Certificate Transparency (CT) information of DNS resolver

Background DNS resolver can support any encrypted DNS scheme, such as DNS over HTTPS (DoH) , DNS over TLS (DoT) , or DNS over QUIC (DoQ) . Certificate hijacking allows attackers to impersonate a legitimate encrypted DNS resolver, see also . Certificate Transparency (CT) is to combat the certificate hijacking issue . This document describes the CT information of the encrypted DNS resolver.

Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in . Basic terms used in this specification are defined in the documents , , , , .

IANA Considerations

DNS Resolver Information Keys Registration specifies a method for DNS resolvers to publish information about themselves. IANA has created a new registry called "DNS Resolver Information Keys" . This document adds a new DNS Resolver Information Key: CT, to present the CT information of the encrypted DNS resolver. Name: CT Value: 1 Meaning: The value indicates that the certificate of the encrypted DNS resolver contains embedded SCTs. Reference: RFC 9162 Name: CT Value: 2 Meaning: The value indicates that the encrypted DNS resolver supports the transparency_info TLS extension. Reference: RFC 9162

Security Considerations DNS clients can get trustworthy DNS resolver information through DNSSEC query or out-of-band configuration. Suppose the DNS clients find the CT value in the trustworthy DNS resolver information. In that case, they can mandate the CT validation in the encrypted communication channel setup process with the encrypted DNS resolver.

Acknowledgements Thanks to all in the DNSOP mailing list.