add                                                          B. Schwartz
Internet-Draft                                                Google LLC
Intended status: Standards Track                            26 July 2021
Expires: 27 January 2022


                Service Binding Mapping for DNS Servers
                       draft-schwartz-svcb-dns-04

Abstract

   The SVCB DNS 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 new transport
   protocols.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the ADD Working Group
   mailing list (add@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/add/.

   Source for this draft and an issue tracker can be found at
   https://github.com/bemasc/svcb-dns.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 27 January 2022.






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Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Name form . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Special case: non-default ports . . . . . . . . . . . . .   3
   4.  Applicable existing SvcParamKeys  . . . . . . . . . . . . . .   4
     4.1.  alpn  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  port  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.3.  Other applicable SvcParamKeys . . . . . . . . . . . . . .   4
   5.  New SvcParamKeys  . . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  dohpath . . . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Limitations . . . . . . . . . . . . . . . . . . . . . . . . .   5
   7.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
     8.1.  Adversary on the query path . . . . . . . . . . . . . . .   6
     8.2.  Adversary on the transport path . . . . . . . . . . . . .   7
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Appendix A.  Mapping Summary  . . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The SVCB record type [SVCB] provides clients with information about
   how to reach alternative endpoints for a service, which may have
   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,



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   manual resolver configuration) and sometimes also a non-default port
   number.

   Use of the SVCB record type requires a mapping document for each
   service type, 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 and DNS over HTTPS.

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.  Name form

   Names are formed using Port-Prefix Naming ([SVCB] Section 2.3), with
   a scheme of "dns".  For example, SVCB records for a DNS service
   identified as "dns1.example.com" would be located at
   "_dns.dns1.example.com".

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 a non-default port number is part of a service identifier, Port-
   Prefix Naming places the port number in an additional a prefix on the
   name.  For example, SVCB records for a DNS service identified as
   "dns1.example.com:9953" would be located 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.







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4.  Applicable existing SvcParamKeys

4.1.  alpn

   This key indicates the set of supported protocols ([SVCB]
   Section 6.1).  There is no default protocol, so the "no-default-alpn"
   key does not apply, and the "alpn" key MUST be present.

   If the protocol set contains any HTTP versions (e.g. "h2", "h3"),
   then the record indicates support for DNS over HTTPS [DOH], and the
   "dohpath" key MUST be present (Section 5.1).  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, ALPN
   negotiation does not defend against cross-protocol downgrade attacks.

4.2.  port

   This key is used to indicate the target port for connection (([SVCB]
   Section 6.2)).  If omitted, the client SHALL use the default port for
   each transport protocol (853 for DNS over TLS [DOT], 443 for DNS over
   HTTPS).

   This key is automatically mandatory if present.  (See Section 7 of
   [SVCB] for the definition of "automatically mandatory".)

4.3.  Other applicable SvcParamKeys

   These SvcParamKeys from [SVCB] apply to the "dns" scheme without
   modification:

   *  ech

   *  ipv4hint

   *  ipv6hint

   Future SvcParamKeys may also be applicable.

5.  New SvcParamKeys








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5.1.  dohpath

   "dohpath" is a single-valued SvcParamKey whose value (both in
   presentation and wire format) is a relative URI Template [RFC6570],
   normally starting with "/".  If the "alpn" SvcParamKey indicates
   support for HTTP, clients MAY construct a DNS over HTTPS URI Template
   by combining the prefix "https://", the service name, the port from
   the "port" key if present, and the "dohpath" value.  (The DNS
   service's original port number MUST NOT be used.)

   Clients SHOULD NOT query for any "HTTPS" RRs when using the
   constructed URI Template.  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 this URI Template 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 must know the intended use in their context.

7.  Examples

   *  A resolver at "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 resolver at "doh.example" that supports only DNS over HTTPS (DNS
      over TLS is not supported):

      _dns.doh.example. 7200 IN SVCB 1 doh.example. (
            alpn=h2 dohpath=/dns-query{?dns} )

   *  A resolver at "resolver.example" that supports

      -  DNS over TLS on "resolver.example" ports 853 (implicit in
         record 1) and 8530 (explicit in record 2), with
         "resolver.example" as the Authentication Domain Name,

      -  DNS over HTTPS at "https://resolver.example/dns-query{?dns}"
         (record 1), and




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      -  an experimental protocol on "fooexp.resolver.example:5353"
         (record 3):

         $ORIGIN resolver.example.
         _dns 7200 IN SVCB 1 @ alpn=dot,h2,h3 dohpath=/dns-query{?dns}
                      SVCB 2 @ alpn=dot port=8530
                      SVCB 3 fooexp port=5353 alpn=foo foo-info=...

