Network Working Group P. Hoffman
Internet-Draft ICANN
Intended status: Experimental P. van Dijk
Expires: 18 December 2021 PowerDNS
16 June 2021
Recursive to Authoritative DNS with Unauthenticated Encryption
draft-ietf-dprive-unauth-to-authoritative-02
Abstract
This document describes a use case and a method for a DNS recursive
resolver to use unauthenticated encryption when communicating with
authoritative servers. The motivating use case for this method is
that more encryption on the Internet is better, and some resolver
operators believe that unauthenticated encryption is better than no
encryption at all. The method described here is optional for both
the recursive resolver and the authoritative server. This method
supports unauthenticated encryption using the same mechanism for
discovery of encryption support for the server as [FULL-AUTH].
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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This Internet-Draft will expire on 18 December 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Use Case for Unauthenticated Encryption . . . . . . . . . 3
1.2. Summary of Protocol . . . . . . . . . . . . . . . . . . . 3
1.3. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4
2. Discovering Whether an Authoritative Server Uses
Encryption . . . . . . . . . . . . . . . . . . . . . . . 4
3. Resolving with Encryption . . . . . . . . . . . . . . . . . . 5
3.1. Resolver Session Failures . . . . . . . . . . . . . . . . 5
4. Serving with Encryption . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
A recursive resolver using traditional DNS over port 53 may wish
instead to use encrypted communication with authoritative servers in
order to limit snooping of its DNS traffic by passive or on-path
attackers. The recursive resolver can use unauthenticated encryption
(defined in [OPPORTUN]) to achieve this goal.
This document describes the use case for unauthenticated encryption
in recursive resolvers in Section 1.1. The encryption method with
authoritative servers can be DNS-over-TLS [DNSOTLS] (DoT), DNS-over-
HTTPS [DNSOHTTPS] (DoH), and/or DNS-over-QUIC [DNSOQUIC] (DoQ), as
described in Section 3.
The document also describes a discovery method that shows if an
authoritative server supports encryption in Section 2.
See [FULL-AUTH] for a description of the use case and a proposed
mechanism for fully-authenticated encryption. See [COMMON] for a
definition of the features that are in common between this document
and [FULL-AUTH].
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NOTE: The draft uses the SVCB record as a discovery mechanism for
encryption by a particular authoritative server. Any record type
that can show multiple types of encryption (currently DoT, DoH, and
DoQ) can be used for discovery. Thus, this record type might change
in the future, depending on the discussion in the DPRIVE WG.
1.1. Use Case for Unauthenticated Encryption
The use case in this document for unauthenticated encryption is
recursive resolver operators who are happy to use encryption with
authoritative servers if doing so doesn't significantly slow down
getting answers, and authoritative server operators that are happy to
use encryption with recursive resolvers if it doesn't cost much. In
this use case, resolvers do not want to return an error for requests
that were sent over an encrypted channel if they would have been able
to give a correct answer using unencrypted transport.
Resolvers and authoritative servers understand that using encryption
costs something, but are willing to absorb the costs for the benefit
of more Internet traffic being encrypted. The extra costs (compared
to using traditional DNS on port 53) include:
* Extra round trips to establish TCP for every session (but not
necessarily for every query)
* Extra round trips for TLS establishment
* Greater CPU use for TLS establishment
* Greater CPU use for encryption after TLS establishment
* Greater memory use for holding TLS state
This use case is not expected to apply to all resolvers or
authoritative servers. For example, according to [RSO_STATEMENT],
some root server operators do not want to be the early adopters for
DNS with encryption. The protocol in this document explicitly allows
authoritative servers to signal when they are ready to begin offering
DNS with encryption.
1.2. Summary of Protocol
This summary gives an overview of how the parts of the protocol work
together.
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* The resolver discovers whether any authoritative server of
interest supports DNS with encryption by querying for the SVCB
records [SVCB]. As described in [DNS-SVCB], SVCB records can
indicate that a server supports encrypted transport of DNS
queries.
NOTE: In this document, the term "SVCB record" is used _only_ for
SVCB records that indicate encryption as described in [DNS-SVCB].
SVCB records that do not have these indicators in the RDATA are
not included in the term "SVCB record" in this document.
* The resolver uses any authoritative server with a SVCB record that
indicates encryption to perform unauthenticated encryption.
* The resolver does not fail to set up encryption if the
authentication in the TLS session fails.
1.3. Definitions
The terms "recursive resolver", "authoritative server", and "classic
DNS" are defined in [DNS-TERM].
"DNS with encryption" means transport of DNS over any of DoT, DoH, or
DoQ. A server that supports DNS with encryption supports transport
over one or more of DoT, DoH, or DoQ.
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 [MUSTSHOULD1] [MUSTSHOULD2] when, and only when, they appear in
all capitals, as shown here.
2. Discovering Whether an Authoritative Server Uses Encryption
A recursive resolver discovers whether an authoritative server
supports DNS with encryption by using the discovery mechanism
described in Section 2.1 of [COMMON]. A resolver MAY also use port
probing, although the mechanism for that is not described here.
