1 Internet Engineering Task Force (IETF) D. Eastlake 3rd
2 Request for Comments: 7873 Huawei
3 Category: Standards Track M. Andrews
4 ISSN: 2070-1721 ISC
5 May 2016
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8 Domain Name System (DNS) Cookies
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10 Abstract
11
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
16 Cookies are tolerant of NAT, NAT-PT (Network Address Translation -
17 Protocol Translation), and anycast and can be incrementally deployed.
18 (Since DNS Cookies are only returned to the IP address from which
19 they were originally received, they cannot be used to generally track
20 Internet users.)
21
22 Status of This Memo
23
24 This is an Internet Standards Track document.
25
26 This document is a product of the Internet Engineering Task Force
27 (IETF). It represents the consensus of the IETF community. It has
28 received public review and has been approved for publication by the
29 Internet Engineering Steering Group (IESG). Further information on
30 Internet Standards is available in Section 2 of RFC 7841.
31
32 Information about the current status of this document, any errata,
33 and how to provide feedback on it may be obtained at
34 http://www.rfc-editor.org/info/rfc7873.
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56 Copyright Notice
57
58 Copyright (c) 2016 IETF Trust and the persons identified as the
59 document authors. All rights reserved.
60
61 This document is subject to BCP 78 and the IETF Trust's Legal
62 Provisions Relating to IETF Documents
63 (http://trustee.ietf.org/license-info) in effect on the date of
64 publication of this document. Please review these documents
65 carefully, as they describe your rights and restrictions with respect
66 to this document. Code Components extracted from this document must
67 include Simplified BSD License text as described in Section 4.e of
68 the Trust Legal Provisions and are provided without warranty as
69 described in the Simplified BSD License.
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111 Table of Contents
112
113 1. Introduction ....................................................4
114 1.1. Contents of This Document ..................................4
115 1.2. Definitions ................................................5
116 2. Threats Considered ..............................................5
117 2.1. Denial-of-Service Attacks ..................................6
118 2.1.1. DNS Amplification Attacks ...........................6
119 2.1.2. DNS Server Denial of Service ........................6
120 2.2. Cache Poisoning and Answer Forgery Attacks .................7
121 3. Comments on Existing DNS Security ...............................7
122 3.1. Existing DNS Data Security .................................7
123 3.2. DNS Message/Transaction Security ...........................8
124 3.3. Conclusions on Existing DNS Security .......................8
125 4. DNS COOKIE Option ...............................................8
126 4.1. Client Cookie .............................................10
127 4.2. Server Cookie .............................................10
128 5. DNS Cookies Protocol Specification .............................11
129 5.1. Originating a Request .....................................11
130 5.2. Responding to a Request ...................................11
131 5.2.1. No OPT RR or No COOKIE Option ......................12
132 5.2.2. Malformed COOKIE Option ............................12
133 5.2.3. Only a Client Cookie ...............................12
134 5.2.4. A Client Cookie and an Invalid Server Cookie .......13
135 5.2.5. A Client Cookie and a Valid Server Cookie ..........13
136 5.3. Processing Responses ......................................14
137 5.4. Querying for a Server Cookie ..............................14
138 6. NAT Considerations and Anycast Server Considerations ...........15
139 7. Operational and Deployment Considerations ......................17
140 7.1. Client and Server Secret Rollover .........................17
141 7.2. Counters ..................................................18
142 8. IANA Considerations ............................................18
143 9. Security Considerations ........................................19
144 9.1. Cookie Algorithm Considerations ...........................20
145 10. Implementation Considerations .................................20
146 11. References ....................................................20
147 11.1. Normative References .....................................20
148 11.2. Informative References ...................................21
149 Appendix A. Example Client Cookie Algorithms ......................23
150 A.1. A Simple Algorithm ........................................23
151 A.2. A More Complex Algorithm ..................................23
152 Appendix B. Example Server Cookie Algorithms ......................23
153 B.1. A Simple Algorithm ........................................23
154 B.2. A More Complex Algorithm ..................................24
155 Acknowledgments ...................................................25
156 Authors' Addresses ................................................25
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165
166 1. Introduction
167
168 As with many core Internet protocols, the Domain Name System (DNS)
169 was originally designed at a time when the Internet had only a small
170 pool of trusted users. As the Internet has grown exponentially to a
171 global information utility, the DNS has increasingly been subject to
172 abuse.
173
174 This document describes DNS Cookies, a lightweight DNS transaction
175 security mechanism specified as an OPT [RFC6891] option. The
176 DNS Cookie mechanism provides limited protection to DNS servers and
177 clients against a variety of increasingly common abuses by off-path
178 attackers. It is compatible with, and can be used in conjunction
179 with, other DNS transaction forgery resistance measures such as those
180 in [RFC5452]. (Since DNS Cookies are only returned to the IP address
181 from which they were originally received, they cannot be used to
182 generally track Internet users.)
183
184 The protection provided by DNS Cookies is similar to that provided by
185 using TCP for DNS transactions. Bypassing the weak protection
186 provided by using TCP requires, among other things, that an off-path
187 attacker guess the 32-bit TCP sequence number in use. Bypassing the
188 weak protection provided by DNS Cookies requires such an attacker to
189 guess a 64-bit pseudorandom "cookie" quantity. Where DNS Cookies are
190 not available but TCP is, falling back to using TCP is reasonable.
191
192 If only one party to a DNS transaction supports DNS Cookies, the
193 mechanism does not provide a benefit or significantly interfere, but
194 if both support it, the additional security provided is automatically
195 available.
196
197 The DNS Cookie mechanism is designed to work in the presence of NAT
198 and NAT-PT (Network Address Translation - Protocol Translation)
199 boxes, and guidance is provided herein on supporting the DNS Cookie
200 mechanism in anycast servers.
201
202 1.1. Contents of This Document
203
204 In Section 2, we discuss the threats against which the DNS Cookie
205 mechanism provides some protection.
