Internet DRAFT - draft-lmo-dprive-phase2-requirements
draft-lmo-dprive-phase2-requirements
DPRIVE J. Livingood
Internet-Draft Comcast
Intended status: Informational A. Mayrhofer
Expires: May 7, 2020 nic.at GmbH
B. Overeinder
NLnet Labs
November 04, 2019
DNS Privacy Requirements for Exchanges between Recursive Resolvers and
Authoritative Servers
draft-lmo-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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 7, 2020.
Copyright Notice
Copyright (c) 2019 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
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction & Scope . . . . . . . . . . . . . . . . . . . . 2
2. Document Development . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Threat Model and Problem Statement . . . . . . . . . . . . . 3
5. Perspectives and Use Cases . . . . . . . . . . . . . . . . . 4
5.1. The User Perspective and Use Cases . . . . . . . . . . . 4
5.2. The Operator Perspective and Use Cases . . . . . . . . . 5
5.3. The Implementor / Software Vendor Perspective and Use
Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Preliminary Requirements . . . . . . . . . . . . . . . . . . 7
6.1. Mandatory Requirements (Proposed) . . . . . . . . . . . . 7
6.2. Optional Requirements (Proposed) . . . . . . . . . . . . 8
6.3. Working Group Discussion Needed . . . . . . . . . . . . . 8
6.4. Prioritization of Requirements . . . . . . . . . . . . . 9
6.5. Opportunistic Upgrade to Encryption . . . . . . . . . . . 9
6.6. Detection of Availability . . . . . . . . . . . . . . . . 10
6.7. Resistance to Downgrade Attack . . . . . . . . . . . . . 10
6.8. End-User Policy Propagation . . . . . . . . . . . . . . . 11
6.9. Performance and Efficiency . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
9. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. lmo-dprive-phase2-requirements-00 . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . 13
10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
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).
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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
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 Development
TEMPORARY SECTION - WILL BE REMOVED BEFORE PUBLISHING 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. The authors plan to continue developing
the document in the lead up to IETF-106, after the draft cut-off
date.
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, potentially privacy-protective protocols such as DoT
provide encryption between the user's stub resolver and a recursive
resolver. This provides (1) protection from observation of end user
DNS queries and responses as well as (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
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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.
5. 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
be identified, and their interests are described in the following
sections:
o Users
o Operators
o Implementors / Software Developers
o Researchers
5.1. 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
which 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
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server. However, the user has never a chance to identify which 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:
5.2. 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:
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.
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* 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
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"?
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5.3. 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
*TODO*: Actual requirements of implementors - essentially, they
follow what Operators need?
6. Preliminary Requirements
The requirements of different interested stakeholders are outlined
below. The parenthetical risks and priority levels are intended only
to spur discussion. But at a high level the requirements may be
summarized as follows:
6.1. Mandatory Requirements (Proposed)
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) (low risk, high priority)
2. Implement DoT between a recursive resolver and single level
domain authoritative servers (high risk, high priority)
3. Implement DNS privacy protections between a recursive resolver
and TLD servers (low risk, low priority)
4. Implement DNS privacy protections between a recursive resolver
and the root servers (low risk, low priority)
5. Implement DoT or other DNS privacy protections in a manner that
enables operators to perform appropriate performance and security
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monitoring, conduct relevant research, etc. (high risk, high
priority)
6. Implement QNAME minimisation in all steps of recursion (medium
risk, medium priority)
7. The legacy unencrypted DNS protocol (e.g. UDP/TCP port 53) MUST
be supported in parallel to DoT (high risk, high priority)
8. Recursive resolvers SHOULD opportunistically upgrade recursive
query transmissions to DoT when an authoritative server is
detected to support DoT (high risk, high priority)
9. TLS 1.3 (or later versions) MUST be supported and downgrades from
TLS 1.3 to prior versions MUST not occur.
6.2. Optional Requirements (Proposed)
1. Implement DoT between a recursive resolver and TLD servers (low
risk, low priority)
2. Implement DoT between a recursive resolver and the root servers
(low risk, low priority)
3. DNSSEC validation SHOULD be performed
4. Users SHOULD have a method for signaling their preferences for
(1) exclusively using DNS privacy & encryption, (2) preferring
DNS privacy & encryption but falling back to un-encrypted DNS as
needed, (3) exclusively using un-encrypted DNS, or other
preferences. (Possible reference to DNSSEC DO bit?)
