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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: May 1, 2016 November 2, 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 86 Acknowledgements..........................................26 88 Appendix A: Example Client Cookie Algorithms..............27 89 A.1 A Simple Algorithm....................................27 90 A.2 A More Complex Algorithm..............................27 92 Table of Contents (continued) 94 Appendix B: Example Server Cookie Algorithms..............28 95 B.1 A Simple Algorithm....................................28 96 B.2 A More Complex Algorithm..............................28 98 Author's Address..........................................30 100 1. Introduction 102 As with many core Internet protocols, the Domain Name System (DNS) 103 was originally designed at a time when the Internet had only a small 104 pool of trusted users. As the Internet has grown exponentially to a 105 global information utility, the DNS has increasingly been subject to 106 abuse. 108 This document describes DNS cookies, a lightweight DNS transaction 109 security mechanism specified as an OPT [RFC6891] option. The DNS 110 cookies mechanism provides limited protection to DNS servers and 111 clients against a variety of increasingly common abuses by off-path 112 attackers. It is compatible with and can be used in conjunction with 113 other DNS transaction forgery resistance measures such as those in 114 [RFC5452]. 116 The protection provided by DNS cookies is similar to that provided by 117 using TCP for DNS transactions. To bypass the weak protection 118 provided by using TCP requires, among other things, that an off-path 119 attacker guessing the 32-bit TCP sequence number in use. To bypass 120 the weak protection provided by DNS Cookies requires such an attacker 121 to guess a 64-bit pseudo-random "cookie" quantity. Where DNS Cookies 122 are not available but TCP is, falling back to using TCP is 123 reasonable. 125 If only one party to a DNS transaction supports DNS cookies, the 126 mechanism does not provide a benefit or significantly interfere; but, 127 if both support it, the additional security provided is automatically 128 available. 130 The DNS cookies mechanism is designed to work in the presence of NAT 131 and NAT-PT boxes and guidance is provided herein on supporting the 132 DNS cookies mechanism in anycast servers. 134 1.1 Contents of This Document 136 In Section 2, we discuss the threats against which the DNS cookie 137 mechanism provides some protection. 139 Section 3 describes existing DNS security mechanisms and why they are 140 not adequate substitutes for DNS cookies. 142 Section 4 describes the COOKIE OPT option. 144 Section 5 provides a protocol description. 146 Section 6 discusses some NAT and anycast related DNS Cookies design 147 considerations. 149 Section 7 discusses incremental deployment considerations. 151 Sections 8 and 9 describe IANA and Security Considerations. 153 1.2 Definitions 155 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 156 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 157 document are to be interpreted as described in [RFC2119]. 159 "Off-path attacker", for a particular DNS client and server, is 160 defined as an attacker who cannot observe the DNS request and 161 response messages between that client and server. 163 "Soft state" indicates information learned or derived by a host which 164 may be discarded when indicated by the policies of that host 165 but can be later re-instantiated if needed. For example, it 166 could be discarded after a period of time or when storage for 167 caching such data becomes full. If operations requiring that 168 soft state continue after it has been discarded, it will be 169 automatically re-generated, albeit at some cost. 171 "Silently discarded" indicates that there are no DNS protocol message 172 consequences; however, it is RECOMMENDED that appropriate 173 network management facilities be included in implementations, 174 such as a counter of the occurrences of each such event type. 176 "IP address" is used herein as a length independent term and includes 177 both IPv4 and IPv6 addresses. 179 2. Threats Considered 181 DNS cookies are intended to provide significant but limited 182 protection against certain attacks by off-path attackers as described 183 below. These attacks include denial-of-service, cache poisoning, and 184 answer forgery. 186 2.1 Denial-of-Service Attacks 188 The typical form of the denial-of-service attacks considered herein 189 is to send DNS requests with forged source IP addresses to a server. 190 The intent can be to attack that server or some other selected host 191 as described below. 193 There are also on-path denial of service attacks that attempt to 194 saturate a server with DNS requests having correct source addresses. 195 Cookies do not protect against such attacks but successful cookie 196 validation improves the probability that the correct source IP 197 address for the requests is known. This facilitates contacting the 198 managers of or taking other actions for the networks from which the 199 requests originate. 