7Internet Engineering Task Force (IETF) D. Eastlake 3rd
8Request for Comments: 7873 Huawei
9Category: Standards Track M. Andrews
14 Domain Name System (DNS) Cookies
18 DNS Cookies are a lightweight DNS transaction security mechanism that
19 provides limited protection to DNS servers and clients against a
20 variety of increasingly common denial-of-service and amplification/
21 forgery or cache poisoning attacks by off-path attackers. DNS
22 Cookies are tolerant of NAT, NAT-PT (Network Address Translation -
23 Protocol Translation), and anycast and can be incrementally deployed.
24 (Since DNS Cookies are only returned to the IP address from which
25 they were originally received, they cannot be used to generally track
30 This is an Internet Standards Track document.
32 This document is a product of the Internet Engineering Task Force
33 (IETF). It represents the consensus of the IETF community. It has
34 received public review and has been approved for publication by the
35 Internet Engineering Steering Group (IESG). Further information on
36 Internet Standards is available in Section 2 of RFC 7841.
38 Information about the current status of this document, any errata,
39 and how to provide feedback on it may be obtained at
40 http://www.rfc-editor.org/info/rfc7873.
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65 Copyright (c) 2016 IETF Trust and the persons identified as the
66 document authors. All rights reserved.
68 This document is subject to BCP 78 and the IETF Trust's Legal
69 Provisions Relating to IETF Documents
70 (http://trustee.ietf.org/license-info) in effect on the date of
71 publication of this document. Please review these documents
72 carefully, as they describe your rights and restrictions with respect
73 to this document. Code Components extracted from this document must
74 include Simplified BSD License text as described in Section 4.e of
75 the Trust Legal Provisions and are provided without warranty as
76 described in the Simplified BSD License.
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121 1. Introduction ....................................................4
122 1.1. Contents of This Document ..................................4
123 1.2. Definitions ................................................5
124 2. Threats Considered ..............................................5
125 2.1. Denial-of-Service Attacks ..................................6
126 2.1.1. DNS Amplification Attacks ...........................6
127 2.1.2. DNS Server Denial of Service ........................6
128 2.2. Cache Poisoning and Answer Forgery Attacks .................7
129 3. Comments on Existing DNS Security ...............................7
130 3.1. Existing DNS Data Security .................................7
131 3.2. DNS Message/Transaction Security ...........................8
132 3.3. Conclusions on Existing DNS Security .......................8
133 4. DNS COOKIE Option ...............................................8
134 4.1. Client Cookie .............................................10
135 4.2. Server Cookie .............................................10
136 5. DNS Cookies Protocol Specification .............................11
137 5.1. Originating a Request .....................................11
138 5.2. Responding to a Request ...................................11
139 5.2.1. No OPT RR or No COOKIE Option ......................12
140 5.2.2. Malformed COOKIE Option ............................12
141 5.2.3. Only a Client Cookie ...............................12
142 5.2.4. A Client Cookie and an Invalid Server Cookie .......13
143 5.2.5. A Client Cookie and a Valid Server Cookie ..........13
144 5.3. Processing Responses ......................................14
145 5.4. Querying for a Server Cookie ..............................14
146 6. NAT Considerations and Anycast Server Considerations ...........15
147 7. Operational and Deployment Considerations ......................17
148 7.1. Client and Server Secret Rollover .........................17
149 7.2. Counters ..................................................18
150 8. IANA Considerations ............................................18
151 9. Security Considerations ........................................19
152 9.1. Cookie Algorithm Considerations ...........................20
153 10. Implementation Considerations .................................20
154 11. References ....................................................20
155 11.1. Normative References .....................................20
156 11.2. Informative References ...................................21
157 Appendix A. Example Client Cookie Algorithms ......................23
158 A.1. A Simple Algorithm ........................................23
159 A.2. A More Complex Algorithm ..................................23
160 Appendix B. Example Server Cookie Algorithms ......................23
161 B.1. A Simple Algorithm ........................................23
162 B.2. A More Complex Algorithm ..................................24
163 Acknowledgments ...................................................25
164 Authors' Addresses ................................................25
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177 As with many core Internet protocols, the Domain Name System (DNS)
178 was originally designed at a time when the Internet had only a small
179 pool of trusted users. As the Internet has grown exponentially to a
180 global information utility, the DNS has increasingly been subject to
183 This document describes DNS Cookies, a lightweight DNS transaction
184 security mechanism specified as an OPT [RFC6891] option. The
185 DNS Cookie mechanism provides limited protection to DNS servers and
186 clients against a variety of increasingly common abuses by off-path
187 attackers. It is compatible with, and can be used in conjunction
188 with, other DNS transaction forgery resistance measures such as those
189 in [RFC5452]. (Since DNS Cookies are only returned to the IP address
190 from which they were originally received, they cannot be used to
191 generally track Internet users.)
193 The protection provided by DNS Cookies is similar to that provided by
194 using TCP for DNS transactions. Bypassing the weak protection
195 provided by using TCP requires, among other things, that an off-path
196 attacker guess the 32-bit TCP sequence number in use. Bypassing the
197 weak protection provided by DNS Cookies requires such an attacker to
198 guess a 64-bit pseudorandom "cookie" quantity. Where DNS Cookies are
199 not available but TCP is, falling back to using TCP is reasonable.
