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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: July 11, 2016 January 12, 2016 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..................................15 74 5.4 QUERYing for a Server Cookie..........................15 76 6. NAT Considerations and AnyCast Server Considerations...17 78 7. Operational and Deployment Considerations..............19 79 7.1 Client and Server Secret Rollover.....................19 80 7.2 Counters..............................................20 82 8. IANA Considerations....................................21 84 9. Security Considerations................................22 85 9.1 Cookie Algorithm Considerations.......................22 87 10. Implementation Considerations.........................24 89 Normative References......................................25 90 Informative References....................................25 92 Acknowledgements..........................................27 94 Table of Contents (continued) 96 Appendix A: Example Client Cookie Algorithms..............28 97 A.1 A Simple Algorithm....................................28 98 A.2 A More Complex Algorithm..............................28 100 Appendix B: Example Server Cookie Algorithms..............29 101 B.1 A Simple Algorithm....................................29 102 B.2 A More Complex Algorithm..............................29 104 Author's Address..........................................31 106 1. Introduction 108 As with many core Internet protocols, the Domain Name System (DNS) 109 was originally designed at a time when the Internet had only a small 110 pool of trusted users. As the Internet has grown exponentially to a 111 global information utility, the DNS has increasingly been subject to 112 abuse. 114 This document describes DNS cookies, a lightweight DNS transaction 115 security mechanism specified as an OPT [RFC6891] option. The DNS 116 cookies mechanism provides limited protection to DNS servers and 117 clients against a variety of increasingly common abuses by off-path 118 attackers. It is compatible with and can be used in conjunction with 119 other DNS transaction forgery resistance measures such as those in 120 [RFC5452]. (Since DNS Cookies are only returned to the IP address 121 from which they were originally received, they cannot be used to 122 generally track Internet users.) 124 The protection provided by DNS cookies is similar to that provided by 125 using TCP for DNS transactions. To bypass the weak protection 126 provided by using TCP requires, among other things, that an off-path 127 attacker guess the 32-bit TCP sequence number in use. To bypass the 128 weak protection provided by DNS Cookies requires such an attacker to 129 guess a 64-bit pseudo-random "cookie" quantity. Where DNS Cookies are 130 not available but TCP is, falling back to using TCP is reasonable. 132 If only one party to a DNS transaction supports DNS cookies, the 133 mechanism does not provide a benefit or significantly interfere; but, 134 if both support it, the additional security provided is automatically 135 available. 137 The DNS cookies mechanism is designed to work in the presence of NAT 138 and NAT-PT boxes and guidance is provided herein on supporting the 139 DNS cookies mechanism in anycast servers. 141 1.1 Contents of This Document 143 In Section 2, we discuss the threats against which the DNS cookie 144 mechanism provides some protection. 146 Section 3 describes existing DNS security mechanisms and why they are 147 not adequate substitutes for DNS cookies. 149 Section 4 describes the COOKIE OPT option. 151 Section 5 provides a protocol description. 153 Section 6 discusses some NAT and anycast related DNS Cookies design 154 considerations. 156 Section 7 discusses incremental deployment considerations. 158 Sections 8 and 9 describe IANA and Security Considerations. 160 1.2 Definitions 162 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 163 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 164 "OPTIONAL" in this document are to be interpreted as described in 165 [RFC2119]. 167 "Off-path attacker", for a particular DNS client and server, is 168 defined as an attacker who cannot observe the DNS request and 169 response messages between that client and server. 171 "Soft state" indicates information learned or derived by a host which 172 may be discarded when indicated by the policies of that host 173 but can be later re-instantiated if needed. For example, it 174 could be discarded after a period of time or when storage for 175 caching such data becomes full. If operations requiring that 176 soft state continue after it has been discarded, it will be 177 automatically re-generated, albeit at some cost. 179 "Silently discarded" indicates that there are no DNS protocol message 180 consequences. 182 "IP address" is used herein as a length independent term and includes 183 both IPv4 and IPv6 addresses. 185 2. Threats Considered 187 DNS cookies are intended to provide significant but limited 188 protection against certain attacks by off-path attackers as described 189 below. These attacks include denial-of-service, cache poisoning, and 190 answer forgery. 192 2.1 Denial-of-Service Attacks 194 The typical form of the denial-of-service attacks considered herein 195 is to send DNS requests with forged source IP addresses to a server. 196 The intent can be to attack that server or some other selected host 197 as described below. 199 There are also on-path denial of service attacks that attempt to 200 saturate a server with DNS requests having correct source addresses. 