Network Working Group P. Hoffman
Internet-Draft ICANN
Intended status: Standards Track November 21, 2018
Expires: May 25, 2019
Associating a DoH Server with a Resolver
draft-hoffman-resolver-associated-doh-06
Abstract
Browsers and web applications may want to know if there are one or
more DoH servers associated with the DNS recursive resolver that the
operating system is already using. This would allow them to get DNS
responses from a resolver that the user (or, more likely, the user's
network administrator) has already chosen. This document describes
two protocols for a resolver to tell a client what its associated DoH
servers are.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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 May 25, 2019.
Copyright Notice
Copyright (c) 2018 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
(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 and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
Hoffman Expires May 25, 2019 [Page 1]
Internet-Draft Resolver Associated DoH November 2018
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Finding the URI Templates of the DoH Servers Associated with
a Resolver . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. DoH Servers by TXT . . . . . . . . . . . . . . . . . . . 4
2.2. DoH Servers by Addresses . . . . . . . . . . . . . . . . 5
2.3. Resolver Addresses by SUDN . . . . . . . . . . . . . . . 5
2.4. Issues Common to "DoH Servers by TXT" and "Resolver
Addresses by SUDN" . . . . . . . . . . . . . . . . . . . 6
3. User Interface . . . . . . . . . . . . . . . . . . . . . . . 6
4. Design Choices . . . . . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 8
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
DoH [RFC8484] requires that one or more DoH servers be configured for
the DoH client. That document does not say how the DoH servers are
found, nor how to select from a list of possible DoH servers, nor
what the user interface (UI) for the configuration should be.
There is a use case for browsers and web applications who have one or
more currently-configured DNS recursive resolvers wanting to use DoH
for DNS resolution instead. For example, the recursive resolver
knows how to give correct answers to DNS queries that contain names
that are only resolvable in the local context. Users typically
configure their DNS recursive resolvers with through manual
configuration (such as manually editing a /etc/named.conf file) or
through automatic configuration from a protocol such as DHCP.
The client that wants to change from its currently-configured Do53
recursive resolver(s) to one or more DoH servers might be the stub
resolver in an operating system, although at this time it is rare
that such stub resolvers can use DoH. A much more likely use case is
a browser or web application that is getting name resolution through
the stub resolver on the computer on which it is running. The user
of the browser might have a preference for using a DoH server, and
Hoffman Expires May 25, 2019 [Page 2]
Internet-Draft Resolver Associated DoH November 2018
they might need to use a DoH server that is associated with the
resolver that the computer is currently using so that its queries for
non-global names are answered correctly. They may also be required
to use only resolvers that are approved by their organization's
network operators.
To address these use cases, this document defines three different
protocols to get the list of URI templates [RFC6570] for the DoH
servers associated with at least one of the resolvers being used by
the operating system on the system on which the application is being
run.
o The first, called "DoH servers by TXT" and described in
Section 2.1, is a new special use domain name (SUDN) [RFC6761]
that can be queried for a TXT RRset. This protocol is most likely
useful only to browsers that can call operating system functions
that in turn query the DNS for text records; web applications can
only query for IP addresses.
o The second, called "DoH servers by addresses" and described in
Section 2.2, is a well-known URI [I-D.nottingham-rfc5785bis] that
can be resolved to return the URI templates. This is useful if
the browser can call operating system functions that will return
the address of the recursive DNS resolver that the operating
system is currently using.
o The third, called "resolver addresses by SUDN" and descrbed in
Section 2.3, is a new SUDN that that can be queried for the IP
address(es) of a resolver. This protocol is useful for a browser
or web application that can query for the addressess associated
with a domain name (such as using the POSIX "getaddrinfo()"
function) but cannot use an operating system function to find
those addresses.
The design choices for this protocol, particularly earlier designs
that were deemed unusable, are described in Section 4.
1.1. Terminology
In this document, "DoT" is used to indicate DNS over TLS as defined
in [RFC7858].
In this document, "Do53" is used to indicate DNS over UDP or TCP as
defined in [RFC1035].
"DoH client" and "DoH server" are defined in [RFC8484].