206
207 Section 3 describes existing DNS security mechanisms and why they are
208 not adequate substitutes for DNS Cookies.
209
210 Section 4 describes the COOKIE option.
211
212 Section 5 provides a protocol description.
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221 Section 6 discusses some NAT considerations and anycast-related
222 DNS Cookies design considerations.
223
224 Section 7 discusses incremental deployment considerations.
225
226 Sections 8 and 9 describe IANA considerations and security
227 considerations, respectively.
228
229 1.2. Definitions
230
231 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
232 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
233 "OPTIONAL" in this document are to be interpreted as described in
234 [RFC2119].
235
236 "Off-path attacker", for a particular DNS client and server, is
237 defined as an attacker who cannot observe the DNS request and
238 response messages between that client and server.
239
240 "Soft state" indicates information that is learned or derived by a
241 host and that may be discarded when indicated by the policies of
242 that host but can be re-instantiated later if needed. For
243 example, it could be discarded after a period of time or when
244 storage for caching such data becomes full. If operations that
245 require soft state continue after the information has been
246 discarded, the information will be automatically regenerated,
247 albeit at some cost.
248
249 "Silently discarded" indicates that there are no DNS protocol message
250 consequences.
251
252 "IP address" is used herein as a length-independent term and includes
253 both IPv4 and IPv6 addresses.
254
255 2. Threats Considered
256
257 DNS Cookies are intended to provide significant but limited
258 protection against certain attacks by off-path attackers, as
259 described below. These attacks include denial of service, cache
260 poisoning, and answer forgery.
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276 2.1. Denial-of-Service Attacks
277
278 The typical form of the denial-of-service attacks considered herein
279 is to send DNS requests with forged source IP addresses to a server.
280 The intent can be to attack that server or some other selected host,
281 as described below.
282
283 There are also on-path denial-of-service attacks that attempt to
284 saturate a server with DNS requests having correct source addresses.
285 Cookies do not protect against such attacks, but successful cookie
286 validation improves the probability that the correct source IP
287 address for the requests is known. This facilitates contacting the
288 managers of the networks from which the requests originate or taking
289 other actions for those networks.
290
291 2.1.1. DNS Amplification Attacks
292
293 A request with a forged source IP address generally causes a response
294 to be sent to that forged IP address. Thus, the forging of many such
295 requests with a particular source IP address can result in enough
296 traffic being sent to the forged IP address to interfere with service
297 to the host at the IP address. Furthermore, it is generally easy in
298 the DNS to create short requests that produce much longer responses,
299 thus amplifying the attack.
300
301 The DNS Cookie mechanism can severely limit the traffic amplification
302 obtained by requests from an attacker that is off the path between
303 the server and the request's source address. Enforced DNS Cookies
304 would make it hard for an off-path attacker to cause any more than
305 rate-limited short error responses to be sent to a forged IP address,
306 so the attack would be attenuated rather than amplified. DNS Cookies
307 make it more effective to implement a rate-limiting scheme for error
308 responses from the server. Such a scheme would further restrict
309 selected host denial-of-service traffic from that server.
310
311 2.1.2. DNS Server Denial of Service
312
313 DNS requests that are accepted cause work on the part of DNS servers.
314 This is particularly true for recursive servers that may issue one or
315 more requests and process the responses thereto, in order to
316 determine their response to the initial request; the situation can be
317 even worse for recursive servers implementing DNSSEC [RFC4033]
318 [RFC4034] [RFC4035], because they may be induced to perform
319 burdensome cryptographic computations in attempts to verify the
320 authenticity of data they retrieve in trying to answer the request.
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331 The computational or communications burden caused by such requests
332 may not depend on a forged source IP address, but the use of such
333 addresses makes
334
335 + the source of the requests causing the denial-of-service attack
336 harder to find and
337
338 + restriction of the IP addresses from which such requests should be
339 honored hard or impossible to specify or verify.
340
341 The use of DNS Cookies should enable a server to reject forged
342 requests from an off-path attacker with relative ease and before any
343 recursive queries or public key cryptographic operations are
344 performed.
345
346 2.2. Cache Poisoning and Answer Forgery Attacks
347
348 The form of the cache poisoning attacks considered is to send forged
349 replies to a resolver. Modern network speeds for well-connected
350 hosts are such that, by forging replies from the IP addresses of a
351 DNS server to a resolver for names that resolver has been induced to
352 resolve or for common names whose resource records have short
353 time-to-live values, there can be an unacceptably high probability of
354 randomly coming up with a reply that will be accepted and cause false
355 DNS information to be cached by that resolver (the Dan Kaminsky
356 attack [Kaminsky]). This can be used to facilitate phishing attacks
357 and other diversions of legitimate traffic to a compromised or
358 malicious host such as a web server.
359
360 With the use of DNS Cookies, a resolver can generally reject such
361 forged replies.
362
363 3. Comments on Existing DNS Security
364
365 Two forms of security have been added to DNS: data security and
366 message/transaction security.
367
368 3.1. Existing DNS Data Security
369
370 DNS data security is one part of DNSSEC and is described in
371 [RFC4033], [RFC4034], [RFC4035], and updates thereto. It provides
372 data origin authentication and authenticated denial of existence.
373 DNSSEC is being deployed and can provide strong protection against
374 forged data and cache poisoning; however, it has the unintended
375 effect of making some denial-of-service attacks worse because of the
376 cryptographic computational load it can require and the increased
377 size in DNS response packets that it tends to produce.
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386 3.2. DNS Message/Transaction Security
387
388 The second form of security that has been added to DNS provides
389 "transaction" security through TSIG [RFC2845] or SIG(0) [RFC2931].
390 TSIG could provide strong protection against the attacks for which
391 the DNS Cookie mechanism provides weaker protection; however, TSIG is
392 non-trivial to deploy in the general Internet because of the burdens
393 it imposes. Among these burdens are pre-agreement and key
394 distribution between client and server, keeping track of server-side
395 key state, and required time synchronization between client and
396 server.
