Interoperable Domain Name System (DNS) Server Cookies
draft-ietf-dnsop-server-cookies-05

DNSOP Working Group                                              O. Sury
Internet-Draft                               Internet Systems Consortium
Updates: 7873 (if approved)                                    W. Toorop
Intended status: Standards Track                              NLnet Labs
Expires: 17 July 2021                                    D. Eastlake 3rd
                                                  Futurewei Technologies
                                                              M. Andrews
                                             Internet Systems Consortium
                                                         13 January 2021

         Interoperable Domain Name System (DNS) Server Cookies
                   draft-ietf-dnsop-server-cookies-05

Abstract

   DNS Cookies, as specified in [RFC7873], are a lightweight DNS
   transaction security mechanism that provide limited protection to DNS
   servers and clients against a variety of amplification denial of
   service, forgery, or cache poisoning attacks by off-path attackers.

   This document updates [RFC7873] with precise directions for creating
   Server Cookies so that an anycast server set including diverse
   implementations will interoperate with standard clients, suggestions
   for constructing Client Cookies in a privacy preserving fashion, and
   suggestions on how to update a Server Secret.  An IANA registry
   listing the methods and associated pseudo random function suitable
   for creating DNS Server Cookies is created, with the method described
   in this document as the first and as of yet only entry.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 17 July 2021.

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Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology and Definitions . . . . . . . . . . . . . . .   3
   2.  Changes to [RFC7873]  . . . . . . . . . . . . . . . . . . . .   4
   3.  Constructing a Client Cookie  . . . . . . . . . . . . . . . .   4
   4.  Constructing a Server Cookie  . . . . . . . . . . . . . . . .   5
     4.1.  The Version Sub-Field . . . . . . . . . . . . . . . . . .   6
     4.2.  The Reserved Sub-Field  . . . . . . . . . . . . . . . . .   6
     4.3.  The Timestamp Sub-Field . . . . . . . . . . . . . . . . .   6
     4.4.  The Hash Sub-Field  . . . . . . . . . . . . . . . . . . .   7
   5.  Updating the Server Secret  . . . . . . . . . . . . . . . . .   8
   6.  Cookie Algorithms . . . . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Security and Privacy Considerations . . . . . . . . . . . . .  10
     8.1.  Client Cookie construction  . . . . . . . . . . . . . . .  10
     8.2.  Server Cookie construction  . . . . . . . . . . . . . . .  11
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  12
   11. Informative References  . . . . . . . . . . . . . . . . . . .  13
   Appendix A.  Test vectors . . . . . . . . . . . . . . . . . . . .  13
     A.1.  Learning a new Server Cookie  . . . . . . . . . . . . . .  13
     A.2.  The same client learning a renewed (fresh) Server
           Cookie  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     A.3.  Another client learning a renewed Server Cookie . . . . .  15
     A.4.  IPv6 query with rolled over secret  . . . . . . . . . . .  16
   Appendix B.  Implementation status  . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

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1.  Introduction

   DNS Cookies, as specified in [RFC7873], are a lightweight DNS
   transaction security mechanism that provide limited protection to DNS
   servers and clients against a variety of denial of service
   amplification, forgery, or cache poisoning attacks by off-path
   attackers.  This document specifies a means of producing
   interoperable Cookies so that an anycast server set including diverse
   implementations can be easily configured to interoperate with
   standard clients.  Also single implementation or non-anycast services
   can benefit from a well-studied standardized algorithm for which the
   behavioural and security characteristics are more widely known.

   The threats considered for DNS Cookies and the properties of the DNS
   Security features other than DNS Cookies are discussed in [RFC7873].

   In [RFC7873] in Section 6 it is "RECOMMENDED for simplicity that the
   same Server Secret be used by each DNS server in a set of anycast
   servers."  However, how precisely a Server Cookie is calculated from
   this Server Secret, is left to the implementation.

   This guidance has led to a gallimaufry of DNS Cookie implementations,
   calculating the Server Cookie in different ways.  As a result, DNS
   Cookies are impractical to deploy on multi-vendor anycast networks,
   because even when all DNS Software share the same secret, as
   RECOMMENDED in Section 6 of [RFC7873], the Server Cookie constructed
   by one implementation cannot generally be validated by another.

   There is no need for DNS client (resolver) Cookies to be
   interoperable across different implementations.  Each client need
   only be able to recognize its own cookies.  However, this document
   does contain recommendations for constructing Client Cookies in a
   client protecting fashion.