201 Cookies do not protect against such attacks but successful cookie 202 validation improves the probability that the correct source IP 203 address for the requests is known. This facilitates contacting the 204 managers of or taking other actions for the networks from which the 205 requests originate. 207 2.1.1 DNS Amplification Attacks 209 A request with a forged IP source address generally causes a response 210 to be sent to that forged IP address. Thus the forging of many such 211 requests with a particular source IP address can result in enough 212 traffic being sent to the forged IP address to interfere with service 213 to the host at the IP address. Furthermore, it is generally easy in 214 the DNS to create short requests that produce much longer responses, 215 thus amplifying the attack. 217 The DNS Cookies mechanism can severely limit the traffic 218 amplification obtained by attacker requests that are off the path 219 between the server and the request's source address. Enforced DNS 220 cookies would make it hard for an off path attacker to cause any more 221 than rate-limited short error responses to be sent to a forged IP 222 address so the attack would be attenuated rather than amplified. DNS 223 cookies make it more effective to implement a rate limiting scheme 224 for error responses from the server. Such a scheme would further 225 restrict selected host denial-of-service traffic from that server. 227 2.1.2 DNS Server Denial-of-Service 229 DNS requests that are accepted cause work on the part of DNS servers. 230 This is particularly true for recursive servers that may issue one or 231 more requests and process the responses thereto, in order to 232 determine their response to the initial request. And the situation 233 can be even worse for recursive servers implementing DNSSEC 234 ([RFC4033] [RFC4034] [RFC4035]) because they may be induced to 235 perform burdensome cryptographic computations in attempts to verify 236 the authenticity of data they retrieve in trying to answer the 237 request. 239 The computational or communications burden caused by such requests 240 may not depend on a forged IP source address, but the use of such 241 addresses makes 242 + the source of the requests causing the denial-of-service attack 243 harder to find and 244 + restriction of the IP addresses from which such requests should 245 be honored hard or impossible to specify or verify. 247 Use of DNS cookies should enable a server to reject forged requests 248 from an off path attacker with relative ease and before any recursive 249 queries or public key cryptographic operations are performed. 251 2.2 Cache Poisoning and Answer Forgery Attacks 253 The form of the cache poisoning attacks considered is to send forged 254 replies to a resolver. Modern network speeds for well-connected hosts 255 are such that, by forging replies from the IP addresses of a DNS 256 server to a resolver for names that resolver has been induced to 257 resolve or for common names whose resource records have short time- 258 to-live values, there can be an unacceptably high probability of 259 randomly coming up with a reply that will be accepted and cause false 260 DNS information to be cached by that resolver (the Dan Kaminsky 261 attack [Kaminsky]). This can be used to facilitate phishing attacks 262 and other diversion of legitimate traffic to a compromised or 263 malicious host such as a web server. 265 With the use of DNS cookies, a resolver can generally reject such 266 forged replies. 268 3. Comments on Existing DNS Security 270 Two forms of security have been added to DNS, data security and 271 message/transaction security. 273 3.1 Existing DNS Data Security 275 DNS data security is one part of DNSSEC and is described in 276 [RFC4033], [RFC4034], [RFC4035], and updates thereto. It provides 277 data origin authentication and authenticated denial of existence. 278 DNSSEC is being deployed and can provide strong protection against 279 forged data and cache poisoning; however, it has the unintended 280 effect of making some denial-of-service attacks worse because of the 281 cryptographic computational load it can require and the increased 282 size in DNS response packets that it tends to produce. 284 3.2 DNS Message/Transaction Security 286 The second form of security that has been added to DNS provides 287 "transaction" security through TSIG [RFC2845] or SIG(0) [RFC2931]. 288 TSIG could provide strong protection against the attacks for which 289 the DNS Cookies mechanism provides weaker protection; however, TSIG 290 is non-trivial to deploy in the general Internet because of the 291 burdens it imposes. Among these burdens are pre-agreement and key 292 distribution between client and server, keeping track of server side 293 key state, and required time synchronization between client and 294 server. 296 TKEY [RFC2930] can solve the problem of key distribution for TSIG but 297 some modes of TKEY impose a substantial cryptographic computation 298 load and can be dependent on the deployment of DNS data security (see 299 Section 3.1). 301 SIG(0) [RFC2931] provides less denial of service protection than TSIG 302 or, in one way, even DNS cookies, because it does not authenticate 303 requests, only complete transactions. In any case, it also depends 304 on the deployment of DNS data security and requires computationally 305 burdensome public key cryptographic operations. 