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If the cache has no positive or negative answers for any SVCB record
for any of a zone's authoritative servers, the resolver MAY send
queries for the SVCB records (and for the A/AAAA records of names
mentioned in those SVCB records) for some or all of the zone's
authoritative servers and wait for a positive response so that the
resolver can use DNS with encryption for the original query. In this
situation, the resolver MAY instead just use classic DNS for the
original query but simultaneously queue queries for the SVCB (and
subsequent A/AAAA) records for some or all of the zone's
authoritative servers so that future queries might be able to use DNS
with encryption.
DNSSEC validation of SVCB RRsets used strictly for this discovery
mechanism is not mandated.
3. Resolving with Encryption
A resolver following this protocol processes the discovery response
using the processing mechanism described in [COMMON].
A resolver following this protocol does not need to authenticate TLS
servers. Thus, when setting up a TLS connection, if the server's
authentication credentials do not match those expected by the
resolver, the resolver continues with the TLS connection. Privacy-
oriented resolvers (defined in [PRIVACY-REC]) following this protocol
MUST NOT indicate that they are using encryption because this
protocol is susceptible to on-path attacks.
3.1. Resolver Session Failures
The following are some of the reasons that a DNS with encryption
session might fail to be set up:
* The resolver receives a TCP RST response
* The resolver does not receive replies to TCP or TLS setup (such as
getting the TCP SYN message, the first TLS message, or completing
TLS handshakes)
* The TLS handshake gets a definitive failure
* The encrypted session fails for reasons other than for
authentication, such as incorrect algorithm choices or TLS record
failures
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4. Serving with Encryption
An authoritative server following this protocol publishes the
discovery records using the serving mechanism described in [COMMON].
5. IANA Considerations
Relevant IANA considerations are covered in [COMMON].
6. Security Considerations
The method described in this document explicitly allows a resolver to
perform DNS communications over traditional unencrypted,
unauthenticated DNS on port 53, if it cannot find an authoritative
server that advertises that it supports encryption. The method
described in this document explicitly allows a resolver using
encryption to choose to allow unauthenticated encryption. In either
of these cases, the resulting communication will be susceptible to
obvious and well-understood attacks from an attacker in the path of
the communications.
7. Acknowledgements
Puneet Sood contributed many ideas to early drafts of this document.
The DPRIVE Working Group has contributed many ideas that keep
shifting the focus and content of this document.
8. References
8.1. Normative References
[COMMON] Dijk, P. V. and P. Hoffman, "Common Features for Encrypted
Recursive to Authoritative DNS", Work in Progress,
Internet-Draft, draft-pp-dprive-common-features-01, 19 May
2021, <https://www.ietf.org/archive/id/draft-pp-dprive-
common-features-01.txt>.
[DNS-SVCB] Schwartz, B., "Service Binding Mapping for DNS Servers",
Work in Progress, Internet-Draft, draft-schwartz-svcb-dns-
03, 19 April 2021, <https://www.ietf.org/archive/id/draft-
schwartz-svcb-dns-03.txt>.
[DNS-TERM] Hoffman, P. and K. Fujiwara, "DNS Terminology", Work in
Progress, Internet-Draft, draft-ietf-dnsop-rfc8499bis-01,
20 November 2020, <https://www.ietf.org/archive/id/draft-
ietf-dnsop-rfc8499bis-01.txt>.
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[FULL-AUTH]
Pauly, T., Rescorla, E., Schinazi, D., and C. A. Wood,
"Signaling Authoritative DNS Encryption", Work in
Progress, Internet-Draft, draft-rescorla-dprive-adox-
latest-00, 26 February 2021,
<https://www.ietf.org/archive/id/draft-rescorla-dprive-
adox-latest-00.txt>.
[MUSTSHOULD1]
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>.
[MUSTSHOULD2]
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>.
[OPPORTUN] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, <https://www.rfc-editor.org/info/rfc7435>.
[SVCB] Schwartz, B., Bishop, M., and E. Nygren, "Service binding
and parameter specification via the DNS (DNS SVCB and
HTTPS RRs)", Work in Progress, Internet-Draft, draft-ietf-
dnsop-svcb-https-06, 16 June 2021,
<https://www.ietf.org/archive/id/draft-ietf-dnsop-svcb-
https-06.txt>.
8.2. Informative References
[DNSOHTTPS]
Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>.
[DNSOQUIC] Huitema, C., Mankin, A., and S. Dickinson, "Specification
of DNS over Dedicated QUIC Connections", Work in Progress,
Internet-Draft, draft-ietf-dprive-dnsoquic-02, 22 February
2021, <https://www.ietf.org/archive/id/draft-ietf-dprive-
dnsoquic-02.txt>.
[DNSOTLS] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.
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[PRIVACY-REC]
Dickinson, S., Overeinder, B., van Rijswijk-Deij, R., and
A. Mankin, "Recommendations for DNS Privacy Service
Operators", BCP 232, RFC 8932, DOI 10.17487/RFC8932,
October 2020, <https://www.rfc-editor.org/info/rfc8932>.
[RSO_STATEMENT]
"Statement on DNS Encryption", 2021, <https://root-
servers.org/media/news/Statement_on_DNS_Encryption.pdf>.
Authors' Addresses
Paul Hoffman
ICANN
Email: paul.hoffman@icann.org
Peter van Dijk
PowerDNS
Email: peter.van.dijk@powerdns.com
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