DPRIVE J. Livingood
Internet-Draft Comcast
Intended status: Informational A. Mayrhofer
Expires: December 18, 2020 nic.at GmbH
B. Overeinder
NLnet Labs
June 16, 2020
DNS Privacy Requirements for Exchanges between Recursive Resolvers and
Authoritative Servers
draft-ietf-dprive-phase2-requirements-01
Abstract
This document provides requirements for adding confidentiality to DNS
exchanges between recursive resolvers and authoritative servers.
Status of This Memo
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Table of Contents
1. Introduction & Scope . . . . . . . . . . . . . . . . . . . . 2
2. Document Work Via GitHub . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Threat Model and Problem Statement . . . . . . . . . . . . . 3
5. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 4
5.1. Mandatory Requirements . . . . . . . . . . . . . . . . . 4
5.2. Optional Requirements . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
8. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 5
9. APPENDIX: Perspectives and Use Cases . . . . . . . . . . . . 5
9.1. The User Perspective and Use Cases . . . . . . . . . . . 6
9.2. The Operator Perspective and Use Cases . . . . . . . . . 6
9.3. The Implementor / Software Vendor Perspective and Use
Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . 9
10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction & Scope
The 2018 approved charter of the IETF DPRIVE Working Group [1]
contains milestones related to confidentiality aspects of DNS
transactions between the iterative resolver and authoritative name
servers.
This is also reflected in the DPRIVE milestones [2], which (as of
October 2019) contains two relevant milestones:
Develop requirements for adding confidentiality to DNS exchanges
between recursive resolvers and authoritative servers (unpublished
document).
Investigate potential solutions for adding confidentiality to DNS
exchanges involving authoritative servers (Experimental).
This document intends to cover the first milestone for defining
requirements for adding confidentiality to DNS exchanges between
recursive resolvers and authoritative servers. This may in turn lead
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to progress in investigating, developing and standardizing potential
experimental methods of meeting those requirements.
The motivation for this work is to extend the confidentiality methods
used between a user's stub resolver and a recursive resolver to the
recursive queries sent by recursive resolvers in response to a DNS
lookup (when a cache miss occurs and the server must perform
recursion to obtain a response to the query). A recursive resolver
will send queries to root servers, to Top Level Domain (TLD) servers,
to authoritative second level domain servers and potentially to other
authoritative DNS servers and each of these query/response
transactions presents an opportunity to extend the confidentiality of
user DNS queries.
2. Document Work Via GitHub
The authors are working on this document via GitHub at
https://github.com/alex-nicat/ietf-dprive-phase2-requirements.
Feedback via pull requests and issues are invited there.
3. Terminology
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.
This document also makes use of DNS Terminology defined in [RFC8499]
4. Threat Model and Problem Statement
Currently, protocols such as DoT provide encryption between the
user's stub resolver and a recursive resolver. This potentially
provides (1) protection from observation of end user DNS queries and
responses, (2) protection from on-the-wire modification DNS queries
or responses (including potentially forcing a downgrade to an
unencrypted communication). Of course, observation and modification
are still possible when performed by the recursive resolver, which
decrypts queries, serves a response from cache or performs recursion
to obtain a response (or synthesizes a response), and then encrypts
the response and sends it back to the user's stub resolver.
But observation and modification threats still exist when a recursive
resolver must perform DNS recursion, from the root to TLD to
authoritative servers. This document specifies requirements for
filling those gaps.
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5. Requirements
The requirements of different interested stakeholders are outlined
below.
5.1. Mandatory Requirements
1. Each implementing party should be able to independently take
incremental steps to meet requirements without the need for
close coordination (e.g. loosely coupled)
2. Use a secure transport protocol between a recursive resolver and
authoritative servers
3. Use a secure transport protocol between a recursive resolver and
TLD servers
4. Use a secure transport protocol between a recursive resolver and
the root servers
5. The secure transport MUST only be established when referential
integrity can be verified, MUST NOT have circular dependencies,
and MUST be easily analyzed for diagnostic purposes.
