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Internet Engineering Task Force (IETF) D. K. Gillmor, Ed.
Request for Comments: 9539 ACLU
Category: Experimental J. Salazar, Ed.
ISSN: 2070-1721
P. Hoffman, Ed.
ICANN
February 2024
Unilateral Opportunistic Deployment of Encrypted
Recursive-to-Authoritative DNS
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 protections provided by the guidance in
this document can be defeated by an active attacker, but they should
be simpler and less risky to deploy than more powerful defenses.
The goal of this document is to simplify and speed up deployment of
opportunistic encrypted transport in the recursive-to-authoritative
hop of the DNS ecosystem. Wider easy deployment of the underlying
encrypted transport on an opportunistic basis may facilitate the
future specification of stronger cryptographic protections against
more-powerful attacks.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are candidates for any level of
Internet Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9539.
Copyright Notice
Copyright (c) 2024 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/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Requirements Language
1.2. Terminology
2. Priorities
2.1. Minimizing Negative Impacts
2.2. Protocol Choices
3. Guidance for Authoritative Servers
3.1. Pooled Authoritative Servers behind a Load Balancer
3.2. Authentication
3.3. Server Name Indication
3.4. Resource Exhaustion
3.5. Pad Responses to Mitigate Traffic Analysis
4. Guidance for Recursive Resolvers
4.1. High-Level Overview
4.2. Maintaining Authoritative State by IP Address
4.3. Overall Recursive Resolver Settings
4.4. Recursive Resolver Requirements
4.5. Authoritative Server Encrypted Transport Connection State
4.6. Probing Policy
4.6.1. Sending a Query over Do53
4.6.2. Receiving a Response over Do53
4.6.3. Initiating a Connection over Encrypted Transport
4.6.4. Establishing an Encrypted Transport Connection
4.6.5. Failing to Establish an Encrypted Transport Connection
4.6.6. Encrypted Transport Failure
4.6.7. Handling Clean Shutdown of an Encrypted Transport
Connection
4.6.8. Sending a Query over Encrypted Transport
4.6.9. Receiving a Response over Encrypted Transport
4.6.10. Resource Exhaustion
4.6.11. Maintaining Connections
4.6.12. Additional Tuning
5. IANA Considerations
6. Privacy Considerations
6.1. Server Name Indication
6.2. Modeling the Probability of Encryption
7. Security Considerations
8. Operational Considerations
9. References
9.1. Normative References
9.2. Informative References
Appendix A. Assessing the Experiment
Appendix B. Defense against Active Attackers
B.1. Signaling Mechanism Properties
B.2. Authentication of Authoritative Servers
B.3. Combining Protocols
Acknowledgements
Authors' Addresses
1. Introduction
This document aims to provide guidance to DNS implementers and
operators 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]), that is,
encrypting things that would otherwise be in the clear, without
interfering with or weakening stronger forms of security.
This document also briefly introduces (but does not try to specify)
how a future protocol might permit defense against an active attacker
in Appendix B.
The protocol described here offers three concrete advantages to the
DNS ecosystem:
* Protection from passive attackers of DNS queries in transit
between recursive and authoritative servers.
* A road map 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 without external
coordination with any of the other parties.
Do53: DNS over port 53 ([RFC1035]) for traditional cleartext
transport.
DoQ: DNS over QUIC ([RFC9250]).
DoT: DNS over TLS ([RFC7858]).
Encrypted transports: DoQ and DoT, collectively.
2. Priorities
The protocol described in this document was developed with two
priorities: minimizing negative impacts and retaining flexibility in
the underlying encrypted transport protocol.
2.1. Minimizing Negative Impacts
The protocol described in this document aims to minimize potentially
negative impacts caused by the probing of encrypted transports for
the systems that adopt the protocol, for the parties that those
systems communicate with, and for uninvolved third parties. The
negative impacts that this protocol specifically tries to minimize
are:
* excessive bandwidth use,
* excessive use of computational resources (CPU and memory in
particular), and
* 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. The DoT specification ([RFC7858]) says that it:
| ...focuses on securing stub-to-recursive traffic, as per the
| charter of the DPRIVE Working Group. It does not prevent future
| applications of the protocol to recursive-to-authoritative
| traffic.
It further says:
| It might work equally between recursive clients and authoritative
| servers...
The DoQ specification ([RFC9250]) says:
| For the recursive to authoritative scenario, authentication
| requirements are unspecified at the time of writing and are the
| subject of ongoing work in the DPRIVE WG.
