wdiff rfc9250v1.txt rfc9250.txt

Internet Engineering Task Force (IETF) C. Huitema
Request for Comments: 9250 Private Octopus Inc.
Category: Standards Track S. Dickinson
ISSN: 2070-1721 Sinodun IT
A. Mankin
Salesforce
May 2022

DNS over Dedicated QUIC Connections

Abstract

This document describes the use of QUIC to provide transport
confidentiality for DNS. The encryption provided by QUIC has similar
properties to those provided by TLS, while QUIC transport eliminates
the head-of-line blocking issues inherent with TCP and provides more
efficient packet-loss recovery than UDP. DNS over QUIC (DoQ) has
privacy properties similar to DNS over TLS (DoT) specified in RFC
7858, and latency characteristics similar to classic DNS over UDP.
This specification describes the use of DNS over QUIC DoQ as a general-
purpose general-purpose
transport for DNS and includes the use of DNS over QUIC DoQ for stub to recursive,
recursive to authoritative, and zone transfer scenarios.

Status of This Memo

This is an Internet Standards Track document.

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). Further information on
Internet Standards is available in 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/rfc9250.

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
2. Key Words
3. Design Considerations
3.1. Provide DNS Privacy
3.2. Design for Minimum Latency
3.3. Middlebox Considerations
3.4. No Server-Initiated Transactions
4. Specifications
4.1. Connection Establishment
4.1.1. Port Selection
4.2. Stream Mapping and Usage
4.2.1. DNS Message IDs
4.3. DoQ Error Codes
4.3.1. Transaction Cancellation
4.3.2. Transaction Errors
4.3.3. Protocol Errors
4.3.4. Alternative Error Codes
4.4. Connection Management
4.5. Session Resumption and 0-RTT
4.6. Message Sizes
5. Implementation Requirements
5.1. Authentication
5.2. Fallback to Other Protocols on Connection Failure
5.3. Address Validation
5.4. Padding
5.5. Connection Handling
5.5.1. Connection Reuse
5.5.2. Resource Management
5.5.3. Using 0-RTT and Session Resumption
5.5.4. Controlling Connection Migration for Privacy
5.6. Processing Queries in Parallel
5.7. Zone Transfer
5.8. Flow Control Mechanisms
6. Security Considerations
7. Privacy Considerations
7.1. Privacy Issues with 0-RTT data
7.2. Privacy Issues with Session Resumption
7.3. Privacy Issues with Address Validation Tokens
7.4. Privacy Issues with Long Duration Sessions
7.5. Traffic Analysis
8. IANA Considerations
8.1. Registration of a DoQ Identification String
8.2. Reservation of a Dedicated Port
8.3. Reservation of an Extended DNS Error Code: Too Early
8.4. DNS-over-QUIC Error Codes Registry
9. References
9.1. Normative References
9.2. Informative References
Appendix A. The NOTIFY Service
Acknowledgements
Authors' Addresses

1. Introduction

Domain Name System (DNS) concepts are specified in "Domain names -
concepts and facilities" [RFC1034]. The transmission of DNS queries
and responses over UDP and TCP is specified in "Domain names -
implementation and specification" [RFC1035].

This document presents a mapping of the DNS protocol over the QUIC
transport [RFC9000] [RFC9001]. DNS over QUIC is referred to here as
DoQ, in line with "DNS Terminology" [DNS-TERMS].

The goals of the DoQ mapping are:

1. Provide the same DNS privacy protection as DNS over TLS (DoT) DoT [RFC7858]. This
includes an option for the client to authenticate the server by
means of an authentication domain name as specified in "Usage
Profiles for DNS over TLS and DNS over DTLS" [RFC8310].

2. Provide an improved level of source address validation for DNS
servers compared to classic DNS over UDP.

3. Provide a transport that does not impose path MTU limitations on
the size of DNS responses it can send.

In order to achieve these goals, and to support ongoing work on
encryption of DNS, the scope of this document includes:

* the "stub to recursive resolver" scenario (also called the "stub
to recursive" scenario in this document)

* the "recursive resolver to authoritative nameserver" scenario, scenario
(also called the "recursive to authoritative" scenario in this
document), and

* the "nameserver to nameserver" scenario (mainly used for zone
transfers (XFR) [RFC1995] [RFC5936]).

In other words, this document specifies QUIC as a general-purpose
transport for DNS.

The specific non-goals of this document are:

1. No attempt is made to evade potential blocking of DNS-over-QUIC DoQ traffic by
middleboxes.

2. No attempt to support server-initiated transactions, which are
used only in DNS Stateful Operations (DSO) [RFC8490].

Specifying the transmission of an application over QUIC requires
specifying how the application's messages are mapped to QUIC streams,
and generally how the application will use QUIC. This is done for
HTTP in "Hypertext Transfer Protocol Version 3 (HTTP/3)" [HTTP/3].
The purpose of this document is to define the way DNS messages can be
transmitted over QUIC.

DNS over HTTP HTTPS (DoH) [RFC8484] can be used with HTTP/3 to get some of
the benefits of QUIC. However, a lightweight direct mapping for DNS over
QUIC DoQ
can be regarded as a more natural fit for both the recursive to
authoritative and zone transfer scenarios, which rarely involve
intermediaries. In these scenarios, the additional overhead of HTTP
is not offset by, for example, benefits of HTTP proxying and caching
behavior.

In this document, Section 3 presents the reasoning that guided the
proposed design. Section 4 specifies the actual mapping of DoQ.
Section 5 presents guidelines on the implementation, usage, and
deployment of DoQ.

2. Key Words

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Design Considerations

This section and its subsections present the design guidelines that
were used for DoQ. While all other sections in this document are
normative, this section is informative in nature.

3.1. Provide DNS Privacy

DoT [RFC7858] defines how to mitigate some of the issues described in
"DNS Privacy Considerations" [RFC9076] by specifying how to transmit
DNS messages over TLS. The "Usage Profiles for DNS over TLS and DNS
over DTLS" [RFC8310] specify Strict and Opportunistic usage profiles
for DoT including how stub resolvers can authenticate recursive
resolvers.

QUIC connection setup includes the negotiation of security parameters
using TLS, as specified in "Using TLS to Secure QUIC" [RFC9001],
enabling encryption of the QUIC transport. Transmitting DNS messages
over QUIC will provide essentially the same privacy protections as
DoT [RFC7858] including Strict and Opportunistic usage profiles
[RFC8310]. Further discussion on this is provided in Section 7.