397
398 TKEY [RFC2930] can solve the problem of key distribution for TSIG,
399 but some modes of TKEY impose a substantial cryptographic computation
400 load and can be dependent on the deployment of DNS data security (see
401 Section 3.1).
402
403 SIG(0) [RFC2931] provides less denial-of-service protection than TSIG
404 or, in one way, even DNS Cookies, because it authenticates complete
405 transactions but does not authenticate requests. In any case, it
406 also depends on the deployment of DNS data security and requires
407 computationally burdensome public key cryptographic operations.
408
409 3.3. Conclusions on Existing DNS Security
410
411 The existing DNS security mechanisms do not provide the services
412 provided by the DNS Cookie mechanism: lightweight message
413 authentication of DNS requests and responses with no requirement for
414 pre-configuration or per-client server-side state.
415
416 4. DNS COOKIE Option
417
418 The DNS COOKIE option is an OPT RR [RFC6891] option that can be
419 included in the RDATA portion of an OPT RR in DNS requests and
420 responses. The option length varies, depending on the circumstances
421 in which it is being used. There are two cases, as described below.
422 Both use the same OPTION-CODE; they are distinguished by their
423 length.
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441 In a request sent by a client to a server when the client does not
442 know the server's cookie, its length is 8, consisting of an 8-byte
443 Client Cookie, as shown in Figure 1.
444
445 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
446 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
447 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
448 | OPTION-CODE = 10 | OPTION-LENGTH = 8 |
449 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
450 | |
451 +-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+
452 | |
453 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
454
455 Figure 1: COOKIE Option, Unknown Server Cookie
456
457 In a request sent by a client when a Server Cookie is known, and in
458 all responses to such a request, the length is variable -- from 16 to
459 40 bytes, consisting of an 8-byte Client Cookie followed by the
460 variable-length (8 bytes to 32 bytes) Server Cookie, as shown in
461 Figure 2. The variability of the option length stems from the
462 variable-length Server Cookie. The Server Cookie is an integer
463 number of bytes, with a minimum size of 8 bytes for security and a
464 maximum size of 32 bytes for convenience of implementation.
465
466 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
467 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
468 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
469 | OPTION-CODE = 10 | OPTION-LENGTH >= 16, <= 40 |
470 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
471 | |
472 +-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+
473 | |
474 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
475 | |
476 / Server Cookie (variable size, 8 to 32 bytes) /
477 / /
478 +-+-+-+-...
479
480 Figure 2: COOKIE Option, Known Server Cookie
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496 4.1. Client Cookie
497
498 The Client Cookie SHOULD be a pseudorandom function of the Client IP
499 Address, the Server IP Address, and a secret quantity known only to
500 the client. This Client Secret SHOULD have at least 64 bits of
501 entropy [RFC4086] and be changed periodically (see Section 7.1). The
502 selection of the pseudorandom function is a matter private to the
503 client, as only the client needs to recognize its own DNS Cookies.
504
505 The Client IP Address is included so that the Client Cookie cannot be
506 used to (1) track a client if the Client IP Address changes due to
507 privacy mechanisms or (2) impersonate the client by some network
508 device that was formerly on path but is no longer on path when the
509 Client IP Address changes due to mobility. However, if the Client IP
510 Address is being changed very often, it may be necessary to fix the
511 Client Cookie for a particular server for several requests, to avoid
512 undue inefficiency due to retries caused by that server not
513 recognizing the Client Cookie.
514
515 For further discussion of the Client Cookie field, see Section 5.1.
516 For example methods of determining a Client Cookie, see Appendix A.
517
518 In order to provide minimal authentication, a client MUST send
519 Client Cookies that will usually be different for any two servers at
520 different IP addresses.
521
522 4.2. Server Cookie
523
524 The Server Cookie SHOULD consist of or include a 64-bit or larger
525 pseudorandom function of the request source (client) IP address, a
526 secret quantity known only to the server, and the request
527 Client Cookie. (See Section 6 for a discussion of why the
528 Client Cookie is used as input to the Server Cookie but the
529 Server Cookie is not used as an input to the Client Cookie.) This
530 Server Secret SHOULD have at least 64 bits of entropy [RFC4086] and
531 be changed periodically (see Section 7.1). The selection of the
532 pseudorandom function is a matter private to the server, as only the
533 server needs to recognize its own DNS Cookies.
534
535 For further discussion of the Server Cookie field, see Section 5.2.
536 For example methods of determining a Server Cookie, see Appendix B.
537 When implemented as recommended, the server need not maintain any
538 cookie-related per-client state.
539
540 In order to provide minimal authentication, a server MUST send
541 Server Cookies that will usually be different for clients at any two
542 different IP addresses or with different Client Cookies.
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551 5. DNS Cookies Protocol Specification
552
553 This section discusses using DNS Cookies in the DNS protocol. The
554 cycle of originating a request, responding to that request, and
555 processing responses is covered in Sections 5.1, 5.2, and 5.3. A
556 de facto extension to QUERY to allow the prefetching of a
557 Server Cookie is specified in Section 5.4. Rollover of the Client
558 Secrets and Server Secrets, and transient retention of the old cookie
559 or secret, are covered in Section 7.1.
560
561 DNS clients and servers SHOULD implement DNS Cookies to decrease
562 their vulnerability to the threats discussed in Section 2.
563
564 5.1. Originating a Request
565
566 A DNS client that implements DNS Cookies includes one DNS
567 COOKIE option containing a Client Cookie in every DNS request
568 it sends, unless DNS Cookies are disabled.
569
570 If the client has a cached Server Cookie for the server against its
571 IP address, it uses the longer cookie form and includes that
572 Server Cookie in the option along with the Client Cookie (Figure 2).
573 Otherwise, it just sends the shorter-form option with a Client Cookie
574 (Figure 1).
