Internet DRAFT - draft-btw-dprive-rfc8484-clarification
draft-btw-dprive-rfc8484-clarification
ADD M. Boucadair
Internet-Draft Orange
Updates: 8484 (if approved) N. Cook
Intended status: Standards Track Open-Xchange
Expires: February 28, 2021 T. Reddy
McAfee
D. Wing
Citrix
August 27, 2020
Supporting Redirection for DNS Queries over HTTPS (DoH)
draft-btw-dprive-rfc8484-clarification-00
Abstract
This document clarifies whether DNS-over-HTTPS (DoH) redirection is
allowed, describes potential issues with redirection in DoH, and
proposes how DoH redirection might be performed.
Status of This Memo
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Copyright Notice
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. RFC8484 Update . . . . . . . . . . . . . . . . . . . . . . . 4
5. Issues with Redirection in DoH . . . . . . . . . . . . . . . 4
6. Service-Level Redirect . . . . . . . . . . . . . . . . . . . 6
6.1. Well-Known URI . . . . . . . . . . . . . . . . . . . . . 6
7. Resource-Level Redirect . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9.1. resinfo Well-Known URI Suffix . . . . . . . . . . . . . . 8
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. Normative References . . . . . . . . . . . . . . . . . . 9
11.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Extending Alternative Services . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
This document clarifies the intent of DNS-over-HTTPS (DoH) [RFC8484]
whether redirection is allowed (Section 4), potential issues with
redirection in DoH (Section 5) and subsequently makes some proposals
for how service-level (Section 6) and resource-level (Section 7)
redirection might be performed.
This document adheres to Section 4.3 of [I-D.ietf-httpbis-bcp56bis]
which discusses the need for protocols using HTTP to specify redirect
handling to avoid interoperability problems.
2. Terminology
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.
"A/AAAA" is used to refer to "A and/or AAAA records".
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3. Discussion
[RFC8484] indicates that the support of HTTP [RFC7540] redirection is
one of DoH design goals (Section 1):
"The described approach is more than a tunnel over HTTP. It
establishes default media formatting types for requests and
responses but uses normal HTTP content negotiation mechanisms for
selecting alternatives that endpoints may prefer in anticipation
of serving new use cases. In addition to this media type
negotiation, it aligns itself with HTTP features such as caching,
redirection, proxying, authentication, and compression.
The integration with HTTP provides a transport suitable for both
existing DNS clients and native web applications seeking access to
the DNS."
Nevertheless, Section 3 of [RFC8484] indicates the following:
"This specification does not extend DNS resolution privileges to
URIs that are not recognized by the DoH client as configured
URIs."
This looks like an internal inconsistency of [RFC8484] that is worth
the clarification: is redirection allowed or not?
Also, Section 3 of [RFC8484] indicates that:
"A DoH client MUST NOT use a different URI simply because it was
discovered outside of the client's configuration (such as through
HTTP/2 server push) or because a server offers an unsolicited
response that appears to be a valid answer to a DNS query."
Nevertheless, [RFC8484] does not:
o specify under which conditions a discovered different URI can be
used.
o describe how a different URI can be discovered using HTTP/2 server
push. The only available example in the mailing list archives
clarifies that server push is an example of unsolicited responses.
The text was updated late in the publication process to address
this comment: https://mailarchive.ietf.org/arch/msg/doh/f_V-tBgB-
KRsLZhttx9tGt75cps/. The example provided in the thread (server
push) is related to the second part of the above excerpt.
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o clarify that unsolicited messages from a configured DoH server
should be excluded.
A clarification is proposed in Section 4. This clarification focuses
on a "different URI" that might be discovered while communicating
with an HTTP server.
Additionally, assuming that redirection is allowed, this
specification recommends how it is achieved. This is required
because redirection to a domain-based URI requires DNS resolution of
that domain name, which creates a potential bootstrapping problem
(e.g., If DoH server is the only configured DNS server, redirecting
the client to a new server by presenting a name will fail).
4. RFC8484 Update
OLD:
A DoH client MUST NOT use a different URI simply because it was
discovered outside of the client's configuration (such as through
HTTP/2 server push) or because a server offers an unsolicited
response that appears to be a valid answer to a DNS query.
NEW
A DoH client MUST NOT use a different URI that was discovered
outside of the client's configuration when communicating with HTTP
servers except via HTTP redirection from a configured URI
(Section 6.4 of [RFC7231]).
