Internet DRAFT - draft-ietf-tls-wkech
draft-ietf-tls-wkech
TLS S. Farrell
Internet-Draft Trinity College Dublin
Intended status: Experimental R. Salz
Expires: 24 April 2024 Akamai Technologies
B. Schwartz
Meta Platforms, Inc.
22 October 2023
A well-known URI for publishing ECHConfigList values.
draft-ietf-tls-wkech-04
Abstract
We define a well-known URI at which an HTTP origin can inform an
authoritative DNS server, or other interested parties, about this
origin's Service Bindings, i.e. its "HTTPS" DNS records. These
instructions can include Encrypted ClientHello (ECH) configurations,
allowing the origin, in collaboration with DNS infrastructure
elements, to publish and rotate its own ECH keys.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 24 April 2024.
Copyright Notice
Copyright (c) 2023 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
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Please review these documents carefully, as they describe your rights
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extracted from this document must include Revised BSD License text as
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Example use of the well-known URI for ECH . . . . . . . . . . 3
4. The origin-svcb well-known URI . . . . . . . . . . . . . . . 4
5. The JSON structure for origin service binding info . . . . . 4
6. Zone factory behaviour . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
10. Normative References . . . . . . . . . . . . . . . . . . . . 7
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Encrypted ClientHello (ECH) [I-D.ietf-tls-esni] for TLS1.3 [RFC8446]
defines a confidentiality mechanism for server names and other
ClientHello content in TLS. For many applications, that requires
publication of ECHConflgList data structures in the DNS. An
ECHConfigList structure contains a list of ECHConfig values. Each
ECHConfig value contains the public component of a key pair that will
typically be periodically (re-)generated by a web server. Many web
infrastructures will have an API that can be used to dynamically
update the DNS RR values containing ECHConfigList values. Some
deployments however, will not, so web deployments could benefit from
a mechanism to use in such cases.
We define such a mechanism here. Note that this is not intended for
universal deployment, but rather for cases where the web server
doesn't have write access to the relevant zone file (or equivalent).
That zone file will eventually include an HTTPS or SVCB RR
[I-D.ietf-tls-svcb-ech] containing the ECHConfigList. This mechanism
is extensible to deliver other kinds of information about the origin,
that can be of use in these circumstances, but is mainly intended to
provide the functionality necessary for ongoing management of ECH
keys.
We use the term "zone factory" for the entity that does have write
access to the zone file. We assume the zone factory (ZF) can also
make HTTPS requests to the web server with the ECH keys. We define a
well-known URI [RFC8615] on the web server that allows the ZF to poll
for changes to ECHConfigList values. For example, if a web server
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generates new ECHConfigList values hourly and publishes those at the
well-known URI, the ZF can poll that URI. When the ZF sees new
values, it can check if those work, and if they do, then update the
zone file and re-publish the zone.
If ECH is being operated in "split-mode" then the web server (back-
end) can similarly poll the ECH front-end at the well-known URI and
then create it's own value to publish for the ZF to read.
[[The source for this draft is in https://github.com/sftcd/wkesni/
Issues and PRs are welcome there too.]]
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.
We define or re-use the following terms:
* Zone factory (ZF): an entity that has write-accsss to the DNS
* Front-end (FE): the web server that has an ECH private value.
This processes the outer ClientHello and attempts ECH decryption.
The name of FE will typically be the public_name value used in an
ECHconfig.
* Back-end (BE): the web server that will process the inner
ClientHello. Note that even if FE and BE are on the same web
server, they almost certainly have different DNS names.
* Shared-mode: this is where FE and BE are the same web server.
* Split-mode: this refers to the case where FE only does ECH
decryption but the TLS session is between the client and BE, which
will typically be on a different host to FE
* regeninterval: the number of seconds after which the value
retrieved after acessing a well-known URI may be changed.
3. Example use of the well-known URI for ECH
An example deployment could be as follows:
1. Shared-mode ECH web server generates new ECHConfigList values
every "regeninterval" seconds via some regular, automated process
(e.g. a cronjob)
2. ECHConfigList values are "current" for an hour, and remain usable
for 3 hours from the time of generation
3. The cronjob updates the ECHConfigList values in a JSON resource
at at https://BE/.well-known/origin-svcb, as shown in Figure 1.
