dprive                                                          T. Pauly
Internet-Draft                                                Apple Inc.
Intended status: Informational                               E. Rescorla
Expires: 27 August 2021                                          Mozilla
                                                             D. Schinazi
                                                              Google LLC
                                                               C.A. Wood
                                                        23 February 2021

                 Signaling Authoritative DNS Encryption


   This document defines a mechanism for signaling that a given
   authoritative DNS server is reachable by encrypted DNS.

Discussion Venues

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

   Discussion of this document takes place on the DNS PRIVate Exchange
   Working Group mailing list (dns-privacy@ietf.org), which is archived
   at https://mailarchive.ietf.org/arch/browse/dns-privacy/.

   Source for this draft and an issue tracker can be found at

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   This Internet-Draft will expire on 27 August 2021.

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Table of Contents

   1.  Introduction
   2.  Conventions and Definitions
   3.  Overview of Operation
   4.  Use of SVCB Records to Signal Encrypted Transport
     4.1.  Caching and lifetime
     4.2.  Authenticating the Server
   5.  Example
   6.  Security Considerations
   7.  IANA Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Authors' Addresses

1.  Introduction

   The IETF has defined a number of mechanisms for carrying DNS queries
   over encrypted transport [DOH] [DOT] [DOQ].  However, there is no
   scalable way for a recursive resolver to know that a given
   authoritative resolver supports encrypted transport, which inhibits
   the deployment of encrypted DNS for queries from recursive resolvers.
   This specification defines a mechanism for carrying that signal,
   using the DNS SVCB [SVCB] record.

2.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.  Overview of Operation

   The mechanism defined in this document works by using the DNS SVCB
   [SVCB] record to indicate that a given server supports TLS.  The
   recursive resolver can obtain these records in two distinct ways:

   *  In the additional data block of the response that referred the
      recursive to the target authoritative server.

   *  By directly resolving a SVCB query for the target authoritative

   As a practical matter, the first of these options is preferred as it
   allows the recursive to learn that the authoritative server supports
   encrypted transport without an additional round trip, as shown below:

Recursive                .com                      ns.example.example
                   Authoritative Server      (Authoritative for example.com)
NS example.com? ------------>

<----- example.com NS ns.example.example
       ns.example.example A
       _dns.ns.example.example SVCB alpn=dot

<--------------  TLS connection to ns.example ------------>
A example.com? ------------------------------------------->

   The recursive resolver starts by contacting the authoritative server
   for .com and asks for the NS records for example.com.  Note that .com
   is not authoritative for the example.com apex, and will not sign the
   NS RRset; see [RFC4035], Section 2.2, and Section 6 for details.  The
   authoritative returns the NS record pointing at ns.example.example
   and also returns a glue records for ns.example.example indicating
   that it supports DNS over TLS (DoT), in much the same way as it might
   have sent an IP address for that server.  This additional record is
   the only difference from the current situation, and allows the
   recursive resolver to know that it can reach ns.example.example over
   encrypted transport.

   Note: SVCB is not presently permitted at the root [REGISTRY].  In the
   interim, recursive resolvers may be preconfigured with the TLS status
   of the resolvers for TLDs.  [[OPEN ISSUE: Do we want to invent some
   other sentinel as a temporary measure?]]

4.  Use of SVCB Records to Signal Encrypted Transport

   Any given authoritative server name can have one or more DNS Server
   SVCB records, as defined in [I-D.schwartz-svcb-dns].

   For instance, the following records would indicate that
   ns.example.example could be reached by either DoT or DoH (over both
   TCP and QUIC).

   _dns.ns.example.example. 7200 IN SVCB 1 ns.example.example. alpn=dot,h2,h3 dohpath=/dns-query{?dns}

   Upon determining that a given nameserver supports a compatible
   encrypted transport, an implementation MUST only use encrypted
   transport for the rest of the cache lifetime for that information and
   MUST hard fail with error if it is unable to establish a connection.
   If multiple encrypted transports are available, an implementation
   SHOULD try all of them before declaring failure.