307 3.3 Conclusions on Existing DNS Security 309 The existing DNS security mechanisms do not provide the services 310 provided by the DNS Cookies mechanism: lightweight message 311 authentication of DNS requests and responses with no requirement for 312 pre-configuration or per client server side state. 314 4. DNS Cookie Option 316 The DNS Cookie Option is an OPT RR [RFC6891] option that can be 317 included in the RDATA portion of an OPT RR in DNS requests and 318 responses. The option length varies depending on the circumstances 319 in which it is being used. There are two cases as described below. 320 Both use the same OPTION-CODE; they are distinguished by their 321 length. 323 In a request sent by a client to a server when the client does not 324 know the server's cookie, its length is 8, consisting of an 8 byte 325 Client Cookie as shown in Figure 1. 327 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 328 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 1 329 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 330 | OPTION-CODE = 10 | OPTION-LENGTH = 8 | 331 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 332 | | 333 +-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+ 334 | | 335 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 337 Figure 1. COOKIE Option, Unknown Server Cookie 339 In a request sent by a client when a server cookie is known and in 340 all responses, the length is variable from 16 to 40 bytes, consisting 341 of an 8 bytes Client Cookie followed by the variable 8 to 32 bytes 342 Server Cookie as shown in Figure 2. The variability of the option 343 length stems from the variable length Server Cookie. The Server 344 Cookie is an integer number of bytes with a minimum size of 8 bytes 345 for security and a maximum size of 32 bytes for implementation 346 convenience. 348 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 349 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 1 350 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 351 | OPTION-CODE = 10 | OPTION-LENGTH >= 16, <= 40 | 352 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 353 | | 354 +-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+ 355 | | 356 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 357 | | 358 / Server Cookie (variable size, 8 to 32 bytes) / 359 / / 360 +-+-+-+-... 362 Figure 2. COOKIE Option, Known Server Cookie 364 4.1 Client Cookie 366 The Client Cookie SHOULD be a pseudo-random function of the server IP 367 address and a secret quantity known only to the client. This client 368 secret SHOULD have at least 64 bits of entropy [RFC4086] and be 369 changed periodically (see Section 7.1). The selection of the pseudo- 370 random function is a matter private to the client as only the client 371 needs to recognize its own DNS cookies. 373 For further discussion of the Client Cookie field, see Section 5.1. 374 For example methods of determining a Client Cookie, see Appendix A. 376 In order to provide minimal authentication, a client MUST send Client 377 Cookies that will usually be different for any two servers at 378 different IP addresses. 380 4.2 Server Cookie 382 The Server Cookie SHOULD consist of or include a 64-bit or larger 383 pseudo-random function of the request source IP address, the request 384 Client Cookie, and a secret quantity known only to the server. (See 385 Section 6 for a discussion of why the Client Cookie is used as input 386 to the Server Cookie but the Server Cookie is not used as an input to 387 the Client Cookie.) This server secret SHOULD have at least 64 bits 388 of entropy [RFC4086] and be changed periodically (see Section 7.1). 389 The selection of the pseudo-random function is a matter private to 390 the server as only the server needs to recognize its own DNS cookies. 392 For further discussion of the Server Cookie field see Section 5.2. 393 For example methods of determining a Server Cookie, see Appendix B. 394 When implemented as recommended, the server need not maintain any 395 cookie related per client state. 397 In order to provide minimal authentication, a server MUST send Server 398 Cookies that will usually be different for clients at any two 399 different IP addresses or with different Client Cookies. 401 5. DNS Cookies Protocol Specification 403 This section discusses using DNS Cookies in the DNS Protocol. The 404 cycle of originating a request, responding to that request, and 405 processing the response are covered in Sections 5.1, 5.2, and 5.3. A 406 de facto extension to QUERY to allow pre-fetching a Server Cookie is 407 specified in Section 5.4. Rollover of the client and server secrets 408 and transient retention of the old cookie or secret is covered in 409 Section 7.1. 411 DNS clients and servers SHOULD implement DNS cookies to decrease 412 their vulnerability to the threats discussed in Section 2. 414 5.1 Originating Requests 416 A DNS client that implements DNS Cookies includes one DNS COOKIE OPT 417 option containing a Client Cookie in every DNS request it sends 418 unless DNS cookies are disabled. 420 If the client has a cached Server Cookie for the server against its 421 IP address it uses the longer cookie form and includes that Server 422 Cookie in the option along with the Client Cookie (Figure 2). 423 Otherwise it just sends the shorter form option with a Client Cookie 424 (Figure 1). 426 5.2 Responding to Request 428 The Server Cookie, when it occurs in a COOKIE OPT option in a 429 request, is intended to weakly assure the server that the request 430 came from a client that is both at the source IP address of the 431 request and using the Client Cookie included in the option. This 432 assurance is provided by the Server Cookie that server sent to that 433 client in an earlier response appearing as the Server Cookie field in 434 the request. 