Internet DRAFT - draft-ietf-dprive-unilateral-probing
draft-ietf-dprive-unilateral-probing
dprive D. K. Gillmor, Ed.
Internet-Draft ACLU
Intended status: Standards Track J. Salazar, Ed.
Expires: 4 September 2023
P. Hoffman, Ed.
ICANN
3 March 2023
Unilateral Opportunistic Deployment of Encrypted Recursive-to-
Authoritative DNS
draft-ietf-dprive-unilateral-probing-05
Abstract
This document sets out steps that DNS servers (recursive resolvers
and authoritative servers) can take unilaterally (without any
coordination with other peers) to defend DNS query privacy against a
passive network monitor. The steps in this document can be defeated
by an active attacker, but should be simpler and less risky to deploy
than more powerful defenses.
The goal of this document is to simplify and speed deployment of
opportunistic encrypted transport in the recursive-to-authoritative
hop of the DNS ecosystem. With wider easy deployment of the
underlying transport on an opportunistic basis, we hope to facilitate
the future specification of stronger cryptographic protections
against more powerful attacks.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://dkg.gitlab.io/dprive-unilateral-probing/. Status information
for this document may be found at https://datatracker.ietf.org/doc/
draft-ietf-dprive-unilateral-probing/.
Discussion of this document takes place on the DPRIVE Working Group
mailing list (mailto:dns-privacy@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/dns-privacy/. Subscribe at
https://www.ietf.org/mailman/listinfo/dns-privacy/.
Source for this draft and an issue tracker can be found at
https://gitlab.com/dkg/dprive-unilateral-probing.
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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
Task Force (IETF). Note that other groups may also distribute
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This Internet-Draft will expire on 4 September 2023.
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
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Priorities . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Minimizing Negative Impacts . . . . . . . . . . . . . . . 5
2.2. Protocol Choices . . . . . . . . . . . . . . . . . . . . 5
3. Guidance for Authoritative Servers . . . . . . . . . . . . . 5
3.1. Pooled Authoritative Servers Behind a Single IP
Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Authentication . . . . . . . . . . . . . . . . . . . . . 6
3.3. Server Name Indication . . . . . . . . . . . . . . . . . 7
3.4. Resource Exhaustion . . . . . . . . . . . . . . . . . . . 7
3.5. Pad Responses to Mitigate Traffic Analysis . . . . . . . 7
4. Guidance for Recursive Resolvers . . . . . . . . . . . . . . 8
4.1. High-level Overview . . . . . . . . . . . . . . . . . . . 8
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4.2. Maintaining Authoritative State by IP Address . . . . . . 8
4.3. Overall Recursive Resolver Settings . . . . . . . . . . . 9
4.4. Recursive Resolver Requirements . . . . . . . . . . . . . 10
4.5. Authoritative Server Encrypted Transport Connection
State . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.5.1. Separate State for Each of the Recursive Resolver's Own
IP Addresses . . . . . . . . . . . . . . . . . . . . 13
4.6. Probing Policy . . . . . . . . . . . . . . . . . . . . . 13
4.6.1. Sending a Query over Do53 . . . . . . . . . . . . . . 14
4.6.2. Receiving a Response over Do53 . . . . . . . . . . . 14
4.6.3. Initiating a Connection over Encrypted Transport . . 15
4.6.4. Establishing an Encrypted Transport Connection . . . 17
4.6.5. Failing to Establish an Encrypted Transport
Connection . . . . . . . . . . . . . . . . . . . . . 17
4.6.6. Encrypted Transport Failure . . . . . . . . . . . . . 18
4.6.7. Handling Clean Shutdown of an Encrypted Transport
Connection . . . . . . . . . . . . . . . . . . . . . 19
4.6.8. Sending a Query over Encrypted Transport . . . . . . 19
4.6.9. Receiving a Response over Encrypted Transport . . . . 20
4.6.10. Resource Exhaustion . . . . . . . . . . . . . . . . . 21
4.6.11. Maintaining Connections . . . . . . . . . . . . . . . 21
4.6.12. Additional Tuning . . . . . . . . . . . . . . . . . . 21
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 22
6.1. Server Name Indication . . . . . . . . . . . . . . . . . 22
6.2. Modelling Probability of Encryption . . . . . . . . . . . 22
7. Security Considerations . . . . . . . . . . . . . . . . . . . 23
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 23
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
9.1. Normative References . . . . . . . . . . . . . . . . . . 23
9.2. Informative References . . . . . . . . . . . . . . . . . 24
Appendix A. Defense Against Active Attackers . . . . . . . . . . 26
A.1. Signalling Mechanism Properties . . . . . . . . . . . . . 26
A.2. Authentication of Authoritative Server . . . . . . . . . 27
A.3. Combining Protocols . . . . . . . . . . . . . . . . . . . 27
Appendix B. Document Considerations . . . . . . . . . . . . . . 27
B.1. Document History . . . . . . . . . . . . . . . . . . . . 28
B.1.1. Substantive Changes from -03 to -04 . . . . . . . . . 28
B.1.2. Substantive Changes from -02 to -03 . . . . . . . . . 28
B.1.3. Substantive Changes from -01 to -02 . . . . . . . . . 28
B.1.4. Substantive Changes from -00 to -01 . . . . . . . . . 28
B.1.5. Substantive Changes from -01 to -02 (now
draft-ietf-dprive-unilateral-probing-00) . . . . . . 29
B.1.6. draft-dkgjsal-dprive-unilateral-probing Substantive
Changes from -00 to -01 . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
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1. Introduction
This document aims to provide guidance to implementers who want to
simply enable protection against passive network observers.
