idnits 2.17.1 draft-ietf-dnsop-cookies-08.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (December 24, 2015) is 3045 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Obsolete informational reference (is this intentional?): RFC 2845 (Obsoleted by RFC 8945) Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 2 comments (--). 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: June 23, 2016 December 24, 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. (Since DNS Cookies are only returned to the IP address from 18 which they were originally received, they cannot be used to generally 19 track Internet users.) 21 Status of This Document 23 This Internet-Draft is submitted to IETF in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Distribution of this document is unlimited. Comments should be sent 27 to the author or the DNSEXT mailing list . 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF), its areas, and its working groups. Note that 31 other groups may also distribute working documents as Internet- 32 Drafts. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 The list of current Internet-Drafts can be accessed at 40 http://www.ietf.org/1id-abstracts.html. The list of Internet-Draft 41 Shadow Directories can be accessed at 42 http://www.ietf.org/shadow.html. 44 Table of Contents 46 1. Introduction............................................4 47 1.1 Contents of This Document..............................4 48 1.2 Definitions............................................5 50 2. Threats Considered......................................6 51 2.1 Denial-of-Service Attacks..............................6 52 2.1.1 DNS Amplification Attacks............................6 53 2.1.2 DNS Server Denial-of-Service.........................7 54 2.2 Cache Poisoning and Answer Forgery Attacks.............7 56 3. Comments on Existing DNS Security.......................8 57 3.1 Existing DNS Data Security.............................8 58 3.2 DNS Message/Transaction Security.......................8 59 3.3 Conclusions on Existing DNS Security...................8 61 4. DNS Cookie Option......................................10 62 4.1 Client Cookie.........................................11 63 4.2 Server Cookie.........................................11 65 5. DNS Cookies Protocol Specification.....................12 66 5.1 Originating Requests..................................12 67 5.2 Responding to Request.................................12 68 5.2.1 No Opt RR or No COOKIE OPT option...................13 69 5.2.2 Malformed COOKIE OPT option.........................13 70 5.2.3 Only a Client Cookie................................13 71 5.2.4 A Client Cookie and an Invalid Server Cookie........14 72 5.2.5 A Client Cookie and a Valid Server Cookie...........14 73 5.3 Processing Responses..................................14 74 5.4 QUERYing for a Server Cookie..........................15 75 5.5 Client and Server Secret Rollover.....................16 77 6. NAT Considerations and AnyCast Server Considerations...17 78 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 89 Acknowledgements..........................................26 91 Appendix A: Example Client Cookie Algorithms..............27 92 A.1 A Simple Algorithm....................................27 93 A.2 A More Complex Algorithm..............................27 95 Table of Contents (continued) 97 Appendix B: Example Server Cookie Algorithms..............28 98 B.1 A Simple Algorithm....................................28 99 B.2 A More Complex Algorithm..............................28 101 Author's Address..........................................30 103 1. Introduction 105 As with many core Internet protocols, the Domain Name System (DNS) 106 was originally designed at a time when the Internet had only a small 107 pool of trusted users. As the Internet has grown exponentially to a 108 global information utility, the DNS has increasingly been subject to 109 abuse. 111 This document describes DNS cookies, a lightweight DNS transaction 112 security mechanism specified as an OPT [RFC6891] option. The DNS 113 cookies mechanism provides limited protection to DNS servers and 114 clients against a variety of increasingly common abuses by off-path 115 attackers. It is compatible with and can be used in conjunction with 116 other DNS transaction forgery resistance measures such as those in 117 [RFC5452]. (Since DNS Cookies are only returned to the IP address 118 from which they were originally received, they cannot be used to 119 generally track Internet users.) 121 The protection provided by DNS cookies is similar to that provided by 122 using TCP for DNS transactions. To bypass the weak protection 123 provided by using TCP requires, among other things, that an off-path 124 attacker guess the 32-bit TCP sequence number in use. To bypass the 125 weak protection provided by DNS Cookies requires such an attacker to 126 guess a 64-bit pseudo-random "cookie" quantity. Where DNS Cookies are 127 not available but TCP is, falling back to using TCP is reasonable. 129 If only one party to a DNS transaction supports DNS cookies, the 130 mechanism does not provide a benefit or significantly interfere; but, 131 if both support it, the additional security provided is automatically 132 available. 134 The DNS cookies mechanism is designed to work in the presence of NAT 135 and NAT-PT boxes and guidance is provided herein on supporting the 136 DNS cookies mechanism in anycast servers. 138 1.1 Contents of This Document 140 In Section 2, we discuss the threats against which the DNS cookie 141 mechanism provides some protection. 143 Section 3 describes existing DNS security mechanisms and why they are 144 not adequate substitutes for DNS cookies. 