   [[OPEN ISSUE: figure out error details]]

4.1.  Caching and lifetime

   Note that in the common case where the name of the target
   authoritative server is out-of-bailiwick [RFC7719] for the referring
   resolver, then the SVCB record may not be retained for future
   queries.  This can create a situation in which a given authoritative
   server will be queried over encrypted transport for one name and over
   unencrypted transport for another.  This is not the end of the world
   (HTTPS has historically operated in this way, with the security
   properties being attached to the reference), but is also not ideal.
   In order to prevent this, resolvers which are also authoritative for
   their own name SHOULD send SVCB glue records in the additional data
   section so that they can be properly cached, and the TTL for these
   SVCB records SHOULD match that of the corresponding NS records in the
   same RRset.

   [[OPEN ISSUE: How often is the case where ns.example.example is not
   authoritative for itself?  Should we encourage people to accept out-
   of-bailiwick responses in that case?]]

4.2.  Authenticating the Server

   Recursive servers MUST authenticate the authoritative server using
   the procedures associated with the relevant protocol, [RFC6125] and
   [RFC2818] for DoT and DoH respectively.  This is in principle
   compatible with having the server authenticated either with the
   WebPKI or with TLSA records [DANE], or both.  In order for this to
   work properly, however, the recursive resolver must know at the time
   it connects whether it will be willing to accept the authoritative
   server's credentials.

   This can be addressed in several ways:

   1.  Require a particular form of authentication (e.g., the WebPKI or
       TLSA records) as mandatory.

   2.  Have the SVCB record indicate what kind(s) of authentication the
       authoritative server supports, allowing the recursive to filter
       out incompatible advertisements.  For instance, the SVCB record
       could contain a key that stated that it only had a WebPKI
       certificate, in which case the resolver could ignore that entry.

   [[OPEN ISSUE: If we have the kind of advertisement indicated in (2)
   above, is not necessary to have an MTI, but it might be desirable to
   promote interoperability.]]

   One challenge with TLSA records in this context is that they may not
   be in the recursive resolver's cache at the time when it wants to
   connect to the authoritative.  This can create added latency if the
   recursive resolver must then first retrieve TLSA records for the
   authoritative.  If we wish for servers to authenticate with DANE, we
   will also probably want some mechanism to carry the TLSA records in
   the TLS handshake, as, for instance defined in

   [[OPEN ISSUE: Resolve this.]]

5.  Example

   A complete example is shown below.

Recursive  x.root-servers.org  ns.a.example         ns.example.example
             (Auth. for .)     (Auth for .example)  (Auth for .example.example)

<== TLS handshake ==>
NS .example? ------->
<- .example NS ns.op.example
   ns.op.example A
   _dns.ns.op.example SVCB alpn=dot

<============ TLS handshake ===========>
NS example.example? ------------------->
<----- example.example NS ns.example.example
       ns.example.example A
       _dns.ns.example.example SVCB alpn=dot

<====================== TLS Handshake ======================>
A www.example.example? ------------------------------------->
<---------------------------- www.example.example A

   In this case, the recursive wants to resolve www.example.example.

   Resolution proceeds in three phases.

   Initially, the recursive connects to the root server.  We assume that
   the recursive knows that the root server is able to do DoT, either
   because it has been preconfigured with his information or because it
   has connected to that root server before.  It performs an NS query
   for ".example" (we are assuming QMIN) and receives:

   *  An NS record pointing to ns.op.example

   *  A glue A record for ns.op.example =

   *  A SVCB record stating that ns.op.example speaks DoT

   Next, the recursive resolver forms a TLS connection to ns.op.example
   and requests an NS record for example.example.  It receives:

   *  An NS record pointing to ns.example.example

   *  A glue A record for ns.example.example =

   *  A SVCB record stating that ns.example.example speaks DoT

   Finally, the recursive resolver forms a TLS connection to
   ns.example.example and request an A record for www.example.example
   and receives the A record of

6.  Security Considerations

   The primary security property delivered by this mechanism is
   confidentiality of the query and response.  As long as (1) all
   queries in the resolution chain, including to the authoritative
   server are encrypted and (2) all resolvers in the resolution chain
   are trustworthy, then even an on-path attacker cannot discover the
   name being resolved or its response.  However, if either of these
   conditions is violated, then an attack is possible:

   *  If the connection to the authoritative server is not encrypted,
      then the request and response can be read directly.

