Domain Name System Operations (dnsop) Working Group        S. Bortzmeyer
Internet-Draft                                                     AFNIC
Updates: 1034,2308 (if approved)                                S. Huque
Intended status: Standards Track                           Verisign Labs
Expires: September 11, 2016                               March 10, 2016

           NXDOMAIN really means there is nothing underneath


   This document states clearly that when a DNS resolver receives a
   response with response code of NXDOMAIN, it means that the domain
   name which is thus denied 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 at Github [1].

   This documents clarifies RFC 1034 and modifies a bit RFC 2308 so it
   updates both of them.

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
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   This Internet-Draft will expire on September 11, 2016.

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

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   ( 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction and background . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Benefits  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Possible issues . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Implementation considerations . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  Implementation status - RFC EDITOR: REMOVE BEFORE PUBLICATION   6
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
     10.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Appendix A.  Why can't we just use the owner name of the returned
                SOA? . . . . . . . . . . . . . . . . . . . . . . . .   9
   Appendix B.  Related approaches . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction and background

   The DNS protocol [RFC1035] defines response code 3 as "Name Error",
   or "NXDOMAIN" [RFC2308], which means that the queried domain name
   does not exist in the DNS.  Since domain names are represented as a
   tree of labels ([RFC1034], Section 3.1), non-existence of a node
   implies non-existence of the entire sub-tree rooted at this node.

   The DNS iterative resolution algorithm precisely interprets the
   NXDOMAIN signal in this manner.  If it encounters an NXDOMAIN
   response code from an authoritative server, it immediately stops
   iteration and returns the NXDOMAIN response to the querier.

   However, in most known existing resolvers today, a cached non-
   existence for a domain is not considered "proof" that there can be no
   child domains underneath.  This is due to an ambiguity in [RFC1034]
   that failed to distinguish Empty Non-Terminal names (ENT) ([RFC7719])
   from nonexistent names.  The distinction became especially important
   for the development of DNSSEC, which provides proof of non-existence.

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   [RFC4035], section, describes how security-aware
   authoritative name servers make the distinction, but no existing RFCs
   describe the behavior for recursive name servers.

   This document specifies that an NXDOMAIN response for a domain name
   means that no child domains underneath the queried name exist either.
   And furthermore, that DNS resolvers should interpret cached non-
   existence in this manner.  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

1.1.  Terminology

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

   "Denied name": the domain name whose existence has been denied by a
   response of rcode NXDOMAIN.  In most cases, it is the QNAME but,
   because of [RFC6604], it is not always the case.

   Other terms are defined in [RFC1034] or [RFC1035] or (like NXDOMAIN
   itself) in the more recent [RFC7719].

2.  Rules

   When an iterative caching DNS resolver receives a response NXDOMAIN,
   it SHOULD store it in its cache and all names and RRsets at or below
   that node SHOULD then be considered to be unreachable.  Subsequent
   queries for such names SHOULD elicit an NXDOMAIN response.

   [TODO: currently under discussion, some people find it dangerous.
   Only if the NXDOMAIN is DNSSEC-validated?  Perhaps the resolver could
   be configured to not apply this rule to TLDs (or root).  See
   Section 7.]

   For example, consider two successive queries to a resolver, with a
   non-existing domain 'foo.example': the first is for 'foo.example'
   (which results in an NXDOMAIN) and the second for ''
   (which also results in an NXDOMAIN).  Many resolvers today will
   forward both queries, as noticed in Section 8.  However, following
   the rules in this document ("NXDOMAIN cut"), a resolver would cache
   the first NXDOMAIN response, as a sign of non-existence, and then
   immediately return an NXDOMAIN response for the second query, without
   transmitting it to an authoritative server.

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   If the first request is for '' and the second for
   '', the first NXDOMAIN response won't tell anything
   about '' and therefore the second query will be
   transmitted as it was before "NXDOMAIN cut" (see Appendix A).

   These rules replace the second paragraph of section 5 of [RFC2308].
   Otherwise, this document does not update any other parts of
   [RFC2308].  The fact that a subtree does not exist is not forever:
   [RFC2308], section 3, already describes the amount of time that an
   NXDOMAIN response may be cached (the "negative TTL").

   If the NXDOMAIN response due to a cached non-existence is from a
   DNSSEC signed zone, then it will have accompanying NSEC or NSEC3
   records that authenticate the non-existence of the name.  For a
   descendant name of the original NXDOMAIN name, the same set of NSEC
   or NSEC3 records proves the non-existence of the descendant name.
   The iterative, caching resolver MUST return these NSEC or NSEC3
   records in the response to the triggering query if the query had the
   DNSSEC OK (DO) bit set.

