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dnsop                                                          W. Kumari
Internet-Draft                                                    Google
Intended status: Standards Track                                  Z. Yan
Expires: December 28, 2016                                         CNNIC
                                                             W. Hardaker
                                                           Parsons, Inc.
                                                           June 26, 2016

               Returning extra answers in DNS responses.


   This document (re)introduces the ability to provide multiple answers
   in a DNS response.

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
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   This Internet-Draft will expire on December 28, 2016.

Copyright Notice

   Copyright (c) 2016 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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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements notation . . . . . . . . . . . . . . . . . .   3
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Returning multiple answers  . . . . . . . . . . . . . . . . .   4
   5.  The EXTRA Resource Record . . . . . . . . . . . . . . . . . .   4
     5.1.  File Format . . . . . . . . . . . . . . . . . . . . . . .   5
     5.2.  Wire Format . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Signaling support . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Stub-Resolver Considerations  . . . . . . . . . . . . . . . .   6
   8.  Use of Additional information . . . . . . . . . . . . . . . .   6
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   10. Security Considerations . . . . . . . . . . . . . . . . . . .   6
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   12. Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Appendix A.  Changes / Author Notes.  . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   In many cases a name being resolved in the DNS provides the reason
   behind why the name is being resolved.  This may allow the
   authoritative nameserver to predict what other answers a recursive
   resolver will soon query for.  By providing multiple answers in the
   response, the authoritative name server operator can assist a caching
   recursive resolver in pre-populating its cache before a stub resolver
   or other client asks for the subsequent queries.  Apart from
   decreasing the latency for end users [RFC6555], this also decreases
   the total number of queries that the recursive resolver needs to send
   and the autorative server needs to answer.

   For example, the domain name administrator of Example Widgets, Inc
   (example.com) knows that the web page at www.example.com contains
   various other resources, including some images (served from
   images.example.com), some Cascading Style Sheets (served from
   css.example.com) and some JavaScript (served from data.example.com).
   An application attempting to resolve www.example.com is very likely
   to be a web browser rendering the page and will likely also need to
   resolve all of these additional names as well.  Providing all of
   these answers in response to a query for www.example.com allows the
   recursive resolver to pre-populate its cache and have these answers
   available immediately when a stub resolver or other DNS client asks
   for them.

   Other examples where this technique may be useful include SMTP (by
   including mail server addresses, SPF and DKIM records when serving

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   the MX record), SRV (by providing the target information in addition
   to the SRV response) and TLSA (by providing any TLSA records
   associated with a name).  This same technique can also be used to
   include both the IPv4 (A) and IPv6 (AAAA) addresses for any singular
   address query.

   This technique, described in this document, is purely an optimization
   and enables a zone publisher to distribute other related answers that
   the client is likely to need along with an answer to the original
   request.  Users get a better experience, recursive resolvers need to
   send less queries, authoritative servers have to answer fewer
   queries, etc.

1.1.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Background

   The existing DNS specifications [RFC1034] allow for supplemental
   information to be included in the "additional" section of the DNS
   response, but in order to defeat cache poisoning attacks most
   implementations either ignore or don't trust additional records they
   didn't ask for.  For more background, see [Ref.Bellovin] and

   Not trusting the information in the additional section was necessary
   since there was no way to authenticate it.  If a resolver queries for
   www.example.com and received answers for www.invalid.com as well, it
   is impossible for a non-validating resolver to tell if these were
   actually from invalid.com or if an attacker was trying to push bad
   information for invalid.com into the resolver's cache.  In a world of
   ubiquitous DNSSEC deployment [Ed note: By the time this document is
   published, there *will* be ubiquitous DNSSEC :-) ], a validating
   resolver can validate, authenticate and trust the records in the
   additional information.

3.  Terminology

   Additional records  Additional records are records that the
      authoritative nameserver has included in the Additional section.

   EXTRTA Resource Record  The EXTRA resource record (defined below)
      carries a list fo additional records to send.

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   Primary query  A Primary query (or primary question) is a QNAME that
      the name server operator would like to return additional answers

   Supporting DNSSEC information  Supporting DNSSEC information is the
      DNSSEC RRSIGs that prove the authenticity of the Additional

   Stub Resolver  The term "Stub Resolver" is used in this document to
      refer to the most common instance of a DNS client sending DNS
      requests to a Recursive Resolver.  However, other DNS clients are
      not excluded from these usages and where we write "Stub Resolver"
      you may read it as "Stub Resolver or other DNS client".

4.  Returning multiple answers

   The authoritative nameserver should include as many of the instructed
   additional records identified by the Extra Resource Record and
   Supporting DNSSEC information as will fit in the response packet.
   These additional records (and Supporting DNSSEC information) are
   appended to the additional section of the response.

   In order to include additional records in a response, these
   conditions need to be met:

   1.  Additional records MUST only be included when the Name Server is
       authoritative for the zone, and the records to be returned are
       DNSSEC signed.

   2.  The supporting DNSSEC information necessary to perform validation
       on the records MUST be included.

   3.  The Authoritative Name Server SHOULD include as many of the
       additional records as will fit in the response.  Additional
       records SHOULD be inserted in the order specified in the
       Additional records list.

   4.  Zone administrators SHOULD only include records identified in the
       EXTRA Resource Records that they expect a client to need.

5.  The EXTRA Resource Record

   To allow a zone content administrator to instruct the name server
   which additional records to serve when it receives a query to a
   label, we introduce the EXTRA Resource Record (RR).  These additional
   records are appended to the additional section (note that the EXTRA
   RR itself is not appended).  The EXTRA resource record MAY still be

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   queried for directly (e.g for debugging), in which case the record
   itself is returned.