436 At a server where DNS Cookies are not implemented and enabled, 437 presence of a COOKIE OPT option is ignored and the server responds as 438 if no COOKIE OPT option had been included in the request. 440 When DNS Cookies are implemented and enabled, there are five 441 possibilities: (1) there is no OPT RR at all in the request or there 442 is a OPT RR but the COOKIE OPT option is absent from the OPT RR; (2) 443 a COOKIE OPT is present but is not a legal length or otherwise 444 malformed; (3) there is a valid length cookie option in the request 445 with no Server Cookie; (4) there is a valid length COOKIE OPT in the 446 request with a Server Cookie but that Server Cookie is invalid; or 447 (5) there is a valid length COOKIE OPT in the request with a correct 448 Server Cookie. 450 The five possibilities are discussed in the subsections below. 452 In all cases of multiple COOKIE OPT options in a request, only the 453 first (the one closest to the DNS header) is considered. All others 454 are ignored. 456 5.2.1 No Opt RR or No COOKIE OPT option 458 If there is no OPT record or no COOKIE OPT option present in the 459 request then the server responds to the request as if the server 460 doesn't implement the COOKIE OPT. 462 5.2.2 Malformed COOKIE OPT option 464 If the COOKIE OPT is too short to contain a Client Cookie then 465 FORMERR is generated. If the COOKIE OPT is longer than that required 466 to hold a COOKIE OPT with just a Client Cookie (8 bytes) but is 467 shorter that the minimum COOKIE OPT with both a Client and Server 468 Cookie (16 bytes) then FORMERR is generated. If the COOKIE OPT is 469 longer than the maximum valid COOKIE OPT (40 bytes) then a FORMERR is 470 generated. 472 In summary, valid cookie lengths are 8 and 16 to 40 inclusive. 474 5.2.3 Only a Client Cookie 476 Based on server policy, including rate limiting, the server chooses 477 one of the following: 479 (1) Silently discard the request. 481 (2) Send a BADCOOKIE error response. 483 (3) Process the request and provide a normal response. The RCODE is 484 NOERROR unless some non-cookie error occurs in processing the 485 request. 487 If the server responds, choosing 2 or 3 above, it SHALL generate its 488 own COOKIE OPT containing both the Client Cookie copied from the 489 request and a Server Cookie it has generated and adds this COOKIE OPT 490 to the response's OPT record. Servers MUST, at least occasionally, 491 respond to such requests to inform the client of the correct Server 492 Cookie. This is necessary so that such a client can bootstrap to the 493 more secure state where requests and responses have recognized Server 494 Cookies and Client Cookies. A server is not expected to maintain per 495 client state to achieve this. For example, it could respond to every 496 Nth request across all clients. 498 If the request was received over TCP, the server SHOULD take the 499 authentication provided by the use of TCP into account and SHOULD 500 choose 3. In this case, if the server is not willing to accept the 501 security provided by TCP as a substitute for the security provided by 502 DNS Cookies but instead chooses 2, there is some danger of an 503 indefinite loop of retries (see Section 5.3). 505 5.2.4 A Client Cookie and an Invalid Server Cookie 507 The server examines the Server Cookie to determine if it is a valid 508 Server Cookie it has generated. This determination normally involves 509 re-calculating the Server Cookie (or the hash part thereof) based on 510 the server secret (or the previous server secret if it has just 511 changed), the received Client Cookie, the client IP address, and 512 possibly other fields -- see Appendix B.2 for an example. If the 513 cookie is invalid, it can be because of a stale Server Cookie, or a 514 client's IP address or Client Cookie changing without the DNS server 515 being aware, or an anycast server cluster that is not consistently 516 configured, or an attempt to spoof the client. 518 The server SHALL process the request as if the invalid Server Cookie 519 was not present as described in Section 5.2.3. 521 5.2.5 A Client Cookie and a Valid Server Cookie 523 When a valid Server Cookie is present in the request the server can 524 assume that the request is from a client that it has talked to before 525 and defensive measures for spoofed UDP requests, if any, are no 526 longer required. 528 The server SHALL process the request and include a COOKIE OPT in the 529 response by (a) copying the complete COOKIE OPT from the request or 530 (b) generating a new COOKIE OPT containing both the Client Cookie 531 copied from the request and a valid Server Cookie it has generated. 533 5.3 Processing Responses 535 The Client Cookie, when it occurs in a COOKIE OPT option in a DNS 536 reply, is intended to weakly assure the client that the reply came 537 from a server at the source IP address used in the response packet 538 because the Client Cookie value is the value that client would send 539 to that server in a request. In a DNS reply with multiple COOKIE OPT 540 options, all but the first (the one closest to the DNS Header) are 541 ignored. 543 A DNS client where DNS cookies are implemented and enabled examines 544 the response for DNS cookies and MUST discard the response if it 545 contains an illegal COOKIE OPT option length or an incorrect Client 546 Cookie value. If the COOKIE OPT option Client Cookie is correct, the 547 client caches the Server Cookie provided even if the response is an 548 error response (RCODE non-zero). 