Independent Submission E. Kinnear

Independent Submission E. Kinnear
Request for Comments: 9230 Apple Inc.
Category: Experimental P. McManus
ISSN: 2070-1721 Fastly
T. Pauly
Apple Inc.
T. Verma
C.A. Wood
Cloudflare
June 2022

Oblivious DNS over HTTPS

Abstract

This document describes a protocol that allows clients to hide their
IP addresses from DNS resolvers via proxying encrypted DNS over HTTPS
(DoH) messages. This improves privacy of DNS operations by not
allowing any one server entity to be aware of both the client IP
address and the content of DNS queries and answers.

This experimental protocol has been developed outside the IETF and is
published here to guide implementation, ensure interoperability among
implementations, and enable wide-scale experimentation.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.

This document defines an Experimental Protocol for the Internet
community. This is a contribution to the RFC Series, independently
of any other RFC stream. The RFC Editor has chosen to publish this
document at its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not candidates for any level of Internet Standard;
see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9230.

Copyright Notice

Copyright (c) 2022 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.

Table of Contents

1. Introduction
1.1. Specification of Requirements
2. Terminology
3. Deployment Requirements
4. HTTP Exchange
4.1. HTTP Request
4.2. HTTP Request Example
4.3. HTTP Response
4.4. HTTP Response Example
4.5. HTTP Metadata
5. Configuration and Public Key Format
6. Protocol Encoding
6.1. Message Format
6.2. Encryption and Decryption Routines
7. Oblivious Client Behavior
8. Oblivious Target Behavior
9. Compliance Requirements
10. Experiment Overview
11. Security Considerations
11.1. Denial of Service
11.2. Proxy Policies
11.3. Authentication
12. IANA Considerations
12.1. Oblivious DoH Message Media Type
13. References
13.1. Normative References
13.2. Informative References
Appendix A. Use of Generic Proxy Services
Acknowledgments
Authors' Addresses

1. Introduction

DNS over HTTPS (DoH) [RFC8484] defines a mechanism to allow DNS
messages to be transmitted in HTTP messages protected with TLS. This
provides improved confidentiality and authentication for DNS
interactions in various circumstances.

While DoH can prevent eavesdroppers from directly reading the
contents of DNS exchanges, clients cannot send DNS queries to and
receive answers from servers without revealing their local IP address
(and thus information about the identity or location of the client)
to the server.

Proposals such as Oblivious DNS [OBLIVIOUS-DNS] increase privacy by
ensuring that no single DNS server is aware of both the client IP
address and the message contents.

This document defines Oblivious DoH, an experimental protocol built
on DoH that permits proxied resolution, in which DNS messages are
encrypted so that no server can independently read both the client IP
address and the DNS message contents.

As with DoH, DNS messages exchanged over Oblivious DoH are fully
formed DNS messages. Clients that want to receive answers that are
relevant to the network they are on without revealing their exact IP
address can thus use the EDNS0 Client Subnet option ([RFC7871],
Section 7.1.2) to provide a hint to the resolver using Oblivious DoH.

This mechanism is intended to be used as one mechanism for resolving
privacy-sensitive content in the broader context of DNS privacy.

This experimental protocol has been developed outside the IETF and is
published here to guide implementation, ensure interoperability among
implementations, and enable wide-scale experimentation. See
Section 10 for more details about the experiment.

1.1. Specification of Requirements

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. Terminology

This document defines the following terms:

Oblivious Client: A client that sends DNS queries to an Oblivious
Target, through an Oblivious Proxy. The Client is responsible for
selecting the combination of Proxy and Target to use for a given
query.

Oblivious Proxy: An HTTP server that proxies encrypted DNS queries
and responses between an Oblivious Client and an Oblivious Target
and is identified by a URI Template [RFC6570] (see Section 4.1).
Note that this Oblivious Proxy is not acting as a full HTTP proxy
but is instead a specialized server used to forward Oblivious DNS
messages.