The protocol described in this document specifies the use of DoT and
DoQ without authentication of the server.
This document does not pursue the use of DNS over HTTPS, commonly
called "DoH" ([RFC8484]), in this context because a DoH client needs
to know the path part of a DoH endpoint URL. Currently, there are no
mechanisms for a DNS recursive resolver to predict the path on its
own, in an opportunistic or unilateral fashion, without incurring an
excessive use of resources. If such mechanisms are later defined,
the protocol in this document can be updated to accommodate them.
3. Guidance for Authoritative Servers
The protocol described in this document is OPTIONAL for authoritative
servers. An authoritative server choosing to implement the protocol
described in this document MUST implement at least one of either DoT
or DoQ on port 853.
An authoritative server choosing to implement the protocol described
in this document MAY require clients to use Application-Layer
Protocol Negotiation (ALPN) (see [RFC7301]). The ALPN strings the
client will use are given in Section 4.4.
An authoritative server implementing DoT or DoQ MUST populate the
response from the same authoritative zone data as the unencrypted DNS
transports. Encrypted transports have their own characteristic
response size that might be different from the unencrypted DNS
transports, so response sizes and related options (e.g., Extension
Mechanisms for DNS (EDNS0)) and flags (e.g., the TrunCation (TC) bit)
might vary based on the transport. In other words, the content of
the responses to a particular query MUST be the same regardless of
the type of transport.
3.1. Pooled Authoritative Servers behind a Load Balancer
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 and
interacting with such a pool likely does not know that it is a pool.
If some members of the pool follow the protocol specified in this
document 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:
* ensure that all members of the pool enable the same encrypted
transport(s) within the span of a few seconds (such as within 30
seconds), or
* ensure that the load balancer maps client requests to pool members
based on client IP addresses, or
* use a load balancer that forwards queries/connections on encrypted
transports to only those members of the pool known (e.g., via
monitoring) to support the given encrypted transport.
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.
One simple deployment approach would just be to provide a self-
issued, regularly updated X.509 certificate. Whether the
certificates used are short-lived or long-lived is up to the
deployment. This mechanism is supported by many TLS and QUIC clients
and will be acceptable for any opportunistic connection. The server
could provide a normal PKI-based certificate, but there is no
advantage to doing so at this time.
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 an 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 5.5 of
[RFC9250] 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 server'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. The ability for the authoritative server to add
padding might be limited, such as by not receiving an EDNS0 option in
the query. Specifically, a DoT server SHOULD use EDNS0 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 [RFC8467].
4. Guidance for Recursive Resolvers
The protocol described in this document is OPTIONAL 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 delays.
4.1. High-Level Overview
In addition to querying on Do53, the recursive resolver will try DoT,
DoQ, or both 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 encrypted transport
protocol that was recently shown to be good.
If the encrypted transport protocol fails, the recursive resolver
falls back to Do53 for that serverIP. When any encrypted transport
fails, the recursive resolver remembers that failure for a reasonable
amount of time to avoid flooding an incompatible server with requests
that it cannot accept. The description of how an encrypted transport
protocol fails is in Section 4.6.4 and the sections following that.
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, and does not describe the other two strategies.
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 destination port unreachable message, such as
during rate limiting).
By associating the state with the authoritative IP address, the
client can minimize the number of accidental delays introduced (see
also Sections 3.1 and 4.5).
For example, consider an authoritative server named ns0.example.com
that is served by two installations: one at 2001:db8::7 that follows
this guidance and one at 2001:db8::8 that is a legacy (cleartext port
53-only) deployment. A recursive client who associates state with
the NS name and reaches 2001:db8::7 first will "learn" that
ns0.example.com supports encrypted transport. A subsequent query
over encrypted transport dispatched to 2001:db8::8 would fail,
potentially delaying the response.
4.3. Overall Recursive Resolver Settings
A recursive resolver implementing the protocol in 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.