550 If the reply extended RCODE is BADCOOKIE and the Client Cookie 551 matches what was sent, it means that the server was unwilling to 552 process the request because it did not have the correct Server Cookie 553 in it. The client SHOULD retry the request using the new Server 554 Cookie from the response. Repeated BADCOOKIE responses to requests 555 that use the Server Cookie provided in the previous response may be 556 an indication that the shared secrets / secret generation method in 557 an anycast cluster of servers are inconsistent. If the reply to a 558 retried request with a fresh Server Cookie is BADCOOKIE, the client 559 SHOULD retry using TCP as the transport since the server will likely 560 process the request normally based on the security provided by TCP 561 (see Section 5.2.3). 563 If the RCODE is some value other than BADCOOKIE, including zero, the 564 further processing of the response proceeds normally. 566 5.4 QUERYing for a Server Cookie 568 In many cases a client will learn the Server Cookie for a server as 569 the side effect of another transaction; however, there may be times 570 when this is not desirable. Therefore a means is provided for 571 obtaining a Server Cookie through an extension to the QUERY opcode 572 for which opcode most existing implementations require that QDCOUNT 573 be one (see Section 4.1.2 of [RFC1035]). 575 For servers with DNS Cookies enabled, the QUERY opcode behavior is 576 extended to support queries with an empty question section (QDCOUNT 577 zero) provided that an OPT record is present with a COOKIE option. 578 Such servers will reply with an empty answer section and a COOKIE 579 option giving the Client Cookie provided in the query and a valid 580 Server Cookie. 582 If such a query provided just a Client Cookie and no Server Cookie, 583 the response SHALL have the RCODE NOERROR. 585 This mechanism can also be used to confirm/re-establish an existing 586 Server Cookie by sending a cached Server Cookie with the Client 587 Cookie. In this case the response SHALL have the RCODE BADCOOKIE if 588 the Server Cookie sent with the query was invalid and the RCODE 589 NOERROR if it was valid. 591 Servers which don't support the COOKIE option will normally send 592 FORMERR in response to such a query, though REFUSED, NOTIMP, and 593 NOERROR without a COOKIE option are also possible in such responses. 595 6. NAT Considerations and AnyCast Server Considerations 597 In the Classic Internet, DNS Cookies could simply be a pseudo-random 598 function of the client IP address and a server secret or the server 599 IP address and a client secret. You would want to compute the Server 600 Cookie that way, so a client could cache its Server Cookie for a 601 particular server for an indefinite amount of time and the server 602 could easily regenerate and check it. You could consider the Client 603 Cookie to be a weak client signature over the server IP address that 604 the client checks in replies and you could extend this signature to 605 cover the request ID, for example, or any other information that is 606 returned unchanged in the reply. 608 But we have this reality called NAT [RFC3022], Network Address 609 Translation (including, for the purposes of this document, NAT-PT, 610 Network Address and Protocol Translation, which has been declared 611 Historic [RFC4966]). There is no problem with DNS transactions 612 between clients and servers behind a NAT box using local IP 613 addresses. Nor is there a problem with NAT translation of internal 614 addresses to external addresses or translations between IPv4 and IPv6 615 addresses, as long as the address mapping is relatively stable. 616 Should the external IP address an internal client is being mapped to 617 change occasionally, the disruption is little more than when a client 618 rolls-over its DNS COOKIE secret. And normally external access to a 619 DNS server behind a NAT box is handled by a fixed mapping which 620 forwards externally received DNS requests to a specific host. 622 However, NAT devices sometimes also map ports. This can cause 623 multiple DNS requests and responses from multiple internal hosts to 624 be mapped to a smaller number of external IP addresses, such as one 625 address. Thus there could be many clients behind a NAT box that 626 appear to come from the same source IP address to a server outside 627 that NAT box. If one of these were an attacker (think Zombie or 628 Botnet), that behind-NAT attacker could get the Server Cookie for 629 some server for the outgoing IP address by just making some random 630 request to that server. It could then include that Server Cookie in 631 the COOKIE OPT of requests to the server with the forged local IP 632 address of some other host and/or client behind the NAT box. 633 (Attacker possession of this Server Cookie will not help in forging 634 responses to cause cache poisoning as such responses are protected by 635 the required Client Cookie.) 