Oblivious DNS Over HTTPS
draft-pauly-dprive-oblivious-doh-02

Document Type Active Internet-Draft (individual)
Authors Eric Kinnear  , Patrick McManus  , Tommy Pauly  , Christopher Wood 
Last updated 2020-10-09
Stream (None)
Intended RFC status (None)
Formats plain text html xml pdf htmlized (tools) htmlized bibtex
Stream Stream state (No stream defined)
Consensus Boilerplate Unknown
RFC Editor Note (None)
IESG IESG state I-D Exists
Telechat date
Responsible AD (None)
Send notices to (None)
Network Working Group                                         E. Kinnear
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                              P. McManus
Expires: 12 April 2021                                            Fastly
                                                                T. Pauly
                                                              Apple Inc.
                                                               C.A. Wood
                                                              Cloudflare
                                                          9 October 2020

                        Oblivious DNS Over HTTPS
                  draft-pauly-dprive-oblivious-doh-02

Abstract

   This document describes an extension to DNS Over HTTPS (DoH) that
   allows hiding client IP addresses via proxying encrypted DNS
   transactions.  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.

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/.

   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 12 April 2021.

Copyright Notice

   Copyright (c) 2020 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

Kinnear, et al.           Expires 12 April 2021                 [Page 1]
Internet-Draft                Oblivious DoH                 October 2020

   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Specification of Requirements . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Deployment Requirements . . . . . . . . . . . . . . . . . . .   3
   4.  HTTP Exchange . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  HTTP Request  . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  HTTP Request Example  . . . . . . . . . . . . . . . . . .   5
     4.3.  HTTP Response . . . . . . . . . . . . . . . . . . . . . .   6
     4.4.  HTTP Response Example . . . . . . . . . . . . . . . . . .   6
   5.  Configuration and Public Key Discovery  . . . . . . . . . . .   6
   6.  Configuration and Public Key Format . . . . . . . . . . . . .   7
   7.  Protocol Encoding . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Message Format  . . . . . . . . . . . . . . . . . . . . .   8
     7.2.  Encryption and Decryption Routines  . . . . . . . . . . .   9
   8.  Oblivious Client Behavior . . . . . . . . . . . . . . . . . .  11
   9.  Oblivious Target Behavior . . . . . . . . . . . . . . . . . .  11
   10. Compliance Requirements . . . . . . . . . . . . . . . . . . .  12
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  12
     11.1.  Denial of Service  . . . . . . . . . . . . . . . . . . .  14
     11.2.  General Proxy Services . . . . . . . . . . . . . . . . .  14
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
     12.1.  Oblivious DoH Message Media Type . . . . . . . . . . . .  14
     12.2.  Oblivious DoH Public Key DNS Parameter . . . . . . . . .  15
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     14.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   DNS Over HTTPS (DoH) [RFC8484] defines a mechanism to allow DNS
   messages to be transmitted in encrypted HTTP messages.  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 and
   receive answers from servers without revealing their local IP
   address, and thus information about the identity or location of the
   client.

Kinnear, et al.           Expires 12 April 2021                 [Page 2]
Internet-Draft                Oblivious DoH                 October 2020

   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 extension to DoH that permits
   proxied resolution, in which DNS messages are encrypted so that no
   DoH server can independently read both the client IP address and the
   DNS message contents.

   This mechanism is intended to be used as one option for resolving
   privacy-sensitive content in the broader context of Adaptive DNS
   [I-D.pauly-dprive-adaptive-dns-privacy].

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 Server:  A DoH server that acts as either an Oblivious
      Proxy or Oblivious Target.

   Oblivious Proxy:  An Oblivious Server that proxies encrypted DNS
      queries and responses between a client and an Oblivious Target.

   Oblivious Target:  An Oblivious Server that receives and decrypts
      encrypted 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 Proxy and
   Target to refer to an Oblivious Proxy and Oblivious Target,
   respectively.

3.  Deployment Requirements

   Oblivious DoH requires, at a minimum:

   *  Two Oblivious Servers, where one can act as a Proxy, and the other
      can act as a Target.