In particular, it focuses on mechanisms that can be adopted
unilaterally by recursive resolvers and authoritative servers,
without any explicit coordination with the other parties. This
guidance provides opportunistic security (see [RFC7435]) --
encrypting things that would otherwise be in the clear, without
interfering with or weakening stronger forms of security.
The document also briefly introduces (but does not try to specify)
how a future protocol might permit defense against an active attacker
in Appendix A.
The protocol described here offers three concrete advantages to the
Internet ecosystem:
* Protection from passive attackers of DNS queries in transit
between recursive and authoritative servers.
* A roadmap for gaining real-world experience at scale with
encrypted protections of this traffic.
* A bridge to some possible future protection against a more
powerful attacker.
1.1. Requirements Language
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.
1.2. Terminology
Unilateral: capable of opportunistic probing deployment without
external coordination with any of the other parties
Do53: traditional cleartext DNS over port 53 ([RFC1035])
DoQ: DNS-over-QUIC ([RFC9250])
DoT: DNS-over-TLS ([RFC7858])
DoH: DNS-over-HTTPS ([RFC8484])
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Encrypted transports: DoQ, DoT, and DoH collectively
2. Priorities
2.1. Minimizing Negative Impacts
This document aims to minimize potentially negative impacts caused by
the probing of encrypted transports -- for the systems that adopt
these guidelines, for the parties that they communicate with, and for
uninvolved third parties. The negative impacts that we specifically
try to minimize are:
* excessive bandwidth use
* excessive use of computational resources (CPU and memory in
particular)
* the potential for amplification attacks (where DNS resolution
infrastructure is wielded as part of a DoS attack)
2.2. Protocol Choices
Although this document focuses specifically on strategies used by DNS
servers, it does not go into detail on the specific protocols used
because those protocols, in particular DoT and DoQ, are described in
other documents.
This document does not pursue the use of DoH in this context, because
a DoH client needs to know the path part of a DoH endpoint URL, and
there are currently no mechanisms for a DNS resolver to predict the
path on its own, in an opportunistic or unilateral fashion, without
incurring in excessive use of resources. If such mechanisms are
later defined, the protocol in this document can be updated to
accomodate them.
3. Guidance for Authoritative Servers
An authoritative server SHOULD implement and deploy DNS-over-TLS
(DoT) on TCP port 853.
An authoritative server SHOULD implement and deploy DNS-over-QUIC
(DoQ) on UDP port 853.
An authoritative server implementing the protocol described in this
document MUST implement at least one of DoT or DoQ on port 853.
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3.1. Pooled Authoritative Servers Behind a Single IP Address
Some authoritative DNS servers are structured as a pool of
authoritatives standing behind a load-balancer that runs on a single
IP address, forwarding queries to members of the pool.
In such a deployment, individual members of the pool typically get
updated independently from each other.
A recursive resolver following the guidance in Section 4 that
interacts with such a pool likely does not know that it is a pool.
If some members of the pool follow this guidance while others do not,
the recursive client might see the pool as a single authoritative
server that sometimes offers and sometimes refuses encrypted
transport.
To avoid incurring additional minor timeouts for such a recursive
resolver, the pool operator SHOULD either:
* ensure that all members of the pool enable the same encrypted
transport(s) within the span of a few seconds, or
* ensure that the load balancer maps client requests to pool members
based on client IP addresses.
Similar concerns apply to authoritative servers responding from an
anycast IP address. As long as the pool of servers is in a
heterogeneous state, any flapping route that switches a given client
IP address to a different responder risks incurring an additional
timeout. Frequent changes of routing for anycast listening IP
addresses are also likely to cause problems for TLS, TCP, or QUIC
connection state as well, so stable routes are important to ensure
that the service remains available and responsive. The servers in a
pool can share session information to increase the likelihood of
successful resumptions.
3.2. Authentication
For unilateral deployment, an authoritative server does not need to
offer any particular form of authentication.
The simplest deployment would simply provide a self-issued,
regularly-updated X.509 certificate. This mechanism is supported by
many TLS and QUIC clients, and will be acceptable for any
opportunistic connection.
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3.3. Server Name Indication
An authoritative DNS server that wants to handle unilateral queries
MAY rely on Server Name Indication (SNI) to select alternate server
credentials. However, such a server MUST NOT serve resource records
that differ based on SNI (or on the lack of SNI) provided by the
client, because a probing recursive resolver that offers SNI might or
might not have used the right server name to get the records it is
looking for.
3.4. Resource Exhaustion
A well-behaved recursive resolver may keep an encrypted connection
open to an authoritative server, to amortize the costs of connection
setup for both parties.
However, some authoritative servers may have insufficient resources
available to keep many connections open concurrently.
To keep resources under control, authoritative servers should
proactively manage their encrypted connections. Section 6.5 of
[RFC9250] ("Connection Handling") offers useful guidance for servers
managing DoQ connections. Section 3.4 of [RFC7858] offers useful
guidance for servers managing DoT connections.
An authoritative server facing unforeseen resource exhaustion SHOULD
cleanly close open connections from recursive resolvers based on the
authoritative's preferred prioritization.