+=============+==================================+===========+
| Name | Description | Suggested |
| | | Default |
+=============+==================================+===========+
| persistence | How long the recursive resolver | 3 days |
| | remembers a successful encrypted | (259200 |
| | transport connection | seconds) |
+-------------+----------------------------------+-----------+
| damping | How long the recursive resolver | 1 day |
| | remembers an unsuccessful | (86400 |
| | encrypted transport connection | seconds) |
+-------------+----------------------------------+-----------+
| timeout | How long the recursive resolver | 4 seconds |
| | waits for an initiated encrypted | |
| | connection to complete | |
+-------------+----------------------------------+-----------+
Table 1: Recursive Resolver System Parameters per
Encrypted Transport
This document uses the notation <transport>-foo to refer to the foo
parameter for the encrypted transport <transport>. 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. Additionally, when describing an arbitrary
encrypted transport, we use E in place of <transport> to generically
mean whatever encrypted transport is in use. For example, when
handling a query sent over encrypted transport E, a reference to
E-timeout should be understood to mean DoT-timeout if the query is
sent over DoT, and to mean DoQ-timeout if the query is sent over DoQ.
This document also assumes that the recursive 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 publish their preferred
values of these parameters.
4.4. Recursive Resolver Requirements
To follow the strategies in this document, 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 DoT. A recursive resolver SHOULD
implement the client side of DoQ.
DoT queries from the recursive resolver MUST target TCP port 853
using an ALPN of "dot". DoQ queries from the recursive resolver MUST
target UDP port 853 using an ALPN of "doq".
While this document focuses on the recursive-to-authoritative hop, a
recursive resolver implementing the strategies in this document
SHOULD also accept queries from its clients over some encrypted
transport unless it only accepts queries from the localhost.
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.
Note that the recursive resolver might 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 2001:db8::100 and
2001:db8::200, it could keep two distinct sets of per-authoritative-
IP state: one for each source address it uses, if the recursive
resolver knows the addresses in use. 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).
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 are dependent on the transport protocol implementation
(such as a TLS session ticket or TLS session ID, a 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:
+===============+======================================+=========+
| Name | Description | Retain |
| | | Across |
| | | Restart |
+===============+======================================+=========+
| session | The associated state of any existing | no |
| | 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 the most recent | yes |
| | connection attempt | |
+---------------+--------------------------------------+---------+
| completed | Timestamp of the most recent | yes |
| | completed handshake (which can | |
| | include one where an existing | |
| | session is resumed) | |
+---------------+--------------------------------------+---------+
| status | Enumerated value of success, 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 of 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 and
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.
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.
This document uses the notation any-E-queries to indicate any query
on an encrypted transport.
4.6. Probing Policy
When a recursive resolver discovers the need for an authoritative
lookup to an authoritative DNS server using that server's IP address
X, it retrieves the connection state records described in Section 4.5
associated with X from its cache.
Some of the subsections that follow offer pseudocode that corresponds
roughly to an asynchronous programming model for a recursive
resolver's interactions with authoritative servers. All subsections
also 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 either of the supported encrypted transports (DoT
or DoQ).
Note that a recursive 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 the method used in the document known as "Happy Eyeballs"
([RFC8305]). However, unlike Happy Eyeballs, when one transport
succeeds, the other connections do not need to be terminated; instead
they can be continued to establish whether the IP address X is
capable of communicating on the relevant transport.
4.6.1. Sending a Query over Do53
For any of the supported encrypted transports E, the recursive
resolver SHOULD NOT send a query to X over Do53 if either of the
following holds true:
* 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.
Otherwise, if there is no outstanding session for any encrypted
transport, and the last successful encrypted transport connection was
long ago, the recursive 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 any response R (a well-formed DNS response, asynchronous
timeout, asynchronous destination port unreachable, etc.) for query Q
arrives at the recursive resolver in cleartext sent over Do53 from an
authoritative server with IP address X, the recursive resolver should
perform the following.
If Q is not in Do53-queries[X]:
* process R no further (do not respond to a cleartext response to a
query that is not outstanding).
Otherwise, if Q was marked as already processed:
* remove Q from Do53-queries[X],
* discard any content from the response, and process R no further.
If R is a well-formed DNS response:
* remove Q from Do53-queries[X],
* process R further, and
* for each supported encrypted transport E:
- if Q is in E-queries[X], then
o mark Q as already processed.
However, if R is malformed or a failure (e.g., a timeout or
destination port unreachable), and
* if Q is not in any of any-E-queries[X], then
- treat this as a failed query (i.e., follow the resolver's
policy for unresponsive or non-compliant authoritatives, such
as falling back to another authoritative server, returning
SERVFAIL to the requesting client, and so on).
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 connection 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 prefers one encrypted transport over
another, but only the unpreferred encrypted transport is in the
established state, it MAY also initiate a new connection to X over
its preferred encrypted transport while concurrently sending the
query over the established encrypted transport E.
Before considering whether to initiate a new connection over an
encrypted transport, the timer should be examined, and its state
possibly refreshed, 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 recursive 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 either 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 ClientHello handshake should not try to resume any session.