   *  If one of the earlier connections is not encrypted, then the
      attacker can substitute their own NS records. records from the
      additional_data, forcing the resolution back to unencrypted mode.

   *  If one of the resolvers is untrustworthy, then they can substitute
      their own NS records.

   DNSSEC signing only partly mitigates these issues because delegations
   at top-level zones are not signed, as per [RFC4035], Section 2.2.  In
   practice, this means a recursive resolver attempting to resolve a
   zone apex query, such as example.com in Section 3, cannot assume the
   NS answer is authentic.  While NS records received from the
   authoritative server may be signed, in order to retrieve them, the
   recursive resolver will have to contact the servers listed by the
   unverified NS records received from the referring server, at which
   point it has leaked the zone apex to the (potentially fake)
   authoritative server (as well as to the referring server).

   If the recursive resolver is attempting to resolve a specific
   subdomain from the resolver (e.g., server-1234.example.com), then it
   may be able to protect against this attack by (1) using query
   minimization [QMIN] and (2) querying the (alleged) authoritative for
   its DNSSEC-signed NS and SVCB records and only once it has received
   those, attempting to retrieve the actual subdomain.  If the domain is
   DNSSEC signed, then this prevents a malicious referring resolver from
   redirecting the recursive resolver to their own authoritative and
   learning the true subdomain.  However, if, as is common, the
   recursive is just trying to resolve the apex name or one of the
   common "service" names such as "www.example.com", then this procedure
   does not provide additional protection.

   Encryption does not mitigate all leakage.  In some circumstances, an
   on-path attacker may learn the identity of the authoritative server
   if, for example, that server only serves a small number of domains.
   The attacker can learn information about what is being resolved by
   observing whether or not server is queried.

   As a secondary property, the mechanism in this document can provide
   some level of integrity for DNS responses, again under the condition
   that each resolver in the chain is trustworthy.  By contrast, DNSSEC
   provides integrity even if the resolvers are untrustworthy.

7.  IANA Considerations

   This document has no IANA actions.

8.  References

8.1.  Normative References

   [DANE]     Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <https://datatracker.ietf.org/doc/html/rfc6698>.

   [DOH]      Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,

   [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://datatracker.ietf.org/doc/html/rfc7858>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://datatracker.ietf.org/doc/html/rfc6125>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://datatracker.ietf.org/doc/html/rfc8174>.

8.2.  Informative References

   [DOQ]      Huitema, C., Mankin, A., and S. Dickinson, "Specification
              of DNS over Dedicated QUIC Connections", Work in Progress,
              Internet-Draft, draft-ietf-dprive-dnsoquic-02, 22 February
              2021, <https://datatracker.ietf.org/doc/html/draft-ietf-

              Shore, M., Barnes, R., Huque, S., and W. Toorop, "A DANE
              Record and DNSSEC Authentication Chain Extension for TLS",
              Work in Progress, Internet-Draft, draft-ietf-tls-dnssec-
              chain-extension-07, 21 March 2018,

              Schwartz, B., "Service Binding Mapping for DNS Servers",
              Work in Progress, Internet-Draft, draft-schwartz-svcb-dns-
              02, 17 February 2021,

   [QMIN]     Bortzmeyer, S., "DNS Query Name Minimisation to Improve
              Privacy", RFC 7816, DOI 10.17487/RFC7816, March 2016,

   [REGISTRY] ICANN, "ICANN Registry Agreement", July 2017,

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,

   [RFC7719]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", RFC 7719, DOI 10.17487/RFC7719, December
              2015, <https://datatracker.ietf.org/doc/html/rfc7719>.

   [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-03, 17 February 2021,


   TODO acknowledge.

Authors' Addresses

   Tommy Pauly
   Apple Inc.

   Email: tpauly@apple.com

   Eric Rescorla

   Email: ekr@rtfm.com

   David Schinazi
   Google LLC

   Email: dschinazi.ietf@gmail.com

   Christopher A. Wood

   Email: caw@heapingbits.net