1.1.  Terminology and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   *  "IP address" is used herein as a length independent term covering
      both IPv4 and IPv6 addresses.

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2.  Changes to [RFC7873]

   In its Appendices A.1 and B.1, [RFC7873] provides example "simple"
   algorithms for computing Client and Server Cookies, respectively.
   These algorithms MUST NOT be used as the resulting cookies are too
   weak when evaluated against modern security standards.

   In its Appendix B.2, [RFC7873] provides an example "more complex"
   server algorithm.  This algorithm is replaced by the interoperable
   specification in Section 4 of this document, which MUST be used by
   Server Cookie implementations.

   This document has suggestions on Client Cookie construction in
   Section 3.  The previous example in Appendix A.2 of [RFC7873] is NOT
   RECOMMENDED.

3.  Constructing a Client Cookie

   The Client Cookie acts as an identifier for a given client and its IP
   address, and needs to be unguessable.  In order to provide minimal
   authentication of the targeted server, a client MUST use a different
   Client Cookie for each different Server IP address.  This complicates
   a server's ability to spoof answers for other DNS servers.  The
   Client Cookie SHOULD have 64-bits of entropy.

   When a server does not support DNS Cookies, the client MUST NOT send
   the same Client Cookie to that same server again.  Instead, it is
   recommended that the client does not send a Client Cookie to that
   server for a certain period, for example five minutes, before it
   retries with a new Client Cookie.

   When a server does support DNS Cookies, the client should store the
   Client Cookie alongside the Server Cookie it registered for that
   server.

   Except for when the Client IP address changes, there is no need to
   change the Client Cookie often.  It is reasonable to change the
   Client Cookie then only if it has been compromised or after a
   relatively long implementation-defined period of time.  The time
   period should be no longer than a year, and in any case Client
   Cookies are not expected to survive a program restart.

   Client-Cookie = 64 bits of entropy

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   Previously, the recommended algorithm to compute the Client Cookie
   included Client IP address as an input to a hashing function.
   However, when implementing the DNS Cookies, several DNS vendors found
   impractical to include the Client IP as the Client Cookie is
   typically computed before the Client IP address is known.  Therefore,
   the requirement to put Client IP address as input was removed.

   However, for privacy reasons, in order to prevent tracking of devices
   across links and to not circumvent IPv6 Privacy Extensions [RFC4941],
   clients MUST NOT re-use a Client or Server Cookie after the Client IP
   address has changed.

   One way to satisfy this requirement for non-re-use is to register the
   Client IP address alongside the Server Cookie when it receives the
   Server Cookie.  In subsequent queries to the server with that Server
   Cookie, the socket MUST be bound to the Client IP address that was
   also used (and registered) when it received the Server Cookie.
   Failure to bind MUST then result in a new Client Cookie.

4.  Constructing a Server Cookie

   The Server Cookie is effectively a Message Authentication Code (MAC).
   The Server Cookie, when it occurs in a COOKIE option in a request, is
   intended to weakly assure the server that the request came from a
   client that is both at the source IP address of the request and using
   the Client Cookie included in the option.  This assurance is provided
   by the Server Cookie that the server (or any other server from the
   anycast set) sent to that client in an earlier response appearing as
   the Server Cookie field in the request (see Section 5.2 of
   [RFC7873]).

   DNS Cookies do not provide protection against "on-path" adversaries
   (see Section 9 of [RFC7873]).  An on path observer that has seen a
   Server Cookie for a client, can abuse that Server Cookie to spoof
   request for that client within the timespan a Server Cookie is valid
   (see Section 4.3).

   The Server Cookie is calculated from the Client Cookie, a series of
   Sub-Fields specified below, the Client IP address, and a Server
   Secret known only to the server, or servers responding on the same
   address in an anycast set.

   For calculation of the Server Cookie, a pseudorandom function is
   RECOMMENDED with the property that an attacker that does not know the
   Server Secret, cannot find (any information about) the Server Secret
   and cannot create a Server Cookie for any combination of - the Client
   Cookie, the series of Sub-Fields specified below and the client IP
   address - for which it has not seen a Server Cookie before.  Because

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   DNS servers need to calculate in order to verify Server Cookies, it
   is RECOMMENDED for the pseudorandom function to be performant.  The
   [SipHash-2-4] pseudorandom function introduced in Section 4.4 fit
   these recommendations.

