Internet DRAFT - draft-pp-recursive-authoritative-opportunistic
draft-pp-recursive-authoritative-opportunistic
Network Working Group P. Hoffman
Internet-Draft ICANN
Intended status: Standards Track 13 January 2021
Expires: 17 July 2021
Recursive to Authoritative DNS with Opportunistic Encryption
draft-pp-recursive-authoritative-opportunistic-04
Abstract
This document describes a use case and a method for a DNS recursive
resolver to use opportunistic encryption (that is, encryption with
optional authentication) when communicating with authoritative
servers. The motivating use case for this method is that more
encryption on the Internet is better, and opportunistic encryption is
better than no encryption at all. The method here is optional for
both the recursive resolver and the authoritative server. Nothing in
this method prevents use cases and methods that can use, or require,
authenticated encryption.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 17 July 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Use Case . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Summary of Protocol . . . . . . . . . . . . . . . . . . . 3
1.3. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4
2. Method for Opportunistic Encryption . . . . . . . . . . . . . 4
2.1. Resolvers . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Authoritative Servers . . . . . . . . . . . . . . . . . . 5
3. Discovering Whether an Authoritative Server Uses
Encryption . . . . . . . . . . . . . . . . . . . . . . . 5
4. The Transport Cache . . . . . . . . . . . . . . . . . . . . . 6
5. Authentication . . . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
A recursive resolver using traditional DNS over port 53 may wish
instead to use encrypted communication with authoritative servers in
order to prevent passive snooping of its DNS traffic. The recursive
resolver can use opportunistic encryption (defined in [RFC7435] to
achieve this goal.
This document describes a use case and a method for recursive
resolvers to use opportunistic encryption. The use case is described
in Section 1.1. The method uses DNS-over-TLS [RFC7858] (DoT) with
authoritative servers in an efficient manner; it is called "ADoT", as
described in [I-D.ietf-dnsop-rfc8499bis]. (( A later version of this
document will probably also describe the use of DNS-over-QUIC
[I-D.ietf-dprive-dnsoquic] (DoQ). ))
Because opportunistic encryption means encryption with optional
authentication, a resolver using the mechanism described here could
achieve authenticated encryption with some authoritative servers,
depending on how authentication for ADoT is defined. To date, there
have been no definition of how a resolver can take advantage of DNS
features that require authentication of authoritative servers. If
those advantages are defined in the future, this document would need
to define the types of authentication for ADoT that would be allowed.
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1.1. Use Case
The use case in this document is recursive resolver operators who are
happy to use TLS [RFC8446] 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.
Both parties 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
* 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
1.2. Summary of Protocol
This protocol has four main parts. This summary gives an overview of
how the work together.
* A resolver that uses this protocol has a cache that it uses to
know whether to attempt using ADoT with a particular authoritative
server, as described in Section 4.
* A resolver fills its transport cache by discovering whether any
authoritative server of interest uses encrypted DNS, as described
in Section 3.
* If there is no entry for that server in the cache, or the cache
says that the authoritative server doesn't support encrypted
transport, the resolver uses classic DNS; otherwise, the resolver
attempts to connect to the authoritative server with ADoT, as
described in Section 2.
* If the TLS session is authenticated and the resolver has use for
this authentication, the resolver can mark responses it gets as
authenticated, as described in Section 5. If the TLS session is
not authenticated, the resolver treats the answers it receives as
if they were received over classic DNS.
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1.3. Definitions
The terms "recursive resolver", "authoritative server", "ADoT", and
"classic DNS" are defined in [I-D.ietf-dnsop-rfc8499bis].
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. Method for Opportunistic Encryption
[RFC7435] defines opportunistic encryption. In this document, the
only difference between normal TLS session establishment and
opportunistic encryption is that the the TLS client (the recursive
resolver) optionally authenticates the server. See Section 5 for a
fuller description of the use of authentication.
2.1. Resolvers
A resolver following this protocol uses its transport cache
(described in Section 4) to decide whether to use classic DNS or this
protocol to contact authoritative servers. If the transport cache
indicates that the authoritative server is known to support encrypted
DNS, the resolver attempts to connect to it with ADoT on port 853.
The resolver is configured with a set of timeouts that it uses when
it is setting up ADoT. This document does has suggested values for
those timeouts; they are marked here with (( timeout_ )). Resolver
software might use these suggested values for defaults, or might
choose their own default values.
(( The proposed default values here are based on research that I have
done but not published. The research is expected to be published
before IETF 110. ))
The resolver MUST fall back to using classic DNS with a server if any
of the following happens when using ADoT:
* The resolver receives a TCP RST response
* The resolver does not receive a reply to the TCP SYN message
within timeout "timeout_syn"; the suggested default is .6 seconds
* The resolver does not receive a reply to its first TLS message
within timeout "timeout_tls_start"; the suggested default (which
includes the TCP startup time) is 2.3 seconds
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* The TLS handshake gets a definitive failure
* The TLS session is set up, but the resolver does not receive a
response to its first DNS query in the TLS session within timeout
"timeout_dns_answ"; the suggested default is 5 seconds (which
includes the TCP and TLS startup times)
* The TLS session fails for reasons other than for authentication,
such as incorrect algorithm choices or TLS record failures
In any of those cases, the resolver needs to update its transport
cache to indicate that the server is not currently available over
DoT. The time-to-live value for that entry, "timeout_ttl", could be
as long as the TTL on the NS RRset.