6. Use a secure transport protocol or other DNS privacy protections
in a manner that enables operators to perform appropriate
performance and security monitoring, conduct relevant research,
etc.
7. The authoritative domain owner or their administrator MUST have
the option to specify their secure transport preferences (e.g.
what specific protocols are supported). This SHALL include a
method to publish a list of secure transport protocols (e.g.
DoH, DoT and other future protocols not yet developed). In
addition this SHALL include whether a secure transport protocol
MUST always be used (non-downgradable) or whether a secure
transport protocol MAY be used on an opportunistic (not strict)
basis.
8. The authoritative domain owner or their administrator MUST have
the option to vary their preferences on an authoritative
nameserver to nameserver basis, due to the fact that
administration of a particular DNS zone may be delegated to
multiple parties (such as several CDNs), each of which may have
different technical capabilities.
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9. The specification of secure transport preferences MUST be
performed using the DNS and MUST NOT depend on non-DNS
protocols.
10. For the secure transport, TLS 1.3 (or later versions) MUST be
supported and downgrades from TLS 1.3 to prior versions MUST not
occur.
5.2. Optional Requirements
1. QNAME minimisation SHOULD be implemented in all steps of
recursion
2. DNSSEC validation SHOULD be performed
3. If an authoritative domain owner or their administrator indicates
that (1) multiple secure transport protocols are available or
that (2) a secure transport and insecure transport are available,
then per the recommendations in [RFC8305] (aka Happy Eyeballs) a
recursive server SHOULD initiate concurrent connections to
available protocols. Consistent with Section 2 of [RFC8305] this
would be: (1) Initiation of asynchronous DNS queries to determine
what transport protocols are supported, (2) Sorting of resolved
destination transport protocols, (3) Initiation of asynchronous
connection attempts, and (4) Establishment of one connection,
which cancels all other attempts.
6. Security Considerations
This entire document concerns the security of DNS traffic, so a
specific section on security is superfluous.
7. IANA Considerations
This document has no actions for IANA.
8. Changelog
Version 00: Updated prior individual draft following IETF-106
feedback
9. APPENDIX: Perspectives and Use Cases
The DNS resolving process involves several entities. These entities
have different interests/requirements, and hence it does make sense
to examine the interests of those entities separately - though in
many cases their interests are aligned. Four different entities can
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be identified, and their interests are described in the following
sections:
o Users
o Operators
o Implementors / Software Developers
o Researchers
9.1. The User Perspective and Use Cases
The privacy and confidentiality of Users (that is, users as in
clients of recursive resolvers, which in turn forward/resolve the
user's DNS requests by contacting authoritative servers) can be
improved in several ways. We call this "minimisation of exposure",
and there are currently three ways to reduce that exposure:
o Qname minimisation [RFC7816], reducing the amount of information
to what is absolutely necessary to resolve a query
o Aggressive NSEC/local auth cache [RFC8198], reducing the amount of
outgoing queries in the first place
o Encryption, removing exposure of information while in transit
As recursors typically forwards queries received from the user to
authoritative servers. This creates a transitive trust between the
user and the recursor, as well as the authoritative server, since
information created by the user is exposed to the authoritative
server. However, the user never has a chance to identify what data
was exposed to which authoritative party (via which path).