When initiating a connection, the recursive 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], and
* 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 send
"early data" from the client in the initial ClientHello in some
contexts. A recursive resolver that initiates a connection over an
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
ClientHello.
There are two complications with selecting or sending an 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 are 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 an SNI to the authoritative
server when attempting encrypted transport.
If the recursive resolver needs to send an SNI to the authoritative
server for some reason not found in this document, using Encrypted
ClientHello ([TLS-ECH]) would 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; however, 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 recursive resolver sets
E-completed[X] to T1 and does the following.
If the handshake completed successfully, the recursive resolver:
* updates E-session[X] so that it is in state established,
* sets E-status[X] to success,
* sets E-last-response[X] to T1,
* sets E-completed[X] to T1, and
* for each query Q in E-queries[X]:
- if early data was accepted and Q is early, then
o sets the status of Q to sent.
- Otherwise:
o sends Q through the session (see Section 4.6.8) and sets 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):
* set E-session[X] to null,
* set E-status[X] to fail,
* set E-completed[X] to T2, and
* for each query Q in E-queries[X]:
- if Q is not present in any other any-E-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, and
* for each query Q in E-queries[X]:
- if Q is not present in any other any-E-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.
4.6.7. 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, and
* for each query Q in E-queries[X]:
- if Q is not present in any other any-E-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.
* Set E-last-activity[X] to T4.
The recursive resolver should also consider the guidance in the
following subsections.
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 EDNS0 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].
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 Section 6.2.1.1 of [RFC7766] (for DoT) and Section 5.6
of [RFC9250] (for DoQ).
4.6.9. Receiving a Response over Encrypted Transport
Even though session-level events on encrypted transports like clean
shutdown (see Section 4.6.7) or encrypted transport failure (see
Section 4.6.6) can happen, some events happen on encrypted transports
that are specific to a query and are not session-wide. This
subsection describes how the recursive resolver deals with events
related to a specific query.
When a query-specific response R (a well-formed DNS response or an
asynchronous timeout) associated with query Q arrives at the
recursive resolver over encrypted transport E from an authoritative
server with IP address X at time T5, the recursive resolver should
perform the following.
If Q is not in E-queries[X]:
* discard the response and process R no further (do not respond to
an encrypted response to a query that is not outstanding).
Otherwise:
* remove Q from E-queries[X],
* set E-last-activity[X] to T5, and
* set E-last-response[X] to T5.
If Q was marked as already processed:
* discard the response and process the response no further.
If R is a well-formed DNS response:
* process R further, and
* for each supported encrypted transport N other than E:
- if Q is in N-queries[X], then
o mark Q as already processed.
* If Q is in Do53-queries[X]:
- mark Q as already processed.
However, if R is malformed or a failure (e.g., timeout), and
* if Q is not in Do53-queries[X] or in any of any-E-queries[X], then
- treat this as a failed query (i.e., follow the resolver's
policy for unresponsive or non-compliant authoritative servers,
such as falling back to another authoritative server, returning
SERVFAIL to the requesting client, and so on).
4.6.10. Resource Exhaustion
To keep resources under control, a recursive resolver should
proactively manage outstanding encrypted connections. Section 5.5 of
[RFC9250] 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 to close outstanding
established sessions based on E-last-activity[X] (i.e, the 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
minimize latency MAY choose to synthesize queries to a particular
authoritative server to keep an 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 states include the following.
* Whether the server accepts "early data" over a transport such as
DoQ.
* Whether the server fails to respond to queries after the handshake
succeeds.
* Tracking the round-trip time of queries to the server.
* Tracking the number of timeouts (compared to the number of
successful queries).
5. IANA Considerations
This document has no IANA actions.
6. Privacy Considerations
6.1. Server Name Indication
A recursive resolver querying an authoritative server over DoT or DoQ
that sends a 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 the queries to one zone from the queries to
the other.
Omitting SNI entirely, or using Encrypted ClientHello to hide the
intended SNI, avoids this additional leakage. However, a series of
queries that leak this information is still an improvement over
cleartext.
6.2. Modeling the Probability of Encryption
Given that there are many parameter choices that can be made by
recursive resolvers and authoritative servers, it is reasonable to
consider the probability that queries would be encrypted. Such a
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 overall DNS privacy value of
implementing the protocol. Thus, it would be useful if recursive
resolvers and authoritative servers reported percentages of queries
sent and received over the different transports.