201 2.1.1 DNS Amplification Attacks 203 A request with a forged IP source address generally causes a response 204 to be sent to that forged IP address. Thus the forging of many such 205 requests with a particular source IP address can result in enough 206 traffic being sent to the forged IP address to interfere with service 207 to the host at the IP address. Furthermore, it is generally easy in 208 the DNS to create short requests that produce much longer responses, 209 thus amplifying the attack. 211 The DNS Cookies mechanism can severely limit the traffic 212 amplification obtained by attacker requests that are off the path 213 between the server and the request's source address. Enforced DNS 214 cookies would make it hard for an off path attacker to cause any more 215 than rate-limited short error responses to be sent to a forged IP 216 address so the attack would be attenuated rather than amplified. DNS 217 cookies make it more effective to implement a rate limiting scheme 218 for error responses from the server. Such a scheme would further 219 restrict selected host denial-of-service traffic from that server. 221 2.1.2 DNS Server Denial-of-Service 223 DNS requests that are accepted cause work on the part of DNS servers. 224 This is particularly true for recursive servers that may issue one or 225 more requests and process the responses thereto, in order to 226 determine their response to the initial request. And the situation 227 can be even worse for recursive servers implementing DNSSEC 228 ([RFC4033] [RFC4034] [RFC4035]) because they may be induced to 229 perform burdensome cryptographic computations in attempts to verify 230 the authenticity of data they retrieve in trying to answer the 231 request. 233 The computational or communications burden caused by such requests 234 may not depend on a forged IP source address, but the use of such 235 addresses makes 236 + the source of the requests causing the denial-of-service attack 237 harder to find and 238 + restriction of the IP addresses from which such requests should 239 be honored hard or impossible to specify or verify. 241 Use of DNS cookies should enable a server to reject forged requests 242 from an off path attacker with relative ease and before any recursive 243 queries or public key cryptographic operations are performed. 245 2.2 Cache Poisoning and Answer Forgery Attacks 247 The form of the cache poisoning attacks considered is to send forged 248 replies to a resolver. Modern network speeds for well-connected hosts 249 are such that, by forging replies from the IP addresses of a DNS 250 server to a resolver for names that resolver has been induced to 251 resolve or for common names whose resource records have short time- 252 to-live values, there can be an unacceptably high probability of 253 randomly coming up with a reply that will be accepted and cause false 254 DNS information to be cached by that resolver (the Dan Kaminsky 255 attack [Kaminsky]). This can be used to facilitate phishing attacks 256 and other diversion of legitimate traffic to a compromised or 257 malicious host such as a web server. 259 With the use of DNS cookies, a resolver can generally reject such 260 forged replies. 262 3. Comments on Existing DNS Security 264 Two forms of security have been added to DNS, data security and 265 message/transaction security. 267 3.1 Existing DNS Data Security 269 DNS data security is one part of DNSSEC and is described in 270 [RFC4033], [RFC4034], [RFC4035], and updates thereto. It provides 271 data origin authentication and authenticated denial of existence. 272 DNSSEC is being deployed and can provide strong protection against 273 forged data and cache poisoning; however, it has the unintended 274 effect of making some denial-of-service attacks worse because of the 275 cryptographic computational load it can require and the increased 276 size in DNS response packets that it tends to produce. 278 3.2 DNS Message/Transaction Security 280 The second form of security that has been added to DNS provides 281 "transaction" security through TSIG [RFC2845] or SIG(0) [RFC2931]. 282 TSIG could provide strong protection against the attacks for which 283 the DNS Cookies mechanism provides weak protection; however, TSIG is 284 non-trivial to deploy in the general Internet because of the burdens 285 it imposes. Among these burdens are pre-agreement and key 286 distribution between client and server, keeping track of server side 287 key state, and required time synchronization between client and 288 server. 290 TKEY [RFC2930] can solve the problem of key distribution for TSIG but 291 some modes of TKEY impose a substantial cryptographic computation 292 load and can be dependent on the deployment of DNS data security (see 293 Section 3.1). 295 SIG(0) [RFC2931] provides less denial of service protection than TSIG 296 or, in one way, even DNS cookies, because it does not authenticate 297 requests, only complete transactions. In any case, it also depends 298 on the deployment of DNS data security and requires computationally 299 burdensome public key cryptographic operations. 301 3.3 Conclusions on Existing DNS Security 303 The existing DNS security mechanisms do not provide the services 304 provided by the DNS Cookies mechanism: lightweight message 305 authentication of DNS requests and responses with no requirement for 306 pre-configuration or per client server side state. 