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Keywords: DoH, DoT, DoQ, DDR, DNR

Internet Engineering Task Force (IETF) B. Schwartz
Request for Comments: 9461 Meta Platforms, Inc.
Category: Standards Track November 2023
ISSN: 2070-1721
Service Binding Mapping for DNS Servers
Abstract
The SVCB DNS resource record type expresses a bound collection of
endpoint metadata, for use when establishing a connection to a named
service. DNS itself can be such a service, when the server is
identified by a domain name. This document provides the SVCB mapping
for named DNS servers, allowing them to indicate support for
encrypted transport protocols.
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/rfc9461.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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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. Conventions and Definitions
3. Identities and Names
3.1. Special Case: Non-default Ports
4. Applicable Existing SvcParamKeys
4.1. "alpn"
4.2. "port"
4.3. Other Applicable SvcParamKeys
5. New SvcParamKey: "dohpath"
6. Limitations
7. Examples
8. Security Considerations
8.1. Adversary on the Query Path
8.1.1. Downgrade Attacks
8.1.2. Redirection Attacks
8.2. Adversary on the Transport Path
9. IANA Considerations
10. References
10.1. Normative References
10.2. Informative References
Appendix A. Mapping Summary
Acknowledgments
Author's Address
1. Introduction
The SVCB resource record (RR) type [SVCB] provides clients with
information about how to reach alternative endpoints for a service.
These endpoints may offer improved performance or privacy properties.
The service is identified by a "scheme" indicating the service type,
a hostname, and, optionally, other information such as a port number.
A DNS server is often identified only by its IP address (e.g., in
DHCP), but in some contexts it can also be identified by a hostname
(e.g., "NS" records, manual resolver configuration) and sometimes
also a non-default port number.
The use of the SVCB RR type requires a mapping document for each
service type (Section 2.4.3 of [SVCB]), indicating how a client for
that service can interpret the contents of the SVCB SvcParams. This
document provides the mapping for the "dns" service type, allowing
DNS servers to offer alternative endpoints and transports, including
encrypted transports like DNS over TLS (DoT) [RFC7858], DNS over
HTTPS (DoH) [RFC8484], and DNS over QUIC (DoQ) [RFC9250].
The SVCB mapping described in this document is intended as a general-
purpose baseline. Subsequent specifications will adapt this
mechanism as needed to support specific configurations (e.g., for
communication between stub resolvers and recursive resolvers).
2. Conventions and Definitions
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. Identities and Names
SVCB record names (i.e., QNAMEs) for DNS services are formed using
Port Prefix Naming (Section 2.3 of [SVCB]), with a scheme of "dns".
For example, SVCB records for a DNS service identified as
dns1.example.com would be queried at _dns.dns1.example.com.
In some use cases, the name used for retrieving these DNS records is
different from the server identity used to authenticate the secure
transport. To distinguish between these, this document uses the
following terms:
Binding authority: The service name (Section 1.3 of [SVCB]) and
optional port number used as input to Port Prefix Naming.
Authentication name: The name used for secure transport
authentication. This MUST be a DNS hostname or a literal IP
address. Unless otherwise specified, this is the service name
from the binding authority.
3.1. Special Case: Non-default Ports
Normally, a DNS service is identified by an IP address or a domain
name. When connecting to the service using unencrypted DNS over UDP
or TCP, clients use the default port number for DNS (53). However,
in rare cases, a DNS service might be identified by both a name and a
port number. For example, the DNS URI scheme [DNSURI] optionally
includes an authority, comprised of a host and a port number (with a
default of 53). DNS URIs normally omit the authority or specify an
IP address, but a hostname and non-default port number are allowed.
When the binding authority specifies a non-default port number, Port
Prefix Naming places the port number in an additional prefix on the
name. For example, if the binding authority is
"dns1.example.com:9953", the client would query for SVCB records at
_9953._dns.dns1.example.com. If two DNS services operating on
different port numbers provide different behaviors, this arrangement
allows them to preserve the distinction when specifying alternative
endpoints.
