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Providing Minimal-Sized Responses to DNS Queries That Have QTYPE=ANY

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8482.
Authors Joe Abley , Ólafur Guðmundsson , Marek Majkowski , Evan Hunt
Last updated 2019-01-10 (Latest revision 2018-08-14)
Replaces draft-dnsop-refuse-any
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Tim Wicinski
Shepherd write-up Show Last changed 2018-07-06
IESG IESG state Became RFC 8482 (Proposed Standard)
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Warren "Ace" Kumari
Send notices to "Tim Wicinski" <>
IANA IANA review state IANA OK - Actions Needed
IANA action state RFC-Ed-Ack
Network Working Group                                           J. Abley
Internet-Draft                                                   Afilias
Updates: 1034, 1035 (if approved)                         O. Gudmundsson
Intended status: Standards Track                            M. Majkowski
Expires: February 15, 2019                               Cloudflare Inc.
                                                                 E. Hunt
                                                         August 14, 2018

  Providing Minimal-Sized Responses to DNS Queries that have QTYPE=ANY


   The Domain Name System (DNS) specifies a query type (QTYPE) "ANY".
   The operator of an authoritative DNS server might choose not to
   respond to such queries for reasons of local policy, motivated by
   security, performance or other reasons.

   The DNS specification does not include specific guidance for the
   behaviour of DNS servers or clients in this situation.  This document
   aims to provide such guidance.

   This document updates RFC 1034 and RFC 1035.

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

   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 February 15, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Motivations for Use of ANY Queries  . . . . . . . . . . . . .   3
   3.  General Approach  . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Behaviour of DNS Responders . . . . . . . . . . . . . . . . .   4
     4.1.  Answer with a Subset of Available RRSets  . . . . . . . .   5
     4.2.  Answer with a Synthesised HINFO RRSet . . . . . . . . . .   5
     4.3.  Answer with Best Guess as to Intention  . . . . . . . . .   6
     4.4.  Behaviour with TCP Transport  . . . . . . . . . . . . . .   6
   5.  Behaviour of DNS Initiators . . . . . . . . . . . . . . . . .   6
   6.  HINFO Considerations  . . . . . . . . . . . . . . . . . . . .   7
   7.  Updates to RFC 1034 and RFC 1035  . . . . . . . . . . . . . .   7
   8.  Implementation Experience . . . . . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     12.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     12.2.  Informative References . . . . . . . . . . . . . . . . .   9
     12.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Appendix A.  Editorial Notes  . . . . . . . . . . . . . . . . . .  10
     A.1.  Change History  . . . . . . . . . . . . . . . . . . . . .  10
       A.1.1.  draft-ietf-dnsop-refuse-any-07  . . . . . . . . . . .  10
       A.1.2.  draft-ietf-dnsop-refuse-any-06  . . . . . . . . . . .  10
       A.1.3.  draft-ietf-dnsop-refuse-any-05  . . . . . . . . . . .  10
       A.1.4.  draft-ietf-dnsop-refuse-any-04  . . . . . . . . . . .  10
       A.1.5.  draft-ietf-dnsop-refuse-any-03  . . . . . . . . . . .  10
       A.1.6.  draft-ietf-dnsop-refuse-any-02  . . . . . . . . . . .  10
       A.1.7.  draft-ietf-dnsop-refuse-any-01  . . . . . . . . . . .  11
       A.1.8.  draft-ietf-dnsop-refuse-any-00  . . . . . . . . . . .  11
       A.1.9.  draft-jabley-dnsop-refuse-any-01  . . . . . . . . . .  11
       A.1.10. draft-jabley-dnsop-refuse-any-00  . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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1.  Introduction

   The Domain Name System (DNS) specifies a query type (QTYPE) "ANY".
   The operator of an authoritative DNS server might choose not to
   respond to such queries for reasons of local policy, motivated by
   security, performance or other reasons.

   The DNS specification [RFC1034] [RFC1035] does not include specific
   guidance for the behaviour of DNS servers or clients in this
   situation.  This document aims to provide such guidance.

1.1.  Terminology

   This document uses terminology specific to the Domain Name System
   (DNS), descriptions of which can be found in [RFC7719].

   In this document, "ANY Query" refers to a DNS meta-query with
   QTYPE=ANY.  An "ANY Response" is a response to such a query.

   In this document, "conventional ANY response" means an ANY response
   that is constructed in accordance with the algorithm documented in
   section 4.3.2 of [RFC1034] and specifically without implementing any
   of the mechanisms described in this document.