146 Section 4 describes the COOKIE OPT option. 148 Section 5 provides a protocol description. 150 Section 6 discusses some NAT and anycast related DNS Cookies design 151 considerations. 153 Section 7 discusses incremental deployment considerations. 155 Sections 8 and 9 describe IANA and Security Considerations. 157 1.2 Definitions 159 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 160 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 161 "OPTIONAL" in this document are to be interpreted as described in 162 [RFC2119]. 164 "Off-path attacker", for a particular DNS client and server, is 165 defined as an attacker who cannot observe the DNS request and 166 response messages between that client and server. 168 "Soft state" indicates information learned or derived by a host which 169 may be discarded when indicated by the policies of that host 170 but can be later re-instantiated if needed. For example, it 171 could be discarded after a period of time or when storage for 172 caching such data becomes full. If operations requiring that 173 soft state continue after it has been discarded, it will be 174 automatically re-generated, albeit at some cost. 176 "Silently discarded" indicates that there are no DNS protocol message 177 consequences; however, it is RECOMMENDED that appropriate 178 network management facilities be included in implementations, 179 such as a counter of the occurrences of each such event type. 181 "IP address" is used herein as a length independent term and includes 182 both IPv4 and IPv6 addresses. 184 2. Threats Considered 186 DNS cookies are intended to provide significant but limited 187 protection against certain attacks by off-path attackers as described 188 below. These attacks include denial-of-service, cache poisoning, and 189 answer forgery. 191 2.1 Denial-of-Service Attacks 193 The typical form of the denial-of-service attacks considered herein 194 is to send DNS requests with forged source IP addresses to a server. 195 The intent can be to attack that server or some other selected host 196 as described below. 198 There are also on-path denial of service attacks that attempt to 199 saturate a server with DNS requests having correct source addresses. 200 Cookies do not protect against such attacks but successful cookie 201 validation improves the probability that the correct source IP 202 address for the requests is known. This facilitates contacting the 203 managers of or taking other actions for the networks from which the 204 requests originate. 206 2.1.1 DNS Amplification Attacks 208 A request with a forged IP source address generally causes a response 209 to be sent to that forged IP address. Thus the forging of many such 210 requests with a particular source IP address can result in enough 211 traffic being sent to the forged IP address to interfere with service 212 to the host at the IP address. Furthermore, it is generally easy in 213 the DNS to create short requests that produce much longer responses, 214 thus amplifying the attack. 216 The DNS Cookies mechanism can severely limit the traffic 217 amplification obtained by attacker requests that are off the path 218 between the server and the request's source address. Enforced DNS 219 cookies would make it hard for an off path attacker to cause any more 220 than rate-limited short error responses to be sent to a forged IP 221 address so the attack would be attenuated rather than amplified. DNS 222 cookies make it more effective to implement a rate limiting scheme 223 for error responses from the server. Such a scheme would further 224 restrict selected host denial-of-service traffic from that server. 226 2.1.2 DNS Server Denial-of-Service 228 DNS requests that are accepted cause work on the part of DNS servers. 229 This is particularly true for recursive servers that may issue one or 230 more requests and process the responses thereto, in order to 231 determine their response to the initial request. And the situation 232 can be even worse for recursive servers implementing DNSSEC 233 ([RFC4033] [RFC4034] [RFC4035]) because they may be induced to 234 perform burdensome cryptographic computations in attempts to verify 235 the authenticity of data they retrieve in trying to answer the 236 request. 238 The computational or communications burden caused by such requests 239 may not depend on a forged IP source address, but the use of such 240 addresses makes 241 + the source of the requests causing the denial-of-service attack 242 harder to find and 243 + restriction of the IP addresses from which such requests should 244 be honored hard or impossible to specify or verify. 246 Use of DNS cookies should enable a server to reject forged requests 247 from an off path attacker with relative ease and before any recursive 248 queries or public key cryptographic operations are performed. 250 2.2 Cache Poisoning and Answer Forgery Attacks 252 The form of the cache poisoning attacks considered is to send forged 253 replies to a resolver. Modern network speeds for well-connected hosts 254 are such that, by forging replies from the IP addresses of a DNS 255 server to a resolver for names that resolver has been induced to 256 resolve or for common names whose resource records have short time- 257 to-live values, there can be an unacceptably high probability of 258 randomly coming up with a reply that will be accepted and cause false 259 DNS information to be cached by that resolver (the Dan Kaminsky 260 attack [Kaminsky]). This can be used to facilitate phishing attacks 261 and other diversion of legitimate traffic to a compromised or 262 malicious host such as a web server. 264 With the use of DNS cookies, a resolver can generally reject such 265 forged replies. 