 | | 351 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 352 | | 353 / Server Cookie (variable size, 8 to 32 bytes) / 354 / / 355 +-+-+-+-... 357 Figure 2. COOKIE Option, Known Server Cookie 359 4.1 Client Cookie 361 The Client Cookie SHOULD be a pseudo-random function of the server IP 362 address and a secret quantity known only to the client. This client 363 secret SHOULD have at least 64 bits of entropy [RFC4086] and be 364 changed periodically (see Section 5.5). The selection of the pseudo- 365 random function is a matter private to the client as only the client 366 needs to recognize its own DNS cookies. 368 For further discussion of the Client Cookie field, see Section 5.1. 369 For example methods of determining a Client Cookie, see Appendix A. 371 In order to provide minimal authentication, a client MUST send client 372 COOKIEs that will usually be different for any two servers at 373 different IP addresses. 375 4.2 Server Cookie 377 The Server Cookie SHOULD consist of or include a 64-bit or larger 378 pseudo-random function of the request source IP address, the request 379 Client Cookie, and a secret quantity known only to the server. (See 380 Section 6 for a discussion of why the Client Cookie is used as input 381 to the Server Cookie but the Server Cookie is not used as an input to 382 the Client Cookie.) This server secret SHOULD have at least 64 bits 383 of entropy [RFC4086] and be changed periodically (see Section 5.5). 384 The selection of the pseudo-random function is a matter private to 385 the server as only the server needs to recognize its own DNS cookies. 387 For further discussion of the Server Cookie field see Section 5.2. 388 For example methods of determining a Server Cookie, see Appendix B. 390 In order to provide minimal authentication, a server MUST send server 391 COOKIEs that will usually be different for clients at any two 392 different IP addresses or with different client COOKIEs. 394 5. DNS Cookies Protocol Specification 396 This section discusses using DNS Cookies in the DNS Protocol. The 397 cycle of originating a request, responding to that request, and 398 processing the response are covered in Sections 5.1, 5.2, and 5.3. A 399 de facto extension to QUERY to allow pre-fetching a Server Cookie is 400 specified in Section 5.4. Rollover of the client and server secrets 401 and transient retention of the old cookie or secret is covered in 402 Section 5.5. 404 DNS clients and servers SHOULD implement DNS cookies to decrease 405 their vulnerability to the threats discussed in Section 2. 407 5.1 Originating Requests 409 A DNS client that implements DNS Cookies includes one DNS COOKIE OPT 410 option containing a Client Cookie in every DNS request it sends 411 unless DNS cookies are disabled. 413 If the client has a cached Server Cookie for the server against its 414 IP address it uses the longer cookie form and includes that Server 415 Cookie in the option along with the Client Cookie (Figure 2). 416 Otherwise it just sends the shorter form option with a Client Cookie 417 (Figure 1). 419 5.2 Responding to Request 421 The Server Cookie, when it occurs in a COOKIE OPT option in a 422 request, is intended to weakly assure the server that the request 423 came from a client that is both at the source IP address of the 424 request and using the Client Cookie included in the option. This weak 425 assurance is provided by the Server Cookie that server sent to that 426 client in an earlier response appearing as the Server Cookie field in 427 the request. 429 At a server where DNS Cookies are not implemented and enabled, 430 presence of a COOKIE OPT option is ignored and the server responds as 431 if no COOKIE OPT option had been included in the request. 433 When DNS Cookies are implemented and enabled, there are five 434 possibilities: (1) there is no OPT RR at all in the request or there 435 is a OPT RR but the the COOKIE OPT option is absent from the OPT RR; 436 (2) a COOKIE OPT is present but is not a legal length or otherwise 437 malformed; (3) there is a valid length cookie option in the request 438 with no Server Cookie; (4) there is a valid length COOKIE OPT in the 439 request with a Server Cookie but that Server Cookie is invalid; or 440 (5) there is a valid length COOKIE OPT in the request with a correct 441 Server Cookie. 443 The five possibilities are discussed in the subsections below. 445 In all cases of multiple COOKIE OPT options in a request, only the 446 first (the one closest to the DNS header) is considered. All others 447 are ignored. 