   In an exchange of DNS messages between two hosts, this document
   refers to the host sending a DNS request as the initiator, and the
   host sending a DNS response as the responder.

   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.

2.  Motivations for Use of ANY Queries

   ANY queries are legitimately used for debugging and checking the
   state of a DNS server for a particular name.

   ANY queries are sometimes used as a attempt to reduce the number of
   queries needed to get information, e.g. to obtain MX, A and AAAA
   RRSets for a mail domain in a single query.  There is no documented
   guidance available for this use case, however, and some
   implementations have been observed not to function as perhaps their
   developers expected.  Implementers that assume that an ANY query will
   ultimately be received by an authoritative server and will fetch all
   existing RRSets, should include a fallback mechanism to use when that
   does not happen.

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   ANY queries are frequently used to exploit the amplification
   potential of DNS servers/resolvers using spoofed source addresses and
   UDP transport (see [RFC5358]).  Having the ability to return small
   responses to such queries makes DNS servers less attractive

   ANY queries are sometimes used to help mine authoritative-only DNS
   servers for zone data, since they are expected to return all RRSets
   for a particular query name.  If a DNS operator prefers to reduce the
   potential for information leaks, they might choose not to send large
   ANY responses.

   Some authoritative-only DNS server implementations require additional
   processing in order to send a conventional ANY response, and avoiding
   that processing expense might be desirable.

3.  General Approach

   This proposal provides a mechanism for an authority server to signal
   that conventional ANY queries are not supported for a particular
   QNAME, and to do so in such a way that is both compatible with and
   triggers desirable behaviour by unmodified clients (e.g.  DNS

   Alternative proposals for dealing with ANY queries have been
   discussed.  One approach proposed using a new RCODE to signal that an
   authoritative server did not answer ANY queries in the standard way.
   This approach was found to have an undesirable effect on both
   resolvers and authoritative-only servers; resolvers receiving an
   unknown RCODE would re-send the same query to all available
   authoritative servers, rather than suppress future such ANY queries
   for the same QNAME.

   This proposal avoids that outcome by returning a non-empty RRSet in
   the ANY response, providing resolvers with something to cache and
   effectively suppressing repeat queries to the same or different
   authority servers.

4.  Behaviour of DNS Responders

   Below are the three different modes of behaviour by DNS responders
   when processing queries with QNAMEs that exist, QCLASS=IN and
   QTYPE=ANY.  Operators/Implementers are free to choose whichever
   mechanism best suits their environment.

   1.  A DNS responder can choose to select one or a larger subset of
       the available RRSets at the QNAME.

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   2.  A DNS responder can return a synthesised HINFO resource record.
       See Section 6 for discussion of the use of HINFO.

   3.  Resolver can try to give out the most likely records the
       requester wants.  This is not always possible and the result
       might well be a large response.

   Except as described below in this section, the DNS responder MUST
   follow the standard algorithms when constructing a response.

4.1.  Answer with a Subset of Available RRSets

   A DNS responder which receives an ANY query MAY decline to provide a
   conventional ANY response, or MAY instead send a response with a
   single RRSet (or a larger subset of available RRSets) in the answer

   The RRSets returned in the answer section of the response MAY consist
   of a single RRSet owned by the name specified in the QNAME.  Where
   multiple RRSets exist, the responder SHOULD choose a small subset of
   those avialable to reduce the amplification potential of the

   If the zone is signed, appropriate RRSIG records MUST be included in
   the answer.

   Note that this mechanism does not provide any signalling to indicate
   to a client that an incomplete subset of the available RRSets has
   been returned.

4.2.  Answer with a Synthesised HINFO RRSet

   If there is no CNAME present at the owner name matching the QNAME,
   the resource record returned in the response MAY instead be
   synthesised, in which case a single HINFO resource record SHOULD be
   returned.  The CPU field of the HINFO RDATA SHOULD be set to RFCXXXX
   [note to RFC Editor, replace with RFC number assigned to this
   document].  The OS field of the HINFO RDATA SHOULD be set to the null
   string to minimize the size of the response.

   The TTL encoded for the synthesised HINFO RR SHOULD be chosen by the
   operator of the DNS responder to be large enough to suppress frequent
   subsequent ANY queries from the same initiator with the same QNAME,
   understanding that a TTL that is too long might make policy changes
   relating to ANY queries difficult to change in the future.  The
   specific value used is hence a familiar balance when choosing TTL for
   any RR in any zone, and be specified according to local policy.