Hoffman Expires May 25, 2019 [Page 3]
Internet-Draft Resolver Associated DoH November 2018
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. Finding the URI Templates of the DoH Servers Associated with a
Resolver
A client (a browser or web application) uses either the protocol in
Section 2.1 or Section 2.2 to get a list of URI templates for the DoH
servers associated with a resolver. The following sub-sections
describe the protocols and have notes that are common to both
protocols.
2.1. DoH Servers by TXT
To find the DoH Servers associated with a resolver, an application
sends that resolver a query for "resolver-associated-doh.arpa" in
class IN with the RRtype of TXT [RFC1035] (that is, the query is
resolver-associated-doh.arpa/IN/TXT).
As described in Section 5, the zone resolver-associated-doh.arpa is
not actually delegated and never will be. The resolver acts as if it
is delegated, and adds its own TXT records to the answer. The
resolver replies with its associated DoH servers as URI templates in
the TXT RRset in the Answer section.
A resolver that understands this protocol MUST send a TXT RRset in
the Answer section. Each TXT record contains one URI template. If a
resolver that understands this protocol has no associated DoH
servers, the TXT RRset contains exactly one record that has an empty
string as the RDATA; that is, the RDLENGTH in that record is 1, and
the RDATA contains just the byte 0x00.
The client uses the TXT records in the response to the resolver-
associated-doh.arpa/IN/TXT query as a list of the URI templates of
the DoH servers associated with the resolver. Note that TXT records
can contain multiple "character-strings" [RFC1035]; for this
protocol, all characters-strings in a TXT record are concatenated to
form a single URI template.
The URI templates of the DoH servers associated with a resolver might
be hosted on the resolver itself, or a resolver hosted by the same
operator, or even hosted somewhere else. The latter could be used by
resolver operators who don't want to host DoH servers but trust
another operator to do so.
Hoffman Expires May 25, 2019 [Page 4]
Internet-Draft Resolver Associated DoH November 2018
2.2. DoH Servers by Addresses
To find the DoH servers associated with a resolver, a browser or web
application uses either an operating system function (if such a
function is available to it) or the process described in Section 2.3
to find one or more IP addreses for the resolver. It uses one or
more of those IP addresses as part of a well-known URI to find out
the URI templates [RFC6570] to use for the DoH server(s) associated
with the resolver.
To find the DoH servers associated with a resolver, the client sends
a query to
https://IPADDRESSGOESHERE/.well-known/doh-servers-associated/
The resolver replies with its associated DoH servers as URI templates
[RFC6570].
[[ Need to describe the media types; likely JSON ]]
[[ Need to talk about what a response with an empty list means ]]
[[ Need to talk about what happens if authentication fails. This is
complicated by the fact that the application doesn't know if the OS-
to-resolver communication is authenticated. ]]
[[ Need to talk about HTTP caching ]]
A client MUST try to establish a new list of DoH servers associated
with a resolver every time the configured resolver in the operating
system changes.
The result of sending this query can be an HTTP redirect to a
different server, such as when a resolver operator doesn't want to
run its own DoH server.
2.3. Resolver Addresses by SUDN
Browsers and web applications are able to use an operating system
function such as gethostbyname() or its equivalents to convert host
names into IP addresses through the stub resolver in the operating
system on which they are running.
For this protocol, the browser or web application uses the SUDN
"resolver-addresses.arpa". As described in Section 5, the zone
resolver-addresses.arpa is not actually delegated and never will be.
The resolver acts as if it is delegated, and returns its own A or
AAAA records in the answer.
Hoffman Expires May 25, 2019 [Page 5]
Internet-Draft Resolver Associated DoH November 2018
2.4. Issues Common to "DoH Servers by TXT" and "Resolver Addresses by
SUDN"
See Section 7 for warnings about sending the DNS queries over a
transport that does not assure data integrity (such as Do53), and
over a transport that does assure data integrity (such as DoT) but in
circumstances where the browser or web application doesn't know the
type of DNS transport being used.
A client MUST re-issue the queries in {#doh_by_txt} and
{#resolver_by_sudn} every time the configured resolver in the
operating system changes.