3.2. Design for Minimum Latency

QUIC is specifically designed to reduce protocol-induced delays, with
features such as:

1. Support for 0-RTT data during session resumption.

2. Support for advanced packet-loss recovery procedures as specified
in "QUIC Loss Detection and Congestion Control" [RFC9002].

3. Mitigation of head-of-line blocking by allowing parallel delivery
of data on multiple streams.

This mapping of DNS to QUIC will take advantage of these features in
three ways:

1. Optional support for sending 0-RTT data during session resumption
(the security and privacy implications of this are discussed in
later sections).

2. Long-lived QUIC connections over which multiple DNS transactions
are performed, generating the sustained traffic required to
benefit from advanced recovery features.

3. Mapping of each DNS Query/Response transaction to a separate
stream, to mitigate head-of-line blocking. This enables servers
to respond to queries "out of order". It also enables clients to
process responses as soon as they arrive, without having to wait
for in-order delivery of responses previously posted by the
server.

These considerations are reflected in the mapping of DNS traffic to
QUIC streams in Section 4.2.

3.3. Middlebox Considerations

Using QUIC might allow a protocol to disguise its purpose from
devices on the network path using encryption and traffic analysis
resistance techniques like padding, traffic pacing, and traffic
shaping. This specification does not include any measures that are
designed to avoid such classification; the padding mechanisms defined
in Section 5.4 are intended to obfuscate the specific records
contained in DNS queries and responses, but not the fact that this is
DNS traffic. Consequently, firewalls and other middleboxes might be
able to distinguish DoQ from other protocols that use QUIC, like
HTTP, and apply different treatment.

The lack of measures in this specification to avoid protocol
classification is not an endorsement of such practices.

3.4. No Server-Initiated Transactions

As stated in Section 1, this document does not specify support for
server-initiated transactions within established DoQ connections.
That is, only the initiator of the DoQ connection may send queries
over the connection.

DSO does support server-initiated transactions within existing
connections. However, DoQ as defined here does not meet the criteria
for an applicable transport for DSO because it does not guarantee in-
order delivery of messages; see Section 4.2 of [RFC8490].

4. Specifications

4.1. Connection Establishment

DoQ connections are established as described in the QUIC transport
specification [RFC9000]. During connection establishment, DoQ
support is indicated by selecting the Application-Layer Protocol
Negotiation (ALPN) token "doq" in the crypto handshake.

4.1.1. Port Selection

By default, a DNS server that supports DoQ MUST listen for and accept
QUIC connections on the dedicated UDP port 853 Section (Section 8), unless
there is a mutual agreement to use another port.

By default, a DNS client desiring to use DoQ with a particular server
MUST establish a QUIC connection to UDP port 853 on the server,
unless there is a mutual agreement to use another port.

DoQ connections MUST NOT use UDP port 53. This recommendation
against use of port 53 for DoQ is to avoid confusion between DoQ and
the use of DNS over UDP [RFC1035]. The risk of confusion exists even
if two parties agreed on port 53, as other parties without knowledge
of that agreement might still try to use that port.

In the stub to recursive scenario, the use of port 443 as a mutually
agreed alternative port can be operationally beneficial, since port
443 is used by many services using QUIC and HTTP-3 and is thus less
likely to be blocked than other ports. Several mechanisms for stubs
to discover recursives offering encrypted transports, including the
use of custom ports, are the subject of ongoing work.

4.2. Stream Mapping and Usage

The mapping of DNS traffic over QUIC streams takes advantage of the
QUIC stream features detailed in Section 2 of [RFC9000], the QUIC
transport specification.

DNS query/response traffic [RFC1034] [RFC1035] follows a simple
pattern in which the client sends a query, and the server provides
one or more responses (multiple responses can occur in zone
transfers).

The mapping specified here requires that the client select a separate
QUIC stream for each query. The server then uses the same stream to
provide all the response messages for that query. In order for
multiple responses to be parsed, a 2-octet length field is used in
exactly the same way as the 2-octet length field defined for DNS over
TCP [RFC1035]. The practical result of this is that the content of
each QUIC stream is exactly the same as the content of a TCP
connection that would manage exactly one query.

All DNS messages (queries and responses) sent over DoQ connections
MUST be encoded as a 2-octet length field followed by the message
content as specified in [RFC1035].

The client MUST select the next available client-initiated
bidirectional stream for each subsequent query on a QUIC connection,
in conformance with the QUIC transport specification [RFC9000].
Packet losses and other network events might cause queries to arrive
in a different order. Servers SHOULD process queries as they arrive,
as not doing so would cause unnecessary delays.

The client MUST send the DNS query over the selected stream and MUST
indicate through the STREAM FIN mechanism that no further data will
be sent on that stream.

The server MUST send the response(s) on the same stream and MUST
indicate, after the last response, through the STREAM FIN mechanism
that no further data will be sent on that stream.

Therefore, a single DNS transaction consumes a single bidirectional
client-initiated stream. This means that the client's first query
occurs on QUIC stream 0, the second on 4, and so on (see Section 2.1
of [RFC9000]).

Servers MAY defer processing of a query until the STREAM FIN has been
indicated on the stream selected by the client.

Servers and clients MAY monitor the number of "dangling" streams.
These are open streams where the following events have not occurred
after implementation-defined timeouts:

* the expected queries or responses have not been received or,

* the expected queries or responses have been received but not the
STREAM FIN

Implementations MAY impose a limit on the number of such dangling
streams. If limits are encountered, implementations MAY close the
connection.

4.2.1. DNS Message IDs

When sending queries over a QUIC connection, the DNS Message ID MUST
be set to 0. The stream mapping for DoQ allows for unambiguous
correlation of queries and responses, so the Message ID field is not
required.

This has implications for proxying DoQ messages to and from other
transports. For example, proxies may have to manage the fact that
DoQ can support a larger number of outstanding queries on a single
connection than, for example, DNS over TCP, because DoQ is not
limited by the Message ID space. This issue already exists for DoH,
where a Message ID of 0 is recommended.

When forwarding a DNS message from DoQ over another transport, a DNS
Message ID MUST be generated according to the rules of the protocol
that is in use. When forwarding a DNS message from another transport
over DoQ, the Message ID MUST be set to 0.