A resolver SHOULD keep a TLS session to a particular server open if
it expects to send additional queries to that server in a short
period of time, "timeout_hold_open". If the server closes the TLS
session, the resolver can re-establish a TLS session of the version
of TLS in use allows for session resumption.
2.2. Authoritative Servers
An authoritative server following this protocol SHOULD support an
ADoT service at port 853 for each IP address on which it offers
service for classic DNS on port 53.
A server MAY close a TLS connection at any time. For example, it can
close the TLS session if it has not received a DNS query in a defined
length of time, "timeout_dns_query". It can also close the TLS
session after it sends a DNS response; however, it might also want to
keep the TLS session open waiting for another DNS query from the
resolver.
3. Discovering Whether an Authoritative Server Uses Encryption
A recursive resolver can discover whether an authoritative server
supports ADoT by attempting to open a TLS session to port 853 of an
IP address for the server. If the server completes the TLS
handshake, the resolver can be fairly confident that the server
supports ADoT.
(( Note that there are likely better ways to do discovery. The
DPRIVE WG requested that this version of this draft only specify
port-probing. Future drafts might describe other methods, and how to
use multiple methods at the same time for discovery, depending on
what the WG chooses for discovery. ))
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The following are indications of failure for the ability to use ADoT
with the server:
* The resolver receives a TCP RST response
* The resolver does not receive a reply to the TCP SYN message
within timeout "timeout_syn"
* The resolver does not receive a reply to its first TLS message
within timeout "timeout_tls_start"
* The TLS handshake gets a definitive failure
4. The Transport Cache
A recursive resolver that attempted to use encrypted transport every
time it connected to any authoritative server would inherently be
slower than one that did not. Similarly, a recursive resolver that
made an external lookup of what secure transports every authoritative
server supports each time it connected would also inherently be
slower than one that did not. Recursive resolver operators desire to
give answers to stub resolvers as quickly as possible, so neither of
these two strategies would make sense.
Instead, recursive resolvers following the method described in this
document MUST keep a cache of relevant information about how DNS-
over-TLS is supported by authoritative servers. This is called a
"transport cache" in this document. The relevant information could
include things such as support for encryption, expected round-trip
times, authentication mechanisms, and so on. The transport cache is
likely to store both positive and negative information about a
server's ability to support encrypted DNS.
The recursive resolver MUST look in its transport cache before
sending DNS queries to an authoritative server. If there is no entry
for an authoritative server in its transport cache, the recursive
resolver MUST use classic DNS over port 53. It MAY then probe for
encrypted transports, and cache that information for later
connections.
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This document explicitly does not mandate the contents of the
transport cache. Different recursive resolver implementers are
likely to have different cache structures based on many factors, such
as research results, active measurements, secure protocols supported,
and customer feedback, There will likely be different strategies for
the time-to-live for parts of the transport cache, such as how often
to refresh the data in the cache, how often to refresh negative data,
whether to prioritize refreshing certain zones or types of zones, and
so on.
This document also explicitly doesn't mandate the strategy for
filling transport caches. Some strategies might include one or more
of "test NS entries from the main cache", "try to send a refresh
query over ADoT", "use external data", "trust a third-party service
for filling the transport cache", and so on.
There are no interoperability issues with different implementors
making different choices for the contents and fill strategies of
their transport caches, and having many different options available
will likely cause the cache designs to get better over time.
5. Authentication
In the opportunistic encryption described here, there is no
requirement, and no advantage, for the recursive resolver to
authenticate the authoritative server because any certificate
authentication failure does not prevent the TLS session from being
set up. If it is easier programmatically for the recursive resolver
to authenticate the authoritative server and then ignore the negative
result for certificate authentication, than to just not authenticate,
the recursive resolver MAY do that.
This document does not describe what to do with successful
authentication of a ADoT TLS session. Some suggestions have been
floated in the DPRIVE WG, but none have been written into drafts. If
there later are reasons to note authentication of the server,
resolvers following this protocol MAY use that authenticated data. ((
Change this paragraph if the WG later defines DNS-related reasons to
authenticate. ))
Later protocols for encrypted resolver-to-authoritative communication
might to require normal TLS authentication. Because of this,
authoritative servers SHOULD use TLS certificates that can be used in
authenticated TLS authentication, such as those issued by trusted
third parties or self-issued certificates that can be authenticated
with DANE [RFC6698] records. However, if an authoritative server
does not care about the use cases for such future protocols, it MAY
use self-issued certificates that cannot be authenticated.
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6. Security Considerations
The method described in this document explicitly allows a stub to
perform DNS communications over traditional unencrypted,
unauthenticated DNS on port 53.
The method described in this document explicitly allows a stub to
choose to allow unauthenticated TLS. In this case, 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 and Peter van Dijk contributed many ideas to early drafts
of this document.
8. References
8.1. 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, <http://www.ietf.org/internet-drafts/
draft-ietf-dnsop-rfc8499bis-01.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>.
[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>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
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8.2. Informative References
[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-01, 20 October
2020, <http://www.ietf.org/internet-drafts/draft-ietf-
dprive-dnsoquic-01.txt>.
[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>.
Author's Address
Paul Hoffman
ICANN
Email: paul.hoffman@icann.org
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