Internet DRAFT - draft-dkgjsal-dprive-unilateral-probing
draft-dkgjsal-dprive-unilateral-probing
dprive D.K. Gillmor
Internet-Draft ACLU
Intended status: Informational J. Salazar
Expires: 30 July 2022 A19
26 January 2022
Unilateral Opportunistic Deployment of Encrypted Recursive-to-
Authoritative DNS
draft-dkgjsal-dprive-unilateral-probing-02
Abstract
This draft 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 draft can be defeated by
an active attacker, but should be simpler and less risky to deploy
than more powerful defenses. The draft also introduces (but does not
try to specify) the semantics of signalling that would permit defense
against an active attacker.
The goal of this draft 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.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 30 July 2022.
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Copyright Notice
Copyright (c) 2022 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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Priorities . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Minimizing Negative Impacts . . . . . . . . . . . . . . . 4
2.2. Protocol Choices . . . . . . . . . . . . . . . . . . . . 4
3. Guidance for Authoritative Servers . . . . . . . . . . . . . 5
3.1. Pooled Authoritative Servers Behind a Single IP
Address . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Authentication . . . . . . . . . . . . . . . . . . . . . 5
3.3. Server Name Indication . . . . . . . . . . . . . . . . . 6
3.4. Resource Exhaustion . . . . . . . . . . . . . . . . . . . 6
4. Guidance for recursive resolvers . . . . . . . . . . . . . . 7
4.1. Overall recursive resolver Settings . . . . . . . . . . . 7
4.2. Recursive Resolver Requirements . . . . . . . . . . . . . 8
4.3. Authoritative Server Encrypted Transport Connection
State . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.3.1. Separate State for Each of the Recursive Resolver's Own
IP Addresses . . . . . . . . . . . . . . . . . . . . 10
4.4. Maintaining Authoritative State by IP Address . . . . . . 10
4.5. Probing Policy . . . . . . . . . . . . . . . . . . . . . 11
4.5.1. Sending a query over Do53 . . . . . . . . . . . . . . 11
4.5.2. Receiving a response over Do53 . . . . . . . . . . . 12
4.5.3. Initiating a connection over encrypted transport . . 12
4.5.4. Establishing an encrypted transport connection . . . 14
4.5.5. Failing to establish an encrypted transport
connection . . . . . . . . . . . . . . . . . . . . . 15
4.5.6. Encrypted transport failure . . . . . . . . . . . . . 15
4.5.7. Handling clean shutdown of encrypted transport
connection . . . . . . . . . . . . . . . . . . . . . 16
4.5.8. Sending a query over encrypted transport . . . . . . 17
4.5.9. Receiving a response over encrypted transport . . . . 17
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4.5.10. Resource Exhaustion . . . . . . . . . . . . . . . . . 18
4.5.11. Maintaining connections . . . . . . . . . . . . . . . 19
5. Signalling for Stronger Defense . . . . . . . . . . . . . . . 19
5.1. Combining Signals with Opportunistic Probing . . . . . . 20
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 20
7.1. Server Name Indication . . . . . . . . . . . . . . . . . 20
8. Security Considerations . . . . . . . . . . . . . . . . . . . 20
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
10.1. Normative References . . . . . . . . . . . . . . . . . . 21
10.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Document Considerations . . . . . . . . . . . . . . 22
A.1. Document History . . . . . . . . . . . . . . . . . . . . 23
A.1.1. Substantive changes from -01 to -02 . . . . . . . . . 23
A.1.2. Substantive changes from -00 to -01 . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
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] and [RFC8174]) when, and only when, they appear in all
capitals, as shown here.