Oblivious Target: An HTTP server that receives and decrypts
encrypted Oblivious Client DNS queries from an Oblivious Proxy and
returns encrypted DNS responses via that same Proxy. In order to
provide DNS responses, the Target can be a DNS resolver, be co-
located with a resolver, or forward to a resolver.

Throughout the rest of this document, we use the terms "Client",
"Proxy", and "Target" to refer to an Oblivious Client, Oblivious
Proxy, and Oblivious Target, respectively.

3. Deployment Requirements

Oblivious DoH requires, at a minimum:

* An Oblivious Proxy server, identified by a URI Template.

* An Oblivious Target server. The Target and Proxy are expected to
be non-colluding (see Section 11).

* One or more Target public keys for encrypting DNS queries sent to
a Target via a Proxy (Section 5). These keys guarantee that only
the intended Target can decrypt Client queries.

The mechanism for discovering and provisioning the Proxy URI Template
and Target public keys is out of scope for this document.

4. HTTP Exchange

Unlike direct resolution, oblivious hostname resolution over DoH
involves three parties:

1. The Client, which generates queries.

2. The Proxy, which receives encrypted queries from the Client and
passes them on to a Target.

3. The Target, which receives proxied queries from the Client via
the Proxy and produces proxied answers.

--- [ Request encrypted with Target public key ] -->
+---------+ +-----------+ +-----------+
| Client +-------------> Oblivious +-------------> Oblivious |
| <-------------+ Proxy <-------------+ Target |
+---------+ +-----------+ +-----------+
<-- [ Response encrypted with symmetric key ] ---

Figure 1: Oblivious DoH Exchange

4.1. HTTP Request

Oblivious DoH queries are created by the Client and are sent to the
Proxy as HTTP requests using the POST method. Clients are configured
with a Proxy URI Template [RFC6570] and the Target URI. The scheme
for both the Proxy URI Template and the Target URI MUST be "https".
The Proxy URI Template uses the Level 3 encoding defined in
Section 1.2 of [RFC6570] and contains two variables: "targethost",
which indicates the hostname of the Target server; and "targetpath",
which indicates the path on which the Target is accessible. Examples
of Proxy URI Templates are shown below:

https://dnsproxy.example/dns-query{?targethost,targetpath}
https://dnsproxy.example/{targethost}/{targetpath}

The URI Template MUST contain both the "targethost" and "targetpath"
variables exactly once and MUST NOT contain any other variables. The
variables MUST be within the path or query components of the URI.
Clients MUST ignore configurations that do not conform to this
template. See Section 4.2 for an example request.

Oblivious DoH messages have no cache value, since both requests and
responses are encrypted using ephemeral key material. Requests and
responses MUST NOT be cached.

Clients MUST set the HTTP Content-Type header to "application/
oblivious-dns-message" to indicate that this request is an Oblivious
DoH query intended for proxying. Clients also SHOULD set this same
value for the HTTP Accept header.

A correctly encoded request has the HTTP Content-Type header
"application/oblivious-dns-message", uses the HTTP POST method, and
contains "targethost" and "targetpath" variables. If the Proxy fails
to match the "targethost" and "targetpath" variables from the path,
it MUST treat the request as malformed. The Proxy constructs the URI
of the Target with the "https" scheme, using the value of
"targethost" as the URI host and the percent-decoded value of
"targetpath" as the URI path. Proxies MUST check that Client
requests are correctly encoded and MUST return a 4xx (Client Error)
if the check fails, along with the Proxy-Status response header with
an "error" parameter of type "http_request_error" [RFC9209].

Proxies MAY choose to not forward connections to non-standard ports.
In such cases, Proxies can indicate the error with a 403 response
status code, along with a Proxy-Status response header with an
"error" parameter of type "http_request_denied" and with an
appropriate explanation in "details".

If the Proxy cannot establish a connection to the Target, it can
indicate the error with a 502 response status code, along with a
Proxy-Status response header with an "error" parameter whose type
indicates the reason. For example, if DNS resolution fails, the
error type might be "dns_timeout", whereas if the TLS connection
fails, the error type might be "tls_protocol_error".