Also, a DoH client MUST ignore an unsolicited response (such as
through HTTP/2 server push) that appears to be a valid answer to a
DNS query unless that response comes from a configured URI (as
described in Section 5.3 of [RFC8484]).
5. Issues with Redirection in DoH
There are several potential issues with redirection in DoH, which are
summarized below.
The first issue to be considered is whether a new document
considering redirection is needed at all. Redirection in HTTP is
done on a per-resource basis; if the only functionality required is
to redirect all requests to an entirely different server under the
same administrative control, then the alternative service mechanism
described in [RFC7838] might be sufficient. However, there are
restrictions on the use of alternative services; specifically the
certificate presented by the alternative service must be valid for
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the origin. This restriction means that alternative services cannot
be used for use-cases such as redirecting the client to a locally
administered DoH server (e.g., resolver or forwarder) which does not
have a certificate valid for the origin. Additionally, alternative
services suffer from the bootstrapping issue described below.
The second issue with using HTTP redirection is bootstrapping; any
client that is relying solely upon a DoH server for resolution must
be able to resolve the domain in the redirect response. Even if a
DoH client has a plaintext DNS resolver configured, using that
resolver is considered as a minimal privacy leakage [RFC8310]. One
possible solution is for the DoH client to use the same server that
returned the redirect response to perform the resolution, however
that may then lead to a further redirect response. Another solution
is for the DoH server to include additional information in the
response, similar to the "glue" records as defined in [RFC7719].
The final issue is that HTTP redirection is done on a per-resource
basis; this presents several problems for DoH:
1. Every GET request with a new query name will require redirection,
which is suboptimal. Indeed, a redirect will only affect a
unique request, and the DoH client will thus need to contact the
origin server for every new request and get redirected, requiring
two roundtrips. Also, permanent redirects [RFC7538] for all
these queries would bloat the client's HTTP cache.
2. Using POST requests would solve the issue. Nevertheless POST
responses are not widely cached as per Section 4.2.3 of
[RFC7231], and mandating the use of POST requests for DoH in
order to enable redirection hardly seems reasonable.
The above issues would seem to indicate that despite the intention of
[RFC8484] to align itself with HTTP redirection, some additional work
is required in order for any other mechanism than alternative
services (e.g., [RFC7838]) to be deployed with confidence.
The rest of this document considers the issue of redirection at two
levels:
1. Service-level Redirect: Similar to alternative services, this
would allow a DoH server to redirect a DoH client to an
alternative service for all future queries, rather than on a per-
resource basis.
2. Resource-Level Redirect: Solving the bootstrapping problem for
regular HTTP redirects. Note that this doesn't solve the caching
issues described above, and does raise the question of whether
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regular HTTP redirection is desirable or worthwhile (i.e., are
there any valid use-cases for resource-level redirection in
DoH?).
6. Service-Level Redirect
We considered two possibilities for service-level redirect:
1. Extending [RFC7838] by relaxing the host authentication checks.
2. Using a well-known URI to return information about alternative
services.
Extending alternative services was considered, but rejected (see
Appendix A for the reasons) in favour of the well-known URI approach.
6.1. Well-Known URI
We propose the use of the well-known URI mechanism [RFC8615], with
the name "resinfo" to retrieve resolver information, which could
include specifying alternative services, through the use of a JSON
object in the response payload. A well-known URI would thus look
like "https://doh.example.com/.well-known/resinfo".
The example in Figure 1 shows what a JSON object might look like that
specified one or more alternative services. The structure of the
response is inspired by Section 4.4.2 of [RFC7975].
Note that the response includes "glue" RR information to allow the
alternative service to be accessed without further DNS queries, and
includes an authenticated domain name to be used for authenticating
the alternative service.
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{
"associated-resolvers": {
"adn": [
{
"name": "cpe123.example.net",
"uri-template": [
"https://cpe123.example.net/dns-query{?dns}"
],
"a": [
"192.0.2.1",
"192.0.2.2"
],
"aaaa": [
"2001:db8::1",
"2001:db8::2"
],
"ttl": 3600
}
]
}
}
Figure 1: Response Example with Glue RR Information
7. Resource-Level Redirect
Notwithstanding the issues with resource-level redirects described in
Section 5, this section describes a proposal for returning the "glue"
RRs required to avoid the bootstrapping issue described in that
section (but not the roundtrip or caching issues).
Servers supporting DoH redirect MUST support returning the redirect
response body mechanism described hereafter.