201 If only one party to a DNS transaction supports DNS Cookies, the
202 mechanism does not provide a benefit or significantly interfere, but
203 if both support it, the additional security provided is automatically
206 The DNS Cookie mechanism is designed to work in the presence of NAT
207 and NAT-PT (Network Address Translation - Protocol Translation)
208 boxes, and guidance is provided herein on supporting the DNS Cookie
209 mechanism in anycast servers.
2111.1. Contents of This Document
213 In Section 2, we discuss the threats against which the DNS Cookie
214 mechanism provides some protection.
216 Section 3 describes existing DNS security mechanisms and why they are
217 not adequate substitutes for DNS Cookies.
219 Section 4 describes the COOKIE option.
221 Section 5 provides a protocol description.
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231 Section 6 discusses some NAT considerations and anycast-related
232 DNS Cookies design considerations.
234 Section 7 discusses incremental deployment considerations.
236 Sections 8 and 9 describe IANA considerations and security
237 considerations, respectively.
241 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
242 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
243 "OPTIONAL" in this document are to be interpreted as described in
246 "Off-path attacker", for a particular DNS client and server, is
247 defined as an attacker who cannot observe the DNS request and
248 response messages between that client and server.
250 "Soft state" indicates information that is learned or derived by a
251 host and that may be discarded when indicated by the policies of
252 that host but can be re-instantiated later if needed. For
253 example, it could be discarded after a period of time or when
254 storage for caching such data becomes full. If operations that
255 require soft state continue after the information has been
256 discarded, the information will be automatically regenerated,
259 "Silently discarded" indicates that there are no DNS protocol message
262 "IP address" is used herein as a length-independent term and includes
263 both IPv4 and IPv6 addresses.
267 DNS Cookies are intended to provide significant but limited
268 protection against certain attacks by off-path attackers, as
269 described below. These attacks include denial of service, cache
270 poisoning, and answer forgery.
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2872.1. Denial-of-Service Attacks
289 The typical form of the denial-of-service attacks considered herein
290 is to send DNS requests with forged source IP addresses to a server.
291 The intent can be to attack that server or some other selected host,
294 There are also on-path denial-of-service attacks that attempt to
295 saturate a server with DNS requests having correct source addresses.
296 Cookies do not protect against such attacks, but successful cookie
297 validation improves the probability that the correct source IP
298 address for the requests is known. This facilitates contacting the
299 managers of the networks from which the requests originate or taking
300 other actions for those networks.
3022.1.1. DNS Amplification Attacks
304 A request with a forged source IP address generally causes a response
305 to be sent to that forged IP address. Thus, the forging of many such
306 requests with a particular source IP address can result in enough
307 traffic being sent to the forged IP address to interfere with service
308 to the host at the IP address. Furthermore, it is generally easy in
309 the DNS to create short requests that produce much longer responses,
310 thus amplifying the attack.
312 The DNS Cookie mechanism can severely limit the traffic amplification
313 obtained by requests from an attacker that is off the path between
314 the server and the request's source address. Enforced DNS Cookies
315 would make it hard for an off-path attacker to cause any more than
316 rate-limited short error responses to be sent to a forged IP address,
317 so the attack would be attenuated rather than amplified. DNS Cookies
318 make it more effective to implement a rate-limiting scheme for error
319 responses from the server. Such a scheme would further restrict
320 selected host denial-of-service traffic from that server.
3222.1.2. DNS Server Denial of Service
324 DNS requests that are accepted cause work on the part of DNS servers.
325 This is particularly true for recursive servers that may issue one or
326 more requests and process the responses thereto, in order to
327 determine their response to the initial request; the situation can be
328 even worse for recursive servers implementing DNSSEC [RFC4033]
329 [RFC4034] [RFC4035], because they may be induced to perform
330 burdensome cryptographic computations in attempts to verify the
331 authenticity of data they retrieve in trying to answer the request.
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343 The computational or communications burden caused by such requests
344 may not depend on a forged source IP address, but the use of such
347 + the source of the requests causing the denial-of-service attack
350 + restriction of the IP addresses from which such requests should be
351 honored hard or impossible to specify or verify.
353 The use of DNS Cookies should enable a server to reject forged
354 requests from an off-path attacker with relative ease and before any
355 recursive queries or public key cryptographic operations are
3582.2. Cache Poisoning and Answer Forgery Attacks
360 The form of the cache poisoning attacks considered is to send forged
361 replies to a resolver. Modern network speeds for well-connected
362 hosts are such that, by forging replies from the IP addresses of a
363 DNS server to a resolver for names that resolver has been induced to
364 resolve or for common names whose resource records have short
365 time-to-live values, there can be an unacceptably high probability of
366 randomly coming up with a reply that will be accepted and cause false
367 DNS information to be cached by that resolver (the Dan Kaminsky
368 attack [Kaminsky]). This can be used to facilitate phishing attacks
369 and other diversions of legitimate traffic to a compromised or
370 malicious host such as a web server.
372 With the use of DNS Cookies, a resolver can generally reject such
3753. Comments on Existing DNS Security
377 Two forms of security have been added to DNS: data security and
378 message/transaction security.
3803.1. Existing DNS Data Security
382 DNS data security is one part of DNSSEC and is described in
383 [RFC4033], [RFC4034], [RFC4035], and updates thereto. It provides
384 data origin authentication and authenticated denial of existence.
385 DNSSEC is being deployed and can provide strong protection against
386 forged data and cache poisoning; however, it has the unintended
387 effect of making some denial-of-service attacks worse because of the
388 cryptographic computational load it can require and the increased
389 size in DNS response packets that it tends to produce.