308 4. DNS Cookie Option 310 The DNS Cookie Option is an OPT RR [RFC6891] option that can be 311 included in the RDATA portion of an OPT RR in DNS requests and 312 responses. The option length varies depending on the circumstances 313 in which it is being used. There are two cases as described below. 314 Both use the same OPTION-CODE; they are distinguished by their 315 length. 317 In a request sent by a client to a server when the client does not 318 know the server's cookie, its length is 8, consisting of an 8 byte 319 Client Cookie as shown in Figure 1. 321 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 322 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 1 323 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 324 | OPTION-CODE = 10 | OPTION-LENGTH = 8 | 325 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 326 | | 327 +-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+ 328 | | 329 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 331 Figure 1. COOKIE Option, Unknown Server Cookie 333 In a request sent by a client when a server cookie is known and in 334 all responses, the length is variable from 16 to 40 bytes, consisting 335 of an 8 bytes Client Cookie followed by the variable 8 to 32 bytes 336 Server Cookie as shown in Figure 2. The variability of the option 337 length stems from the variable length Server Cookie. The Server 338 Cookie is an integer number of bytes with a minimum size of 8 bytes 339 for security and a maximum size of 32 bytes for implementation 340 convenience. 342 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 343 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 1 344 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 345 | OPTION-CODE = 10 | OPTION-LENGTH >= 16, <= 40 | 346 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 347 | | 348 +-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+ 349 | | 350 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 351 | | 352 / Server Cookie (variable size, 8 to 32 bytes) / 353 / / 354 +-+-+-+-... 356 Figure 2. COOKIE Option, Known Server Cookie 358 4.1 Client Cookie 360 The Client Cookie SHOULD be a pseudo-random function of the server IP 361 address and a secret quantity known only to the client. This client 362 secret SHOULD have at least 64 bits of entropy [RFC4086] and be 363 changed periodically (see Section 5.5). The selection of the pseudo- 364 random function is a matter private to the client as only the client 365 needs to recognize its own DNS cookies. 367 For further discussion of the Client Cookie field, see Section 5.1. 368 For example methods of determining a Client Cookie, see Appendix A. 370 In order to provide minimal authentication, a client MUST send Client 371 Cookies that will usually be different for any two servers at 372 different IP addresses. 374 4.2 Server Cookie 376 The Server Cookie SHOULD consist of or include a 64-bit or larger 377 pseudo-random function of the request source IP address, the request 378 Client Cookie, and a secret quantity known only to the server. (See 379 Section 6 for a discussion of why the Client Cookie is used as input 380 to the Server Cookie but the Server Cookie is not used as an input to 381 the Client Cookie.) This server secret SHOULD have at least 64 bits 382 of entropy [RFC4086] and be changed periodically (see Section 5.5). 383 The selection of the pseudo-random function is a matter private to 384 the server as only the server needs to recognize its own DNS cookies. 386 For further discussion of the Server Cookie field see Section 5.2. 387 For example methods of determining a Server Cookie, see Appendix B. 389 In order to provide minimal authentication, a server MUST send Server 390 Cookies that will usually be different for clients at any two 391 different IP addresses or with different Client Cookies. 393 5. DNS Cookies Protocol Specification 395 This section discusses using DNS Cookies in the DNS Protocol. The 396 cycle of originating a request, responding to that request, and 397 processing the response are covered in Sections 5.1, 5.2, and 5.3. A 398 de facto extension to QUERY to allow pre-fetching a Server Cookie is 399 specified in Section 5.4. Rollover of the client and server secrets 400 and transient retention of the old cookie or secret is covered in 401 Section 5.5. 403 DNS clients and servers SHOULD implement DNS cookies to decrease 404 their vulnerability to the threats discussed in Section 2. 406 5.1 Originating Requests 408 A DNS client that implements DNS Cookies includes one DNS COOKIE OPT 409 option containing a Client Cookie in every DNS request it sends 410 unless DNS cookies are disabled. 412 If the client has a cached Server Cookie for the server against its 413 IP address it uses the longer cookie form and includes that Server 414 Cookie in the option along with the Client Cookie (Figure 2). 415 Otherwise it just sends the shorter form option with a Client Cookie 416 (Figure 1). 