449 5.2.1 No Opt RR or No COOKIE OPT option 451 If there is no OPT record or no COOKIE OPT option present in the 452 request then the server responds to the request as if the server 453 doesn't implement the COOKIE OPT. 455 5.2.2 Malformed COOKIE OPT option 457 If the COOKIE OPT is too short to contain a Client Cookie then 458 FORMERR is generated. If the COOKIE OPT is longer than that required 459 to hold a COOKIE OPT with just a Client Cookie (8) but is shorter 460 that the minimum COOKIE OPT with both a Client and Server Cookie (16) 461 then FORMERR is generated. If the COOKIE OPT is longer than the 462 maximum valid COOKIE OPT (40) then a FORMERR is generated. 464 In summary, valid cookie lengths are 8 and 16 to 40 inclusive. 466 5.2.3 Only a Client Cookie 468 Based on server policy, including rate limiting, the server chooses 469 one of the following: 471 (1) Silently discard the request. 473 (2) Send a BADCOOKIE error response. 475 (3) Process the request and provide a normal response. The RCODE is 476 NOERROR unless some non-cookie error occurs in processing the 477 request. 479 If the server responds, choosing 2 or 3 above, it SHALL generate its 480 own COOKIE OPT containing both the Client Cookie copied from the 481 request and a Server Cookie it has generated and adds this COOKIE OPT 482 to the response's OPT record. Servers MUST, at least occasionally, 483 respond to such requests to inform the client of the correct Server 484 Cookie. This is necessary so that such a client can bootstrap to the 485 weakly secure state where requests and responses have recognized 486 Server Cookies and Client Cookies. A server is not expected to 487 maintain per client state to achieve this. For example, it could 488 respond to every Nth request across all clients. 490 If the request was received over TCP, the server SHOULD take the weak 491 authentication provided by the use of TCP into account and SHOULD 492 choose 3. In this case, if the server is not willing to accept the 493 weak security provided by TCP as a substitute for the weak security 494 provided by DNS Cookies but instead chooses 2, there is some danger 495 of an indefinite loop of retries (see Section 5.3). 497 5.2.4 A Client Cookie and an Invalid Server Cookie 499 The server examines the Server Cookie to determine if it is a valid 500 Server Cookie it has generated. This examination will result in a 501 determination of whether the Server Cookie is valid or not. If the 502 cookie is invalid, it can be because of a stale Server Cookie, or a 503 client's IP address or Client Cookie changing without the DNS server 504 being aware, or an anycast server cluster that is not consistently 505 configured, or an attempt to spoof the client. 507 The server SHALL process the request as if the invalid Server Cookie 508 was not present as described in Section 5.2.3. 510 5.2.5 A Client Cookie and a Valid Server Cookie 512 When a valid Server Cookie is present in the request the server can 513 assume that the request is from a client that it has talked to before 514 and defensive measures for spoofed UDP requests, if any, are no 515 longer required. 517 The server SHALL process the request and include a COOKIE OPT in the 518 response by (a) copying the complete COOKIE OPT from the request or 519 (b) generating a new COOKIE OPT containing both the Client Cookie 520 copied from the request and a valid Server Cookie it has generated. 522 5.3 Processing Responses 524 The Client Cookie, when it occurs in a COOKIE OPT option in a DNS 525 reply, is intended to weakly assure the client that the reply came 526 from a server at the source IP address used in the response packet 527 because the Client Cookie value is the value that client would send 528 to that server in a request. In a DNS reply with multiple COOKIE OPT 529 options, all but the first (the one closest to the DNS Header) are 530 ignored. 532 A DNS client where DNS cookies are implemented and enabled examines 533 the response for DNS cookies and MUST discard the response if it 534 contains an illegal COOKIE OPT option length or an incorrect Client 535 Cookie value. If the COOKIE OPT option Client Cookie is correct, the 536 client caches the Server Cookie provided even if the response is an 537 error response (RCODE non-zero). 539 If the reply extended RCODE is BADCOOKIE and the Client Cookie 540 matches what was sent, it means that the server was unwilling to 541 process the request because it did not have the correct Server Cookie 542 in it. The client SHOULD retry the request using the new Server 543 Cookie from the response. Repeated BADCOOKIE responses to requests 544 that use the Server Cookie provided in the previous response may be 545 an indication that the shared secrets / secret generation method in 546 an anycast cluster of servers are inconsistent. If the reply to a 547 retried request with a fresh Server Cookie is BADCOOKIE, the client 548 SHOULD retry using TCP as the transport since the server will likely 549 process the request normally based on the weak security provided by 550 TCP (see Section 5.2.3). 552 If the RCODE is some value other than BADCOOKIE, including zero, the 553 further processing of the response proceeds normally. 