Using DNSSEC Authentication of Named Entities (DANE) with DNS Service Bindings (SVCB) and QUIC

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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 records when one or more CNAME records are present, aliasing either the hostname or the initial TLSA query name, and the resulting server names used in TLS or DTLS.¶

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 document 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.¶

The contents of this document apply equally to all SVCB-compatible record types, such as SVCB and HTTPS records. For brevity, the abbrevation "SVCB" is used to refer to these record types generally.¶

3. Using DANE with Service Bindings (SVCB)

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 document 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).¶

Resolution security is assessed according to the criteria in Section 4.1 of [RFC6698].¶

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]. However, domain owners SHOULD NOT place a CNAME record on a SVCB TargetName, as this arrangement is unusual, inefficient, and at risk for deprecation in a future revision.¶

If any TLSA QNAME is aliased by a CNAME, clients MUST follow the TLSA CNAME to complete the resolution of the TLSA records. (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. Adding a TLSA protocol prefix for QUIC

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

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

When this text was written, 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 document updates the above sentence as follows:¶

• The transport names defined for this protocol are "tcp" (TLS over TCP [RFC8446]), "udp" (DTLS [RFC9147]), "sctp" (TLS over SCTP [RFC3436]), and "quic" (QUIC [RFC9000]).¶

5. Operational considerations

alias.example.                  HTTPS 0 xyz.provider.example.
www.alias.example.              CNAME xyz.provider.example.
xyz.provider.example.           HTTPS 1 . alpn=h2 ...
xyz.provider.example.           A     192.0.2.1
_443._tcp.xyz.provider.example. TLSA  ...

_dns.dns.example. HTTPS 1 xyz.provider.example. ( alpn=h2 ...
                                                  dohpath=/doh{?dns} )

_443._tcp.xyz.provider.example. CNAME dane-central.provider.example.
dane-central.provider.example.  TLSA  ...

6. Security Considerations

The use of TLSA records specified in this document is independent for each SVCB connection attempt. In environments where DANE is optional, this means that the client might use DANE for some connection attempts but not others when processing a single SVCB RRSet.¶

This document only specifies the use of TLSA records when all relevant DNS records (including SVCB, TLSA, and CNAME records) were resolved securely. If any of these resolutions were insecure (as defined in Section 4.3 of [RFC4035]), the client MUST NOT rely on the TLSA record for connection security. However, if the client would otherwise have used an insecure plaintext transport, it MAY use an insecure resolution result to achieve opportunistic security.¶

Certain protocols that can run over TLS, such as HTTP/1.0, do not confirm the name of the service after connecting. With DANE, these protocols are subject to an Unknown Key Share (UKS) attack, in which the client believes it is connecting to the attacker's domain, but is actually connecting to an unaffiliated victim domain [I-D.barnes-dane-uks-00]. Clients SHOULD NOT use DANE with vulnerable protocols. (HTTP/1.1 and later and encrypted DNS are not normally vulnerable to UKS attacks, but see Appendix A for some important exceptions.)¶

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.cdn.example.
xyz.cdn.example.     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.cdn.example.

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

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

_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

8. IANA Considerations

IANA is requested to add the following entry to the "Underscored and Globally Scoped DNS Node Names" registry ([RFC8552], Section 4):¶

Appendix A. Unknown Key-Share Attacks

In the Unknown Key-Share (UKS) Attack [I-D.barnes-dane-uks-00], a hostile domain ("attacker.example") claims the IP addresses and TLSA records of another domain ("victim.example"). A client who attempts to connect to "attacker.example" will actually be connecting to "victim.example".¶

The client then sends some commands or requests over this connection. If the server rejects these requests, or if the attacker could have forwarded these requests itself, the attack confers no advantage. However, if the client issues commands that the attacker could not have issued, and the victim does not ignore these requests, then the attack could change state at the victim server, reveal confidential information to the attacker (e.g., via same-origin data sharing in the client), or waste resources.¶

Here are some examples of requests that the attacker likely could not have issued themselves:¶

• Requests authenticated using a TLS Client Certificate or other credential that is bound to the connection but not the domain.¶

• Requests that are only permitted if they appear to come from a particular IP range.¶

This section lists some protocols that can be used with SVCB, analyzes their vulnerability to this attack, and indicates any resulting restrictions on their use:¶

• HTTP/0.9 and HTTP/1.0: Vulnerable¶

  • Clients MUST NOT use TLS Client Authentication with DANE and these protocol versions.¶

    • Example attack: "https://attacker.example/" fetches "/profile" in Javascript. The second request is directed to "victim.example" and authenticated by a client certificate, revealing the user's profile to the attacker.¶

  • Use of these protocol versions with DANE is NOT RECOMMENDED.¶

• HTTP/1.1 and later: Slightly Vulnerable¶

  • The CONNECT method ([RFC9110], Section 3.6) MUST NOT be used on a connection authenticated with DANE.¶

    • Example attack: "attacker.example" advertises a CONNECT proxy service to existing customers of the "victim.example" proxy, which is access-controlled by client IP. To reduce its own operating costs, "attacker.example" uses UKS to send users back to "victim.example", resulting in the attacker's service appearing to work but silently consuming clients' transfer quota on "victim.example".¶

  • Clients MAY use all other methods with DANE, including Extended CONNECT [RFC8441]. These methods are defended from misdirection attacks by server verification of the Host or :authority header ([RFC9110], Section 7.4).¶

• DNS over TLS, DTLS, or QUIC [RFC7858][RFC8094][RFC9250]:¶

  • For resolution: Not Vulnerable¶

    • DNS resolution does not change state at the server, reveal confidential information to the attacker, or waste significant resources.¶

  • For other uses: Mitigation Required¶

    • When using DNS for other purposes such as zone transfers [RFC9103], clients relying on DANE for server authentication MUST NOT use a client certificate that is authorized by multiple potentially hostile servers.¶

Acknowledgments

TODO acknowledge.¶

Authors' Addresses