418 5.2 Responding to Request 420 The Server Cookie, when it occurs in a COOKIE OPT option in a 421 request, is intended to weakly assure the server that the request 422 came from a client that is both at the source IP address of the 423 request and using the Client Cookie included in the option. This weak 424 assurance is provided by the Server Cookie that server sent to that 425 client in an earlier response appearing as the Server Cookie field in 426 the request. 428 At a server where DNS Cookies are not implemented and enabled, 429 presence of a COOKIE OPT option is ignored and the server responds as 430 if no COOKIE OPT option had been included in the request. 432 When DNS Cookies are implemented and enabled, there are five 433 possibilities: (1) there is no OPT RR at all in the request or there 434 is a OPT RR but the the COOKIE OPT option is absent from the OPT RR; 435 (2) a COOKIE OPT is present but is not a legal length or otherwise 436 malformed; (3) there is a valid length cookie option in the request 437 with no Server Cookie; (4) there is a valid length COOKIE OPT in the 438 request with a Server Cookie but that Server Cookie is invalid; or 439 (5) there is a valid length COOKIE OPT in the request with a correct 440 Server Cookie. 442 The five possibilities are discussed in the subsections below. 444 In all cases of multiple COOKIE OPT options in a request, only the 445 first (the one closest to the DNS header) is considered. All others 446 are ignored. 448 5.2.1 No Opt RR or No COOKIE OPT option 450 If there is no OPT record or no COOKIE OPT option present in the 451 request then the server responds to the request as if the server 452 doesn't implement the COOKIE OPT. 454 5.2.2 Malformed COOKIE OPT option 456 If the COOKIE OPT is too short to contain a Client Cookie then 457 FORMERR is generated. If the COOKIE OPT is longer than that required 458 to hold a COOKIE OPT with just a Client Cookie (8) but is shorter 459 that the minimum COOKIE OPT with both a Client and Server Cookie (16) 460 then FORMERR is generated. If the COOKIE OPT is longer than the 461 maximum valid COOKIE OPT (40) then a FORMERR is generated. 463 In summary, valid cookie lengths are 8 and 16 to 40 inclusive. 465 5.2.3 Only a Client Cookie 467 Based on server policy, including rate limiting, the server chooses 468 one of the following: 470 (1) Silently discard the request. 472 (2) Send a BADCOOKIE error response. 474 (3) Process the request and provide a normal response. The RCODE is 475 NOERROR unless some non-cookie error occurs in processing the 476 request. 478 If the server responds, choosing 2 or 3 above, it SHALL generate its 479 own COOKIE OPT containing both the Client Cookie copied from the 480 request and a Server Cookie it has generated and adds this COOKIE OPT 481 to the response's OPT record. Servers MUST, at least occasionally, 482 respond to such requests to inform the client of the correct Server 483 Cookie. This is necessary so that such a client can bootstrap to the 484 weakly secure state where requests and responses have recognized 485 Server Cookies and Client Cookies. A server is not expected to 486 maintain per client state to achieve this. For example, it could 487 respond to every Nth request across all clients. 489 If the request was received over TCP, the server SHOULD take the weak 490 authentication provided by the use of TCP into account and SHOULD 491 choose 3. In this case, if the server is not willing to accept the 492 weak security provided by TCP as a substitute for the weak security 493 provided by DNS Cookies but instead chooses 2, there is some danger 494 of an indefinite loop of retries (see Section 5.3). 496 5.2.4 A Client Cookie and an Invalid Server Cookie 498 The server examines the Server Cookie to determine if it is a valid 499 Server Cookie it has generated. This examination will result in a 500 determination of whether the Server Cookie is valid or not. If the 501 cookie is invalid, it can be because of a stale Server Cookie, or a 502 client's IP address or Client Cookie changing without the DNS server 503 being aware, or an anycast server cluster that is not consistently 504 configured, or an attempt to spoof the client. 506 The server SHALL process the request as if the invalid Server Cookie 507 was not present as described in Section 5.2.3. 509 5.2.5 A Client Cookie and a Valid Server Cookie 511 When a valid Server Cookie is present in the request the server can 512 assume that the request is from a client that it has talked to before 513 and defensive measures for spoofed UDP requests, if any, are no 514 longer required. 516 The server SHALL process the request and include a COOKIE OPT in the 517 response by (a) copying the complete COOKIE OPT from the request or 518 (b) generating a new COOKIE OPT containing both the Client Cookie 519 copied from the request and a valid Server Cookie it has generated. 521 5.3 Processing Responses 523 The Client Cookie, when it occurs in a COOKIE OPT option in a DNS 524 reply, is intended to weakly assure the client that the reply came 525 from a server at the source IP address used in the response packet 526 because the Client Cookie value is the value that client would send 527 to that server in a request. In a DNS reply with multiple COOKIE OPT 528 options, all but the first (the one closest to the DNS Header) are 529 ignored. 