In the case of unanticipated resource exhaustion, close connections
until resources are back in control. A reasonable prioritization
scheme would be to close connections with no outstanding queries,
ordered by idle time (longest idle time gets closed first), then
close connections with outstanding queries, ordered by age of
outstanding query (oldest outstanding query gets closed first).
When resources are especially tight, the authoritative server may
also decline to accept new connections over encrypted transport.
3.5. Pad Responses to Mitigate Traffic Analysis
To increase the anonymity set for each response, the authoritative
server SHOULD use a sensible padding mechanism for all responses it
sends when possible (this might be limited by e.g. not receiving an
EDNS(0) option in the query). Specifically, a DoT server SHOULD use
EDNS(0) padding [RFC7830] if possible, and a DoQ server SHOULD follow
the guidance in Section 5.4 of [RFC9250]. How much to pad is out of
scope of this document, but a reasonable suggestion can be found in
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[RFC8467].
4. Guidance for Recursive Resolvers
This section outlines a probing policy suitable for unilateral
adoption by any recursive resolver. Following this policy should not
result in failed resolutions or significant delay.
4.1. High-level Overview
In addition to querying on Do53, the recursive resolver will try
either or both of DoT and DoQ concurrently. The recursive resolver
remembers what opportunistic encrypted transport protocols have
worked recently based on a (clientIP, serverIP, protocol) tuple.
If a query needs to go to a given authoritative server, and the
recursive resolver remembers a recent successful encrypted transport
to that server, then it doesn't send the query over Do53 at all.
Rather, it only sends the query using the recently-good encrypted
transport protocol.
If the encrypted transport protocol fails, the recursive resolver
falls back to Do53 for that tuple. When any encrypted transport
fails, the recursive resolver remembers that failure for a reasonable
amount of time to avoid flooding a non-compatible server with
requests that it cannot accept.
See the subsections below for a more detailed description of this
protocol.
4.2. Maintaining Authoritative State by IP Address
In designing a probing strategy, the recursive resolver could record
its knowledge about any given authoritative server with different
strategies, including at least:
* the authoritative server's IP address,
* the authoritative server's name (the NS record used), or
* the zone that contains the record being looked up.
This document encourages the first strategy, to minimize timeouts or
accidental delays. This document does not describe the other two
strategies because the first is strongly encouraged.
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A timeout (accidental delay) is most likely to happen when the
recursive client believes that the authoritative server offers
encrypted transport, but the actual server reached declines encrypted
transport (or worse, filters the incoming traffic and does not even
respond with an ICMP port closed message).
By associating state with the IP address, the recursive client is
most able to avoid reaching a heterogeneous deployment.
For example, consider an authoritative server named ns0.example.com
that is served by two installations (with two A records), one at
192.0.2.7 that follows this guidance, and one at 192.0.2.8 that is a
legacy (cleartext port 53-only) deployment. A recursive client who
associates state with the NS name and reaches .7 first will "learn"
that ns0.example.com supports encrypted transport. A subsequent
query over encrypted transport dispatched to .8 would fail,
potentially delaying the response.
By associating the state with the authoritative IP address, the
client can minimize the number of accidental delays introduced (see
also Section 4.5.1 and Section 3.1).
4.3. Overall Recursive Resolver Settings
A recursive resolver implementing this document needs to set system-
wide values for some default parameters. These parameters may be set
independently for each supported encrypted transport, though a simple
implementation may keep the parameters constant across encrypted
transports.
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+=============+===================================+===========+
| Name | Description | Suggested |
| | | Default |
+=============+===================================+===========+
| persistence | How long should the recursive | 3 days |
| | resolver remember successful | (259200 |
| | encrypted transport connections? | seconds) |
+-------------+-----------------------------------+-----------+
| damping | How long should the recursive | 1 day |
| | resolver remember unsuccessful | (86400 |
| | encrypted transport connections? | seconds) |
+-------------+-----------------------------------+-----------+
| timeout | How long should the recursive | 4 seconds |
| | resolver wait for an initiated | |
| | encrypted connection to complete? | |
+-------------+-----------------------------------+-----------+
Table 1: Recursive resolver system parameters per encrypted
transport
This document uses the notation E-foo to refer to the foo parameter
for the encrypted transport E.
For example DoT-persistence would indicate the length of time that
the recursive resolver will remember that an authoritative server had
a successful connection over DoT.
This document also assumes that the resolver maintains a list of
outstanding cleartext queries destined for the authoritative server's
IP address X. This list is referred to as Do53-queries[X]. This
document does not attempt to describe the specific operation of
sending and receiving cleartext DNS queries (Do53) for a recursive
resolver. Instead it describes a "bolt-on" mechanism that extends
the recursive resolver's operation on a few simple hooks into the
recursive resolver's existing handling of Do53.
Implementers or deployers of DNS recursive resolvers that follow the
strategies in this document are encouraged to report their preferred
values of these parameters.
4.4. Recursive Resolver Requirements
To follow this guidance, a recursive resolver MUST implement at least
one of either DoT or DoQ in its capacity as a client of authoritative
nameservers.
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A recursive resolver SHOULD implement the client side of DNS-over-TLS
(DoT). A recursive resolver SHOULD implement the client side of DNS-
over-QUIC (DoQ).
DoT queries from the recursive resolver MUST target TCP port 853,
with an ALPN of "dot". DoQ queries from the recursive resolver MUST
target UDP port 853, with an ALPN of "doq". ALPN is described in
[RFC7301].
