A well-known URI for publishing ECHConfigList values.

Internet-Draft	Well-Known URI for ECH	July 2023
Farrell, et al.	Expires 5 January 2024	[Page]

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

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This Internet-Draft will expire on 5 January 2024.¶

Copyright Notice

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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-dnsop-svcb-https] containing an ECHConfigList. This mechanism is extensible to deliver other kinds of information about the origin, but in this specification it only provides the functionality necessary to configure ECH.¶

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 propose use of a well-known URI [RFC8615] on the web server that allows ZF to poll for changes to ECHConfigList values. For example, if a web server generates new ECHConfigList values hourly and publishes those at the well-known URI, ZF can poll that URI. When ZF sees new values, it can check if those work, and if they do, then update the zone file and re-publish the zone.¶

[[The source for this draft is in https://github.com/sftcd/wkesni/ Issues and PRs are welcome there too.]]¶

3. Example use of the well-known URI for ECH

An example deployment could be as follows:¶

Web server generates new ECHConfigList values every "regeninterval" seconds via some regular, automated process (e.g. a cronjob)¶
ECHConfigList values are "current" for an hour, and remain usable for 3 hours from the time of generation¶
The cronjob updates the ECHConfigList values in a JSON resource at at https://$ORIGIN/.well-known/origin-svcb, as shown in Figure 1.¶
On the zone factory, another regularaly executed job uses an HTTP client to retrieve this JSON resource. It also attempts to connect to the origin using these ECH values and confirms that they are working.¶
The zone factory observes that the JSON resource has a regeninterval of 3600 seconds, and chooses a DNS TTL of 1800. It updates the zone file for $ORIGIN and re-publishes the zone containing only the new ECHConfigList values.¶
When regeninterval seconds have passed, the zone factory attempts to refresh its cached copy of the JSON resource. If the resource has changed, it repeats this process.¶

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

        {
            "alias": "cdn.example.net:443"
        }

Figure 2: Sample JSON with aliasing

The JSON file at the well-known URI MUST contain an object with either an "endpoints" key or an "alias" key. If the "endpoints" key is present, its value is an array whose elements represent HTTPS records in ServiceMode. 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 ".".¶
port: The value is a non-negative integer, corresponding to the value of the "port" SvcParamKey.¶
ech: The value is a string containing an ECHConfigList encoded in Base64 [RFC4648], corresponding to the value of the "ech" SvcParamKey.¶

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?]]¶

If the object contains an "alias" key, its value MUST be an "authority" (Section 3.2 of [RFC3986]). This indicates that $ORIGIN is hosted on the same endpoints as this target, and is equivalent to an HTTPS AliasMode record. A zone factory might implement this directive by publishing an AliasMode record, publishing a CNAME record, copying HTTPS records from the target zone, or fetching https://$TARGET/.well-known/origin-svcb" (if it exists).¶

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

The zone factory SHOULD check that the presented endpoints work and provide access to $ORIGIN 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. [[I guess that calls for the zone factory to know of a "safe" URL on $ORIGIN to try, or maybe it could use HTTP HEAD? Figuring that out is TBD. The ZF could also try a GREASEd ECH and see if the retry-configs it gets back is one of the ECHConfig values in the ECHConfigList.]]¶

A careful zone factory could explode the ECHConfigList value presented into "singleton" values with one public key in each and test each for each endpoint.¶

The zone factory SHOULD publish all the endpoints that are presented in the JSON file that pass the check above.¶

The zone factory 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 zone factory MUST attempt to refresh the JSON object and regenerate the zone before this time. This aims to ensure that ECHConfigs are not used longer than intended by the origin.¶

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 backend 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.¶

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>.
[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>.
[RFC3986]: Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005, <https://www.rfc-editor.org/info/rfc3986>.
[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-16, 6 April 2023, <https://datatracker.ietf.org/doc/html/draft-ietf-tls-esni-16>.
[I-D.ietf-dnsop-svcb-https]: Schwartz, B. M., Bishop, M., and E. Nygren, "Service binding and parameter specification via the DNS (DNS SVCB and HTTPS RRs)", Work in Progress, Internet-Draft, draft-ietf-dnsop-svcb-https-12, 11 March 2023, <https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-svcb-https-12>.

A well-known URI for publishing ECHConfigList values.

Abstract

Status of This Memo

Copyright Notice

Table of Contents

1. Introduction

2. Terminology

3. Example use of the well-known URI for ECH

4. The origin-svcb well-known URI

5. The JSON structure for origin service binding info

6. Zone factory behaviour

7. Security Considerations

8. Acknowledgements

9. IANA Considerations

10. Normative References

Appendix A. Change Log

Authors' Addresses