531 A DNS client where DNS cookies are implemented and enabled examines 532 the response for DNS cookies and MUST discard the response if it 533 contains an illegal COOKIE OPT option length or an incorrect Client 534 Cookie value. If the COOKIE OPT option Client Cookie is correct, the 535 client caches the Server Cookie provided even if the response is an 536 error response (RCODE non-zero). 538 If the reply extended RCODE is BADCOOKIE and the Client Cookie 539 matches what was sent, it means that the server was unwilling to 540 process the request because it did not have the correct Server Cookie 541 in it. The client SHOULD retry the request using the new Server 542 Cookie from the response. Repeated BADCOOKIE responses to requests 543 that use the Server Cookie provided in the previous response may be 544 an indication that the shared secrets / secret generation method in 545 an anycast cluster of servers are inconsistent. If the reply to a 546 retried request with a fresh Server Cookie is BADCOOKIE, the client 547 SHOULD retry using TCP as the transport since the server will likely 548 process the request normally based on the weak security provided by 549 TCP (see Section 5.2.3). 551 If the RCODE is some value other than BADCOOKIE, including zero, the 552 further processing of the response proceeds normally. 554 5.4 QUERYing for a Server Cookie 556 In many cases a client will learn the Server Cookie for a server as 557 the side effect of another transaction; however, there may be times 558 when this is not desirable. Therefore a means is provided for 559 obtaining a Server Cookie through an extension to the QUERY opcode 560 for which opcode most existing implementations require that QDCOUNT 561 be one (see Section 4.1.2 of [RFC1035]). 563 For servers with DNS Cookies enabled, the QUERY opcode behavior is 564 extended to support queries with a empty question section (QDCOUNT 565 zero) provided that an OPT record is present with a COOKIE option. 566 Such servers will reply with an empty answer section and a COOKIE 567 option giving the Client Cookie provided in the query and a valid 568 Server Cookie. 570 If such a query provided just a Client Cookie and no Server Cookie, 571 the response SHALL have the RCODE NOERROR. 573 This mechanism can also be used to confirm/re-establish a existing 574 Server Cookie by sending a cached Server Cookie with the Client 575 Cookie. In this case the response SHALL have the RCODE BADCOOKIE if 576 the Server Cookie sent with the query was invalid and the RCODE 577 NOERROR if it was valid. 579 Servers which don't support the COOKIE option will normally send 580 FORMERR in response to such a query, though REFUSED, NOTIMP, and 581 NOERROR without a COOKIE option are also possible in such responses. 583 5.5 Client and Server Secret Rollover 585 The longer a secret is used, the higher the probability it has been 586 compromised. Thus clients and servers MUST NOT continue to use the 587 same secret in new requests and responses for more than 36 days and 588 SHOULD NOT continue to do so for more than 26 hours. These values are 589 chosen to assure that a secret will not be used for longer than about 590 a month and normally no longer than one day. The odd values are to 591 allow for long holiday weekends and daylight savings time shifts and 592 the like while still staying within the limits. 594 Many clients rolling over their secret at the same time could briefly 595 increase server traffic and exactly predictable rollover times for 596 clients or servers might facilitate guessing attacks. For example, an 597 attacker might increase the priority of attacking secrets they 598 believe will be in effect for an extended period of time. To avoid 599 rollover synchronization and predictability, it is RECOMMENDED that 600 pseudorandom jitter in the range of plus zero to minus at least 40% 601 be applied to the time until a scheduled rollover of a DNS cookie 602 secret. 604 It is RECOMMENDED that a client keep the Client Cookie it is 605 expecting in a reply associated with the outstanding request to avoid 606 rejection of replies due to a bad Client Cookie right after a change 607 in the client secret. It is RECOMMENDED that a server retain its 608 previous secret for a period of time not less than 1 second or more 609 than 5 minutes, after a change in its secret, and consider requests 610 with Server Cookies based on its previous secret to have a correct 611 Server Cookie during that time. 613 When a server or client starts receiving an increased level of 614 requests with bad server cookies or replies with bad client cookies, 615 it would be reasonable for it to believe it is likely under attack 616 and it should consider a more frequent rollover of its secret. More 617 rapid rollover decreases the benefit to a cookie guessing attacker if 618 they succeed in guessing a cookie. 