Bootstrapping TLS Encrypted ClientHello with DNS Service Bindings

Overview The Service Bindings framework allows server operators to publish a detailed description of their service in the Domain Name System using SVCB or HTTPS records. Each SVCB record describes a single "alternative endpoint", and contains a collection of "SvcParams" that can be extended with new kinds of information that may be of interest to a client. Clients can use the SvcParams to improve the privacy, security, and performance of their connection to this endpoint. This specification defines a new SvcParam to enable the use of TLS Encrypted ClientHello in TLS-based protocols. This SvcParam can be used in SVCB, HTTPS or any future SVCB-compatible DNS records, and is intended to serve as the primary bootstrap mechanism for ECH.

SvcParam for ECH configuration The "ech" SvcParamKey is defined for conveying the ECH configuration of an alternative endpoint. It is applicable to all TLS-based protocols (including DTLS and QUIC version 1 ) unless otherwise specified. In wire format, the value of the parameter is an ECHConfigList (), including the redundant length prefix. In presentation format, the value is the ECHConfigList in Base 64 Encoding (). Base 64 is used here to simplify integration with TLS server software. To enable simpler parsing, this SvcParam MUST NOT contain escape sequences.

Server behavior When publishing a record containing an "ech" parameter, the publisher MUST ensure that all IP addresses of TargetName correspond to servers that have access to the corresponding private key or are authoritative for the public name. (See for more details about the public name.) Otherwise, connections will fail entirely.

Client behavior This section describes client behavior in using ECH configurations provided in SVCB or HTTPS records.

Disabling fallback The SVCB-optional client behavior specified in () permits clients to fall back to a direct connection if all SVCB options fail. This behavior is not suitable for ECH, because fallback would negate the privacy benefits of ECH. Accordingly, ECH-capable SVCB-optional clients MUST switch to SVCB-reliant connection establishment if SVCB resolution succeeded (as defined in ) and all alternative endpoints have an "ech" SvcParam.

ClientHello construction When ECH is in use, the TLS ClientHello is divided into an unencrypted "outer" and an encrypted "inner" ClientHello. The outer ClientHello is an implementation detail of ECH, and its contents are controlled by the ECHConfig in accordance with . The inner ClientHello is used for establishing a connection to the service, so its contents may be influenced by other SVCB parameters. For example, the requirements related to ALPN protocol identifiers in apply only to the inner ClientHello. Similarly, it is the inner ClientHello whose Server Name Indication identifies the desired service.

Performance optimizations Prior to retrieving the SVCB records, the client does not know whether they contain an "ech" parameter. As a latency optimization, clients MAY prefetch DNS records that will only be used if this parameter is not present (i.e. only in SVCB-optional mode). The "ech" SvcParam alters the contents of the TLS ClientHello if it is present. Therefore, clients that support ECH MUST NOT issue any TLS ClientHello until after SVCB resolution has completed. (See ).

Interaction with HTTP Alt-Svc HTTP clients that implement both HTTP Alt-Svc and the HTTPS record type can use them together, provided that they only perform connection attempts that are "consistent" with both sets of parameters (). At the time of writing, there is no defined parameter related to ECH for Alt-Svc. Accordingly, a connection attempt that uses ECH is considered "consistent" with an Alt-Svc Field Value that does not mention ECH. Origins that publish an "ech" SvcParam in their HTTPS record SHOULD also publish an HTTPS record with the "ech" SvcParam for every alt-authority offered in its Alt-Svc Field Values. Otherwise, clients might reveal the name of the server in an unencrypted ClientHello to an alt-authority. If all HTTPS records for an alt-authority contain "ech" SvcParams, the client MUST adopt SVCB-reliant behavior (as in ) for that RRSet. This precludes the use of certain connections that Alt-Svc would otherwise allow, as discussed in .

Security Considerations A SVCB RRSet containing some RRs with "ech" and some without is vulnerable to a downgrade attack: a network intermediary can block connections to the endpoints that support ECH, causing the client to fall back to a non-ECH endpoint. This configuration is NOT RECOMMENDED. Zone owners who do use such a mixed configuration SHOULD mark the RRs with "ech" as more preferred (i.e. lower SvcPriority value) than those without, in order to maximize the likelihood that ECH will be used in the absence of an active adversary. Use of ECH yields an anonymity set of cardinality equal to the number of ECH-enabled server domains supported by a given client-facing server. Thus, even with an encrypted ClientHello, an attacker who can enumerate the set of ECH-enabled domains supported by a client-facing server can guess the correct SNI with probability at least 1/K, where K is the size of this ECH-enabled server anonymity set. This probability may be increased via traffic analysis or other mechanisms.

IANA Considerations IANA is instructed to modify the Service Binding (SVCB) Parameter Keys Registry entry for "ech" as follows:

Number	Name	Meaning	Format Reference	Change Controller
5	ech	TLS Encrypted ClientHello Config	(This document)	IETF