Nameserver Access Modes with Encryption Held in Alphanumeric Configuration Keys

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Table of Contents

1. 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.¶

2. Background

[I-D.draft-schwartz-svcb-dns] defines how to use SVCB records to describe the secure transport protocols supported by a DNS server. [I-D.draft-ietf-dprive-unauth-to-authoritative] describes the use of such records on the names of nameservers (the "NS name") to enable opportunistic encryption of recursive-to-authoritative DNS queries. Resolvers are permitted to fetch SVCB records asynchronously and cache them, resulting in "partial opportunistic encryption": even without an active adversary forcing a downgrade, queries will sometimes be sent in cleartext. Participating authoritative nameservers and recursive resolvers would have to be modified to make use of these records.¶

When the child zone is DNSSEC-signed, publishing a SVCB record of this kind is technically sufficient to enable authenticated encryption. However, in order to support reliable authentication, recursive resolvers would have to query for a SVCB record on every signed delegation, and wait for a response before issuing their intended query. We call this behavior a "synchronous binding check".¶

Many validating resolvers might not be willing to enable a "synchronous binding check" behavior, as this would slow down resolution of many existing domains in order to enable a new feature (authenticated encryption) that is not yet used at all. To enable authenticated encryption without this general performance loss, [I-D.draft-rescorla-dprive-adox-latest] proposes to deliver the SVCB records from the parent, in the delegation response. This avoids the need for a binding check, at the cost of additionally requiring modifications to the parent nameserver, which must provide these extra records in delegation responses.¶

Providing these additional records is sufficient to enable "full opportunistic encryption": the transport is always encrypted in the absence of an active adversary. However, these records are not protected by DNSSEC, so the child can only achieve fully authenticated encryption if the parent also implements fully authenticated encryption or otherwise protects the delivery of these records.¶

Even if this approach is standardized, many parent zones may not support delivery of SVCB records in delegation responses in the near future. To enable the broadest use of encrypted transport, we may need an interim solution that can be deployed more easily.¶

3. Proposal

We propose to indicate a nameserver's support for encrypted transports using a signal encoded in its name. This signal takes two forms: a "flag" and a "menu".¶

• QUESTION: Do we need both of these forms, or should we drop one?¶

We note that encoding semantics in DNS labels is a hack, but believe that the privacy benefits outweigh the ick factor.¶

In either form, the signal helps resolvers to acquire a SVCB RRSet for the nameserver. Resolvers use this RRSet as specified in [I-D.draft-rescorla-dprive-adox-latest].¶

_dns.svcb--qt.ns3.example. IN SVCB 1 svcb--qt.ns3.example. alpn=doq
_dns.svcb--qt.ns3.example. IN SVCB 2 svcb--qt.ns3.example. alpn=dot

4. Security Considerations

NS names received during delegation are not protected by DNSSEC. Therefore, just like in [I-D.draft-rescorla-dprive-adox-latest], this scheme only enables authenticated encryption if the parent domain can provide authentication without DNSSEC validation, e.g. using a secure transport or Zone Digest [RFC8976].¶

• QUESTION: Do we expect to have parent zones that can provide authenticated NS names but cannot provide authenticated SVCB records in delegation responses? (Maybe the root, with ZONEMD?) If not, does this proposal provide enough value?¶

5. Operational Considerations

It is possible that an existing NS name already matches the "flag" pattern. Such a "false positive flag" will result in a small performance loss due to the unnecessary synchronous binding check, but will not otherwise impair functionality.¶

If a pre-existing NS name contains the menu pattern, that nameserver will become unreachable by resolvers implementing this specification. The authors believe that no such nameservers are currently deployed, and such servers are unlikely to be deployed by accident.¶

6. IANA Considerations

IANA is requested to create a new registry entitled "Authoritative Server Transport In-Name Signal Characters", with the following fields:¶

• Character: a digit or lower-case letter¶
• SvcParams: a valid SVCB SvcParams set in presentation format¶

The registry policy is TBD.¶

The initial contents (DO NOT USE, subject to change) are as follows:¶

Appendix A. Comparison with related designs

Several other designs have been proposed to encode a transport upgrade signal in an existing record type.¶

Acknowledgments

TODO¶

Authors' Addresses