Internet DRAFT - draft-ietf-ohai-svcb-config
draft-ietf-ohai-svcb-config
Oblivious HTTP Application Intermediation T. Pauly
Internet-Draft Apple Inc.
Intended status: Standards Track T. Reddy
Expires: 8 April 2024 Nokia
6 October 2023
Discovery of Oblivious Services via Service Binding Records
draft-ietf-ohai-svcb-config-07
Abstract
This document defines a parameter that can be included in SVCB and
HTTPS DNS resource records to denote that a service is accessible
using Oblivious HTTP, by offering an Oblivious Gateway Resource
through which to access the target. This document also defines a
mechanism to learn the key configuration of the discovered Oblivious
Gateway Resource.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-ohai-svcb-config/.
Discussion of this document takes place on the Oblivious HTTP
Application Intermediation Working Group mailing list
(mailto:ohai@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/ohai/. Subscribe at
https://www.ietf.org/mailman/listinfo/ohai/.
Source for this draft and an issue tracker can be found at
https://github.com/ietf-wg-ohai/draft-ohai-svcb-config.
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/.
Pauly & Reddy Expires 8 April 2024 [Page 1]
Internet-Draft Oblivious Services in SVCB October 2023
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 8 April 2024.
Copyright Notice
Copyright (c) 2023 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 Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
3. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 4
4. The ohttp SvcParamKey . . . . . . . . . . . . . . . . . . . . 4
4.1. Use in HTTPS service RRs . . . . . . . . . . . . . . . . 5
4.2. Use in DNS server SVCB RRs . . . . . . . . . . . . . . . 5
4.2.1. Use with DDR . . . . . . . . . . . . . . . . . . . . 6
4.2.2. Use with DNR . . . . . . . . . . . . . . . . . . . . 6
5. Gateway Location . . . . . . . . . . . . . . . . . . . . . . 7
6. Key Configuration Fetching . . . . . . . . . . . . . . . . . 7
7. Security and Privacy Considerations . . . . . . . . . . . . . 8
7.1. Key Targeting Attacks . . . . . . . . . . . . . . . . . . 9
7.2. dohpath Targeting Attacks . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8.1. SVCB Service Parameter . . . . . . . . . . . . . . . . . 9
8.2. Well-Known URI . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
Pauly & Reddy Expires 8 April 2024 [Page 2]
Internet-Draft Oblivious Services in SVCB October 2023
1. Introduction
Oblivious HTTP [OHTTP] allows clients to encrypt messages exchanged
with an Oblivious Target Resource (target). The messages are
encapsulated in encrypted messages to an Oblivious Gateway Resource
(gateway), which offers Oblivious HTTP access to the target. The
gateway is accessed via an Oblivious Relay Resource (relay), which
proxies the encapsulated messages to hide the identity of the client.
Overall, this architecture is designed in such a way that the relay
cannot inspect the contents of messages, and the gateway and target
cannot learn the client's identity from a single transaction.
Since Oblivious HTTP deployments typically involve very specific
coordination between clients, relays, and gateways, the key
configuration is often shared in a bespoke fashion. However, some
deployments involve clients discovering targets and their associated
gateways more dynamically. For example, a network might operate a
DNS resolver that provides more optimized or more relevant DNS
answers and is accessible using Oblivious HTTP, and might want to
advertise support for Oblivious HTTP via mechanisms like Discovery of
Designated Resolvers ([DDR]) and Discovery of Network-designated
Resolvers ([DNR]). Clients can access these gateways through trusted
relays.
This document defines a way to use DNS resource records (RRs) to
advertise that an HTTP service supports Oblivious HTTP. This
advertisement is a parameter that can be included in SVCB and HTTPS
DNS RRs [SVCB] (Section 4). The presence of this parameter indicates
that a service can act as a target and has a gateway that can provide
access to the target.
The client learns the URI to use for the gateway using a well-known
URI suffix [WELLKNOWN], "ohttp-gateway", which is accessed on the
target (Section 5). This means that for deployments that support
this kind of discovery, the gateway and target resources need to be
located on the same host.
