SVCB for DNS servers: Service Binding Mapping for DNS Servers

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
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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