While this document focuses on the recursive-to-authoritative hop, a
recursive resolver implementing these strategies SHOULD also accept
queries from its clients over some encrypted transport (current
common transports are DoH or DoT).
4.5. Authoritative Server Encrypted Transport Connection State
The recursive resolver SHOULD keep a record of the state for each
authoritative server it contacts, indexed by the IP address of the
authoritative server and the encrypted transports supported by the
recursive resolver. In addition, the recursive resolver SHOULD also
keep a record of its own IP addresses used for queries, as described
in Section 4.5.1.
In addition to tracking the state of connection attempts and
outcomes, a recursive resolver SHOULD record the state of established
sessions for encrypted protocols. The details of how sessions are
identified is dependent on the transport protocol implementation
(such as TLS session ticket or TLS session ID, QUIC connection ID,
and so on). The use of session resumption as recommended here is
limited somewhat because the tickets are only stored within the
context defined by the (clientIP, serverIP, protocols) tuples used to
track client-server interaction by the recursive resolver in a table
like the one below. However, session resumption still offers the
ability to optimize the handshake in some circumstances.
Each record should contain the following fields for each supported
encrypted transport, each of which would initially be null:
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+===============+==========================================+========+
| Name | Description | Retain |
| | | Across |
| | | Reset |
+===============+==========================================+========+
| session | The associated state of any | no |
| | existing, established session (the | |
| | structure of this value is dependent | |
| | on the encrypted transport | |
| | implementation). If session is not | |
| | null, it may be in one of two | |
| | states: pending or established | |
+---------------+------------------------------------------+--------+
| initiated | Timestamp of most recent connection | yes |
| | attempt | |
+---------------+------------------------------------------+--------+
| completed | Timestamp of most recent completed | yes |
| | handshake (which can include one | |
| | where an existing session is | |
| | resumed) | |
+---------------+------------------------------------------+--------+
| status | Enumerated value of success or fail | yes |
| | or timeout, associated with the | |
| | completed handshake | |
+---------------+------------------------------------------+--------+
| last-response | A timestamp of the most recent | yes |
| | response received on the connection | |
+---------------+------------------------------------------+--------+
| resumptions | A stack of resumption tickets (and | yes |
| | associated parameters) that could be | |
| | used to resume a prior successful | |
| | session | |
+---------------+------------------------------------------+--------+
| queries | A queue of queries intended for this | no |
| | authoritative server, each of which | |
| | has additional status early, unsent, | |
| | or sent | |
+---------------+------------------------------------------+--------+
| last-activity | A timestamp of the most recent | no |
| | activity on the connection | |
+---------------+------------------------------------------+--------+
Table 2: Recursive resolver state per authoritative IP, per encrypted
transport
Note that the session fields in aggregate constitute a pool of open
connections to different servers.
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With the exception of the session, queries, and last-activity fields,
this cache information should be kept across restart of the server
unless explicitly cleared by administrative action.
This document uses the notation E-foo[X] to indicate the value of
field foo for encrypted transport E to IP address X.
For example, DoT-initiated[192.0.2.4] represents the timestamp when
the most recent DoT connection packet was sent to IP address
192.0.2.4.
4.5.1. Separate State for Each of the Recursive Resolver's Own IP
Addresses
Note that the recursive resolver should record this per-
authoritative-IP state for each source IP address it uses as it sends
its queries. For example, if a recursive resolver can send a packet
to authoritative servers from IP addresses 192.0.2.100 and
192.0.2.200, it should keep two distinct sets of per-authoritative-IP
state, one for each source address it uses. Keeping these state
tables distinct for each source address makes it possible for a
pooled authoritative server behind a load balancer to do a partial
rollout while minimizing accidental timeouts (see Section 3.1).
4.6. Probing Policy
When a recursive resolver discovers the need for an authoritative
lookup to an authoritative DNS server using IP address X, it
retrieves the records associated with X from its cache.
The following sections presume that the time of the discovery of the
need for lookup is time T0.
If any of the records discussed here are absent, they are treated as
null.
The recursive resolver must decide whether to initially send a query
over Do53, or over any of the supported encrypted transports (DoT or
DoQ).
Note that a resolver might initiate this query via any or all of the
known transports. When multiple queries are sent, the initial
packets for each connection can be sent concurrently, similar to
"Happy Eyeballs" ([RFC8305]). However, unlike Happy Eyeballs, when
one transport succeeds, the other connections do not need to be
terminated, but can instead be continued to establish whether the IP
address X is capable of communicating on the relevant transport.
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4.6.1. Sending a Query over Do53
For any of the supported encrypted transports E, if either of the
following holds true, the resolver SHOULD NOT send a query to X over
Do53:
* E-session[X] is in the established state, or
* E-status[X] is success, and (T0 - E-last-response[X]) <
persistence
This indicates that one successful connection to a server that the
client then closed cleanly would result in the client not sending the
next query over Do53, regardless of how long in the past that was.
Otherwise, if there is no outstanding session for any encrypted
transport, and the last successful encrypted transport connection was
long ago, the resolver sends a query to X over Do53. When it does
so, it inserts a handle for the query in Do53-queries[X].