575
576 5.2. Responding to a Request
577
578 The Server Cookie, when it occurs in a COOKIE option in a request, is
579 intended to weakly assure the server that the request came from a
580 client that is both at the source IP address of the request and using
581 the Client Cookie included in the option. This assurance is provided
582 by the Server Cookie that server sent to that client in an earlier
583 response appearing as the Server Cookie field in the request.
584
585 At a server where DNS Cookies are not implemented and enabled, the
586 presence of a COOKIE option is ignored and the server responds as if
587 no COOKIE option had been included in the request.
588
589 When DNS Cookies are implemented and enabled, there are five
590 possibilities:
591
592 (1) There is no OPT RR at all in the request, or there is an OPT RR
593 but the COOKIE option is absent from the OPT RR.
594
595 (2) A COOKIE option is present but is not a legal length or is
596 otherwise malformed.
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606 (3) There is a COOKIE option of valid length in the request with no
607 Server Cookie.
608
609 (4) There is a COOKIE option of valid length in the request with a
610 Server Cookie, but that Server Cookie is invalid.
611
612 (5) There is a COOKIE option of valid length in the request with a
613 correct Server Cookie.
614
615 These five possibilities are discussed in the subsections below.
616
617 In all cases of multiple COOKIE options in a request, only the first
618 (the one closest to the DNS header) is considered. All others are
619 ignored.
620
621 5.2.1. No OPT RR or No COOKIE Option
622
623 If there is no OPT record or no COOKIE option present in the request,
624 then the server responds to the request as if the server doesn't
625 implement the COOKIE option.
626
627 5.2.2. Malformed COOKIE Option
628
629 If the COOKIE option is too short to contain a Client Cookie, then
630 FORMERR is generated. If the COOKIE option is longer than that
631 required to hold a COOKIE option with just a Client Cookie (8 bytes)
632 but is shorter than the minimum COOKIE option with both a
633 Client Cookie and a Server Cookie (16 bytes), then FORMERR is
634 generated. If the COOKIE option is longer than the maximum valid
635 COOKIE option (40 bytes), then FORMERR is generated.
636
637 In summary, valid cookie lengths are 8 and 16 to 40 inclusive.
638
639 5.2.3. Only a Client Cookie
640
641 Based on server policy, including rate limiting, the server chooses
642 one of the following:
643
644 (1) Silently discard the request.
645
646 (2) Send a BADCOOKIE error response.
647
648 (3) Process the request and provide a normal response. The RCODE is
649 NOERROR, unless some non-cookie error occurs in processing the
650 request.
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661 If the server responds choosing (2) or (3) above, it SHALL generate
662 its own COOKIE option containing both the Client Cookie copied from
663 the request and a Server Cookie it has generated, and it will add
664 this COOKIE option to the response's OPT record. Servers MUST, at
665 least occasionally, respond to such requests to inform the client of
666 the correct Server Cookie. This is necessary so that such a client
667 can bootstrap to the more secure state where requests and responses
668 have recognized Server Cookies and Client Cookies. A server is not
669 expected to maintain per-client state to achieve this. For example,
670 it could respond to every Nth request across all clients.
671
672 If the request was received over TCP, the server SHOULD take the
673 authentication provided by the use of TCP into account and SHOULD
674 choose (3). In this case, if the server is not willing to accept the
675 security provided by TCP as a substitute for the security provided by
676 DNS Cookies but instead chooses (2), there is some danger of an
677 indefinite loop of retries (see Section 5.3).
678
679 5.2.4. A Client Cookie and an Invalid Server Cookie
680
681 The server examines the Server Cookie to determine if it is a valid
682 Server Cookie that it had generated previously. This determination
683 normally involves recalculating the Server Cookie (or the Hash part
684 thereof) based on the Server Secret (or the previous Server Secret,
685 if it has just changed); the received Client Cookie; the Client IP
686 Address; and, possibly, other fields. See Appendix B.2 for an
687 example. If the cookie is invalid, it could be because
688
689 + it is too old
690
691 + a client's IP address or Client Cookie changed, and the DNS server
692 is not aware of the change
693
694 + an anycast cluster of servers is not consistently configured, or
695
696 + an attempt to spoof the client has occurred
697
698 The server SHALL process the request as if the invalid Server Cookie
699 was not present, as described in Section 5.2.3.
700
701 5.2.5. A Client Cookie and a Valid Server Cookie
702
703 When a valid Server Cookie is present in the request, the server can
704 assume that the request is from a client that it has talked to before
705 and defensive measures for spoofed UDP requests, if any, are no
706 longer required.
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716 The server SHALL process the request and include a COOKIE option in
717 the response by (a) copying the complete COOKIE option from the
718 request or (b) generating a new COOKIE option containing both the
719 Client Cookie copied from the request and a valid Server Cookie it
720 has generated.
721
722 5.3. Processing Responses
723
724 The Client Cookie, when it occurs in a COOKIE option in a DNS reply,
725 is intended to weakly assure the client that the reply came from a
726 server at the source IP address used in the response packet, because
727 the Client Cookie value is the value that client would send to that
728 server in a request. In a DNS reply with multiple COOKIE options,
729 all but the first (the one closest to the DNS header) are ignored.
730
731 A DNS client where DNS Cookies are implemented and enabled examines
732 the response for DNS Cookies and MUST discard the response if it
733 contains an illegal COOKIE option length or an incorrect
734 Client Cookie value. If the client is expecting the response to
735 contain a COOKIE option and it is missing, the response MUST be
736 discarded. If the COOKIE option Client Cookie is correct, the client
737 caches the Server Cookie provided, even if the response is an error
738 response (RCODE non-zero).
739
740 If the extended RCODE in the reply is BADCOOKIE and the Client Cookie
741 in the reply matches what was sent, it means that the server was
742 unwilling to process the request because it did not have the correct
743 Server Cookie in it. The client SHOULD retry the request using the
744 new Server Cookie from the response. Repeated BADCOOKIE responses to
745 requests that use the Server Cookie provided in the previous response
746 may be an indication that either the shared secrets or the method for
747 generating secrets in an anycast cluster of servers is inconsistent.