267 3. Comments on Existing DNS Security 269 Two forms of security have been added to DNS, data security and 270 message/transaction security. 272 3.1 Existing DNS Data Security 274 DNS data security is one part of DNSSEC and is described in 275 [RFC4033], [RFC4034], [RFC4035], and updates thereto. It provides 276 data origin authentication and authenticated denial of existence. 277 DNSSEC is being deployed and can provide strong protection against 278 forged data and cache poisoning; however, it has the unintended 279 effect of making some denial-of-service attacks worse because of the 280 cryptographic computational load it can require and the increased 281 size in DNS response packets that it tends to produce. 283 3.2 DNS Message/Transaction Security 285 The second form of security that has been added to DNS provides 286 "transaction" security through TSIG [RFC2845] or SIG(0) [RFC2931]. 287 TSIG could provide strong protection against the attacks for which 288 the DNS Cookies mechanism provides weaker protection; however, TSIG 289 is non-trivial to deploy in the general Internet because of the 290 burdens it imposes. Among these burdens are pre-agreement and key 291 distribution between client and server, keeping track of server side 292 key state, and required time synchronization between client and 293 server. 295 TKEY [RFC2930] can solve the problem of key distribution for TSIG but 296 some modes of TKEY impose a substantial cryptographic computation 297 load and can be dependent on the deployment of DNS data security (see 298 Section 3.1). 300 SIG(0) [RFC2931] provides less denial of service protection than TSIG 301 or, in one way, even DNS cookies, because it does not authenticate 302 requests, only complete transactions. In any case, it also depends 303 on the deployment of DNS data security and requires computationally 304 burdensome public key cryptographic operations. 306 3.3 Conclusions on Existing DNS Security 308 The existing DNS security mechanisms do not provide the services 309 provided by the DNS Cookies mechanism: lightweight message 310 authentication of DNS requests and responses with no requirement for 311 pre-configuration or per client server side state. 313 4. DNS Cookie Option 315 The DNS Cookie Option is an OPT RR [RFC6891] option that can be 316 included in the RDATA portion of an OPT RR in DNS requests and 317 responses. The option length varies depending on the circumstances 318 in which it is being used. There are two cases as described below. 319 Both use the same OPTION-CODE; they are distinguished by their 320 length. 322 In a request sent by a client to a server when the client does not 323 know the server's cookie, its length is 8, consisting of an 8 byte 324 Client Cookie as shown in Figure 1. 326 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 327 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 1 328 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 329 | OPTION-CODE = 10 | OPTION-LENGTH = 8 | 330 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 331 | | 332 +-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+ 333 | | 334 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 336 Figure 1. COOKIE Option, Unknown Server Cookie 338 In a request sent by a client when a server cookie is known and in 339 all responses, the length is variable from 16 to 40 bytes, consisting 340 of an 8 bytes Client Cookie followed by the variable 8 to 32 bytes 341 Server Cookie as shown in Figure 2. The variability of the option 342 length stems from the variable length Server Cookie. The Server 343 Cookie is an integer number of bytes with a minimum size of 8 bytes 344 for security and a maximum size of 32 bytes for implementation 345 convenience. 347 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 348 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 1 349 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 350 | OPTION-CODE = 10 | OPTION-LENGTH >= 16, <= 40 | 351 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 352 | | 353 +-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+ 354 | | 355 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 356 | | 357 / Server Cookie (variable size, 8 to 32 bytes) / 358 / / 359 +-+-+-+-... 361 Figure 2. COOKIE Option, Known Server Cookie 363 4.1 Client Cookie 365 The Client Cookie SHOULD be a pseudo-random function of the server IP 366 address and a secret quantity known only to the client. This client 367 secret SHOULD have at least 64 bits of entropy [RFC4086] and be 368 changed periodically (see Section 5.5). The selection of the pseudo- 369 random function is a matter private to the client as only the client 370 needs to recognize its own DNS cookies. 372 For further discussion of the Client Cookie field, see Section 5.1. 373 For example methods of determining a Client Cookie, see Appendix A. 375 In order to provide minimal authentication, a client MUST send Client 376 Cookies that will usually be different for any two servers at 377 different IP addresses. 379 4.2 Server Cookie 381 The Server Cookie SHOULD consist of or include a 64-bit or larger 382 pseudo-random function of the request source IP address, the request 383 Client Cookie, and a secret quantity known only to the server. (See 384 Section 6 for a discussion of why the Client Cookie is used as input 385 to the Server Cookie but the Server Cookie is not used as an input to 386 the Client Cookie.) This server secret SHOULD have at least 64 bits 387 of entropy [RFC4086] and be changed periodically (see Section 5.5). 388 The selection of the pseudo-random function is a matter private to 389 the server as only the server needs to recognize its own DNS cookies. 391 For further discussion of the Server Cookie field see Section 5.2. 