555 5.4 QUERYing for a Server Cookie 557 In many cases a client will learn the Server Cookie for a server as 558 the side effect of another transaction; however, there may be times 559 when this is not desirable. Therefore a means is provided for 560 obtaining a Server Cookie through an extension to the QUERY opcode 561 for which opcode most existing implementations require that QDCOUNT 562 be one (see Section 4.1.2 of [RFC1035]). 564 For servers with DNS Cookies enabled, the QUERY opcode behavior is 565 extended to support queries with a empty question section (QDCOUNT 566 zero) provided that an OPT record is present with a COOKIE option. 567 Such servers will reply with an empty answer section and a COOKIE 568 option giving the Client Cookie provided in the query and a valid 569 Server Cookie. 571 If such a query provided just a Client Cookie and no Server Cookie, 572 the response SHALL have the RCODE NOERROR. 574 This mechanism can also be used to confirm/re-establish a existing 575 Server Cookie by sending a cached Server Cookie with the Client 576 Cookie. In this case the response SHALL have the RCODE BADCOOKIE if 577 the Server Cookie sent with the query was invalid and the RCODE 578 NOERROR if it was valid. 580 Servers which don't support the COOKIE option will normally send 581 FORMERR in response to such a query, though REFUSED, NOTIMP, and 582 NOERROR without a COOKIE option are also possible in such responses. 584 5.5 Client and Server Secret Rollover 586 The longer a secret is used, the higher the probability it has been 587 compromised. Thus clients and servers MUST NOT continue to use the 588 same secret in new requests and responses for more than 36 days and 589 SHOULD NOT continue to do so for more than 26 hours. These values are 590 chosen to assure that a secret will not be used for longer than about 591 a month and normally no longer than one day. The odd values are to 592 allow for long holiday weekends and daylight savings time shifts and 593 the like while still staying within the limits. 595 Many clients rolling over their secret at the same time could briefly 596 increase server traffic and exactly predictable rollover times for 597 clients or servers might facilitate guessing attacks. For example, an 598 attacker might increase the priority of attacking secrets they 599 believe will be in effect for an extended period of time. To avoid 600 rollover synchronization and predictability, it is RECOMMENDED that 601 pseudorandom jitter in the range of plus zero to minus at least 40% 602 be applied to the time until a scheduled rollover of a DNS cookie 603 secret. 605 It is RECOMMENDED that a client keep the Client Cookie it is 606 expecting in a reply associated with the outstanding request to avoid 607 rejection of replies due to a bad Client Cookie right after a change 608 in the client secret. It is RECOMMENDED that a server retain its 609 previous secret for a period of time not less than 1 second or more 610 than 5 minutes, after a change in its secret, and consider requests 611 with Server Cookies based on its previous secret to have a correct 612 Server Cookie during that time. 614 When a server or client starts receiving an increased level of 615 requests with bad server cookies or replies with bad client cookies, 616 it would be reasonable for it to believe it is likely under attack 617 and it should consider a more frequent rollover of its secret. More 618 rapid rollover decreases the benefit to a cookie guessing attacker if 619 they succeed in guessing a cookie. 621 6. NAT Considerations and AnyCast Server Considerations 623 In the Classic Internet, DNS Cookies could simply be a pseudo-random 624 function of the client IP address and a server secret or the server 625 IP address and a client secret. You would want to compute the Server 626 Cookie that way, so a client could cache its Server Cookie for a 627 particular server for an indefinitely amount of time and the server 628 could easily regenerate and check it. You could consider the Client 629 Cookie to be a weak client signature over the server IP address that 630 the client checks in replies and you could extend this weak signature 631 to cover the request ID, for example, or any other information that 632 is returned unchanged in the reply. 634 But we have this reality called NAT [RFC3022], Network Address 635 Translation (including, for the purposes of this document, NAT-PT, 636 Network Address and Protocol Translation, which has been declared 637 Historic [RFC4966]). There is no problem with DNS transactions 638 between clients and servers behind a NAT box using local IP 639 addresses. Nor is there a problem with NAT translation of internal 640 addresses to external addresses or translations between IPv4 and IPv6 641 addresses, as long as the address mapping is relatively stable. 