DNS Server Selection: DNS Server Information with Assertion Token

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Table of Contents

1. Introduction

[RFC7626] discusses DNS privacy considerations in both "on the wire" (Section 2.4 of [RFC7626]) and "in the server" (Section 2.5 of [RFC7626]) contexts. Examples of protocols that provide encrypted channels between DNS clients and servers are DNS-over-HTTPS (DoH) [RFC8484], DNS-over-TLS (DoT) [RFC7858], and DNS-over-QUIC (DoQ) [I-D.ietf-dprive-dnsoquic].¶

DNS clients can discover and authenticate encrypted DNS servers provided by a local network, for example using the techniques proposed in [I-D.ietf-add-dnr] and [I-D.ietf-add-ddr]. If the mechanism used to discover the encrypted DNS server is insecure, the DNS client needs evidence about the encrypted server to assess its trustworthiness and a way to appraise such evidence. The mechanism specified in this document can be used by the DNS client to cryptographically identify if it is connecting to an encrypted DNS server hosted by a specific organization (e.g., ISP or Enterprise). This strengthens the protection as clients can detect and reject connections to encrypted DNS servers hosted by attackers.¶

2. Terminology

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.¶

This document makes use of the terms defined in [RFC8499] and [I-D.ietf-dnsop-terminology-ter].¶

'Encrypted DNS' refers to a DNS protocol that provides an encrypted channel between a DNS client and server (e.g., DoT, DoH, or DoQ).¶

The terms 'Evidence', 'Verifier', 'Background Check', 'Relying Party', 'Appraisal Policy', and 'Attestation Results' are defined in [I-D.ietf-rats-architecture].¶

3. Resolver Assertion Token (REAT): Overview

The mechanism used in this specification resembles the Background-Check Model discussed in Sections 5.2 and 5.3 of Remote attestation procedure (RATS) Architecture [I-D.ietf-rats-architecture]. RATS enables a relying party to establish a level of confidence in the trustworthiness of a remote peer through the creation of Evidence to assess the peer's trustworthiness, and an Appraisal Policy for such Evidence.¶

In this document, the Relying Party is the DNS client and the Attester is the encrypted DNS server. The Encrypted DNS servers MAY use "Domain Validation" (DV) certificates for certificate-based server authentication in TLS connections.¶

The DNS server's resolver information needs to be validated and signed. This signature is called an Attestation Result [I-D.ietf-rats-architecture]. This validation can be performed by the DNS operator itself (signed by the DNS operator's certificate) acting as a verifier or performed by an external Verifier (signed by that external Verifier). The signing certificate can to be an Extended Validation (EV) certificate issued by a public CA in specific scenarios listed below. An EV certificate is issued by the public CA after a thorough Background Check to verify the requesting organization's legal identity. If the signing certificate is a EV certificate, it leaves the client with a better audit trail of the organization hosting the DNS server in comparison with the DV certificate.¶

The use of EV certificate is needed in the following scenarios:¶

• It helps the client to avoid sending DNS queries to an Encrypted DNS server hosted by an attacker discovered insecurely (e.g., using DHCP/RA or DNS). For example, an attacker can get a domain name, domain-validated public certificate from a CA and host a Encrypted DNS server. Furthermore, an attacker can use a public IP address, get an 'IP address'-validated public certificate from a CA and host a Encrypted DNS server.¶
• It can be used by the client to identify the Encrypted DNS server is hosted by a legal organization.¶

The use of EV certificate is not required in the following scenarios:¶

• If the Encrypted DNS server can only be discovered securely (e.g., using IKEv2 [I-D.btw-add-ipsecme-ike]), the signing certificate need not be an EV certificate.¶
• Secure Zero Touch Provisioning [RFC8572] defines a bootstrapping strategy for enabling a networking device to securely obtain the required configuration information with no user input. If the encrypted DNS server is insecurely discovered and not preconfigured in the networking device, the DNS client on the networking device can validate the Resolver Assertion Token signature using the owner certificate as per Section 3.2 of [RFC8572].¶

JSON Web Token (JWT) [RFC7519] and JSON Web Signature (JWS) [RFC7515] and related specifications define a standard token format that can be used as a way of encapsulating claimed or asserted information with an associated digital signature using X.509 based certificates. JWT provides a set of claims in JSON format that can accommodate asserted resolver information of the Encrypted DNS server. Additionally, JWS provides a path for updating methods and cryptographic algorithms used for the associated digital signatures.¶

JWS defines the use of JSON data structures in a specified canonical format for signing data corresponding to JOSE header, JWS Payload, and JWS Signature. The next sections define the header and claims that MUST be minimally used with JWT and JWS for resolver assertion token.¶