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 ([RFC8305]) when the recursive
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: they should treat it as
a stepping stone en route to stronger defense.
In particular, a recursive resolver following the guidance in this
document 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 signaling 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. Operational Considerations
As recursive resolvers implement this protocol, authoritative servers
will see more probing on port 853 of IP addresses that are associated
with NS records. Such probing of an authoritative server should
generally not cause any significant problems. If the authoritative
server is not supporting this protocol, it will not respond on port
853; if it is supporting this protocol, it will act accordingly.
However, a system that is a public recursive resolver that supports
DoT and/or DoQ may also have an IP address that is associated with NS
records. This could be accidental (such as a glue record with the
wrong target address) or intentional. In such a case, a recursive
resolver following this protocol will look for authoritative answers
to ports 53 and 853 on that IP address. Additionally, the DNS server
answering on port 853 would need to be able to differentiate queries
for recursive answers from queries for authoritative answers (e.g.,
by having the authoritative server handle all queries that have the
Recursion Desired (RD) flag unset).
As discussed in Section 7, the protocol described in this document
provides no defense against active attackers. On a network where a
captive portal is operating, some communications may be actively
intercepted (e.g., when the network tries to redirect a user to
complete an interaction with a captive portal server). A recursive
resolver operating on a node that performs captive portal detection
and Internet connectivity checks SHOULD delay encrypted transport
probes to authoritative servers until after the node's Internet
connectivity check policy has been satisfied.
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/info/rfc2119>.
[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/info/rfc7301>.
[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>.
[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>.
[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/info/rfc9250>.
9.2. 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>.
[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>.
[RFC7672] 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/info/rfc7672>.
[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/info/rfc7766>.
[RFC7830] Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830,
DOI 10.17487/RFC7830, May 2016,
<https://www.rfc-editor.org/info/rfc7830>.
[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>.
[RFC8460] 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/info/rfc8460>.
[RFC8461] 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/info/rfc8461>.
[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>.
[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/info/rfc8806>.
[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/info/rfc9102>.
[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/info/rfc9156>.
[TLS-ECH] Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
Encrypted Client Hello", Work in Progress, Internet-Draft,
draft-ietf-tls-esni-17, 9 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
esni-17>.
[DNS-ER] Arends, R. and M. Larson, "DNS Error Reporting", Work in
Progress, Internet-Draft, draft-ietf-dnsop-dns-error-
reporting-07, 17 November 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-
dns-error-reporting-07>.
Appendix A. Assessing the Experiment
This document is an Experimental RFC. In order to assess the success
of the experiment, some key metrics could be collected by the
technical community about the deployment of the protocol in this
document. These metrics will be collected in recursive resolvers,
authoritative servers, and the networks containing them. Some key
metrics include the following.
* Comparison of the CPU and memory use between Do53 and encrypted
transports.
* Comparison of the query response rates between Do53 and encrypted
transports.
* Measurement of server authentication successes and failures.
* Measurement and descriptions of observed attack traffic, if any.
* Comparison of transactional bandwidth (ingress/egress, packets per
second, bytes per second) between Do53 and encrypted transports.
Appendix B. 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.
Therefore, such a future protocol would need at least three major
distinctions from the protocol described in this document:
* A signaling mechanism that tells the recursive 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 [RFC8461] or [RFC7672].
B.1. Signaling Mechanism Properties
To defend against an active attacker, the signaling 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 signaling mechanism itself would have to be resistant to
downgrade attacks from active attackers.
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, signaled intent to
offer encrypted transport is more likely to be scoped by the queried
zone in the DNS or by the nameserver name than by the 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 signaling 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 those described in [RFC8460] or [DNS-ER].
B.2. Authentication of Authoritative Servers
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.
* DNS-Based Authentication of Named Entities (DANE) (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.
B.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 wants 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.
Acknowledgements
Many people contributed to the development of this document beyond
the authors, including Alexander Mayrhofer, Brian Dickson, Christian
Huitema, Dhruv Dhody, Eric Nygren, Erik Kline, Florian Obser, Haoyu
Song, Jim Reid, Kris Shrishak, Peter Thomassen, Peter van Dijk, Ralf
Weber, Rich Salz, Robert Evans, Sara Dickinson, Scott Hollenbeck,
Stephane Bortzmeyer, Yorgos Thessalonikefs, and the DPRIVE Working
Group.
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
Paul Hoffman (editor)
ICANN
United States of America
Email: paul.hoffman@icann.org