4.6.2. Receiving a Response over Do53
When a response R for query Q arrives at the recursive resolver in
cleartext sent over Do53 from authoritative server with IP address X,
the recursive resolver should:
If Q is not in Do53-queries[X]:
* Discard R and process it no further (do not respond to a cleartext
response to a query that is not outstanding)
Otherwise:
* Remove Q from Do53-queries[X]
If R is successful:
* If Q is in Do53-queries[X]:
- Return R to the requesting client
* For each supported encrypted transport E:
- If Q is in E-queries[X]:
o Remove Q from E-queries[X]
But if R is unsuccessful (e.g. SERVFAIL):
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* if Q is not in any of *-queries[X]:
- Return SERVFAIL to the client
4.6.3. Initiating a Connection over Encrypted Transport
If any E-session[X] is in the established state, the recursive
resolver SHOULD NOT initiate a new or resume a previous connection to
X over Do53 or E, but should instead send queries to X through the
existing session (see Section 4.6.8).
If the recursive resolver has a preferred encrypted transport, but
only a different transport is in the established state, it MAY also
initiate a new connection to X over its preferred transport while
concurrently sending the query over the established transport E.
Before considering whether to initiate a new connection over an
encrypted transport, the timer should examine and possibly refresh
its state for encrypted transport E to authoritative IP address X:
* if E-session[X] is in state pending, and
* T0 - E-initiated[X] > E-timeout, then
- set E-session[X] to null and
- set E-status[X] to timeout
When resources are available to attempt a new encrypted transport,
the resolver should only initiate a new connection to X over E as
long as one of the following holds true:
* E-status[X] is success, or
* E-status[X] is fail or timeout and (T0 - E-completed[X]) >
damping, or
* E-status[X] is null and E-initiated[X] is null
When initiating a session to X over encrypted transport E, if
E-resumptions[X] is not empty, one ticket should be popped off the
stack and used to try to resume a previous session. Otherwise, the
initial Client Hello handshake should not try to resume any session.
When initiating a connection, the resolver should take the following
steps:
* set E-initiated[X] to T0
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* store a handle for the new session (which should have pending
state) in E-session[X]
* insert a handle for the query that prompted this connection in
E-queries[X], with status unsent or early, as appropriate (see
below).
4.6.3.1. Early Data
Modern encrypted transports like TLS 1.3 offer the chance to store
"early data" from the client into the initial Client Hello in some
contexts. A resolver that initiates a connection over a encrypted
transport according to this guidance in a context where early data is
possible SHOULD send the DNS query that prompted the connection in
the early data, according to the sending guidance in Section 4.6.8.
If it does so, the status of Q in E-queries[X] should be set to early
instead of unsent.
4.6.3.2. Resumption Tickets
When initiating a new connection (whether by resuming an old session
or not), the recursive resolver SHOULD request a session resumption
ticket from the authoritative server. If the authoritative server
supplies a resumption ticket, the recursive resolver pushes it into
the stack at E-resumptions[X].
4.6.3.3. Server Name Indication
For modern encrypted transports like TLS 1.3, most client
implementations expect to send a Server Name Indication (SNI) in the
Client Hello.
There are two complications with selecting or sending SNI in this
unilateral probing:
* Some authoritative servers are known by more than one name;
selecting a single name to use for a given connection may be
difficult or impossible.
* In most configurations, the contents of the SNI field is exposed
on the wire to a passive adversary. This potentially reveals
additional information about which query is being made, based on
the NS of the query itself.
To avoid additional leakage and complexity, a recursive resolver
following this guidance SHOULD NOT send SNI to the authoritative when
attempting encrypted transport.
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If the recursive resolver needs to send SNI to the authoritative for
some reason not found in this document, it is RECOMMENDED that it
implements Encrypted Client Hello ([I-D.ietf-tls-esni]) to reduce
leakage.
4.6.3.4. Authoritative Server Authentication
Because this probing policy is unilateral and opportunistic, the
client connecting under this policy MUST accept any certificate
presented by the server. If the client cannot verify the server's
identity, it MAY use that information for reporting, logging, or
other analysis purposes. But it MUST NOT reject the connection due
to the authentication failure, as the result would be falling back to
cleartext, which would leak the content of the session to a passive
network monitor.
4.6.4. Establishing an Encrypted Transport Connection
When an encrypted transport connection actually completes (e.g., the
TLS handshake completes) at time T1, the resolver sets E-completed[X]
to T1 and does the following:
If the handshake completed successfully:
* update E-session[X] so that it is in state established
* set E-status[X] to success
* set E-last-response[X] to T1
* set E-completed[X] to T1
* for each query Q in E-queries[X]:
- if early data was accepted and Q is early,
o set the status of Q to sent
- otherwise:
o send Q through the session (see Section 4.6.8), and set the
status of Q to sent
4.6.5. Failing to Establish an Encrypted Transport Connection
If, at time T2 an encrypted transport handshake completes with a
failure (e.g. a TLS alert),
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* set E-session[X] to null
* set E-status[X] to fail
* set E-completed[X] to T2
* for each query Q in E-queries[X]:
- if Q is not present in any other *-queries[X] or in
Do53-queries[X], add Q to Do53-queries[X] and send query Q to X
over Do53.
Note that this failure will trigger the recursive resolver to fall
back to cleartext queries to the authoritative server at IP address
X. It will retry encrypted transport to X once the damping timer has
elapsed.
4.6.6. Encrypted Transport Failure
Once established, an encrypted transport might fail for a number of
reasons (e.g., decryption failure, or improper protocol sequence).
If this happens:
* set E-session[X] to null
* set E-status[X] to fail
* for each query Q in E-queries[X]:
- if Q is not present in any other *-queries[X] or in
Do53-queries[X], add Q to Do53-queries[X] and send query Q to X
over Do53.
