Network Working Group P. Hoffman
Internet-Draft ICANN
Intended status: Standards Track March 24, 2019
Expires: September 25, 2019
Associating a DoH Server with a Resolver
draft-ietf-doh-resolver-associated-doh-03
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. It also describes a protocol for a client to find out
the address of the resolver it is using, if it cannot find that
address by an operating system API or some other means.
Status of This Memo
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This Internet-Draft will expire on September 25, 2019.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. DoH Servers from HTTPS . . . . . . . . . . . . . . . . . . . 4
3. DoH Servers from DNS . . . . . . . . . . . . . . . . . . . . 5
4. Resolver Addresses from DNS . . . . . . . . . . . . . . . . . 6
5. Lists of DoH Servers . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Earlier Design Choices . . . . . . . . . . . . . . . 10
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
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 to want the DNS
recursive resolver(s) configured in the operating system to use DoH
for DNS resolution instead of normal DNS, but to do so to at a DoH
server specified by the configured resolver. For example, a
recursive resolver configured by the operating system may know how to
give correct answers to DNS queries that contain names that are only
resolvable in the local context, or resolve differently in the local
context. Similarly, the recursive resolver configured in the
operating system may implement security policies such as malware
prevention that are not implemented in the same way in DoH servers
not affiliated with the user's organization. Users typically
configure their DNS recursive resolvers with through automatic
configuration from a protocol such as DHCP; much less often, they use
manual configuration (such as manually editing a /etc/resolv.conf
file).
The expected use cases for DoH are browsers and web applications that
would otherwise get their DNS service from the resolver configured by
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the operating system. The user of the client might have a preference
for using a DoH server for the benefits that DoH brings, and they
might need to use a DoH server that is associated with the resolver
that the computer is currently using for the reasons listed above.
In a common scenario, user may be required to use only resolvers that
are approved by their organization's network operators.
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.
To address these use cases, this document defines protocols to get
the list of URI templates [RFC6570] or addresses 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 "DoH servers from HTTPS", described in Section 2, is a well-known
URI [I-D.nottingham-rfc5785bis] that can be resolved to return the
URI templates in an HTTP response.
o "DoH servers from DNS", described in Section 3, is a new special
use domain name (SUDN) [RFC6761] that can be queried to return the
URI templates as a TXT RRset.
o "resolver addresses from DNS", described in Section 4, is a new
SUDN that that can be queried to return the addresses as A and
AAAA RRsets.
For these protocols 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"). Alternatively, these protocols
might be the default for a browser, and naming specific other DoH
servers might be done with a UI.
The protocols described here are meant for a browser to be able to
start using DoH based on its current interactions with the resolver
from the operating system on which the browser is running: they are
not expected to work without being able to do DNS resolution. Even
"DoH servers from HTTPS", which ostensibly only needs an IP address,
is likely to need DNS resolution for things like OCSP servers during
the setup of TLS.
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1.1. Terminology
In this document, "client" means either a browser or web application.
When one or the other is named explicitly,
In this document, "browser" means any application that can open ports
in the operating system. This odd usage of the term "browser" is
adopted from [RFC8484].
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].
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. DoH Servers from HTTPS
To find the URI templates for DoH servers associated with a resolver
whose address is already known, a browser or web application can use
the well-known URI described in this section. To find the IP address
of the resolver used by the operating system on which a browser is
running, a browser can use either an operating system function (if
such a function is available to it) or the process described in
Section 4. (A web application can also use the process described in
Section 4, and thus use this well-known URI.)
To find the DoH servers associated with a resolver, the client MUST
use the following query:
https://IPADDRESSGOESHERE/.well-known/doh-servers-associated/
The resolver replies with its associated DoH servers as URI templates
[RFC6570]. The HTTP response header MUST contain an appropriate HTTP
status code as defined in [RFC7231]. Successful responses MUST set
the Content-Type set to "application/json".
The returned JSON object [RFC8259] MUST contain a member whose name
is "associated-resolvers" and whose value is a JSON array. The array
contains zero or more JSON strings, each of which is a single URI
template.
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If the array of URI templates returned is empty, that indicates that
the resolver does not have any DoH servers associated with it.
For example, the returned object might look like:
{ "associated-resolvers":
[ "https://dnsserver.example.net/dns-query{?dns}",
"https://webhost.example.net/a/b/c/dns-query{?dns}" ]
}
If there are no associated DoH servers, the returned object would
look like:
{ "associated-resolvers": [ ] }
If the TLS authentication for the query fails, the browser MUST abort
the connection without sending the HTTP request, and it cannot assume
anything about whether the resolver has any DoH servers associated
with it.
