ECH in DNS: Bootstrapping TLS Encrypted ClientHello with DNS Service Bindings
How a server publishes its Encrypted ClientHello settings in DNS so clients can find them before connecting.
Schwartz, B., Bishop, M., and E. Nygren, "Bootstrapping TLS Encrypted ClientHello with DNS Service Bindings", RFC 9848, DOI 10.17487/RFC9848, March 2026, <https://www.rfc-editor.org/info/rfc9848>.
@misc{rfc9848,
series = {Request for Comments},
number = 9848,
howpublished = {RFC 9848},
publisher = {RFC Editor},
doi = {10.17487/RFC9848},
url = {https://www.rfc-editor.org/info/rfc9848},
author = {B. Schwartz and M. Bishop and E. Nygren},
title = {{Bootstrapping TLS Encrypted ClientHello with DNS Service Bindings}},
pagetotal = 9,
year = 2026,
month = mar,
} RFC text
Contents
Internet Engineering Task Force (IETF) B. Schwartz
Request for Comments: 9848 Meta Platforms, Inc.
Category: Standards Track M. Bishop
ISSN: 2070-1721 E. Nygren
Akamai Technologies
March 2026
Bootstrapping TLS Encrypted ClientHello with DNS Service Bindings
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 resource
record (RR).
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9848.
Copyright Notice
Copyright (c) 2026 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.
Table of Contents
1. Overview
2. Terminology
3. SvcParam for ECH Configuration
4. Requirements for Server Deployments
5. Requirements for Client Implementations
5.1. Disabling Fallback
5.2. ClientHello Construction
5.3. Performance Optimizations
6. Interaction with HTTP Alt-Svc
7. Examples
8. Security Considerations
9. IANA Considerations
10. References
10.1. Normative References
10.2. Informative References
Authors' Addresses
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] and [BCP219]) 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. 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
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 (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.
This example uses line wrapping per [RFC8792].
ech="AEj+DQBEAQAgACAdd+scUi0IYFsXnUIU7ko2Nd9+F8M26pAGZVpz/KrWPgAEAAE\
AAWQVZWNoLXNpdGVzLmV4YW1wbGUubmV0AAA="
Figure 1: ECH SvcParam with a public_name of "ech-sites.example.net"
4. Requirements for 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
This section describes client behavior in using ECH configurations
provided in SVCB or HTTPS records.
5.1. 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.
5.2. 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 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
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].)
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 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 5.1) for
that resource record set (RRSet). This precludes the use of certain
connections that 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 with the Same Configuration on the
Apex and Web Domain
The example above is compatible with clients that do or do not
support HTTPS records.
$ORIGIN heterogeneous.example. ; Example zone with 2 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 to Choose Between Two
Server Pools with Different ECH 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 for an 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 is 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
$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
8. 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, 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., 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 (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, 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
IANA has modified the entry for "ech" in the "DNS SVCB Service
Parameter Keys (SvcParamKeys)" registry in the "DNS Service Bindings
(SVCB)" registry group as follows:
+========+======+====================+============+===========+
| Number | Name | Meaning | Change | Reference |
| | | | Controller | |
+========+======+====================+============+===========+
| 5 | ech | TLS Encrypted | IETF | RFC 9848 |
| | | ClientHello Config | | |
+--------+------+--------------------+------------+-----------+
Table 1
10. References
10.1. Normative References
[ECH] Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
Encrypted Client Hello", RFC 9849, DOI 10.17487/RFC9849,
March 2026, <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/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/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/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/info/rfc8174>.
[RFC9364] Hoffman, P., "DNS Security Extensions (DNSSEC)", BCP 237,
RFC 9364, DOI 10.17487/RFC9364, February 2023,
<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/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", Lecture Notes in Computer Science, vol.
8555, pp. 143-163, DOI 10.1007/978-3-319-08506-7_8, 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/info/rfc7838>.
[RFC8792] Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,
"Handling Long Lines in Content of Internet-Drafts and
RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020,
<https://www.rfc-editor.org/info/rfc8792>.
[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/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/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
AI ELI5
On this page
§1Overview
The Service Bindings framework (RFC 9460) lets server operators publish a description of their service in the DNS using SVCB or HTTPS records. Each record describes one alternative endpoint and holds a set of SvcParams that clients can use to improve the privacy, security and performance of their connection.
RFC 9848 defines one new SvcParam for TLS Encrypted ClientHello (ECH, RFC 9849). It can be used in SVCB, HTTPS or any future SVCB-compatible record, and is intended to be the primary bootstrap mechanism for ECH.
A server can leave a note in the public phone book about how to talk to it. This document adds a new line to that note: the details needed to hide who you are visiting.
§3The ech SvcParam
The ech SvcParamKey carries the ECH configuration of an alternative endpoint. It applies to all schemes that use TLS-based protocols, including DTLS (RFC 9147) and QUIC version 1 (RFC 9001), unless otherwise specified.