4.3. DoQ Error Codes

The following error codes are defined for use when abruptly
terminating streams, for use as application protocol error codes when
aborting reading of streams, or for immediately closing connections:

DOQ_NO_ERROR (0x0): No error. This is used when the connection or
stream needs to be closed, but there is no error to signal.

DOQ_INTERNAL_ERROR (0x1): The DoQ implementation encountered an
internal error and is incapable of pursuing the transaction or the
connection.

DOQ_PROTOCOL_ERROR (0x2): The DoQ implementation encountered a
protocol error and is forcibly aborting the connection.

DOQ_REQUEST_CANCELLED (0x3): A DoQ client uses this to signal that
it wants to cancel an outstanding transaction.

DOQ_EXCESSIVE_LOAD (0x4): A DoQ implementation uses this to signal
when closing a connection due to excessive load.

DOQ_UNSPECIFIED_ERROR (0x5): A DoQ implementation uses this in the
absence of a more specific error code.

DOQ_ERROR_RESERVED (0xd098ea5e): An alternative error code used for
tests.

See Section 8.4 for details on registering new error codes.

4.3.1. Transaction Cancellation

In QUIC, sending STOP_SENDING requests that a peer cease transmission
on a stream. If a DoQ client wishes to cancel an outstanding
request, it MUST issue a QUIC STOP_SENDING, and it SHOULD use the
error code DOQ_REQUEST_CANCELLED. It MAY use a more specific error
code registered according to Section 8.4. The STOP_SENDING request
may be sent at any time but will have no effect if the server
response has already been sent, in which case the client will simply
discard the incoming response. The corresponding DNS transaction
MUST be abandoned.

Servers that receive STOP_SENDING act in accordance with Section 3.5
of [RFC9000]. Servers SHOULD NOT continue processing a DNS
transaction if they receive a STOP_SENDING.

Servers MAY impose implementation limits on the total number or rate
of cancellation requests. If limits are encountered, servers MAY
close the connection. In this case, servers wanting to help client
debugging MAY use the error code DOQ_EXCESSIVE_LOAD. There is always
a trade-off between helping good faith clients debug issues and
allowing denial-of-service attackers to test server defenses;
depending on circumstances servers might very well choose to send
different error codes.

Note that this mechanism provides a way for secondaries to cancel a
single zone transfer occurring on a given stream without having to
close the QUIC connection.

Servers MUST NOT continue processing a DNS transaction if they
receive a RESET_STREAM request from the client before the client
indicates the STREAM FIN. The server MUST issue a RESET_STREAM to
indicate that the transaction is abandoned unless:

* it has already done so for another reason or

* it has already both sent the response and indicated the STREAM
FIN.

4.3.2. Transaction Errors

Servers normally complete transactions by sending a DNS response (or
responses) on the transaction's stream, including cases where the DNS
response indicates a DNS error. For example, a client SHOULD be
notified of a Server Failure (SERVFAIL, [RFC1035]) SHOULD be notified to the client by sending
back through a response
with the Response Code set to SERVFAIL.

If a server is incapable of sending a DNS response due to an internal
error, it SHOULD issue a QUIC RESET_STREAM frame. The error code
SHOULD be set to DOQ_INTERNAL_ERROR. The corresponding DNS
transaction MUST be abandoned. Clients MAY limit the number of
unsolicited QUIC Stream Resets RESET_STREAM frames received on a connection before
choosing to close the connection.

Note that this mechanism provides a way for primaries to abort a
single zone transfer occurring on a given stream without having to
close the QUIC connection.

4.3.3. Protocol Errors

Other error scenarios can occur due to malformed, incomplete, or
unexpected messages during a transaction. These include (but are not
limited to):

* a client or server receives a message with a non-zero Message ID

* a client or server receives a STREAM FIN before receiving all the
bytes for a message indicated in the 2-octet length field

* a client receives a STREAM FIN before receiving all the expected
responses

* a server receives more than one query on a stream

* a client receives a different number of responses on a stream than
expected (e.g., multiple responses to a query for an A record)

* a client receives a STOP_SENDING request

* the client or server does not indicate the expected STREAM FIN
after sending requests or responses (see Section 4.2)

* an implementation receives a message containing the edns-tcp-
keepalive EDNS(0) Option [RFC7828] (see Section 5.5.2)

* a client or a server attempts to open a unidirectional QUIC stream

* a server attempts to open a server-initiated bidirectional QUIC
stream

* receiving a server receives a "replayable" transaction in O-RTT 0-RTT data (for
servers not willing to handle this case, see Section 4.5)

If a peer encounters such an error condition, it is considered a
fatal error. It SHOULD forcibly abort the connection using QUIC's
CONNECTION_CLOSE mechanism and SHOULD use the DoQ error code
DOQ_PROTOCOL_ERROR. In some cases, it MAY instead silently abandon
the connection, which uses fewer of the local resources but makes
debugging at the offending node more difficult.

It is noted that the restrictions on use of the above EDNS(0) options option
has implications for proxying messages from TCP/DoT/DoH over DoQ.

4.3.4. Alternative Error Codes

This specification describes specific error codes in Sections 4.3.1,
4.3.2, and 4.3.3. These error codes are meant to facilitate
investigation of failures and other incidents. New error codes may
be defined in future versions of DoQ or registered as specified in
Section 8.4.

Because new error codes can be defined without negotiation, use of an
error code in an unexpected context or receipt of an unknown error
code MUST be treated as equivalent to DOQ_UNSPECIFIED_ERROR.

Implementations MAY wish to test the support for the error code
extension mechanism by using error codes not listed in this document,
or they MAY use DOQ_ERROR_RESERVED.

4.4. Connection Management

Section 10 of [RFC9000], the QUIC transport specification, specifies
that connections can be closed in three ways:

* idle timeout

* immediate close

* stateless reset

Clients and servers implementing DoQ SHOULD negotiate use of the idle
timeout. Closing on idle timeout is done without any packet
exchange, which minimizes protocol overhead. Per Section 10.1 of
[RFC9000], the QUIC transport specification, the effective value of
the idle timeout is computed as the minimum of the values advertised
by the two endpoints. Practical considerations on setting the idle
timeout are discussed in Section 5.5.2.

Clients SHOULD monitor the idle time incurred on their connection to
the server, defined by the time spent since the last packet from the
server has been received. When a client prepares to send a new DNS
query to the server, it SHOULD check whether the idle time is
sufficiently lower than the idle timer. If it is, the client SHOULD
send the DNS query over the existing connection. If not, the client
SHOULD establish a new connection and send the query over that
connection.