620 6. NAT Considerations and AnyCast Server Considerations 622 In the Classic Internet, DNS Cookies could simply be a pseudo-random 623 function of the client IP address and a server secret or the server 624 IP address and a client secret. You would want to compute the Server 625 Cookie that way, so a client could cache its Server Cookie for a 626 particular server for an indefinitely amount of time and the server 627 could easily regenerate and check it. You could consider the Client 628 Cookie to be a weak client signature over the server IP address that 629 the client checks in replies and you could extend this weak signature 630 to cover the request ID, for example, or any other information that 631 is returned unchanged in the reply. 633 But we have this reality called NAT [RFC3022], Network Address 634 Translation (including, for the purposes of this document, NAT-PT, 635 Network Address and Protocol Translation, which has been declared 636 Historic [RFC4966]). There is no problem with DNS transactions 637 between clients and servers behind a NAT box using local IP 638 addresses. Nor is there a problem with NAT translation of internal 639 addresses to external addresses or translations between IPv4 and IPv6 640 addresses, as long as the address mapping is relatively stable. 641 Should the external IP address an internal client is being mapped to 642 change occasionally, the disruption is little more than when a client 643 rolls-over its DNS COOKIE secret. And normally external access to a 644 DNS server behind a NAT box is handled by a fixed mapping which 645 forwards externally received DNS requests to a specific host. 647 However, NAT devices sometimes also map ports. This can cause 648 multiple DNS requests and responses from multiple internal hosts to 649 be mapped to a smaller number of external IP addresses, such as one 650 address. Thus there could be many clients behind a NAT box that 651 appear to come from the same source IP address to a server outside 652 that NAT box. If one of these were an attacker (think Zombie or 653 Botnet), that behind-NAT attacker could get the Server Cookie for 654 some server for the outgoing IP address by just making some random 655 request to that server. It could then include that Server Cookie in 656 the COOKIE OPT of requests to the server with the forged local IP 657 address of some other host and/or client behind the NAT box. 658 (Attacker possession of this Server Cookie will not help in forging 659 responses to cause cache poisoning as such responses are protected by 660 the required Client Cookie.) 662 To fix this potential defect, it is necessary to distinguish 663 different clients behind a NAT box from the point of view of the 664 server. It is for this reason that the Server Cookie is specified as 665 a pseudo-random function of both the request source IP address and 666 the Client Cookie. From this inclusion of the Client Cookie in the 667 calculation of the Server Cookie, it follows that a stable Client 668 Cookie, for any particular server, is needed. If, for example, the 669 request ID was included in the calculation of the Client Cookie, it 670 would normally change with each request to a particular server. This 671 would mean that each request would have to be sent twice: first to 672 learn the new Server Cookie based on this new Client Cookie based on 673 the new ID and then again using this new Client Cookie to actually 674 get an answer. Thus the input to the Client Cookie computation must 675 be limited to the server IP address and one or more things that 676 change slowly such as the client secret. 678 In principle, there could be a similar problem for servers, not due 679 to NAT but due to mechanisms like anycast which may cause requests to 680 a DNS server at an IP address to be delivered to any one of several 681 machines. (External requests to a DNS server behind a NAT box usually 682 occur via port forwarding such that all such requests go to one 683 host.) However, it is impossible to solve this the way the similar 684 problem was solved for NATed clients; if the Server Cookie was 685 included in the calculation of the Client Cookie the same way the 686 Client Cookie is included in the Server Cookie, you would just get an 687 almost infinite series of errors as a request was repeatedly retried. 689 For servers accessed via anycast to successfully support DNS COOKIES, 690 the server clones must either all use the same server secret or the 691 mechanism that distributes requests to them must cause the requests 692 from a particular client to go to a particular server for a 693 sufficiently long period of time that extra requests due to changes 694 in Server Cookie resulting from accessing different server machines 695 are not unduly burdensome. (When such anycast-accessed servers act 696 as recursive servers or otherwise act as clients they normally use a 697 different unique address to source their requests to avoid confusion 698 in the delivery of responses.) 700 For simplicity, it is RECOMMENDED that the same server secret be used 701 by each DNS server in a set of anycast servers. If there is limited 702 time skew in updating this secret in different anycast servers, this 703 can be handled by a server accepting requests containing a Server 704 Cookie based on either its old or new secret for the maximum likely 705 time period of such time skew (see also Section 5.5). 