4. Applicable Existing SvcParamKeys
4.1. "alpn"
This key indicates the set of supported protocols (Section 7.1 of
[SVCB]). There is no default protocol, so the "no-default-alpn" key
does not apply. If the "alpn" SvcParamKey is absent, the client MUST
treat the SVCB record as "incompatible" (as defined in Section 8 of
[SVCB]) unless some other recognized SvcParam indicates a supported
protocol.
If the protocol set contains any HTTP versions (e.g., "h2", "h3"),
then the record indicates support for DoH and the "dohpath" key MUST
be present (Section 5). All keys specified for use with the HTTPS
record are also permissible and apply to the resulting HTTP
connection.
If the protocol set contains protocols with different default ports
and no "port" key is specified, then protocols are contacted
separately on their default ports. Note that in this configuration,
Application-Layer Protocol Negotiation (ALPN) negotiation does not
defend against cross-protocol downgrade attacks.
4.2. "port"
This key is used to indicate the target port for connection
(Section 7.2 of [SVCB]). If omitted, the client SHALL use the
default port number for each transport protocol (853 for DoT and DoQ,
443 for DoH).
This key is automatically mandatory for this binding. This means
that a client that does not respect the "port" key MUST ignore any
SVCB record that contains this key. (See Section 8 of [SVCB] for the
definition of "automatically mandatory".)
Support for the "port" key can be unsafe if the client has implicit
elevated access to some network service (e.g., a local service that
is inaccessible to remote parties) and that service uses a TCP-based
protocol other than TLS. A hostile DNS server might be able to
manipulate this service by causing the client to send a specially
crafted TLS Server Name Indication (SNI) or session ticket that can
be misparsed as a command or exploit. To avoid such attacks, clients
SHOULD NOT support the "port" key unless one of the following
conditions applies:
* The client is being used with a DNS server that it trusts not to
attempt this attack.
* The client is being used in a context where implicit elevated
access cannot apply.
* The client restricts the set of allowed TCP port values to exclude
any ports where a confusion attack is likely to be possible (e.g.,
the "bad ports" list from Section 2.9 ("Port blocking") of
[FETCH]).
4.3. Other Applicable SvcParamKeys
These SvcParamKeys from [SVCB] apply to the "dns" scheme without
modification:
* mandatory
* ipv4hint
* ipv6hint
Future SvcParamKeys might also be applicable.
5. New SvcParamKey: "dohpath"
"dohpath" is a single-valued SvcParamKey whose value (in both
presentation format and wire format) MUST be a URI Template in
relative form ([RFC6570], Section 1.1) encoded in UTF-8 [RFC3629].
If the "alpn" SvcParam indicates support for HTTP, "dohpath" MUST be
present. The URI Template MUST contain a "dns" variable, and MUST be
chosen such that the result after DoH URI Template expansion
(Section 6 of [RFC8484]) is always a valid and functional ":path"
value ([RFC9113], Section 8.3.1).
When using this SVCB record, the client MUST send any DoH requests to
the HTTP origin identified by the "https" scheme, the authentication
name, and the port from the "port" SvcParam (if present). HTTP
requests MUST be directed to the resource resulting from DoH URI
Template expansion of the "dohpath" value.
Clients SHOULD NOT query for any HTTPS RRs when using "dohpath".
Instead, the SvcParams and address records associated with this SVCB
record SHOULD be used for the HTTPS connection, with the same
semantics as an HTTPS RR. However, for consistency, service
operators SHOULD publish an equivalent HTTPS RR, especially if
clients might learn about this DoH service through a different
channel.
6. Limitations
This document is concerned exclusively with the DNS transport and
does not affect or inform the construction or interpretation of DNS
messages. For example, nothing in this document indicates whether
the service is intended for use as a recursive or authoritative DNS
server. Clients need to know the intended use of services based on
their context.