This document also defines a way to fetch a gateway's key
configuration from the gateway (Section 6).
This mechanism does not aid in the discovery of relays; relay
configuration is out of scope for this document. Models in which
this discovery mechanism is applicable are described in Section 3.
Pauly & Reddy Expires 8 April 2024 [Page 3]
Internet-Draft Oblivious Services in SVCB October 2023
2. Conventions and Definitions
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.
3. Applicability
There are multiple models in which the discovery mechanism defined in
this document can be used.
* Upgrading regular (non-proxied) HTTP to Oblivious HTTP. In this
model, the client intends to communicate with a specific target
service, and prefers to use Oblivious HTTP if it is available.
The target service has a gateway that it offers to allow access
using Oblivious HTTP. Once the client learns about the gateway,
it "upgrades" its requests from non-proxied HTTP to Oblivious HTTP
to access the target service.
* Discovering alternative Oblivious HTTP services. In this model,
the client has a default target service that it uses. For
example, this may be a public DNS resolver that is accessible over
Oblivious HTTP. The client is willing to use alternative target
services if they are discovered, which may provide more optimized
or more relevant responses.
In both deployment models, the client is configured with a relay that
it trusts for Oblivious HTTP transactions. This relay either needs
to provide generic access to gateways, or provide a service to
clients to allow them to check which gateways are accessible.
4. The ohttp SvcParamKey
The "ohttp" SvcParamKey is used to indicate that a service described
in an SVCB RR can be accessed as a target using an associated
gateway. The service that is queried by the client hosts one or more
target resources.
In order to access the service's target resources using Oblivious
HTTP, the client needs to send encapsulated messages to the gateway
resource and the gateway's key configuration (both of which can be
retrieved using the method described in Section 6).
Both the presentation and wire format values for the "ohttp"
parameter MUST be empty.
Pauly & Reddy Expires 8 April 2024 [Page 4]
Internet-Draft Oblivious Services in SVCB October 2023
Services can include the "ohttp" parameter in the mandatory parameter
list if the service is only accessible using Oblivious HTTP. Marking
the "ohttp" parameter as mandatory will cause clients that do not
understand the parameter to ignore that SVCB RR. Including the
"ohttp" parameter without marking it mandatory advertises a service
that is optionally available using Oblivious HTTP. Note also that
multiple SVCB RRs can be provided to indicate separate
configurations.
The media type to use for encapsulated requests made to a target
service depends on the scheme of the SVCB RR. This document defines
the interpretation for the "https" [SVCB] and "dns" [DNS-SVCB]
schemes. Other schemes that want to use this parameter MUST define
the interpretation and meaning of the configuration.
4.1. Use in HTTPS service RRs
For the "https" scheme, which uses the HTTPS RR type instead of SVCB,
the presence of the "ohttp" parameter means that the target being
described is an Oblivious HTTP service that is accessible using the
default "message/bhttp" media type [OHTTP] [BINARY-HTTP].
For example, an HTTPS service RR for svc.example.com that supports
Oblivious HTTP could look like this:
svc.example.com. 7200 IN HTTPS 1 . ( alpn=h2 ohttp )
A similar RR for a service that only supports Oblivious HTTP could
look like this:
svc.example.com. 7200 IN HTTPS 1 . ( mandatory=ohttp ohttp )
4.2. Use in DNS server SVCB RRs
For the "dns" scheme, as defined in [DNS-SVCB], the presence of the
"ohttp" parameter means that the DNS server being described has a DNS
over HTTP (DoH) [DOH] service that can be accessed using Oblivious
HTTP. Requests to the resolver are sent to the gateway using binary
HTTP with the default "message/bhttp" media type [BINARY-HTTP],
containing inner requests that use the "application/dns-message"
media type [DOH].
If the "ohttp" parameter is included in an DNS server SVCB RR, the
"alpn" MUST include at least one HTTP value (such as "h2" or "h3").