707 7. Deployment 709 The DNS cookies mechanism is designed for incremental deployment and 710 to complement the orthogonal techniques in [RFC5452]. Either or both 711 techniques can be deployed independently at each DNS server and 712 client. 714 In particular, a DNS server or client that implements the DNS COOKIE 715 mechanism can interoperate successfully with a DNS client or server 716 that does not implement this mechanism although, of course, in this 717 case it will not get the benefit of the mechanism and the server 718 involved might choose to severely rate limit responses. When such a 719 server or client interoperates with a client or server which also 720 implements the DNS cookies mechanism, they get the weak security 721 benefits of the DNS Cookies mechanism. 723 8. IANA Considerations 725 IANA has assigned the following OPT option value: 727 Value Name Status Reference 728 -------- ------ -------- --------------- 729 10 COOKIE Standard [this document] 731 IANA has assigned the following DNS error code as an early 732 allocation: 734 RCODE Name Description Reference 735 -------- --------- ------------------------- --------------- 736 23 BADCOOKIE Bad/missing server cookie [this document] 738 9. Security Considerations 740 DNS Cookies provide a weak form of authentication of DNS requests and 741 responses. In particular, they provide no protection against "on- 742 path" adversaries; that is, they provide no protection against any 743 adversary that can observe the plain text DNS traffic, such as an on- 744 path router, bridge, or any device on an on-path shared link (unless 745 the DNS traffic in question on that path is encrypted). 747 For example, if a host is connected via an unsecured IEEE Std 802.11 748 link (Wi-Fi), any device in the vicinity that could receive and 749 decode the 802.11 transmissions must be considered "on-path". On the 750 other hand, in a similar situation but one where 802.11 Robust 751 Security (WPAv2) is appropriately deployed on the Wi-Fi network 752 nodes, only the Access Point via which the host is connecting is "on- 753 path" as far as the 802.11 link is concerned. 755 Despite these limitations, deployment of DNS Cookies on the global 756 Internet is expected to provide a significant reduction in the 757 available launch points for the traffic amplification and denial of 758 service forgery attacks described in Section 2 above. 760 Should stronger message/transaction security be desired, it is 761 suggested that TSIG or SIG(0) security be used (see Section 3.2); 762 however, it may be useful to use DNS Cookies in conjunction with 763 these features. In particular, DNS Cookies could screen out many DNS 764 messages before the cryptographic computations of TSIG or SIG(0) are 765 required and, if SIG(0) is in use, DNS Cookies could usefully screen 766 out many requests given that SIG(0) does not screen requests but only 767 authenticates the response of complete transactions. 769 9.1 Cookie Algorithm Considerations 771 The cookie computation algorithm for use in DNS Cookies SHOULD be 772 based on a pseudo-random function at least as strong as 64-bit FNV 773 (Fowler-Noll-Vo [FNV]) because an excessively weak or trivial 774 algorithm could enable adversaries to guess cookies. However, in 775 light of the weak plain-text token security provided by DNS Cookies, 776 a strong cryptography hash algorithm may not be warranted in many 777 cases, and would cause an increased computational burden. 778 Nevertheless there is nothing wrong with using something stronger, 779 for example, HMAC-SHA256-64 [RFC6234], assuming a DNS processor has 780 adequate computational resources available. DNS processors that feel 781 the need for somewhat stronger security without a significant 782 increase in computational load should consider more frequent changes 783 in their client and/or server secret; however, this does require more 784 frequent generation of a cryptographically strong random number 785 [RFC4086]. See Appendices A and B for specific examples of cookie 786 computation algorithms. 788 10. Implementation Considerations 790 The DNS Cookie Option specified herein is implemented in BIND 9.10 791 using a experimental option code. 793 Normative References 795 [RFC1035] - Mockapetris, P., "Domain names - implementation and 796 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 797 November 1987, . 799 [RFC2119] - Bradner, S., "Key words for use in RFCs to Indicate 800 Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, 801 March 1997, . 803 [RFC4086] - Eastlake 3rd, D., Schiller, J., and S. Crocker, 804 "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 805 10.17487/RFC4086, June 2005, . 808 [RFC6891] - Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms 809 for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/RFC6891, 810 April 2013, . 812 Informative References 814 [FNV] - G. Fowler, L. C. Noll, K.-P. Vo, D. Eastlake, "The FNV Non- 815 Cryptographic Hash Algorithm", draft-eastlake-fnv, work in 816 progress. 818 [Kaminsky] - Olney, M., P. Mullen, K. Miklavicic, "Dan Kaminsky's 819 2008 DNS Vulnerability", 25 July 2008, 820 . 823 [RFC2845] - Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B. 824 Wellington, "Secret Key Transaction Authentication for DNS 825 (TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000, 826 . 