1.2. Terminology
* "unilateral" means capable of opportunistic probing deployment
without external coordination with any of the other parties
* Do53 refers to traditional cleartext DNS over port 53 ([RFC1035])
* DoQ refers to DNS-over-QUIC ([I-D.ietf-dprive-dnsoquic])
* DoT refers to DNS-over-TLS ([RFC7858])
* DoH refers to DNS-over-HTTPS ([RFC8484])
* Encrypted transports refers to DoQ, DoT, and DoH collectively
2. Priorities
This document aims to provide guidance to implementers who want to
simply enable protection against passive network observers.
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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.
2.1. Minimizing Negative Impacts
It also 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 in the "second
hump" of the DNS camel, and for uninvolved third parties. The
negative impacts that we specifically try to minimize are:
* excessive bandwidth use
* excessive computational resources (CPU and memory in particular)
* amplification attacks (where DNS resolution infrastructure is
wielded as part of a DoS attack)
2.2. Protocol Choices
While this document focuses specifically on strategies used by DNS
servers, it does not go into detail on the specific protocols used,
as 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. For instance, a recursive
resolver in theory could guess the full path to a queried IP address
by trying all the URL paths that the client has in records and see if
one of those works, but even though it can be expected that this
would work 99% of the time with fewer than 100 probes, this technique
would likely incur in excessive resource consumption potentially
leading to vulnerabilities and amplification attacks. The authors of
this draft particularly welcome ideas and contributions from the
community that lead to a suitable mechanism for unilaterally probing
for DoH-capable authoritative servers, for later consideration in
this or other drafts.
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3. Guidance for Authoritative Servers
An authoritative server SHOULD implement and deploy DNS-over-TLS
(DoT) on TCP port 853.
An authoritative server MAY implement and deploy DNS-over-QUIC (DoQ)
on UDP port 853.
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 are updated to follow this guidance while
others are 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
heterogenous 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.
3.2. Authentication
For unilateral deployment, an authoritative server does not need to
offer any particular form of authentication.
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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.
Possible alternate forms of server authentication 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 (potentially including the TLS handshake)
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, as a probing recursive resolver that offers SNI might or
might not have used the right server name to get the records it's
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
[I-D.ietf-dprive-dnsoquic] ("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 unforseen resource exhaustion SHOULD
cleanly close open connections from recursive resolvers based on the
authoritative's preferred prioritization.
In the case of unanticipated resource exhaustion, a reasonable
prioritization scheme would be to close connections in this order,
until resources are back in control:
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* connections with no outstanding queries, ordered by idle time
(longest idle time gets closed first)
* 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.
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. Overall recursive resolver Settings
A recursive resolver implementing this draft must 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.
+=============+===================================+===========+
| 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.
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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
resolver'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.
4.2. 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.
A recursive resolver SHOULD implement the client side of DNS-over-TLS
(DoT). A recursive resolver MAY 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.
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).
4.3. 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.
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 | N |
| | 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 | Y |
| | attempt | |
+---------------+------------------------------------------+--------+
| completed | Timestamp of most recent completed | Y |
| | handshake | |
+---------------+------------------------------------------+--------+
| status | Enumerated value of success or fail | Y |
| | or timeout, associated with the | |
| | completed handshake | |
+---------------+------------------------------------------+--------+
| resumptions | A stack of resumption tickets (and | Y |
| | associated parameters) that could be | |
| | used to resume a prior successful | |
| | connection | |
+---------------+------------------------------------------+--------+
| queries | A queue of queries intended for this | N |
| | authoritative server, each of which | |
| | has additional status early, unsent, | |
| | or sent | |
+---------------+------------------------------------------+--------+
| last-activity | A timestamp of the most recent | N |
| | 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.
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.
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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.
4.3.1. 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 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).
4.4. 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 draft encourages the first strategy, to minimize timeouts or
accidental delays.
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 heterogenous 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
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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 4.3.1 and Section 3.1).
4.5. 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 know to 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 corresponding on the relevant transport.
4.5.1. 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 (T - E-completed[X]) < persistence
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].
