Oblivious DNS Over HTTPS

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Table of Contents

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.¶

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].¶

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.¶
• 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 symmetric key   ] ---

: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>

: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>

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

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

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 document specifies one discovery mechanism for public keys 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 that use this discovery mechansim MUST only use keys that were retrieved from records protected by DNSSEC [RFC4033] to encrypt messages to a Target.¶

Servers SHOULD 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.¶

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 0xff05: 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 0xff05.¶

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 [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

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

ObliviousDoHMessagePlaintext ObliviousDoHQuery;
ObliviousDoHMessagePlaintext ObliviousDoHResponse;

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

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

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

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

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

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

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

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).¶
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; 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.¶

10. Compliance Requirements

Oblivious DoH uses draft-08 of HPKE for public key encryption [I-D.irtf-cfrg-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 [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 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.¶

12. IANA Considerations

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 Nafeez Ahamed, Elliot Briggs, Marwan Fayed, Frederic Jacobs, Tommy Jensen Paul Schmitt, Brian Swander, Tanya Verma, and Peter Wu for the feedback and input.¶

Authors' Addresses