   Changing the Server Secret regularly is RECOMMENDED but, when a
   secure pseudorandom function is used, it need not be changed too
   frequently.  For example once a month would be adequate.  See
   Section 5 on operator and implementation guidelines for updating a
   Server Secret.

   The 128-bit Server Cookie consists of Sub-Fields: a 1 octet Version
   Sub-Field, a 3 octet Reserved Sub-Field, a 4 octet Timestamp Sub-
   Field and an 8 octet Hash Sub-Field.

    0                   1                   2                   3
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Version    |                   Reserved                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Hash                              |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.1.  The Version Sub-Field

   The Version Sub-Field prescribes the structure and Hash calculation
   formula.  This document defines Version 1 to be the structure and way
   to calculate the Hash Sub-Field as defined in this Section.

4.2.  The Reserved Sub-Field

   The value of the Reserved Sub-Field is reserved for future versions
   of server side Cookie construction.  On construction it MUST be set
   to zero octets.  On Server Cookie verification the server MUST NOT
   enforce those fields to be zero and the Hash should be computed with
   the received value as described in Section 4.4.

4.3.  The Timestamp Sub-Field

   The Timestamp value prevents Replay Attacks and MUST be checked by
   the server to be within a defined period of time.  The DNS server
   SHOULD allow Cookies within 1 hour period in the past and 5 minutes
   into the future to allow operation of low volume clients and some
   limited time skew between the DNS servers in the anycast set.

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   The Timestamp value specifies a date and time in the form of a 32-bit
   *unsigned* number of seconds elapsed since 1 January 1970 00:00:00
   UTC, ignoring leap seconds, in network byte order.  All comparisons
   involving these fields MUST use "Serial number arithmetic", as
   defined in [RFC1982].  The [RFC1982] specifies how the differences
   should be handled.  This handles any relative time window less than
   68 years, at any time in the future (2038 or 2106 or 2256 or 22209 or
   later.)

   The DNS server SHOULD generate a new Server Cookie at least if the
   received Server Cookie from the client is more than half an hour old,
   but MAY generate a new cookie more often than that.

4.4.  The Hash Sub-Field

   It's important that all the DNS servers use the same algorithm for
   computing the Server Cookie.  This document defines the Version 1 of
   the server side algorithm to be:

   Hash = SipHash-2-4(
       Client Cookie | Version | Reserved | Timestamp | Client-IP,
       Server Secret )

   where "|" indicates concatenation.

   Notice that Client-IP is used for hash generation even though it is
   not included in the cookie value itself.  Client-IP can be either 4
   bytes for IPv4 or 16 bytes for IPv6.  The length of all the
   concatenated elements (the input into [SipHash-2-4]) MUST be either
   precisely 20 bytes in case of an IPv4 Client-IP or precisely 32 bytes
   in case of an IPv6 Client-IP.

   When a DNS server receives a Server Cookie version 1 for validation,
   the length of the received COOKIE option MUST be precisely 24 bytes:
   8 bytes for the Client Cookie plus 16 bytes for the Server Cookie.
   Verification of the length of the received COOKIE option is REQUIRED
   to guarantee the length of the input into [SipHash-2-4] to be
   precisely 20 bytes in case of an IPv4 Client-IP and precisely 32
   bytes in case of an IPv6 Client-IP.  This ensures that the input into
   [SipHash-2-4] is an injective function of the elements making up the
   input, and thereby prevents data substitution attacks.  More
   specifically, this prevents a 36 byte COOKIE option coming from an
   IPv4 Client-IP to be validated as if it were coming from an IPv6
   Client-IP.

   The Server Secret MUST be configurable to make sure that servers in
   an anycast network return consistent results.

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5.  Updating the Server Secret

   Changing the Server Secret regularly is RECOMMENDED.  All servers in
   an anycast set must be able to verify the Server Cookies constructed
   by all other servers in that anycast set at all times.  Therefore it
   is vital that the Server Secret is shared among all servers before it
   is used to generate Server Cookies on any server.

   Also, to maximize maintaining established relationships between
   clients and servers, an old Server Secret should be valid for
   verification purposes for a specific period.

   To facilitate this, deployment of a new Server Secret MUST be done in
   three stages:

   Stage 1
      The new Server Secret is deployed on all the servers in an anycast
      set by the operator.

