Specification of DNS over Dedicated QUIC Connections

4.1. Scope is Limited to the Stub to Resolver Scenario

Usage scenarios for the DNS protocol can be broadly classified in three groups: stub to recursive resolver, recursive resolver to authoritative server, and server to server. This design focuses only on the "stub to recursive resolver" scenario following the approach taken in DoT [RFC7858] and "Usage Profiles for DNS over TLS and DNS over DTLS" [RFC8310].¶

QUESTION: Should this document specify any aspects of configuration of discoverability differently to DoT?¶

No attempt is made to address the recursive to authoritative scenarios. Authoritative resolvers are discovered dynamically through NS records. It is noted that at the time of writing work is ongoing in the DPRIVE working group to attempt to address the analogous problem for DoT [I-D.ietf-dprive-phase2-requirements]. In the absence of an agreed way for authoritative to signal support for QUIC transport, recursive resolvers would have to resort to some trial and error process. At this stage of QUIC deployment, this would be mostly errors, and does not seem attractive. This could change in the future.¶

The DNS protocol is also used for zone transfers. In the AXFR zone transfer scenario [RFC5936], the client emits a single AXFR query, and the server responds with a series of AXFR responses. This creates a unique profile, in which a query results in several responses. Supporting that profile would complicate the mapping of DNS queries over QUIC streams. Zone transfers are not used in the stub to recursive scenario that is the focus here, and seem to be currently well served by using DNS over TCP. There is no attempt to support either AXFR or IXFR in this proposed mapping of DNS to QUIC.¶

4.2. Provide DNS Privacy

DoT [RFC7858] defines how to mitigate some of the issues described in "DNS Privacy Considerations" [RFC7626] 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" [I-D.ietf-quic-tls], 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 9.¶

4.3. Design for Minimum Latency

QUIC is specifically designed to reduce the delay between HTTP queries and HTTP responses. This is achieved through three main components:¶

1. Support for 0-RTT data during session resumption.¶
2. Support for advanced error recovery procedures as specified in "QUIC Loss Detection and Congestion Control" [I-D.ietf-quic-recovery].¶
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. Fast resumption of QUIC connections to manage the disconnect-on-idle feature of QUIC without incurring retransmission time-outs.¶
4. 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 will be reflected in the mapping of DNS traffic to QUIC streams in Section 5.2.¶

4.4. No Specific Middlebox Bypass Mechanism

The mapping of DNS over QUIC is defined for minimal overhead and maximum performance. This means a different traffic profile than HTTP3 over QUIC. This difference can be noted by firewalls and middleboxes. There may be environments in which HTTP3 over QUIC will be able to pass through, but DoQ will be blocked by these middle boxes.¶

4.5. No Server Initiated Transactions

As stated in Section 1, this document does not specify support for server initiated transactions because these are not relevant for the "stub to recursive resolver" scenario. Note that "DNS Stateful Operations" (DSO) [RFC8490] are only applicable for DNS over TCP and DNS over TLS. DSO is not applicable to DNS over HTTP since HTTP has its own mechanism for managing sessions, and this is incompatible with the DSO; the same is true for DNS over QUIC.¶

5.1. Connection Establishment

DoQ connections are established as described in the QUIC transport specification [I-D.ietf-quic-transport]. During connection establishment, DoQ support is indicated by selecting the ALPN token "doq" in the crypto handshake.¶

5.2. Stream Mapping and Usage

The mapping of DNS traffic over QUIC streams takes advantage of the QUIC stream features detailed in Section 2 of the QUIC transport specification [I-D.ietf-quic-transport].¶

The stub to resolver DNS traffic follows a simple pattern in which the client sends a query, and the server provides a response. This design specifies that for each subsequent query on a QUIC connection the client MUST select the next available client-initiated bidirectional stream, in conformance with the QUIC transport specification [I-D.ietf-quic-transport].¶

