TLS Working Group B. Schwartz Internet-Draft Meta Platforms, Inc. Intended status: Standards Track M. Bishop Expires: 21 February 2025 E. Nygren Akamai Technologies 20 August 2024 Bootstrapping TLS Encrypted ClientHello with DNS Service Bindings draft-ietf-tls-svcb-ech-04 Abstract To use TLS Encrypted ClientHello (ECH) the client needs to learn the ECH configuration for a server before it attempts a connection to the server. This specification provides a mechanism for conveying the ECH configuration information via DNS, using a SVCB or HTTPS record. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 21 February 2025. Copyright Notice Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Schwartz, et al. Expires 21 February 2025 [Page 1] Internet-Draft ECH in SVCB August 2024 Table of Contents 1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. SvcParam for ECH configuration . . . . . . . . . . . . . . . 2 3. Server behavior . . . . . . . . . . . . . . . . . . . . . . . 3 4. Client behavior . . . . . . . . . . . . . . . . . . . . . . . 3 4.1. Disabling fallback . . . . . . . . . . . . . . . . . . . 3 4.2. ClientHello construction . . . . . . . . . . . . . . . . 3 4.3. Performance optimizations . . . . . . . . . . . . . . . . 4 5. Interaction with HTTP Alt-Svc . . . . . . . . . . . . . . . . 4 5.1. Security Considerations . . . . . . . . . . . . . . . . . 4 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 7.1. Normative References . . . . . . . . . . . . . . . . . . 5 7.2. Informative References . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 1. Overview The Service Bindings framework [SVCB] allows server operators to publish a detailed description of their service in the Domain Name System [RFC1034] 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 [ECH] 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. 2. SvcParam for ECH configuration The "ech" SvcParamKey is defined for conveying the ECH configuration of an alternative endpoint. It is applicable to all schemes that use TLS-based protocols (including DTLS [RFC9147] and QUIC version 1 [RFC9001]) unless otherwise specified. In wire format, the value of the parameter is an ECHConfigList (Section 4 of [ECH]), including the redundant length prefix. In presentation format, the value is the ECHConfigList in Base 64 Encoding (Section 4 of [RFC4648]). Base 64 is used here to simplify integration with TLS server software. To enable simpler parsing, this SvcParam MUST NOT contain escape sequences. Schwartz, et al. Expires 21 February 2025 [Page 2] Internet-Draft ECH in SVCB August 2024 ech="AEj+DQBEAQAgACAdd+scUi0IYFsXnUIU7ko2Nd9+F8M26pAGZVpz/KrWPgAEAAEAAWQ VZWNoLXNpdGVzLmV4YW1wbGUubmV0AAA=" Figure 1: ECH SvcParam with a public_name of "ech-sites.example.com 3. 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 Section 7.2.2 of [ECH] for more details about the public name.) Otherwise, connections will fail entirely. These servers SHOULD support a protocol version that is compatible with ECH. At the time of writing, the compatible versions are TLS 1.3, DTLS 1.3, and QUIC version 1. If the server does not support a compatible version, each connection attempt will have to be retried, delaying the connection and wasting resources. 4. Client behavior This section describes client behavior in using ECH configurations provided in SVCB or HTTPS records. 4.1. Disabling fallback The SVCB-optional client behavior specified in (Section 3 of [SVCB]) 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 Section 3 of [SVCB]) and all alternative endpoints have an "ech" SvcParam. 4.2. 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 [ECH]. 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 Section 7.1.2 of [SVCB] apply only to the inner ClientHello. Similarly, it is the inner ClientHello whose Server Name Indication (SNI) identifies the desired service. Schwartz, et al. Expires 21 February 2025 [Page 3] Internet-Draft ECH in SVCB August 2024 4.3. 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 Section 5.1 of [SVCB]). 5. Interaction with HTTP Alt-Svc HTTP clients that implement both HTTP Alt-Svc [RFC7838] and the HTTPS record type [SVCB] can use them together, provided that they only perform connection attempts that are "consistent" with both sets of parameters (Section 9.3 of [SVCB]). 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 Section 4.1) for that RRSet. This precludes the use of certain connections that Alt- Svc would otherwise allow, as discussed in Section 9.3 of [SVCB]. 5.1. 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. In an idealized deployment, ECH protects the SNI with an anonymity set consisting of all the ECH-enabled server domains supported by a given client-facing server. Accordingly, an attacker who can Schwartz, et al. Expires 21 February 2025 [Page 4] Internet-Draft ECH in SVCB August 2024 enumerate this set can always guess the encrypted SNI with probability 1/K, where K is the number of domains in the set. In practice, this probability may be increased via traffic analysis, popularity weighting, and other mechanisms. An attacker who can prevent SVCB resolution can deny clients any associated security benefits. A hostile recursive resolver can always deny service to SVCB queries, but network intermediaries can often prevent resolution as well, even when the client and recursive resolver validate DNSSEC and use a secure transport. These downgrade attacks can prevent a client from being aware that "ech" is configured which could result in the client sending the ClientHello in cleartext. To prevent downgrades, Section 3.1 of [SVCB] recommends that clients abandon the connection attempt when such an attack is detected. 6. IANA Considerations IANA is instructed to modify the Service Binding (SVCB) Parameter Keys Registry entry for "ech" as follows: +========+======+====================+===========+============+ | Number | Name | Meaning | Format | Change | | | | | Reference | Controller | +========+======+====================+===========+============+ | 5 | ech | TLS Encrypted | (This | IETF | | | | ClientHello Config | document) | | +--------+------+--------------------+-----------+------------+ Table 1 7. References 7.1. Normative References [ECH] Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS Encrypted Client Hello", Work in Progress, Internet-Draft, draft-ietf-tls-esni-20, 4 August 2024, . [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, . [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, . Schwartz, et al. Expires 21 February 2025 [Page 5] Internet-Draft ECH in SVCB August 2024 [SVCB] Schwartz, B. M., Bishop, M., and E. Nygren, "Service Binding and Parameter Specification via the DNS (SVCB and HTTPS Resource Records)", Work in Progress, Internet- Draft, draft-ietf-dnsop-svcb-https-12, 11 March 2023, . 7.2. Informative References [RFC7838] Nottingham, M., McManus, P., and J. Reschke, "HTTP Alternative Services", RFC 7838, DOI 10.17487/RFC7838, April 2016, . [RFC9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021, . [RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The Datagram Transport Layer Security (DTLS) Protocol Version 1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022, . Authors' Addresses Ben Schwartz Meta Platforms, Inc. Email: ietf@bemasc.net Mike Bishop Akamai Technologies Email: mbishop@evequefou.be Erik Nygren Akamai Technologies Email: erik+ietf@nygren.org Schwartz, et al. Expires 21 February 2025 [Page 6]