Upon receipt of requests from a Proxy, Targets MUST validate that the
request has the HTTP Content-Type header "application/oblivious-dns-
message" and uses the HTTP POST method. Targets can respond with a
4xx response status code if this check fails.

4.2. HTTP Request Example

The following example shows how a Client requests that a Proxy,
"dnsproxy.example", forward an encrypted message to
"dnstarget.example". The URI Template for the Proxy is
"https://dnsproxy.example/dns-query{?targethost,targetpath}". The
URI for the Target is "https://dnstarget.example/dns-query".

:method = POST
:scheme = https
:authority = dnsproxy.example
:path = /dns-query?targethost=dnstarget.example&targetpath=/dns-query
accept = application/oblivious-dns-message
content-type = application/oblivious-dns-message
content-length = 106

<Bytes containing an encrypted Oblivious DNS query>

The Proxy then sends the following request on to the Target:

:method = POST
:scheme = https
:authority = dnstarget.example
:path = /dns-query
accept = application/oblivious-dns-message
content-type = application/oblivious-dns-message
content-length = 106

<Bytes containing an encrypted Oblivious DNS query>

4.3. HTTP Response

The response to an Oblivious DoH query is generated by the Target.
It MUST set the Content-Type HTTP header to "application/oblivious-
dns-message" for all successful responses. The body of the response
contains an encrypted DNS message; see Section 6.

The response from a Target MUST set the Content-Type HTTP header to
"application/oblivious-dns-message", and that same type MUST be used
on all successful responses sent by the Proxy to the Client. A
Client MUST only consider a response that contains the Content-Type
header before processing the payload. A response without the
appropriate header MUST be treated as an error and be handled
appropriately. All other aspects of the HTTP response and error
handling are inherited from standard DoH.

Proxies forward responses from the Target to the Client, without any
modifications to the body or status code. The Proxy also SHOULD add
a Proxy-Status response header with a "received-status" parameter
indicating that the status code was generated by the Target.

Note that if a Client receives a 3xx status code and chooses to
follow a redirect, the subsequent request MUST also be performed
through a Proxy in order to avoid directly exposing requests to the
Target.

Requests that cannot be processed by the Target result in 4xx (Client
Error) responses. If the Target and Client keys do not match, it is
an authorization failure (HTTP status code 401; see Section 15.5.2 of
[HTTP]). Otherwise, if the Client's request is invalid, such as in
the case of decryption failure, wrong message type, or
deserialization failure, this is a bad request (HTTP status code 400;
see Section 15.5.1 of [HTTP]).

Even in the case of DNS responses indicating failure, such as
SERVFAIL or NXDOMAIN, a successful HTTP response with a 2xx status
code is used as long as the DNS response is valid. This is identical
to how DoH [RFC8484] handles HTTP response codes.

4.4. HTTP Response Example

The following example shows a 2xx (Successful) response that can be
sent from a Target to a Client via a Proxy.

:status = 200
content-type = application/oblivious-dns-message
content-length = 154

<Bytes containing an encrypted Oblivious DNS response>

4.5. HTTP Metadata

Proxies forward requests and responses between Clients and Targets as
specified in Section 4.1. Metadata sent with these messages could
inadvertently weaken or remove Oblivious DoH privacy properties.
Proxies MUST NOT send any Client-identifying information about
Clients to Targets, such as "Forwarded" HTTP headers [RFC7239].
Additionally, Clients MUST NOT include any private state in requests
to Proxies, such as HTTP cookies. See Section 11.3 for related
discussion about Client authentication information.

5. Configuration and Public Key Format

In order to send a message to a Target, the Client needs to know a
public key to use for encrypting its queries. The mechanism for
discovering this configuration is out of scope for this document.

Servers ought to rotate public keys regularly. It is RECOMMENDED
that servers rotate keys every day. Shorter rotation windows reduce
the anonymity set of Clients that might use the public key, whereas
longer rotation windows widen the time frame of possible compromise.