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3993.2. DNS Message/Transaction Security
401 The second form of security that has been added to DNS provides
402 "transaction" security through TSIG [RFC2845] or SIG(0) [RFC2931].
403 TSIG could provide strong protection against the attacks for which
404 the DNS Cookie mechanism provides weaker protection; however, TSIG is
405 non-trivial to deploy in the general Internet because of the burdens
406 it imposes. Among these burdens are pre-agreement and key
407 distribution between client and server, keeping track of server-side
408 key state, and required time synchronization between client and
411 TKEY [RFC2930] can solve the problem of key distribution for TSIG,
412 but some modes of TKEY impose a substantial cryptographic computation
413 load and can be dependent on the deployment of DNS data security (see
416 SIG(0) [RFC2931] provides less denial-of-service protection than TSIG
417 or, in one way, even DNS Cookies, because it authenticates complete
418 transactions but does not authenticate requests. In any case, it
419 also depends on the deployment of DNS data security and requires
420 computationally burdensome public key cryptographic operations.
4223.3. Conclusions on Existing DNS Security
424 The existing DNS security mechanisms do not provide the services
425 provided by the DNS Cookie mechanism: lightweight message
426 authentication of DNS requests and responses with no requirement for
427 pre-configuration or per-client server-side state.
431 The DNS COOKIE option is an OPT RR [RFC6891] option that can be
432 included in the RDATA portion of an OPT RR in DNS requests and
433 responses. The option length varies, depending on the circumstances
434 in which it is being used. There are two cases, as described below.
435 Both use the same OPTION-CODE; they are distinguished by their
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455 In a request sent by a client to a server when the client does not
456 know the server's cookie, its length is 8, consisting of an 8-byte
457 Client Cookie, as shown in Figure 1.
459 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
460 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
461 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
462 | OPTION-CODE = 10 | OPTION-LENGTH = 8 |
463 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
465 +-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+
467 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
469 Figure 1: COOKIE Option, Unknown Server Cookie
471 In a request sent by a client when a Server Cookie is known, and in
472 all responses to such a request, the length is variable -- from 16 to
473 40 bytes, consisting of an 8-byte Client Cookie followed by the
474 variable-length (8 bytes to 32 bytes) Server Cookie, as shown in
475 Figure 2. The variability of the option length stems from the
476 variable-length Server Cookie. The Server Cookie is an integer
477 number of bytes, with a minimum size of 8 bytes for security and a
478 maximum size of 32 bytes for convenience of implementation.
480 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
481 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
482 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
483 | OPTION-CODE = 10 | OPTION-LENGTH >= 16, <= 40 |
484 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
486 +-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+
488 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
490 / Server Cookie (variable size, 8 to 32 bytes) /
494 Figure 2: COOKIE Option, Known Server Cookie
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513 The Client Cookie SHOULD be a pseudorandom function of the Client IP
514 Address, the Server IP Address, and a secret quantity known only to
515 the client. This Client Secret SHOULD have at least 64 bits of
516 entropy [RFC4086] and be changed periodically (see Section 7.1). The
517 selection of the pseudorandom function is a matter private to the
518 client, as only the client needs to recognize its own DNS Cookies.
520 The Client IP Address is included so that the Client Cookie cannot be
521 used to (1) track a client if the Client IP Address changes due to
522 privacy mechanisms or (2) impersonate the client by some network
523 device that was formerly on path but is no longer on path when the
524 Client IP Address changes due to mobility. However, if the Client IP
525 Address is being changed very often, it may be necessary to fix the
526 Client Cookie for a particular server for several requests, to avoid
527 undue inefficiency due to retries caused by that server not
528 recognizing the Client Cookie.
530 For further discussion of the Client Cookie field, see Section 5.1.
531 For example methods of determining a Client Cookie, see Appendix A.
533 In order to provide minimal authentication, a client MUST send
534 Client Cookies that will usually be different for any two servers at
535 different IP addresses.
539 The Server Cookie SHOULD consist of or include a 64-bit or larger
540 pseudorandom function of the request source (client) IP address, a
541 secret quantity known only to the server, and the request
542 Client Cookie. (See Section 6 for a discussion of why the
543 Client Cookie is used as input to the Server Cookie but the
544 Server Cookie is not used as an input to the Client Cookie.) This
545 Server Secret SHOULD have at least 64 bits of entropy [RFC4086] and
546 be changed periodically (see Section 7.1). The selection of the
547 pseudorandom function is a matter private to the server, as only the
548 server needs to recognize its own DNS Cookies.
550 For further discussion of the Server Cookie field, see Section 5.2.
551 For example methods of determining a Server Cookie, see Appendix B.
552 When implemented as recommended, the server need not maintain any
553 cookie-related per-client state.
555 In order to provide minimal authentication, a server MUST send
556 Server Cookies that will usually be different for clients at any two
557 different IP addresses or with different Client Cookies.
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5675. DNS Cookies Protocol Specification
569 This section discusses using DNS Cookies in the DNS protocol. The
570 cycle of originating a request, responding to that request, and
571 processing responses is covered in Sections 5.1, 5.2, and 5.3. A
572 de facto extension to QUERY to allow the prefetching of a
573 Server Cookie is specified in Section 5.4. Rollover of the Client
574 Secrets and Server Secrets, and transient retention of the old cookie
575 or secret, are covered in Section 7.1.