748 If the reply to a retried request with a fresh Server Cookie is
749 BADCOOKIE, the client SHOULD retry using TCP as the transport, since
750 the server will likely process the request normally based on the
751 security provided by TCP (see Section 5.2.3).
752
753 If the RCODE is some value other than BADCOOKIE, including zero, the
754 further processing of the response proceeds normally.
755
756 5.4. Querying for a Server Cookie
757
758 In many cases, a client will learn the Server Cookie for a server as
759 the "side effect" of another transaction; however, there may be times
760 when this is not desirable. Therefore, a means is provided for
761 obtaining a Server Cookie through an extension to the QUERY opcode
762 for which opcode most existing implementations require that QDCOUNT
763 be one (1) (see Section 4.1.2 of [RFC1035]).
764
765
766
767 Eastlake & Andrews Standards Track [Page 14]
768 RFC 7873 DNS Cookies May 2016
769
770
771 For servers with DNS Cookies enabled, the QUERY opcode behavior is
772 extended to support queries with an empty Question Section (a QDCOUNT
773 of zero (0)), provided that an OPT record is present with a COOKIE
774 option. Such servers will send a reply that has an empty
775 Answer Section and has a COOKIE option containing the Client Cookie
776 and a valid Server Cookie.
777
778 If such a query provided just a Client Cookie and no Server Cookie,
779 the response SHALL have the RCODE NOERROR.
780
781 This mechanism can also be used to confirm/re-establish an existing
782 Server Cookie by sending a cached Server Cookie with the
783 Client Cookie. In this case, the response SHALL have the RCODE
784 BADCOOKIE if the Server Cookie sent with the query was invalid and
785 the RCODE NOERROR if it was valid.
786
787 Servers that don't support the COOKIE option will normally send
788 FORMERR in response to such a query, though REFUSED, NOTIMP, and
789 NOERROR without a COOKIE option are also possible in such responses.
790
791 6. NAT Considerations and Anycast Server Considerations
792
793 In the classic Internet, DNS Cookies could simply be a pseudorandom
794 function of the Client IP Address and a Server Secret or the Server
795 IP Address and a Client Secret. You would want to compute the
796 Server Cookie that way, so a client could cache its Server Cookie for
797 a particular server for an indefinite amount of time and the server
798 could easily regenerate and check it. You could consider the
799 Client Cookie to be a weak client signature over the Server IP
800 Address that the client checks in replies, and you could extend this
801 signature to cover the request ID, for example, or any other
802 information that is returned unchanged in the reply.
803
804 But we have this reality called "NAT" [RFC3022] (including, for the
805 purposes of this document, NAT-PT, which has been declared Historic
806 [RFC4966]). There is no problem with DNS transactions between
807 clients and servers behind a NAT box using local IP addresses. Nor
808 is there a problem with NAT translation of internal addresses to
809 external addresses or translations between IPv4 and IPv6 addresses,
810 as long as the address mapping is relatively stable. Should the
811 external IP address to which an internal client is being mapped
812 change occasionally, the disruption is little more than when a client
813 rolls over its COOKIE secret. Also, external access to a DNS server
814 behind a NAT box is normally handled by a fixed mapping that forwards
815 externally received DNS requests to a specific host.
816
817
818
819
820
821
822 Eastlake & Andrews Standards Track [Page 15]
823 RFC 7873 DNS Cookies May 2016
824
825
826 However, NAT devices sometimes also map ports. This can cause
827 multiple DNS requests and responses from multiple internal hosts to
828 be mapped to a smaller number of external IP addresses, such as one
829 address. Thus, there could be many clients behind a NAT box that
830 appear to come from the same source IP address to a server outside
831 that NAT box. If one of these were an attacker (think "zombie" or
832 "botnet") behind a NAT box, that attacker could get the Server Cookie
833 for some server for the outgoing IP address by just making some
834 random request to that server. It could then include that
835 Server Cookie in the COOKIE option of requests to the server with the
836 forged local IP address of some other host and/or client behind the
837 NAT box. (An attacker's possession of this Server Cookie will not
838 help in forging responses to cause cache poisoning, as such responses
839 are protected by the required Client Cookie.)
840
841 To fix this potential defect, it is necessary to distinguish
842 different clients behind a NAT box from the point of view of the
843 server. This is why the Server Cookie is specified as a pseudorandom
844 function of both the request source IP address and the Client Cookie.
845 From this inclusion of the Client Cookie in the calculation of the
846 Server Cookie, it follows that, for any particular server, a stable
847 Client Cookie is needed. If, for example, the request ID was
848 included in the calculation of the Client Cookie, it would normally
849 change with each request to a particular server. This would mean
850 that each request would have to be sent twice: first, to learn the
851 new Server Cookie based on this new Client Cookie based on the new
852 ID, and then again using this new Client Cookie to actually get an
853 answer. Thus, the input to the Client Cookie computation must be
854 limited to the Server IP Address and one or more things that change
855 slowly, such as the Client Secret.
856
857 In principle, there could be a similar problem for servers, not due
858 to NAT but due to mechanisms like anycast that may cause requests to
859 a DNS server at an IP address to be delivered to any one of several
860 machines. (External requests to a DNS server behind a NAT box
861 usually occur via port forwarding such that all such requests go to
862 one host.) However, it is impossible to solve this in the way that
863 the similar problem was solved for NATed clients; if the
864 Server Cookie was included in the calculation of the Client Cookie in
865 the same way that the Client Cookie is included in the Server Cookie,
866 you would just get an almost infinite series of errors as a request
867 was repeatedly retried.