642 Should the external IP address an internal client is being mapped to 643 change occasionally, the disruption is little more than when a client 644 rolls-over its DNS COOKIE secret. And normally external access to a 645 DNS server behind a NAT box is handled by a fixed mapping which 646 forwards externally received DNS requests to a specific host. 648 However, NAT devices sometimes also map ports. This can cause 649 multiple DNS requests and responses from multiple internal hosts to 650 be mapped to a smaller number of external IP addresses, such as one 651 address. Thus there could be many clients behind a NAT box that 652 appear to come from the same source IP address to a server outside 653 that NAT box. If one of these were an attacker (think Zombie or 654 Botnet), that behind-NAT attacker could get the Server Cookie for 655 some server for the outgoing IP address by just making some random 656 request to that server. It could then include that Server Cookie in 657 the COOKIE OPT of requests to the server with the forged local IP 658 address of some other host and/or client behind the NAT box. 659 (Attacker possession of this Server Cookie will not help in forging 660 responses to cause cache poisoning as such responses are protected by 661 the required Client Cookie.) 663 To fix this potential defect, it is necessary to distinguish 664 different clients behind a NAT box from the point of view of the 665 server. It is for this reason that the Server Cookie is specified as 666 a pseudo-random function of both the request source IP address and 667 the Client Cookie. From this inclusion of the Client Cookie in the 668 calculation of the Server Cookie, it follows that a stable Client 669 Cookie, for any particular server, is needed. If, for example, the 670 request ID was included in the calculation of the Client Cookie, it 671 would normally change with each request to a particular server. This 672 would mean that each request would have to be sent twice: first to 673 learn the new Server Cookie based on this new Client Cookie based on 674 the new ID and then again using this new Client Cookie to actually 675 get an answer. Thus the input to the Client Cookie computation must 676 be limited to the server IP address and one or more things that 677 change slowly such as the client secret. 679 In principle, there could be a similar problem for servers, not due 680 to NAT but due to mechanisms like anycast which may cause requests to 681 a DNS server at an IP address to be delivered to any one of several 682 machines. (External requests to a DNS server behind a NAT box usually 683 occur via port forwarding such that all such requests go to one 684 host.) However, it is impossible to solve this the way the similar 685 problem was solved for NATed clients; if the Server Cookie was 686 included in the calculation of the Client Cookie the same way the 687 Client Cookie is included in the Server Cookie, you would just get an 688 almost infinite series of errors as a request was repeatedly retried. 690 For servers accessed via anycast to successfully support DNS COOKIES, 691 the server clones must either all use the same server secret or the 692 mechanism that distributes requests to them must cause the requests 693 from a particular client to go to a particular server for a 694 sufficiently long period of time that extra requests due to changes 695 in Server Cookie resulting from accessing different server machines 696 are not unduly burdensome. (When such anycast-accessed servers act 697 as recursive servers or otherwise act as clients they normally use a 698 different unique address to source their requests to avoid confusion 699 in the delivery of responses.) 701 For simplicity, it is RECOMMENDED that the same server secret be used 702 by each DNS server in a set of anycast servers. If there is limited 703 time skew in updating this secret in different anycast servers, this 704 can be handled by a server accepting requests containing a Server 705 Cookie based on either its old or new secret for the maximum likely 706 time period of such time skew (see also Section 5.5). 708 7. Deployment 710 The DNS cookies mechanism is designed for incremental deployment and 711 to complement the orthogonal techniques in [RFC5452]. Either or both 712 techniques can be deployed independently at each DNS server and 713 client. 715 In particular, a DNS server or client that implements the DNS COOKIE 716 mechanism can interoperate successfully with a DNS client or server 717 that does not implement this mechanism although, of course, in this 718 case it will not get the benefit of the mechanism and the server 719 involved might choose to severely rate limit responses. When such a 720 server or client interoperates with a client or server which also 721 implements the DNS cookies mechanism, they get the weak security 722 benefits of the DNS Cookies mechanism. 