392 For example methods of determining a Server Cookie, see Appendix B. 393 When implemented as recommended, the server need not maintain any 394 cookie related per client state. 396 In order to provide minimal authentication, a server MUST send Server 397 Cookies that will usually be different for clients at any two 398 different IP addresses or with different Client Cookies. 400 5. DNS Cookies Protocol Specification 402 This section discusses using DNS Cookies in the DNS Protocol. The 403 cycle of originating a request, responding to that request, and 404 processing the response are covered in Sections 5.1, 5.2, and 5.3. A 405 de facto extension to QUERY to allow pre-fetching a Server Cookie is 406 specified in Section 5.4. Rollover of the client and server secrets 407 and transient retention of the old cookie or secret is covered in 408 Section 5.5. 410 DNS clients and servers SHOULD implement DNS cookies to decrease 411 their vulnerability to the threats discussed in Section 2. 413 5.1 Originating Requests 415 A DNS client that implements DNS Cookies includes one DNS COOKIE OPT 416 option containing a Client Cookie in every DNS request it sends 417 unless DNS cookies are disabled. 419 If the client has a cached Server Cookie for the server against its 420 IP address it uses the longer cookie form and includes that Server 421 Cookie in the option along with the Client Cookie (Figure 2). 422 Otherwise it just sends the shorter form option with a Client Cookie 423 (Figure 1). 425 5.2 Responding to Request 427 The Server Cookie, when it occurs in a COOKIE OPT option in a 428 request, is intended to weakly assure the server that the request 429 came from a client that is both at the source IP address of the 430 request and using the Client Cookie included in the option. This 431 assurance is provided by the Server Cookie that server sent to that 432 client in an earlier response appearing as the Server Cookie field in 433 the request. 435 At a server where DNS Cookies are not implemented and enabled, 436 presence of a COOKIE OPT option is ignored and the server responds as 437 if no COOKIE OPT option had been included in the request. 439 When DNS Cookies are implemented and enabled, there are five 440 possibilities: (1) there is no OPT RR at all in the request or there 441 is a OPT RR but the the COOKIE OPT option is absent from the OPT RR; 442 (2) a COOKIE OPT is present but is not a legal length or otherwise 443 malformed; (3) there is a valid length cookie option in the request 444 with no Server Cookie; (4) there is a valid length COOKIE OPT in the 445 request with a Server Cookie but that Server Cookie is invalid; or 446 (5) there is a valid length COOKIE OPT in the request with a correct 447 Server Cookie. 449 The five possibilities are discussed in the subsections below. 451 In all cases of multiple COOKIE OPT options in a request, only the 452 first (the one closest to the DNS header) is considered. All others 453 are ignored. 455 5.2.1 No Opt RR or No COOKIE OPT option 457 If there is no OPT record or no COOKIE OPT option present in the 458 request then the server responds to the request as if the server 459 doesn't implement the COOKIE OPT. 461 5.2.2 Malformed COOKIE OPT option 463 If the COOKIE OPT is too short to contain a Client Cookie then 464 FORMERR is generated. If the COOKIE OPT is longer than that required 465 to hold a COOKIE OPT with just a Client Cookie (8) but is shorter 466 that the minimum COOKIE OPT with both a Client and Server Cookie (16) 467 then FORMERR is generated. If the COOKIE OPT is longer than the 468 maximum valid COOKIE OPT (40) then a FORMERR is generated. 470 In summary, valid cookie lengths are 8 and 16 to 40 inclusive. 472 5.2.3 Only a Client Cookie 474 Based on server policy, including rate limiting, the server chooses 475 one of the following: 477 (1) Silently discard the request. 479 (2) Send a BADCOOKIE error response. 481 (3) Process the request and provide a normal response. The RCODE is 482 NOERROR unless some non-cookie error occurs in processing the 483 request. 485 If the server responds, choosing 2 or 3 above, it SHALL generate its 486 own COOKIE OPT containing both the Client Cookie copied from the 487 request and a Server Cookie it has generated and adds this COOKIE OPT 488 to the response's OPT record. Servers MUST, at least occasionally, 489 respond to such requests to inform the client of the correct Server 490 Cookie. This is necessary so that such a client can bootstrap to the 491 more secure state where requests and responses have recognized Server 492 Cookies and Client Cookies. A server is not expected to maintain per 493 client state to achieve this. For example, it could respond to every 494 Nth request across all clients. 496 If the request was received over TCP, the server SHOULD take the 497 authentication provided by the use of TCP into account and SHOULD 498 choose 3. In this case, if the server is not willing to accept the 499 security provided by TCP as a substitute for the security provided by 500 DNS Cookies but instead chooses 2, there is some danger of an 501 indefinite loop of retries (see Section 5.3). 503 5.2.4 A Client Cookie and an Invalid Server Cookie 505 The server examines the Server Cookie to determine if it is a valid 506 Server Cookie it has generated. This examination will result in a 507 determination of whether the Server Cookie is valid or not. If the 508 cookie is invalid, it can be because of a stale Server Cookie, or a 509 client's IP address or Client Cookie changing without the DNS server 510 being aware, or an anycast server cluster that is not consistently 511 configured, or an attempt to spoof the client. 513 The server SHALL process the request as if the invalid Server Cookie 514 was not present as described in Section 5.2.3. 