An Oblivious DNS public key configuration is a structure encoded,
using TLS-style encoding [RFC8446], as follows:

struct {
uint16 kem_id;
uint16 kdf_id;
uint16 aead_id;
opaque public_key<1..2^16-1>;
} ObliviousDoHConfigContents;

struct {
uint16 version;
uint16 length;
select (ObliviousDoHConfig.version) {
case 0x0001: ObliviousDoHConfigContents contents;
}
} ObliviousDoHConfig;

ObliviousDoHConfig ObliviousDoHConfigs<1..2^16-1>;

The ObliviousDoHConfigs structure contains one or more
ObliviousDoHConfig structures in decreasing order of preference.
This allows a server to support multiple versions of Oblivious DoH
and multiple sets of Oblivious DoH parameters.

An ObliviousDoHConfig structure contains a versioned representation
of an Oblivious DoH configuration, with the following fields.

version: The version of Oblivious DoH for which this configuration
is used. Clients MUST ignore any ObliviousDoHConfig structure
with a version they do not support. The version of Oblivious DoH
specified in this document is 0x0001.

length: The length, in bytes, of the next field.

contents: An opaque byte string whose contents depend on the
version. For this specification, the contents are an
ObliviousDoHConfigContents structure.

An ObliviousDoHConfigContents structure contains the information
needed to encrypt a message under
ObliviousDoHConfigContents.public_key such that only the owner of the
corresponding private key can decrypt the message. The values for
ObliviousDoHConfigContents.kem_id, ObliviousDoHConfigContents.kdf_id,
and ObliviousDoHConfigContents.aead_id are described in Section 7 of
[HPKE]. The fields in this structure are as follows:

kem_id: The hybrid public key encryption (HPKE) key encapsulation
mechanism (KEM) identifier corresponding to public_key. Clients
MUST ignore any ObliviousDoHConfig structure with a key using a
KEM they do not support.

kdf_id: The HPKE key derivation function (KDF) identifier
corresponding to public_key. Clients MUST ignore any
ObliviousDoHConfig structure with a key using a KDF they do not
support.

aead_id: The HPKE authenticated encryption with associated data
(AEAD) identifier corresponding to public_key. Clients MUST
ignore any ObliviousDoHConfig structure with a key using an AEAD
they do not support.

public_key: The HPKE public key used by the Client to encrypt
Oblivious DoH queries.

6. Protocol Encoding

This section includes encoding and wire format details for Oblivious
DoH, as well as routines for encrypting and decrypting encoded
values.

6.1. Message Format

There are two types of Oblivious DoH messages: Queries (0x01) and
Responses (0x02). Both messages carry the following information:

1. A DNS message, which is either a Query or Response, depending on
context.

2. Padding of arbitrary length, which MUST contain all zeros.

They are encoded using the following structure:

struct {
opaque dns_message<1..2^16-1>;
opaque padding<0..2^16-1>;
} ObliviousDoHMessagePlaintext;

Both Query and Response messages use the ObliviousDoHMessagePlaintext
format.

ObliviousDoHMessagePlaintext ObliviousDoHQuery;
ObliviousDoHMessagePlaintext ObliviousDoHResponse;

An encrypted ObliviousDoHMessagePlaintext parameter is carried in an
ObliviousDoHMessage message, encoded as follows:

struct {
uint8 message_type;
opaque key_id<0..2^16-1>;
opaque encrypted_message<1..2^16-1>;
} ObliviousDoHMessage;

The ObliviousDoHMessage structure contains the following fields:

message_type: A one-byte identifier for the type of message. Query
messages use message_type 0x01, and Response messages use
message_type 0x02.

key_id: The identifier of the corresponding
ObliviousDoHConfigContents key. This is computed as
Expand(Extract("", config), "odoh key id", Nh), where config is
the ObliviousDoHConfigContents structure and Extract, Expand, and
Nh are as specified by the HPKE cipher suite KDF corresponding to
config.kdf_id.

encrypted_message: An encrypted message for the Oblivious Target
(for Query messages) or Client (for Response messages).
Implementations MAY enforce limits on the size of this field,
depending on the size of plaintext DNS messages. (DNS queries,
for example, will not reach the size limit of 2^16-1 in practice.)