5.1.  File Format

   The format of the Extra RR is:

   label EXTRA "label,type; label,type; label,type; ..."

   For example, if the operator of example.com would like to also return
   A record answers for images.example.com, css.html.example.com and
   both an A and AAAA for data.example.com when queried for
   www.example.com, they would create the following record:

   www.example.com.  EXTRA "images,A; css,A; data,A; data,AAA;"

   The entries in the EXRTA list are ordered.  An authoritative
   nameserver SHOULD insert the records in the order listed when filling
   the response packet.  This is to allow the operator to express a
   preference in case all the records will not fit in the response.  The
   TTL of the records added to the Additional section are MUST be the
   same as if queried directly.

   In some cases a zone content administrator might not know what all
   additional records clients need.  For example, the owner of
   www.example.com may have outsourced his DNS operations to a third
   party.  DNS administrators may be able to mine their query logs, and
   see that, in a large majority of cases, a recursive server asks for
   foo.example.com and then very soon after asks for bar.example.com,
   and so may decide to optimize this by opportunistically returning bar
   when queried for foo.  This functionality could also be included in
   the authoritative name server software itself, but discussions of
   these are outside the scope of this document.

5.2.  Wire Format

   The wire format of the EXTRA RR is the same as the wire format for a
   TXT RR:

       /                   TXT-DATA                    /

   Where TXT-DATA is one or more <character-string>s.

   The Extra RR has RR type TBD [RFC Editor: insert the IANA assigned
   value and delete this note]

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6.  Signaling support

   Recursive Resolvers (or other DNS clients) that support EXTRA records
   MAY signal this by setting the OPT record's EXTRA bit (bit NN [TBD:
   assigned by IANA] in the EDNS0 extension header to 1.

7.  Stub-Resolver Considerations

   No modifications need to be made to stub-resolvers to get the
   predominate benefit of this protocol, since the majority of the speed
   gain will take place between the validating recursive resolver and
   the authoritative name server.  However, stub resolvers may choose to
   support this technique, and / or may query directly for the EXTRA RR
   if it wants to pre-query for data that will likely be needed in the
   process of supporting its application.

8.  Use of Additional information

   When receiving additional records in the additional section, a
   resolver follows certain rules:

   1.  Additional records MUST be validated before being used.

   2.  Additional records SHOULD be annotated in the cache as having
       been received as Additional records.

   3.  Additional records SHOULD have lower priority in the cache than
       answers received because they were requested.  This is to help
       evict Additional records from the cache first (to help prevent
       cache filling attacks).

   4.  Recursive resolvers MAY choose to ignore Additional records for
       any reason, including CPU or cache space concerns, phase of the
       moon, etc.  It may choose to accept all, some or none of the
       Additional records.

9.  IANA Considerations

   This document contains the following IANA assignment requirements:

   1.  The EXTRA bit discussed in Section 6 needs to be allocated.

10.  Security Considerations

   Additional records will make DNS responses even larger than they are
   currently, leading to larger records that can be used in DNS
   reflection attacks.  One could mitigate this by only serving
   responses to EXTRA requests over TCP or when using Cookies [RFC5395],

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   although there is no easy way to signal this to a client other than
   through the use of the truncate bit.

   A malicious authoritative server could include a large number of
   extra records (and associated DNSSEC information) and attempt to DoS
   the recursive by making it do lots of DNSSEC validation.  However,
   this is not considered a realistic threat; CPU for validation is
   cheap compared to bandwidth.  This can be mitigated by allowing the
   recursive resolver to ignore Additional records whenever it considers
   itself under attack or its CPU resources are otherwise over-

   This specification requires that the all of the Additional records
   are signed, and all necessary DNSSEC information for validation be
   included to avoid cache poisoning attacks.

11.  Acknowledgements

   The authors wish to thank some folk.

12.  Normative References

              Bellovin, S., "Using the Domain Name System for System
              Break-Ins", 1995,

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

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

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,

   [RFC5395]  Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations", RFC 5395, DOI 10.17487/RFC5395, November
              2008, <http://www.rfc-editor.org/info/rfc5395>.

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, DOI 10.17487/RFC6555, April
              2012, <http://www.rfc-editor.org/info/rfc6555>.

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   [RFC7873]  Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
              Cookies", RFC 7873, DOI 10.17487/RFC7873, May 2016,

Appendix A.  Changes / Author Notes.

   [RFC Editor: Please remove this section before publication ]

   From -00 to -01.

   o  Nothing changed in the template!

   From -02 to -3:

      Sat down and rewrote and cleaned up large sections of text.

      Changed name of RR from Additional to EXTRA (the term "Additional"
      is overloaded in general)

      Clarified that stub resolvers and other clients MAY use this

      Attempted to clarify that the individual RRs are added to the
      response, not the EXTRA record itself.  The EXTRA RR can be
      queried directly.

Authors' Addresses

   Warren Kumari
   1600 Amphitheatre Parkway
   Mountain View, CA  94043

   Email: warren@kumari.net

   Zhiwei Yan
   No.4 South 4th Street, Zhongguancun
   Beijing  100190
   P. R. China

   Email: yanzhiwei@cnnic.cn

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   Wes Hardaker
   Parsons, Inc.
   P.O. Box 382
   Davis, CA  95617

   Email: ietf@hardakers.net

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