516 5.2.5 A Client Cookie and a Valid Server Cookie 518 When a valid Server Cookie is present in the request the server can 519 assume that the request is from a client that it has talked to before 520 and defensive measures for spoofed UDP requests, if any, are no 521 longer required. 523 The server SHALL process the request and include a COOKIE OPT in the 524 response by (a) copying the complete COOKIE OPT from the request or 525 (b) generating a new COOKIE OPT containing both the Client Cookie 526 copied from the request and a valid Server Cookie it has generated. 528 5.3 Processing Responses 530 The Client Cookie, when it occurs in a COOKIE OPT option in a DNS 531 reply, is intended to weakly assure the client that the reply came 532 from a server at the source IP address used in the response packet 533 because the Client Cookie value is the value that client would send 534 to that server in a request. In a DNS reply with multiple COOKIE OPT 535 options, all but the first (the one closest to the DNS Header) are 536 ignored. 538 A DNS client where DNS cookies are implemented and enabled examines 539 the response for DNS cookies and MUST discard the response if it 540 contains an illegal COOKIE OPT option length or an incorrect Client 541 Cookie value. If the COOKIE OPT option Client Cookie is correct, the 542 client caches the Server Cookie provided even if the response is an 543 error response (RCODE non-zero). 545 If the reply extended RCODE is BADCOOKIE and the Client Cookie 546 matches what was sent, it means that the server was unwilling to 547 process the request because it did not have the correct Server Cookie 548 in it. The client SHOULD retry the request using the new Server 549 Cookie from the response. Repeated BADCOOKIE responses to requests 550 that use the Server Cookie provided in the previous response may be 551 an indication that the shared secrets / secret generation method in 552 an anycast cluster of servers are inconsistent. If the reply to a 553 retried request with a fresh Server Cookie is BADCOOKIE, the client 554 SHOULD retry using TCP as the transport since the server will likely 555 process the request normally based on the security provided by TCP 556 (see Section 5.2.3). 558 If the RCODE is some value other than BADCOOKIE, including zero, the 559 further processing of the response proceeds normally. 561 5.4 QUERYing for a Server Cookie 563 In many cases a client will learn the Server Cookie for a server as 564 the side effect of another transaction; however, there may be times 565 when this is not desirable. Therefore a means is provided for 566 obtaining a Server Cookie through an extension to the QUERY opcode 567 for which opcode most existing implementations require that QDCOUNT 568 be one (see Section 4.1.2 of [RFC1035]). 570 For servers with DNS Cookies enabled, the QUERY opcode behavior is 571 extended to support queries with a empty question section (QDCOUNT 572 zero) provided that an OPT record is present with a COOKIE option. 573 Such servers will reply with an empty answer section and a COOKIE 574 option giving the Client Cookie provided in the query and a valid 575 Server Cookie. 577 If such a query provided just a Client Cookie and no Server Cookie, 578 the response SHALL have the RCODE NOERROR. 580 This mechanism can also be used to confirm/re-establish a existing 581 Server Cookie by sending a cached Server Cookie with the Client 582 Cookie. In this case the response SHALL have the RCODE BADCOOKIE if 583 the Server Cookie sent with the query was invalid and the RCODE 584 NOERROR if it was valid. 586 Servers which don't support the COOKIE option will normally send 587 FORMERR in response to such a query, though REFUSED, NOTIMP, and 588 NOERROR without a COOKIE option are also possible in such responses. 590 5.5 Client and Server Secret Rollover 592 The longer a secret is used, the higher the probability it has been 593 compromised. Thus clients and servers MUST NOT continue to use the 594 same secret in new requests and responses for more than 36 days and 595 SHOULD NOT continue to do so for more than 26 hours. These values are 596 chosen to assure that a secret will not be used for longer than about 597 a month and normally no longer than one day. The odd values are to 598 allow for long holiday weekends and daylight savings time shifts and 599 the like while still staying within the limits. 601 Many clients rolling over their secret at the same time could briefly 602 increase server traffic and exactly predictable rollover times for 603 clients or servers might facilitate guessing attacks. For example, an 604 attacker might increase the priority of attacking secrets they 605 believe will be in effect for an extended period of time. To avoid 606 rollover synchronization and predictability, it is RECOMMENDED that 607 pseudorandom jitter in the range of plus zero to minus at least 40% 608 be applied to the time until a scheduled rollover of a DNS cookie 609 secret. 611 It is RECOMMENDED that a client keep the Client Cookie it is 612 expecting in a reply associated with the outstanding request to avoid 613 rejection of replies due to a bad Client Cookie right after a change 614 in the client secret. It is RECOMMENDED that a server retain its 615 previous secret for a period of time not less than 1 second or more 616 than 5 minutes, after a change in its secret, and consider requests 617 with Server Cookies based on its previous secret to have a correct 618 Server Cookie during that time. When a server responds to a request 619 containing a old Server Cookie that the server is treating as 620 correct, the server MUST include a new Server Cookie in its response. 