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 on the same stream, and MUST indicate through the STREAM FIN mechanism that no further data will be sent on that stream.¶

Therefore, a single client initiated DNS transaction consumes a single stream. This means that the client's first query occurs on QUIC stream 0, the second on 4, and so on.¶

5.3. DoQ Error Codes

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

DOQ_NO_ERROR (0x00):

No error. This is used when the connection or stream needs to be closed, but there is no error to signal.¶

DOQ_INTERNAL_ERROR (0x01):

The DoQ implementation encountered an internal error and is incapable of pursuing the transaction or the connection¶

5.4. Connection Management

Section 10 of the QUIC transport specification [I-D.ietf-quic-transport] specifies that connections can be closed in three ways:¶

• idle timeout¶
• immediate close¶
• stateless reset¶

Clients and servers implementing DNS over QUIC SHOULD negotiate use of the idle timeout. Closing on idle timeout is done without any packet exchange, which minimizes protocol overhead. Per section 10.2 of 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 6.6.2.¶

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 will check whether the idle time is sufficient lower than the idle timer. If it is, the client will send the DNS query over the existing connection. If not, the client will establish a new connection and send the query over that connection.¶

Clients MAY discard their connection to the server before the idle timeout expires. If they do that, they SHOULD close the connection explicitly, using QUIC's CONNECTION_CLOSE mechanisms, and indicating the Application reason "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 MAY receive a stateless reset indication. If a connection fails, all queries in progress over the connection MUST be considered failed, and a Server Failure (SERVFAIL, [RFC1035]) SHOULD be notified to the initiator of the transaction.¶

5.5. Connection Resume and 0-RTT

A stub resolver MAY take advantage of the connection resume mechanisms supported by QUIC transport [I-D.ietf-quic-transport] and QUIC TLS [I-D.ietf-quic-tls]. Stub resolvers SHOULD consider potential privacy issues associated with session resume before deciding to use this mechanism. These privacy issues are detailed in Section 9.2.¶

When resuming a session, a stub resolver MAY take advantage of the 0-RTT mechanism supported by QUIC. The 0-RTT mechanism MUST NOT be used to send data that is not "replayable" transactions. For example, a stub resolver MAY transmit a Query as 0-RTT, but MUST NOT transmit an Update.¶

5.6. Message Sizes

DoQ Queries and 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 message length field in DNS over TCP [RFC1035] and DNS over TLS [RFC7858], and by the definition of the "application/dns-message" for DNS over HTTP [RFC8484]. DoQ enforces the same restriction.¶

The flow control mechanism of QUIC control how much data can be sent by QUIC nodes at a given time. The initial values of per stream flow control parameters is defined by two transport parameters:¶

• initial_max_stream_data_bidi_local: when set by the client, specifies the amount of data that servers can send on a "response" stream without waiting for a MAX_STREAM_DATA frame.¶
• initial_max_stream_data_bidi_remote: when set by the server, specifies the amount of data that clients can send on a "query" stream without waiting for a MAX_STREAM_DATA frame.¶

For better performance, it is RECOMMENDED that clients and servers set each of these two parameters to a value of 65535 or greater.¶

The Extension Mechanisms for DNS (EDNS) [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.¶

6.1. 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]. There is no need to authenticate the client's identity in either scenario.¶

6.2. Fall Back to Other Protocols on Connection Failure

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

DNS clients SHOULD remember server IP addresses that don't support DoQ, including timeouts, connection refusals, and QUIC handshake failures, and not request DoQ from them for a reasonable period (such as one hour per server). DNS clients following an out-of-band key-pinned privacy profile ([RFC7858]) MAY be more aggressive about retrying DoQ connection failures.¶

6.3. Address Validation

Section 8 of the QUIC transport specification [I-D.ietf-quic-transport] 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 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 the QUIC transport specification [I-D.ietf-quic-transport]). 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 the QUIC transport specification [I-D.ietf-quic-transport]). 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.¶