577 DNS clients and servers SHOULD implement DNS Cookies to decrease
578 their vulnerability to the threats discussed in Section 2.
5805.1. Originating a Request
582 A DNS client that implements DNS Cookies includes one DNS
583 COOKIE option containing a Client Cookie in every DNS request
584 it sends, unless DNS Cookies are disabled.
586 If the client has a cached Server Cookie for the server against its
587 IP address, it uses the longer cookie form and includes that
588 Server Cookie in the option along with the Client Cookie (Figure 2).
589 Otherwise, it just sends the shorter-form option with a Client Cookie
5925.2. Responding to a Request
594 The Server Cookie, when it occurs in a COOKIE option in a request, is
595 intended to weakly assure the server that the request came from a
596 client that is both at the source IP address of the request and using
597 the Client Cookie included in the option. This assurance is provided
598 by the Server Cookie that server sent to that client in an earlier
599 response appearing as the Server Cookie field in the request.
601 At a server where DNS Cookies are not implemented and enabled, the
602 presence of a COOKIE option is ignored and the server responds as if
603 no COOKIE option had been included in the request.
605 When DNS Cookies are implemented and enabled, there are five
608 (1) There is no OPT RR at all in the request, or there is an OPT RR
609 but the COOKIE option is absent from the OPT RR.
611 (2) A COOKIE option is present but is not a legal length or is
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623 (3) There is a COOKIE option of valid length in the request with no
626 (4) There is a COOKIE option of valid length in the request with a
627 Server Cookie, but that Server Cookie is invalid.
629 (5) There is a COOKIE option of valid length in the request with a
630 correct Server Cookie.
632 These five possibilities are discussed in the subsections below.
634 In all cases of multiple COOKIE options in a request, only the first
635 (the one closest to the DNS header) is considered. All others are
6385.2.1. No OPT RR or No COOKIE Option
640 If there is no OPT record or no COOKIE option present in the request,
641 then the server responds to the request as if the server doesn't
642 implement the COOKIE option.
6445.2.2. Malformed COOKIE Option
646 If the COOKIE option is too short to contain a Client Cookie, then
647 FORMERR is generated. If the COOKIE option is longer than that
648 required to hold a COOKIE option with just a Client Cookie (8 bytes)
649 but is shorter than the minimum COOKIE option with both a
650 Client Cookie and a Server Cookie (16 bytes), then FORMERR is
651 generated. If the COOKIE option is longer than the maximum valid
652 COOKIE option (40 bytes), then FORMERR is generated.
654 In summary, valid cookie lengths are 8 and 16 to 40 inclusive.
6565.2.3. Only a Client Cookie
658 Based on server policy, including rate limiting, the server chooses
659 one of the following:
661 (1) Silently discard the request.
663 (2) Send a BADCOOKIE error response.
665 (3) Process the request and provide a normal response. The RCODE is
666 NOERROR, unless some non-cookie error occurs in processing the
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679 If the server responds choosing (2) or (3) above, it SHALL generate
680 its own COOKIE option containing both the Client Cookie copied from
681 the request and a Server Cookie it has generated, and it will add
682 this COOKIE option to the response's OPT record. Servers MUST, at
683 least occasionally, respond to such requests to inform the client of
684 the correct Server Cookie. This is necessary so that such a client
685 can bootstrap to the more secure state where requests and responses
686 have recognized Server Cookies and Client Cookies. A server is not
687 expected to maintain per-client state to achieve this. For example,
688 it could respond to every Nth request across all clients.
690 If the request was received over TCP, the server SHOULD take the
691 authentication provided by the use of TCP into account and SHOULD
692 choose (3). In this case, if the server is not willing to accept the
693 security provided by TCP as a substitute for the security provided by
694 DNS Cookies but instead chooses (2), there is some danger of an
695 indefinite loop of retries (see Section 5.3).
6975.2.4. A Client Cookie and an Invalid Server Cookie
699 The server examines the Server Cookie to determine if it is a valid
700 Server Cookie that it had generated previously. This determination
701 normally involves recalculating the Server Cookie (or the Hash part
702 thereof) based on the Server Secret (or the previous Server Secret,
703 if it has just changed); the received Client Cookie; the Client IP
704 Address; and, possibly, other fields. See Appendix B.2 for an
705 example. If the cookie is invalid, it could be because
709 + a client's IP address or Client Cookie changed, and the DNS server
710 is not aware of the change
712 + an anycast cluster of servers is not consistently configured, or
714 + an attempt to spoof the client has occurred
716 The server SHALL process the request as if the invalid Server Cookie
717 was not present, as described in Section 5.2.3.
7195.2.5. A Client Cookie and a Valid Server Cookie
721 When a valid Server Cookie is present in the request, the server can
722 assume that the request is from a client that it has talked to before
723 and defensive measures for spoofed UDP requests, if any, are no
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735 The server SHALL process the request and include a COOKIE option in
736 the response by (a) copying the complete COOKIE option from the
737 request or (b) generating a new COOKIE option containing both the
738 Client Cookie copied from the request and a valid Server Cookie it
7415.3. Processing Responses
743 The Client Cookie, when it occurs in a COOKIE option in a DNS reply,
744 is intended to weakly assure the client that the reply came from a
745 server at the source IP address used in the response packet, because
746 the Client Cookie value is the value that client would send to that
747 server in a request. In a DNS reply with multiple COOKIE options,
748 all but the first (the one closest to the DNS header) are ignored.