      Each server learns the new Server Secret, but keeps using the
      previous Server Secret to generate Server Cookies.

      Server Cookies constructed with the both the new Server Secret and
      with the previous Server Secret are considered valid when
      verifying.

      After stage 1 completed, all the servers in the anycast set have
      learned the new Server Secret, and can verify Server Cookies
      constructed with it, but keep generating Server Cookies with the
      old Server Secret.

   Stage 2
      This stage is initiated by the operator after the Server Cookie is
      present on all members in the anycast set.

      When entering Stage 2, servers start generating Server Cookies
      with the new Server Secret.  The previous Server Secret is not yet
      removed/forgotten about.

      Server Cookies constructed with the both the new Server Secret and
      with the previous Server Secret are considered valid when
      verifying.

   Stage 3
      This stage is initiated by the operator when it can be assumed
      that most clients have obtained a Server Cookie derived from the
      new Server Secret.

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      With this stage, the previous Server Secret can be removed and
      MUST NOT be used anymore for verifying.

      We RECOMMEND the operator to wait at least a period to be the
      longest TTL in the zones served by the server plus 1 hour after it
      initiated Stage 2, before initiating Stage 3.

      The operator SHOULD wait at least longer than the period clients
      are allowed to use the same Server Cookie, which SHOULD be 1 hour,
      see Section 4.3.

6.  Cookie Algorithms

   [SipHash-2-4] is a pseudorandom function suitable as Message
   Authentication Code.  This document REQUIRES compliant DNS server to
   use SipHash-2-4 as a mandatory and default algorithm for DNS Cookies
   to ensure interoperability between the DNS Implementations.

   The construction method and pseudorandom function used in calculating
   and verifying the Server Cookies are determined by the initial
   version byte and by the length of the Server Cookie.  Additional
   pseudorandom or construction algorithms for Server Cookies might be
   added in the future.

7.  IANA Considerations

   IANA is requested to create a registry on the "Domain Name System
   (DNS) Parameters" IANA web page as follows:

   Registry Name: DNS Server Cookie Methods
   Assignment Policy: Expert Review
   Reference: [this document], [RFC7873]
   Note: Server Cookie method (construction and pseudorandom algorithm)
   are determined by the Version in the first byte of the Cookie and by
   the Cookie size.  Server Cookie size is limited to the inclusive
   range of 8 to 32 bytes.

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        +=========+=======+=======================================+
        | Version |  Size | Method                                |
        +=========+=======+=======================================+
        |       0 |  8-32 | reserved                              |
        +---------+-------+---------------------------------------+
        |       1 |  8-15 | unassigned                            |
        +---------+-------+---------------------------------------+
        |       1 |    16 | SipHash-2-4 [this document] Section 4 |
        +---------+-------+---------------------------------------+
        |       1 | 17-32 | unassigned                            |
        +---------+-------+---------------------------------------+
        |   2-239 |  8-32 | unassigned                            |
        +---------+-------+---------------------------------------+
        | 240-254 |  8-32 | private use                           |
        +---------+-------+---------------------------------------+
        |     255 |  8-32 | reserved                              |
        +---------+-------+---------------------------------------+

                                  Table 1

8.  Security and Privacy Considerations

   DNS Cookies provide limited protection to DNS servers and clients
   against a variety of denial of service amplification, forgery or
   cache poisoning attacks by off-path attackers.  They provide no
   protection against on-path adversaries that can observe the plaintext
   DNS traffic.  An on-path adversary that can observe a Server Cookie
   for a client and server interaction, can use that Server Cookie for
   denial of service amplification, forgery or cache poisoning attacks
   directed at that client for the lifetime of the Server Cookie.

8.1.  Client Cookie construction

   In [RFC7873] it was RECOMMENDED to construct a Client Cookie by using
   a pseudorandom function of the Client IP address, the Server IP
   address, and a secret quantity known only to the client.  The Client
   IP address was included to ensure that a client could not be tracked
   if its IP address changes due to privacy mechanisms or otherwise.

   In this document, we changed Client Cookie construction to be just 64
   bits of entropy newly created for each new upstream server the client
   connects to.  As a consequence additional care needs to be taken to
   prevent tracking of clients.  To prevent tracking, a new Client
   Cookie for a server MUST be created whenever the Client IP address
   changes.