622 When a server or client starts receiving an increased level of 623 requests with bad server cookies or replies with bad client cookies, 624 it would be reasonable for it to believe it is likely under attack 625 and it should consider a more frequent rollover of its secret. More 626 rapid rollover decreases the benefit to a cookie guessing attacker if 627 they succeed in guessing a cookie. 629 6. NAT Considerations and AnyCast Server Considerations 631 In the Classic Internet, DNS Cookies could simply be a pseudo-random 632 function of the client IP address and a server secret or the server 633 IP address and a client secret. You would want to compute the Server 634 Cookie that way, so a client could cache its Server Cookie for a 635 particular server for an indefinitely amount of time and the server 636 could easily regenerate and check it. You could consider the Client 637 Cookie to be a weak client signature over the server IP address that 638 the client checks in replies and you could extend this signature to 639 cover the request ID, for example, or any other information that is 640 returned unchanged in the reply. 642 But we have this reality called NAT [RFC3022], Network Address 643 Translation (including, for the purposes of this document, NAT-PT, 644 Network Address and Protocol Translation, which has been declared 645 Historic [RFC4966]). There is no problem with DNS transactions 646 between clients and servers behind a NAT box using local IP 647 addresses. Nor is there a problem with NAT translation of internal 648 addresses to external addresses or translations between IPv4 and IPv6 649 addresses, as long as the address mapping is relatively stable. 650 Should the external IP address an internal client is being mapped to 651 change occasionally, the disruption is little more than when a client 652 rolls-over its DNS COOKIE secret. And normally external access to a 653 DNS server behind a NAT box is handled by a fixed mapping which 654 forwards externally received DNS requests to a specific host. 656 However, NAT devices sometimes also map ports. This can cause 657 multiple DNS requests and responses from multiple internal hosts to 658 be mapped to a smaller number of external IP addresses, such as one 659 address. Thus there could be many clients behind a NAT box that 660 appear to come from the same source IP address to a server outside 661 that NAT box. If one of these were an attacker (think Zombie or 662 Botnet), that behind-NAT attacker could get the Server Cookie for 663 some server for the outgoing IP address by just making some random 664 request to that server. It could then include that Server Cookie in 665 the COOKIE OPT of requests to the server with the forged local IP 666 address of some other host and/or client behind the NAT box. 667 (Attacker possession of this Server Cookie will not help in forging 668 responses to cause cache poisoning as such responses are protected by 669 the required Client Cookie.) 671 To fix this potential defect, it is necessary to distinguish 672 different clients behind a NAT box from the point of view of the 673 server. It is for this reason that the Server Cookie is specified as 674 a pseudo-random function of both the request source IP address and 675 the Client Cookie. From this inclusion of the Client Cookie in the 676 calculation of the Server Cookie, it follows that a stable Client 677 Cookie, for any particular server, is needed. If, for example, the 678 request ID was included in the calculation of the Client Cookie, it 679 would normally change with each request to a particular server. This 680 would mean that each request would have to be sent twice: first to 681 learn the new Server Cookie based on this new Client Cookie based on 682 the new ID and then again using this new Client Cookie to actually 683 get an answer. Thus the input to the Client Cookie computation must 684 be limited to the server IP address and one or more things that 685 change slowly such as the client secret. 687 In principle, there could be a similar problem for servers, not due 688 to NAT but due to mechanisms like anycast which may cause requests to 689 a DNS server at an IP address to be delivered to any one of several 690 machines. (External requests to a DNS server behind a NAT box usually 691 occur via port forwarding such that all such requests go to one 692 host.) However, it is impossible to solve this the way the similar 693 problem was solved for NATed clients; if the Server Cookie was 694 included in the calculation of the Client Cookie the same way the 695 Client Cookie is included in the Server Cookie, you would just get an 696 almost infinite series of errors as a request was repeatedly retried. 698 For servers accessed via anycast to successfully support DNS COOKIES, 699 the server clones must either all use the same server secret or the 700 mechanism that distributes requests to them must cause the requests 701 from a particular client to go to a particular server for a 702 sufficiently long period of time that extra requests due to changes 703 in Server Cookie resulting from accessing different server machines 704 are not unduly burdensome. (When such anycast-accessed servers act 705 as recursive servers or otherwise act as clients they normally use a 706 different unique address to source their requests to avoid confusion 707 in the delivery of responses.) 709 For simplicity, it is RECOMMENDED that the same server secret be used 710 by each DNS server in a set of anycast servers. If there is limited 711 time skew in updating this secret in different anycast servers, this 712 can be handled by a server accepting requests containing a Server 713 Cookie based on either its old or new secret for the maximum likely 714 time period of such time skew (see also Section 5.5). 