750 A DNS client where DNS Cookies are implemented and enabled examines
751 the response for DNS Cookies and MUST discard the response if it
752 contains an illegal COOKIE option length or an incorrect
753 Client Cookie value. If the client is expecting the response to
754 contain a COOKIE option and it is missing, the response MUST be
755 discarded. If the COOKIE option Client Cookie is correct, the client
756 caches the Server Cookie provided, even if the response is an error
757 response (RCODE non-zero).
759 If the extended RCODE in the reply is BADCOOKIE and the Client Cookie
760 in the reply matches what was sent, it means that the server was
761 unwilling to process the request because it did not have the correct
762 Server Cookie in it. The client SHOULD retry the request using the
763 new Server Cookie from the response. Repeated BADCOOKIE responses to
764 requests that use the Server Cookie provided in the previous response
765 may be an indication that either the shared secrets or the method for
766 generating secrets in an anycast cluster of servers is inconsistent.
767 If the reply to a retried request with a fresh Server Cookie is
768 BADCOOKIE, the client SHOULD retry using TCP as the transport, since
769 the server will likely process the request normally based on the
770 security provided by TCP (see Section 5.2.3).
772 If the RCODE is some value other than BADCOOKIE, including zero, the
773 further processing of the response proceeds normally.
7755.4. Querying for a Server Cookie
777 In many cases, a client will learn the Server Cookie for a server as
778 the "side effect" of another transaction; however, there may be times
779 when this is not desirable. Therefore, a means is provided for
780 obtaining a Server Cookie through an extension to the QUERY opcode
781 for which opcode most existing implementations require that QDCOUNT
782 be one (1) (see Section 4.1.2 of [RFC1035]).
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791 For servers with DNS Cookies enabled, the QUERY opcode behavior is
792 extended to support queries with an empty Question Section (a QDCOUNT
793 of zero (0)), provided that an OPT record is present with a COOKIE
794 option. Such servers will send a reply that has an empty
795 Answer Section and has a COOKIE option containing the Client Cookie
796 and a valid Server Cookie.
798 If such a query provided just a Client Cookie and no Server Cookie,
799 the response SHALL have the RCODE NOERROR.
801 This mechanism can also be used to confirm/re-establish an existing
802 Server Cookie by sending a cached Server Cookie with the
803 Client Cookie. In this case, the response SHALL have the RCODE
804 BADCOOKIE if the Server Cookie sent with the query was invalid and
805 the RCODE NOERROR if it was valid.
807 Servers that don't support the COOKIE option will normally send
808 FORMERR in response to such a query, though REFUSED, NOTIMP, and
809 NOERROR without a COOKIE option are also possible in such responses.
8116. NAT Considerations and Anycast Server Considerations
813 In the classic Internet, DNS Cookies could simply be a pseudorandom
814 function of the Client IP Address and a Server Secret or the Server
815 IP Address and a Client Secret. You would want to compute the
816 Server Cookie that way, so a client could cache its Server Cookie for
817 a particular server for an indefinite amount of time and the server
818 could easily regenerate and check it. You could consider the
819 Client Cookie to be a weak client signature over the Server IP
820 Address that the client checks in replies, and you could extend this
821 signature to cover the request ID, for example, or any other
822 information that is returned unchanged in the reply.
824 But we have this reality called "NAT" [RFC3022] (including, for the
825 purposes of this document, NAT-PT, which has been declared Historic
826 [RFC4966]). There is no problem with DNS transactions between
827 clients and servers behind a NAT box using local IP addresses. Nor
828 is there a problem with NAT translation of internal addresses to
829 external addresses or translations between IPv4 and IPv6 addresses,
830 as long as the address mapping is relatively stable. Should the
831 external IP address to which an internal client is being mapped
832 change occasionally, the disruption is little more than when a client
833 rolls over its COOKIE secret. Also, external access to a DNS server
834 behind a NAT box is normally handled by a fixed mapping that forwards
835 externally received DNS requests to a specific host.
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847 However, NAT devices sometimes also map ports. This can cause
848 multiple DNS requests and responses from multiple internal hosts to
849 be mapped to a smaller number of external IP addresses, such as one
850 address. Thus, there could be many clients behind a NAT box that
851 appear to come from the same source IP address to a server outside
852 that NAT box. If one of these were an attacker (think "zombie" or
853 "botnet") behind a NAT box, that attacker could get the Server Cookie
854 for some server for the outgoing IP address by just making some
855 random request to that server. It could then include that
856 Server Cookie in the COOKIE option of requests to the server with the
857 forged local IP address of some other host and/or client behind the
858 NAT box. (An attacker's possession of this Server Cookie will not
859 help in forging responses to cause cache poisoning, as such responses
860 are protected by the required Client Cookie.)
862 To fix this potential defect, it is necessary to distinguish
863 different clients behind a NAT box from the point of view of the
864 server. This is why the Server Cookie is specified as a pseudorandom
865 function of both the request source IP address and the Client Cookie.
866 From this inclusion of the Client Cookie in the calculation of the
867 Server Cookie, it follows that, for any particular server, a stable
868 Client Cookie is needed. If, for example, the request ID was
869 included in the calculation of the Client Cookie, it would normally
870 change with each request to a particular server. This would mean
871 that each request would have to be sent twice: first, to learn the
872 new Server Cookie based on this new Client Cookie based on the new
873 ID, and then again using this new Client Cookie to actually get an
874 answer. Thus, the input to the Client Cookie computation must be
875 limited to the Server IP Address and one or more things that change
876 slowly, such as the Client Secret.