Kinnear, et al.           Expires 12 April 2021                 [Page 3]
Internet-Draft                Oblivious DoH                 October 2020

   *  Public keys for encrypting DNS queries that are passed from a
      client through a Proxy to a Target (Section 6).  These keys
      guarantee that only the intended Target can decrypt client
      queries.

   The mechanism for discovering and provisioning the DoH URI Templates
   and public keys is via parameters added to DNS resource records.  The
   mechanism for discovering the public key is described in Section 5.
   The mechanism for discovering a DoH URI Template is described in
   [I-D.pauly-add-resolver-discovery].

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 client symmetric key ] ---

                     Figure 1: Obvlivious DoH Exchange

4.1.  HTTP Request

   Oblivious DoH queries are created by the Client, and sent to the
   Proxy.  Requests to the Proxy indicate which DoH server to use as a
   Target by specifying two variables: "targethost", which indicates the
   host name of the Target server, and "targetpath", which indicates the
   path on which the Target's DoH server is running.  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.  Clients SHOULD
   prefer using HTTP methods and headers that will prevent unhelpful
   cache storage of these exchanges (i.e., preferring POST instead of
   GET).

Kinnear, et al.           Expires 12 April 2021                 [Page 4]
Internet-Draft                Oblivious DoH                 October 2020

   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.

   Upon receiving a request that contains a "application/oblivious-dns-
   message" Content-Type, the DoH server looks for the "targethost" and
   "targetpath" variables.  If the variables are not present, then it is
   the target of the query, and it can decrypt the query (Section 7).
   If the variables are present, then the DoH server is acting as a
   Proxy.  If it is a proxy, it is expected to send the request on to
   the Target using the URI template constructed as "https://targethost/
   targetpath".

4.2.  HTTP Request Example

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

:method = POST
:scheme = https
:authority = dnsproxy.example.net
:path = /dns-query?targethost=dnstarget.example.net&targetpath=/dns-query
accept = application/oblivious-dns-message
cache-control = no-cache, no-store
content-type = application/oblivious-dns-message
content-length = 106

<Bytes containing the encrypted payload for an Oblivious DNS query>

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

   :method = POST
   :scheme = https
   :authority = dnstarget.example.net
   :path = /dns-query
   accept = application/oblivious-dns-message
   cache-control = no-cache, no-store
   content-type = application/oblivious-dns-message
   content-length = 106

   <Bytes containing the encrypted payload for an Oblivious DNS query>

Kinnear, et al.           Expires 12 April 2021                 [Page 5]
Internet-Draft                Oblivious DoH                 October 2020

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 a DNS message that is encrypted with the client's symmetric
   key (Section 7).

   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.

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 the encrypted payload for an Oblivious DNS response>

   Requests that cannot be processed result in 4xx (Client Error)
   responses.

5.  Configuration and Public Key Discovery

   In order to use a DoH server as a Target, the client must know a
   public key to use for encrypting its queries.  This key can be
   discovered using the SVCB or HTTPSSVC record type
   ([I-D.ietf-dnsop-svcb-https]) for a name owned by the server.

   The Service Binding key name is "odohconfig" (Section 12).  If
   present, this key/value pair contains the public key to use when
   encrypting Oblivious DoH messages that will be targeted at a DoH
   server.  The format of the key is defined in (Section 6).

   Clients MUST only use keys that were retrieved from records protected
   by DNSSEC [RFC4033] to encrypt messages to a Target.

Kinnear, et al.           Expires 12 April 2021                 [Page 6]
Internet-Draft                Oblivious DoH                 October 2020

6.  Configuration and Public Key Format

   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 0xff02: 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 "0xff02".

   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.

Kinnear, et al.           Expires 12 April 2021                 [Page 7]
Internet-Draft                Oblivious DoH                 October 2020

   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
   [I-D.irtf-cfrg-hpke] Section 7.  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.

