Network Working Group P. Hoffman
Internet-Draft ICANN
Intended status: Experimental P. van Dijk
Expires: 3 October 2021 PowerDNS
1 April 2021
Recursive to Authoritative DNS with Unauthenticated Encryption
draft-ietf-dprive-opportunistic-adotq-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
[I-D.rescorla-dprive-adox-latest].
NOTE: The file name for this draft, draft-ietf-dprive-opportunistic-
adotq, is now incorrect. This draft only covers unauthenticated
encryption, not opportunistic encryption.
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
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 3 October 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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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 . . . . . . . . . . . . . . . . 6
4. Serving with Encryption . . . . . . . . . . . . . . . . . . . 7
5. Resolvers Reporting Errors to Authoritative Servers . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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 [RFC7435]) 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 [RFC7858] (DoT), DNS-over-
HTTPS [RFC8484] (DoH), and/or DNS-over-QUIC
[I-D.ietf-dprive-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 [I-D.rescorla-dprive-adox-latest] for a description of the use
case and a proposed mechanism for fully-authenticated encryption.
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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 [I-D.ietf-dnsop-svcb-https]. As described in
[I-D.schwartz-svcb-dns], 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
[I-D.schwartz-svcb-dns]. 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 [I-D.ietf-dnsop-rfc8499bis].
"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 [RFC2119] [RFC8174] 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 looking for a cached SVCB record for
the name of the authoritative server (with "_dns" prefix) with a
positive answer. A cached SVCB record with a negative answer
indicates that the authoritative server does not support any
encrypted transport. Positive and negative responses for SVCB
queries are cached the same way as for all other DNS resource
records.
See [I-D.rescorla-dprive-adox-latest] for examples of querying for NS
records and for SVCB records, and the interpretation of positive
answers.
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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 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 records
for some or all of the zone's authoritative servers so that future
queries might be able to use DNS with encryption.
Discovery using SVCB records differs between resolvers using
unauthenticated encryption and those using fully-authenticated
encryption (described in [I-D.rescorla-dprive-adox-latest]). If the
resolver is using unauthenticated encryption, the SVCB records do not
need to be DNSSEC-signed.
DNSSEC validation of SVCB RRsets used strictly for this discovery
mechanism is not mandated.
As described in [I-D.rescorla-dprive-adox-latest], these records may
be in the resolver's cache because they came in the Additional
section of a query for the NS records of a zone. This document does
not rely on that feature being standardized or operationally present
to work.
Because some authoritative servers or middleboxes are misconfigured,
requests for unknown RRtypes might be ignored by them. Resolvers
should be ready to deal with timeouts or other bad responses to their
SVCB queries.
3. Resolving with Encryption
A resolver following this protocol MUST use SVCB records in its cache
to decide whether to use classic DNS or encryption to contact
authoritative servers for a zone. If any of the SVCB records in the
cache for the authoritative servers for a zone are positive
responses, the resolver uses any of those servers for encryption. A
resolver MUST NOT attempt encryption for a server that has a negative
response in its cache for the associated SVCB record.
If all of the SVCB records for the authoritative servers in the cache
for a zone are negative responses, the resolver MUST use classic
(unencrypted) DNS instead of encryption. Similarly, if none of the
SVCB records for the authoritative servers in the cache have
information about encrypted services as described in
[I-D.schwartz-svcb-dns], the resolver MUST use classic (unencrypted)
DNS instead of encryption.
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If there are any SVCB records in the cache for the authoritative
servers for a zone with a positive response, the resolver MUST try
each indicated authoritative server using DNS with encryption until
it successfully sets up a connection. The resolver only attempts to
use the encrypted transports that are in the associated SVCB record
for the authoritative server. Reasons for TLS failures are listed in
Section 3.1.
After a DNS with encryption session is set up, the resolver uses that
authoritative server for whatever query about the zone it was going
to send. If a resolver cannot set up a DNS with encryption session
with any of the authoritative servers, it MUST attempt to perform the
resolution over classic (unencrypted) DNS as it would have without
encryption.
A resolver SHOULD keep a DNS with encryption session to a particular
server open if it expects to send additional queries to that server
in a short period of time. If the server closes the DNS with
encryption session, the resolver can possibly re-establish a DNS with
encryption session using encrypted session resumption. [RFC7766]
says "both clients and servers SHOULD support connection reuse" for
TCP connections, and that advice could apply as well for DNS with
encryption even though DNS with encryption has greater overhead for
saving state.
Privacy-oriented resolvers (defined in [RFC8932]) 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 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 operator of an authoritative server following this protocol SHOULD
publish SVCB records as described in Section 2. If they cannot
publish such records, the security properties of their authoritative
servers will not be found. If an operator wants to test serving
using encryption, they can publish SVCB records with short TTLs and
then stop serving with encryption after removing the SVCB records and
waiting for the TTLs to expire.
An operator of authoritative servers for a zone that is following
this protocol MAY support encryption towards any IP address on which
it offers service for classic DNS on port 53. It is acceptable for
such an operator to only offer encryption on some of the named
authoritative servers, such as when the operator is determining how
far to roll out encrypted service.
A server MAY close an encrypted connection at any time. For example,
it can close the session if it has not received a DNS query in a
defined length of time. The server MAY close an encrypted session
after it sends a DNS response; however, it might also want to keep
the session open waiting for another DNS query from the resolver.
[RFC7766] says "both clients and servers SHOULD support connection
reuse" for TCP connections, and that advice could apply as well for
DNS with encryption even though DNS with encryption has greater
overhead for saving state.
5. Resolvers Reporting Errors to Authoritative Servers
Resolvers should have a method of telling authoritative servers that
there are problems with the encrypted service they are offering.
There is a proposal that the DNSOP Working Group adopt
[I-D.arends-dns-error-reporting], which would enable such reporting.
(( Clearly, more will need to go here. ))
6. IANA Considerations
(( Update registration for TCP/853 to also include ADoT ))
(( Maybe other updates for DoH and DoQ ))
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7. 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.
An authoritative server that wants to only serve data to resolvers
that using fully-authenticated encryption as described in
[I-D.rescorla-dprive-adox-latest] cannot differentiate between those
resolvers and resolvers using the mechanisms described in this
document.
8. 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.
9. References
9.1. Normative References
[I-D.ietf-dnsop-rfc8499bis]
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>.
[I-D.ietf-dnsop-svcb-https]
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-04, 17 March 2021,
<https://www.ietf.org/archive/id/draft-ietf-dnsop-svcb-
https-04.txt>.
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[I-D.rescorla-dprive-adox-latest]
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>.
[I-D.schwartz-svcb-dns]
Schwartz, B., "Service Binding Mapping for DNS Servers",
Work in Progress, Internet-Draft, draft-schwartz-svcb-dns-
02, 17 February 2021, <https://www.ietf.org/archive/id/
draft-schwartz-svcb-dns-02.txt>.
[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>.
[RFC7435] 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>.
[RFC7766] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
D. Wessels, "DNS Transport over TCP - Implementation
Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,
<https://www.rfc-editor.org/info/rfc7766>.
[RFC7858] 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>.
[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>.
[RFC8484] 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>.
9.2. Informative References
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[I-D.arends-dns-error-reporting]
Arends, R. and M. Larson, "DNS Error Reporting", Work in
Progress, Internet-Draft, draft-arends-dns-error-
reporting-00, 30 October 2020,
<https://www.ietf.org/archive/id/draft-arends-dns-error-
reporting-00.txt>.
[I-D.ietf-dprive-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>.
[RFC8932] 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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