724 8. IANA Considerations 726 IANA has assigned the following OPT option value: 728 Value Name Status Reference 729 -------- ------ -------- --------------- 730 10 COOKIE Standard [this document] 732 IANA has assigned the following DNS error code as an early 733 allocation: 735 RCODE Name Description Reference 736 -------- --------- ------------------------- --------------- 737 23 BADCOOKIE Bad/missing server cookie [this document] 739 9. Security Considerations 741 DNS Cookies provide a weak form of authentication of DNS requests and 742 responses. In particular, they provide no protection against "on- 743 path" adversaries; that is, they provide no protection against any 744 adversary that can observe the plain text DNS traffic, such as an on- 745 path router, bridge, or any device on an on-path shared link (unless 746 the DNS traffic in question on that path is encrypted). 748 For example, if a host is connected via an unsecured IEEE Std 802.11 749 link (Wi-Fi), any device in the vicinity that could receive and 750 decode the 802.11 transmissions must be considered "on-path". On the 751 other hand, in a similar situation but one where 802.11 Robust 752 Security (WPAv2) is appropriately deployed on the Wi-Fi network 753 nodes, only the Access Point via which the host is connecting is "on- 754 path" as far as the 802.11 link is concerned. 756 Despite these limitations, deployment of DNS Cookies on the global 757 Internet is expected to provide a significant reduction in the 758 available launch points for the traffic amplification and denial of 759 service forgery attacks described in Section 2 above. 761 Should stronger message/transaction security be desired, it is 762 suggested that TSIG or SIG(0) security be used (see Section 3.2); 763 however, it may be useful to use DNS Cookies in conjunction with 764 these features. In particular, DNS Cookies could screen out many DNS 765 messages before the cryptographic computations of TSIG or SIG(0) are 766 required and, if SIG(0) is in use, DNS Cookies could usefully screen 767 out many requests given that SIG(0) does not screen requests but only 768 authenticates the response of complete transactions. 770 9.1 Cookie Algorithm Considerations 772 The cookie computation algorithm for use in DNS Cookies SHOULD be 773 based on a pseudo-random function at least as strong as 64-bit FNV 774 (Fowler-Noll-Vo [FNV]) because an excessively weak or trivial 775 algorithm could enable adversaries to guess cookies. However, in 776 light of the weak plain-text token security provided by DNS Cookies, 777 a strong cryptography hash algorithm may not be warranted in many 778 cases, and would cause an increased computational burden. 779 Nevertheless there is nothing wrong with using something stronger, 780 for example, HMAC-SHA256-64 [RFC6234], assuming a DNS processor has 781 adequate computational resources available. DNS processors that feel 782 the need for somewhat stronger security without a significant 783 increase in computational load should consider more frequent changes 784 in their client and/or server secret; however, this does require more 785 frequent generation of a cryptographically strong random number 786 [RFC4086]. See Appendices A and B for specific examples of cookie 787 computation algorithms. 789 10. Implementation Considerations 791 The DNS Cookie Option specified herein is implemented in BIND 9.10 792 using a experimental option code. 794 Normative References 796 [RFC1035] - Mockapetris, P., "Domain names - implementation and 797 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 798 November 1987, . 800 [RFC2119] - Bradner, S., "Key words for use in RFCs to Indicate 801 Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, 802 March 1997, . 804 [RFC4086] - Eastlake 3rd, D., Schiller, J., and S. Crocker, 805 "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 806 10.17487/RFC4086, June 2005, . 809 [RFC6891] - Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms 810 for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/RFC6891, 811 April 2013, . 813 Informative References 815 [FNV] - G. Fowler, L. C. Noll, K.-P. Vo, D. Eastlake, "The FNV Non- 816 Cryptographic Hash Algorithm", draft-eastlake-fnv, work in 817 progress. 819 [Kaminsky] - Olney, M., P. Mullen, K. Miklavicic, "Dan Kaminsky's 820 2008 DNS Vulnerability", 25 July 2008, 821 . 824 [RFC2845] - Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B. 825 Wellington, "Secret Key Transaction Authentication for DNS 826 (TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000, 827 . 829 [RFC2930] - Eastlake 3rd, D., "Secret Key Establishment for DNS (TKEY 830 RR)", RFC 2930, DOI 10.17487/RFC2930, September 2000, 831 . 833 [RFC2931] - Eastlake 3rd, D., "DNS Request and Transaction Signatures 834 ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September 2000, 835 . 