Note: "MUST" is used here because resolving the redirect name
using Do53 will fail in some configurations, e.g.,
https://wiki.mozilla.org/Trusted_Recursive_Resolver
(network.trr.mode=3).
Concretely, the DoH server returns in the response body a DNS
response with an 'application/dns-message' media type as specified in
Section 6 of [RFC8484], containing any A and AAAA records for the
domain name in the redirect URI, including any CNAMEs.
For example, if the redirect URI contains the domain name
"redirect.example.com", and "redirect.example.com" is a CNAME
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pointing to "real.example.com", then an example response body would
contain:
o A CNAME record for "redirect.example.com"
o Any A records for "real.example.com"
o Any AAAA records for "real.example.com"
This approach is simple; no client or server support of server push
is required, and it is also more efficient in terms of the amount of
data transmitted.
8. Security Considerations
DoH-related security considerations are discussed in Section 9 of
[RFC8484].
Section 9 of [RFC7838] describes security considerations related to
the use of alternate services. Relaxing the host authentication
requirements would certainly warrant additional security
considerations.
9. IANA Considerations
9.1. resinfo Well-Known URI Suffix
This document requests IANA to assign the following well-known URI
from the registry available at https://www.iana.org/assignments/well-
known-uris/well-known-uris.xhtml.
URI suffix: resinfo
Change controller: IETF
Specification document(s): This document
Status: permanent
10. Acknowledgements
Many thanks to Christian Jacquenet, Philippe Fouquart, and Ben
Schwartz for the comments.
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11. References
11.1. Normative References
[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>.
[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>.
[RFC7538] Reschke, J., "The Hypertext Transfer Protocol Status Code
308 (Permanent Redirect)", RFC 7538, DOI 10.17487/RFC7538,
April 2015, <https://www.rfc-editor.org/info/rfc7538>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[RFC7838] Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
April 2016, <https://www.rfc-editor.org/info/rfc7838>.
[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>.
[RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
for DNS over TLS and DNS over DTLS", RFC 8310,
DOI 10.17487/RFC8310, March 2018,
<https://www.rfc-editor.org/info/rfc8310>.
[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>.
[RFC8615] Nottingham, M., "Well-Known Uniform Resource Identifiers
(URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
<https://www.rfc-editor.org/info/rfc8615>.
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11.2. Informative References
[I-D.ietf-httpbis-bcp56bis]
Nottingham, M., "Building Protocols with HTTP", draft-
ietf-httpbis-bcp56bis-09 (work in progress), November
2019.
[RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 7719, DOI 10.17487/RFC7719, December
2015, <https://www.rfc-editor.org/info/rfc7719>.
[RFC7975] Niven-Jenkins, B., Ed. and R. van Brandenburg, Ed.,
"Request Routing Redirection Interface for Content
Delivery Network (CDN) Interconnection", RFC 7975,
DOI 10.17487/RFC7975, October 2016,
<https://www.rfc-editor.org/info/rfc7975>.
Appendix A. Extending Alternative Services
Section 9.2 of [RFC7838] discusses the possibilities for attackers to
hijack the communication to an origin. This is the justification for
the requirement in Section 2.1 of [RFC7838] that "Clients MUST have
reasonable assurances that the alternative service is under control
of and valid for the whole origin.".
However, when a DoH server presents an alternative DoH service to a
DoH client, both the origin and alternative service, as well as the
DNS queries and responses, must be, by definition, resistant to MITM
attacks. Thus it could be argued that in these circumstances,
relaxing the host authentication requirements is justified. The
relaxation could be limited, e.g., still requiring some relationship
between the origin and the alternative, or unlimited, allowing no
such relationship to exist.
However the bootstrapping issues described in Section 5 still apply,
and there is no mechanism for the DoH server to specify an
authenticated domain name to use to authenticate the alternative
service, making this proposal unsuitable for deployment.
Authors' Addresses
Mohamed Boucadair
Orange
Rennes 35000
France
Email: mohamed.boucadair@orange.com
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Neil Cook
Open-Xchange
UK
Email: neil.cook@noware.co.uk
Tirumaleswar Reddy
McAfee, Inc.
Embassy Golf Link Business Park
Bangalore, Karnataka 560071
India
Email: TirumaleswarReddy_Konda@McAfee.com
Dan Wing
Citrix Systems, Inc.
USA
Email: dwing-ietf@fuggles.com
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