Note that this failure will trigger the recursive resolver to fall
back to cleartext queries to the authoritative server at IP address
X. It will retry encrypted transport to X once the damping timer has
elapsed.
For example, What if a TCP timeout closes an idle DoT connection?
What if a QUIC stream ends up timing out but other streams on the
same QUIC connection are going through? Do the described scenarios
cover the case when an encrypted transport's port is made
unavailable/closed?
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4.6.7. Handling Clean Shutdown of an Encrypted Transport Connection
At time T3, the recursive resolver may find that authoritative server
X cleanly closes an existing outstanding connection (most likely due
to resource exhaustion, see Section 3.4).
When this happens:
* set E-session[X] to null
* for each query Q in E-queries[X]:
- if Q is not present in any other *-queries[X] or in
Do53-queries[X], add Q to Do53-queries[X] and send query Q to X
over Do53.
Note that this premature shutdown will trigger the recursive resolver
to fall back to cleartext queries to the authoritative server at IP
address X. Any subsequent query to X will retry the encrypted
connection promptly.
4.6.8. Sending a Query over Encrypted Transport
When sending a query to an authoritative server over encrypted
transport at time T4, the recursive resolver should take a few
reasonable steps to ensure privacy and efficiency.
After sending query Q, the recursive resolver should ensure that Q's
state in E-queries[X] is set to sent.
The recursive resolver also sets E-last-activity[X] to T4.
In addition, the recursive resolver should consider the guidance in
the following sections.
4.6.8.1. Pad Queries to Mitigate Traffic Analysis
To increase the anonymity set for each query, the recursive resolver
SHOULD use a sensible padding mechanism for all queries it sends.
Specifically, a DoT client SHOULD use EDNS(0) padding [RFC7830], and
a DoQ client SHOULD follow the guidance in Section 5.4 of [RFC9250].
How much to pad is out of scope of this document, but a reasonable
suggestion can be found in [RFC8467].
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4.6.8.2. Send Queries in Separate Channels
When multiple queries are multiplexed on a single encrypted transport
to a single authoritative server, the recursive resolver SHOULD
pipeline queries and MUST be capable of receiving responses out of
order. For guidance on how to best achieve this on a given encrypted
transport, see [RFC7766] (for DoT) and [RFC9250] (for DoQ).
4.6.9. Receiving a Response over Encrypted Transport
When a response R for query Q arrives at the recursive resolver over
encrypted transport E from authoritative server with IP address X at
time T5, the recursive resolver should:
If Q is not in E-queries[X]:
* Discard R and process it no further (do not respond to a encrypted
response to a query that is not outstanding)
Otherwise:
* Remove Q from E-queries[X]
* Set E-last-activity[X] to T5
* Set E-last-response[X] to T5
If R is successful:
* Return R to the requesting client
* For each supported encrypted transport N other than E:
- If Q is in N-queries[X]:
o Remove Q from N-queries[X]
* If Q is in Do53-queries[X]:
- Remove Q from Do53-queries[X]
But if R is unsuccessful (e.g. SERVFAIL):
* If Q is not in Do53-queries[X] or in any of *-queries[X]:
- Return SERVFAIL to the requesting client
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4.6.10. Resource Exhaustion
To keep resources under control, a recursive resolver should
proactively manage outstanding encrypted connections. Section 6.5 of
[RFC9250] ("Connection Handling") offers useful guidance for clients
managing DoQ connections. Section 3.4 of [RFC7858] offers useful
guidance for clients managing DoT connections.
Even with sensible connection management, a recursive resolver doing
unilateral probing may find resources unexpectedly scarce, and may
need to close some outstanding connections.
In such a situation, the recursive resolver SHOULD use a reasonable
prioritization scheme to close outstanding connections.
One reasonable prioritization scheme would be:
* close outstanding established sessions based on E-last-activity[X]
(oldest timestamp gets closed first)
Note that when resources are limited, a recursive resolver following
this guidance may also choose not to initiate new connections for
encrypted transport.
4.6.11. Maintaining Connections
Some recursive resolvers looking to amortize connection costs, and to
minimize latency MAY choose to synthesize queries to a particular
resolver to keep a encrypted transport session active.
A recursive resolver that adopts this approach should try to align
the synthesized queries with other optimizations. For example, a
recursive resolver that "pre-fetches" a particular resource record to
keep its cache "hot" can send that query over an established
encrypted transport session.
4.6.12. Additional Tuning
A recursive resolver's state table for an authoritative server can
contain additional information beyond what is described above. The
recursive resolver might use that additional state to change the way
it interacts with the authoritative server in the future. Some
examples of additional state include:
* Whether the server accepts "early data" over a transport such as
DoQ
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* Whether the server fails to respond to queries after the handshake
succeeds:
* Track the RTT (round trip time) of queries to the server
* Track the number of timeouts (compared to the number of successful
queries)
5. IANA Considerations
IANA does not need to do anything for implementers to adopt the
guidance found in this document.
6. Privacy Considerations
6.1. Server Name Indication
A recursive resolver querying an authoritative server over DoT or DoQ
that sends Server Name Indication (SNI) in the clear in the
cryptographic handshake leaks information about the intended query to
a passive network observer.
In particular, if two different zones refer to the same nameserver IP
addresses via differently-named NS records, a passive network
observer can distinguish queries to one zone from the queries to the
other.