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   Unfortunately, tracking Client IP address changes is impractical with
   servers that do not support DNS Cookies.  To prevent tracking of
   clients with non DNS Cookie supporting servers, a client MUST NOT
   send a previously sent Client Cookie to a server not known to support
   DNS Cookies.  To prevent the creation of a new Client Cookie for each
   query to an non DNS Cookies supporting server, it is RECOMMENDED to
   not send a Client Cookie to that server for a certain period, for
   example five minutes.

   Summarizing:

   *  In order to provide minimal authentication, a client MUST use a
      different Client Cookie for each different Server IP address.

   *  To prevent tracking of clients, a new Client Cookie MUST be
      created when the Client IP address changes.

   *  To prevent tracking of clients by a non DNS Cookie supporting
      server, a client MUST NOT send a previously sent Client Cookie to
      a server in the absence of an associated Server Cookie.

   Note that it is infeasible for a client to detect change of the
   public IP address when the client is behind a routing device
   performing Network Address Translation (NAT).  A server may track the
   public IP address of that routing device performing the NAT.
   Preventing tracking of the public IP of a NAT performing routing
   device is beyond the scope of this document.

8.2.  Server Cookie construction

   [RFC7873] did not give a precise recipe for constructing Server
   Cookies, but did recommend usage of a pseudorandom function strong
   enough to prevent guessing of cookies.  In this document SipHash-2-4
   is assigned as the pseudorandom function to be used for version 1
   Server Cookies.  SipHash-2-4 is considered sufficiently strong for
   the immediate future, but predictions about future development in
   cryptography and cryptanalysis are beyond the scope of this document.

   The precise structure of version 1 Server Cookies is defined in this
   document.  Portion of the structure is made up of unhashed data
   elements which are exposed in clear text to an on-path observer.
   These unhashed data elements are taken along as input to the SipHash-
   2-4 function of which the result is the other portion of the Server
   Cookie, so the unhashed portion of the Server Cookie can not by
   changed by an on-path attacking without also recalculating the hashed
   portion for which the Server Secret needs to be known.

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   One of the elements in the unhashed portion of version 1 Server
   Cookies is a Timestamp used to prevent Replay Attacks.  Servers
   verifying version 1 Server Cookies need to have access to a reliable
   time value to compare with the Timestamp value, that cannot be
   altered by an attacker.  Furthermore, all servers participating in an
   anycast set that validate version 1 Server Cookies need to have their
   clocks synchronized.

   The cleartext Timestamp data element reveal to an on-path adversary
   using an observed Server Cookie to attack the client for which the
   Server Cookie was constructed (as shown in the first paragraph of
   this Section), the lifetime the observed Server Cookie can be used
   for the attack.

   In addition to the Security Considerations in this section, the
   Security Considerations section of [RFC7873] still apply.

9.  Acknowledgements

   Thanks to Witold Krecicki and Pieter Lexis for valuable input,
   suggestions and text and above all for implementing a prototype of an
   interoperable DNS Cookie in Bind9, Knot and PowerDNS during the
   hackathon of IETF104 in Prague.  Thanks for valuable input and
   suggestions go to Ralph Dolmans, Bob Harold, Daniel Salzman, Martin
   Hoffmann, Mukund Sivaraman, Petr Spacek, Loganaden Velvindron, Bob
   Harold, Philip Homburg, Tim Wicinski and Brian Dickson.

10.  Normative References

   [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
              DOI 10.17487/RFC1982, August 1996,
              <https://www.rfc-editor.org/info/rfc1982>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <https://www.rfc-editor.org/info/rfc3339>.

   [RFC7873]  Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
              Cookies", RFC 7873, DOI 10.17487/RFC7873, May 2016,
              <https://www.rfc-editor.org/info/rfc7873>.

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [SipHash-2-4]
              Aumasson, J. and D. J. Bernstein, "SipHash: a fast short-
              input PRF", Progress in Cryptology - INDOCRYPT
              2012. Lecture Notes in Computer Science, vol 7668.
              Springer., 2012,
              <https://doi.org/10.1007/978-3-642-34931-7_28>.

11.  Informative References

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
              <https://www.rfc-editor.org/info/rfc4941>.

Appendix A.  Test vectors

A.1.  Learning a new Server Cookie

   A resolver (client) sending from IPv4 address 198.51.100.100, sends a
   query for "example.com" to an authoritative server listening on
   192.0.2.53 from which it has not yet learned the server cookie.

