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

Network Working Group                                           J. Abley
Internet-Draft                                                 Dyn, Inc.
Updates: 1035 (if approved)                               O. Gudmundsson
Intended status: Standards Track                            M. Majkowski
Expires: August 12, 2017                                 Cloudflare Inc.
                                                       February 08, 2017


  Providing Minimal-Sized Responses to DNS Queries that have QTYPE=ANY
                     draft-ietf-dnsop-refuse-any-04

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 August 12, 2017.

Copyright Notice

   Copyright (c) 2017 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
     4.1.  Select one RRSet mode . . . . . . . . . . . . . . . . . .   4
     4.2.  Synthesised HINFO RRset . . . . . . . . . . . . . . . . .   5
     4.3.  Guess intention . . . . . . . . . . . . . . . . . . . . .   5
     4.4.  Responding to queries over TCP  . . . . . . . . . . . . .   5
   5.  Behaviour of DNS Initiators . . . . . . . . . . . . . . . . .   6
   6.  HINFO Considerations  . . . . . . . . . . . . . . . . . . . .   6
   7.  Updates to RFC 1035 . . . . . . . . . . . . . . . . . . . . .   6
   8.  Implementation Experience . . . . . . . . . . . . . . . . . .   7
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     12.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     12.2.  Informative References . . . . . . . . . . . . . . . . .   8
     12.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Appendix A.  Editorial Notes  . . . . . . . . . . . . . . . . . .   8
     A.1.  Change History  . . . . . . . . . . . . . . . . . . . . .   8
       A.1.1.  draft-ietf-dnsop-refuse-any-04  . . . . . . . . . . .   8
       A.1.2.  draft-ietf-dnsop-refuse-any-03  . . . . . . . . . . .   9
       A.1.3.  draft-ietf-dnsop-refuse-any-02  . . . . . . . . . . .   9
       A.1.4.  draft-ietf-dnsop-refuse-any-01  . . . . . . . . . . .   9
       A.1.5.  draft-ietf-dnsop-refuse-any-00  . . . . . . . . . . .   9
       A.1.6.  draft-jabley-dnsop-refuse-any-01  . . . . . . . . . .   9
       A.1.7.  draft-jabley-dnsop-refuse-any-00  . . . . . . . . . .   9
   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.






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   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 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.

4.  Behaviour of DNS Responders

   Below are the three different modes of behaviour by DNS responders
   for names that exists that are used, listed in the order of
   preference.  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 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 likely
       RRsets may add up to a large answer.

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

4.1.  Select one RRSet mode

   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.




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   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 the zone is signed RRSIG records MUST be included in the answer

4.2.  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 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.

4.3.  Guess 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 of CNAME or (MX A and AAAA)
   RRsets that are present.  This is not a guess but a heuristic that
   seems to work well in practice.  The main drawback is the size of the
   answer.

   As in the first one if the zone is signed RRSIG MUST be returned if
   there the DO bit is set on query.

4.4.  Responding to queries over TCP

   There has been a desire to specify that a ANY query over TCP get full
   response.  This document does not specify that as that is best left
   to the operator to decide.  Implementers SHOULD provide an option for
   operators to specify behavior over TCP.



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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.

   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.  Similarly it is fine to
   replay back exactly what Authoritative server returned to ANY query.

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
   type.

   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 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.  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 have the same potential as ANY queries of generating
   large answers as well as extra work.  DNS implementations are free to
   not return all RRSIG records.  In the wild there are implementations
   that return REFUSE, others return single RRSIG, etc.  This document
   recommends returning a single RRSIG in this case.



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8.  Implementation Experience

   In October 2015 Cloudflare Authoritative Name server 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, his code is now in the current
   release.

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:

   +------+-------+-------------------------------+--------------------+
   | 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.








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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>.

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-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



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   behavior over TCP.  The document is clear in how it updates from
   RFC1035.  Minor adjustments for readability and remove redundancy.

A.1.2.  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.1.3.  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.1.4.  draft-ietf-dnsop-refuse-any-01

   Add IANA Considerations

A.1.5.  draft-ietf-dnsop-refuse-any-00

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

A.1.6.  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.1.7.  draft-jabley-dnsop-refuse-any-00

   Initial draft circulated for comment.

Authors' Addresses

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

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




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   Olafur Gudmundsson
   Cloudflare Inc.

   Email: olafur+ietf@cloudflare.com


   Marek Majkowski
   Cloudflare Inc.

   Email: marek@cloudflare.com









































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