   *  A nameserver at "ns.example" whose service configuration is
      published on a different domain:

      $ORIGIN example.
      _dns.ns 7200 IN SVCB 0 _dns.ns.nic

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.

   Clients MUST authenticate the server to its name during secure
   transport establishment.  This name is the hostname used to construct
   the original SVCB query, and cannot be influenced by the SVCB record
   contents.  Accordingly, this draft does not mandate the use of
   DNSSEC.  This draft also does not specify how clients authenticate
   the name (e.g. selection of roots of trust), which might vary
   according to the context.

   Although this adversary cannot alter the authentication name of the
   service, it does have control of the port number and "dohpath" value.
   As a result, the adversary can direct DNS queries for $HOSTNAME to
   any port on $HOSTNAME, and any path on "https://$HOSTNAME", even if
   $HOSTNAME is not actually a DNS server.  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".  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.

   To mitigate this attack, a client of this SVCB mapping MUST NOT
   provide client authentication for DNS queries, except to servers that
   it specifically knows are not vulnerable to such attacks, and a DoH
   service operator MUST ensure that all unauthenticated DoH requests to



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   its origin maintain the DoH service's privacy guarantees, regardless
   of the path.  Also, if an alternative service endpoint sends an
   invalid response to a DNS query, the client SHOULD NOT send more
   queries to that endpoint.

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 ([SVCB] Section 3), 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 use of this specification is
   optional, clients SHOULD switch to SVCB-reliant behavior if SVCB
   resolution succeeds.  Specifications making using of this mapping MAY
   adjust this fallback behavior to suit their requirements.

9.  IANA Considerations

   Per [SVCB] IANA would be directed to add the following entry to the
   SVCB Service Parameters registry.

   +========+=========+==============================+=================+
   | Number | Name    | Meaning                      | Reference       |
   +========+=========+==============================+=================+
   | TBD    | dohpath | DNS over HTTPS path template | (This           |
   |        |         |                              | document)       |
   +--------+---------+------------------------------+-----------------+

                                  Table 1

   Per [Attrleaf], IANA would be directed to add the following entry to
   the DNS Underscore Global Scoped Entry Registry:

        +=========+============+===============+=================+
        | RR TYPE | _NODE NAME | Meaning       | Reference       |
        +=========+============+===============+=================+
        | SVCB    | _dns       | DNS SVCB info | (This document) |
        +---------+------------+---------------+-----------------+

                                 Table 2

10.  References

10.1.  Normative References




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   [DOH]      Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/rfc/rfc8484>.

   [DOT]      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/rfc/rfc7858>.

   [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/rfc/rfc2119>.

   [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/rfc/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/rfc/rfc8174>.

   [SVCB]     Schwartz, B., Bishop, M., and E. Nygren, "Service binding
              and parameter specification via the DNS (DNS SVCB and
              HTTPS RRs)", Work in Progress, Internet-Draft, draft-ietf-
              dnsop-svcb-https-06, 16 June 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-
              svcb-https-06>.

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/rfc/rfc8552>.

   [DNSURI]   Josefsson, S., "Domain Name System Uniform Resource
              Identifiers", RFC 4501, DOI 10.17487/RFC4501, May 2006,
              <https://www.rfc-editor.org/rfc/rfc4501>.

Appendix A.  Mapping Summary

   This table serves as a non-normative summary of the DNS mapping for
   SVCB.






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       +=================+========================================+
       +=================+========================================+
       | *Mapped scheme* | "dns"                                  |
       +-----------------+----------------------------------------+
       | *RR type*       | SVCB (64)                              |
       +-----------------+----------------------------------------+
       | *Name prefix*   | "_dns" for port 53, else "_$PORT._dns" |
       +-----------------+----------------------------------------+
       | *Required keys* | alpn                                   |
       +-----------------+----------------------------------------+
       | *Automatically  | port                                   |
       | Mandatory Keys* |                                        |
       +-----------------+----------------------------------------+
       | *Special        | Supports all HTTPS RR SvcParamKeys     |
       | behaviors*      |                                        |
       +-----------------+----------------------------------------+
       |                 | Overrides the HTTPS RR for DoH         |
       +-----------------+----------------------------------------+
       |                 | Default port is per-transport          |
       +-----------------+----------------------------------------+
       |                 | No encrypted -> cleartext fallback     |
       +-----------------+----------------------------------------+

                                 Table 3

Acknowledgments

   Thanks to the many reviewers and contributors, including Daniel
   Migault, Paul Hoffman, Matt Norhoff, Peter van Dijk, Eric Rescorla,
   and Andreas Schulze.

Author's Address

   Benjamin Schwartz
   Google LLC

   Email: bemasc@google.com














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