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   If the DNS query includes DO=1 and the QNAME corresponds to a zone
   that is known by the responder to be signed, a valid RRSIG for the
   RRSets in the answer (or authority if answer is empty) section MUST
   be returned.  In the case of DO=0, the RRSIG SHOULD be omitted.

   A system that receives an HINFO response SHOULD NOT infer that the
   response was generated according to this specification and apply any
   special processing of the response, since in general it is not
   possible to tell with certainty whether the HINFO RRSet received was
   synthesised.  In particular, systems SHOULD NOT rely upon the HINFO
   RDATA described in this seection to distinguish between synthesised
   and non-synthesised HINFO RRSets.

4.3.  Answer with Best Guess as to Intention

   In some cases it is possible to guess what the initiator wants in the
   answer (but not always).  Some implementations have implemented the
   spirit of this document by returning all RRSets of RRType CNAME, MX,
   A and AAAA that are present at the owner name but suppressing others.
   This heuristic seems to work well in practice, satisfying the needs
   of some applications whilst suppressing other RRSets such as TXT and
   DNSKEY that can often contribute to large responses.  Whilst some
   applications may be satisfied by this behaviour, the resulting
   responses in the general case are larger than the approaches
   described in Section 4.1 and Section 4.2.

   As before, if the zone is signed and the DO bit is set on the
   corresponding query, an RRSIG RRSet MUST be included in the response.

4.4.  Behaviour with TCP Transport

   A DNS responder MAY behave differently when processing ANY queries
   received over different transport, e.g. by providing a conventional
   ANY response over TCP whilst using one of the other mechanisms
   specified in this document in the case where a query was received
   using UDP.

   Implementers SHOULD provide configuration options to allow operators
   to specify different behaviour over UDP and TCP.

5.  Behaviour of DNS Initiators

   A DNS initiator which sends a query with QTYPE=ANY and receives a
   response containing an HINFO resource record or a single RRset, as
   described in Section 4, MAY cache the response in the normal way.
   Such cached resource records SHOULD be retained in the cache
   following normal caching semantics, as it would with any other
   response received from a DNS responder.

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   A DNS initiator MAY suppress queries with QTYPE=ANY in the event that
   the local cache contains a matching HINFO resource record with
   RDATA.CPU field, as described in Section 4.  A DNS initiator MAY
   instead respond to such queries with the contents of the local cache
   in the usual way.

6.  HINFO Considerations

   It is possible that the synthesised HINFO RRSet in an ANY response,
   once cached by the initiator, might suppress subsequent queries from
   the same initiator with QTYPE=HINFO.  Thus the use of HINFO in this
   proposal would hence have effectively mask the HINFO RRSet present in
   the zone.

   Authority-server operators who serve zones that rely upon
   conventional use of the HINFO RRTYPE SHOULD sensibly choose the
   "single RRset" method described in this document or select another

   The HINFO RRTYPE is believed to be rarely used in the DNS at the time
   of writing, based on observations made at recursive servers,
   authority servers and in passive DNS.

7.  Updates to RFC 1034 and RFC 1035

   This document extends the specification for processing ANY queries
   described in section 4.3.2 of [RFC1034].

   It is important to note that returning a subset of available RRSets
   when processing an ANY query is legitimate and consistent with
   [RFC1035]; it can be argued that ANY does not always mean ALL, as
   used in section 3.2.3 of [RFC1035].  The main difference here is that
   the TC bit SHOULD NOT be set on the response indicating that this is
   not a complete answer.

   This document describes optional behaviour for both DNS initiators
   and responders, and implementation of the guidance provided by this
   document is OPTIONAL.

   RRSIG queries (i.e. queries with QTYPE=RRSIG) are similar to ANY
   queries in the sense that they have the potential to generate large
   responses as well as extra work for the responders that process them,
   e.g. in the case where signatures are generated on-the-fly.  RRSIG
   RRSets are not usually obtained using such explicit queries, but are
   rather included in the responses for other RRSets that the RRSIGs
   cover.  This document does not specify appropriate behaviour for
   RRSIG queries, but note that future such advice might well benefit

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   from consistency with and experience of the approaches for ANY
   queries described here.

8.  Implementation Experience

   In October 2015 Cloudflare Authoritative Name server implementation
   implemented the HINFO response.  A few minor problems were reported
   and have since been resolved.