878 In principle, there could be a similar problem for servers, not due
879 to NAT but due to mechanisms like anycast that may cause requests to
880 a DNS server at an IP address to be delivered to any one of several
881 machines. (External requests to a DNS server behind a NAT box
882 usually occur via port forwarding such that all such requests go to
883 one host.) However, it is impossible to solve this in the way that
884 the similar problem was solved for NATed clients; if the
885 Server Cookie was included in the calculation of the Client Cookie in
886 the same way that the Client Cookie is included in the Server Cookie,
887 you would just get an almost infinite series of errors as a request
888 was repeatedly retried.
890 For servers accessed via anycast, to successfully support
891 DNS Cookies, either (1) the server clones must all use the same
892 Server Secret or (2) the mechanism that distributes requests to the
893 server clones must cause the requests from a particular client to go
894 to a particular server for a sufficiently long period of time that
898Eastlake & Andrews Standards Track [Page 16]
900RFC 7873 DNS Cookies May 2016
903 extra requests due to changes in Server Cookies resulting from
904 accessing different server machines are not unduly burdensome. (When
905 such anycast-accessed servers act as recursive servers or otherwise
906 act as clients, they normally use a different unique address to
907 source their requests, to avoid confusion in the delivery of
910 For simplicity, it is RECOMMENDED that the same Server Secret be used
911 by each DNS server in a set of anycast servers. If there is limited
912 time skew in updating this secret in different anycast servers, this
913 can be handled by a server accepting requests containing a
914 Server Cookie based on either its old or new secret for the maximum
915 likely time period of such time skew (see also Section 7.1).
9177. Operational and Deployment Considerations
919 The DNS Cookie mechanism is designed for incremental deployment and
920 to complement the orthogonal techniques in [RFC5452]. Either or both
921 techniques can be deployed independently at each DNS server and
922 client. Thus, installation at the client and server end need not be
925 In particular, a DNS server or client that implements the DNS Cookie
926 mechanism can interoperate successfully with a DNS client or server
927 that does not implement this mechanism, although, of course, in this
928 case it will not get the benefit of the mechanism and the server
929 involved might choose to severely rate-limit responses. When such a
930 server or client interoperates with a client or server that also
931 implements the DNS Cookie mechanism, these servers and clients get
932 the security benefits of the DNS Cookie mechanism.
9347.1. Client and Server Secret Rollover
936 The longer a secret is used, the higher the probability that it has
937 been compromised. Thus, clients and servers are configured with a
938 lifetime setting for their secret, and they roll over to a new secret
939 when that lifetime expires, or earlier due to deliberate jitter as
940 described below. The default lifetime is one day, and the maximum
941 permitted is one month. To be precise and to make it practical to
942 stay within limits despite long holiday weekends, daylight saving
943 time shifts, and the like, clients and servers MUST NOT continue to
944 use the same secret in new requests and responses for more than
945 36 days and SHOULD NOT continue to do so for more than 26 hours.
947 Many clients rolling over their secret at the same time could briefly
948 increase server traffic, and exactly predictable rollover times for
949 clients or servers might facilitate guessing attacks. For example,
950 an attacker might increase the priority of attacking secrets they
954Eastlake & Andrews Standards Track [Page 17]
956RFC 7873 DNS Cookies May 2016
959 believe will be in effect for an extended period of time. To avoid
960 rollover synchronization and predictability, it is RECOMMENDED that
961 pseudorandom jitter in the range of plus zero to minus at least 40%
962 be applied to the time until a scheduled rollover of a COOKIE secret.
964 It is RECOMMENDED that a client keep the Client Cookie it is
965 expecting in a reply until there is no longer an outstanding request
966 associated with that Client Cookie that the client is tracking. This
967 avoids rejection of replies due to a bad Client Cookie right after a
968 change in the Client Secret.
970 It is RECOMMENDED that a server retain its previous secret after a
971 rollover to a new secret for a configurable period of time not less
972 than 1 second or more than 300 seconds, with a default configuration
973 of 150 seconds. Requests with Server Cookies based on its previous
974 secret are treated as a correct Server Cookie during that time. When
975 a server responds to a request containing an old Server Cookie that
976 the server is treating as correct, the server MUST include a new
977 Server Cookie in its response.
981 It is RECOMMENDED that implementations include counters of the
982 occurrences of the various types of requests and responses described
9858. IANA Considerations
987 IANA has assigned the following DNS EDNS0 option code:
989 Value Name Status Reference
990 -------- ------ -------- ---------------
991 10 COOKIE Standard RFC 7873
993 IANA has assigned the following DNS error code as an early allocation
996 RCODE Name Description Reference
997 -------- --------- ------------------------- ---------------
998 23 BADCOOKIE Bad/missing Server Cookie RFC 7873
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10159. Security Considerations
1017 DNS Cookies provide a weak form of authentication of DNS requests and
1018 responses. In particular, they provide no protection against
1019 "on-path" adversaries; that is, they provide no protection against
1020 any adversary that can observe the plaintext DNS traffic, such as an
1021 on-path router, bridge, or any device on an on-path shared link
1022 (unless the DNS traffic in question on that path is encrypted).