In order for DoH-capable recursive resolvers to function as Oblivious
HTTP targets, their associated gateways need to be accessible via a
client-trusted relay. DoH recursive resolvers used with the
Pauly & Reddy Expires 8 April 2024 [Page 5]
Internet-Draft Oblivious Services in SVCB October 2023
discovery mechanisms described in this section can either be publicly
accessible, or specific to a network. In general, only publicly
accessible DoH recursive resolvers will work as Oblivious HTTP
targets, unless there is a coordinated deployment with a relay to
access the network-specific DoH recursive resolvers.
4.2.1. Use with DDR
Clients can discover that a DoH recursive resolvers support Oblivious
HTTP using DDR, either by querying _dns.resolver.arpa to a locally
configured resolver or by querying using the name of a resolver
[DDR].
For example, a DoH service advertised over DDR can be annotated as
supporting resolution via Oblivious HTTP using the following RR:
_dns.resolver.arpa 7200 IN SVCB 1 doh.example.net (
alpn=h2 dohpath=/dns-query{?dns} ohttp )
Clients still need to perform verification of oblivious DoH servers,
specifically the TLS certificate checks described in Section 4.2 of
[DDR]. Since the gateway and target resources for discovered
oblivious services need to be on the same host, this means that the
client needs to verify that the certificate presented by the gateway
passes the required checks. These checks can be performed when
looking up the configuration on the gateway as described in
Section 6, which can either be done directly or via the relay or
another proxy to avoid exposing client IP addresses.
Opportunistic discovery [DDR], where only the IP address is
validated, SHOULD NOT be used in general with Oblivious HTTP, since
this mode primarily exists to support resolvers that use private or
local IP addresses, which will usually not be accessible when using a
relay. If a configuration occurs where the resolver is accessible,
but cannot use certificate-based validation, the client MUST ensure
that the relay only accesses the gateway and target using the
unencrypted resolver's original IP address.
For the case of DoH recursive resolvers, clients also need to ensure
that they are not being targeted with unique DoH paths that would
reveal their identity. See Section 7 for more discussion.
4.2.2. Use with DNR
The SvcParamKeys defined in this document also can be used with
Discovery of Network-designated Resolvers (DNR) [DNR]. In this case,
the oblivious configuration and path parameters can be included in
DHCP and Router Advertisement messages.
Pauly & Reddy Expires 8 April 2024 [Page 6]
Internet-Draft Oblivious Services in SVCB October 2023
While DNR does not require the same kind of verification as DDR,
clients that learn about DoH recursive resolvers still need to ensure
that they are not being targeted with unique DoH paths that would
reveal their identity. See Section 7 for more discussion.
5. Gateway Location
Once a client has discovered that a service supports Oblivious HTTP
via the "ohttp" parameter in a SVCB or HTTPS RR, it needs to be able
to send requests via a relay to the correct gateway location.
This document defines a well-known resource ([WELLKNOWN]), "/.well-
known/ohttp-gateway", which is an Oblivious Gateway Resource
available on the same host as the target resource.
Some servers might not want to operate the gateway on a well-known
URI. In such cases, these servers can use 3xx redirection responses
(Section 15.4 of [HTTP]) to direct clients and relays to the correct
location of the gateway. Such redirects would apply both to requests
made to fetch key configurations (as defined in Section 6) and to
encapsulated requests made via a relay.
If a client receives a redirect when fetching the key configuration
from the well-known gateway resource, it MUST NOT communicate the
redirected gateway URI to the relay as the location of the gateway to
use. Doing so would allow the gateway to target clients by encoding
unique or client-identifying values in the redirected URI. Instead,
relays being used with dynamically discovered gateways MUST use the
well-known gateway resource and follow any redirects independently of
redirects that clients received. The relay can remember such
redirects across oblivious requests for all clients in order to avoid
added latency.
6. Key Configuration Fetching
Clients also need to know the key configuration of a gateway before
encapsulating and sending requests to the relay.