   An implementation of the subset-mode response to ANY queries was
   implemented in NSD 4.1 in 2016.

   An implementation of a single RRSet response to an ANY query was made
   for BIND9 by Tony Finch, and that functionality was subsequently made
   available in production releases starting in BIND 9.11.

9.  Security Considerations

   Queries with QTYPE=ANY are frequently observed as part of reflection
   attacks, since a relatively small query can be used to elicit a large
   response; this is a desirable characteristic if the goal is to
   maximize the amplification potential of a DNS server as part of a
   volumetric attack.  The ability of a DNS operator to suppress such
   responses on a particular server makes that server a less useful

   The optional behaviour described in this document to reduce the size
   of responses to queries with QTYPE=ANY is compatible with the use of
   DNSSEC by both initiator and responder.

10.  IANA Considerations

   The IANA is requested to update the Resource Record (RR) TYPEs
   Registry [1] entry as follows:

   | Type | Value | Meaning                       | Reference          |
   | *    | 255   | A request for some or all     | [RFC1035][RFC6895] |
   |      |       | records the server has        | [This Document]    |
   |      |       | available                     |                    |

11.  Acknowledgements

   David Lawrence provided valuable observations and concrete
   suggestions.  Jeremy Laidman helped make the document better.  Tony
   Finch realized that this document was valuable and implemented it

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   while under attack.  Richard Gibson identified areas where more
   detail and accuracy was useful.  A large number of other people also
   provided comments and suggestions we thank them all for the feedback.

12.  References

12.1.  Normative References

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

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

12.2.  Informative References

   [RFC5358]  Damas, J. and F. Neves, "Preventing Use of Recursive
              Nameservers in Reflector Attacks", BCP 140, RFC 5358,
              DOI 10.17487/RFC5358, October 2008,

   [RFC6895]  Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
              April 2013, <>.

   [RFC7719]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", RFC 7719, DOI 10.17487/RFC7719, December
              2015, <>.

12.3.  URIs


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Appendix A.  Editorial Notes

   This section (and sub-sections) to be removed prior to publication.

A.1.  Change History

A.1.1.  draft-ietf-dnsop-refuse-any-07

   Address AD's concerns: more colour to describe updates to 1034/1035
   in the abstract; don't rely upon HINFO RDATA formatting; language
   cleanup around guess intent.  Add Evan as author (originator of the
   "choose one record" response idea).

A.1.2.  draft-ietf-dnsop-refuse-any-06

   Update RFC 1034 as well as RFC 1035; define the term "conventional
   ANY response"; soften and qualify ANY does not mean ALL; note that
   the subset mode response lacks signalling.

A.1.3.  draft-ietf-dnsop-refuse-any-05

   Minor editorial changes.  Soften advice on RRSIG queries.  Version

A.1.4.  draft-ietf-dnsop-refuse-any-04

   These are the changes requested during WGLC.  The title has been
   updated for readability The behavior section now contains description
   of three different approaches in order of preference.  Text added on
   behavior over TCP.  The document is clear in how it updates from
   RFC1035.  Minor adjustments for readability and remove redundancy.

A.1.5.  draft-ietf-dnsop-refuse-any-03

   Change section name to "Updates to RFC1034", few minor grammar
   changes suggested by Matthew Pounsett and Tony Finch.

   Text clarifications, reflecting experience, added implementation

A.1.6.  draft-ietf-dnsop-refuse-any-02

   Added suggestion to call out RRSIG is optional when DO=0.

   Number of text suggestions from Jeremy Laidman.

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A.1.7.  draft-ietf-dnsop-refuse-any-01

   Add IANA Considerations

A.1.8.  draft-ietf-dnsop-refuse-any-00

   Re-submitted with a different name following adoption at the dnsop WG
   meeting convened at IETF 94.

A.1.9.  draft-jabley-dnsop-refuse-any-01

   Make signing of RRSets in answers from signed zones mandatory.

   Document the option of returning an existing RRSet in place of a
   synthesised one.

A.1.10.  draft-jabley-dnsop-refuse-any-00

   Initial draft circulated for comment.

Authors' Addresses

   Joe Abley
   300-184 York Street
   London, ON  N6A 1B5

   Phone: +1 519 670 9327

   Olafur Gudmundsson
   Cloudflare Inc.


   Marek Majkowski
   Cloudflare Inc.


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   Evan Hunt
   950 Charter St
   Redwood City, CA  94063


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