868
869 For servers accessed via anycast, to successfully support
870 DNS Cookies, either (1) the server clones must all use the same
871 Server Secret or (2) the mechanism that distributes requests to the
872 server clones must cause the requests from a particular client to go
873 to a particular server for a sufficiently long period of time that
874
875
876
877 Eastlake & Andrews Standards Track [Page 16]
878 RFC 7873 DNS Cookies May 2016
879
880
881 extra requests due to changes in Server Cookies resulting from
882 accessing different server machines are not unduly burdensome. (When
883 such anycast-accessed servers act as recursive servers or otherwise
884 act as clients, they normally use a different unique address to
885 source their requests, to avoid confusion in the delivery of
886 responses.)
887
888 For simplicity, it is RECOMMENDED that the same Server Secret be used
889 by each DNS server in a set of anycast servers. If there is limited
890 time skew in updating this secret in different anycast servers, this
891 can be handled by a server accepting requests containing a
892 Server Cookie based on either its old or new secret for the maximum
893 likely time period of such time skew (see also Section 7.1).
894
895 7. Operational and Deployment Considerations
896
897 The DNS Cookie mechanism is designed for incremental deployment and
898 to complement the orthogonal techniques in [RFC5452]. Either or both
899 techniques can be deployed independently at each DNS server and
900 client. Thus, installation at the client and server end need not be
901 synchronized.
902
903 In particular, a DNS server or client that implements the DNS Cookie
904 mechanism can interoperate successfully with a DNS client or server
905 that does not implement this mechanism, although, of course, in this
906 case it will not get the benefit of the mechanism and the server
907 involved might choose to severely rate-limit responses. When such a
908 server or client interoperates with a client or server that also
909 implements the DNS Cookie mechanism, these servers and clients get
910 the security benefits of the DNS Cookie mechanism.
911
912 7.1. Client and Server Secret Rollover
913
914 The longer a secret is used, the higher the probability that it has
915 been compromised. Thus, clients and servers are configured with a
916 lifetime setting for their secret, and they roll over to a new secret
917 when that lifetime expires, or earlier due to deliberate jitter as
918 described below. The default lifetime is one day, and the maximum
919 permitted is one month. To be precise and to make it practical to
920 stay within limits despite long holiday weekends, daylight saving
921 time shifts, and the like, clients and servers MUST NOT continue to
922 use the same secret in new requests and responses for more than
923 36 days and SHOULD NOT continue to do so for more than 26 hours.
924
925 Many clients rolling over their secret at the same time could briefly
926 increase server traffic, and exactly predictable rollover times for
927 clients or servers might facilitate guessing attacks. For example,
928 an attacker might increase the priority of attacking secrets they
929
930
931
932 Eastlake & Andrews Standards Track [Page 17]
933 RFC 7873 DNS Cookies May 2016
934
935
936 believe will be in effect for an extended period of time. To avoid
937 rollover synchronization and predictability, it is RECOMMENDED that
938 pseudorandom jitter in the range of plus zero to minus at least 40%
939 be applied to the time until a scheduled rollover of a COOKIE secret.
940
941 It is RECOMMENDED that a client keep the Client Cookie it is
942 expecting in a reply until there is no longer an outstanding request
943 associated with that Client Cookie that the client is tracking. This
944 avoids rejection of replies due to a bad Client Cookie right after a
945 change in the Client Secret.
946
947 It is RECOMMENDED that a server retain its previous secret after a
948 rollover to a new secret for a configurable period of time not less
949 than 1 second or more than 300 seconds, with a default configuration
950 of 150 seconds. Requests with Server Cookies based on its previous
951 secret are treated as a correct Server Cookie during that time. When
952 a server responds to a request containing an old Server Cookie that
953 the server is treating as correct, the server MUST include a new
954 Server Cookie in its response.
955
956 7.2. Counters
957
958 It is RECOMMENDED that implementations include counters of the
959 occurrences of the various types of requests and responses described
960 in Section 5.
961
962 8. IANA Considerations
963
964 IANA has assigned the following DNS EDNS0 option code:
965
966 Value Name Status Reference
967 -------- ------ -------- ---------------
968 10 COOKIE Standard RFC 7873
969
970 IANA has assigned the following DNS error code as an early allocation
971 per [RFC7120]:
972
973 RCODE Name Description Reference
974 -------- --------- ------------------------- ---------------
975 23 BADCOOKIE Bad/missing Server Cookie RFC 7873
976
977
978
979
980
981
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987 Eastlake & Andrews Standards Track [Page 18]
988 RFC 7873 DNS Cookies May 2016
989
990
991 9. Security Considerations
992
993 DNS Cookies provide a weak form of authentication of DNS requests and
994 responses. In particular, they provide no protection against
995 "on-path" adversaries; that is, they provide no protection against
996 any adversary that can observe the plaintext DNS traffic, such as an
997 on-path router, bridge, or any device on an on-path shared link
998 (unless the DNS traffic in question on that path is encrypted).
999
1000 For example, if a host is connected via an unsecured IEEE Std. 802.11
1001 link (Wi-Fi), any device in the vicinity that could receive and
1002 decode the 802.11 transmissions must be considered "on path". On the
1003 other hand, in a similar situation but one where 802.11 Robust
1004 Security (WPA2, also called "Wi-Fi Protected Access 2") is
1005 appropriately deployed on the Wi-Fi network nodes, only the
1006 Access Point via which the host is connecting is "on path" as far as
1007 the 802.11 link is concerned.
1008
1009 Despite these limitations, deployment of DNS Cookies on the global
1010 Internet is expected to provide a significant reduction in the
1011 available launch points for the traffic amplification and denial-of-
1012 service forgery attacks described in Section 2 above.
1013
1014 Work is underway in the IETF DPRIVE working group to provide
1015 confidentiality for DNS requests and responses that would be
1016 compatible with DNS Cookies.
1017
1018 Should stronger message/transaction security be desired, it is
1019 suggested that TSIG or SIG(0) security be used (see Section 3.2);
1020 however, it may be useful to use DNS Cookies in conjunction with
1021 these features. In particular, DNS Cookies could screen out many DNS
1022 messages before the cryptographic computations of TSIG or SIG(0) are
1023 required, and if SIG(0) is in use, DNS Cookies could usefully screen
1024 out many requests given that SIG(0) does not screen requests but only
1025 authenticates the response of complete transactions.