1024 For example, if a host is connected via an unsecured IEEE Std. 802.11
1025 link (Wi-Fi), any device in the vicinity that could receive and
1026 decode the 802.11 transmissions must be considered "on path". On the
1027 other hand, in a similar situation but one where 802.11 Robust
1028 Security (WPA2, also called "Wi-Fi Protected Access 2") is
1029 appropriately deployed on the Wi-Fi network nodes, only the
1030 Access Point via which the host is connecting is "on path" as far as
1031 the 802.11 link is concerned.
1033 Despite these limitations, deployment of DNS Cookies on the global
1034 Internet is expected to provide a significant reduction in the
1035 available launch points for the traffic amplification and denial-of-
1036 service forgery attacks described in Section 2 above.
1038 Work is underway in the IETF DPRIVE working group to provide
1039 confidentiality for DNS requests and responses that would be
1040 compatible with DNS Cookies.
1042 Should stronger message/transaction security be desired, it is
1043 suggested that TSIG or SIG(0) security be used (see Section 3.2);
1044 however, it may be useful to use DNS Cookies in conjunction with
1045 these features. In particular, DNS Cookies could screen out many DNS
1046 messages before the cryptographic computations of TSIG or SIG(0) are
1047 required, and if SIG(0) is in use, DNS Cookies could usefully screen
1048 out many requests given that SIG(0) does not screen requests but only
1049 authenticates the response of complete transactions.
1051 An attacker that does not know the Server Cookie could do a variety
1052 of things, such as omitting the COOKIE option or sending a random
1053 Server Cookie. In general, DNS servers need to take other measures,
1054 including rate-limiting responses, to protect from abuse in such
1055 cases. See further information in Section 5.2.
1057 When a server or client starts receiving an increased level of
1058 requests with bad Server Cookies or replies with bad Client Cookies,
1059 it would be reasonable for it to believe that it is likely under
1060 attack, and it should consider a more frequent rollover of its
1061 secret. More rapid rollover decreases the benefit to a
1062 cookie-guessing attacker if they succeed in guessing a cookie.
1066Eastlake & Andrews Standards Track [Page 19]
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10719.1. Cookie Algorithm Considerations
1073 The cookie computation algorithm for use in DNS Cookies SHOULD be
1074 based on a pseudorandom function at least as strong as 64-bit FNV
1075 (Fowler/Noll/Vo [FNV]), because an excessively weak or trivial
1076 algorithm could enable adversaries to guess cookies. However, in
1077 light of the lightweight plaintext token security provided by
1078 DNS Cookies, a strong cryptography hash algorithm may not be
1079 warranted in many cases and would cause an increased computational
1080 burden. Nevertheless, there is nothing wrong with using something
1081 stronger -- for example, HMAC-SHA-256 [RFC6234] truncated to 64 bits,
1082 assuming that a DNS processor has adequate computational resources
1083 available. DNS implementations or applications that need somewhat
1084 stronger security without a significant increase in computational
1085 load should consider more frequent changes in their client and/or
1086 Server Secret; however, this does require more frequent generation of
1087 a cryptographically strong random number [RFC4086]. See Appendices A
1088 and B for specific examples of cookie computation algorithms.
109010. Implementation Considerations
1092 The DNS COOKIE option specified herein is implemented in BIND 9.10
1093 using an experimental option code. BIND 9.10.3 (and later) use the
1094 allocated option code.
109811.1. Normative References
1100 [RFC1035] Mockapetris, P., "Domain names - implementation and
1101 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
1102 November 1987, <http://www.rfc-editor.org/info/rfc1035>.
1104 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
1105 Requirement Levels", BCP 14, RFC 2119,
1106 DOI 10.17487/RFC2119, March 1997,
1107 <http://www.rfc-editor.org/info/rfc2119>.
1109 [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
1110 "Randomness Requirements for Security", BCP 106, RFC 4086,
1111 DOI 10.17487/RFC4086, June 2005,
1112 <http://www.rfc-editor.org/info/rfc4086>.
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1124RFC 7873 DNS Cookies May 2016
1127 [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
1128 for DNS (EDNS(0))", STD 75, RFC 6891,
1129 DOI 10.17487/RFC6891, April 2013,
1130 <http://www.rfc-editor.org/info/rfc6891>.
1132 [RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code
1133 Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120,
1134 January 2014, <http://www.rfc-editor.org/info/rfc7120>.
113611.2. Informative References
1138 [FNV] Fowler, G., Noll, L., Vo, K., and D. Eastlake 3rd, "The
1139 FNV Non-Cryptographic Hash Algorithm", Work in Progress,
1140 draft-eastlake-fnv-10, October 2015.
1142 [Kaminsky] Olney, M., Mullen, P., and K. Miklavcic, "Dan Kaminsky's
1143 2008 DNS Vulnerability", July 2008, <https://www.ietf.org/
1144 mail-archive/web/dnsop/current/pdf2jgx6rzxN4.pdf>.
1146 [RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
1147 Wellington, "Secret Key Transaction Authentication for DNS
1148 (TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000,
1149 <http://www.rfc-editor.org/info/rfc2845>.
