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: 6 September 2023 Nokia
5 March 2023
Discovery of Oblivious Services via Service Binding Records
draft-ietf-ohai-svcb-config-01
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-
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on 6 September 2023.
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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 records . . . . . . . . . . . . . . 5
4.2. Use in DNS server SVCB records . . . . . . . . . . . . . 5
4.2.1. Use with DDR . . . . . . . . . . . . . . . . . . . . 6
4.2.2. Use with DNR . . . . . . . . . . . . . . . . . . . . 7
5. Gateway Location . . . . . . . . . . . . . . . . . . . . . . 7
6. Key Configuration Fetching . . . . . . . . . . . . . . . . . 7
7. Security and Privacy Considerations . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8.1. SVCB Service Parameter . . . . . . . . . . . . . . . . . 9
8.2. Well-Known URI . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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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 gates access to the target. The gateway is access
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
discover the client's identity.
Since Oblivious HTTP deployments will often involve very specific
coordination between clients, relays, and gateways, the key
configuration can often be shared in a bespoke fashion. However,
some deployments involve clients discovering oblivious targets and
their assoicated 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]). Clients can work
with trusted relays to access these gateways.
This document defines a way to use DNS records to advertise that an
HTTP service supports Oblivious HTTP. This indication is a parameter
that can be included in SVCB and HTTPS DNS resource records [SVCB]
(Section 4). The presence of this parameter indicates that a service
can act as an oblivious target and has an oblivious gateway that can
provide access to the target.
The client learns the URI to use for the oblivious gateway using a
well-known URI [WELLKNOWN], "ohttp-gateway", which is accessed on the
oblivious 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 an oblivious gateway's key
configuration from the oblivious gateway (Section 6).
This mechanism does not aid in the discovery of oblivious relays;
relay configuration is out of scope for this document. Models in
which this discovery mechanism is applicable are described in
Section 3.
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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 non-oblivious 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 an oblivious gateway that it offers to allow access
using oblivious HTTP. Once the client learns about the oblivious
gateway, it "upgrades" to using oblivious HTTP to access the
target service.
* Discovering alternative oblivious HTTP services. In this model,
the client has a default oblivious 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
oblivious target services if they are discovered, which may
provide more optimized or more relevant responses.
In both of these deployment models, the client is assumed to already
know of an oblivious relay that it trusts and works with. This
oblivious relay either needs to provide generic access to oblivious
gateways, or provide a service to clients to allow them to check
which gateways are accessible.
4. The ohttp SvcParamKey
The "ohttp" SvcParamKey (Section 8) is used to indicate that a
service described in an SVCB record can be accessed as an oblivious
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 obliviously, 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.
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The "ohttp" parameter can be included in the mandatory parameter list
to ensure that clients that do not support oblivious access do not
try to use the service. Services that mark the "ohttp" parameter as
mandatory can, therefore, indicate that the service might not be
accessible in a non-oblivious fashion. Services that are intended to
be accessed either obliviously or directly SHOULD NOT mark the
"ohttp" parameter as mandatory. Note that since multiple SVCB
responses can be provided for a single query, the oblivious and non-
oblivious versions of a single service can have different SVCB
records to support different names or properties.
The media type to use for encapsulated requests made to a target
service depends on the scheme of the SVCB record. 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 records
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 record for svc.example.com that
supports an oblivious gateway could look like this:
svc.example.com. 7200 IN HTTPS 1 . ( alpn=h2 ohttp )
A similar record for a service that only supports oblivious
connectivity could look like this:
svc.example.com. 7200 IN HTTPS 1 . ( mandatory=ohttp ohttp )
4.2. Use in DNS server SVCB records
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 oblivious 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 record,
the "alpn" MUST include at least one HTTP value (such as "h2" or
"h3").
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In order for DoH servers to function as oblivious targets, their
associated gateways need to be accessible via an oblivious relay.
DoH servers used with the discovery mechanisms described in this
section can either be publicly accessible, or specific to a network.
In general, only publicly accessible DoH servers will work as
oblivious targets, unless there is a coordinated deployment with an
oblivious relay that is also hosted within a network.
4.2.1. Use with DDR
Clients can discover that a DoH server 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 oblivious resolution using the following record:
_dns.resolver.arpa 7200 IN SVCB 1 doh.example.net (
alpn=h2 dohpath=/dns-query{?dns} ohttp )
Clients still need to perform some verification of oblivious DoH
servers, such as the TLS certificate check described in [DDR]. This
certificate check can be done 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. Since the oblivious gateway that is discovered
dynamically uses a well-known URI on the same host as the target, as
described in Section 6, the certificate evaluation for the connection
to well-known gateway URI also covers the name of the target DoH
server.
