Using Service Bindings with DANE

Status of This Memo

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Table of Contents

1. Introduction

The DNS-Based Authentication of Named Entities specification [RFC7671] explains how clients locate the TLSA record for a service of interest, starting with knowledge of the service's hostname, transport, and port number. These are concatenated, forming a name like _8080._tcp.example.com. It also specifies how clients should locate the TLSA record when one or more CNAME records are present, aliasing either the hostname or the TLSA record's name, and the resulting server names used in TLS.¶

There are various DNS records other than CNAME that add indirection to the host resolution process, requiring similar specifications. Thus, [RFC7672] describes how DANE interacts with MX records, and [RFC7673] describes its interaction with SRV records.¶

This draft describes the interaction of DANE with indirection via Service Bindings [SVCB], i.e. SVCB-compatible records such as SVCB and HTTPS. It also explains how to use DANE with new TLS-based transports such as QUIC.¶

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. Using DANE with Service Bindings

Section 6 of [RFC7671] says:¶

• With protocols that support explicit transport redirection via DNS MX records, SRV records, or other similar records, the TLSA base domain is based on the redirected transport endpoint rather than the origin domain.¶

This draft applies the same logic to SVCB-compatible records. Specifically, if SVCB resolution was entirely secure (including any AliasMode records and/or CNAMEs), then for each connection attempt derived from a SVCB-compatible record,¶

• The initial TLSA base domain MUST be the final SVCB TargetName used for this connection attempt. (Names appearing earlier in a resolution chain are not used.)¶
• The transport prefix MUST be the transport of this connection attempt (possibly influenced by the "alpn" SvcParam).¶
• The port prefix MUST be the port number of this connection attempt (possibly influenced by the "port" SvcParam).¶

If the initial TLSA base domain is the start of a secure CNAME chain, clients MUST first try to use the end of the chain as the TLSA base domain, with fallback to the initial base domain, as described in Section 7 of [RFC7671].¶

If any TLSA QNAME is aliased by a CNAME, clients MUST follow the TLSA CNAME to complete the resolution of the TLSA record. (This does not alter the TLSA base domain.)¶

If a TLSA RRSet is securely resolved, the client MUST set the SNI to the TLSA base domain of the RRSet. In usage modes other than DANE-EE(3), the client MUST validate that the certificate covers this base domain, and MUST NOT require it to cover any other domain.¶

If the client has SVCB-optional behavior (as defined in Section 3 of [SVCB]), it MUST use the standard DANE logic described in Section 4.1 of [RFC6698] when falling back to non-SVCB connection.¶

4. Updating the TLSA protocol prefixes

Section 3 of [RFC6698] defined the protocol prefix used for constructing TLSA QNAMEs, and said:¶

• The transport names defined for this protocol are "tcp", "udp", and "sctp".¶

At that time, there was exactly one TLS-based protocol defined for each of these transports. However, with the introduction of QUIC [RFC9000], there are now multiple TLS-derived protocols that can operate over UDP, even on the same port. To distinguish the availability and configuration of DTLS and QUIC, this draft Updates the above sentence as follows:¶

• The transport names defined for this protocol are "tcp" (TLS over TCP [RFC8446]), "udp" (DTLS [I-D.draft-ietf-tls-dtls13]), "sctp" (TLS over SCTP [RFC3436]), and "quic" (QUIC [RFC9000]).¶

5. Operational considerations

alias.net.                  HTTPS 0 xyz.provider.com.
www.alias.net.              CNAME xyz.provider.com.
xyz.provider.com.           HTTPS 1 . alpn=h2 ...
xyz.provider.com.           A     192.0.2.1
_443._tcp.xyz.provider.com. TLSA  <provider keys>

_443._tcp.xyz.provider.com. CNAME dane-central.provider.com.
dane-central.provider.com.  TLSA  <provider keys>

6. Security Considerations

This document specifies the use of TLSA as a property of each connection attempt. In environments where DANE is optional, this means that the fallback procedure might use DANE for some conection attempts but not others.¶

This document only specifies the use of TLSA records when the SVCB records were resolved securely. Use of TLSA records in conjunction with insecurely resolved SVCB records is not safe in general, although there may be some configurations where it is appropriate (e.g. when only opportunistic security is available).¶

7. Examples

The following examples demonstrate Service Binding interaction with TLSA base domain selection.¶

All of the RRSets below are assumed fully-secure with all related DNSSEC record types omitted for brevity.¶

api.example.com. HTTPS 1 .

api.example.com.     HTTPS 0 svc4.example.net.
svc4.example.net.    HTTPS 0 xyz.example-cdn.com.
xyz.example-cdn.com. A     192.0.2.1

www.example.com.  CNAME api.example.com.
api.example.com.  HTTPS 1 svc4.example.net alpn=h2,h3 port=8443
svc4.example.net. CNAME xyz.example-cdn.com.

_8443._foo.api.example.com. SVCB 1 api.example.com.

_8443._foo.api.example.com. SVCB 0 svc4.example.net.
svc4.example.net.           SVCB 1 .
svc4.example.net.           A    192.0.2.1

_8443._foo.api.example.com. SVCB 0 svc4.example.net.
svc4.example.net. SVCB 3 . alpn=foo,bar port=8004

_dns.dns.example.com. SVCB 1 dns.example.com. alpn=dot

_dns.dns.example.com. SVCB 0 dns.my-dns-host.net.
dns.my-dns-host.net.  SVCB 1 . alpn=dot

8. IANA Considerations

IANA is instructed to add the following entry to the "Underscored and Globally Scoped DNS Node Names" registry:¶

Acknowledgments

TODO acknowledge.¶

Authors' Addresses