Status of This Memo

This is an Internet Standards Track document.¶

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9018.¶

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 and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶

Table of Contents

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 behavioral 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 Section 6 of [RFC7873], for simplicity, it is "RECOMMENDED 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 shares 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.¶

2. Changes to RFC 7873

Appendices A.1 and B.1 of [RFC7873] provide 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.¶

Appendix B.2 of [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 then reasonable to change the Client Cookie 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

Previously, the recommended algorithm to compute the Client Cookie included the Client IP address as an input to a hashing function. However, when implementing the DNS Cookies, several DNS vendors found it 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 the 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 reuse a Client or Server Cookie after the Client IP address has changed.¶

One way to satisfy this requirement for non-reuse 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 and that appears as the Server Cookie field in the weakly authenticated 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 time span 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 that is known only to the server or only to the set of servers at the same anycast address.¶

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 DNS servers need to use the pseudorandom function in order to verify Server Cookies, it is RECOMMENDED that it be efficient to calculate. The pseudorandom function described in [SipHash-2-4] and introduced in Section 4.4 of this document fits these recommendations.¶

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

The 128-bit Server Cookie consists of the following 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                              |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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 both the new Server Secret and the previous Server Secret are considered valid when verifying.¶

After stage 1 is 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.¶

Server Cookies constructed with both the new Server Secret and 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.¶

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

It is RECOMMENDED that the operator wait, after initiating Stage 2 and before initiating Stage 3, at least a period of time equal to the longest TTL in the zones served by the server plus 1 hour.¶

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 a Message Authentication Code. It is REQUIRED that a compliant DNS server 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 has created a registry under the "Domain Name System (DNS) Parameters" heading as follows:¶

Registry Name:

DNS Server Cookie Methods¶

Assignment Policy:

Expert Review¶

Reference:

[RFC9018], [RFC7873]¶

Note:

A Server Cookie method (construction and pseudorandom algorithm) is 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.¶

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.¶

Appendix A. Test Vectors

;; 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

;; 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

;; 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

;; 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

;; 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

;; 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

;; 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

;; 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 document. Unbound and NSD have a Proof-of-Concept implementation that has been tested for interoperability during the hackathon at IETF 104 in Prague. Construction of privacy maintaining Client Cookies according to the directions in this document 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.¶

Acknowledgements

Thanks to Witold Krecicki and Pieter Lexis for valuable input, suggestions, text, and above all for implementing a prototype of an interoperable DNS Cookie in Bind9, Knot, and PowerDNS during the hackathon at IETF 104 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.¶

Authors' Addresses