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
an oblivious relay. If a configuration occurs where the resolver is
accessible, but cannot use certificate-based validation, the client
needs to ensure that the oblivious relay only accesses the gateway
and target using the unencrypted resolver's original IP address.
For the case of DoH servers, 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.
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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.
While DNR does not require the same kind of verification as DDR,
clients that learn about DoH servers 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
Clients that know a service is available as an oblivious target via
discovery through the "ohttp" parameter in a SVCB or HTTPS record
need to know the location of the associated oblivious gateway before
sending oblivious requests.
By default, the oblivious gateway for an oblivious target is defined
as a well-known resource ([WELLKNOWN]) on the target, "/.well-known/
ohttp-gateway".
Commonly, servers will not want to actually operate the oblivious
gateway on a well-known URI. In such cases, servers can use 3xx
redirection responses (Section 15.4 of [HTTP]) to direct clients and
relays to the correct location of the oblivious gateway.
Generally, the first request a client will make will be a GET request
to discover the key configuration, described in Section 6. This
initial request also provides a convenient way for clients to learn
about the redirect from the well-known resource, if there is a
redirect. When clients work with their oblivious relays to send
oblivious requests to the gateway, clients can communicate this
redirected gateway URI.
6. Key Configuration Fetching
Clients also need to know the key configuration of an oblivious
gateway before sending oblivious requests.
In order to fetch the key configuration of an oblivious gateway
discovered in the manner described in Section 5, the client issues a
GET request to the URI of the gateway specifying the "application/
ohttp-keys" ([OHTTP]) media type in the Accept header.
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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
Oblivious gateways that coordinate with targets that advertise
oblivious 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".
Clients can either fetch this key configuration directly, or do so
via a proxy in order to avoid the server discovering information
about the client's identity. See Section 7 for more discussion of
avoiding key targeting attacks.
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 support are just hints, a client can mitigate this by
always checking for oblivious 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 discovering designated oblivious DoH servers 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 [SVCBDNS].
For oblivious DoH servers, an attacker could use unique DoH path
values to target or identify specific clients. Clients can mitigate
such attacks in several ways. Some options include: only allow
common DoH paths (such as the de-facto default "/dns-query{?dns}");
performing consistency checks by fetching the information about the
resolver over multiple resolution paths; or coordinating with a
trusted oblivious relay to validate that DoH paths are common across
clients using the same gateway.
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
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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 analysis of the options
for ensuring the key configurations are consistent between different
clients. Clients SHOULD employ some technique to mitigate key
targeting attack. Oblivious gateways that are detected to use
targeted key configurations per-client MUST NOT be used.
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 obfuscate its IP address or location (and not merely
decouple its specific requests from its IP address), or revealing its
IP address will increase the risk of a key targeting attack (if a
gateway service is trying to differentiate traffic across client IP
addresses), a proxy or similar mechanism can be used to fetch the
gateway's configuration.
8. IANA Considerations
8.1. SVCB Service Parameter
IANA is requested to add the following entry to the SVCB Service
Parameters registry ([SVCB]).
+========+=======+=================================+===========+
| Number | Name | Meaning | Reference |
+========+=======+=================================+===========+
| TBD | ohttp | Denotes that a service operates | (This |
| | | an Oblivious HTTP target | document) |
+--------+-------+---------------------------------+-----------+
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
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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", Work in Progress, Internet-Draft, draft-ietf-
httpbis-binary-message-06, 6 July 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
binary-message-06>.
[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-13, 13 August
2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
add-dnr-13>.
[DNS-SVCB] Schwartz, B. M., "Service Binding Mapping for DNS
Servers", Work in Progress, Internet-Draft, draft-ietf-
add-svcb-dns-07, 11 August 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-add-
svcb-dns-07>.
[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>.
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[OHTTP] Thomson, M. and C. A. Wood, "Oblivious HTTP", Work in
Progress, Internet-Draft, draft-ietf-ohai-ohttp-06, 15
December 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-ohai-ohttp-06>.
[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-11, 11 October 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-
svcb-https-11>.
[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
[CONSISTENCY]
Davidson, A., Finkel, M., Thomson, M., and C. A. Wood,
"Key Consistency and Discovery", Work in Progress,
Internet-Draft, draft-wood-key-consistency-03, 17 August
2022, <https://datatracker.ietf.org/doc/html/draft-wood-
key-consistency-03>.
[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>.
[SVCBDNS] Schwartz, B. M., "Service Binding Mapping for DNS
Servers", Work in Progress, Internet-Draft, draft-ietf-
add-svcb-dns-07, 11 August 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-add-
svcb-dns-07>.
Authors' Addresses
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Tommy Pauly
Apple Inc.
Email: tpauly@apple.com
Tirumaleswar Reddy
Nokia
Email: kondtir@gmail.com
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