The contents of ObliviousDoHMessage.encrypted_message depend on
ObliviousDoHMessage.message_type. In particular,
ObliviousDoHMessage.encrypted_message is an encryption of an
ObliviousDoHQuery message if the message is a Query and an encryption
of ObliviousDoHResponse if the message is a Response.

6.2. Encryption and Decryption Routines

Clients use the following utility functions for encrypting a Query
and decrypting a Response as described in Section 7.

* encrypt_query_body: Encrypt an Oblivious DoH query.

def encrypt_query_body(pkR, key_id, Q_plain):
enc, context = SetupBaseS(pkR, "odoh query")
aad = 0x01 || len(key_id) || key_id
ct = context.Seal(aad, Q_plain)
Q_encrypted = enc || ct
return Q_encrypted

* decrypt_response_body: Decrypt an Oblivious DoH response.

def decrypt_response_body(context, Q_plain, R_encrypted, resp_nonce):
aead_key, aead_nonce = derive_secrets(context, Q_plain, resp_nonce)
aad = 0x02 || len(resp_nonce) || resp_nonce
R_plain, error = Open(key, nonce, aad, R_encrypted)
return R_plain, error

The derive_secrets function is described below.

Targets use the following utility functions in processing queries and
producing responses as described in Section 8.

* setup_query_context: Set up an HPKE context used for decrypting an
Oblivious DoH query.

def setup_query_context(skR, key_id, Q_encrypted):
enc || ct = Q_encrypted
context = SetupBaseR(enc, skR, "odoh query")
return context

* decrypt_query_body: Decrypt an Oblivious DoH query.

def decrypt_query_body(context, key_id, Q_encrypted):
aad = 0x01 || len(key_id) || key_id
enc || ct = Q_encrypted
Q_plain, error = context.Open(aad, ct)
return Q_plain, error

* derive_secrets: Derive keying material used for encrypting an
Oblivious DoH response.

def derive_secrets(context, Q_plain, resp_nonce):
secret = context.Export("odoh response", Nk)
salt = Q_plain || len(resp_nonce) || resp_nonce
prk = Extract(salt, secret)
key = Expand(odoh_prk, "odoh key", Nk)
nonce = Expand(odoh_prk, "odoh nonce", Nn)
return key, nonce

The random(N) function returns N cryptographically secure random
bytes from a good source of entropy [RFC4086]. The max(A, B)
function returns A if A > B, and B otherwise.

* encrypt_response_body: Encrypt an Oblivious DoH response.

def encrypt_response_body(R_plain, aead_key, aead_nonce, resp_nonce):
aad = 0x02 || len(resp_nonce) || resp_nonce
R_encrypted = Seal(aead_key, aead_nonce, aad, R_plain)
return R_encrypted

7. Oblivious Client Behavior

Let M be a DNS message (query) a Client wishes to protect with
Oblivious DoH. When sending an Oblivious DoH Query for resolving M
to an Oblivious Target with ObliviousDoHConfigContents config, a
Client does the following:

1. Creates an ObliviousDoHQuery structure, carrying the message M
and padding, to produce Q_plain.

2. Deserializes config.public_key to produce a public key pkR of
type config.kem_id.

3. Computes the encrypted message as Q_encrypted =
encrypt_query_body(pkR, key_id, Q_plain), where key_id is as
computed in Section 6. Note also that len(key_id) outputs the
length of key_id as a two-byte unsigned integer.

4. Outputs an ObliviousDoHMessage message Q, where Q.message_type =
0x01, Q.key_id carries key_id, and Q.encrypted_message =
Q_encrypted.

The Client then sends Q to the Proxy according to Section 4.1. Once
the Client receives a response R, encrypted as specified in
Section 8, it uses decrypt_response_body to decrypt
R.encrypted_message (using R.key_id as a nonce) and produce R_plain.
Clients MUST validate R_plain.padding (as all zeros) before using
R_plain.dns_message.