Also, Users would want to be informed about the status of the
connections which were made on their behalf, which adds a fourth
point
Encryption/privacy status signaling
*TODO*: Actual requirements - what do users "want"? Start below:
9.2. The Operator Perspective and Use Cases
Operators of authoritative services have to provide stable and fast
DNS services, and interact with a wide range of clients, not all of
them authoritative servers. The operator side actually consists of
two sides:
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o The "upstream" facing side of recursive resolvers
o The "downstream" side of authoritative servers
Those two sides are typically operated by different entities, but
many entities operate "both sides". Even though that is discouraged
(*TODO* source), the two sides might even be operated on the same
nameserver.
o Maybe different technical perspectives for operators
* Intelligence (sharing information)
* SLD popularity for marketing
o Focus initially on Second Level Domains (SLDs) initially
* Is there a difference for TLDs vs. SLDs from a "protocol"
perspective?
o Monitoring and aggregated data analysis
o Signaling provisioning information
* New record type for finding authoritative server key and
authentication? Use SRV? (Being able to use different servers
for serving up DNS-over-{TCP,UDP} vs DNS-over-TLS responses may
be valuable.
* Signal secure transport details (DNS-over-TLS, DNS-over-QUIC,
EncryptedSNI, connectionless, etc.), perhaps in an extensible
manner? Minimize RTTs and reduce need for trials.
* Large provider use cases where the NS names are out of
bailiwick for the zone (e.g. small number of distinct NS
records serving 100k+ zones)
o EDNS client subnet (JL: Not sure ECS crosses the cost/benefit
threshold to be included as a requirement and many CDNs that run
auth servers will likely say ECS is quite operationally important)
o Decide between TLS and connectionless (such as COSE-based
messages)
o Costs of TLS connection vs. connectionless
* Technical solution, e.g. encryption of the DNS query, shouldn't
enable an attack vector for DDoS or resource exhaustion. For
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example, only if the client uses DNS-over-TLS, the upstream
query to the authoritative will be over DNS-over-TLS also. If
the client uses UDP, the resolver won't invest resources in
DNS-over-TLS to prevent a potential resource exhaustion attack.
* Reuse connection state (if any) and examine resumption
considerations
* Minimize server-side state (eg, with session tickets)
* Need empirical studies on capacity, traffic, attack vectors
* Evaluate impact on architecture and footprint expansion
* Analyze optimal persistent connection time/time-out
* Analyze optimal number of persistent connections recursive
resolvers should maintain
* Consider operational concerns with respect to capabilities
signaling
* Develop a profile that has operational advantages for operators
*TODO*: Actual requirements - what do operators "want"?
9.3. The Implementor / Software Vendor Perspective and Use Cases
Implementer requirements follows requirements from user and operator
perspectives:
o Non-functional requirements, e.g. diversity of implementations
o Horizontal vs. vertical scaling, for example similar to http
servers
o Use of DANE [RFC6698] for authentication: strict vs. opportunistic
o Incremental deployment
o Cache reuse vs. downgrade? Does the cache need to be partitioned?
When can an in-cache answer retrieved via cleartext be served
encrypted to a recursive query?
o (Use of TCP fast open) - but this might be a requirement for the
actual encryption protocol
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*TODO*: Actual requirements of implementors - essentially, they
follow what Operators need?
10. References
10.1. 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>.
[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>.
[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.
10.2. Informative References
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <https://www.rfc-editor.org/info/rfc6698>.
[RFC7816] Bortzmeyer, S., "DNS Query Name Minimisation to Improve
Privacy", RFC 7816, DOI 10.17487/RFC7816, March 2016,
<https://www.rfc-editor.org/info/rfc7816>.
[RFC8198] Fujiwara, K., Kato, A., and W. Kumari, "Aggressive Use of
DNSSEC-Validated Cache", RFC 8198, DOI 10.17487/RFC8198,
July 2017, <https://www.rfc-editor.org/info/rfc8198>.
[RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/info/rfc8305>.
10.3. URIs
[1] https://datatracker.ietf.org/doc/charter-ietf-dprive/
[2] https://datatracker.ietf.org/wg/dprive/about/
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Acknowledgments
TODO
Authors' Addresses
Jason Livingood
Comcast
Email: Jason_Livingood@comcast.com
Alexander Mayrhofer
nic.at GmbH
Email: alex.mayrhofer.ietf@gmail.com
Benno Overeinder
NLnet Labs
Email: benno@NLnetLabs.nl
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