Clients MAY discard their connections to the server before the idle
timeout expires. A client that has outstanding queries SHOULD close
the connection explicitly using QUIC's CONNECTION_CLOSE mechanism and
the DoQ error code DOQ_NO_ERROR.

Clients and servers MAY close the connection for a variety of other
reasons, indicated using QUIC's CONNECTION_CLOSE. Client and servers
that send packets over a connection discarded by their peer might
receive a stateless reset indication. If a connection fails, all the
in-progress transactions on that connection MUST be abandoned.

4.5. Session Resumption and 0-RTT

A client MAY take advantage of the session resumption and 0-RTT
mechanisms supported by QUIC transport [RFC9000] and QUIC TLS
[RFC9001] if the server supports them. Clients SHOULD consider
potential privacy issues associated with session resumption before
deciding to use this mechanism and specifically evaluate the trade-
offs presented in the various sections of this document. The privacy
issues are detailed in Sections 7.1 and 7.2, and the implementation
considerations are discussed in Section 5.5.3.

The 0-RTT mechanism MUST NOT be used to send DNS requests that are
not "replayable" transactions. In this specification, only
transactions that have an OPCODE of QUERY or NOTIFY are considered
replayable; therefore, other OPCODES MUST NOT be sent in 0-RTT data.
See Appendix A for a detailed discussion of why NOTIFY is included
here.

Servers MAY support session resumption, and MAY do that with or
without supporting 0-RTT, using the mechanisms described in
Section 4.6.1 of [RFC9001]. Servers supporting 0-RTT MUST NOT
immediately process non-replayable transactions received in 0-RTT
data but instead MUST adopt one of the following behaviors:

* Queue the offending transaction and only process it after the QUIC
handshake has been completed, as defined in Section 4.1.1 of
[RFC9001].

* Reply to the offending transaction with a response code REFUSED
and an Extended DNS Error Code (EDE) "Too Early" using the
extended RCODE mechanisms defined in [RFC6891] and the extended
DNS errors defined in [RFC8914]; see Section 8.3.

* Close the connection with the error code DOQ_PROTOCOL_ERROR.

4.6. Message Sizes

DoQ Queries queries and Responses responses are sent on QUIC streams, which in theory
can carry up to 2^62 bytes. However, DNS messages are restricted in
practice to a maximum size of 65535 bytes. This maximum size is
enforced by the use of a two-octet 2-octet message length field in DNS over TCP
[RFC1035] and DNS over TLS DoT [RFC7858], and by the definition of the
"application/dns-message" for DNS over HTTP DoH [RFC8484]. DoQ enforces the same
restriction.

The Extension Mechanisms for DNS (EDNS) (EDNS(0)) [RFC6891] allow peers to
specify the UDP message size. This parameter is ignored by DoQ. DoQ
implementations always assume that the maximum message size is 65535
bytes.

5. Implementation Requirements

5.1. Authentication

For the stub to recursive resolver scenario, the authentication requirements
are the same as described in DoT [RFC7858] and "Usage Profiles for
DNS over TLS and DNS over DTLS" [RFC8310]. [RFC8932] states that DNS
privacy services SHOULD provide credentials that clients can use to
authenticate the server. Given this, and to align with the
authentication model for DoH, DoQ stubs SHOULD use a Strict
authentication usage
profile. Client authentication for the encrypted stub to recursive
scenario is not described in any DNS RFC.

For zone transfer, the authentication requirements are the same as
described in [RFC9103].

For the recursive resolver to authoritative nameserver scenario, authentication
requirements are unspecified at the time of writing and are the
subject of ongoing work in the DPRIVE WG.

5.2. Fallback to Other Protocols on Connection Failure

If the establishment of the DoQ connection fails, clients MAY attempt
to fall back to DoT and then potentially clear text, cleartext, as specified in
DoT [RFC7858] and "Usage Profiles for DNS over TLS and DNS over DTLS"
[RFC8310], depending on their privacy usage profile.

DNS clients SHOULD remember server IP addresses that don't support
DoQ. Mobile clients might also remember the lack of DoQ support by
given IP addresses on a per-context basis (e.g., per network or
provisioning domain).

Timeouts, connection refusals, and QUIC handshake failures are
indicators that a server does not support DoQ. Clients SHOULD NOT
attempt DoQ queries to a server that does not support DoQ for a
reasonable period (such as one hour per server). DNS clients
following an out-of-band key-pinned privacy usage profile [RFC7858] MAY be
more aggressive about retrying after DoQ connection failures.

5.3. Address Validation

Section 8 of [RFC9000], the QUIC transport specification, defines
Address Validation procedures to avoid servers being used in address
amplification attacks. DoQ implementations MUST conform to this
specification, which limits the worst case worst-case amplification to a factor
3.

DoQ implementations SHOULD consider configuring servers to use the
Address Validation using Retry Packets procedure defined in
Section 8.1.2 of [RFC9000], the QUIC transport specification. This
procedure imposes a 1-RTT delay for verifying the return routability
of the source address of a client, similar to the DNS Cookies
mechanism [RFC7873].

DoQ implementations that configure Address Validation using Retry
Packets SHOULD implement the Address Validation for Future
Connections procedure defined in Section 8.1.3 of [RFC9000], the QUIC
transport specification. This defines how servers can send NEW_TOKEN
frames to clients after the client address is validated in order to
avoid the 1-RTT penalty during subsequent connections by the client
from the same address.

5.4. Padding

Implementations MUST protect against the traffic analysis attacks
described in Section 7.5 by the judicious injection of padding. This
could be done either by padding individual DNS messages using the
EDNS(0) Padding Option [RFC7830] or by padding QUIC packets (see
Section 19.1 of [RFC9000]).

In theory, padding at the QUIC packet level could result in better
performance for the equivalent protection, because the amount of
padding can take into account non-DNS frames such as acknowledgements
or flow control updates, and also because QUIC packets can carry
multiple DNS messages. However, applications can only control the
amount of padding in QUIC packets if the implementation of QUIC
exposes adequate APIs. This leads to the following recommendations:

* If the implementation of QUIC exposes APIs to set a padding
policy, DNS over QUIC DoQ SHOULD use that API to align the packet length to a
small set of fixed sizes.