The REsolver Assertion Token (REAT) specifically uses this token format and defines claims that convey the resolver information of Encrypted DNS server.¶

The client can retrieve the REAT object using the RESINFO RRtype defined in [I-D.reddy-add-resolver-info] and QNAME of the domain name that is used to authenticate the DNS server (referred to as ADN in [RFC8310]). If the special use domain name "resolver.arpa" defined in [I-D.ietf-add-ddr] is used to discover the Encrypted DNS server, the client can retrieve the REAT object using the RESINFO RRtype and QNAME of the special use domain name.¶

The signature of REAT object MUST be validated by the DNS client. If signature is invalid, the REAT object is rejected. If signature is valid and signer is trusted, the DNS client can use that encrypted DNS server.¶

4. REAT Header

The JWS token header is a JOSE header (Section 4 of [RFC7515]) that defines the type and encryption algorithm used in the token.¶

The REAT header MUST include, at a minimum, the header parameters defined in Sections 4.1, 4.2, and 4.3.¶

{
  "typ":"rat",
  "alg":"ES256",
  "x5u":"https://cert.example.com/rat.cer"
}

5. REAT Payload

The token claims consist of the resolver information of the DNS server that needs to be verified at the DNS client. These claims follow the definition of a JWT claim (Section 4 of [RFC7519]) and are encoded as defined by the JWS Payload (Section 3 of [RFC7515]).¶

REAT defines the use of a standard JWT-defined claim as well as custom claims corresponding to the DoT or DoH servers.¶

Claim names MUST use the US-ASCII character set. Claim values MAY contain characters that are outside the ASCII range, however they MUST follow the default JSON serialization defined in Section 7 of [RFC7519].¶

{
  "server":{
      "adn":"example.com"
  },
  "iat":1443208345,
  "exp":1443640345,
  "resinfo": {
     "qnameminimization":false,
  }
}

6. REAT Signature

The signature of the REAT is created as specified in Section 5.1 of [RFC7515] (Steps 1 through 6). REAT MUST use the JWS Protected Header.¶

For the JWS Payload and the JWS Protected Header, the lexicographic ordering and white space rules described in Section 4 and Section 5, and JSON serialization rules in Section 8 MUST be followed.¶

The REAT is cryptographically signed by the domain hosting the DNS server and optionally by a third party who performed privacy and security audit of the DNS server.¶

The resolver information is attested using "Extended Validation" (EV) certificate to avoid bad actors taking advantage of this mechanism to advertise encrypted DNS servers for illegitimate and fraudulent purposes meant to trick DNS clients into believing that they are using a legitimate encrypted DNS server hosted to provide privacy for DNS transactions.¶

Alternatively, a DNS client has to be configured to trust the leaf of the signer of the REAT object. That is, trust of the signer MUST NOT be determined by validating the signer via the OS or the browser trust chain because that would allow any arbitrary entity to operate a DNS server and assert any sort of resolver information.¶

Appendix A provides an example of how to follow the steps to create the JWS Signature.¶

JWS JSON serialization (Step 7 in Section 5.1 of [RFC7515]) is supported for REAT to enable multiple signatures to be applied to the REAT object. For example, the REAT object can be cryptographically signed by the domain hosting the DNS server and by a third party who performed privacy and security audit of the DNS server.¶

Appendix B includes an example of the full JWS JSON serialization representation with multiple signatures.¶

Section 5.1 of [RFC7515] (Step 8) describes the method to create the final JWS Compact Serialization form of the REAT Token.¶

7. Extending REAT

REAT includes the minimum set of claims needed to securely assert the resolver information of the DNS server. JWT supports a mechanism to add additional asserted or signed information by simply adding new claims. REAT can be extended beyond the defined base set of claims to represent other DNS server information requiring assertion or validation. Specifying new claims follows the baseline JWT procedures (Section 10.1 of [RFC7519]). Understanding new claims on the DNS client is optional. The creator of a REAT object cannot assume that the DNS client will understand the new claims.¶

8. Deterministic JSON Serialization

JSON objects can include spaces and line breaks, and key value pairs can occur in any order. It is therefore a non-deterministic string format. In order to make the digital signature verification work deterministically, the JSON representation of the JWS Protected Header object and JWS Payload object MUST be computed as follows.¶

The JSON object MUST follow the following rules. These rules are based on the thumbprint of a JSON Web Key (JWK) as defined in Section 3 of [RFC7638] (Step 1).¶

1. The JSON object MUST contain no whitespace or line breaks before or after any syntactic elements.¶
2. JSON objects MUST have the keys ordered lexicographically by the Unicode [UNICODE] code points of the member names.¶
3. JSON value literals MUST be lowercase.¶
4. JSON numbers are to be encoded as integers unless the field is defined to be encoded otherwise.¶
5. Encoding rules MUST be applied recursively to member values and array values.¶