828 [RFC2930] - Eastlake 3rd, D., "Secret Key Establishment for DNS (TKEY 829 RR)", RFC 2930, DOI 10.17487/RFC2930, September 2000, 830 . 832 [RFC2931] - Eastlake 3rd, D., "DNS Request and Transaction Signatures 833 ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September 2000, 834 . 836 [RFC3022] - Srisuresh, P. and K. Egevang, "Traditional IP Network 837 Address Translator (Traditional NAT)", RFC 3022, DOI 838 10.17487/RFC3022, January 2001, . 841 [RFC4033] - Arends, R., Austein, R., Larson, M., Massey, D., and S. 842 Rose, "DNS Security Introduction and Requirements", RFC 4033, 843 DOI 10.17487/RFC4033, March 2005, . 846 [RFC4034] - Arends, R., Austein, R., Larson, M., Massey, D., and S. 847 Rose, "Resource Records for the DNS Security Extensions", RFC 848 4034, DOI 10.17487/RFC4034, March 2005, . 851 [RFC4035] - Arends, R., Austein, R., Larson, M., Massey, D., and S. 852 Rose, "Protocol Modifications for the DNS Security Extensions", 853 RFC 4035, DOI 10.17487/RFC4035, March 2005, . 856 [RFC4966] - Aoun, C. and E. Davies, "Reasons to Move the Network 857 Address Translator - Protocol Translator (NAT-PT) to Historic 858 Status", RFC 4966, DOI 10.17487/RFC4966, July 2007, 859 . 861 [RFC5452] - Hubert, A. and R. van Mook, "Measures for Making DNS More 862 Resilient against Forged Answers", RFC 5452, DOI 863 10.17487/RFC5452, January 2009, . 866 [RFC6234] - Eastlake 3rd, D. and T. Hansen, "US Secure Hash 867 Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI 868 10.17487/RFC6234, May 2011, . 871 Acknowledgements 873 The suggestions and contributions of the following are gratefully 874 acknowledged: 876 Bob Harold, Paul Hoffman, Gayle Noble, Tim Wicinski 878 The document was prepared in raw nroff. All macros used were defined 879 within the source file. 881 Appendix A: Example Client Cookie Algorithms 883 A.1 A Simple Algorithm 885 An simple example method to compute Client Cookies is the FNV-64 886 [FNV] of the server IP address and the client secret. That is 888 Client Cookie = FNV-64 ( Client Secret | Server IP Address ) 890 where "|" indicates concatenation. 892 A.2 A More Complex Algorithm 894 A more complex algorithm to calculate Client Cookies is given below. 895 It uses more computational resources than the simpler algorithm shown 896 in A.1. 898 Client Cookie = HMAC-SHA256-64 ( Client Secret, 899 Server IP Address ) 901 Appendix B: Example Server Cookie Algorithms 903 B.1 A Simple Algorithm 905 An example of a simple method producing a 64-bit Server Cookie is the 906 FNV-64 [FNV] of the request IP address, the Client Cookie, and the 907 server secret. That is 909 Server Cookie = 910 FNV-64 ( Server Secret | Request IP Address | Client Cookie ) 912 where "|" represents concatenation. 914 B.2 A More Complex Algorithm 916 Since the Server Cookie has a variable size, the server can store 917 various information in that field as long as it is hard for an 918 adversary to guess the entire quantity used for weak authentication. 919 There should be 64 bits of entropy in the Server Cookie; for example 920 it could have a sub-field of 64-bits computed pseudo-randomly with 921 the server secret as one of the inputs to the pseudo-random function. 922 Types of additional information that could be stored include a time 923 stamp and/or a nonce. 925 The example below is one variation for the Server Cookie that has 926 been implemented in a beta release of BIND where the Server Cookie is 927 128 bits composed as follows: 929 Sub-field Size 930 --------- --------- 931 Nonce 32 bits 932 Time 32 bits 933 Hash 64 bits 935 With this algorithm, the server sends a new 128-bit cookie back with 936 every request. The Nonce field assures a low probability that there 937 would be a duplicate. 939 The Time field gives the server time and makes it easy to reject old 940 cookies. 942 The Hash part of the Server Cookie is the hard-to-guess part. In the 943 beta release of BIND, its computation can be configured to use AES, 944 HMAC-SHA1, or, as shown below, HMAC-SHA256: 946 hash = 947 HMAC-SHA256-64 ( Server Secret, 948 (Client Cookie | nonce | time | client IP Address) ) 950 where "|" represents concatenation. 952 Author's Address 954 Donald E. Eastlake 3rd 955 Huawei Technologies 956 155 Beaver Street 957 Milford, MA 01757 USA 959 Telephone: +1-508-333-2270 960 EMail: d3e3e3@gmail.com 962 Mark Andrews 963 Internet Systems Consortium 964 950 Charter Street 965 Redwood City, CA 94063 USA 967 Email: marka@isc.org 969 Copyright, Disclaimer, and Additional IPR Provisions 971 Copyright (c) 2015 IETF Trust and the persons identified as the 972 document authors. All rights reserved. 974 This document is subject to BCP 78 and the IETF Trust's Legal 975 Provisions Relating to IETF Documents 976 (http://trustee.ietf.org/license-info) in effect on the date of 977 publication of this document. Please review these documents 978 carefully, as they describe your rights and restrictions with respect 979 to this document. Code Components extracted from this document must 980 include Simplified BSD License text as described in Section 4.e of 981 the Trust Legal Provisions and are provided without warranty as 982 described in the Simplified BSD License.