Internet DRAFT - draft-farrell-tls-wkesni
draft-farrell-tls-wkesni
TLS S. Farrell
Internet-Draft Trinity College Dublin
Intended status: Experimental 24 May 2022
Expires: 25 November 2022
A well-known URI for publishing ECHConfigList values.
draft-farrell-tls-wkesni-03
Abstract
We propose use of a well-known URI at which web servers can publish
ECHConfigList values as a way to help get those published in the DNS.
Status of This Memo
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This Internet-Draft will expire on 25 November 2022.
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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 ech well-known URI . . . . . . . . . . . . . . . . . . . 3
5. The JSON structure for ECHConfigList values . . . . . . . . . 4
6. Zone factory behaviour . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
10. Normative References . . . . . . . . . . . . . . . . . . . . 6
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
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.
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 is in https://github.com/sftcd/wkesni/ PRs are
welcome there too.]]
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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. Example use of the well-known URI for ECH
An example deployment could be as follows:
* Web server generates new ECHConfigList values hourly at N past the
hour via a cronjob
* ECHConfigList values are "current" for an hour, published with a
TTL of 1800, and remain usable for 3 hours from the time of
generation
* Web server has a set of "backend" sites - the DNS name for each
such site is here represented as $BACKEND, which will end up as an
SNI value to be encrypted inside an ECH extension
* Web server has a "front-end" site ($FRONT), where $FRONT will
typically be the DNS name used in the ECHConfigList public_name
field for ECHConfig version 0xff0d
* A cronjob creates creates a JSON file for each backend site at
https://$FRONT/.well-known/ech/$BACKEND.json
* Each JSON file contains an array with the ECHConfigList values
values for that particular $BACKEND as shown in Figure 1 - the
values in Figure 1 with ellipses are the values we want to
eventually see in the DNS
* On the zone factory, a cronjob runs at N+3 past the hour, it knows
all the names involved and checks to see if the content at those
well-known URIs has changed or not
* If the content has changed the cronjob attempts to use the
ECHConfigList values, and for each $BACKEND where that works, it
updates the zone file and re-publishes the zone containing only
the new ECHConfigList values
4. The ech well-known URI
When a web server ($FRONT) wants to publish ECHConfigList information
for a backend site ($BACKEND) then it provides the JSON content
defined in Section 5 at: https://$FRONT/.well-known/ech/$BACKEND.json
The well-known URI defined here MUST be an https URL and therefore
the zone factory verifies the correct $FRONT is being accessed. If
there is any failure in accessing the well-known URI, then the zone
factory MUST NOT modify the zone.
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5. The JSON structure for ECHConfigList values
[[Since the specifics of the JSON structure in Figure 1 are very
likely to change, this is mostly TBD. What is here for now, is what
the author has currently implemented simply because it worked ok and
was easy to do:-) One issue raised as a result of the dispatch
presentation is whether or not anything beyond the ECHConfigList
might make sense to represent in the JSON response. One example
could be the inner ClientHello ALPN extension, if that might somehow
be useful to the TLS cilent (which really should know in that case).
The scope in that respect and the correct level of generality to
cover here is something to consider as this evolves.]]
[
{
"desired-ttl": 1800,
"ports": [ 443, 8413 ],
"echconfiglist": "AD7+DQA65wAgAC..AA=="
},
{
"desired-ttl": 1800,
"ports": [ 443, 8413 ],
"echconfiglist": "AD7+DQA65wAgAC..AA=="
}
]
Figure 1: Sample JSON
The JSON file at the well-known URI MUST contain an array with one or
more elements. Each element of the array MUST have these fields:
* desired-ttl: contains a number indicating the TTL that the web
server would like to see used for this RR. The zone factory MUST
NOT use a longer TTL.
* ports: this has a list of the TCP ports on which the web server
with the relevant key pair will listen (needed to produce the
correct zone file).
* ECHConfigList: contains the value to be used as a base64 encoded
string.
The JSON file contains an array for a couple of reasons:
* As TLS authentication doesn't really distinguish ports, servers on
the same host could in any case cheat on one another, so we may as
well just read one JSON file per name.
* Different ports could map to different sets of ECHConfig values
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* As ECHConfigList is (regrettably:-) an extensible structure, it
may be necessary to publish different ECHConfigList values to get
best interoperability.
6. Zone factory behaviour
The zone factory SHOULD check that the presented ECHConfigList values
work with the $BACKEND server before publication. A "special" 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
$BACKEND 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 port claimed.
The zone factory SHOULD publish all the ECHConfigList values that are
presented in the JSON file, and that pass the check above.
The zone factory SHOULD only publish ECHConfigList values that are in
the latest version of the JSON file. This leaves the control of
"expiry" with the web server, so long as the ECHConfigList values
presented actually work. [[An alternative could be to have the new
values just be appended to the zone, but that'd require some form of
"notAfter" value in the JSON file which seems unnecessary and more
complex.]]
The SCVB and HTTPS RR specification [I-D.ietf-dnsop-svcb-https]
defines how and where the ECHConfigList values for $BACKEND needs to
be published in the DNS. The zone factory is assumed to be in
control of how ECHConfigList values are included in such RRs.
A possibly interesting (unintended) consequence of this design is
that once a TLS client has first gotten an ECHConfigList from the DNS
for $BACKEND with the ECHConfigList structure containing the
public_name field, the TLS client would know both $FRONT and $BACKEND
and so could later probe for this .well-known as an alternative to
doing so via DoT/DoH. Probably not something a web browser might do,
but could be fun for other applications maybe.
[[The extent to which retry-configs could be used for a similar
purpose might be worth considering. But the JSON stuff here may
still be needed if implementations (such as mine:-) tend to only
return one ECHConfig in retry-configs.]]
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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:-)
8. Acknowledgements
Thanks to Niall O'Reilly for a quick review of -00.
9. IANA Considerations
[[TBD: IANA registration of a .well-known. Also TBD - how to handle
I18N for $FRONT and $BACKEND within such a URL.]]
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>.
[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-14, 13 February 2022,
<https://www.ietf.org/archive/id/draft-ietf-tls-esni-
14.txt>.
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[I-D.ietf-dnsop-svcb-https]
Schwartz, B., 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-09, 6 May 2022,
<https://www.ietf.org/archive/id/draft-ietf-dnsop-svcb-
https-09.txt>.
Appendix A. Change Log
[[RFC editor: please remove this before publication.]]
From -02 to -03:
* noted scope issue
From -01 to -02:
* General changes from ESNI to ECH.
From -00 to -01:
* Re-structured a bit after re-reading rfc8615
Author's Address
Stephen Farrell
Trinity College Dublin
Dublin
2
Ireland
Phone: +353-1-896-2354
Email: stephen.farrell@cs.tcd.ie
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