716 7. Deployment 718 The DNS cookies mechanism is designed for incremental deployment and 719 to complement the orthogonal techniques in [RFC5452]. Either or both 720 techniques can be deployed independently at each DNS server and 721 client. 723 In particular, a DNS server or client that implements the DNS COOKIE 724 mechanism can interoperate successfully with a DNS client or server 725 that does not implement this mechanism although, of course, in this 726 case it will not get the benefit of the mechanism and the server 727 involved might choose to severely rate limit responses. When such a 728 server or client interoperates with a client or server which also 729 implements the DNS cookies mechanism, they get the security benefits 730 of the DNS Cookies mechanism. 732 8. IANA Considerations 734 IANA has assigned the following OPT option value: 736 Value Name Status Reference 737 -------- ------ -------- --------------- 738 10 COOKIE Standard [this document] 740 IANA has assigned the following DNS error code as an early 741 allocation: 743 RCODE Name Description Reference 744 -------- --------- ------------------------- --------------- 745 23 BADCOOKIE Bad/missing server cookie [this document] 747 9. Security Considerations 749 DNS Cookies provide a weak form of authentication of DNS requests and 750 responses. In particular, they provide no protection against "on- 751 path" adversaries; that is, they provide no protection against any 752 adversary that can observe the plain text DNS traffic, such as an on- 753 path router, bridge, or any device on an on-path shared link (unless 754 the DNS traffic in question on that path is encrypted). 756 For example, if a host is connected via an unsecured IEEE Std 802.11 757 link (Wi-Fi), any device in the vicinity that could receive and 758 decode the 802.11 transmissions must be considered "on-path". On the 759 other hand, in a similar situation but one where 802.11 Robust 760 Security (WPAv2) is appropriately deployed on the Wi-Fi network 761 nodes, only the Access Point via which the host is connecting is "on- 762 path" as far as the 802.11 link is concerned. 764 Despite these limitations, deployment of DNS Cookies on the global 765 Internet is expected to provide a significant reduction in the 766 available launch points for the traffic amplification and denial of 767 service forgery attacks described in Section 2 above. 769 Should stronger message/transaction security be desired, it is 770 suggested that TSIG or SIG(0) security be used (see Section 3.2); 771 however, it may be useful to use DNS Cookies in conjunction with 772 these features. In particular, DNS Cookies could screen out many DNS 773 messages before the cryptographic computations of TSIG or SIG(0) are 774 required and, if SIG(0) is in use, DNS Cookies could usefully screen 775 out many requests given that SIG(0) does not screen requests but only 776 authenticates the response of complete transactions. 778 An attacker that does not know the Server Cookie could do a variety 779 of things, such as omitting the COOKIE OPT option or sending a random 780 Server Cookie. In general, DNS servers need to take other measures, 781 including rate limiting responses, to protect from abuse in such 782 cases. See further information in Section 5.2. 784 9.1 Cookie Algorithm Considerations 786 The cookie computation algorithm for use in DNS Cookies SHOULD be 787 based on a pseudo-random function at least as strong as 64-bit FNV 788 (Fowler-Noll-Vo [FNV]) because an excessively weak or trivial 789 algorithm could enable adversaries to guess cookies. However, in 790 light of the lightweight plain-text token security provided by DNS 791 Cookies, a strong cryptography hash algorithm may not be warranted in 792 many cases, and would cause an increased computational burden. 793 Nevertheless there is nothing wrong with using something stronger, 794 for example, HMAC-SHA256 [RFC6234] truncated to 64 bits, assuming a 795 DNS processor has adequate computational resources available. DNS 796 processors that feel the need for somewhat stronger security without 797 a significant increase in computational load should consider more 798 frequent changes in their client and/or server secret; however, this 799 does require more frequent generation of a cryptographically strong 800 random number [RFC4086]. See Appendices A and B for specific examples 801 of cookie computation algorithms. 803 10. Implementation Considerations 805 The DNS Cookie Option specified herein is implemented in BIND 9.10 806 using a experimental option code. 808 Normative References 810 [RFC1035] - Mockapetris, P., "Domain names - implementation and 811 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 812 November 1987, . 814 [RFC2119] - Bradner, S., "Key words for use in RFCs to Indicate 815 Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, 816 March 1997, . 818 [RFC4086] - Eastlake 3rd, D., Schiller, J., and S. Crocker, 819 "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 820 10.17487/RFC4086, June 2005, . 823 [RFC6891] - Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms 824 for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/RFC6891, 825 April 2013, . 827 Informative References 829 [FNV] - G. Fowler, L. C. Noll, K.-P. Vo, D. Eastlake, "The FNV Non- 830 Cryptographic Hash Algorithm", draft-eastlake-fnv, work in 831 progress. 833 [Kaminsky] - Olney, M., P. Mullen, K. Miklavicic, "Dan Kaminsky's 834 2008 DNS Vulnerability", 25 July 2008, 835 . 838 [RFC2845] - Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B. 839 Wellington, "Secret Key Transaction Authentication for DNS 840 (TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000, 841 . 