On the wire, the value is an ECHConfigList from the ECH specification, including its redundant length prefix. In presentation format, it is that list in Base 64 (RFC 4648), chosen to simplify integration with TLS server software. To keep parsing simple, the value MUST NOT contain escape sequences.
The new line in the note is a long string of letters and numbers. It is a packed-up set of instructions that the visitor's computer knows how to unpack.
§4Server Requirements
A publisher of a record with an ech parameter MUST ensure that every IP address of the TargetName belongs to a server that has the matching private key or is authoritative for the public name. Otherwise, connections will fail entirely.
Those servers SHOULD support a protocol version compatible with ECH. At the time of writing these are TLS 1.3, DTLS 1.3 and QUIC version 1. Without one, each connection attempt has to be retried, which delays the connection and wastes resources.
If the note says to use a certain secret lock, every door the note points to must have the matching key. Otherwise visitors get stuck outside.
§5Client Behavior
No fallback. Normally an SVCB-optional client may fall back to a direct connection if all SVCB options fail. For ECH this is unsuitable, since fallback would negate its privacy benefit. So an ECH-capable, SVCB-optional client MUST switch to SVCB-reliant connection establishment if SVCB resolution succeeded and all alternative endpoints have an ech SvcParam.
Two ClientHellos. With ECH, the ClientHello is split into an unencrypted outer one and an encrypted inner one. The outer one is controlled by the ECHConfig. The inner one is used to reach the service, so other SVCB parameters can shape it. For example, the ALPN rules from the SVCB specification apply only to the inner ClientHello, and its Server Name Indication names the desired service.
Ordering. Because the ech parameter changes the ClientHello, ECH-capable clients MUST NOT send any TLS ClientHello until SVCB resolution has completed. As a latency optimization, clients MAY prefetch DNS records that are only used when ech is absent.
Once the phone book says to use the secret lock, the visitor must not try the unlocked side door instead. And they must look in the phone book before knocking at all.
- Query SVCB or HTTPS recordMust finish before any ClientHello is sent.
- Record with ech SvcParamCarries the ECHConfigList.
- ClientHello (outer, with encrypted inner)
§6Interaction with HTTP Alt-Svc
HTTP clients that support both Alt-Svc (RFC 7838) and HTTPS records can use them together if they only make connection attempts consistent with both. No ECH parameter is defined for Alt-Svc, so an attempt that uses ECH counts as consistent with an Alt-Svc value that does not mention ECH.
An origin that publishes ech in its HTTPS record SHOULD also publish an HTTPS record with ech for every alt-authority in its Alt-Svc values. Otherwise clients might reveal the server name in an unencrypted ClientHello to an alt-authority. If all HTTPS records for an alt-authority carry ech, the client MUST use SVCB-reliant behavior for that record set.
If the server also lists backup addresses, each backup needs its own lock details too, or a visitor could end up shouting the secret name at an unprepared door.
§7Deployment Examples
The RFC gives zone-file examples for common setups.
- A simple zone with the same
HTTPS 1 . ech=...record on the apex and onwww, which works for clients with or without HTTPS record support. - A service with two server pools that have different ECH configurations, published as two SVCB records.
- An aliasing-based CDN, where the customer zone points to the CDN's pool name and the CDN publishes the
echvalue. - A domain reachable only with ECH, using
mandatory=ech. - A multi-CDN customer that switches between two CDN pools on the server side.
- A DNS server that supports ECH, combining
echwithalpnanddohpath.
The document shows sample phone-book entries for different kinds of websites, from a tiny one with one server to a big one that uses several delivery companies.
$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§8Security Considerations
Mixed record sets. A set containing some records with ech and some without is open to a downgrade attack: a network intermediary can block the ECH endpoints and push the client to a non-ECH endpoint. This is NOT RECOMMENDED, though it may be unavoidable when combining providers or rolling out in stages. Zone owners who do it SHOULD give the ech records a lower SvcPriority value, meaning more preferred.
Limits of ECH. The protection is limited to an anonymity set: all ECH-enabled domains behind a client-facing server that share a configuration. An attacker who can enumerate the set can guess the encrypted name with probability at least 1/K for K domains, and traffic analysis may do better.
DNS still sees the name. This specification does not hide the server name from the DNS resolver. Without authenticated encryption between client and resolver, an on-path attacker can also learn it.
Blocked lookups. An attacker who prevents SVCB resolution can deny the client the benefits, even with DNSSEC and a secure transport, and the client may then send its ClientHello in cleartext. The SVCB specification recommends abandoning the connection attempt when such an attack is detected.
Hiding the name in the secret note does not hide it from the phone book keeper, and someone could tear out the page so visitors never see the secret lock details.