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4.5.2. Receiving a response over Do53
When a successful response R is received in cleartext from
authoritative server X for a query Q that was sent over Do53, the
recursive resolver should:
* If Q is in Do53-queries[X]:
- Return R to the requesting client
* Remove Q from Do53-queries[X]
* 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):
* If Q is in Do53-queries[X]:
- Remove Q from Do53-queries[X]
* if Q is not in any of *-queries[X]:
- Return SERVFAIL to the client
4.5.3. Initiating a connection over encrypted transport
If any E-session[X] is in the established, the recursive resolver
SHOULD NOT initiate a new connection to X over any other transport,
but should instead send a query through the existing session (see
Section 4.5.8). FIXME: What if there's a preferred transport, but
the established session does not correspond to that preferred
transport?
Otherwise, 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
* T - E-initiated[X] > E-timeout, then
- set E-session[X] to null and
- set E-status[X] to timeout
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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 (T - 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
* 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).
4.5.3.1. 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 4.5.8.
If it does so, the status of Q in E-queries[X] should be set to early
instead of unsent.
4.5.3.2. 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].
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4.5.3.3. 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.
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.
4.5.3.4. Authoritative Server Authentication
A recursive resolver following this guidance MAY attempt to verify
the server's identity by X.509 certificate or DANE. When doing so,
the identity would presumably be based on the NS name used for a
given query.
However, since 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.
4.5.4. 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:
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If the handshake completed successfully:
* update E-session[X] so that it is in state established
* set E-status[X] to success
* 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 4.5.8), and set the
status of Q to sent
* set E-last-activity[X] to T1
4.5.5. Failing to establish an encrypted transport connection
If, at time T2 an encrypted transport handshake completes with a
failure (e.g. a TLS alert),
* 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.
4.5.6. 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:
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* 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. FIXME: should a resumption ticket be used here for
this previously successful connection?
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.
FIXME: are there specific forms of failure that we might handle
differently? 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?
4.5.7. Handling clean shutdown of 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.
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4.5.8. 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.
When sending query Q, the recursive resolver should ensure that its
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 following
guidance:
4.5.8.1. Avoid EDNS client subnet
To protect the privacy of the client, the recursive resolver SHOULD
NOT send EDNS(0) Client Subnet information to the authoritative
server ([RFC7871]) unless explicitly authorized to do so by the
client.
4.5.8.2. Pad to standard policy
To increase the anonymity set for each query, the recursive resolver
SHOULD use EDNS(0) padding according to policies described in
[RFC8467].
4.5.8.3. Send queries in separate channels
When multiple queries are multiplexed on a single encrypted transport
to a single authoritative server, the recursive resolver MUST offer
distinct query ID fields for every outstanding query on a connection,
and MUST be capable of receiving responses out of order.
To the extent that the encrypted transport can avoid head-of-line
blocking (e.g. QUIC can use a separate stream per query) the
recursive resolver SHOULD avoid head-of-line blocking.
4.5.9. 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, if Q is in E-queries[X], the recursive resolver takes the
following steps:
* Remove R from E-queries[X]
* Set E-last-activity[X] to T5
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* If R is successful:
- send R to the requesting client
- For each supported encrypted transport N other than E:
o If Q is in N-queries[X]:
+ Remove Q from N-queries[X]
- If Q is in Do53-queries[X]:
o Remove Q from Do53-queries[X]
* Otherwise (R is unsuccessful, e.g., SERVFAIL):
- If Q is not in Do53-queries[X] or any other *-queries[X]:
o Return SERVFAIL to the requesting client FIXME: What
response should be sent to the clients in the case that
extended DNS errors are used in an authoritative's response?
4.5.10. Resource Exhaustion
To keep resources under control, a recursive resolver should
proactively manage outstanding encrypted connections. Section 6.5 of
[I-D.ietf-dprive-dnsoquic] ("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 managment, 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.
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4.5.11. 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.
5. Signalling for Stronger Defense
This draft _does not_ contemplate the specification of any form of
coordinated signalling between authoritative servers and recursive
resolvers, as such measures would not be unilateral.