843 [RFC2930] - Eastlake 3rd, D., "Secret Key Establishment for DNS (TKEY 844 RR)", RFC 2930, DOI 10.17487/RFC2930, September 2000, 845 . 847 [RFC2931] - Eastlake 3rd, D., "DNS Request and Transaction Signatures 848 ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September 2000, 849 . 851 [RFC3022] - Srisuresh, P. and K. Egevang, "Traditional IP Network 852 Address Translator (Traditional NAT)", RFC 3022, DOI 853 10.17487/RFC3022, January 2001, . 856 [RFC4033] - Arends, R., Austein, R., Larson, M., Massey, D., and S. 857 Rose, "DNS Security Introduction and Requirements", RFC 4033, 858 DOI 10.17487/RFC4033, March 2005, . 861 [RFC4034] - Arends, R., Austein, R., Larson, M., Massey, D., and S. 862 Rose, "Resource Records for the DNS Security Extensions", RFC 863 4034, DOI 10.17487/RFC4034, March 2005, . 866 [RFC4035] - Arends, R., Austein, R., Larson, M., Massey, D., and S. 867 Rose, "Protocol Modifications for the DNS Security Extensions", 868 RFC 4035, DOI 10.17487/RFC4035, March 2005, . 871 [RFC4966] - Aoun, C. and E. Davies, "Reasons to Move the Network 872 Address Translator - Protocol Translator (NAT-PT) to Historic 873 Status", RFC 4966, DOI 10.17487/RFC4966, July 2007, 874 . 876 [RFC5452] - Hubert, A. and R. van Mook, "Measures for Making DNS More 877 Resilient against Forged Answers", RFC 5452, DOI 878 10.17487/RFC5452, January 2009, . 881 [RFC6234] - Eastlake 3rd, D. and T. Hansen, "US Secure Hash 882 Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI 883 10.17487/RFC6234, May 2011, . 886 Acknowledgements 888 The suggestions and contributions of the following are gratefully 889 acknowledged: 891 Bob Harold, Paul Hoffman, Yoav Nir, Gayle Noble, Tim Wicinski 893 The document was prepared in raw nroff. All macros used were defined 894 within the source file. 896 Appendix A: Example Client Cookie Algorithms 898 A.1 A Simple Algorithm 900 An simple example method to compute Client Cookies is the FNV-64 901 [FNV] of the server IP address and the client secret. That is 903 Client Cookie = FNV-64 ( Server IP Address | Client Secret ) 905 where "|" indicates concatenation. (If the order of the items 906 concatenated above were reversed, it might be possible to reduce the 907 computational effort by pre-computing FNV-64 through the bytes of the 908 Client Secret but this would reduce the strength of the Client Cookie 909 and is NOT RECOMMENDED.) 911 A.2 A More Complex Algorithm 913 A more complex algorithm to calculate Client Cookies is given below. 914 It uses more computational resources than the simpler algorithm shown 915 in A.1. 917 Client Cookie = HMAC-SHA256-64 ( Client Secret, 918 Server IP Address ) 920 Appendix B: Example Server Cookie Algorithms 922 B.1 A Simple Algorithm 924 An example of a simple method producing a 64-bit Server Cookie is the 925 FNV-64 [FNV] of the request IP address, the Client Cookie, and the 926 server secret. 928 Server Cookie = 929 FNV-64 ( Request IP Address | Client Cookie | Server Secret ) 931 where "|" represents concatenation. (If the order of the items 932 concatenated above were reversed, it might be possible to reduce the 933 computational effort by pre-computing FNV-64 through the bytes of the 934 Sever Secret and Client Cookie but this would reduce the strength of 935 the Server Cookie and is NOT RECOMMENDED.) 937 B.2 A More Complex Algorithm 939 Since the Server Cookie has a variable size, the server can store 940 various information in that field as long as it is hard for an 941 adversary to guess the entire quantity used for authentication. There 942 should be 64 bits of entropy in the Server Cookie; for example it 943 could have a sub-field of 64-bits computed pseudo-randomly with the 944 server secret as one of the inputs to the pseudo-random function. 945 Types of additional information that could be stored include a time 946 stamp and/or a nonce. 948 The example below is one variation for the Server Cookie that has 949 been implemented in BIND 9.10.3 (and later) releases where the Server 950 Cookie is 128 bits composed as follows: 952 Sub-field Size 953 --------- --------- 954 Nonce 32 bits 955 Time 32 bits 956 Hash 64 bits 958 With this algorithm, the server sends a new 128-bit cookie back with 959 every request. The Nonce field assures a low probability that there 960 would be a duplicate. 962 The Time field gives the server time and makes it easy to reject old 963 cookies. 965 The Hash part of the Server Cookie is the hard-to-guess part. In the 966 beta release of BIND, its computation can be configured to use AES, 967 HMAC-SHA1, or, as shown below, HMAC-SHA256: 969 hash = 970 HMAC-SHA256-64 ( Server Secret, 971 (Client Cookie | nonce | time | client IP Address) ) 973 where "|" represents concatenation. 975 Author's Address 977 Donald E. Eastlake 3rd 978 Huawei Technologies 979 155 Beaver Street 980 Milford, MA 01757 USA 982 Telephone: +1-508-333-2270 983 EMail: d3e3e3@gmail.com 985 Mark Andrews 986 Internet Systems Consortium 987 950 Charter Street 988 Redwood City, CA 94063 USA 990 Email: marka@isc.org 992 Copyright, Disclaimer, and Additional IPR Provisions 994 Copyright (c) 2015 IETF Trust and the persons identified as the 995 document authors. All rights reserved. 997 This document is subject to BCP 78 and the IETF Trust's Legal 998 Provisions Relating to IETF Documents 999 (http://trustee.ietf.org/license-info) in effect on the date of 1000 publication of this document. Please review these documents 1001 carefully, as they describe your rights and restrictions with respect 1002 to this document. Code Components extracted from this document must 1003 include Simplified BSD License text as described in Section 4.e of 1004 the Trust Legal Provisions and are provided without warranty as 1005 described in the Simplified BSD License.