637 To fix this potential defect, it is necessary to distinguish 638 different clients behind a NAT box from the point of view of the 639 server. It is for this reason that the Server Cookie is specified as 640 a pseudo-random function of both the request source IP address and 641 the Client Cookie. From this inclusion of the Client Cookie in the 642 calculation of the Server Cookie, it follows that a stable Client 643 Cookie, for any particular server, is needed. If, for example, the 644 request ID was included in the calculation of the Client Cookie, it 645 would normally change with each request to a particular server. This 646 would mean that each request would have to be sent twice: first to 647 learn the new Server Cookie based on this new Client Cookie based on 648 the new ID and then again using this new Client Cookie to actually 649 get an answer. Thus the input to the Client Cookie computation must 650 be limited to the server IP address and one or more things that 651 change slowly such as the client secret. 653 In principle, there could be a similar problem for servers, not due 654 to NAT but due to mechanisms like anycast which may cause requests to 655 a DNS server at an IP address to be delivered to any one of several 656 machines. (External requests to a DNS server behind a NAT box usually 657 occur via port forwarding such that all such requests go to one 658 host.) However, it is impossible to solve this the way the similar 659 problem was solved for NATed clients; if the Server Cookie was 660 included in the calculation of the Client Cookie the same way the 661 Client Cookie is included in the Server Cookie, you would just get an 662 almost infinite series of errors as a request was repeatedly retried. 664 For servers accessed via anycast to successfully support DNS COOKIES, 665 the server clones must either all use the same server secret or the 666 mechanism that distributes requests to them must cause the requests 667 from a particular client to go to a particular server for a 668 sufficiently long period of time that extra requests due to changes 669 in Server Cookie resulting from accessing different server machines 670 are not unduly burdensome. (When such anycast-accessed servers act 671 as recursive servers or otherwise act as clients they normally use a 672 different unique address to source their requests to avoid confusion 673 in the delivery of responses.) 675 For simplicity, it is RECOMMENDED that the same server secret be used 676 by each DNS server in a set of anycast servers. If there is limited 677 time skew in updating this secret in different anycast servers, this 678 can be handled by a server accepting requests containing a Server 679 Cookie based on either its old or new secret for the maximum likely 680 time period of such time skew (see also Section 7.1). 682 7. Operational and Deployment Considerations 684 The DNS cookies mechanism is designed for incremental deployment and 685 to complement the orthogonal techniques in [RFC5452]. Either or both 686 techniques can be deployed independently at each DNS server and 687 client. Thus installation at the client and server end need not be 688 synchronized. 690 In particular, a DNS server or client that implements the DNS COOKIE 691 mechanism can interoperate successfully with a DNS client or server 692 that does not implement this mechanism although, of course, in this 693 case it will not get the benefit of the mechanism and the server 694 involved might choose to severely rate limit responses. When such a 695 server or client interoperates with a client or server which also 696 implements the DNS cookies mechanism, they get the security benefits 697 of the DNS Cookies mechanism. 699 7.1 Client and Server Secret Rollover 701 The longer a secret is used, the higher the probability it has been 702 compromised. Thus clients and servers are configured with a lifetime 703 for their secret and rollover to a new secret when that lifetime 704 expires or earlier due to deliberate jitter as described below. The 705 default lifetime is one day and the maximum permitted is one month. 706 To be precise and to make it practical to stay within limits despite 707 long holiday weekends and daylight savings time shifts and the like, 708 clients and servers MUST NOT continue to use the same secret in new 709 requests and responses for more than 36 days and SHOULD NOT continue 710 to do so for more than 26 hours. 712 Many clients rolling over their secret at the same time could briefly 713 increase server traffic and exactly predictable rollover times for 714 clients or servers might facilitate guessing attacks. For example, an 715 attacker might increase the priority of attacking secrets they 716 believe will be in effect for an extended period of time. To avoid 717 rollover synchronization and predictability, it is RECOMMENDED that 718 pseudorandom jitter in the range of plus zero to minus at least 40% 719 be applied to the time until a scheduled rollover of a DNS cookie 720 secret. 722 It is RECOMMENDED that a client keep the Client Cookie it is 723 expecting in a reply associated with the outstanding request to avoid 724 rejection of replies due to a bad Client Cookie right after a change 725 in the client secret. It is RECOMMENDED that a server retain its 726 previous secret after a rollover to a new secret for a configurable 727 period of time not less than 1 second or more than 5 minutes with 728 default configuration of 2 1/2 minutes. Requests with Server Cookies 729 based on its previous secret are treated as a correct Server Cookie 730 during that time. When a server responds to a request containing a 731 old Server Cookie that the server is treating as correct, the server 732 MUST include a new Server Cookie in its response. 734 7.2 Counters 736 It is RECOMMENDED that implementations include counters of the 737 occurrences of the various types of requests and responses described 738 in Section 5. 