Encoding DNS-over-TLS (DoT) Subject Public Key Info (SPKI) in Name Server name

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Table of Contents

1. Introduction

This document describes a mechanism to exchange the Subject Public Key Info (SPKI) ([RFC5280] Section 4.1.2.7) fingerprint associated with a DNS-over-TLS (DoT [RFC7858]) authoritative server by encoding it as part of its name. The fingerprint can thereafter be used to validate the certificate received from the DoT server as well as being able to discover support for DoT on the server.¶

2. Terminology

A server that supports DNS-over-TLS is called a "DoT server" to differentiate it from a "DNS Server" (one that provides DNS service over any other protocol), likewise, a client that supports this protocol is called a "DoT client"¶

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.¶

3. Validating a remote DoT server

While DoT provides protection against eavesdropping and on-path tampering of the DNS queries exchanged with an authoritative server, a recursive server that is talking to a remote DoT server needs a mechanism to authenticate that the name server it is communicating with is indeed the one that the authority of the zone manages or has delegated responsibility to.¶

A common mechanism is to have TLS certificates issued by "Certification Authorities" (CAs), those CA public keys are used as trust anchors, and through a chain of trust, a leaf TLS certificate can be validated. Any CA is able to issue a certificate for any domain, which can have its drawbacks ([RFC6698] Section 1.1).¶

Another method is to leverage DANE/TLSA ([RFC6698]), in which case a recursive resolver would be provided the certificate or SPKI hash over DNS and validate it using DNSSEC ([RFC4033], [RFC4034], and [RFC4035]).¶

This document describes a mechanism to signal to a recursive resolver that DoT is supported by the authoritative name server as well as providing a fingerprint of the SPKI to expect from the name server, this is done by formatting a special first label for the name servers. Recursive servers that understand the naming convention detailed in this document will be able to upgrade their connection to the authoritative server to TLS, while the ones that don't will transparently use the name servers as a standard UDP/53 and TCP/53 servers. This format is heavily inspired from [dnscurve].¶

4. Encoding data in a domain name label

A label is limited to a maximum of 63 octets ([RFC1035] Section 2.3.4) and has a limited set of characters that can be used ([RFC1035] Section 2.3.1), limiting both the amount of data that can be embedded in a label as well as the encoding format.¶

The set of character used by Base32 encoding ([RFC4648] Section 6), without padding character, is suitable to be used in a label. Base32 encodes a 5-bit group into 1 byte which allows to encode up to 39 bytes within the 63 bytes space of a label.¶

floor(63 * 5 / 8)

While this limits what can be encoded in a label, there is enough space to store the hash produced by sha256 which requires 32 bytes, leaving 7 bytes to spare.¶

<label> ::= <dot-header> <b32-spki-fingerprint>
<dot-header> ::= "dot-"
<b32-spki-fingerprint> ::= base32encode(<spki-fingerprint>)
<spki-fingerprint> ::= sha256(<spki>)
<spki> ::= der-encoded binary structure of SubjectPublicKeyInfo

openssl x509 -in /path/to/cert.pem  -pubkey -noout | \
openssl pkey -pubin -outform der | \
openssl dgst -sha256 -binary | \
base32 | tr -d '=' | tr '[:upper:]' '[:lower:]'
tpwxmgqdaurcqxqsckxvdq5sty3opxlgcbjj43kumdq62kpqr72a

...
...
;; QUESTION SECTION:
;example.com.      IN      NS

;; AUTHORITY SECTION:
example.com. 3600  IN      NS      dot-tpwxmgqdaurcqxqsckxvdq5sty3opxlgcbjj43kumdq62kpqr72a.a.example.com.
example.com. 3600  IN      NS      dot-tpwxmgqdaurcqxqsckxvdq5sty3opxlgcbjj43kumdq62kpqr72a.b.example.com.

;; ADDITIONAL SECTION:
dot-tpwxmgqdaurcqxqsckxvdq5sty3opxlgcbjj43kumdq62kpqr72a.a.example.com. 3600 IN A 192.0.2.1
dot-tpwxmgqdaurcqxqsckxvdq5sty3opxlgcbjj43kumdq62kpqr72a.b.example.com. 3600 IN AAAA 2001:DB8::1
...
...

5. Security Considerations

TODO Security¶

6. IANA Considerations

TODO: This document requires IANA actions (new RR type).¶

Acknowledgments

TODO acknowledge.¶

Author's Address