7.  Protocol Encoding

7.1.  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;

Kinnear, et al.           Expires 12 April 2021                 [Page 8]
Internet-Draft                Oblivious DoH                 October 2020

   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).

   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.

7.2.  Encryption and Decryption Routines

   Clients use the following utility functions for encrypting a Query
   and decrypting a Response as described in Section 8.

   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.

Kinnear, et al.           Expires 12 April 2021                 [Page 9]
Internet-Draft                Oblivious DoH                 October 2020

   def decrypt_response_body(context, Q_plain, R_encrypted):
     key, nonce = derive_secrets(context, Q_plain)
     aad = 0x02 || 0x0000 // 0x0000 represents a 0-length KeyId
     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 9.

   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):
     odoh_secret = context.Export("odoh secret", 32)
     odoh_prk = Extract(Q_plain, odoh_secret)
     key = Expand(odoh_prk, "odoh key", Nk)
     nonce = Expand(odoh_prk, "odoh nonce", Nn)
     return key, nonce

   encrypt_response_body: Encrypt an Oblivious DoH response.

   def encrypt_response_body(R_plain, answer_key, answer_nonce):
     aad = 0x02 || 0x0000 // 0x0000 represents a 0-length KeyId
     R_encrypted = Seal(answer_key, answer_nonce, aad, R_plain)
     return R_encrypted

Kinnear, et al.           Expires 12 April 2021                [Page 10]
Internet-Draft                Oblivious DoH                 October 2020

8.  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.  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 Section 7.  Note also that "len(key_id)" outputs the
       length of "key_id" as a two-byte unsigned integer.

   4.  Output a 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 9, it uses "decrypt_response_body" to decrypt
   "R.encrypted_message" and produce R_plain.  Clients MUST validate
   "R_plain.padding" (as all zeros) before using R_plain.dns_message.

9.  Oblivious Target Behavior

   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 400 (Client Error) 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)".

Kinnear, et al.           Expires 12 April 2021                [Page 11]
Internet-Draft                Oblivious DoH                 October 2020

   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.  Compute "answer_key, answer_nonce = derive_secrets(context,
       Q_plain)".

   7.  Compute "R_encrypted = encrypt_response_body(R_plain, answer_key,
       answer_nonce)".  The "key_id" field used for encryption is empty,
       yielding "0x0000" as part of the AAD.  Also, the "Seal" function
       is that which is associated with the HPKE AEAD.

   8.  Output a "ObliviousDoHMessage" message "R" where "R.message_type
       = 0x02", "R.key_id = nil", and "R.encrypted_message =
       R_encrypted".

   The Target then sends "R" in a 2xx (Successful) response to the Proxy
   according to 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.

10.  Compliance Requirements

   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 [I-D.irtf-cfrg-hpke],
      Section 7.1)

   *  KDF: HKDF-SHA256 (see [I-D.irtf-cfrg-hpke], Section 7.2)

   *  AEAD: AES-128-GCM (see [I-D.irtf-cfrg-hpke], Section 7.3)

11.  Security Considerations

   DISCLAIMER: this is a work in progress draft and has not yet seen
   significant security analysis.

   Oblivious DoH aims to keep knowledge of the true query origin and its
   contents known to only 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

Kinnear, et al.           Expires 12 April 2021                [Page 12]
Internet-Draft                Oblivious DoH                 October 2020

   either P or T, but not C1 or C2.  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 may 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 recommend 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.

Kinnear, et al.           Expires 12 April 2021                [Page 13]
Internet-Draft                Oblivious DoH                 October 2020

11.1.  Denial of Service

   Malicious clients (or proxies) may send bogus Oblivious DoH queries
   to targets as a Denial-of-Service (DoS) attack.  Target servers may
   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 may throttle specific clients in response to these
   errors.

   Malicious Targets or Proxies may 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.

11.2.  General Proxy Services

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

   1.  Tor is also meant as a generic connection-level anonymity system,
       and thus seems overly complex and costly for the purpose of
       proxying individual DoH queries.  In contrast, Oblivious DoH is a
       lightweight extension to standard 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.