837 [RFC3022] - Srisuresh, P. and K. Egevang, "Traditional IP Network 838 Address Translator (Traditional NAT)", RFC 3022, DOI 839 10.17487/RFC3022, January 2001, . 842 [RFC4033] - Arends, R., Austein, R., Larson, M., Massey, D., and S. 843 Rose, "DNS Security Introduction and Requirements", RFC 4033, 844 DOI 10.17487/RFC4033, March 2005, . 847 [RFC4034] - Arends, R., Austein, R., Larson, M., Massey, D., and S. 848 Rose, "Resource Records for the DNS Security Extensions", RFC 849 4034, DOI 10.17487/RFC4034, March 2005, . 852 [RFC4035] - Arends, R., Austein, R., Larson, M., Massey, D., and S. 853 Rose, "Protocol Modifications for the DNS Security Extensions", 854 RFC 4035, DOI 10.17487/RFC4035, March 2005, . 857 [RFC4966] - Aoun, C. and E. Davies, "Reasons to Move the Network 858 Address Translator - Protocol Translator (NAT-PT) to Historic 859 Status", RFC 4966, DOI 10.17487/RFC4966, July 2007, 860 . 862 [RFC5452] - Hubert, A. and R. van Mook, "Measures for Making DNS More 863 Resilient against Forged Answers", RFC 5452, DOI 864 10.17487/RFC5452, January 2009, . 867 [RFC6234] - Eastlake 3rd, D. and T. Hansen, "US Secure Hash 868 Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI 869 10.17487/RFC6234, May 2011, . 872 Acknowledgements 874 The suggestions and contributions of the following are gratefully 875 acknowledged: 877 Bob Harold, Paul Hoffman, Gayle Noble, Tim Wicinski 879 The document was prepared in raw nroff. All macros used were defined 880 within the source file. 882 Appendix A: Example Client Cookie Algorithms 884 A.1 A Simple Algorithm 886 An simple example method to compute Client Cookies is the FNV-64 887 [FNV] of the server IP address and the client secret. That is 889 Client Cookie = FNV-64 ( Client Secret | Server IP Address ) 891 where "|" indicates concatenation. 893 A.2 A More Complex Algorithm 895 A more complex algorithm to calculate Client Cookies is given below. 896 It uses more computational resources than the simpler algorithm shown 897 in A.1. 899 Client Cookie = HMAC-SHA256-64 ( Client Secret, 900 Server IP Address ) 902 Appendix B: Example Server Cookie Algorithms 904 B.1 A Simple Algorithm 906 An example of a simple method producing a 64-bit Server Cookie is the 907 FNV-64 [FNV] of the request IP address, the Client Cookie, and the 908 server secret. That is 910 Server Cookie = 911 FNV-64 ( Server Secret | Request IP Address | Client Cookie ) 913 where "|" represents concatenation. 915 B.2 A More Complex Algorithm 917 Since the Server Cookie has a variable size, the server can store 918 various information in that field as long as it is hard for an 919 adversary to guess the entire quantity used for weak authentication. 920 There should be 64 bits of entropy in the Server Cookie; for example 921 it could have a sub-field of 64-bits computed pseudo-randomly with 922 the server secret as one of the inputs to the pseudo-random function. 923 Types of additional information that could be stored include a time 924 stamp and/or a nonce. 926 The example below is one variation for the Server Cookie that has 927 been implemented in a beta release of BIND where the Server Cookie is 928 128 bits composed as follows: 930 Sub-field Size 931 --------- --------- 932 Nonce 32 bits 933 Time 32 bits 934 Hash 64 bits 936 With this algorithm, the server sends a new 128-bit cookie back with 937 every request. The Nonce field assures a low probability that there 938 would be a duplicate. 940 The Time field gives the server time and makes it easy to reject old 941 cookies. 943 The Hash part of the Server Cookie is the hard-to-guess part. In the 944 beta release of BIND, its computation can be configured to use AES, 945 HMAC-SHA1, or, as shown below, HMAC-SHA256: 947 hash = 948 HMAC-SHA256-64 ( Server Secret, 949 (Client Cookie | nonce | time | client IP Address) ) 951 where "|" represents concatenation. 953 Author's Address 955 Donald E. Eastlake 3rd 956 Huawei Technologies 957 155 Beaver Street 958 Milford, MA 01757 USA 960 Telephone: +1-508-333-2270 961 EMail: d3e3e3@gmail.com 963 Mark Andrews 964 Internet Systems Consortium 965 950 Charter Street 966 Redwood City, CA 94063 USA 968 Email: marka@isc.org 970 Copyright, Disclaimer, and Additional IPR Provisions 972 Copyright (c) 2015 IETF Trust and the persons identified as the 973 document authors. All rights reserved. 975 This document is subject to BCP 78 and the IETF Trust's Legal 976 Provisions Relating to IETF Documents 977 (http://trustee.ietf.org/license-info) in effect on the date of 978 publication of this document. Please review these documents 979 carefully, as they describe your rights and restrictions with respect 980 to this document. Code Components extracted from this document must 981 include Simplified BSD License text as described in Section 4.e of 982 the Trust Legal Provisions and are provided without warranty as 983 described in the Simplified BSD License.