8. Oblivious Target Behavior

Targets that receive a Query message Q decrypt and process it as
follows:

1. Look up the ObliviousDoHConfigContents information according to
Q.key_id. If no such key exists, the Target MAY discard the
query, and if so, it MUST return a 401 (Unauthorized) response to
the Proxy. Otherwise, let skR be the private key corresponding
to this public key, or one chosen for trial decryption.

2. Compute context = setup_query_context(skR, Q.key_id,
Q.encrypted_message).

3. Compute Q_plain, error = decrypt_query_body(context, Q.key_id,
Q.encrypted_message).

4. If no error was returned and Q_plain.padding is valid (all
zeros), resolve Q_plain.dns_message as needed, yielding a DNS
message M. Otherwise, if an error was returned or the padding
was invalid, return a 400 (Client Error) response to the Proxy.

5. Create an ObliviousDoHResponseBody structure, carrying the
message M and padding, to produce R_plain.

6. Create a fresh nonce resp_nonce = random(max(Nn, Nk)).

7. Compute aead_key, aead_nonce = derive_secrets(context, Q_plain,
resp_nonce).

8. Compute R_encrypted = encrypt_response_body(R_plain, aead_key,
aead_nonce, resp_nonce). The key_id field used for encryption
carries resp_nonce in order for Clients to derive the same
secrets. Also, the Seal function is the function that is
associated with the HPKE AEAD.

9. Output an ObliviousDoHMessage message R, where R.message_type =
0x02, R.key_id = resp_nonce, and R.encrypted_message =
R_encrypted.

The Target then sends R in a 2xx (Successful) response to the Proxy;
see Section 4.3. The Proxy forwards the message R without
modification back to the Client as the HTTP response to the Client's
original HTTP request. In the event of an error (non-2xx status
code), the Proxy forwards the Target error to the Client; see
Section 4.3.

9. Compliance Requirements

Oblivious DoH uses HPKE for public key encryption [HPKE]. In the
absence of an application profile standard specifying otherwise, a
compliant Oblivious DoH implementation MUST support the following
HPKE cipher suite:

KEM: DHKEM(X25519, HKDF-SHA256) (see [HPKE], Section 7.1)

KDF: HKDF-SHA256 (see [HPKE], Section 7.2)

AEAD: AES-128-GCM (see [HPKE], Section 7.3)

10. Experiment Overview

This document describes an experimental protocol built on DoH. The
purpose of this experiment is to assess deployment configuration
viability and related performance impacts on DNS resolution by
measuring key performance indicators such as resolution latency.
Experiment participants will test various parameters affecting
service operation and performance, including mechanisms for discovery
and configuration of DoH Proxies and Targets, as well as performance
implications of connection reuse and pools where appropriate. The
results of this experiment will be used to influence future protocol
design and deployment efforts related to Oblivious DoH, such as
Oblivious HTTP [OHTP]. Implementations of DoH that are not involved
in the experiment will not recognize this protocol and will not
participate in the experiment. It is anticipated that the use of
Oblivious DoH will be widespread and that this experiment will be of
long duration.

11. Security Considerations

Oblivious DoH aims to keep knowledge of the true query origin and its
contents known only to Clients. As a simplified model, consider a
case where there exist two Clients C1 and C2, one Proxy P, and one
Target T. Oblivious DoH assumes an extended Dolev-Yao style attacker
[Dolev-Yao] that can observe all network activity and can adaptively
compromise either P or T, but not C1 or C2. Note that compromising
both P and T is equivalent to collusion between these two parties in
practice. Once compromised, the attacker has access to all session
information and private key material. (This generalizes to
arbitrarily many Clients, Proxies, and Targets, with the constraints
that (1) not all Targets and Proxies are simultaneously compromised
and (2) at least two Clients are left uncompromised.) The attacker
is prohibited from sending Client-identifying information, such as IP
addresses, to Targets. (This would allow the attacker to trivially
link a query to the corresponding Client.)

In this model, both C1 and C2 send Oblivious DoH queries Q1 and Q2,
respectively, through P to T, and T provides answers A1 and A2. The
attacker aims to link C1 to (Q1, A1) and C2 to (Q2, A2),
respectively. The attacker succeeds if this linkability is possible
without any additional interaction. (For example, if T is
compromised, it could return a DNS answer corresponding to an entity
it controls and then observe the subsequent connection from a Client,
learning its identity in the process. Such attacks are out of scope
for this model.)