* If padding at the QUIC packet level is not available or not used,
DNS over QUIC
DoQ MUST ensure that all DNS queries and responses are padded to a
small set of fixed sizes, using the EDNS(0) padding extension as
specified in [RFC7830].

Implementations might choose not to use a QUIC API for padding if it
is significantly simpler to reuse existing DNS message padding logic
that is applied to other encrypted transports.

In the absence of a standard policy for padding sizes,
implementations SHOULD follow the recommendations of the Experimental
status "Padding Policies for Extension Mechanisms for DNS (EDNS(0))"
[RFC8467]. While Experimental, these recommendations are referenced
because they are implemented and deployed for DoT and provide a way
for implementations to be fully compliant with this specification.

5.5. Connection Handling

"DNS Transport over TCP - Implementation Requirements" [RFC7766]
provides updated guidance on DNS over TCP, some of which is
applicable to DoQ. This section provides similar advice on
connection handling for DoQ.

5.5.1. Connection Reuse

Historic implementations of DNS clients are known to open and close
TCP connections for each DNS query. To amortize connection setup
costs, both clients and servers SHOULD support connection reuse by
sending multiple queries and responses over a single persistent QUIC
connection.

In order to achieve performance on par with UDP, DNS clients SHOULD
send their queries concurrently over the QUIC streams on a QUIC
connection. That is, when a DNS client sends multiple queries to a
server over a QUIC connection, it SHOULD NOT wait for an outstanding
reply before sending the next query.

5.5.2. Resource Management

Proper management of established and idle connections is important to
the healthy operation of a DNS server.

An implementation of DoQ SHOULD follow best practices similar to
those specified for DNS over TCP [RFC7766], in particular with regard
to:

* Concurrent Connections (Section 6.2.2 of [RFC7766], updated by
Section 6.4 of [RFC9103])

* Security Considerations (Section 10 of [RFC7766])

Failure to do so may lead to resource exhaustion and denial of
service.

Clients that want to maintain long duration DoQ connections SHOULD
use the idle timeout mechanisms defined in Section 10.1 of [RFC9000],
the QUIC transport specification. Clients and servers MUST NOT send
the edns-tcp-keepalive EDNS(0) Option [RFC7828] in any messages sent
on a DoQ connection (because it is specific to the use of TCP/TLS as
a transport).

This document does not make specific recommendations for timeout
values on idle connections. Clients and servers should reuse and/or
close connections depending on the level of available resources.
Timeouts may be longer during periods of low activity and shorter
during periods of high activity.

5.5.3. Using 0-RTT and Session Resumption

Using 0-RTT for DNS over QUIC DoQ has many compelling advantages. Clients can
establish connections and send queries without incurring a connection
delay. Servers can thus negotiate low values of the connection
timers, which reduces the total number of connections that they need
to manage. They can do that because the clients that use 0-RTT will
not incur latency penalties if new connections are required for a
query.

Session resumption and 0-RTT data transmission create privacy risks
detailed in Sections 7.1 and 7.2. The following recommendations are
meant to reduce the privacy risks while enjoying the performance
benefits of 0-RTT data, subject to the restrictions specified in
Section 4.5.

Clients SHOULD use resumption tickets only once, as specified in
Appendix C.4 of [RFC8446]. By default, clients SHOULD NOT use
session resumption if the client's connectivity has changed.

Clients could receive address validation tokens from the server using
the NEW_TOKEN mechanism; see Section 8 of [RFC9000]. The associated
tracking risks are mentioned in Section 7.3. Clients SHOULD only use
the address validation tokens when they are also using session
resumption thus avoiding additional tracking risks.

Servers SHOULD issue session resumption tickets with a sufficiently
long lifetime (e.g., 6 hours), so that clients are not tempted to
either keep the connection alive or frequently poll the server to
renew session resumption tickets. Servers SHOULD implement the anti-
replay mechanisms specified in Section 8 of [RFC8446].

5.5.4. Controlling Connection Migration for Privacy

DoQ implementations might consider using the connection migration
features defined in Section 9 of [RFC9000]. These features enable
connections to continue operating as the client's connectivity
changes. As detailed in Section 7.4, these features trade-off trade off
privacy for latency. By default, clients SHOULD be configured to
prioritize privacy and start new sessions if their connectivity
changes.

5.6. Processing Queries in Parallel

As specified in Section 7 of [RFC7766] "DNS Transport over TCP -
Implementation Requirements", resolvers are RECOMMENDED to support
the preparing of responses in parallel and sending them out of order.
In DoQ, they do that by sending responses on their specific stream as
soon as possible, without waiting for availability of responses for
previously opened streams.

5.7. Zone Transfer

[RFC9103] specifies zone transfer over TLS (XoT) and includes updates
to [RFC1995] (IXFR), [RFC5936] (AXFR), and [RFC7766]. Considerations
relating to the reuse of XoT connections described there apply
analogously to zone transfers performed using DoQ connections. One
reason for reiterating such specific guidance is the lack of
effective connection reuse in existing TCP/TLS zone transfer
implementations today. The following recommendations apply:

* DoQ servers MUST be able to handle multiple concurrent IXFR
requests on a single QUIC connection.

* DoQ servers MUST be able to handle multiple concurrent AXFR
requests on a single QUIC connection.

* DoQ implementations SHOULD

- use the same QUIC connection for both AXFR and IXFR requests to
the same primary

- send those requests in parallel as soon as they are queued,
i.e., do not wait for a response before sending the next query
on the connection (this is analogous to pipelining requests on
a TCP/TLS connection)

- send the response(s) for each request as soon as they are
available, i.e., response streams MAY be sent intermingled

5.8. Flow Control Mechanisms

Servers and clients manage flow control using the mechanisms defined
in Section 4 of [RFC9000]. These mechanisms allow clients and
servers to specify how many streams can be created, how much data can
be sent on a stream, and how much data can be sent on the union of
all streams. For DNS over QUIC, DoQ, controlling how many streams are created
allows servers to control how many new requests the client can send
on a given connection.

Flow control exists to protect endpoint resources. For servers,
global and per-stream flow control limits control how much data can
be sent by clients. The same mechanisms allow clients to control how
much data can be sent by servers. Values that are too small will
unnecessarily limit performance. Values that are too large might
expose endpoints to overload or memory exhaustion. Implementations
or deployments will need to adjust flow control limits to balance
these concerns. In particular, zone transfer implementations will
need to control these limits carefully to ensure both large and
concurrent zone transfers are well managed.