{
  "server":{
      "adn":"example.com"
  },
  "iat":1443208345,
  "exp":1443640345,
  "resinfo": {
     "qnameminimization":false,
  }
}

{"exp":1443640345,"iat":1443208345,
"resinfo":{"qnameminimization":false},
"server":{"adn":"example.com"}}

9. Using RESINFO Responses

This document defines the following entry for the IANA DNS Resolver Information Registry that is defined in [I-D.reddy-add-resolver-info].¶

• The "attested-resinfo" name contains the full REAT object. The REAT header, REAT payload, and REAT signature components comprise a full REAT object. If the "attested-resinfo" name is conveyed to the client, the server need not convey the attributes "resinfourl", "identityurl", "extendeddnserror" and "qnameminimization" attributes separately as that resolver information will be extracted by the client from the REAT payload.¶

10. Security Considerations

The use of REAT object based on the validation of the digital signature and the associated certificate requires consideration of the authentication and authority or reputation of the signer to attest the resolver information of the DNS server being asserted. Bad actors can host encrypted DNS servers to invade the privacy of the user. Bad actor can get a domain name, host encrypted DNS servers, and get the DNS server certificate signed by a CA. The resolver information will have to be attested using EV certificates or a REAT object signer trusted by the DNS client to prevent the attack.¶

The CA that issued the EV certificate does not attest the resolver information. The organization hosting the DNS server attests the resolver information using the EV certificate and the client uses the EV certificate to identify the organization (e.g., ISP or Enterprise) hosting the DNS server.¶

If the REAT object is asserted by a third party, it can do a "time of check" but the DNS server is susceptible of "time of use" attack. For example, changes to the DNS server can cause a disagreement between the auditor and the DNS server operation, hence the REAT object needs to be also asserted by the domain hosting the DNS server. In addition, the REAT object needs to have a short expiration time (e.g., 7 days) to ensure the DNS server's domain re-asserts the resolver information and limits the damage from change in behaviour and mis-issuance.¶

11. IANA Considerations

12. Acknowledgments

This specification leverages some of the work that has been done in [RFC8225]. Thanks to Tommy Jensen, Ted Lemon, Paul Wouters, Neil Cook, Vittorio Bertola, Vinny Parla, Chris Box, Ben Schwartz and Shashank Jain for the discussion and comments.¶

Appendix A. Example of ES256-based REAT JWS Serialization and Signature

For REAT, there will always be a JWS with the following members:¶

• 'protected', with the value BASE64URL(UTF8(JWS Protected Header))¶
• 'payload', with the value BASE64URL (JWS Payload)¶
• 'signature', with the value BASE64URL(JWS Signature)¶

This example will follow the steps in JWS [RFC7515] Section 5.1, steps 1-6 and 8 and incorporates the additional serialization steps required for REAT.¶

Step 1 for JWS references the JWS Payload, an example REAT Payload is as follows:¶

{
  "server":{
      "adn":"example.com"
  },
  "iat":1443208345,
  "exp":1443640345,
  "resinfo": {
     "qnameminimization":false
  }
}

This would be serialized to the form (with line break used for display purposes only):¶

{"exp":1443640345,"iat":1443208345,"resinfo":{
"qnameminimization":false},"server":{"adn":"example.com"}}

Step 2 Computes the BASE64URL(JWS Payload) producing this value (with line break used for display purposes only):¶

eyJleHAiOjE0NDM2NDAzNDUsImlhdCI6MTQ0MzIwODM0NSwicmVzaW5mbyI6ey
JxbmFtZW1pbmltaXphdGlvbiI6ZmFsc2V9LCJzZXJ2ZXIiOnsiYWRuIjoiZXhh
bXBsZS5jb20ifX0

For Step 3, an example REAT Protected Header comprising the JOSE Header is as follows:¶

{
  "alg":"ES256",
  "typ":"rat",
  "x5u":"https://cert.example.com/rat.cer"
}

This would be serialized to the form (with line break used for display purposes only):¶

{"alg":"ES256","typ":"rat","x5u":"https://cert.example.com
/rat.cer"}

Step 4 Performs the BASE64URL(UTF8(JWS Protected Header)) operation and encoding produces this value (with line break used for display purposes only):¶

eyJhbGciOiJFUzI1NiIsInR5cCI6InJhdCIsIng1dSI6Imh0dHBzOi8vY2VydC5l
eGFtcGxlLmNvbS9yYXQuY2VyIn0

Step 5 and Step 6 performs the computation of the digital signature of the REAT Signing Input ASCII(BASE64URL(UTF8(JWS Protected Header)) || '.' || BASE64URL(JWS Payload)) using ES256 as the algorithm and the BASE64URL(JWS Signature).¶

d1g7szj0roHsWe8psCzYVl4QdN2b7pQnq8EJhc4j3GOJj2NE6M9Em6aidtycnFJ5
mRj3ojiUfVF6rK5RksD0rg