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4. On the ZF, another regularaly executed job uses an HTTP client to
retrieve this JSON resource. It also attempts to connect to BE
using these ECH values and confirms that they are working.
5. The ZF observes that the JSON resource has a regeninterval of
3600 seconds, and chooses a DNS TTL of 1800. It updates the zone
file for BE and re-publishes the zone containing only the new
ECHConfigList values.
6. When regeninterval seconds have passed, the ZF attempts to
refresh its cached copy of the JSON resource. If the resource
has changed, it repeats this process.
4. The origin-svcb well-known URI
If BE wants to convey information to the Zone Factory, it publishes
the JSON content defined in Section 5 at: https://BE/.well-known/
origin-svcb
The well-known URI defined here MUST be an https URL and therefore
the ZF verifies the correct BE is being accessed. If no new ECH
value resulting "works," then the zone factory SHOULD NOT modify the
zone.
Note that a consequence of the URL above is that back-ends that wish
to use different ECH settings are very likely to have to use
different "DocRoot" settings.
5. The JSON structure for origin service binding info
[[The JSON structure is a work in progress.]]
{
"endpoints": [{
"regeninterval": 3600,
"priority": 1,
"target": "cdn.example.",
"ech": "AD7+DQA65wAgAC..AA=="
}, {
"regeninterval": 3600,
"priority": 1,
"port": 8413,
"ech": "AD7+DQA65wAgAC..AA=="
}]
}
Figure 1: Sample JSON for ECH without aliases
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{
"endpoints": [{
"alias": "cdn1.example.net",
"regeninterval": 108000
}, {
"alias": "cdn2.example.com",
"regeninterval": 108000
}]
}
Figure 2: Sample JSON with aliasing
The JSON file at the well-known URI MUST contain an object with an
"endpoints" key that contains an array of objects that each map to a
desired HTTPS/SVCB resource record that the back-end would like to
see published. The "endpoints" array its value is an array whose
elements each represent HTTPS records in ServiceMode or AliasMode as
described below. Each element MAY contain one or more keys from the
JSON HTTP Origin Info registry (see IANA Considerations). The
initial registry entries are:
* regeninterval: the number of seconds between key generation
actions at the origin, i.e. a replacement ECHConfigList may be
generated this often.
* priority: The value is a positive integer corresponding to the
SvcPriority. If omitted, the zone factory SHOULD infer
numerically increasing SvcPriority from the order of the endpoints
array.
* target: The value is a string containing a fully qualified domain
name, corresponding to the HTTPS record's TargetName. The default
value is ".".
* alias: The value MUST be a DNS name that could be used as the
TargetName of an HTTPS resource record. This indicates that the
back-end is hosted on the same endpoints as this target, and is
equivalent to an HTTPS AliasMode record. The ZF might implement
this directive by publishing an AliasMode record, publishing a
CNAME record, copying HTTPS records from the target zone, or
fetching https://FE/.well-known/origin-svcb" (if it exists). In
this case, the regeninterval indicates that BE does not plan to
change the content at the URL for at least that number of seconds.
If an alias entry is present then any ech and port entries (if
also present) MUST be ignored.
* ech: The value is a string containing an ECHConfigList encoded in
Base64 [RFC4648], corresponding to the value of the "ech"
SvcParamKey.
* port: The value is a non-negative integer, corresponding to the
value of the "port" SvcParamKey.
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* alpn: The value is a string corresponding to the value of the
"alpn" SvcParamKey. Typically this is likely to be set per back-
end.
* ipv4hints: The value is a string corresponding to the value of the
"ipv4hints" SvcParamKey. Typically this is likely to be set per
front-end.
* ipv6hints: The value is a string corresponding to the value of the
"ipv6hints" SvcParamKey. Typically this is likely to be set per
front-end.
An empty endpoint object corresponds to an HTTPS record with inferred
SvcPriority, TargetName=".", and no ECH support. An empty record of
this kind can be useful as a simple way to make use of the HTTPS RR
type's HSTS behavior.