Initial values of parameters control how many requests and how much
data can be sent by clients and servers at the beginning of the
connection. These values are specified in transport parameters
exchanged during the connection handshake. The parameter values
received in the initial connection also control how many requests and
how much data can be sent by clients using 0-RTT data in a resumed
connection. Using too small values of these initial parameters would
restrict the usefulness of allowing 0-RTT data.

6. Security Considerations

A Threat Analysis of the Domain Name System is found in [RFC3833].
This analysis was written before the development of DoT, DoH, and
DoQ, and probably needs to be updated.

The security considerations of DoQ should be comparable to those of
DoT [RFC7858]. DoT as specified in [RFC7858] only addresses the stub
to recursive resolver scenario, but the considerations about person-
in-the-middle person-in-the-
middle attacks, middleboxes, and caching of data from clear-
text cleartext
connections also apply for DoQ to the resolver to authoritative
server scenario. As stated in Section 5.1, the authentication
requirements for securing zone transfer using DoQ are the same as
those for zone transfer over DoT; therefore, the general security
considerations are entirely analogous to those described in
[RFC9103].

DoQ relies on QUIC, which itself relies on TLS 1.3 and thus supports
by default the protections against downgrade attacks described in
[BCP195]. QUIC-specific issues and their mitigations are described
in Section 21 of [RFC9000].

7. Privacy Considerations

The general considerations of encrypted transports provided in "DNS
Privacy Considerations" [RFC9076] apply to DoQ. The specific
considerations provided there do not differ between DoT and DoQ, and
they are not discussed further here. Similarly, "Recommendations for
DNS Privacy Service Operators" [RFC8932] (which covers operational,
policy, and security considerations for DNS privacy services) is also
applicable to DoQ services.

QUIC incorporates the mechanisms of TLS 1.3 [RFC8446], and this
enables QUIC transmission of "0-RTT" data. This can provide
interesting latency gains, but it raises two concerns:

1. Adversaries could replay the 0-RTT data and infer its content
from the behavior of the receiving server.

2. The 0-RTT mechanism relies on TLS session resumption, which can
provide linkability between successive client sessions.

These issues are developed in Sections 7.1 and 7.2.

7.1. Privacy Issues with 0-RTT data

The 0-RTT data can be replayed by adversaries. That data may trigger
queries by a recursive resolver to authoritative resolvers.
Adversaries may be able to pick a time at which the recursive
resolver outgoing traffic is observable and thus find out what name
was queried for in the 0-RTT data.

This risk is in fact a subset of the general problem of observing the
behavior of the recursive resolver discussed in "DNS Privacy
Considerations" [RFC9076]. The attack is partially mitigated by
reducing the observability of this traffic. The mandatory replay
protection mechanisms in TLS 1.3 [RFC8446] limit but do not eliminate
the risk of replay. 0-RTT packets can only be replayed within a
narrow window, which is only wide enough to account for variations in
clock skew and network transmission.

The recommendation for TLS 1.3 [RFC8446] is that the capability to
use 0-RTT data should be turned off by default and only enabled if
the user clearly understands the associated risks. In the case of
DoQ, allowing 0-RTT data provides significant performance gains, and
there is a concern that a recommendation to not use it would simply
be ignored. Instead, a set of practical recommendations is provided
in Sections 4.5 and 5.5.3.

The specifications in Section 4.5 block the most obvious risks of
replay attacks, as they only allow for transactions that will not
change the long-term state of the server.

The attacks described above apply to the stub resolver to recursive
resolver scenario, but similar attacks might be envisaged in the
recursive resolver to authoritative resolver scenario, and the same
mitigations apply.

7.2. Privacy Issues with Session Resumption

The QUIC session resumption mechanism reduces the cost of re-
establishing sessions and enables 0-RTT data. There is a linkability
issue associated with session resumption, if the same resumption
token is used several times. Attackers on path between client and
server could observe repeated usage of the token and use that to
track the client over time or over multiple locations.

The session resumption mechanism allows servers to correlate the
resumed sessions with the initial sessions and thus to track the
client. This creates a virtual long duration session. The series of
queries in that session can be used by the server to identify the
client. Servers can most probably do that already if the client
address remains constant, but session resumption tickets also enable
tracking after changes of the client's address.

The recommendations in Section 5.5.3 are designed to mitigate these
risks. Using session tickets only once mitigates the risk of
tracking by third parties. Refusing to resume a session if addresses
change mitigates the incremental risk of tracking by the server (but
the risk of tracking by IP address remains).

The privacy trade-offs here may be context specific. Stub resolvers
will have a strong motivation to prefer privacy over latency since
they often change location. However, recursive resolvers that use a
small set of static IP addresses are more likely to prefer the
reduced latency provided by session resumption and may consider this
a valid reason to use resumption tickets even if the IP address
changed between sessions.

Encrypted zone transfer ([RFC9103]) explicitly does not attempt to
hide the identity of the parties involved in the transfer; at the
same time, such transfers are not particularly latency sensitive.
This means that applications supporting zone transfers may decide to
apply the same protections as stub to recursive applications.

7.3. Privacy Issues with Address Validation Tokens

QUIC specifies address validation mechanisms in Section 8 of
[RFC9000]. Use of an address validation token allows QUIC servers to
avoid an extra RTT for new connections. Address validation tokens
are typically tied to an IP address. QUIC clients normally only use
these tokens when setting up a new connection from a previously used
address. However, clients are not always aware that they are using a
new address. This could be due to NAT, or because the client does
not have an API available to check if the IP address has changed
(which can be quite often for IPv6). There is a linkability risk if
clients mistakenly use address validation tokens after unknowingly
moving to a new location.

The recommendations in Section 5.5.3 mitigates this risk by tying the
usage of the NEW_TOKEN to that of session resumption, though this
recommendation does not cover the case where the client is unaware of
the address change.

7.4. Privacy Issues with Long Duration Sessions

A potential alternative to session resumption is the use of long
duration sessions: if a session remains open for a long time, new
queries can be sent without incurring connection establishment
delays. It is worth pointing out that the two solutions have similar
privacy characteristics. Session resumption may allow servers to
keep track of the IP addresses of clients, but long duration sessions
have the same effect.

In particular, a DoQ implementation might take advantage of the
connection migration features of QUIC to maintain a session even if
the client's connectivity changes, for example, if the client
migrates from a Wi-Fi connection to a cellular network connection and
then to another Wi-Fi connection. The server would be able to track
the client location by monitoring the succession of IP addresses used
by the long duration connection.

