Returning extra answers in DNS responses.

Versions: 00 01 02 03 04 05                                             
dnsop                                                          W. Kumari
Internet-Draft                                                    Google
Intended status: Standards Track                                  Z. Yan
Expires: July 15, 2015                                             CNNIC
                                                             W. Hardaker
                                                           Parsons, Inc.
                                                        January 11, 2015

             Returning multiple answers in a DNS response.


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

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on July 15, 2015.

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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  . . . . . . . . . . . . . . . . .   3
   5.  Additional records pseudo-RR  . . . . . . . . . . . . . . . .   4
   6.  Signalling support  . . . . . . . . . . . . . . . . . . . . .   5
   7.  Use of Additional information . . . . . . . . . . . . . . . .   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     11.2.  Informative References . . . . . . . . . . . . . . . . .   7
   Appendix A.  Changes / Author Notes.  . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Often the name being resolved in the DNS provides information about
   why the name is being resolved, allowing the authoritative name
   server operator to predict what other answers the client will soon
   query for.  By providing multiple answers in the response, the
   authoritative name server operator can ensure that the recursive
   server that the client is using has all the answers in its cache.

   For example, the name server operator of Example Widgets, Inc
   ( knows that the web page at
   contains various resources, including some images (served from, some Cascading Style Sheets (served from and some JavaScript (  A client
   attempting to resolve is very likely to be a web
   browser, rendering the page, and so will need to also resolve all of
   the other names for these other resources.  Providing all of these
   answers in response to a query for allows the
   recursive server to populate its cache and have all of the answers
   available when the client asks for them.

   Other examples where this technique is useful include SMTP (including
   the mail server address when serving the MX record), SRV (providing
   the target information in addition to the SRV response) and TLSA
   (providing any TLSA records associated with a name).

   This is purely an optimization - by providing all of other, related
   answers that the client is likely to need along with the answer that
   they requested, users get a better experience, iterative servers need

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   to perform 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 allow for additional 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 information (other than for "glue").
   For some more background, see [Ref.Bellovin], [RFC1034], [RFC2181].

   Not trusting the information in the additional section was necessary
   because there was no way to authenticate it.  If you queried for and got back answers for you couldn't
   tell if these were actually from or if an attacker was
   trying to get bad information for into your cache.  In a
   world of ubiquitous DNSSEC deployment [Ed note: By the time this
   document is published, there *will* be ubiquitous DNSSEC :-) ] the
   iterative server can validate the information and trust it.

3.  Terminology

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

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

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

4.  Returning multiple answers

   The authoritative nameserver should include as many of the instructed
   Additional records and Supporting information as will fit in the
   response packet.

   In order to include Additional records in a response, certain
   conditions need to be met.  [Ed note: Some discussion on each rule is

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   1.  Additional records MUST only be included when the primary name is
       DNSSEC secured.

   2.  Additional records MUST only be served over TCP connections.
       This is to mitigate Denial of Service reflection attacks.[1]

   3.  Additional records MUST be leaf records at the same node in the
       DNS tree[2]

   4.  The DNSSEC supporting information must be included.  This is the
       RRSIGs required to validate the Additional record information.

   5.  All of the records MUST be signed with the same DNSSEC keys.

   6.  The authoritative nameserver SHOULD include as many of the
       additional records as will fit in the response.  Each Additional
       record MUST have its matching Supporting information.  Additional
       records MUST be inserted in the order specified in the Additional
       records list.

   7.  Operators SHOULD only include Additional answers that they expect
       a client to actually need. [3]

   [Ed note 1: The above MAY be troll bait.  I'm not really sure if this
   is a good idea or not - moving folk towards TCP is probably a good
   idea, and this is somewhat of an optional record type.  Then again,
   special handing (TCP only) for a record would be unusual.  Additional
   records could cause responses to become really large, but there are
   already enough large records that can be used for reflection attacks
   that we can just give up on the whole "keep responses as small as
   possible" ship.  ]

   [Ed note 2: This is poorly worded.  I mumbled about bailiwick,
   subdomains, etc but nothing I could come up with was better.  Also,
   is this rule actually needed?  I *think* it would be bad for .com
   servers to be able to include Additional records for, but perhaps <handwave>public-suffix-
   list?! This rule also makes it easier to decide what all DNSSEC
   information is required.]