740 8. IANA Considerations 742 IANA has assigned the following OPT option value: 744 Value Name Status Reference 745 -------- ------ -------- --------------- 746 10 COOKIE Standard [this document] 748 IANA has assigned the following DNS error code as an early 749 allocation: 751 RCODE Name Description Reference 752 -------- --------- ------------------------- --------------- 753 23 BADCOOKIE Bad/missing server cookie [this document] 755 9. Security Considerations 757 DNS Cookies provide a weak form of authentication of DNS requests and 758 responses. In particular, they provide no protection against "on- 759 path" adversaries; that is, they provide no protection against any 760 adversary that can observe the plain text DNS traffic, such as an on- 761 path router, bridge, or any device on an on-path shared link (unless 762 the DNS traffic in question on that path is encrypted). 764 For example, if a host is connected via an unsecured IEEE Std 802.11 765 link (Wi-Fi), any device in the vicinity that could receive and 766 decode the 802.11 transmissions must be considered "on-path". On the 767 other hand, in a similar situation but one where 802.11 Robust 768 Security (WPA2) is appropriately deployed on the Wi-Fi network nodes, 769 only the Access Point via which the host is connecting is "on-path" 770 as far as the 802.11 link is concerned. 772 Despite these limitations, deployment of DNS Cookies on the global 773 Internet is expected to provide a significant reduction in the 774 available launch points for the traffic amplification and denial of 775 service forgery attacks described in Section 2 above. 777 Should stronger message/transaction security be desired, it is 778 suggested that TSIG or SIG(0) security be used (see Section 3.2); 779 however, it may be useful to use DNS Cookies in conjunction with 780 these features. In particular, DNS Cookies could screen out many DNS 781 messages before the cryptographic computations of TSIG or SIG(0) are 782 required and, if SIG(0) is in use, DNS Cookies could usefully screen 783 out many requests given that SIG(0) does not screen requests but only 784 authenticates the response of complete transactions. 786 An attacker that does not know the Server Cookie could do a variety 787 of things, such as omitting the COOKIE OPT option or sending a random 788 Server Cookie. In general, DNS servers need to take other measures, 789 including rate limiting responses, to protect from abuse in such 790 cases. See further information in Section 5.2. 792 When a server or client starts receiving an increased level of 793 requests with bad server cookies or replies with bad client cookies, 794 it would be reasonable for it to believe it is likely under attack 795 and it should consider a more frequent rollover of its secret. More 796 rapid rollover decreases the benefit to a cookie guessing attacker if 797 they succeed in guessing a cookie. 799 9.1 Cookie Algorithm Considerations 801 The cookie computation algorithm for use in DNS Cookies SHOULD be 802 based on a pseudo-random function at least as strong as 64-bit FNV 803 (Fowler-Noll-Vo [FNV]) because an excessively weak or trivial 804 algorithm could enable adversaries to guess cookies. However, in 805 light of the lightweight plain-text token security provided by DNS 806 Cookies, a strong cryptography hash algorithm may not be warranted in 807 many cases, and would cause an increased computational burden. 808 Nevertheless there is nothing wrong with using something stronger, 809 for example, HMAC-SHA256 [RFC6234] truncated to 64 bits, assuming a 810 DNS processor has adequate computational resources available. DNS 811 processors that feel the need for somewhat stronger security without 812 a significant increase in computational load should consider more 813 frequent changes in their client and/or server secret; however, this 814 does require more frequent generation of a cryptographically strong 815 random number [RFC4086]. See Appendices A and B for specific examples 816 of cookie computation algorithms. 818 10. Implementation Considerations 820 The DNS Cookie Option specified herein is implemented in BIND 9.10 821 using an experimental option code. 823 Normative References 825 [RFC1035] - Mockapetris, P., "Domain names - implementation and 826 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 827 November 1987, . 829 [RFC2119] - Bradner, S., "Key words for use in RFCs to Indicate 830 Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, 831 March 1997, . 833 [RFC4086] - Eastlake 3rd, D., Schiller, J., and S. Crocker, 834 "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 835 10.17487/RFC4086, June 2005, . 838 [RFC6891] - Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms 839 for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/RFC6891, 840 April 2013, . 842 Informative References 844 [FNV] - G. Fowler, L. C. Noll, K.-P. Vo, D. Eastlake, "The FNV Non- 845 Cryptographic Hash Algorithm", draft-eastlake-fnv, work in 846 progress. 848 [Kaminsky] - Olney, M., P. Mullen, K. Miklavicic, "Dan Kaminsky's 849 2008 DNS Vulnerability", 25 July 2008, 850 . 853 [RFC2845] - Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B. 854 Wellington, "Secret Key Transaction Authentication for DNS 855 (TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000, 856 . 858 [RFC2930] - Eastlake 3rd, D., "Secret Key Establishment for DNS (TKEY 859 RR)", RFC 2930, DOI 10.17487/RFC2930, September 2000, 860 . 862 [RFC2931] - Eastlake 3rd, D., "DNS Request and Transaction Signatures 863 ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September 2000, 864 . 