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title: "Oblivious DNS Over HTTPS" abbrev: Oblivious DoH docname: draft-pauly-dprive-oblivious-doh-latest date: category: exp

ipr: trust200902 keyword: Internet-Draft

stand_alone: yes pi: [toc, sortrefs, symrefs]

author:

ins: E. Kinnear
name: Eric Kinnear
org: Apple Inc.
street: One Apple Park Way
city: Cupertino, California 95014
country: United States of America
email: [email protected]
  • ins: P. McManus name: Patrick McManus org: Fastly email: [email protected]
  • ins: T. Pauly name: Tommy Pauly org: Apple Inc. street: One Apple Park Way city: Cupertino, California 95014 country: United States of America email: [email protected]
  • ins: T. Verma name: Tanya Verma org: Cloudflare street: 101 Townsend St city: San Francisco country: United States of America email: [email protected]
  • ins: C. A. Wood name: Christopher A. Wood org: Cloudflare street: 101 Townsend St city: San Francisco country: United States of America email: [email protected]

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

--- middle

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 ({{?I-D.annee-dprive-oblivious-dns}}) increase privacy by ensuring 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 is developed outside the IETF and is published here to guide implementation, ensure interoperability among implementations, and enable wide-scale experimentation. See {{experiment}} for more details about the experiment.

Specification of Requirements

{::boilerplate bcp14}

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 {{oblivious-request}}). 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.

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 {{security-considerations}}).
  • One or more Target public keys for encrypting DNS queries send to a Target via a Proxy ({{publickey}}). 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 of this document.

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   ] ---

{: #fig-doh-exchange title="Obvlivious DoH Exchange"}

HTTP Request {#oblivious-request}

Oblivious DoH queries are created by the Client, and 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 host name 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 {{request-example}} 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" {{!I-D.ietf-httpbis-proxy-status}}.

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", along 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 failed 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.

HTTP Request Example {#request-example}

The following example shows how a Client requests that a Proxy, "dnsproxy.example", forwards 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>

HTTP Response {#oblivious-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 {{encryption}}.

The response from a Target MUST set the Content-Type HTTP header to "application/oblivious-dns-message" which MUST be forwarded by the Proxy to the Client. A Client MUST only consider a response which contains the Content-Type header in the response 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 client, without any modifications to the body or status code. The Proxy also SHOULD add a Proxy-Status response header with an "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 3.1 of {{!RFC7235}}). 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 6.5.1 of {{!RFC7231}}).

Even in 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.

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>

HTTP Metadata

Proxies forward requests and responses between Clients and Targets as specified in {{oblivious-request}}. 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 {{authentication}} for related discussion about Client authentication information.

Configuration and Public Key Format {#publickey}

In order 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 of 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 timeframe 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 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 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=I-D.irtf-cfrg-hpke}}. The fields in this structure are as follows:

kem_id : The HPKE 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 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 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.

Protocol Encoding {#encryption}

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

Message Format {#encoding}

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 is carried in a 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 a ObliviousDoHQuery if the message is a Query, and ObliviousDoHResponse if the message is a Response.

Encryption and Decryption Routines

Clients use the following utility functions for encrypting a Query and decrypting a Response as described in {{odoh-client}}.

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 {{odoh-target}}.

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

Oblivious Client Behavior {#odoh-client}

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. Create an ObliviousDoHQuery structure, carrying the message M and padding, to produce Q_plain.
  2. Deserialize config.public_key to produce a public key pkR of type config.kem_id.
  3. Compute the encrypted message as Q_encrypted = encrypt_query_body(pkR, key_id, Q_plain), where key_id is as computed in {{encryption}}. Note also that len(key_id) outputs the length of key_id as a two-byte unsigned integer.
  4. Output 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 {{oblivious-request}}. Once the Client receives a response R, encrypted as specified in {{odoh-target}}, 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.

Oblivious Target Behavior {#odoh-target}

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

  1. Look up the ObliviousDoHConfigContents 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 that which 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 {{oblivious-response}}. 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 {{oblivious-response}}.

Compliance Requirements {#compliance}

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)

Experiment Overview {#experiment}

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=I-D.draft-ietf-ohai-ohttp}}. 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 use of Oblivious DoH and the duration of this experiment to be widespread.

Security Considerations {#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 exists two Clients C1 and C2, one proxy P, and one Target T. Oblivious DoH assumes an extended Dolev-Yao style attacker which 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 not all Targets and Proxies are simultaneously compromised, and 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 an 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 for choosing padding length in {{!RFC8467}}.

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

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 either implement some form of rate limiting or client authentication to limit abuse; see {{authentication}}.

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 above, malicious Targets and Proxies are out of scope for the threat model.

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.

Authentication {#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 Unauthorized response codes due to Client key mismatch; see {{oblivious-response}}.

IANA Considerations {#iana}

This document makes changes to the "Multipurpose Internet Mail Extensions (MIME) and Media Types" registry. The changes are described in the following subsection.

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 {{encoding}}.

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

Interoperability considerations: This document specifies format of conforming messages and the interpretation thereof; see {{encoding}}.

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 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

Acknowledgments

This work is inspired by Oblivious DNS {{?I-D.annee-dprive-oblivious-dns}}. Thanks to all of the authors of that document. Thanks to Nafeez Ahamed, Elliot Briggs, Marwan Fayed, Frederic Jacobs, Tommy Jensen, Jonathan Hoyland, Paul Schmitt, Brian Swander, Erik Nygren, and Peter Wu for the feedback and input.

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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 in comparison to Oblivious DoH:

  1. Tor is meant to be a generic connection-level anonymity system, and 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 which need increased privacy.

  2. As a one-hop proxy, Oblivious DoH encourages connection-less 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.