Oblivious DoH security prevents such linkability. Informally, this
means:

1. Queries and answers are known only to Clients and Targets in
possession of the corresponding response key and HPKE keying
material. In particular, Proxies know the origin and destination
of an oblivious query, yet do not know the plaintext query.
Likewise, Targets know only the oblivious query origin, i.e., the
Proxy, and the plaintext query. Only the Client knows both the
plaintext query contents and destination.

2. Target resolvers cannot link queries from the same Client in the
absence of unique per-Client keys.

Traffic analysis mitigations are outside the scope of this document.
In particular, this document does not prescribe padding lengths for
ObliviousDoHQuery and ObliviousDoHResponse messages. Implementations
SHOULD follow the guidance in [RFC8467] for choosing padding length.

Oblivious DoH security does not depend on Proxy and Target
indistinguishability. Specifically, an on-path attacker could
determine whether a connection to a specific endpoint is used for
oblivious or direct DoH queries. However, this has no effect on the
confidentiality goals listed above.

11.1. Denial of Service

Malicious Clients (or Proxies) can send bogus Oblivious DoH queries
to Targets as a Denial-of-Service (DoS) attack. Target servers can
throttle processing requests if such an event occurs. Additionally,
since Targets provide explicit errors upon decryption failure, i.e.,
if ciphertext decryption fails or if the plaintext DNS message is
malformed, Proxies can throttle specific Clients in response to these
errors. In general, however, Targets trust Proxies to not overwhelm
the Target, and it is expected that Proxies implement either some
form of rate limiting or client authentication to limit abuse; see
Section 11.3.

Malicious Targets or Proxies can send bogus answers in response to
Oblivious DoH queries. Response decryption failure is a signal that
either the Proxy or Target is misbehaving. Clients can choose to
stop using one or both of these servers in the event of such failure.
However, as noted above, malicious Targets and Proxies are out of
scope for the threat model.

11.2. Proxy Policies

Proxies are free to enforce any forwarding policy they desire for
Clients. For example, they can choose to only forward requests to
known or otherwise trusted Targets.

Proxies that do not reuse connections to Targets for many Clients may
allow Targets to link individual queries to unknown Targets. To
mitigate this linkability vector, it is RECOMMENDED that Proxies pool
and reuse connections to Targets. Note that this benefits
performance as well as privacy, since queries do not incur any delay
that might otherwise result from Proxy-to-Target connection
establishment.

11.3. Authentication

Depending on the deployment scenario, Proxies and Targets might
require authentication before use. Regardless of the authentication
mechanism in place, Proxies MUST NOT reveal any Client authentication
information to Targets. This is required so Targets cannot uniquely
identify individual Clients.

Note that if Targets require Proxies to authenticate at the HTTP or
application layer before use, this ought to be done before attempting
to forward any Client query to the Target. This will allow Proxies
to distinguish 401 (Unauthorized) response codes due to
authentication failure from 401 response codes due to Client key
mismatch; see Section 4.3.

12. IANA Considerations

This document makes changes to the "Media Types" registry. The
changes are described in the following subsection.

12.1. Oblivious DoH Message Media Type

This document registers a new media type, "application/oblivious-dns-
message".

Type name: application

Subtype name: oblivious-dns-message

Required parameters: N/A

Optional parameters: N/A

Encoding considerations: This is a binary format, containing
encrypted DNS requests and responses encoded as
ObliviousDoHMessage values, as defined in Section 6.1.

Security considerations: See this document. The content is an
encrypted DNS message, and not executable code.

Interoperability considerations: This document specifies the format
of conforming messages and the interpretation thereof; see
Section 6.1.

Published specification: This document

Applications that use this media type: This media type is intended
to be used by Clients wishing to hide their DNS queries when using
DNS over HTTPS.

Additional information: N/A

Person and email address to contact for further information: See the
Authors' Addresses section.