The recommendation in Section 5.5.4 mitigates the privacy concerns
related to long duration sessions using multiple client addresses.

7.5. Traffic Analysis

Even though QUIC packets are encrypted, adversaries can gain
information from observing packet lengths, in both queries and
responses, as well as packet timing. Many DNS requests are emitted
by web browsers. Loading a specific web page may require resolving
dozens of DNS names. If an application adopts a simple mapping of
one query or response per packet, or "one QUIC STREAM frame per
packet", then the succession of packet lengths may provide enough
information to identify the requested site.

Implementations SHOULD use the mechanisms defined in Section 5.4 to
mitigate this attack.

8. IANA Considerations

8.1. Registration of a DoQ Identification String

This document creates a new registration for the identification of
DoQ in the "TLS Application-Layer Protocol Negotiation (ALPN)
Protocol IDs" registry [RFC7301].

The "doq" string identifies DoQ:

Protocol: DoQ

Identification Sequence: 0x64 0x6F 0x71 ("doq")

Specification: This document

8.2. Reservation of a Dedicated Port

For both TCP and UDP, port 853 is currently reserved for "DNS query-
response protocol run over TLS/DTLS" [RFC7858].

However, the specification for DNS over DTLS (DoD) [RFC8094] is
experimental, limited to stub to resolver, and no implementations or
deployments currently exist to the authors' knowledge (even though
several years have passed since the specification was published).

This specification additionally reserves the use of UDP port 853 for
DoQ. QUIC version 1 was designed to be able to coexist with other
protocols on the same port, including DTLS; see Section 17.2 of
[RFC9000]. This means that deployments that serve DNS over DTLS DoD and
DNS over QUIC DoQ (QUIC
version 1) on the same port will be able to demultiplex the two due
to the second most significant bit in each UDP payload. Such
deployments ought to check the signatures of future versions or
extensions (e.g., [GREASING-QUIC]) of QUIC and DTLS before deploying
them to serve DNS on the same port.

IANA has updated the following value in the "Service Name and
Transport Protocol Port Number Registry" in the System range. The
registry for that range requires IETF Review or IESG Approval
[RFC6335].

Service Name: domain-s

Port Number: 853

Transport Protocol(s): UDP

Assignee: IESG

Contact: IETF Chair

Description: DNS query-response protocol run over DTLS or QUIC

Reference: [RFC7858][RFC8094] This document

Additionally, IANA has updated the Description field for the
corresponding TCP port 853 allocation to be "DNS query-response
protocol run over TLS" and removed [RFC8094] from the TCP
allocation's Reference field for consistency and clarity.

8.3. Reservation of an Extended DNS Error Code: Too Early

IANA has registered the following value in the "Extended DNS Error
Codes" registry [RFC8914]:

INFO-CODE: 26

Purpose: Too Early

Reference: This document

8.4. DNS-over-QUIC Error Codes Registry

IANA has added a registry for "DNS-over-QUIC Error Codes" on the
"Domain Name System (DNS) Parameters" web page.

The "DNS-over-QUIC Error Codes" registry governs a 62-bit space.
This space is split into three regions that are governed by different
policies:

* Permanent registrations for values between 0x00 and 0x3f (in
hexadecimal; inclusive), which are assigned using Standards Action
or IESG Approval as defined in Sections 4.9 and 4.10 of [RFC8126]

* Permanent registrations for values larger than 0x3f, which are
assigned using the Specification Required policy ([RFC8126])

* Provisional registrations for values larger than 0x3f, which
require Expert Review, as defined in Section 4.5 of [RFC8126].

Provisional reservations share the range of values larger than 0x3f
with some permanent registrations. This is by design to enable
conversion of provisional registrations into permanent registrations
without requiring changes in deployed systems. (This design is
aligned with the principles set in Section 22 of [RFC9000].)

Registrations in this registry MUST include the following fields:

Value: The assigned codepoint

Status: "Permanent" or "Provisional"

Contact: Contact details for the registrant

In addition, permanent registrations MUST include:

Error: A short mnemonic for the parameter

Specification: A reference to a publicly available specification for
the value (optional for provisional registrations)

Description: A brief description of the error code semantics, which
MAY be a summary if a specification reference is provided

Provisional registrations of codepoints are intended to allow for
private use and experimentation with extensions to DNS over QUIC. DoQ. However,
provisional registrations could be reclaimed and reassigned for other
purposes. In addition to the parameters listed above, provisional
registrations MUST include:

Date: The date of last update to the registration

A request to update the date on any provisional registration can be
made without review from the designated expert(s).

The initial content of this registry is shown in Table 1 and all
entries share the following fields:

Status: Permanent

Contact: DPRIVE WG

Specification: Section 4.3

+============+=======================+=============================+
| Value | Error | Description |
+============+=======================+=============================+
| 0x0 | DOQ_NO_ERROR | No error |
+------------+-----------------------+-----------------------------+
| 0x1 | DOQ_INTERNAL_ERROR | Implementation error |
+------------+-----------------------+-----------------------------+
| 0x2 | DOQ_PROTOCOL_ERROR | Generic protocol violation |
+------------+-----------------------+-----------------------------+
| 0x3 | DOQ_REQUEST_CANCELLED | Request cancelled by client |
+------------+-----------------------+-----------------------------+
| 0x4 | DOQ_EXCESSIVE_LOAD | Closing a connection for |
| | | excessive load |
+------------+-----------------------+-----------------------------+
| 0x5 | DOQ_UNSPECIFIED_ERROR | No error reason specified |
+------------+-----------------------+-----------------------------+
| 0xd098ea5e | DOQ_ERROR_RESERVED | Alternative error code used |
| | | for tests |
+------------+-----------------------+-----------------------------+

Table 1: Initial DNS-over-QUIC Error Codes Entries

9. References

9.1. Normative References

[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.

[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>.

[RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
DOI 10.17487/RFC1995, August 1996,
<https://www.rfc-editor.org/info/rfc1995>.

[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>.

[RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
(AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
<https://www.rfc-editor.org/info/rfc5936>.

[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013,
<https://www.rfc-editor.org/info/rfc6891>.

[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>.

[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>.

[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>.

[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.

[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>.

[RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
for DNS over TLS and DNS over DTLS", RFC 8310,
DOI 10.17487/RFC8310, March 2018,
<https://www.rfc-editor.org/info/rfc8310>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

[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>.

