TLS Working Group
Internet Engineering Task Force (IETF) B. Schwartz
Internet-Draft
Request for Comments: 9848 Meta Platforms, Inc.
Intended status:
Category: Standards Track M. Bishop
Expires: 18 December 2025
ISSN: 2070-1721 E. Nygren
Akamai Technologies
16 June
December 2025
Bootstrapping TLS Encrypted ClientHello with DNS Service Bindings
draft-ietf-tls-svcb-ech-08
Abstract
To use TLS Encrypted ClientHello (ECH) (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 resource
record (RR).
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 18 December 2025.
https://www.rfc-editor.org/info/rfc9848.
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Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. SvcParam for ECH configuration . . . . . . . . . . . . . . . 3 Configuration
4. Requirements for server deployments . . . . . . . . . . . . . 3 Server Deployments
5. Requirements for client implementations . . . . . . . . . . . 3 Client Implementations
5.1. Disabling fallback . . . . . . . . . . . . . . . . . . . 4 Fallback
5.2. ClientHello construction . . . . . . . . . . . . . . . . 4 Construction
5.3. Performance optimizations . . . . . . . . . . . . . . . . 4 Optimizations
6. Interaction with HTTP Alt-Svc . . . . . . . . . . . . . . . . 4
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 5
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Overview
The Service Bindings framework [SVCB] allows server operators to
publish a detailed description of their service in the Domain Name
System (DNS) (see [RFC1034], [RFC1034] and [BCP219]) using SVCB or HTTPS
records. Each SVCB record describes a single "alternative endpoint", 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 HTTPS, or any future SVCB-compatible DNS records, records
and is intended to serve as the primary bootstrap mechanism for ECH.
2. Terminology
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. SvcParam for ECH configuration Configuration
The "ech" SvcParamKey conveys 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
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.
ech="AEj+DQBEAQAgACAdd+scUi0IYFsXnUIU7ko2Nd9+F8M26pAGZVpz/KrWPgAEAAEAAWQ
VZWNoLXNpdGVzLmV4YW1wbGUubmV0AAA="
Figure 1: ECH SvcParam with a public_name of "ech-sites.example.net". "ech-sites.example.net"
4. Requirements for server deployments Server Deployments
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 Sections 6.1.7 and 8.1.1 of
[ECH] for requirements related to 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.
5. Requirements for client implementations Client Implementations
This section describes client behavior in using ECH configurations
provided in SVCB or HTTPS records.
5.1. Disabling fallback Fallback
The SVCB-optional client behavior specified in (Section Section 3 of [SVCB]) [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
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.
5.2. ClientHello construction 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 Application-Layer Protocol
Negotiation (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.
5.3. Performance optimizations 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. (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]). [SVCB].)
6. 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 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. 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 5.1) for
that RRSet. resource record set (RRSet). This precludes the use of certain
connections that Alt-
Svc Alt-Svc would otherwise allow, as discussed in
Section 9.3 of [SVCB].
7. Examples
$ORIGIN simple.example. ; Simple example zone
@ 300 IN A 192.0.2.1
AAAA 2001:db8::1
HTTPS 1 . ech=ABC...
www 300 IN A 192.0.2.1
AAAA 2001:db8::1
HTTPS 1 . ech=ABC...
Figure 2: Simple example zone Example Zone with the same configuration Same Configuration on the
apex
Apex and web domain. It Web Domain
The example above is compatible with clients that do or do not
support HTTPS records.
$ORIGIN heterogeneous.example. ; Example zone with two pools of servers
pool1 300 IN A 192.0.2.1
AAAA 2001:db8:1::a
pool2 300 IN A 192.0.2.2
AAAA 2001:db8:2::a
service 300 IN SVCB 1 pool1 ech=ABC...
SVCB 1 pool2 ech=DEF...
A 192.0.2.1
A 192.0.2.2
AAAA 2001:db8:1::a
AAAA 2001:db8:2::a
Figure 3: Service that allows clients That Allows Clients to choose between two
server pools Choose Between Two
Server Pools with different Different ECH configurations. Configurations
$ORIGIN cdn.example. ; CDN operator zone
pool 300 IN A 192.0.2.1
AAAA 2001:db8::1
HTTPS 1 . ech=ABC...
$ORIGIN customer.example. ; CDN customer's zone
@ 3600 IN HTTPS 0 pool.cdn.example.
; Apex IP records for compatibility with clients that do not support
; HTTPS records.
@ 300 IN A 192.0.2.1
AAAA 2001:db8::1
www 300 IN CNAME pool.cdn.example.
Figure 4: ECH usage pattern Usage Pattern for an aliasing-based CDN. Aliasing-Based CDN
$ORIGIN secret.example. ; High confidentiality zone
www 3600 IN HTTPS 1 backend ech=ABC... mandatory=ech
backend 300 IN A 192.0.2.1
AAAA 2001:db8::1
Figure 5: A domain that Domain That is only reachable using ECH. Only Reachable Using ECH
$ORIGIN cdn1.example. ; First CDN operator zone
pool 300 IN A 192.0.2.1
AAAA 2001:db8::1
HTTPS 1 . ech=ABC...
$ORIGIN cdn2.example. ; Second CDN operator zone
pool 300 IN A 192.0.2.2
AAAA 2001:db8::2
HTTPS 1 . ech=DEF...
;; Multi-CDN customer zone (version 1)
$ORIGIN customer.example.