However, the draft highlights the needs of a signaling mechanism for
stronger defense.
We highlight the following questions for other specifications to
solve:
* What does the signal need to contain?
- type of transport? (DoQ? DoT? DoH?)
- error reporting if secure, authenticated connection fails (how
to report? similar to TLSRPT?)
- whether to hard-fail if encrypted communication isn't available
- cryptographic authentication of authoritative server (e.g.
pubkeys) vs. names vs. domain?
* How should the signal be presented?
- SVCB RR or "surprising" DS RR
* How should the signal be scoped?
- per-nameserver (by NS), per-nameserver (by IP address, via in-
addr.arpa), or per-domain?
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5.1. Combining Signals with Opportunistic Probing
FIXME: How do the signals get combined with the above opportunistic
probing policy? Can we specify that without needing to specify the
signalling mechanism itself?
6. IANA Considerations
IANA does not need to do anything for implementers to adopt the
guidance found in this draft.
7. Privacy Considerations
7.1. 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 ECH 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.
8. Security Considerations
The guidance in this draft 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 draft should increase
deployment of opportunistic encrypted DNS transport between recursive
resolvers and authoritative servers at little operational risk.
However, implementers should not rely on the guidance in this draft
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-
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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 draft.
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, local root zone,
etc, to reduce the overall leakage of query information that could
infringe on the client's privacy.
9. Acknowledgements
Many people contributed to the development of this draft beyond the
authors, including Brian Dickson, Christian Huitema, Eric Nygren, Jim
Reid, Kris Shrishak, Paul Hoffman, Ralf Weber, Robert Evans, and the
DPRIVE working group.
10. References
10.1. 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/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>.
10.2. Informative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[I-D.ietf-dprive-dnsoquic]
Huitema, C., Dickinson, S., and A. Mankin, "DNS over
Dedicated QUIC Connections", Work in Progress, Internet-
Draft, draft-ietf-dprive-dnsoquic-08, 11 January 2022,
<https://www.ietf.org/archive/id/draft-ietf-dprive-
dnsoquic-08.txt>.
[I-D.ietf-tls-esni]
Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
Encrypted Client Hello", Work in Progress, Internet-Draft,
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draft-ietf-tls-esni-13, 12 August 2021,
<https://www.ietf.org/archive/id/draft-ietf-tls-esni-
13.txt>.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, <https://www.rfc-editor.org/info/rfc7435>.
[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/info/rfc7858>.
[RFC7871] Contavalli, C., van der Gaast, W., Lawrence, D., and W.
Kumari, "Client Subnet in DNS Queries", RFC 7871,
DOI 10.17487/RFC7871, May 2016,
<https://www.rfc-editor.org/info/rfc7871>.
[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/info/rfc8305>.
[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/info/rfc8467>.
[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>.
Appendix A. Document Considerations
[ RFC Editor: please remove this section before publication ]
This document is currently edited as markdown. Minor editorial
changes can be suggested via merge requests at
https://gitlab.com/dkg/dprive-unilateral-probing or by e-mail to the
editor. Please direct all significant commentary to the public IETF
DPRIVE mailing list: dprive@ietf.org
The authors' latest draft can be read online in html
(https://dkg.gitlab.io/dprive-unilateral-probing/) or pdf
(https://dkg.gitlab.io/dprive-unilateral-probing/unilateral-
probing.pdf) or text (https://dkg.gitlab.io/dprive-unilateral-
probing/unilateral-probing.txt) formats.
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A.1. Document History
A.1.1. Substantive changes from -01 to -02
* 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
A.1.2. 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
American Civil Liberties Union
125 Broad St.
New York, NY, 10004
United States of America
Email: dkg@fifthhorseman.net
Joey Salazar
ARTICLE 19
108-114 Golden Lane
London
EC1Y 0TL
United Kingdom
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Email: joeygsal@gmail.com
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