5. Authoritative domain administrators SHOULD have a method for
signaling their preferences for (1) exclusively using DNS privacy
& encryption, (2) preferring DNS privacy & encryption but falling
back to un-encrypted DNS as needed, (3) exclusively using un-
encrypted DNS, or other preferences. (Possible reference to
DNSSEC DO bit?)
6.3. Working Group Discussion Needed
o Provisioning impacts - operators and vendors say implementation
must be zero-provisioning. What does that mean and how should
that be articulated as a requirement?
o Signaling: Provide some method to signal not just binary support
DoT / do not support to allow for certain QTYPES or whatever to
use DoT while others may not (e.g. an auth server may want to say
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in high load that some low risk or low priority queries fallback
to unencrypted comms). Is this signaling or negotiation? Perhaps
the requirement is ultimately about "Load Shedding" or "Load
Management".
o Trust anchor/authority: Should this depend only on the DNS, such
as DANE, or Certification Authorities? See discussion at
https://github.com/alex-nicat/ietf-dprive-phase2-requirements/
issues/13
o Rather than say DNS privacy methods should we specifically say no
ECS (or not fine-grained ECS), and to do QNAME minimization?
o There is a new signaling draft at https://tools.ietf.org/html/
draft-levine-dprive-signal-00 and a prior one at
https://tools.ietf.org/html/draft-bortzmeyer-dprive-step-2-05 -
are these informative for our requirements?
o Is signaling good and/or necessary.
6.4. Prioritization of Requirements
The preliminary requirements above each have varying levels of risk
and so can be prioritized based on that risk. As a result, the
highest risk area is the one that involves the greatest potential for
surveillance and modification based on the details of the specific
step of recursion. This suggests the highest risk and thus highest
priority is between a recursive server and first level authoritative
server. Lower risks are to TLDs and root servers, with
correspondingly lower priority. Support for monitoring and
compliance are also high risk since this is operationally critical,
and thus should also be considered high priority.
6.5. Opportunistic Upgrade to Encryption
Opportunistically upgrading to use encryption may be the most viable
path to deploy new DNS encryption protocols. This may enable
deployment to occur incrementally and without tightly coupled
coordination across a diverse global group of very different
potential implementors.
EDITORIAL NOTE: This paragraph may be unnecessary and could be cut.
The exact method by which a recursive resolver determines whether an
authoritative server supports DoT has not been specified in this
document. But it seems reasonable to imagine that a recursive server
might be able to probe authoritative servers on TCP/853 using the DoT
protocol and then build a cached list of servers that support DoT so
that subsequent queries will upgrade to use DoT (and can fallback if
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DoT connections subsequently fail). It seems also possible to
imagine a method might exist for an authoritative domain to use a TBD
resource record or other method to specify whether DoT is supported.
6.6. Detection of Availability
EDITORIAL NOTE: This section was just moved up. May need some better
integration later on.
Recursive resolvers typically communicate with many authoritative
nameservers. Not every authoritative nameserver will support DoT and
not every recursive resolver will support every requirement. How
should a recursive resolver determine whether DoT is supported for
example? (There may be multiple ways, or none)
What scope/granularity should such an availability marker have?
o by zone ("all authoritative nameservers in the "example.net" zone
support private queries from resolvers")
o by identified nameserver ("the nameserver "a.ns.example.net"
supports private queries from resolvers")
o by IP address ("any nameservers that resolve to 192.0.2.13 support
private queries from resolvers")
Note that if there is no signal for availability, recursors could
still opportunistically try the DNS privacy mechanism, as this is
employed by some stub resolvers when they contact their designated
recursors.
Should a signal of availability also indicate a preference for
privacy over availability? i.e., are there distinct ways to signal
"DNS-privacy is available" separately from "Only contact this server
via DNS-privacy if you understand this signal (though we may continue
to support non-private DNS queries for clients that don't understand
it)".