   The DNS requests and replies shown in this Appendix, are in a "dig"
   like format.  The content of the DNS COOKIE Option is shown in
   hexadecimal format after "; COOKIE:".

   ;; Sending:
   ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 57406
   ;; flags:; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

   ;; OPT PSEUDOSECTION:
   ; EDNS: version: 0, flags:; udp: 4096
   ; COOKIE: 2464c4abcf10c957
   ;; QUESTION SECTION:
   ;example.com.                IN      A

   ;; QUERY SIZE: 52

   The authoritative nameserver (server) is configured with the
   following secret: e5e973e5a6b2a43f48e7dc849e37bfcf (as hex data).

   It receives the query at Wed Jun 5 10:53:05 UTC 2019.

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   The content of the DNS COOKIE Option that the server will return is
   shown below in hexadecimal format after "; COOKIE:".

   The Timestamp field Section 4.3 in the returned Server Cookie has
   value 1559731985.  In [RFC3339] format this is 2019-06-05
   10:53:05+00:00.

   ;; Got answer:
   ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 57406
   ;; flags: qr aa; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

   ;; OPT PSEUDOSECTION:
   ; EDNS: version: 0, flags:; udp: 4096
   ; COOKIE: 2464c4abcf10c957010000005cf79f111f8130c3eee29480 (good)
   ;; QUESTION SECTION:
   ;example.com.                IN      A

   ;; ANSWER SECTION:
   example.com.         86400   IN      A       192.0.2.34

   ;; Query time: 6 msec
   ;; SERVER: 192.0.2.53#53(192.0.2.53)
   ;; WHEN: Wed Jun  5 10:53:05 UTC 2019
   ;; MSD SIZE  rcvd: 84

A.2.  The same client learning a renewed (fresh) Server Cookie

   40 minutes later, the same resolver (client) queries the same server
   for "example.org".  It reuses the Server Cookie it learned in the
   previous query.

   The Timestamp field in that previously learned Server Cookie, which
   is now send along in the request, was and is 1559731985.  In
   [RFC3339] format this is 2019-06-05 10:53:05+00:00.

   ;; Sending:
   ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 50939
   ;; flags:; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

   ;; OPT PSEUDOSECTION:
   ; EDNS: version: 0, flags:; udp: 4096
   ; COOKIE: 2464c4abcf10c957010000005cf79f111f8130c3eee29480
   ;; QUESTION SECTION:
   ;example.org.                IN      A

   ;; QUERY SIZE: 52

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   The authoritative nameserver (server) now generates a new Server
   Cookie.  The server SHOULD do this because it can see the Server
   Cookie send by the client is older than half an hour Section 4.3, but
   it is also fine for a server to generate a new Server Cookie sooner,
   or even for every answer.

   The Timestamp field in the returned new Server Cookie has value
   1559734385, which in [RFC3339] format is 2019-06-05 11:33:05+00:00.

   ;; Got answer:
   ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 50939
   ;; flags: qr aa; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

   ;; OPT PSEUDOSECTION:
   ; EDNS: version: 0, flags:; udp: 4096
   ; COOKIE: 2464c4abcf10c957010000005cf7a871d4a564a1442aca77 (good)
   ;; QUESTION SECTION:
   ;example.org.                IN      A

   ;; ANSWER SECTION:
   example.org.         86400   IN      A       192.0.2.34

   ;; Query time: 6 msec
   ;; SERVER: 192.0.2.53#53(192.0.2.53)
   ;; WHEN: Wed Jun  5 11:33:05 UTC 2019
   ;; MSD SIZE  rcvd: 84

A.3.  Another client learning a renewed Server Cookie

   Another resolver (client) with IPv4 address 203.0.113.203 sends a
   request to the same server with a valid Server Cookie that it learned
   before (at Wed Jun 5 09:46:25 UTC 2019).

   The Timestamp field in Server Cookie in the request has value
   1559727985, which in [RFC3339] format is 2019-06-05 09:46:25+00:00.

   Note that the Server Cookie has Reserved bytes set, but is still
   valid with the configured secret; the Hash part is calculated taking
   along the Reserved bytes.