1026
1027 An attacker that does not know the Server Cookie could do a variety
1028 of things, such as omitting the COOKIE option or sending a random
1029 Server Cookie. In general, DNS servers need to take other measures,
1030 including rate-limiting responses, to protect from abuse in such
1031 cases. See further information in Section 5.2.
1032
1033 When a server or client starts receiving an increased level of
1034 requests with bad Server Cookies or replies with bad Client Cookies,
1035 it would be reasonable for it to believe that it is likely under
1036 attack, and it should consider a more frequent rollover of its
1037 secret. More rapid rollover decreases the benefit to a
1038 cookie-guessing attacker if they succeed in guessing a cookie.
1039
1040
1041
1042 Eastlake & Andrews Standards Track [Page 19]
1043 RFC 7873 DNS Cookies May 2016
1044
1045
1046 9.1. Cookie Algorithm Considerations
1047
1048 The cookie computation algorithm for use in DNS Cookies SHOULD be
1049 based on a pseudorandom function at least as strong as 64-bit FNV
1050 (Fowler/Noll/Vo [FNV]), because an excessively weak or trivial
1051 algorithm could enable adversaries to guess cookies. However, in
1052 light of the lightweight plaintext token security provided by
1053 DNS Cookies, a strong cryptography hash algorithm may not be
1054 warranted in many cases and would cause an increased computational
1055 burden. Nevertheless, there is nothing wrong with using something
1056 stronger -- for example, HMAC-SHA-256 [RFC6234] truncated to 64 bits,
1057 assuming that a DNS processor has adequate computational resources
1058 available. DNS implementations or applications that need somewhat
1059 stronger security without a significant increase in computational
1060 load should consider more frequent changes in their client and/or
1061 Server Secret; however, this does require more frequent generation of
1062 a cryptographically strong random number [RFC4086]. See Appendices A
1063 and B for specific examples of cookie computation algorithms.
1064
1065 10. Implementation Considerations
1066
1067 The DNS COOKIE option specified herein is implemented in BIND 9.10
1068 using an experimental option code. BIND 9.10.3 (and later) use the
1069 allocated option code.
1070
1071 11. References
1072
1073 11.1. Normative References
1074
1075 [RFC1035] Mockapetris, P., "Domain names - implementation and
1076 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
1077 November 1987, <http://www.rfc-editor.org/info/rfc1035>.
1078
1079 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
1080 Requirement Levels", BCP 14, RFC 2119,
1081 DOI 10.17487/RFC2119, March 1997,
1082 <http://www.rfc-editor.org/info/rfc2119>.
1083
1084 [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
1085 "Randomness Requirements for Security", BCP 106, RFC 4086,
1086 DOI 10.17487/RFC4086, June 2005,
1087 <http://www.rfc-editor.org/info/rfc4086>.
1088
1089
1090
1091
1092
1093
1094
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1097 Eastlake & Andrews Standards Track [Page 20]
1098 RFC 7873 DNS Cookies May 2016
1099
1100
1101 [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
1102 for DNS (EDNS(0))", STD 75, RFC 6891,
1103 DOI 10.17487/RFC6891, April 2013,
1104 <http://www.rfc-editor.org/info/rfc6891>.
1105
1106 [RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code
1107 Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120,
1108 January 2014, <http://www.rfc-editor.org/info/rfc7120>.
1109
1110 11.2. Informative References
1111
1112 [FNV] Fowler, G., Noll, L., Vo, K., and D. Eastlake 3rd, "The
1113 FNV Non-Cryptographic Hash Algorithm", Work in Progress,
1114 draft-eastlake-fnv-10, October 2015.
1115
1116 [Kaminsky] Olney, M., Mullen, P., and K. Miklavcic, "Dan Kaminsky's
1117 2008 DNS Vulnerability", July 2008, <https://www.ietf.org/
1118 mail-archive/web/dnsop/current/pdf2jgx6rzxN4.pdf>.
1119
1120 [RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
1121 Wellington, "Secret Key Transaction Authentication for DNS
1122 (TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000,
1123 <http://www.rfc-editor.org/info/rfc2845>.
1124
1125 [RFC2930] Eastlake 3rd, D., "Secret Key Establishment for DNS
1126 (TKEY RR)", RFC 2930, DOI 10.17487/RFC2930,
1127 September 2000, <http://www.rfc-editor.org/info/rfc2930>.
1128
1129 [RFC2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures
1130 ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931,
1131 September 2000, <http://www.rfc-editor.org/info/rfc2931>.
1132
1133 [RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
1134 Address Translator (Traditional NAT)", RFC 3022,
1135 DOI 10.17487/RFC3022, January 2001,
1136 <http://www.rfc-editor.org/info/rfc3022>.
1137
1138 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
1139 Rose, "DNS Security Introduction and Requirements",
1140 RFC 4033, DOI 10.17487/RFC4033, March 2005,
1141 <http://www.rfc-editor.org/info/rfc4033>.
1142
1143 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
1144 Rose, "Resource Records for the DNS Security Extensions",
1145 RFC 4034, DOI 10.17487/RFC4034, March 2005,
1146 <http://www.rfc-editor.org/info/rfc4034>.
1147
1148
1149
1150
1151
1152 Eastlake & Andrews Standards Track [Page 21]
1153 RFC 7873 DNS Cookies May 2016
1154
1155
1156 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
1157 Rose, "Protocol Modifications for the DNS Security
1158 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
1159 <http://www.rfc-editor.org/info/rfc4035>.