Intended usage: COMMON

Restrictions on usage: N/A

Author: Tommy Pauly ([email protected])

Change controller: IETF

Provisional registration? (standards tree only): No

13. References

13.1. Normative References

[HPKE] Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
February 2022, <https://www.rfc-editor.org/info/rfc9180>.

[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/info/rfc9110>.

[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>.

[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.

[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>.

[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>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

[RFC8467] Mayrhofer, A., "Padding Policies for Extension Mechanisms
for DNS (EDNS(0))", RFC 8467, DOI 10.17487/RFC8467,
October 2018, <https://www.rfc-editor.org/info/rfc8467>.

[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>.

[RFC9209] Nottingham, M. and P. Sikora, "The Proxy-Status HTTP
Response Header Field", RFC 9209, DOI 10.17487/RFC9209,
June 2022, <https://www.rfc-editor.org/info/rfc9209>.

13.2. Informative References

[Dolev-Yao]
Dolev, D. and A. C. Yao, "On the Security of Public Key
Protocols", IEEE Transactions on Information Theory, Vol.
IT-29, No. 2, DOI 10.1109/TIT.1983.1056650, March 1983,
<https://www.cs.huji.ac.il/~dolev/pubs/dolev-yao-ieee-
01056650.pdf>.

[OBLIVIOUS-DNS]
Edmundson, A., Schmitt, P., Feamster, N., and A. Mankin,
"Oblivious DNS - Strong Privacy for DNS Queries", Work in
Progress, Internet-Draft, draft-annee-dprive-oblivious-
dns-00, 2 July 2018,
<https://datatracker.ietf.org/doc/html/draft-annee-dprive-
oblivious-dns-00>.

[OHTP] Thomson, M. and C.A. Wood, "Oblivious HTTP", Work in
Progress, Internet-Draft, draft-ietf-ohai-ohttp-01, 15
February 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-ohai-ohttp-01>.

[RFC7239] Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
RFC 7239, DOI 10.17487/RFC7239, June 2014,
<https://www.rfc-editor.org/info/rfc7239>.

[RFC7871] Contavalli, C., van der Gaast, W., Lawrence, D., and W.
Kumari, "Client Subnet in DNS Queries", RFC 7871,
DOI 10.17487/RFC7871, May 2016,
<https://www.rfc-editor.org/info/rfc7871>.

Appendix A. Use of Generic Proxy Services

Using DoH over anonymizing proxy services such as Tor can also
achieve the desired goal of separating query origins from their
contents. However, there are several reasons why such systems are
undesirable as contrasted with Oblivious DoH:

1. Tor is meant to be a generic connection-level anonymity system,
and it incurs higher latency costs and protocol complexity for
the purpose of proxying individual DNS queries. In contrast,
Oblivious DoH is a lightweight protocol built on DoH, implemented
as an application-layer proxy, that can be enabled as a default
mode for users that need increased privacy.

2. As a one-hop proxy, Oblivious DoH encourages connectionless
proxies to mitigate Client query correlation with few round
trips. In contrast, multi-hop systems such as Tor often run
secure connections (TLS) end to end, which means that DoH servers
could track queries over the same connection. Using a fresh DoH
connection per query would incur a non-negligible penalty in
connection setup time.

Acknowledgments

This work is inspired by Oblivious DNS [OBLIVIOUS-DNS]. Thanks to
all of the authors of that document. Thanks to Nafeez Ahamed, Elliot
Briggs, Marwan Fayed, Jonathan Hoyland, Frederic Jacobs, Tommy
Jensen, Erik Nygren, Paul Schmitt, Brian Swander, and Peter Wu for
their feedback and input.

Authors' Addresses

Eric Kinnear
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
Email: [email protected]

Patrick McManus
Fastly
Email: [email protected]

Tommy Pauly
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
Email: [email protected]

Tanya Verma
Cloudflare
101 Townsend St
San Francisco, California 94107
United States of America
Email: [email protected]

Christopher A. Wood
Cloudflare
101 Townsend St
San Francisco, California 94107
United States of America
Email: [email protected]