6.4. DNS Message IDs

When sending queries over a QUIC connection, the DNS Message ID MUST be set to zero.¶

6.5. Padding

There are mechanisms specified for padding individual DNS messages in "The EDNS(0) Padding Option" [RFC7830] and for padding within QUIC packets (see Section 8.6 of the QUIC transport specification [I-D.ietf-quic-transport]).¶

Implementations SHOULD NOT use DNS options for padding individual DNS messages, because QUIC transport MAY transmit multiple STREAM frames containing separate DNS messages in a single QUIC packet. Instead, implementations SHOULD use QUIC PADDING frames to align the packet length to a small set of fixed sizes, aligned with the recommendations of the "Padding Policies for Extension Mechanisms for DNS (EDNS(0))" [RFC8467].¶

6.6. 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 attempts to specify which and how those considerations apply to DoQ.¶

6.7. Processing Queries in Parallel

As specified in Section 7 of "DNS Transport over TCP - Implementation Requirements" [RFC7766], 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.¶

6.8. Flow Control Mechanisms

Servers and Clients manage flow control as specified in QUIC.¶

Servers MAY use the "maximum stream ID" option of the QUIC transport to limit the number of streams opened by the client. This mechanism will effectively limit the number of DNS queries that a client can send on a single DoQ connection.¶

7.1. Performance Measurements

To our knowledge, no benchmarking studies comparing DoT, DoH and DoQ are published yet. However anecdotal evidence from the AdGuard DoQ recursive resolver deployment indicates that it performs well compared to the other encrypted protocols, particularly in mobile environments. Reasons given for this include that DoQ¶

• Uses less bandwidth due to a more efficient handshake (and due to less per message overhead when compared to DoH).¶
• Performs better in mobile environments due to the increased resilience to packet loss¶
• Can maintain connections as users move between mobile networks via its connection management¶

9.1. 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" [RFC7626]. The attack is partially mitigated by reducing the observability of this traffic. However, the risk is amplified for 0-RTT data, because the attacker might replay it at chosen times, several times.¶

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.¶

QUESTION: Should 0-RTT only be used with Opportunistic profiles (i.e. disabled by default for Strict only)?¶

9.2. Privacy Issues With Session Resume

The QUIC session resume mechanism reduces the cost of re-establishing sessions and enables 0-RTT data. There is a linkability issue associated with session resume, if the same resume token is used several times, but this risk is mitigated by the mechanisms incorporated in QUIC and in TLS 1.3. With these mechanisms, clients and servers can cooperate to avoid linkability by third parties. However, the server will always be able to link the resumed session to the initial session. This creates a virtual long duration session. The series of queries in that session can be used by the server to identify the client.¶

Enabling the server to link client sessions through session resume is probably not a large additional risk if the client's connectivity did not change between the sessions, since the two sessions can probably be correlated by comparing the IP addresses. On the other hand, if the addresses did change, the client SHOULD consider whether the linkability risk exceeds the performance benefits. This evaluation will obviously depend on the level of trust between stub and recursive.¶

9.3. 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 dozen 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 6.5 to mitigate this attack.¶

10.1. Registration of DoQ Identification String

This document creates a new registration for the identification of DoQ in the "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¶

10.2. Reservation of Dedicated Port

IANA is required to add the following value to the "Service Name and Transport Protocol Port Number Registry" in the System Range. The registry for that range requires IETF Review or IESG Approval protocol run over QUIC'.[RFC6335], and such a review was requested using the early allocation process [RFC7120] for the well-known UDP port in this document. Since port 853 is reserved for 'DNS query-response protocol run over TLS' consideration is requested for reserving port 8853 for 'DNS query-response¶

   Service Name           dns-over-quic
   Port Number            8853
   Transport Protocol(s)  UDP
   Assignee               IESG
   Contact                IETF Chair
   Description            DNS query-response protocol run over QUIC
   Reference              This document