ADD M. Boucadair
Internet-Draft Orange
Updates: 8484 (if approved) N. Cook
Intended status: Standards Track Open-Xchange
Expires: October 19, 2020 T. Reddy
McAfee
D. Wing
Citrix
April 17, 2020
Supporting Redirect Responses in DNS Queries over HTTPS (DoH)
draft-btw-add-rfc8484-clarification-01
Abstract
This document clarifies whether DNS-over-HTTPS (DoH) redirection is
allowed and specifies how redirection is thus performed.
Status of This Memo
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This Internet-Draft will expire on October 19, 2020.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 2
4. RFC8484 Update . . . . . . . . . . . . . . . . . . . . . . . 4
5. Resolving the Redirect Domain . . . . . . . . . . . . . . . . 4
6. Applicability to DoH Server Redirect . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
10.1. Normative References . . . . . . . . . . . . . . . . . . 7
10.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Server Push . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
This document clarifies the intent of DNS-over-HTTPS (DoH) [RFC8484]
whether redirection is allowed (Section 4), and subsequently
specifies how redirection is performed (Section 5).
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".
3. Discussion
[RFC8484] indicates that the support of HTTP 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
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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.
o clarify that unsolicited messages from a trusted 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, specifically regarding
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inline resolution of any domain name in the redirect URI. 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).
5. Resolving the Redirect Domain
Redirection in DoH is slightly different from "regular" HTTP
redirection, in that the DoH server may be the only configured DNS
resolver for the client (e.g., as per Section 7.1 of [RFC8310]). In
that case, and assuming the redirect URI uses a domain name, the
client will be unable to contact the URI returned in the redirect
response unless the DoH server provides the resolution information
for that domain as part of the response. Even if a DoH client has a
plaintext DNS resolver configured, using that resolver is considered
as a minimal privacy leakage [RFC8310].
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 (especially for the redirection discussion in
Section 6) will fail in some configurations, e.g.,
https://wiki.mozilla.org/Trusted_Recursive_Resolver
(network.trr.mode=3).
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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
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.
An early version of this document considered the use of server push
to provide the client with the required A/AAAA information
(Appendix A). Nevertheless, such proposal has issues as discussed in
Section 4.14 of [I-D.ietf-httpbis-bcp56bis].
6. Applicability to DoH Server Redirect
This section specifies how DoH server redirection can be safely used
to present a different URI to a requesting DoH client (Section 4).
To that aim, the DoH server may use HTTP redirection (Section 6.4 in
[RFC7231] or [RFC7538]) and the mechanism discussed in Section 5 to
inform the client about the new URI and location of the DoH server.
The mechanism discussed in [RFC7838] MAY be implemented by a DoH
server if the DoH service is authoritatively available at a separate
network location. This mechanism requires the alternative service to
present a certificate for the origin's host name. Nevertheless,
[RFC7838] is not an option for some redirection scenarios (e.g.,
Section 7 of [I-D.btw-add-home]). Additional complications arise to
provide redirection for the latter scenarios:
1. Every GET request with a new query name will require redirection,
which is suboptimal. Indeed, a redirect will only affect a
request and the DoH client will need to contact the base server
for every request and get redirected. Also, permanent redirects
for all these queries would also bloat the client's HTTP cache.
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2. Using POST would solve the issue. Nevertheless POST responses
are not widely cached as per Section 4.2.3 of [RFC7231].
3. What about relaxing [RFC7838] so that the alternate service
presents a certificate of a sub-domain of the Origin?
4. A solution that provides the same benefits as POST but without
the caching issues is needed. Such solution must then rely upon
HTTP GET method. A candidate solution using GET is described
hereafter.
5. At bootstrap, the DoH client sends a GET request against a well-
known URI (can also be used to retrieve the URI Templates
[I-D.ietf-dnsop-resolver-information]). The server can redirect
the client to an alternate server. The server's response will
include: the Authentication Domain Name (ADN) of the redirect
server, a list of IP addresses to locate the redirect server, a
list of URI Templates (e.g., https://cpe123.example.net/dns-
query{?dns}), CNAME, etc. Subsequent queries will be sent to the
redirected server.
An example of such response is depicted in Figure 1. The
structure of the response is inspired by Section 4.4.2 of
[RFC7975].
{
"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
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Unlike the GET discussed in the first bullet, this approach does
not bloat the cache.
7. 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.
DNS clients that ignore authentication failures and accept spoofed
certificates will be subject to attacks (e.g., redirect to malicious
servers, intercept sensitive data).
8. IANA Considerations
This document does not request any action from IANA.
9. Acknowledgements
Many thanks to Christian Jacquenet, Philippe Fouquart, and Ben
Schwartz for the comments.
10. References
10.1. 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>.
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[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>.
10.2. Informative References
[I-D.btw-add-home]
Boucadair, M., Reddy.K, T., Wing, D., and N. Cook, "DNS-
over-HTTPS and DNS-over-TLS Server Discovery and
Deployment Considerations for Home Networks", draft-btw-
add-home-05 (work in progress), April 2020.
[I-D.ietf-dnsop-resolver-information]
Sood, P., Arends, R., and P. Hoffman, "DNS Resolver
Information Self-publication", draft-ietf-dnsop-resolver-
information-01 (work in progress), February 2020.
[I-D.ietf-httpbis-bcp56bis]
Nottingham, M., "Building Protocols with HTTP", draft-
ietf-httpbis-bcp56bis-09 (work in progress), November
2019.
[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. Server Push
The DoH specification allows the use of server push to send DNS
responses (Section 5.3 of [RFC8484]). The typical use case for
server push is when the server knows that the client will need to
make a request for a resource, and so provides the answer to that
request via the server push mechanism. Sending answers to queries
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implies that the DoH server performs those queries itself, or
retrieves them from its cache.
In this case, the DoH server knows that the DoH client will need to
resolve the domain returned in the redirect URI. Therefore, after
receiving the initial request which would lead to a redirect
response, but before returning the response, the server sends a push
promise frame (Section 8.2.1 of [RFC7540]) request URL to retrieve
the A/AAAA resource records for the domain in the redirect response
(for example, if the domain has both A and AAAA records, two push
promise frames would be sent). Any intermediate CNAME records would
result in additional push promise frames. Promise requests cannot
contain a request body as specified in Section 8.2.1 of [RFC7540],
thus they use the GET method specified in Sections 4.1 and 6 of
[RFC8484]. The A/AAAA responses are then sent in separate streams as
specified in Section 8.2.2 of [RFC7540]. Finally, the redirect
response itself is sent.
An example of the use of server push for redirection is shown in
Figure 2.
DoH client DoH server
| |
|<===== Connect & TLS Negotiation ======================>|
|====== DNS Request for example.com/A ==================>|
|<===== Push Promise: GET redirect.example.com/A ========|
|<===== Push Promise: GET redirect.example.com/AAAA =====|
|<===== Redirect Response: https://redirect.example.com =|
|<===== Push Response for redirect.example.com/A ========|
|<===== Push Response for redirect.example.com/AAAA======|
| ... |
Figure 2: Redirect using Server Push
The advantage of using server push to provide the DNS resolution
information of the redirect domain is that, assuming that the DoH
client already supports unsolicited server push messages, then this
approach should work without any changes.
Disadvantages include the possibility that DoH clients do not support
server push.
Authors' Addresses
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Mohamed Boucadair
Orange
Rennes 35000
France
Email: mohamed.boucadair@orange.com
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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