Omitting SNI entirely, or using Encrypted Client Hello to hide the
intended SNI, avoids this additional leakage. However, a series of
queries that leak this information is still an improvement over the
all-cleartext status quo at the time of this document.
6.2. Modelling Probability of Encryption
Given that there are many parameter choices that can be made by
recursive resolvers and authoritative servers, it is reasonable to
ask what is the probability that queries are being encrypted. The
resulting measurement would also certainly be affected by the types
of queries being sent by the recursive resolver, which in turn is
also related to the types of queries that are sent to the recursive
resolver by the stub resolvers and forwarders downstream. Doing this
type of research would be valuable to the DNS community after initial
implementation by a variety of recursive resolvers and authoritative
servers because it would help assess the value of implementing the
protocol.
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7. Security Considerations
The guidance in this document provides defense against passive
network monitors for most queries. It does not defend against active
attackers. It can also leak some queries and their responses due to
"happy eyeballs" optimizations when the resolver's cache is cold.
Implementation of the guidance in this document should increase
deployment of opportunistic encrypted DNS transport between recursive
resolvers and authoritative servers at little operational risk.
However, implementers cannot rely on the guidance in this document
for robust defense against active attackers, but should treat it as a
stepping stone en route to stronger defense.
In particular, a recursive resolver following this guidance can
easily be forced by an active attacker to fall back to cleartext DNS
queries. Or, an active attacker could position itself as a machine-
in-the-middle, which the recursive resolver would not defend against
or detect due to lack of server authentication. Defending against
these attacks without risking additional unexpected protocol failures
would require signalling and coordination that are out of scope for
this document.
This guidance is only one part of operating a privacy-preserving DNS
ecosystem. A privacy-preserving recursive resolver should adopt
other practices as well, such as QNAME minimization ([RFC9156]),
local root zone ([RFC8806]), etc, to reduce the overall leakage of
query information that could infringe on the client's privacy.
8. Acknowledgements
Many people contributed to the development of this document beyond
the authors, including Alexander Mayrhofer, Brian Dickson, Christian
Huitema, Eric Nygren, Jim Reid, Kris Shrishak, Ralf Weber, Robert
Evans, Sara Dickinson, and the DPRIVE working group.
9. References
9.1. 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/rfc/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/rfc/rfc8174>.
[RFC9250] Huitema, C., Dickinson, S., and A. Mankin, "DNS over
Dedicated QUIC Connections", RFC 9250,
DOI 10.17487/RFC9250, May 2022,
<https://www.rfc-editor.org/rfc/rfc9250>.
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/rfc/rfc7858>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/rfc/rfc7301>.
9.2. Informative References
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, <https://www.rfc-editor.org/rfc/rfc7435>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/rfc/rfc1035>.
[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/rfc/rfc8484>.
[RFC7830] Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830,
DOI 10.17487/RFC7830, May 2016,
<https://www.rfc-editor.org/rfc/rfc7830>.
[RFC8467] Mayrhofer, A., "Padding Policies for Extension Mechanisms
for DNS (EDNS(0))", RFC 8467, DOI 10.17487/RFC8467,
October 2018, <https://www.rfc-editor.org/rfc/rfc8467>.
[RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/rfc/rfc8305>.
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[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-15, 3 October 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
esni-15>.
[RFC7766] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
D. Wessels, "DNS Transport over TCP - Implementation
Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,
<https://www.rfc-editor.org/rfc/rfc7766>.
[RFC9156] Bortzmeyer, S., Dolmans, R., and P. Hoffman, "DNS Query
Name Minimisation to Improve Privacy", RFC 9156,
DOI 10.17487/RFC9156, November 2021,
<https://www.rfc-editor.org/rfc/rfc9156>.
[RFC8806] Kumari, W. and P. Hoffman, "Running a Root Server Local to
a Resolver", RFC 8806, DOI 10.17487/RFC8806, June 2020,
<https://www.rfc-editor.org/rfc/rfc8806>.
[MTA-STS] Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
and J. Jones, "SMTP MTA Strict Transport Security (MTA-
STS)", RFC 8461, DOI 10.17487/RFC8461, September 2018,
<https://www.rfc-editor.org/rfc/rfc8461>.
[DANE-SMTP]
Dukhovni, V. and W. Hardaker, "SMTP Security via
Opportunistic DNS-Based Authentication of Named Entities
(DANE) Transport Layer Security (TLS)", RFC 7672,
DOI 10.17487/RFC7672, October 2015,
<https://www.rfc-editor.org/rfc/rfc7672>.
[TLSRPT] Margolis, D., Brotman, A., Ramakrishnan, B., Jones, J.,
and M. Risher, "SMTP TLS Reporting", RFC 8460,
DOI 10.17487/RFC8460, September 2018,
<https://www.rfc-editor.org/rfc/rfc8460>.
[DNS-Error-Reporting]
Arends, R. and M. Larson, "DNS Error Reporting", Work in
Progress, Internet-Draft, draft-ietf-dnsop-dns-error-
reporting-04, 3 February 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-
dns-error-reporting-04>.
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[RFC9102] Dukhovni, V., Huque, S., Toorop, W., Wouters, P., and M.
Shore, "TLS DNSSEC Chain Extension", RFC 9102,
DOI 10.17487/RFC9102, August 2021,
<https://www.rfc-editor.org/rfc/rfc9102>.
Appendix A. Defense Against Active Attackers
The protocol described in this document provides no defense against
active attackers. A future protocol for recursive-to-authoritative
DNS might want to provide such protection.
