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Versions: (draft-dnsop-refuse-any) 00 01 02 03 04 05 06

Network Working Group                                           J. Abley
Internet-Draft                                                 Dyn, Inc.
Updates: 1035 (if approved)                               O. Gudmundsson
Intended status: Standards Track                            M. Majkowski
Expires: May 20, 2017                                    Cloudflare Inc.
                                                       November 16, 2016


    Providing Minimal-Sized Responses to DNS Queries with QTYPE=ANY
                     draft-ietf-dnsop-refuse-any-03

Abstract

   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.

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 http://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 May 20, 2017.

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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   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
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Motivations . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  General Approach  . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Behaviour of DNS Responders . . . . . . . . . . . . . . . . .   4
   5.  Behaviour of DNS Initiators . . . . . . . . . . . . . . . . .   5
   6.  HINFO Considerations  . . . . . . . . . . . . . . . . . . . .   5
   7.  Updates to RFC 1035 . . . . . . . . . . . . . . . . . . . . .   6
   8.  Implementation Experience . . . . . . . . . . . . . . . . . .   6
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     12.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     12.2.  Informative References . . . . . . . . . . . . . . . . .   7
     12.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Appendix A.  Editorial Notes  . . . . . . . . . . . . . . . . . .   8
     A.1.  Change History  . . . . . . . . . . . . . . . . . . . . .   8
       A.1.1.  draft-ietf-dnsop-refuse-any-03  . . . . . . . . . . .   8
       A.1.2.  draft-ietf-dnsop-refuse-any-02  . . . . . . . . . . .   8
       A.1.3.  draft-ietf-dnsop-refuse-any-01  . . . . . . . . . . .   8
       A.1.4.  draft-ietf-dnsop-refuse-any-00  . . . . . . . . . . .   8
       A.1.5.  draft-jabley-dnsop-refuse-any-01  . . . . . . . . . .   8
       A.1.6.  draft-jabley-dnsop-refuse-any-00  . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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.







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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 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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY" and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  Motivations

   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.  Although there is no documented guidance
   available for this use case and some implementations have been
   observed that appear not to function as perhaps their developers
   expected.  For any developer that assumes that ANY query will be sent
   to authoritative server to fetch all RRSets, they need to include a
   fallback when that does not happen.

   ANY queries are also 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
   amplifiers.

   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 MAY choose to 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.







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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
   resolvers).

   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 caused them to 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.

   This proposal specifies two different modes of behaviour by DNS
   responders for names that exists.  Operators/Implementers are free to
   choose whichever mechanism best suits their environment.

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

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

4.  Behaviour of DNS Responders

   A DNS responder which receives an ANY query MAY decline to provide a
   conventional response, or MAY instead send a response with a single
   RRSet in the answer section.

   The RRSet returned in the answer section of the response MAY be a
   single RRSet owned by the name specified in the QNAME.  Where
   multiple RRSets exist, the responder SHOULD choose a small one(s) to
   reduce its amplification potential.

   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



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   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 a synthesised 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.

   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.

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

5.  Behaviour of DNS Initiators

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

   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.

6.  HINFO Considerations

   In the case where a zone that contains HINFO RRSets is served from an
   authority server that does not provide conventional ANY responses.
   It is possible that the HINFO RRSet in an ANY response, once cached
   by the initiator, might suppress subsequent queries from the same
   initiator with QTYPE=HINFO.  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 MAY sensibly choose not to
   deploy the mechanism described in this document or select another
   type.






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

7.  Updates to RFC 1035

   It is important to note that returning a subset of available RRSets
   when processing an ANY query is legitimate and consistent with
   [RFC1035]; ANY does not mean ALL.

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

   RRSIG queries have the same potential as ANY queries of generating
   large answers as well as extra work.  DNS implementations are free to
   not return all RRSIGS.  In the wild there are implimentations that
   return REFUSE, others return single RRSIG, etc.

8.  Implementation Experience

   In October 2015 Cloudflare Authoritative Nameserver implementation
   implemented the HINFO response.  Few minor problems have been
   reported and worked out.  NSD has for a while implemented a sub-set
   response.  A Bind user implemented this draft suggestion of returning
   only single RRset during an attack.

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
   amplifier.

   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:






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

11.  Acknowledgements

   Evan Hunt and 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 while under attack.  A large number of people have
   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,
              <http://www.rfc-editor.org/info/rfc1034>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <http://www.rfc-editor.org/info/rfc1035>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

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,
              <http://www.rfc-editor.org/info/rfc5358>.

   [RFC6895]  Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
              April 2013, <http://www.rfc-editor.org/info/rfc6895>.

   [RFC7719]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", RFC 7719, DOI 10.17487/RFC7719, December
              2015, <http://www.rfc-editor.org/info/rfc7719>.




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12.3.  URIs

   [1] http://www.iana.org/assignments/dns-parameters/dns-
       parameters.xhtml#dns-parameters-4

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-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
   experience.

A.1.2.  draft-ietf-dnsop-refuse-any-02

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

   Number of text suggestions from Jeremy Laidman

A.1.3.  draft-ietf-dnsop-refuse-any-01

   Add IANA Considerations

A.1.4.  draft-ietf-dnsop-refuse-any-00

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

A.1.5.  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.1.6.  draft-jabley-dnsop-refuse-any-00

   Initial draft circulated for comment.







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Authors' Addresses

   Joe Abley
   Dyn, Inc.
   103-186 Albert Street
   London, ON  N6A 1M1
   Canada

   Phone: +1 519 670 9327
   Email: jabley@dyn.com


   Olafur Gudmundsson
   Cloudflare Inc.

   Email: olafur+ietf@cloudflare.com


   Marek Majkowski
   Cloudflare Inc.

   Email: marek@cloudflare.com





























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