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Updates:
Keywords: Client, Hash
Internet Engineering Task Force (IETF) O. Sury
Request for Comments: 9018 Internet Systems Consortium
Updates: 7873 W. Toorop
Category: Standards Track NLnet Labs
ISSN: 2070-1721 D. Eastlake 3rd
Futurewei Technologies
M. Andrews
Internet Systems Consortium
April 2021
Interoperable Domain Name System (DNS) Server Cookies
Abstract
DNS Cookies, as specified in RFC 7873, 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 updates RFC 7873 with precise directions for creating
Server Cookies so that an anycast server set including diverse
implementations will interoperate with standard clients, with
suggestions for constructing Client Cookies in a privacy-preserving
fashion, and with suggestions on how to update a Server Secret. An
IANA registry listing the methods and associated pseudorandom
function suitable for creating DNS Server Cookies has been created
with the method described in this document as the first and, as of
the time of publication, only entry.
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
1.1. Terminology and Definitions
2. Changes to RFC 7873
3. Constructing a Client Cookie
4. Constructing a Server Cookie
4.1. The Version Sub-Field
4.2. The Reserved Sub-Field
4.3. The Timestamp Sub-Field
4.4. The Hash Sub-Field
5. Updating the Server Secret
6. Cookie Algorithms
7. IANA Considerations
8. Security and Privacy Considerations
8.1. Client Cookie Construction
8.2. Server Cookie Construction
9. References
9.1. Normative References
9.2. Informative References
Appendix A. Test Vectors
A.1. Learning a New Server Cookie
A.2. The Same Client Learning a Renewed (Fresh) Server Cookie
A.3. Another Client Learning a Renewed Server Cookie
A.4. IPv6 Query with Rolled Over Secret
Appendix B. Implementation Status
Acknowledgements
Authors' Addresses
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.
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.
Note: "IP address" is used herein as a length-independent term
covering both IPv4 and IPv6 addresses.
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 [RFC8981],
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 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 a 1-hour period in the past and a
5-minute period into the future to allow operation of low-volume
clients and some limited time skew between the DNS servers in the
anycast set.
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]. [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, 2106, 2256, 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 it 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 the case of an IPv4 Client-IP and precisely 32
bytes in the 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.
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.
+=========+=======+=================================+
| Version | Size | Method |
+=========+=======+=================================+
| 0 | 8-32 | Reserved |
+---------+-------+---------------------------------+
| 1 | 8-15 | Unassigned |
+---------+-------+---------------------------------+
| 1 | 16 | SipHash-2-4 [RFC9018] Section 4 |
+---------+-------+---------------------------------+
| 1 | 17-32 | Unassigned |
+---------+-------+---------------------------------+
| 2-239 | 8-32 | Unassigned |
+---------+-------+---------------------------------+
| 240-254 | 8-32 | Reserved for Private Use |
+---------+-------+---------------------------------+
| 255 | 8-32 | Reserved |
+---------+-------+---------------------------------+
Table 1: DNS Server Cookie Methods
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.
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 a non-DNS Cookie-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 a change in 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 it did recommend usage of a pseudorandom function strong
enough to prevent the 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. A portion of the structure is made up of unhashed data
elements that are exposed in cleartext 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 cannot be
changed by an on-path attacker without also recalculating the hashed
portion for which the Server Secret needs to be known.
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, one that cannot be altered by an attacker, to compare
with the Timestamp value. Furthermore, all servers participating in
an anycast set that validate version 1 Server Cookies need to have
their clocks synchronized.
For an on-path adversary observing a Server Cookie (as mentioned in
the first paragraph of Section 8), the cleartext Timestamp data
element reveals the lifetime during which the observed Server Cookie
can be used to attack the client.
In addition to the Security Considerations in this section, the
Security Considerations section of [RFC7873] still applies.
9. References
9.1. 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>.
[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, December
2012, <https://doi.org/10.1007/978-3-642-34931-7_28>.
9.2. Informative References
[RFC8981] Gont, F., Krishnan, S., Narten, T., and R. Draves,
"Temporary Address Extensions for Stateless Address
Autoconfiguration in IPv6", RFC 8981,
DOI 10.17487/RFC8981, February 2021,
<https://www.rfc-editor.org/info/rfc8981>.
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 on Wed Jun 5 10:53:05 UTC 2019.
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 the format described in [RFC3339], 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 sent along in the request, was and is 1559731985. In the
format of [RFC3339], 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
The authoritative nameserver (server) now generates a new Server
Cookie. The server SHOULD do this because it can see the Server
Cookie sent 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 the format of [RFC3339], 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 (on Wed Jun 5 09:46:25 UTC 2019).
The Timestamp field of the Server Cookie in the request has value
1559727985, which, in the format of [RFC3339], 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.
;; 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, i.e., with the Reserved bits set to zero.
The Timestamp field in the returned new Server Cookie has value
1559734700, which, in the format of [RFC3339], 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
(on 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.
The Timestamp field in the Server Cookie in the request has value
1559741817, which, in the format of [RFC3339], 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 the format of [RFC3339], is 2019-06-05
13:39:21+00:00.
;; 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
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
2386 Panoramic Circle
Apopka, FL 32703
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