This appendix assumes that the use case for that future protocol is a
recursive resolver that wants to prevent an active attack on
communication between it and an authoritative server that has
committed to offering encrypted DNS transport. An inherent part of
this use case is that the recursive resolver would want to respond
with a SERVFAIL response to its client if it cannot make an
authenticated encrypted connection to any of the authoritative
nameservers for a name.
However, an authoritative server that merely offers encrypted
transport (for example, by following the guidance in Section 3) has
made no such commitment, and no recursive resolver that prioritizes
delivery of DNS records to its clients would want to "fail closed"
unilaterally.
So such a future protocol would need at least three major
distinctions from the protocol described in this document:
* A signaling mechanism that tells the resolver that the
authoritative server intends to offer authenticated encryption
* Authentication of the authoritative server
* A way to combine defense against an active attacker with the
defenses described in this document
This can be thought of as a DNS analog to [MTA-STS] or [DANE-SMTP].
A.1. Signalling Mechanism Properties
To defend against an active attacker, the signalling mechanism needs
to be able to indicate that the recursive resolver should "fail
closed" if it cannot authenticate the server for a particular query.
The signalling mechanism itself would have to be resistant to
downgrade attacks from active attackers.
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One open question is how such a signal should be scoped. While this
document scopes opportunistic state about encrypted transport based
on the IP addresses of the client and server, signalled intent to
offer encrypted transport is more likely to be scoped by queried zone
in the DNS, or by nameserver name than by IP address.
A reasonable authoritative server operator or zone administrator
probably doesn't want to risk breaking anything when they first
enable the signal. Therefore, a signalling mechanism should probably
also offer a means to report problems to the authoritative server
operator without the client failing closed. Such a mechanism is
likely to be similar to [TLSRPT] or [DNS-Error-Reporting].
A.2. Authentication of Authoritative Server
Forms of server authentication might include:
* an X.509 Certificate issued by a widely-known certification
authority associated with the common NS names used for this
authoritative server
* DANE authentication (to avoid infinite recursion, the DNS records
necessary to authenticate could be transmitted in the TLS
handshake using the DNSSEC Chain Extension (see [RFC9102]))
A recursive resolver would have to verify the server's identity.
When doing so, the identity would presumably be based on the NS name
used for a given query or the IP address of the server.
A.3. Combining Protocols
If this protocol gains reasonable adoption, and a newer protocol that
can offer defense against an active attacker were available,
deployment is likely to be staggered and incomplete. This means that
an operator that want to maximize confidentiality for their users
will want to use both protocols together.
Any new stronger protocol should consider how it interacts with the
opportunistic protocol defined here, so that operators are not faced
with the choice between widespread opportunistic protection against
passive attackers (this document) and more narrowly-targeted
protection against active attackers.
Appendix B. Document Considerations
[ RFC Editor: please remove this section before publication ]
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B.1. Document History
B.1.1. Substantive Changes from -03 to -04
* Clarified that "completed handshake" in Table 2 also includes
resumed sessions.
* In Section 4.6.1, specified not to use Do53 even when the last
successful connection is far in the past.
* In Section 4.6.3, clarified that an established connection in the
established state should not be resumed prematurely.
B.1.2. Substantive Changes from -02 to -03
* Added an Additional Tuning section on recursive resolvers
recording other data about an authoritative server's capabilities
for future interactions (thank you again Sara Dickinson!).
Feedback from operators on how the extra information would be used
by a recursive resolver that retains such an expanded state table
is particularly welcome.
* Added more text about sharing session state information.
* Added section on modelling the probability of encryption as a
future task.
B.1.3. Substantive Changes from -01 to -02
* Removed EDNS Client Subnet recommendations from the probing
policy: recursive resolvers implementing the guidance provided in
this draft intend to enhance privacy for their users' queries, and
although ECS is a valuable feature, it represents a privacy risk.
Therefore, a future document encompassing a "how to add privacy"
approach could serve for better discussion on this discrepancy
(thank you Puneet Sood!).
* Added text on padding queries and responses to mitigate traffic
analysis (thank you Sara Dickinson!).
* Put draft on standards track
B.1.4. Substantive Changes from -00 to -01
* Moved discussion of non-opportunistic encryption to an appendix
* Clarify state transitions when sending over encrypted transport
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* Introduced new field E-last-response[X] for comparison with
persistence
B.1.5. Substantive Changes from -01 to -02 (now draft-ietf-dprive-
unilateral-probing-00)
* Clarify that deployment to a pool does not need to be strictly
simultaneous
* Explain why authoritatives need to serve the same records
regardless of SNI
* Defer to external, protocol-specific references for resource
management
* Clarify that probed connections must not fail due to
authentication failure
B.1.6. draft-dkgjsal-dprive-unilateral-probing Substantive Changes from
-00 to -01
* Fallback to cleartext when encrypted transport fails.
* Reduce default timeout to 4s
* Clarify SNI guidance: OK for selecting server credentials, not OK
for changing answers
* Document ALPN and port numbers
* Justify sorting recursive resolver state by authoritative IP
address
Authors' Addresses
Daniel Kahn Gillmor (editor)
American Civil Liberties Union
125 Broad St.
New York, NY, 10004
United States of America
Email: dkg@fifthhorseman.net
Joey Salazar (editor)
Alajuela
20201
Costa Rica
Email: joeygsal@gmail.com
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Paul Hoffman (editor)
ICANN
United States of America
Email: paul.hoffman@icann.org
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