866 [RFC3022] - Srisuresh, P. and K. Egevang, "Traditional IP Network 867 Address Translator (Traditional NAT)", RFC 3022, DOI 868 10.17487/RFC3022, January 2001, . 871 [RFC4033] - Arends, R., Austein, R., Larson, M., Massey, D., and S. 872 Rose, "DNS Security Introduction and Requirements", RFC 4033, 873 DOI 10.17487/RFC4033, March 2005, . 876 [RFC4034] - Arends, R., Austein, R., Larson, M., Massey, D., and S. 877 Rose, "Resource Records for the DNS Security Extensions", RFC 878 4034, DOI 10.17487/RFC4034, March 2005, . 881 [RFC4035] - Arends, R., Austein, R., Larson, M., Massey, D., and S. 882 Rose, "Protocol Modifications for the DNS Security Extensions", 883 RFC 4035, DOI 10.17487/RFC4035, March 2005, . 886 [RFC4966] - Aoun, C. and E. Davies, "Reasons to Move the Network 887 Address Translator - Protocol Translator (NAT-PT) to Historic 888 Status", RFC 4966, DOI 10.17487/RFC4966, July 2007, 889 . 891 [RFC5452] - Hubert, A. and R. van Mook, "Measures for Making DNS More 892 Resilient against Forged Answers", RFC 5452, DOI 893 10.17487/RFC5452, January 2009, . 896 [RFC6234] - Eastlake 3rd, D. and T. Hansen, "US Secure Hash 897 Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI 898 10.17487/RFC6234, May 2011, . 901 Acknowledgements 903 The suggestions and contributions of the following are gratefully 904 acknowledged: 906 Bob Harold, Paul Hoffman, Yoav Nir, Gayle Noble, Dan Romascanu, 907 Tim Wicinski, Peter Yee 909 The document was prepared in raw nroff. All macros used were defined 910 within the source file. 912 Appendix A: Example Client Cookie Algorithms 914 A.1 A Simple Algorithm 916 A simple example method to compute Client Cookies is the FNV-64 [FNV] 917 of the server IP address and the client secret. That is 919 Client Cookie = FNV-64 ( Server IP Address | Client Secret ) 921 where "|" indicates concatenation. (If the order of the items 922 concatenated above were reversed, it might be possible to reduce the 923 computational effort by pre-computing FNV-64 through the bytes of the 924 Client Secret but this would reduce the strength of the Client Cookie 925 and is NOT RECOMMENDED.) 927 A.2 A More Complex Algorithm 929 A more complex algorithm to calculate Client Cookies is given below. 930 It uses more computational resources than the simpler algorithm shown 931 in A.1. 933 Client Cookie = HMAC-SHA256-64 ( Client Secret, 934 Server IP Address ) 936 Appendix B: Example Server Cookie Algorithms 938 B.1 A Simple Algorithm 940 An example of a simple method producing a 64-bit Server Cookie is the 941 FNV-64 [FNV] of the request IP address, the Client Cookie, and the 942 server secret. 944 Server Cookie = 945 FNV-64 ( Request IP Address | Client Cookie | Server Secret ) 947 where "|" represents concatenation. (If the order of the items 948 concatenated above were reversed, it might be possible to reduce the 949 computational effort by pre-computing FNV-64 through the bytes of the 950 Sever Secret and Client Cookie but this would reduce the strength of 951 the Server Cookie and is NOT RECOMMENDED.) 953 B.2 A More Complex Algorithm 955 Since the Server Cookie has a variable size, the server can store 956 various information in that field as long as it is hard for an 957 adversary to guess the entire quantity used for authentication. There 958 should be 64 bits of entropy in the Server Cookie; for example it 959 could have a sub-field of 64-bits computed pseudo-randomly with the 960 server secret as one of the inputs to the pseudo-random function. 961 Types of additional information that could be stored include a time 962 stamp and/or a nonce. 964 The example below is one variation for the Server Cookie that has 965 been implemented in BIND 9.10.3 (and later) releases where the Server 966 Cookie is 128 bits composed as follows: 968 Sub-field Size 969 --------- --------- 970 Nonce 32 bits 971 Time 32 bits 972 Hash 64 bits 974 With this algorithm, the server sends a new 128-bit cookie back with 975 every request. The Nonce field assures a low probability that there 976 would be a duplicate. 978 The Time field gives the server time and makes it easy to reject old 979 cookies. 981 The Hash part of the Server Cookie is the hard-to-guess part. In BIND 982 9.10.3 (and later), its computation can be configured to use AES, 983 HMAC-SHA1, or, as shown below, HMAC-SHA256: 985 hash = 986 HMAC-SHA256-64 ( Server Secret, 987 (Client Cookie | nonce | time | client IP Address) ) 989 where "|" represents concatenation. 991 Author's Address 993 Donald E. Eastlake 3rd 994 Huawei Technologies 995 155 Beaver Street 996 Milford, MA 01757 USA 998 Telephone: +1-508-333-2270 999 EMail: d3e3e3@gmail.com 1001 Mark Andrews 1002 Internet Systems Consortium 1003 950 Charter Street 1004 Redwood City, CA 94063 USA 1006 Email: marka@isc.org 1008 Copyright, Disclaimer, and Additional IPR Provisions 1010 Copyright (c) 2016 IETF Trust and the persons identified as the 1011 document authors. All rights reserved. 1013 This document is subject to BCP 78 and the IETF Trust's Legal 1014 Provisions Relating to IETF Documents 1015 (http://trustee.ietf.org/license-info) in effect on the date of 1016 publication of this document. Please review these documents 1017 carefully, as they describe your rights and restrictions with respect 1018 to this document. Code Components extracted from this document must 1019 include Simplified BSD License text as described in Section 4.e of 1020 the Trust Legal Provisions and are provided without warranty as 1021 described in the Simplified BSD License.