6.7. Resistance to Downgrade Attack
When a connection is opportunistically upgraded to DoT, if a fallback
to unencrypted DNS can be possible via a downgrade attack by blocking
or modifying TCP/853 communications. In such cases, it may be best
to establish a mechanism whereby the authoritative domain can specify
their preferred behaviour. This may range from only use DoT and do
not fallback to unencrypted DNS, to opportunistically use DoT but
fallback in failure, to do not use DoT. The email application layer
protocols have similar methods for asserting how email from a
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particular domain should be treated, so following some of the lessons
learned there is likely a good idea. Compare HSTS [RFC6797]?
6.8. End-User Policy Propagation
EDITORIAL NOTE: This section was just moved up. May need some better
integration later on.
Like any multi-party protocol (e.g. SMTP), the end user's
preferences or policies might or might not be respected by later hops
in the chain. But if we have a way to express those preferences, we
offer cooperating resolvers at least an opportunity to respect them.
WG DISCUSS: Is it better to let auth domains assert whether fallback
should be permitted or is that an end user preference or both? The
email world might suggest the former while the DNSSEC world the
latter. Or specify the standardization of the preferences and their
communication and leave it to implementors to decide whether or how
to treat those signals?
What sorts of preferences or policy might an end-user want to
express? for example:
o do not identify my general location (e.g. don't send my subnet
information (ECS) [RFC7871] data about me when talking to
authoritative servers), accepting that reduced localization may
result in less localized responses from authoritative Content
Delivery Network (CDN) servers and thus slower access to content
o prefer DNS privacy over reduced latency (i.e., do not try to do
speedups - try opportunistic privacy first and fall back to
cleartext only if that fails)
o never do non-private authoritative queries on my behalf (for any
external queries you need to do to resolve this request, require
strict, well-authenticated DNS privacy)
How specifically are these preferences be expressed by the client?
(e.g. new EDNS0 [RFC6891] options?) Should the recursor have a way
to indicate whether:
o they are capable of honoring them?
o they intend to honor them?
o they _did_ honor them over the course of a specific lookup?
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If a resolver merely forwards a request to another recursor, should
it also propagate those preferences/policy? if so, how?
This seems similar to [I-D.ietf-uta-smtp-require-tls].
To implement end-user policies, support for signaling of DNS server
capabilities is helpful, see for example
[I-D.edmonds-dnsop-capabilities].
6.9. Performance and Efficiency
o Can authoritative server operators limit resource-exhaustion
attacks against private DNS mechanisms from having an impact on
traditional (non-private) authoritative DNS availability? (JL:
seems easy to implement per host connection limits and implement
other standard DDoS protections - again for a later BCP doc)
o What are best practices for authoritative server operators that
can minimize latency and unavailability?
o What are best practices for recursors?
7. Security Considerations
At this point of the document, the authors have not yet discussed
security considerations in detail, as the whole document tends to
deal with user privacy, which can be considered part of security. :)
8. IANA Considerations
This document has no actions for IANA.
9. Changelog
Note to RFC editor: Remove this entire section before publication.
9.1. lmo-dprive-phase2-requirements-00
Initial version
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>.
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[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
[I-D.edmonds-dnsop-capabilities]
Edmonds, R., "Signaling DNS Capabilities", draft-edmonds-
dnsop-capabilities-00 (work in progress), July 2017.
[I-D.ietf-uta-smtp-require-tls]
Fenton, J., "SMTP Require TLS Option", draft-ietf-uta-
smtp-require-tls-09 (work in progress), August 2019.
[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>.
[RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
Transport Security (HSTS)", RFC 6797,
DOI 10.17487/RFC6797, November 2012,
<https://www.rfc-editor.org/info/rfc6797>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013,
<https://www.rfc-editor.org/info/rfc6891>.
[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>.
[RFC7871] Contavalli, C., van der Gaast, W., Lawrence, D., and W.
Kumari, "Client Subnet in DNS Queries", RFC 7871,
DOI 10.17487/RFC7871, May 2016,
<https://www.rfc-editor.org/info/rfc7871>.
[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>.
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10.3. URIs
[1] https://datatracker.ietf.org/doc/charter-ietf-dprive/
[2] https://datatracker.ietf.org/wg/dprive/about/
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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