Not all features of this specification will be applicable or
effective in all contexts:
* If the authentication name is received over an insecure channel
(e.g., a glue NS record), this specification cannot prevent the
client from connecting to an attacker.
* Different transports might prove to be popular for different
purposes (e.g., querying a recursive resolver vs. an authoritative
server). Implementors are not obligated to implement all the
defined transports, although doing so is beneficial for
compatibility.
* Where resolution speed is a high priority, the SVCB TargetName
SHOULD follow the convention described in Section 10.2 of [SVCB],
and the use of AliasMode records (Section 2.4.2 of [SVCB]) is NOT
RECOMMENDED.
7. Examples
* A resolver known as simple.example that supports DNS over TLS on
port 853 (implicitly, as this is its default port):
_dns.simple.example. 7200 IN SVCB 1 simple.example. alpn=dot
* A DoH-only resolver at https://doh.example/dns-query{?dns}. (DNS
over TLS is not supported.):
_dns.doh.example. 7200 IN SVCB 1 doh.example. (
alpn=h2 dohpath=/dns-query{?dns} )
* A resolver known as resolver.example that supports:
- DoT on resolver.example ports 853 (implicit in record 1) and
8530 (explicit in record 2), with "resolver.example" as the
Authentication Domain Name,
- DoQ on resolver.example port 853 (record 1),
- DoH at https://resolver.example/q{?dns} (record 1), and
- an experimental protocol on fooexp.resolver.example:5353
(record 3):
_dns.resolver.example. 7200 IN \
SVCB 1 resolver.example. alpn=dot,doq,h2,h3 dohpath=/q{?dns}
SVCB 2 resolver.example. alpn=dot port=8530
SVCB 3 fooexp.resolver.example. \
port=5353 alpn=foo foo-info=...
* A name server named ns.example. whose service configuration is
published on a different domain:
_dns.ns.example. 7200 IN SVCB 0 _dns.ns.nic.example.
8. Security Considerations
8.1. Adversary on the Query Path
This section considers an adversary who can add or remove responses
to the SVCB query.
During secure transport establishment, clients MUST authenticate the
server to its authentication name, which is not influenced by the
SVCB record contents. Accordingly, this document does not mandate
the use of DNSSEC. This document also does not specify how clients
authenticate the name (e.g., selection of roots of trust), as this
procedure might vary according to the context.
8.1.1. Downgrade Attacks
This attacker cannot impersonate the secure endpoint, but it can
forge a response indicating that the requested SVCB records do not
exist. For a SVCB-reliant client ([SVCB], Section 3), this only
results in a denial of service. However, SVCB-optional clients will
generally fall back to insecure DNS in this case, exposing all DNS
traffic to attacks.
8.1.2. Redirection Attacks
SVCB-reliant clients always enforce the Authentication Domain Name,
but they are still subject to attacks using the transport, port
number, and "dohpath" value, which are controlled by this adversary.
By changing these values in the SVCB answers, the adversary can
direct DNS queries for $HOSTNAME to any port on $HOSTNAME and any
path on "https://$HOSTNAME". If the DNS client uses shared TLS or
HTTP state, the client could be correctly authenticated (e.g., using
a TLS client certificate or HTTP cookie).
This behavior creates a number of possible attacks for certain server
configurations. For example, if https://$HOSTNAME/upload accepts any
POST request as a public file upload, the adversary could forge a
SVCB record containing dohpath=/upload{?dns}. This would cause the
client to upload and publish every query, resulting in unexpected
storage costs for the server and privacy loss for the client.
Similarly, if two DoH endpoints are available on the same origin and
the service has designated one of them for use with this
specification, this adversary can cause clients to use the other
endpoint instead.
To mitigate redirection attacks, a client of this SVCB mapping MUST
NOT identify or authenticate itself when performing DNS queries,
except to servers that it specifically knows are not vulnerable to
such attacks. If an endpoint sends an invalid response to a DNS
query, the client SHOULD NOT send more queries to that endpoint and
MAY log this error. Multiple DNS services MUST NOT share a hostname
identifier (Section 3) unless they are so similar that it is safe to
allow an attacker to choose which one is used.