Step 8 describes how to create the final REAT token, concatenating the values in the order Header.Payload.Signature with period ('.') characters. For the above example values this would produce the following (with line breaks between period used for readability purposes only):¶

eyJhbGciOiJFUzI1NiIsInR5cCI6InJhdCIsIng1dSI6Imh0dHBzOi8vY2VydC5l
eGFtcGxlLmNvbS9yYXQuY2VyIn0
.

eyJhbGciOiJFUzI1NiIsInR5cCI6InJhdCIsIng1dSI6Imh0dHBzOi8vY2VydC5l
eGFtcGxlLmNvbS9yYXQuY2VyIn0
.
d1g7szj0roHsWe8psCzYVl4QdN2b7pQnq8EJhc4j3GOJj2NE6M9Em6aidtycnFJ5
mRj3ojiUfVF6rK5RksD0rg

-----BEGIN PRIVATE KEY-----
MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgevZzL1gdAFr88hb2
OF/2NxApJCzGCEDdfSp6VQO30hyhRANCAAQRWz+jn65BtOMvdyHKcvjBeBSDZH2r
1RTwjmYSi9R/zpBnuQ4EiMnCqfMPWiZqB4QdbAd0E7oH50VpuZ1P087G
-----END PRIVATE KEY-----

-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEEVs/o5+uQbTjL3chynL4wXgUg2R9
q9UU8I5mEovUf86QZ7kOBIjJwqnzD1omageEHWwHdBO6B+dFabmdT9POxg==
-----END PUBLIC KEY-----

Appendix B. Complete JWS JSON Serialization Representation with multiple Signatures

The JWS payload used in this example as follows.¶

{
  "server":{
      "adn":"example.com"
  },
  "iat":1443208345,
  "exp":1443640345,
  "resinfo": {
     "qnameminimization":false
  }
}

This would be serialized to the form (with line break used for display purposes only):¶

{"exp":1443640345,"iat":1443208345,"resinfo":{
"qnameminimization":false},"server":{"adn":"example.com"}}

The JWS protected Header value used for the first signature is same as that used in the example in Appendix A. The X.509 private key used for generating the first signature is same as that used in the example in Appendix A.1.¶

The JWS Protected Header value used for the second signature is:¶

{
  "alg":"ES384",
  "typ":"rat",
  "x5u":"https://cert.audit-example.com/rat.cer"
}

The complete JWS JSON Serialization for these values is as follows (with line breaks within values for display purposes only):¶

{
  "payload":
       "eyJhbGciOiJFUzI1NiIsInR5cCI6InJhdCIsIng1dSI6Imh0dHBzOi8vY2VydC5l
        eGFtcGxlLmNvbS9yYXQuY2VyIn0",
  "signatures":[
       {"protected":"eyJhbGciOiJFUzI1NiIsInR5cCI6InJhdCIsIng1dSI6Imh0dHBz
        Oi8vY2VydC5leGFtcGxlLmNvbS9yYXQuY2VyIn0",
        "signature":"d1g7szj0roHsWe8psCzYVl4QdN2b7pQnq8EJhc4j3GOJj2NE6M9E
        m6aidtycnFJ5mRj3ojiUfVF6rK5RksD0rg"},
       {"protected":"eyJhbGciOiJFUzM4NCIsInR5cCI6InJhdCIsIng1dSI6Imh0dHB
        zOi8vY2VydC5hdWRpdC1leGFtcGxlLmNvbS9yYXQuY2VyIn0",
        "signature":"GnKuEEFql_Y8HdZl_mqd027DlziGRXFHvjMoY_ukX-M0k5v2jSL
        vsQAYOGdKFnt3JY6t938HfBV1onsWerNhgceMJpx5hAsl-xus3fmNY8K1g6QK39
        hn2Dhbleeeyp0f"}]
}

-----BEGIN PRIVATE KEY-----
MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgevZzL1gdAFr88hb2
OF/2NxApJCzGCEDdfSp6VQO30hyhRANCAAQRWz+jn65BtOMvdyHKcvjBeBSDZH2r
1RTwjmYSi9R/zpBnuQ4EiMnCqfMPWiZqB4QdbAd0E7oH50VpuZ1P087G
-----END PRIVATE KEY-----

-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEEVs/o5+uQbTjL3chynL4wXgUg2R9
q9UU8I5mEovUf86QZ7kOBIjJwqnzD1omageEHWwHdBO6B+dFabmdT9POxg==
-----END PUBLIC KEY-----

Authors' Addresses