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   ;; Sending:
   ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 34736
   ;; flags:; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

   ;; OPT PSEUDOSECTION:
   ; EDNS: version: 0, flags:; udp: 4096
   ; COOKIE: fc93fc62807ddb8601abcdef5cf78f71a314227b6679ebf5
   ;; QUESTION SECTION:
   ;example.com.                IN      A

   ;; QUERY SIZE: 52

   The authoritative nameserver (server) replies with a freshly
   generated Server Cookie for this client conformant with this
   specification; so with the Reserved bits set to zero.

   The Timestamp field in the returned new Server Cookie has value
   1559734700, which in [RFC3339] format is 2019-06-05 11:38:20+00:00.

   ;; Got answer:
   ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 34736
   ;; flags: qr aa; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

   ;; OPT PSEUDOSECTION:
   ; EDNS: version: 0, flags:; udp: 4096
   ; COOKIE: fc93fc62807ddb86010000005cf7a9acf73a7810aca2381e (good)
   ;; QUESTION SECTION:
   ;example.com.                IN      A

   ;; ANSWER SECTION:
   example.com.         86400   IN      A       192.0.2.34

   ;; Query time: 6 msec
   ;; SERVER: 192.0.2.53#53(192.0.2.53)
   ;; WHEN: Wed Jun  5 11:38:20 UTC 2019
   ;; MSD SIZE  rcvd: 84

A.4.  IPv6 query with rolled over secret

   The query below is from a client with IPv6 address
   2001:db8:220:1:59de:d0f4:8769:82b8 to a server with IPv6 address
   2001:db8:8f::53.  The client has learned a valid Server Cookie before
   (at Wed Jun 5 13:36:57 UTC 2019) when the Server had the secret:
   dd3bdf9344b678b185a6f5cb60fca715.  The server now uses a new secret,
   but it can still validate the Server Cookie provided by the client as
   the old secret has not expired yet.

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   The Timestamp field in the Server Cookie in the request has value
   1559741817, which in [RFC3339] format is 2019-06-05 13:36:57+00:00.

   ;; Sending:
   ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 6774
   ;; flags:; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

   ;; OPT PSEUDOSECTION:
   ; EDNS: version: 0, flags:; udp: 4096
   ; COOKIE: 22681ab97d52c298010000005cf7c57926556bd0934c72f8
   ;; QUESTION SECTION:
   ;example.net.                IN      A

   ;; QUERY SIZE: 52

   The authoritative nameserver (server) replies with a freshly
   generated server cookie for this client with its new secret:
   445536bcd2513298075a5d379663c962

   The Timestamp field in the returned new Server Cookie has value
   1559741961, which in [RFC3339] format is .

   ;; Got answer:
   ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 6774
   ;; flags: qr aa; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

   ;; OPT PSEUDOSECTION:
   ; EDNS: version: 0, flags:; udp: 4096
   ; COOKIE: 22681ab97d52c298010000005cf7c609a6bb79d16625507a (good)
   ;; QUESTION SECTION:
   ;example.net.                IN      A

   ;; ANSWER SECTION:
   example.net.         86400   IN      A       192.0.2.34

   ;; Query time: 6 msec
   ;; SERVER: 2001:db8:8f::53#53(2001:db8:8f::53)
   ;; WHEN: Wed Jun  5 13:36:57 UTC 2019
   ;; MSD SIZE  rcvd: 84

Appendix B.  Implementation status

   At the time of writing, BIND from version 9.16 and Knot DNS from
   version 2.9.0 create Server Cookies according to the recipe described
   in this draft.  Unbound and NSD have an Proof of Concept
   implementation that has been tested for interoperability during the
   hackathon at the IETF104 in Prague.  Construction of privacy
   maintaining Client Cookies according to the directions in this draft

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   have been implemented in the getdns library and will be in the
   upcoming getdns-1.6.1 release and in Stubby version 0.3.1.

Authors' Addresses

   Ondrej Sury
   Internet Systems Consortium
   Czechia

   Email: ondrej@isc.org

   Willem Toorop
   NLnet Labs
   Science Park 400
   1098 XH Amsterdam
   Netherlands

   Email: willem@nlnetlabs.nl

   Donald E. Eastlake 3rd
   Futurewei Technologies
   1424 Pro Shop Court
   Davenport,  FL 33896
   United States of America

   Phone: +1-508-333-2270
   Email: d3e3e3@gmail.com

   Mark Andrews
   Internet Systems Consortium
   950 Charter Street
   Redwood City,  CA 94063
   United States of America

   Email: marka@isc.org

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