@ 3600 IN HTTPS 0 pool.cdn1.example.
; Apex IP records for compatibility with clients that do not support
; HTTPS records.
@ 300 IN A 192.0.2.1
AAAA 2001:db8::1
www 3600 IN CNAME pool.cdn1.example.
;; Multi-CDN customer zone (version 2)
@ 3600 IN HTTPS 0 pool.cdn2.example.
@ 300 IN A 192.0.2.2
AAAA 2001:db8::2
www 3600 IN CNAME pool.cdn2.example.
Figure 6: Multi-CDN configuration using server-side selection. Configuration Using Server-Side Selection
$ORIGIN dns.example. ; DNS server example.
@ 3600 IN A 192.0.2.1
AAAA 2001:db8::1
HTTPS 1 . ech=ABC... alpn=h3 dohpath=/q{?dns}
_dns 3600 IN SVCB 1 @ ech=ABC... alpn=dot,doq,h3 dohpath=/q{?dns}
Figure 7: Example of a DNS server that supports ECH. Server That Supports ECH
8. Security Considerations
A SVCB RRSet containing some RRs with "ech" and some without is
vulnerable to a downgrade attack: a 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, but it may be unavoidable when combining endpoints from
different providers or conducting a staged rollout. Zone owners who
do use such a mixed configuration SHOULD mark the RRs with "ech" as
more preferred (i.e. (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.
When Encrypted ClientHello is deployed, the inner TLS SNI is
protected from disclosure to attackers. However, there are still
many ways that an attacker might infer the SNI. Even in an idealized
deployment, ECH's protection is limited to an anonymity set
consisting of all the ECH-enabled server domains supported by a given
client-facing server that share an ECH configuration. An attacker
who can enumerate this set can always guess the encrypted SNI with a
probability of at least 1/K, where K is the number of domains in the
set. Some attackers may achieve much greater accuracy using traffic
analysis, popularity weighting, and other mechanisms (see e.g., (e.g., see
[CLINIC]).
ECH ensures that TLS does not disclose the SNI, but the same
information is also present in the DNS queries used to resolve the
server's hostname. This specification does not conceal the server
name from the DNS resolver. If DNS messages are sent between the
client and resolver without authenticated encryption, an attacker on
this path can also learn the destination server name. A similar
attack applies if the client can be linked to a request from the
resolver to a DNS authority.
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 [RFC9364] and use a secure transport. These
downgrade attacks can prevent a client from being aware that "ech" is
configured
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.
9. IANA Considerations
In
IANA has modified the entry for "ech" in the "DNS SVCB Service
Parameter Keys (SvcParamKeys)" registry on in the "DNS Service Bindings
(SVCB)" page, IANA is instructed to modify
the entry for "ech" registry group as follows:
+========+======+====================+===========+============+
+========+======+====================+============+===========+
| Number | Name | Meaning | Format | Change | Reference |
| | | Reference | Controller |
+========+======+====================+===========+============+ |
+========+======+====================+============+===========+
| 5 | ech | TLS Encrypted | (This | IETF | RFC 9848 |
| | | ClientHello Config | document) | |
+--------+------+--------------------+-----------+------------+
+--------+------+--------------------+------------+-----------+
Table 1
10. References
10.1. 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-25, 14 June RFC 9849, DOI 10.17487/RFC9849,
December 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
esni-25>. <https://www.rfc-editor.org/info/rfc9849>.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/rfc/rfc1034>.
<https://www.rfc-editor.org/info/rfc1034>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/rfc/rfc4648>.
<https://www.rfc-editor.org/info/rfc4648>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>. <https://www.rfc-editor.org/info/rfc8174>.
[RFC9364] Hoffman, P., "DNS Security Extensions (DNSSEC)", BCP 237,
RFC 9364, DOI 10.17487/RFC9364, February 2023,
<https://www.rfc-editor.org/rfc/rfc9364>.
<https://www.rfc-editor.org/info/rfc9364>.
[SVCB] Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
and Parameter Specification via the DNS (SVCB and HTTPS
Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
November 2023, <https://www.rfc-editor.org/rfc/rfc9460>. <https://www.rfc-editor.org/info/rfc9460>.
10.2. Informative References
[BCP219] Best Current Practice 219,
<https://www.rfc-editor.org/info/bcp219>.
At the time of writing, this BCP comprises the following:
Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
RFC 9499, DOI 10.17487/RFC9499, March 2024,
<https://www.rfc-editor.org/info/rfc9499>.
[CLINIC] Miller, B., Huang, L., Joseph, A., and J. Tygar, "I Know
Why You Went to the Clinic: Risks and Realization of HTTPS
Traffic Analysis", Springer International Publishing, Lecture Notes in Computer Science Science, vol.
8555, pp. 143-163, DOI 10.1007/978-3-319-08506-7_8, ISBN ["9783319085050",
"9783319085067"], 2014,
<https://doi.org/10.1007/978-3-319-08506-7_8>.
[RFC7838] Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
April 2016, <https://www.rfc-editor.org/rfc/rfc7838>. <https://www.rfc-editor.org/info/rfc7838>.
[RFC9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
<https://www.rfc-editor.org/rfc/rfc9001>.
<https://www.rfc-editor.org/info/rfc9001>.
[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,
<https://www.rfc-editor.org/rfc/rfc9147>.
<https://www.rfc-editor.org/info/rfc9147>.
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