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NXDOMAIN really means there is nothing underneath

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
Author Stéphane Bortzmeyer
Last updated 2015-11-06
Replaced by draft-ietf-dnsop-nxdomain-cut, RFC 8020
RFC stream (None)
Stream Stream state (No stream defined)
Consensus boilerplate Unknown
RFC Editor Note (None)
IESG IESG state I-D Exists
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Send notices to (None)
Domain Name System Operations (dnsop) Working Group        S. Bortzmeyer
Internet-Draft                                                     AFNIC
Intended status: Standards Track                        November 6, 2015
Expires: May 9, 2016

           NXDOMAIN really means there is nothing underneath


   This document states clearly that when a DNS resolver receives a
   response with status code NXDOMAIN, it means that the name in the
   question section AND ALL THE NAMES UNDER IT do not exist.

   REMOVE BEFORE PUBLICATION: this document should be discussed in the
   IETF DNSOP (DNS Operations) group, through its mailing list.  The
   source of the document, as well as a list of open issues, is
   currently kept on at Github [1].

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 May 9, 2016.

Copyright Notice

   Copyright (c) 2015 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
   ( 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

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   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 and background . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Terminology  . . . . . . . . . . . . . . . .   2
   2.  Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Benefits  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Possible issues . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Future work . . . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  Implementation status - RFC EDITOR: REMOVE BEFORE PUBLICATION   5
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   5
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   6
     10.2.  Informative References . . . . . . . . . . . . . . . . .   6
     10.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction and background

   In virtually all existing resolvers, a cached NXDOMAIN is not
   considered "proof" that there can be no child domains underneath.
   This is due to an ambiguity in [RFC1034] that failed to distinguish
   ENT (empty nonterminal domain names,
   [I-D.ietf-dnsop-dns-terminology]) from nonexistent names.  For
   DNSSEC, the IETF had to distinguish this case ([RFC4034], section, but the implication on non-DNSSEC resolvers wasn't fully

   This document dictates that NXDOMAIN means NXDOMAIN for all child
   domains.  Since the domain names are organized in a tree, it is a
   simple consequence of the tree structure: non-existence of a node
   implies non-existence of the entire sub-tree rooted at this node.

1.1.  Requirements Terminology

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

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2.  Rules

   When searching downward in its cache, an iterative caching DNS
   resolver SHOULD stop searching if it encounters a cached NXDOMAIN.
   The response to the triggering query should be NXDOMAIN.

   When an iterative caching DNS resolver stores an NXDOMAIN in its
   cache, all names and RRsets at or below that node SHOULD be deleted
   since they will have become unreachable.

   By implication, a stream of queries foo.example, then, where foo.example does not exist would normally
   cause both queries to be forwarded to example's nameservers.
   Following this recommended practice of "NXDOMAIN cut", the second
   query and indeed any other query for names at or below foo.example
   would not be forwarded.

3.  Benefits

   The main benefit is a better efficiency of the caches.  In the
   example above, we send only one query instead of two, the second one
   being answered from the cache.

   The correct behavior (in [RFC1034] and made clearer in this document)
   is specially useful when combined with QNAME minimisation
   [I-D.ietf-dnsop-qname-minimisation] since it will allow to stop
   searching as soon as a NXDOMAIN is encountered.

   NXDOMAIN cut may also help with random QNAME attacks
   [joost-dnsterror] [balakrichenan-dafa888].  In these attacks, queries
   are sent for a QNAME composed of a fixed suffix ("" in one
   of the articles above), which is typically nonexistent, and a random
   prefix, different for each request.  A resolver receiving these
   requests have to forward them to the authoritative servers.  With
   NXDOMAIN cut, we would just have to send to the resolver a query for
   the fixed suffix, the resolver would get a NXDOMAIN and then would
   stop forwarding the queries.  (It would be better if the SOA record
   in the NXDOMAIN response were sufficient to find the non-existing
   domain but this is more delicate, see Section 5.)