1151 [RFC2930] Eastlake 3rd, D., "Secret Key Establishment for DNS
1152 (TKEY RR)", RFC 2930, DOI 10.17487/RFC2930,
1153 September 2000, <http://www.rfc-editor.org/info/rfc2930>.
1155 [RFC2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures
1156 ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931,
1157 September 2000, <http://www.rfc-editor.org/info/rfc2931>.
1159 [RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
1160 Address Translator (Traditional NAT)", RFC 3022,
1161 DOI 10.17487/RFC3022, January 2001,
1162 <http://www.rfc-editor.org/info/rfc3022>.
1164 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
1165 Rose, "DNS Security Introduction and Requirements",
1166 RFC 4033, DOI 10.17487/RFC4033, March 2005,
1167 <http://www.rfc-editor.org/info/rfc4033>.
1169 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
1170 Rose, "Resource Records for the DNS Security Extensions",
1171 RFC 4034, DOI 10.17487/RFC4034, March 2005,
1172 <http://www.rfc-editor.org/info/rfc4034>.
1178Eastlake & Andrews Standards Track [Page 21]
1180RFC 7873 DNS Cookies May 2016
1183 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
1184 Rose, "Protocol Modifications for the DNS Security
1185 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
1186 <http://www.rfc-editor.org/info/rfc4035>.
1188 [RFC4966] Aoun, C. and E. Davies, "Reasons to Move the Network
1189 Address Translator - Protocol Translator (NAT-PT) to
1190 Historic Status", RFC 4966, DOI 10.17487/RFC4966,
1191 July 2007, <http://www.rfc-editor.org/info/rfc4966>.
1193 [RFC5452] Hubert, A. and R. van Mook, "Measures for Making DNS
1194 More Resilient against Forged Answers", RFC 5452,
1195 DOI 10.17487/RFC5452, January 2009,
1196 <http://www.rfc-editor.org/info/rfc5452>.
1198 [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
1199 (SHA and SHA-based HMAC and HKDF)", RFC 6234,
1200 DOI 10.17487/RFC6234, May 2011,
1201 <http://www.rfc-editor.org/info/rfc6234>.
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1236RFC 7873 DNS Cookies May 2016
1239Appendix A. Example Client Cookie Algorithms
1241A.1. A Simple Algorithm
1243 A simple example method to compute Client Cookies is the FNV64 [FNV]
1244 of the Client IP Address, the Server IP Address, and the Client
1248 FNV64( Client IP Address | Server IP Address | Client Secret )
1250 where "|" indicates concatenation. Some computational resources may
1251 be saved by pre-computing FNV64 through the Client IP Address. (If
1252 the order of the items concatenated above is changed to put the
1253 Server IP Address last, it might be possible to further reduce the
1254 computational effort by pre-computing FNV64 through the bytes of both
1255 the Client IP Address and the Client Secret, but this would reduce
1256 the strength of the Client Cookie and is NOT RECOMMENDED.)
1258A.2. A More Complex Algorithm
1260 A more complex algorithm to calculate Client Cookies is given below.
1261 It uses more computational resources than the simpler algorithm shown
1265 HMAC-SHA256-64( Client IP Address | Server IP Address,
1268Appendix B. Example Server Cookie Algorithms
1270B.1. A Simple Algorithm
1272 An example of a simple method producing a 64-bit Server Cookie is the
1273 FNV64 [FNV] of the request IP address, the Client Cookie, and the
1277 FNV64( Client IP Address | Client Cookie | Server Secret )
1279 where "|" represents concatenation. (If the order of the items
1280 concatenated was changed, it might be possible to reduce the
1281 computational effort by pre-computing FNV64 through the bytes of the
1282 Server Secret and Client Cookie, but this would reduce the strength
1283 of the Server Cookie and is NOT RECOMMENDED.)
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1292RFC 7873 DNS Cookies May 2016
1295B.2. A More Complex Algorithm
1297 Since the Server Cookie has a variable size, the server can store
1298 various information in that field as long as it is hard for an
1299 adversary to guess the entire quantity used for authentication.
1300 There should be 64 bits of entropy in the Server Cookie; for example,
1301 it could have a sub-field of 64 bits computed pseudorandomly with the
1302 Server Secret as one of the inputs to the pseudorandom function.
1303 Types of additional information that could be stored include a
1304 timestamp and/or a nonce.
1306 The example below is one variation of the Server Cookie that has been
1307 implemented in BIND 9.10.3 (and later) releases, where the
1308 Server Cookie is 128 bits, composed as follows:
1316 With this algorithm, the server sends a new 128-bit cookie back with
1317 every request. The Nonce field assures a low probability that there
1318 would be a duplicate.
1320 The Time field gives the server time and makes it easy to reject old
1323 The Hash part of the Server Cookie is the part that is hard to guess.
1324 In BIND 9.10.3 (and later), its computation can be configured to use
1325 AES, HMAC-SHA-1, or, as shown below, HMAC-SHA-256:
1328 HMAC-SHA256-64( Server Secret,
1329 (Client Cookie | Nonce | Time | Client IP Address) )
1331 where "|" represents concatenation.
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1353 The suggestions and contributions of the following are gratefully
1356 Alissa Cooper, Bob Harold, Paul Hoffman, David Malone, Yoav Nir,
1357 Gayle Noble, Dan Romascanu, Tim Wicinski, and Peter Yee
1361 Donald E. Eastlake 3rd
1367 Phone: +1-508-333-2270
1368 Email: d3e3e3@gmail.com
1372 Internet Systems Consortium
1374 Redwood City, CA 94063
1377 Email: marka@isc.org
1402Eastlake & Andrews Standards Track [Page 25]