DNSOP Working Group O. Sury
Internet-Draft Internet Systems Consortium
Updates: 7873 (if approved) W. Toorop
Intended status: Standards Track NLnet Labs
Expires: September 12, 2019 March 11, 2019
Algorithms for Domain Name System (DNS) Cookies construction
draft-sury-toorop-dns-cookies-algorithms-00
Abstract
[RFC7873] left the construction of Server Cookies to the discretion
of the DNS Server (implementer) which has resulted in a gallimaufry
of different implementations. As a result, DNS Cookies are
impractical to deploy on multi-vendor anycast networks, because the
Server Cookie constructed by one implementation cannot be validated
by another.
This document provides precise directions for creating Server Cookies
to address this issue. Furthermore, [FNV] is obsoleted as a suitable
Hash function for calculating DNS Cookies. [SipHash-2.4] is
introduced as a new REQUIRED Hash function for calculating DNS
Cookies.
This document updates [RFC7873]
Status of This Memo
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This Internet-Draft will expire on September 12, 2019.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Contents of this document . . . . . . . . . . . . . . . . 3
1.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 3
2. Constructing a Client Cookie . . . . . . . . . . . . . . . . 3
3. Constructing a Server Cookie . . . . . . . . . . . . . . . . 3
3.1. The Version Sub-Field . . . . . . . . . . . . . . . . . . 4
3.2. The Cookie algo Sub-Field . . . . . . . . . . . . . . . . 4
3.3. The Reserved Sub-Field . . . . . . . . . . . . . . . . . 4
3.4. The Timestamp Sub-Field . . . . . . . . . . . . . . . . . 4
3.5. The Hash Sub-Field . . . . . . . . . . . . . . . . . . . 5
4. Cookie Algorithms . . . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Normative References . . . . . . . . . . . . . . . . . . 6
6.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
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 let to DNS Cookie implementations, calculating the
Server Cookie in different ways. This causes problems with anycast
deployments with DNS Software from multiple vendors, because even
when all DNS Software would share the same secret, as RECOMMENDED in
Section 6. of [RFC7873], they all produce different Server Cookies
based on that secret and (at least) the Client Cookie and Client IP
Address.
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1.1. Contents of this document
In Section Section 2 instructions for constructing a Client Cookie
are given
In Section Section 3 instructions for constructing a Server Cookie
are given
In Section Section 4 the different hash functions usable for DNS
Cookie construction are listed. [FNV] and HMAC-SHA-256-64 [RFC6234]
are obsoleted and AES [RFC5649] and [SipHash-2.4] are introduced as a
REQUIRED hash function for DNS Cookie implementations.
1.2. 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.
2. Constructing a Client Cookie
The Client Cookie is a nonce and should be treated as such. For
simplicity, it can be calculated from Client IP Address, Server IP
Address and a secret known only to the Client. The Client Cookie
SHOULD have at least 64-bits of entropy. If a secure pseudorandom
function (like SipHash24) is used there's no need to change Client
secret periodically and change the Client secret only if it has been
compromised.
It's recommended but not required that a pseudorandom function is
used to construct the Client Cookie:
Client-Cookie = MAC_Algorithm(
Client IP Address | Server IP Address, Client Secret )
where "|" indicates concatenation.
3. Constructing a Server Cookie
The Server Cookie is effectively message authentication code (MAC)
and should be treated as such.
The Server Cookie is not required to be changed periodically if a
secure pseudorandom function is used.
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The 128-bit Server Cookie consists of Sub-Fields: a 1 octet Version
Sub-Field, a 1 octet Cookie Algorithm Sub-Field, a 2 octet Reserved
Sub-Field, a 4 octet Timestamp Sub-Field and a 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 | Cookie Algo | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hash |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.1. 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.
3.2. The Cookie algo Sub-Field
The Cookie Algo value defines what algorithm function to use for
calculating the Hash Sub-Field as described in Section 3.5. The
values are described in Section 4.
3.3. The Reserved Sub-Field
The value of the Reserved Sub-Field is reserved for future versions
of Server Side Cookie construction. Even though the value has no
specific meaning in this Version, note that it *is* used in
determining the Hash value as described in Section 3.5.
3.4. 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 certain
time skew between the DNS servers in the anycast.
The DNS Server SHOULD generate new Server Cookie at least if the
received Server Cookie from the Client is older than half an hour.
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3.5. 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 = Cookie_Algorithm(
Client Cookie | Version | Cookie Algo | Reserved | TimeStamp,
Server Secret )
4. Cookie Algorithms
Implementation recommendations for Cookie Algorithms [DNSCOOKIE-
IANA]:
+--------+-----------------+---------------+---------------+
| Number | Mnemonics | Client Cookie | Server Cookie |
+--------+-----------------+---------------+---------------+
| 1 | FNV | MUST NOT | MUST NOT |
| 2 | HMAC-SHA-256-64 | MUST NOT | MUST NOT |
| 3 | AES | MAY | MAY |
| 4 | SIPHASH24 | MUST | MUST |
+--------+-----------------+---------------+---------------+
[FNV] is a Non-Cryptographic Hash Algorithm and this document
obsoletes the usage of FNV in DNS Cookies.
HMAC-SHA-256-64 is an HMAC-SHA-256 [RFC6234] algorithm reduced to
64-bit. This particular algorithm was implemented in BIND, but it
was never the default algorithm and the computational costs makes it
unsuitable to be used in DNS Cookies. Therefore this document
obsoletes the usage of HMAC-SHA-256 algorithm in the DNS Cookies.
The AES algorithm [RFC5649] has been the default DNS Cookies
algorithm in BIND until version x.y.z, and other implementations MAY
implement AES algorithm as implemented in BIND for backwards
compatibility. However it's recommended that new implementations
implement only a pseudorandom functions for DNS Cookies, in this
document that would be SipHash24.
[SipHash-2.4] is a pseudorandom function suitable as message
authentication code, and this document REQUIRES compliant DNS Server
to use SipHash24 as a mandatory and default algorithm for DNS Cookies
to ensure interoperability between the DNS Implementations.
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5. IANA Considerations
IANA is requested to create and maintain a sub-registry (the "DNS
Cookie Algorithm" registry) of the "Domain Name System (DNS)
Parameters" registry. The initial values for this registry are
described in Section 4.
6. References
6.1. Normative References
[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>.
[RFC5649] Housley, R. and M. Dworkin, "Advanced Encryption Standard
(AES) Key Wrap with Padding Algorithm", RFC 5649,
DOI 10.17487/RFC5649, September 2009,
<https://www.rfc-editor.org/info/rfc5649>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/info/rfc6234>.
[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>.
6.2. Informative References
[FNV] Fowler, G., Noll, L., Vo, K., Eastlake, D., and T. Hansen,
"The FNV Non-Cryptographic Hash Algorithm",
<https://datatracker.ietf.org/doc/draft-eastlake-fnv>.
[SipHash-2.4]
Aumasson, J. and D. Bernstein, "SipHash: a fast short-
input PRF", 2012, <https://131002.net/siphash/>.
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Authors' Addresses
Ondrej Sury
Internet Systems Consortium
CZ
Email: ondrej@isc.org
Willem Toorop
NLnet Labs
Science Park 400
Amsterdam 1098 XH
Netherlands
Email: willem@nlnetlabs.nl
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