A client using this protocol 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 HTTP query follows all the normal rules for HTTP. Thus, the
result of sending this query can be an HTTP redirect to a different
server. Also, the result of the query might be served from an HTTP
cache.
3. DoH Servers from DNS
To find the URI templates for DoH servers associated with a resolver,
a browser 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). 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 cannot currently call such operating system functions.
As described in Section 6, the zone resolver-associated-doh.arpa is
not actually delegated and never will be. The resolver that receives
this query 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. The
resolver can generate this reply with special code to capture queries
for "resolver-associated-doh.arpa"; if the resolver can be configured
to also be authoritative for some zones, it can use that
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configuration to actually be authoritative for "resolver-associated-
doh.arpa".
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.
An example of the TXT RRset (in DNS master file format) might be:
$ORIGIN resolver-associated-doh.arpa.
IN TXT "https://dnsserver.example.net/dns-query{?dns}"
IN TXT "https://webhost.example.net/a/b/c/dns-query{?dns}"
If there are no associated DoH servers, an example of the the TXT
RRset (in DNS master file format) might be:
$ORIGIN resolver-associated-doh.arpa.
IN TXT ""
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.
A client using this protocol MUST try to establish a new list of DoH
servers associated with a resolver every time the configured resolver
in the operating system changes.
4. Resolver Addresses from DNS
Browsers which cannot get the IP address(es) of the resolver
configured by the operating system using APIs are still 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. Web
applications also can convert host names to IP addresses. Either can
use a new SUDN to find the address(es) of the resolvers configured by
the operating system.
A browser or web application uses it normal interface for getting IP
addresses for a hostname, and uses the SUDN "resolver-addresses.arpa"
as the hostname.
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As described in Section 6, the zone resolver-addresses.arpa is not
actually delegated and never will be. The resolver acts as if that
name is delegated, and returns its own A or AAAA addresses in the
records in the answer. The resolver can generate this reply with
special code to capture queries for "resolver-addresses.arpa"; if the
resolver can be configured to also be authoritative for some zones,
it can use that configuration to actually be authoritative for
"resolver-addresses.arpa".
A client using this protocol MUST try to establish a new list of DoH
servers associated with a resolver every time the configured resolver
in the operating system changes.
5. Lists of DoH Servers
The "DoH servers from HTTPS" "DoH servers from DNS" return lists of
DoH servers, and those lists can have more than one element. The DoH
client can choose any of the servers in the list; that is, there is
no inherent "preference" for any of the serers returned. From a
mathematical viewpoint, the lists can better be considered as sets.
6. 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.
Before this draft is complete, mail will be sent to wellknown-uri-
review@ietf.org in order to be registered in the "Well-Known URIs"
registry at IANA. The mail will contain the following:
URI suffix: doh-servers-associated
Change controller: IETF
Specification document(s): draft-ietf-doh-resolver-associated-doh
Status: permanent
7. Privacy Considerations
Allowing a user to use DoH to a server associated with the resolver
in use by the operating system on the user's machine, instead of
using Do53 to to the resolver in use by the operating system on the
user's machine, can increase communication privacy because of the TLS
protection. However, using a DoH server can also reduce overall
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privacy because both TLS and HTTPS allow for user identification in
ways that plain Do53 does not.
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.
8. Security Considerations
If DNS queries sent from stub resolvers to recursive resolvers are
not sent over transports that assure data integrity and server
authentication, the "DoH servers from DNS" and "Resolver addresses
from DNS" protocols are susceptible to on-path attackers directing a
user to a DoH server that is not actually associated with their
resolver. Do53 is not a secure transport, and neither is DoT using
the opportunistic profile.
The DNS responses used in "DoH servers from DNS" and "Resolver
addresses from DNS" cannot be validated with DNSSEC [RFC4033], and
thus even a validating stub resolver would treat them the same as any
other DNS responses in unsigned zones.
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 if 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.
9. References
9.1. Normative References
[I-D.nottingham-rfc5785bis]
Nottingham, M., "Well-Known Uniform Resource Identifiers
(URIs)", draft-nottingham-rfc5785bis-09 (work in
progress), February 2019.
[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>.
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[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/>.
9.2. Informative References
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/info/rfc4033>.
[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>.
[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>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
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[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
Appendix A. Earlier Design Choices
The primary use case for these protocols 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.
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.
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.
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 versions of the
protocol before it came to the DOH Working Group.
Author's Address
Paul Hoffman
ICANN
Email: paul.hoffman@icann.org
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