[[TODO: What does the zone factory do if it encounters an
unrecognized field?]]
This arrangement provides the following important properties:
* Origins can indicate that different ECHConfigs are used on
different ports.
* Origins can indicate that multiple CDNs are in use, each with its
own ECHConfig.
* Origins that simply alias to a single target can indicate this
without copying the ECHConfig and other parameters, which can
interfere with key rotation and other maintenance.
* "port" and "target" are generally sufficient to uniquely identify
a ServiceMode record, so zone factories can use the endpoint list
to add ECH to pre-existing ServiceMode records that may have other
SvcParams.
6. Zone factory behaviour
ZF SHOULD check that the presented endpoints work and provide access
to BE before publication. A bespoke TLS client may be needed for
this check, that does not require the ECHConfigList value to have
already been published in the DNS. In order to make such checks, the
ZF SHOULD attempt to access the well-known URI defined here.
A careful ZF implementation could explode the ECHConfigList value
presented into "singleton" values with one public key in each, and
then test each of those separately.
ZF SHOULD publish all the endpoints that are presented in the JSON
file that pass the checks above.
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ZF SHOULD set a DNS TTL short enough so that any cached DNS resource
records are likely to have expired before the JSON object's content
is likely to have changed. The ZF MUST attempt to refresh the JSON
object and regenerate the zone before this time. This aims to ensure
that ECHConfig values are not used longer than intended by BE.
7. Security Considerations
This document defines another way to publish ECHConfigList values.
If the wrong keys were read from here and published in the DNS, then
clients using ECH would do the wrong thing, likely resulting in
denial of service, or a privacy leak, or worse, when TLS clients
attempt to use ECH with a back-end web site. So: Don't do that:-)
Although this configuration resource MAY be publicly accessible,
general HTTP clients SHOULD NOT attempt to use this resource in lieu
of HTTPS records queries through their preferred DNS server for the
following reasons:
* The bootstrap connection would not be able to use ECH, so it would
reveal all the information that ECH seeks to protect.
* The origin could serve the user with a uniquely identifying
configuration, potentially resulting in an unexpected tracking
vector.
8. Acknowledgements
Thanks to Niall O'Reilly for a quick review of -00.
Stephen Farrell's work on this specification was supported in part by
the Open Technology Fund.
9. IANA Considerations
[[TBD: IANA registration of a .well-known. Also TBD - how to handle
I18N for $FRONTEND and $BACKEND within such a URL.]]
If approved, this specification requests the creation of an IANA
registry named "JSON HTTP Origin Info" with a Standards Action
registration policy, containing a field named "Name" whose value is a
UTF-8 string.
10. Normative References
[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>.
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[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>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8615] Nottingham, M., "Well-Known Uniform Resource Identifiers
(URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
<https://www.rfc-editor.org/info/rfc8615>.
[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>.
[I-D.ietf-tls-esni]
Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
Encrypted Client Hello", Work in Progress, Internet-Draft,
draft-ietf-tls-esni-17, 9 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
esni-17>.
[I-D.ietf-tls-svcb-ech]
Schwartz, B. M., Bishop, M., and E. Nygren, "Bootstrapping
TLS Encrypted ClientHello with DNS Service Bindings", Work
in Progress, Internet-Draft, draft-ietf-tls-svcb-ech-00,
26 September 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-tls-svcb-ech-00>.
Appendix A. Change Log
[[RFC editor: please remove this before publication.]]
The -00 WG draft replaces draft-farrell-tls-wkesni-03.
Version 01 changed from a special-purpose design, carrying only
ECHConfigs and port numbers, to a more general approach based on
Service Bindings.
Version 02 is just a keep-alive
Version 03 reflects some local implementation experience with -02
Version 04 matches a proof-of-concept bash script implementation and
results of IETF-117 discussion.
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Authors' Addresses
Stephen Farrell
Trinity College Dublin
Dublin
2
Ireland
Phone: +353-1-896-2354
Email: stephen.farrell@cs.tcd.ie
Rich Salz
Akamai Technologies
Email: rsalz@akamai.com
Benjamin Schwartz
Meta Platforms, Inc.
Email: ietf@bemasc.net
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