[[ What if there is a list of DoH servers? Pick one (how?) or jump
arond? ]]
3. User Interface
For this protocol to be useful in a browser, the browser needs to
have an entry in its configuration interface where the allowed DoH
servers are listed that indicates that a DoH server from the
configured Do53 or DoT resolver is allowed. That wording might say
something like "DoH server associated with my current resolver" (or
"servidor DoH asociado con mi resolucion actual" or "serveur DoH
associe a mon resolveur actuel").
4. Design Choices
The primary use case for this protocol is a browser or web
application that is getting name resolution through the stub resolver
on the computer on which it is running wanting to switch its name
resolution to DoH. A secondary use case is an operating system that
wants to make a similar switch.
An earlier design suggestion was to use a new RRtype with a query to
./IN/NEWRRTYPE. However, it was pointed out that this would not work
going through stub resolvers that validate DNSSEC.
An earlier design suggestion was to use DHCP to tell the operating
system the DoH servers that the stub resolver might use. That
protocol is orthogonal to the one in this document in that it
addresses a different use case. If both the protocol in this
document and a DHCP-based protocol are standardized, they could co-
exist. However, there is no current mechanism for a stub resolver to
tell a browser, or a web application, what DoH server the stub
resolver is using, so DoH configuration in the stub resolver would
not prevent the browser from trying to find a DoH server on its own.
Hoffman Expires May 25, 2019 [Page 6]
Internet-Draft Resolver Associated DoH November 2018
An earlier design suggestion was to use an EDNS0 [RFC6891] extension.
The design chosen in this document meets the use case better because
applications cannot communicate EDNS0 extensions to the stub
resolver.
5. IANA Considerations
IANA will record the domain name "resolver-associated-doh.arpa" in
the "Special-Use Domain Names" registry [SUDN]. IANA MUST NOT
delegate resolver-associated-doh.arpa in the .arpa zone.
IANA will record the domain name "resolver-addresses.arpa" in the
"Special-Use Domain Names" registry [SUDN]. IANA MUST NOT delegate
resolver-addresses.arpa in the .arpa zone.
[[ When this document settles down, need to register ".well-known/
doh-servers-associated" as specified in [I-D.nottingham-rfc5785bis].
]]
6. Privacy Considerations
Allowing a user to use DoH instead of Do53 increases communication
privacy because of the TLS protection.
When a Do53 or DoT server indicates that a particular DoH server is
associated with it, the client might assume that the DoH server has
the same information privacy policies as the Do53 or DoT server.
Therefore, a Do53 or DoT server SHOULD NOT recommend a DoH server
unless that DoH server has the same (or better) information privacy
policy as the Do53 or DoT server.
A browser that has both a stub resolver stack and a TLS stack that is
independent of HTTP could make a DOT connection to the resolver being
used by the operating system.
7. Security Considerations
There is currently no way for an application to know whether the
operating system's stub resolver is using a transport that assures
data integrity such as DoT.
Even is an application could determine the use of a transport like
DoT, the application would also need to know whether the transport
was authenticated or was simply chosen opportunistically.
Hoffman Expires May 25, 2019 [Page 7]
Internet-Draft Resolver Associated DoH November 2018
8. References
8.1. Normative References
[I-D.nottingham-rfc5785bis]
Nottingham, M., "Well-Known Uniform Resource Identifiers
(URIs)", draft-nottingham-rfc5785bis-08 (work in
progress), October 2018.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[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>.
[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570,
DOI 10.17487/RFC6570, March 2012,
<https://www.rfc-editor.org/info/rfc6570>.
[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>.
[SUDN] "Special-Use Domain Names", n.d.,
<https://www.iana.org/assignments/
special-use-domain-names/>.
8.2. Informative References
[RFC6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
RFC 6761, DOI 10.17487/RFC6761, February 2013,
<https://www.rfc-editor.org/info/rfc6761>.
Hoffman Expires May 25, 2019 [Page 8]
Internet-Draft Resolver Associated DoH November 2018
[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>.
Acknowledgments
The use case in this document was inspired by discussions and the
DRIU BoF at IETF 102 and later in the DNSOP Working Group. Vladimir
Cunat, Philip Homburg, Shumon Huque, Martin Thomson, Eric Rescorla,
and Tony Finch offered useful advice to improve early versions of the
protocol.
Author's Address
Paul Hoffman
ICANN
Email: paul.hoffman@icann.org
Hoffman Expires May 25, 2019 [Page 9]