Oblivious DNS Over HTTPS

Introduction DNS Over HTTPS (DoH) 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 () 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 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 mechanism for resolving privacy-sensitive content in the broader context of DNS privacy. This experimental extension is developed outside the IETF and is published here to guide implementation, ensure interoperability among implementations, and enable wide-scale experimentation.

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 when, and only when, they appear in all capitals, as shown here.

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.

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 (). 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 out of scope of this document.

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

The Client, which generates queries.
The Proxy, which receives encrypted queries from the client and passes them on to a Target.
The Target, which receives proxied queries from the client via the Proxy and produces proxied answers.

Obvlivious DoH Exchange +---------+ +-----------+ +-----------+ | Client +-------------> Oblivious +-------------> Oblivious | | <-------------+ Proxy <-------------+ Target | +---------+ +-----------+ +-----------+ <-- [ Response encrypted with symmetric key ] --- ]]>

HTTP Request Oblivious DoH queries are created by the Client, and sent to the Proxy as an HTTP request using the POST method. 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 for an example request. Oblivious DoH messages have no cache value since both requests and responses are encrypted using ephemeral key material. Clients SHOULD indicate this using the "Cache-Control" header with "no-cache" and "no-store" specified . 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. 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" . A correctly encoded request has the HTTP Content-Type header "application/oblivious-dns-message", and HTTP method POST. If the proxy does not operate as a target, then the request must additionally contain "targethost" and "targetpath" variables. 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 (). 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". Note that "targethost" MAY contain a port. Proxies MAY choose to not forward connections to non-standard ports. In such cases, proxies MUST return a 4xx (Client Error) response to the client request, along with Proxy-Status response header with an "error" parameter of type "http_request_error". If the proxy cannot establish a connection to "targethost", it MUST return a 502 (Bad Gateway) response to the client request, along with 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". Proxies SHOULD choose an error type that best captures the connection failure.

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". ]]> The Proxy then sends the following request on to the Target: ]]>

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 . 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 MUST forward any Target responses with 2xx, 4xx, or 5xx response codes unmodified to the client. Target responses with 1xx response codes MUST NOT be forwarded to the client. If a proxy receives a successful response from a target without the "application/oblivious-dns-message" HTTP Content-Type header, it MUST return a 502 (Bad Gateway) response to the client request, along with Proxy-Status response header with an "error" parameter of type "http_protocol_error". 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 ). 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 ). 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 similar to how DoH handles HTTP response codes. In case of server error, the usual HTTP status code 500 (see Section 6.6.1 of ) applies.

HTTP Response Example The following example shows a 2xx (Successful) response that can be sent from a Target to a client via a Proxy. ]]>

HTTP Metadata Proxies forward requests and responses between clients and targets as specified in . Metadata sent with these messages may 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 . Additionally, clients MUST NOT include any private state in requests to proxies, such as HTTP cookies. See for related discussion about client authentication information.

Configuration and Public Key Format In order to use a DoH server as a Target, the client must know a public key to use for encrypting its queries. The mechanism for discovering this configuration is out of scope of this document. 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. An Oblivious DNS public key configuration is a structure encoded, using TLS-style encoding , as follows: ; } 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. 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

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

A DNS message, which is either a Query or Response, depending on context.
Padding of arbitrary length which MUST contain all zeros.

They are encoded using the following structure: ; opaque padding<0..2^16-1>; } ObliviousDoHMessagePlaintext; ]]> Both Query and Response messages use the ObliviousDoHMessagePlaintext format. An encrypted ObliviousDoHMessagePlaintext is carried in a ObliviousDoHMessage message, encoded as follows: ; 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 . encrypt_query_body: Encrypt an Oblivious DoH query. decrypt_response_body: Decrypt an Oblivious DoH response. The derive_secrets function is described below. Targets use the following utility functions in processing queries and producing responses as described in . setup_query_context: Set up an HPKE context used for decrypting an Oblivious DoH query. decrypt_query_body: Decrypt an Oblivious DoH query. derive_secrets: Derive keying material used for encrypting an Oblivious DoH response. The random(N) function returns N cryptographically secure random bytes from a good source of entropy . The max(A, B) function returns A if A > B, and B otherwise. encrypt_response_body: Encrypt an Oblivious DoH response.

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:

Create an ObliviousDoHQuery structure, carrying the message M and padding, to produce Q_plain.
Deserialize config.public_key to produce a public key pkR of type config.kem_id.
Compute the encrypted message as Q_encrypted = encrypt_query_body(pkR, key_id, Q_plain), where key_id is as computed in . Note also that len(key_id) outputs the length of key_id as a two-byte unsigned integer.
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 . Once the client receives a response R, encrypted as specified in , 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 Targets that receive a Query message Q decrypt and process it as follows:

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.
Compute context = setup_query_context(skR, Q.key_id, Q.encrypted_message).
Compute Q_plain, error = decrypt_query_body(context, Q.key_id, Q.encrypted_message).
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.
Create an ObliviousDoHResponseBody structure, carrying the message M and padding, to produce R_plain.
Create a fresh nonce response_nonce = random(max(Nn, Nk)).
Compute aead_key, aead_nonce = derive_secrets(context, Q_plain, response_nonce).
Compute R_encrypted = encrypt_response_body(R_plain, aead_key, aead_nonce, response_nonce). The key_id field used for encryption carries response_nonce in order for clients to derive the same secrets. Also, the Seal function is that which is associated with the HPKE AEAD.
Output a ObliviousDoHMessage message R where R.message_type = 0x02, R.key_id = response_nonce, and R.encrypted_message = R_encrypted.

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

Compliance Requirements Oblivious DoH uses HPKE for public key encryption . 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 , Section 7.1)
KDF: HKDF-SHA256 (see , Section 7.2)
AEAD: AES-128-GCM (see , Section 7.3)

Security Considerations 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:

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.
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 . 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) 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.

Proxy Policies Proxies are free to enforce any forwarding policy they desire for clients. For example, they may only forward requests to known or otherwise trusted targets.

Authentication Depending on the deployment scenario, Proxies and Targets MAY 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 SHOULD 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 .

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:

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.
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.

IANA Considerations

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, 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: 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 tpauly@apple.com Change controller: Tommy Pauly tpauly@apple.com Provisional registration? (standards tree only): No

Acknowledgments This work is inspired by 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.