If a client fetches the key configuration directly from the gateway,
it will expose identifiers like a client IP address to the gateway.
The privacy and security implications of fetching the key
configuration are discussed more in Section 7. Clients can use an
HTTP proxy to hide their IP addresses when fetching key
configurations. Clients can also perform consistency checks to
validate that they are not receiving unique key configurations, as
discussed in Section 7.1.
Pauly & Reddy Expires 8 April 2024 [Page 7]
Internet-Draft Oblivious Services in SVCB October 2023
In order to fetch the key configuration of a gateway discovered in
the manner described in Section 5, the client issues a GET request
(either through a proxy or directly) to the URI of the gateway
specifying the "application/ohttp-keys" ([OHTTP]) media type in the
Accept header.
For example, if the client knows an oblivious gateway URI,
"https://svc.example.com/.well-known/ohttp-gateway", it could fetch
the key configuration with the following request:
GET /.well-known/ohttp-gateway HTTP/1.1
Host: svc.example.com
Accept: application/ohttp-keys
Gateways that coordinate with targets that advertise Oblivious HTTP
support SHOULD support GET requests for their key configuration in
this manner, unless there is another out-of-band configuration model
that is usable by clients. Gateways respond with their key
configuration in the response body, with a content type of
"application/ohttp-keys".
7. Security and Privacy Considerations
Attackers on a network can remove SVCB information from cleartext DNS
answers that are not protected by DNSSEC [DNSSEC]. This can
effectively downgrade clients. However, since SVCB indications for
Oblivious HTTP support are just hints, a client can mitigate this by
always checking for a gateway configuration (Section 6) on the well-
known gateway location (Section 5). Use of encrypted DNS along with
DNSSEC can also be used as a mitigation.
When clients fetch a gateway's configuration (Section 6), they can
expose their identity in the form of an IP address if they do not
connect via a proxy or some other IP-hiding mechanism. In some
circumstances, this might not be a privacy concern, since revealing
that a particular client IP address is preparing to use an Oblivious
HTTP service can be expected. However, if a client is otherwise
trying to hide its IP address or location (and not merely decouple
its specific requests from its IP address), or if revealing its IP
address facilitates key targeting attacks (if a gateway service uses
IP addresses to associate specific configurations with specific
clients), a proxy or similar mechanism can be used to fetch the
gateway's configuration.
Pauly & Reddy Expires 8 April 2024 [Page 8]
Internet-Draft Oblivious Services in SVCB October 2023
When discovering designated oblivious DoH recursive resolvers using
this mechanism, clients need to ensure that the designation is
trusted in lieu of being able to directly check the contents of the
gateway server's TLS certificate. See Section 4.2.1 for more
discussion, as well as the Security Considerations of [DNS-SVCB].
7.1. Key Targeting Attacks
As discussed in [OHTTP], client requests using Oblivious HTTP can
only be linked by recognizing the key configuration. In order to
prevent unwanted linkability and tracking, clients using any key
configuration discovery mechanism need to be concerned with attacks
that target a specific user or population with a unique key
configuration.
There are several approaches clients can use to mitigate key
targeting attacks. [CONSISTENCY] provides an overview of the options
for ensuring the key configurations are consistent between different
clients. Clients SHOULD employ some technique to mitigate key
targeting attacks, such as the option of confirming the key with a
shared proxy as described in [CONSISTENCY]. If a client detects that
a gateway is using per-client targeted key configuration, the client
can stop using the gateway, and potentially report the targeting
attack to let other clients avoid using this gateway in the future.
7.2. dohpath Targeting Attacks
For oblivious DoH servers, an attacker could use unique dohpath
values to target or identify specific clients. This attack is very
similar to the generic OHTTP key targeting attack described above.