[RFC8914] Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
Lawrence, "Extended DNS Errors", RFC 8914,
DOI 10.17487/RFC8914, October 2020,
<https://www.rfc-editor.org/info/rfc8914>.

[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.

[RFC9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
<https://www.rfc-editor.org/info/rfc9001>.

[RFC9103] Toorop, W., Dickinson, S., Sahib, S., Aras, P., and A.
Mankin, "DNS Zone Transfer over TLS", RFC 9103,
DOI 10.17487/RFC9103, August 2021,
<https://www.rfc-editor.org/info/rfc9103>.

9.2. Informative References

[BCP195] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, May 2015.

Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
1.1", BCP 195, RFC 8996, March 2021.

<https://www.rfc-editor.org/info/bcp195>

[DNS-TERMS]
Hoffman, P. and K. Fujiwara, "DNS Terminology", Work in
Progress, Internet-Draft, draft-ietf-dnsop-rfc8499bis-03,
28 September 2021, <https://datatracker.ietf.org/doc/html/
draft-ietf-dnsop-rfc8499bis-03>.

[DNS0RTT] Kahn Gillmor, D., "DNS + 0-RTT", Message to DNS-Privacy WG
mailing list, 6 April 2016, <https://www.ietf.org/mail-
archive/web/dns-privacy/current/msg01276.html>.

[GREASING-QUIC]
Thomson, M., "Greasing the QUIC Bit", Work in Progress,
Internet-Draft, draft-ietf-quic-bit-grease-02, 10 November
2021, <https://datatracker.ietf.org/doc/html/draft-ietf-
quic-bit-grease-02>.

[HTTP/3] Bishop, M., Ed., "Hypertext Transfer Protocol Version 3
(HTTP/3)", RFC 9114, DOI 10.17487/RFC9114, May 2022,
<https://www.rfc-editor.org/info/rfc9114>. Work in Progress, Internet-Draft, draft-ietf-
quic-http-34, 2 February 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-quic-
http-34>.

[RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone
Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,
August 1996, <https://www.rfc-editor.org/info/rfc1996>.

[RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain
Name System (DNS)", RFC 3833, DOI 10.17487/RFC3833, August
2004, <https://www.rfc-editor.org/info/rfc3833>.

[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>.

[RFC7828] Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
edns-tcp-keepalive EDNS0 Option", RFC 7828,
DOI 10.17487/RFC7828, April 2016,
<https://www.rfc-editor.org/info/rfc7828>.

[RFC7873] Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
Cookies", RFC 7873, DOI 10.17487/RFC7873, May 2016,
<https://www.rfc-editor.org/info/rfc7873>.

[RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
Transport Layer Security (DTLS)", RFC 8094,
DOI 10.17487/RFC8094, February 2017,
<https://www.rfc-editor.org/info/rfc8094>.

[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>.

[RFC8490] Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
Lemon, T., and T. Pusateri, "DNS Stateful Operations",
RFC 8490, DOI 10.17487/RFC8490, March 2019,
<https://www.rfc-editor.org/info/rfc8490>.

[RFC8932] Dickinson, S., Overeinder, B., van Rijswijk-Deij, R., and
A. Mankin, "Recommendations for DNS Privacy Service
Operators", BCP 232, RFC 8932, DOI 10.17487/RFC8932,
October 2020, <https://www.rfc-editor.org/info/rfc8932>.

[RFC9002] Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
May 2021, <https://www.rfc-editor.org/info/rfc9002>.

[RFC9076] Wicinski, T., Ed., "DNS Privacy Considerations", RFC 9076,
DOI 10.17487/RFC9076, July 2021,
<https://www.rfc-editor.org/info/rfc9076>.

Appendix A. The NOTIFY Service

This appendix discusses why it is considered acceptable to send
NOTIFY (see [RFC1996]) in 0-RTT data.

Section 4.5 says "The 0-RTT mechanism SHOULD MUST NOT be used to send DNS
requests that are not "replayable" transactions". This specification
supports sending a NOTIFY in 0-RTT data because although a NOTIFY
technically changes the state of the receiving server, the effect of
replaying NOTIFYs has negligible impact in practice.

NOTIFY messages prompt a secondary to either send an SOA query or an
XFR request to the primary on the basis that a newer version of the
zone is available. It has long been recognized that NOTIFYs can be
forged and, in theory, used to cause a secondary to send repeated
unnecessary requests to the primary. For this reason, most
implementations have some form of throttling of the SOA/XFR queries
triggered by the receipt of one or more NOTIFYs.

[RFC9103] describes the privacy risks associated with both NOTIFY and
SOA queries and does not include addressing those risks within the
scope of encrypting zone transfers. Given this, the privacy benefit
of using DoQ for NOTIFY is not clear, but for the same reason,
sending NOTIFY as 0-RTT data has no privacy risk above that of
sending it using cleartext DNS.

Acknowledgements

This document liberally borrows text from the HTTP/3 specification
[HTTP/3] edited by Mike Bishop and from the DoT specification
[RFC7858] authored by Zi Hu, Liang Zhu, John Heidemann, Allison
Mankin, Duane Wessels, and Paul Hoffman.

The privacy issue with 0-RTT data and session resumption was analyzed
by Daniel Kahn Gillmor (DKG) in a message to the IETF DPRIVE Working
Group [DNS0RTT].

Thanks to Tony Finch for an extensive review of the initial draft
version of this document, and to Robert Evans for the discussion of
0-RTT privacy issues. Early reviews by Paul Hoffman and Martin
Thomson and interoperability tests conducted by Stephane Bortzmeyer
helped improve the definition of the protocol.

Thanks also to Martin Thomson and Martin Duke for their later reviews
focusing on the low-level QUIC details, which helped clarify several
aspects of DoQ. Thanks to Andrey Meshkov, Loganaden Velvindron,
Lucas Pardue, Matt Joras, Mirja Kuelewind, Brian Trammell, and
Phillip Hallam-Baker for their reviews and contributions.

Authors' Addresses

Christian Huitema
Private Octopus Inc.
427 Golfcourse Rd
Friday Harbor, WA 98250
United States of America
Email: huitema@huitema.net

Sara Dickinson
Sinodun IT
Oxford Science Park
Oxford
OX4 4GA
United Kingdom
Email: sara@sinodun.com

Allison Mankin
Salesforce
Email: allison.mankin@gmail.com