12.  IANA Considerations

12.1.  Oblivious DoH Message Media Type

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

   Type name: application

Kinnear, et al.           Expires 12 April 2021                [Page 14]
Internet-Draft                Oblivious DoH                 October 2020

   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, as defined in this document.

   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.

   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: None

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

   Intended usage: COMMON

   Restrictions on usage: None

   Author: IETF

   Change controller: IETF

12.2.  Oblivious DoH Public Key DNS Parameter

   This document adds a parameter ("odohconfig") to the "Service Binding
   (SVCB) Parameter" registry [I-D.ietf-dnsop-svcb-https].  The
   allocation request is 32769, taken from the to the First Come First
   Served range.

   If present, the "odohconfig" parameter contains a ObliviousDoHConfigs
   structure.  In wire format, the value of the parameter is an
   ObliviousDoHConfigs vector, including the redundant length prefix.
   In presentation format, the value is encoded in [base64].

   Name:  odohconfig

Kinnear, et al.           Expires 12 April 2021                [Page 15]
Internet-Draft                Oblivious DoH                 October 2020

   SvcParamKey:  32769

   Meaning:  An ObliviousDoHConfigs structure.

   Reference:  This document.

13.  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 Elliot Briggs, Marwan Fayed, Frederic
   Jacobs, Tommy Jensen Paul Schmitt, and Brian Swander for the feedback
   and input.

14.  References

14.1.  Normative References

   [base64]   Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [I-D.ietf-dnsop-svcb-https]
              Schwartz, B., 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-01, 13 July 2020, <http://www.ietf.org/
              internet-drafts/draft-ietf-dnsop-svcb-https-01.txt>.

   [I-D.irtf-cfrg-hpke]
              Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
              Public Key Encryption", Work in Progress, Internet-Draft,
              draft-irtf-cfrg-hpke-05, 30 July 2020,
              <http://www.ietf.org/internet-drafts/draft-irtf-cfrg-hpke-
              05.txt>.

   [I-D.pauly-add-resolver-discovery]
              Pauly, T., Kinnear, E., Wood, C., McManus, P., and T.
              Jensen, "Adaptive DNS Resolver Discovery", Work in
              Progress, Internet-Draft, draft-pauly-add-resolver-
              discovery-01, 13 July 2020, <http://www.ietf.org/internet-
              drafts/draft-pauly-add-resolver-discovery-01.txt>.

Kinnear, et al.           Expires 12 April 2021                [Page 16]
Internet-Draft                Oblivious DoH                 October 2020

   [I-D.pauly-dprive-adaptive-dns-privacy]
              Kinnear, E., Pauly, T., Wood, C., and P. McManus,
              "Adaptive DNS: Improving Privacy of Name Resolution", Work
              in Progress, Internet-Draft, draft-pauly-dprive-adaptive-
              dns-privacy-01, 1 November 2019, <http://www.ietf.org/
              internet-drafts/draft-pauly-dprive-adaptive-dns-privacy-
              01.txt>.

   [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>.

   [RFC4033]  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/info/rfc4033>.

   [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>.

14.2.  Informative References

   [I-D.annee-dprive-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, <http://www.ietf.org/internet-drafts/
              draft-annee-dprive-oblivious-dns-00.txt>.

Authors' Addresses

   Eric Kinnear
   Apple Inc.
   One Apple Park Way

Kinnear, et al.           Expires 12 April 2021                [Page 17]
Internet-Draft                Oblivious DoH                 October 2020

   Cupertino, California 95014,
   United States of America

   Email: ekinnear@apple.com

   Patrick McManus
   Fastly

   Email: mcmanus@ducksong.com

   Tommy Pauly
   Apple Inc.
   One Apple Park Way
   Cupertino, California 95014,
   United States of America

   Email: tpauly@apple.com

   Christopher A. Wood
   Cloudflare
   101 Townsend St
   San Francisco,
   United States of America

   Email: caw@heapingbits.net

Kinnear, et al.           Expires 12 April 2021                [Page 18]