Interoperable Domain Name System (DNS) Server Cookies

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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 and amplification, forgery, or cache poisoning attacks by off-path attackers. This document specifies a means of producing interoperable strong cookies so that an anycast server set including diverse implementations can be easily configured to interoperate with standard clients.¶

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.¶

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 is a cryptographic nonce and should be treated as such. It is RECOMMENDED to create a new Client Cookie for each new upstream server a Client connects to. 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 period of time such as no longer than a year. 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 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 track Client IP addresses, 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 MAY 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) and should be treated as such. 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 servers responding on the same address in an anycast set.¶

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                              |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Hash = SipHash2.4(
    Client Cookie | Version | Reserved | Timestamp | Client-IP,
    Server Secret )

5. Updating the Server Secret

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.¶

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 learned the new Server Secret.¶

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 half an 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 half an 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.¶

8. Security and Privacy Considerations

DNS Cookies provide limited protection to DNS servers and clients against a variety of denial-of-service and 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 amplification and denial-of-service forgery attacks for the lifetime of the Server Cookie.¶

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.¶

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 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 minute.¶

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 for a non DNS Cookie supporting server, a client MUST NOT send a previously sent Client Cookie to that server, unless it can track Client IP Address changes for those servers too.¶

Besides the Client Cookie construction, this update on [RFC7873] does not introduce any new characteristics to DNS Cookies operations and the Security Considerations section of [RFC7873] still applies.¶

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.¶

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 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 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