8.2. Adversary on the Transport Path
This section considers an adversary who can modify network traffic
between the client and the alternative service (identified by the
TargetName).
For a SVCB-reliant client, this adversary can only cause a denial of
service. However, because DNS is unencrypted by default, this
adversary can execute a downgrade attack against SVCB-optional
clients. Accordingly, when the use of this specification is
optional, clients SHOULD switch to SVCB-reliant behavior if SVCB
resolution succeeds. Specifications making use of this mapping MAY
adjust this fallback behavior to suit their requirements.
9. IANA Considerations
Per [SVCB], IANA has added the following entry to the "Service
Parameter Keys (SvcParamKeys)" registry.
+======+=======+================+=========+============+===========+
|Number|Name | Meaning |Format | Change | Reference |
| | | |Reference| Controller | |
+======+=======+================+=========+============+===========+
| 7 |dohpath| DNS-over-HTTPS |RFC 9461 | IETF | RFC 9461 |
| | | path template | | | |
+------+-------+----------------+---------+------------+-----------+
Table 1
Per [Attrleaf], IANA has added the following entry to the DNS
"Underscored and Globally Scoped DNS Node Names" registry:
+=========+============+===========+
| RR Type | _NODE NAME | Reference |
+=========+============+===========+
| SVCB | _dns | RFC 9461 |
+---------+------------+-----------+
Table 2
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570,
DOI 10.17487/RFC6570, March 2012,
<https://www.rfc-editor.org/info/rfc6570>.
[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>.
[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>.
[RFC9113] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
DOI 10.17487/RFC9113, June 2022,
<https://www.rfc-editor.org/info/rfc9113>.
[SVCB] Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
and Parameter Specification via the DNS (SVCB and HTTPS
Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
November 2023, <https://www.rfc-editor.org/info/rfc9460>.
10.2. Informative References
[Attrleaf] Crocker, D., "Scoped Interpretation of DNS Resource
Records through "Underscored" Naming of Attribute Leaves",
BCP 222, RFC 8552, DOI 10.17487/RFC8552, March 2019,
<https://www.rfc-editor.org/info/rfc8552>.
[DNSURI] Josefsson, S., "Domain Name System Uniform Resource
Identifiers", RFC 4501, DOI 10.17487/RFC4501, May 2006,
<https://www.rfc-editor.org/info/rfc4501>.
[FETCH] WHATWG, "Fetch Living Standard", October 2023,
<https://fetch.spec.whatwg.org/>.
[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>.
[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>.
Appendix A. Mapping Summary
This table serves as a non-normative summary of the DNS mapping for
SVCB.
+-----------------+------------------------------------+
| *Mapped scheme* | "dns" |
+-----------------+------------------------------------+
| *RR type* | SVCB (64) |
+-----------------+------------------------------------+
| *Name prefix* | _dns for port 53, else _$PORT._dns |
+-----------------+------------------------------------+
| *Required keys* | alpn or equivalent |
+-----------------+------------------------------------+
| *Automatically | port |
| mandatory keys* | |
+-----------------+------------------------------------+
| *Special | Supports all HTTPS RR SvcParamKeys |
| behaviors* | |
+-----------------+------------------------------------+
| | Overrides the HTTPS RR for DoH |
+-----------------+------------------------------------+
| | Default port is per-transport |
+-----------------+------------------------------------+
| | Cleartext fallback is discouraged |
+-----------------+------------------------------------+
Table 3
Acknowledgments
Thanks to the many reviewers and contributors, including Andrew
Campling, Peter van Dijk, Paul Hoffman, Daniel Migault, Matt
Nordhoff, Eric Rescorla, Andreas Schulze, and Éric Vyncke.
Author's Address
Benjamin Schwartz
Meta Platforms, Inc.
Email: ietf@bemasc.net