1160
1161 [RFC4966] Aoun, C. and E. Davies, "Reasons to Move the Network
1162 Address Translator - Protocol Translator (NAT-PT) to
1163 Historic Status", RFC 4966, DOI 10.17487/RFC4966,
1164 July 2007, <http://www.rfc-editor.org/info/rfc4966>.
1165
1166 [RFC5452] Hubert, A. and R. van Mook, "Measures for Making DNS
1167 More Resilient against Forged Answers", RFC 5452,
1168 DOI 10.17487/RFC5452, January 2009,
1169 <http://www.rfc-editor.org/info/rfc5452>.
1170
1171 [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
1172 (SHA and SHA-based HMAC and HKDF)", RFC 6234,
1173 DOI 10.17487/RFC6234, May 2011,
1174 <http://www.rfc-editor.org/info/rfc6234>.
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
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1207 Eastlake & Andrews Standards Track [Page 22]
1208 RFC 7873 DNS Cookies May 2016
1209
1210
1211 Appendix A. Example Client Cookie Algorithms
1212
1213 A.1. A Simple Algorithm
1214
1215 A simple example method to compute Client Cookies is the FNV64 [FNV]
1216 of the Client IP Address, the Server IP Address, and the Client
1217 Secret:
1218
1219 Client Cookie =
1220 FNV64( Client IP Address | Server IP Address | Client Secret )
1221
1222 where "|" indicates concatenation. Some computational resources may
1223 be saved by pre-computing FNV64 through the Client IP Address. (If
1224 the order of the items concatenated above is changed to put the
1225 Server IP Address last, it might be possible to further reduce the
1226 computational effort by pre-computing FNV64 through the bytes of both
1227 the Client IP Address and the Client Secret, but this would reduce
1228 the strength of the Client Cookie and is NOT RECOMMENDED.)
1229
1230 A.2. A More Complex Algorithm
1231
1232 A more complex algorithm to calculate Client Cookies is given below.
1233 It uses more computational resources than the simpler algorithm shown
1234 in Appendix A.1.
1235
1236 Client Cookie =
1237 HMAC-SHA256-64( Client IP Address | Server IP Address,
1238 Client Secret )
1239
1240 Appendix B. Example Server Cookie Algorithms
1241
1242 B.1. A Simple Algorithm
1243
1244 An example of a simple method producing a 64-bit Server Cookie is the
1245 FNV64 [FNV] of the request IP address, the Client Cookie, and the
1246 Server Secret.
1247
1248 Server Cookie =
1249 FNV64( Client IP Address | Client Cookie | Server Secret )
1250
1251 where "|" represents concatenation. (If the order of the items
1252 concatenated was changed, it might be possible to reduce the
1253 computational effort by pre-computing FNV64 through the bytes of the
1254 Server Secret and Client Cookie, but this would reduce the strength
1255 of the Server Cookie and is NOT RECOMMENDED.)
1256
1257
1258
1259
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1262 Eastlake & Andrews Standards Track [Page 23]
1263 RFC 7873 DNS Cookies May 2016
1264
1265
1266 B.2. A More Complex Algorithm
1267
1268 Since the Server Cookie has a variable size, the server can store
1269 various information in that field as long as it is hard for an
1270 adversary to guess the entire quantity used for authentication.
1271 There should be 64 bits of entropy in the Server Cookie; for example,
1272 it could have a sub-field of 64 bits computed pseudorandomly with the
1273 Server Secret as one of the inputs to the pseudorandom function.
1274 Types of additional information that could be stored include a
1275 timestamp and/or a nonce.
1276
1277 The example below is one variation of the Server Cookie that has been
1278 implemented in BIND 9.10.3 (and later) releases, where the
1279 Server Cookie is 128 bits, composed as follows:
1280
1281 Sub-field Size
1282 --------- ---------
1283 Nonce 32 bits
1284 Time 32 bits
1285 Hash 64 bits
1286
1287 With this algorithm, the server sends a new 128-bit cookie back with
1288 every request. The Nonce field assures a low probability that there
1289 would be a duplicate.
1290
1291 The Time field gives the server time and makes it easy to reject old
1292 cookies.
1293
1294 The Hash part of the Server Cookie is the part that is hard to guess.
1295 In BIND 9.10.3 (and later), its computation can be configured to use
1296 AES, HMAC-SHA-1, or, as shown below, HMAC-SHA-256:
1297
1298 hash =
1299 HMAC-SHA256-64( Server Secret,
1300 (Client Cookie | Nonce | Time | Client IP Address) )
1301
1302 where "|" represents concatenation.
1303
1304
1305
1306
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1317 Eastlake & Andrews Standards Track [Page 24]
1318 RFC 7873 DNS Cookies May 2016
1319
1320
1321 Acknowledgments
1322
1323 The suggestions and contributions of the following are gratefully
1324 acknowledged:
1325
1326 Alissa Cooper, Bob Harold, Paul Hoffman, David Malone, Yoav Nir,
1327 Gayle Noble, Dan Romascanu, Tim Wicinski, and Peter Yee
1328
1329 Authors' Addresses
1330
1331 Donald E. Eastlake 3rd
1332 Huawei Technologies
1333 155 Beaver Street
1334 Milford, MA 01757
1335 United States
1336
1337 Phone: +1-508-333-2270
1338 Email: d3e3e3@gmail.com
1339
1340
1341 Mark Andrews
1342 Internet Systems Consortium
1343 950 Charter Street
1344 Redwood City, CA 94063
1345 United States
1346
1347 Email: marka@isc.org
1348
1349
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1373
The IETF is responsible for the creation and maintenance of the DNS RFCs. The ICANN DNS RFC annotation project provides a forum for collecting community annotations on these RFCs as an aid to understanding for implementers and any interested parties. The annotations displayed here are not the result of the IETF consensus process.
This RFC is included in the DNS RFCs annotation project whose home page is here.
RFC9018 is a significant update throughout this document. As the abstract of RFC 9018 says:
This document updates RFC 7873 with precise directions for creating Server Cookies so that an anycast server set including diverse implementations will interoperate with standard clients, with suggestions for constructing Client Cookies in a privacy-preserving fashion, and with suggestions on how to update a Server Secret.