   [Ed note 3: This is not enforceable. ]

5.  Additional records pseudo-RR

   To allow the authoritative nameserver operator to configure what
   additional records to serve when it receives a query to a label, we
   introduce the Additional pseudo Resource Record (RR).  This is a
   pseudo-record as it provides instruction to the authoritative

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   nameserver, and does not appear on the wire.  [Ed note: I had
   originally considered a comment, or some sort of format where we
   listed additional records under the primary one, but we a: wanted it
   to survive zone transfers, and b: not trip up zone file parsers. ]

   The format of the Additional pseudo-RR is:

   label ADD "label,typel; label,type; label,type; ..."

   For example, if the operator of would like to also return
   A record answers for, and both an
   A and AAAA for when queried for he
   would enter:

   www ADD "images,A; css,A; data,A; data,AAA;"

   The entries in the ADD list are ordered.  An authoritative nameserver
   MUST attempt to 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 to not fit.  The TTL of the
   records added to the Additional section are the same as if queried

   In some cases the operator might not really know what all additional
   records clients need.  For example, the owner of may
   have outsourced his DNS operations to a third party.  The DNS
   operator may be able to mine their query logs, and see that, in a
   large majority of cases, a recursive server asks for
   and then very soon after asks for, 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 re outside the
   scope of this document.

6.  Signalling support

   Iterative nameservers that support Additional records signal this by
   setting the Z bit (bit 25 of the DNS header).

   [RFC5395] Section 2.1 says:

   There have been ancient DNS implementations for which the Z bit
   being on in a query meant that only a response from the primary
   server for a zone is acceptable. It is believed that current
   DNS implementations ignore this bit.

   Assigning a meaning to the Z bit requires an IETF Standards Action.

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   [ Ed note: Hey, was worth a try :-) I'm fine with an EDNS0 bit
   instead... ]

7.  Use of Additional information

   When receiving Additional information, an iterative server 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, and help stop
       cache filling attacks.

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

8.  IANA Considerations

   This document contains no IANA considerations.Template: Fill this in!

9.  Security Considerations

   Additional records will make DNS responses even larger than they are
   currently, leading to more large records that can be used for DNS
   reflection attacks.  We mitigate this by only serving these over TCP.

   A malicious authorative server could include a large number of
   Additional records (and associated DNSSEC information) and attempt to
   DoS the recursive by making it do lots of DNSSEC validation.  I don't
   view this as a very serious threat (CPU for validation is cheap
   compared to bandwith), but we mitigate this by allowing the iterative
   to ignore Additional records whenever it wants.

   By requiring the ALL of the Additional records are signed, and all
   necessary DNSSEC information for validation be included we avoid
   cache poisoning (I hope :-))

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10.  Acknowledgements

   The authors wish to thank some folk.

11.  References

11.1.  Normative References

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

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

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

   [RFC5395]  Eastlake, D., "Domain Name System (DNS) IANA
              Considerations", RFC 5395, November 2008.

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

11.2.  Informative References

              Manderson, T., Vegoda, L., and S. Kent, "RPKI Objects
              issued by IANA", draft-ietf-sidr-iana-objects-03 (work in
              progress), May 2011.

Appendix A.  Changes / Author Notes.

   [RFC Editor: Please remove this section before publication ]

   From -00 to -01.

   o  Nothing changed in the template!

Authors' Addresses

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   Warren Kumari
   1600 Amphitheatre Parkway
   Mountain View, CA  94043


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


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


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