A client can avoid these targeting attacks by only allowing a single
dohpath value, such as the commonly used "/dns-query{?dns}" or
another pre-known value. If the client allows arbitrary dohpath
values, it SHOULD mitigate targeting attacks with a consistency
check, such as using a mechanism described in [CONSISTENCY] to
validate the dohpath value with another source. Clients might choose
to only employ a consistency check on a percentage of discovery
events, depending on the capacity of consistency check options and
their deployment threat model.
8. IANA Considerations
8.1. SVCB Service Parameter
This document adds the following entry to the SVCB Service Parameters
registry ([SVCB]). The definition of this parameter is in Section 4.
Pauly & Reddy Expires 8 April 2024 [Page 9]
Internet-Draft Oblivious Services in SVCB October 2023
+=============+=======+=================================+===========+
| Number | Name | Meaning | Reference |
+=============+=======+=================================+===========+
| 8 (Early | ohttp | Denotes that a | (This |
| Allocation) | | service operates an | document) |
| | | Oblivious HTTP target | |
+-------------+-------+---------------------------------+-----------+
Table 1
8.2. Well-Known URI
IANA is requested to add one new entry in the "Well-Known URIs"
registry [WELLKNOWN].
URI suffix: ohttp-gateway
Change controller: IETF
Specification document: This document
Status: permanent
Related information: N/A
9. References
9.1. Normative References
[BINARY-HTTP]
Thomson, M. and C. A. Wood, "Binary Representation of HTTP
Messages", RFC 9292, DOI 10.17487/RFC9292, August 2022,
<https://www.rfc-editor.org/rfc/rfc9292>.
[DDR] Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T.
Jensen, "Discovery of Designated Resolvers", Work in
Progress, Internet-Draft, draft-ietf-add-ddr-10, 5 August
2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
add-ddr-10>.
[DNR] Boucadair, M., Reddy.K, T., Wing, D., Cook, N., and T.
Jensen, "DHCP and Router Advertisement Options for the
Discovery of Network-designated Resolvers (DNR)", Work in
Progress, Internet-Draft, draft-ietf-add-dnr-16, 27 April
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
add-dnr-16>.
Pauly & Reddy Expires 8 April 2024 [Page 10]
Internet-Draft Oblivious Services in SVCB October 2023
[DNS-SVCB] Schwartz, B. M., "Service Binding Mapping for DNS
Servers", Work in Progress, Internet-Draft, draft-ietf-
add-svcb-dns-09, 26 June 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-add-
svcb-dns-09>.
[DOH] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/rfc/rfc8484>.
[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/rfc/rfc9110>.
[OHTTP] Thomson, M. and C. A. Wood, "Oblivious HTTP", Work in
Progress, Internet-Draft, draft-ietf-ohai-ohttp-10, 25
August 2023, <https://datatracker.ietf.org/doc/html/draft-
ietf-ohai-ohttp-10>.
[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/rfc/rfc2119>.
[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/rfc/rfc8174>.
[SVCB] Schwartz, B. M., Bishop, M., and E. Nygren, "Service
binding and parameter specification via the DNS (DNS SVCB
and HTTPS RRs)", Work in Progress, Internet-Draft, draft-
ietf-dnsop-svcb-https-12, 11 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-
svcb-https-12>.
[WELLKNOWN]
Nottingham, M., "Well-Known Uniform Resource Identifiers
(URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
<https://www.rfc-editor.org/rfc/rfc8615>.
9.2. Informative References
Pauly & Reddy Expires 8 April 2024 [Page 11]
Internet-Draft Oblivious Services in SVCB October 2023
[CONSISTENCY]
Davidson, A., Finkel, M., Thomson, M., and C. A. Wood,
"Key Consistency and Discovery", Work in Progress,
Internet-Draft, draft-ietf-privacypass-key-consistency-01,
10 July 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-privacypass-key-consistency-01>.
[DNSSEC] 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/rfc/rfc4033>.
Authors' Addresses
Tommy Pauly
Apple Inc.
Email: tpauly@apple.com
Tirumaleswar Reddy
Nokia
Email: kondtir@gmail.com
Pauly & Reddy Expires 8 April 2024 [Page 12]
