Signaling Authoritative DNS Encryption

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Table of Contents

1. Introduction

The IETF has defined a number of mechanisms for carrying DNS queries over encrypted transport [DOH] [DOT] [DOQ]. However, there is no scalable way for a recursive resolver to know that a given authoritative resolver supports encrypted transport, which inhibits the deployment of encrypted DNS for queries from recursive resolvers. This specification defines a mechanism for carrying that signal, using the DNS SVCB [SVCB] record.¶

2. Conventions and Definitions

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. Overview of Operation

The mechanism defined in this document works by using the DNS SVCB [SVCB] record to indicate that a given server supports TLS. The recursive resolver can obtain these records in two distinct ways:¶

• In the additional data block of the response that referred the recursive to the target authoritative server.¶
• By directly resolving a SVCB query for the target authoritative resolver.¶

As a practical matter, the first of these options is preferred as it allows the recursive to learn that the authoritative server supports encrypted transport without an additional round trip, as shown below:¶

Recursive                .com                      ns.example.example
                   Authoritative Server      (Authoritative for example.com)
NS example.com? ------------>

<----- example.com NS ns.example.example
       ns.example.example A 192.0.2.1
       _dns.ns.example.example SVCB alpn=dot

<--------------  TLS connection to ns.example ------------>
A example.com? ------------------------------------------->

The recursive resolver starts by contacting the authoritative server for .com and asks for the NS records for example.com. Note that .com is not authoritative for the example.com apex, and will not sign the NS RRset; see [RFC4035], Section 2.2, and Section 6 for details. The authoritative returns the NS record pointing at ns.example.example and also returns a glue records for ns.example.example indicating that it supports DNS over TLS (DoT), in much the same way as it might have sent an IP address for that server. This additional record is the only difference from the current situation, and allows the recursive resolver to know that it can reach ns.example.example over encrypted transport.¶

Note: SVCB is not presently permitted at the root [REGISTRY]. In the interim, recursive resolvers may be preconfigured with the TLS status of the resolvers for TLDs. [[OPEN ISSUE: Do we want to invent some other sentinel as a temporary measure?]]¶

4. Use of SVCB Records to Signal Encrypted Transport

Any given authoritative server name can have one or more DNS Server SVCB records, as defined in [I-D.schwartz-svcb-dns].¶

For instance, the following records would indicate that ns.example.example could be reached by either DoT or DoH (over both TCP and QUIC).¶

   _dns.ns.example.example. 7200 IN SVCB 1 ns.example.example. alpn=dot,h2,h3 dohpath=/dns-query{?dns}

Upon determining that a given nameserver supports a compatible encrypted transport, an implementation MUST only use encrypted transport for the rest of the cache lifetime for that information and MUST hard fail with error if it is unable to establish a connection. If multiple encrypted transports are available, an implementation SHOULD try all of them before declaring failure.¶

[[OPEN ISSUE: figure out error details]]¶

5. Example

A complete example is shown below.¶

Recursive  x.root-servers.org  ns.a.example         ns.example.example
             (Auth. for .)     (Auth for .example)  (Auth for .example.example)

<== TLS handshake ==>
NS .example? ------->
<- .example NS ns.op.example
   ns.op.example A 198.51.100.1
   _dns.ns.op.example SVCB alpn=dot


<============ TLS handshake ===========>
NS example.example? ------------------->
<----- example.example NS ns.example.example
       ns.example.example A 203.0.113.1
       _dns.ns.example.example SVCB alpn=dot

<====================== TLS Handshake ======================>
A www.example.example? ------------------------------------->
<---------------------------- www.example.example A 192.0.2.1

In this case, the recursive wants to resolve www.example.example.¶

Resolution proceeds in three phases.¶

Initially, the recursive connects to the root server. We assume that the recursive knows that the root server is able to do DoT, either because it has been preconfigured with his information or because it has connected to that root server before. It performs an NS query for ".example" (we are assuming QMIN) and receives:¶

• An NS record pointing to ns.op.example¶
• A glue A record for ns.op.example = 198.51.100.1¶
• A SVCB record stating that ns.op.example speaks DoT¶

Next, the recursive resolver forms a TLS connection to ns.op.example and requests an NS record for example.example. It receives:¶

• An NS record pointing to ns.example.example¶
• A glue A record for ns.example.example = 203.0.113.1¶
• A SVCB record stating that ns.example.example speaks DoT¶

Finally, the recursive resolver forms a TLS connection to ns.example.example and request an A record for www.example.example and receives the A record of 192.0.2.1.¶

6. Security Considerations

The primary security property delivered by this mechanism is confidentiality of the query and response. As long as (1) all queries in the resolution chain, including to the authoritative server are encrypted and (2) all resolvers in the resolution chain are trustworthy, then even an on-path attacker cannot discover the name being resolved or its response. However, if either of these conditions is violated, then an attack is possible:¶

• If the connection to the authoritative server is not encrypted, then the request and response can be read directly.¶
• If one of the earlier connections is not encrypted, then the attacker can substitute their own NS records. records from the additional_data, forcing the resolution back to unencrypted mode.¶
• If one of the resolvers is untrustworthy, then they can substitute their own NS records.¶

DNSSEC signing only partly mitigates these issues because delegations at top-level zones are not signed, as per [RFC4035], Section 2.2. In practice, this means a recursive resolver attempting to resolve a zone apex query, such as example.com in Section 3, cannot assume the NS answer is authentic. While NS records received from the authoritative server may be signed, in order to retrieve them, the recursive resolver will have to contact the servers listed by the unverified NS records received from the referring server, at which point it has leaked the zone apex to the (potentially fake) authoritative server (as well as to the referring server).¶

If the recursive resolver is attempting to resolve a specific subdomain from the resolver (e.g., server-1234.example.com), then it may be able to protect against this attack by (1) using query minimization [QMIN] and (2) querying the (alleged) authoritative for its DNSSEC-signed NS and SVCB records and only once it has received those, attempting to retrieve the actual subdomain. If the domain is DNSSEC signed, then this prevents a malicious referring resolver from redirecting the recursive resolver to their own authoritative and learning the true subdomain. However, if, as is common, the recursive is just trying to resolve the apex name or one of the common "service" names such as "www.example.com", then this procedure does not provide additional protection.¶

Encryption does not mitigate all leakage. In some circumstances, an on-path attacker may learn the identity of the authoritative server if, for example, that server only serves a small number of domains. The attacker can learn information about what is being resolved by observing whether or not server is queried.¶

As a secondary property, the mechanism in this document can provide some level of integrity for DNS responses, again under the condition that each resolver in the chain is trustworthy. By contrast, DNSSEC provides integrity even if the resolvers are untrustworthy.¶

7. IANA Considerations

This document has no IANA actions.¶

Acknowledgments

TODO acknowledge.¶

Authors' Addresses