   Since the principles set in this document are so great, why are the
   rules of Section 2 SHOULD and not MUST?  This is because some
   resolver may have a cache which is NOT organized as a tree (but, for
   instance, as a dictionary) and therefore have a good reason to ignore

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4.  Possible issues

   Let's assume the TLD example exists but foobar.example is not
   delegated (so the example's name servers will reply NXDOMAIN for a
   query about anything.foobar.example).  A system administrator decides
   to name the internal machines of his organization under
   office.foobar.example and use a trick of his resolver to forward
   requests about this zone to his local authoritative name servers.
   NXDOMAIN cut would create problems here, since, depending on the
   order of requests to the resolver, it may have cached the NXDOMAIN
   from example and therefore "deleted" everything under.  This document
   assumes that such setup is rare and does not need to be supported.

   Another issue that may happen: today, we see broken authoritative
   name servers which reply to ENT ([I-D.ietf-dnsop-dns-terminology],
   section 6) with NXDOMAIN instead of the normal NODATA
   ([I-D.ietf-dnsop-dns-terminology], section 3).

   today is (which exists) where querying yields NXDOMAIN.  Another example is,
   redirected to while a query for edu- returns NXDOMAIN.

   Such name servers are definitely broken and have always been.  They
   MUST be fixed.  Given the advantages of NXDOMAIN cuts, there is
   little reason to support this behavior.

5.  Future work

   In this document, we deduce the non-existence of a domain only for
   NXDOMAIN answers where the QNAME was this exact domain.  If a
   resolver sends a query to the name servers of the TLD example, and
   asks the MX record for www.foobar.example, and receives a NXDOMAIN,
   it can only register the fact that www.foobar.example (and everything
   underneath) does not exist.  Even if the accompanying SOA record is
   for example only, it may be dangerous to infer that foobar.example is
   nonexistent.  TODO explain why.

   In the future, deducing the non-existence of a node from the SOA in
   the NXDOMAIN reply may certainly help with random qnames attacks but
   this is out-of-scope for this document.

6.  IANA Considerations

   This document has no actions for IANA.

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7.  Security Considerations

   The technique described here may help against a denial-of-service
   attack named "random qnames" and described in Section 3.  Apart from
   that, it is believed to have no security consequences.

   If a resolver does not validate the answers with DNSSEC, it can of
   course be poisoned with a false NXDOMAIN, thus "deleting" a part of
   the domain name tree.  This denial-of-service attack is already
   possible with the rules of this document (but "NXDOMAIN cut" may
   increase its effects).  The only solution is to use DNSSEC.


   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC6982].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may

   According to [RFC6982], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

   As of today, all existing DNS resolvers are conservative: they
   consider a NXDOMAIN as only significant for the name itself, not for
   the names under.  All current recursive servers will upstream a query
   for out-of-cache even if their cache contains an

9.  Acknowledgments

   The text of this document was mostly copied from
   [I-D.vixie-dnsext-resimprove], section 3.  Thanks to its authors,
   Paul Vixie, Roland Joffe and Frederico Neves.

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10.  References

10.1.  Normative References

   [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,

              Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", draft-ietf-dnsop-dns-terminology-05 (work in
              progress), September 2015.

10.2.  Informative References

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,

   [RFC6982]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", RFC 6982,
              DOI 10.17487/RFC6982, July 2013,

              Vixie, P., Joffe, R., and F. Neves, "Improvements to DNS
              Resolvers for Resiliency, Robustness, and Responsiveness",
              draft-vixie-dnsext-resimprove-00 (work in progress), June

              Bortzmeyer, S., "DNS query name minimisation to improve
              privacy", draft-ietf-dnsop-qname-minimisation-07 (work in
              progress), October 2015.

              Joost, M., "About DNS Attacks and ICMP Destination
              Unreachable Reports", December 2014,

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              Balakrichenan, S., "Disturbance in the DNS - "Random
              qnames", the dafa888 DoS attack"", October 2014,

10.3.  URIs


Author's Address

   Stephane Bortzmeyer
   1, rue Stephenson
   Montigny-le-Bretonneux  78180

   Phone: +33 1 39 30 83 46

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