   Warning: if there is a chain of CNAME (or DNAME), the name which does
   not exist is the last of the chain ([RFC6604]) and not the QNAME.
   The NXDOMAIN stored in the cache is for the denied name, not always
   for the QNAME.

3.  Benefits

   The main benefit is a better efficiency of the caches.  In the
   example above, the resolver 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 a resolver to
   stop searching as soon as an NXDOMAIN is encountered.

   NXDOMAIN cut may also help mitigate certain types of random QNAME
   attacks [joost-dnsterror] [balakrichenan-dafa888], where there is a
   fixed suffix which does not exist.  In these attacks against the
   authoritative name server, queries are sent to resolvers 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, a
   system administrator 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

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   record in the NXDOMAIN response were sufficient to find the non-
   existing domain but it is not the case, see Appendix A.)

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 uses 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 non-
   existence from example and therefore "deleted" everything under.
   This document assumes that such setup is rare and does not need to be

   Another issue that may happen: today, we see broken authoritative
   name servers which reply to ENT ([RFC7719], section 6) with NXDOMAIN
   instead of the normal NODATA ([RFC7719], 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 wrong and have always been.  Their
   behaviour is incompatible with DNSSEC.  Given the advantages of
   NXDOMAIN cuts, there is little reason to support this behavior.

5.  Implementation considerations

   This section is non-normative, and is composed only of various things
   which may be useful for implementors.  A recursive resolver may
   implement its cache in many ways.  The most obvious one is a tree
   data structure, because it fits the data model of domain names.  But,
   in practice, other implementations are possible, as well as various
   optimisations (such as a tree, augmented by an index of some common
   domain names).

   If a resolver implements its cache as a tree (without any
   optimisation), one way to follow the rules of Section 2 is, when
   receiving the NXDOMAIN, to prune the subtree of positive cache
   entries at that node, or to delete all individual cache entries for
   names below that node.  Then, when searching downward in its cache,
   this iterative caching DNS resolver stop searching if it encounters a
   cached non-existence.

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   Some resolver may have a cache which is NOT organized as a tree (but,
   for instance, as a dictionary) and therefore have a reason to ignore
   the rules of Section 2.  So these rules are a SHOULD and not a MUST.

6.  IANA Considerations

   This document has no actions for IANA.

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, or if the
   zone is not signed, the resolver 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 without 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, practically all existing DNS resolvers are conservative
   by default: they consider a NXDOMAIN as only significant for the name
   itself, not for the names under.  Almost all the current recursive
   servers will send upstream a query for out-of-cache
   even if their cache contains an NXDOMAIN for

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   There are a few exceptions.  The Unbound resolver has a configuration
   parameter called "harden-below-nxdomain" [2], which if set to "yes"
   turns on NXDOMAIN cut behavior ("only DNSSEC-secure nxdomains are
   used", see Section 7).  The PowerDNS recursor has optional partial
   support for NXDOMAIN cut, for the root domain only, with its "root-
   nx-trust" setting, described as [3] "If set, an NXDOMAIN from the
   root-servers will serve as a blanket NXDOMAIN for the entire TLD the
   query belonged to.  The effect of this is far fewer queries to the

9.  Acknowledgments

   The main idea is in this document is taken from
   [I-D.vixie-dnsext-resimprove], section 3, "Stopping Downward Cache
   Search on NXDOMAIN".  Thanks to its authors, Paul Vixie, Rodney
   Joffe, and Frederico Neves.  Additionally Tony Finch, John Levine,
   Jinmei Tatuya, and Duane Wessels provided valuable feedback and

10.  References

10.1.  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,

   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
              NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,

   [RFC6604]  Eastlake 3rd, D., "xNAME RCODE and Status Bits
              Clarification", RFC 6604, DOI 10.17487/RFC6604, April
              2012, <>.

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10.2.  Informative References

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, 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,

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

              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-09 (work in
              progress), January 2016.

              Fujiwara, K. and A. Kato, "Aggressive use of NSEC/NSEC3",
              draft-fujiwara-dnsop-nsec-aggressiveuse-02 (work in
              progress), October 2015.

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

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

10.3.  URIs



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Appendix A.  Why can't we just use the owner name of the returned SOA?

   In this document, we deduce the non-existence of a domain only for
   NXDOMAIN answers where the denied name 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, one cannot infer that foobar.example is
   nonexistent.  The accompanying SOA indicates the apex of the zone,
   not the closest existing domain name.

   RFC-EDITOR: REMOVE BEFORE PUBLICATION: to use a real example today,
   ask the authoritative name servers of the TLD fr about  The SOA will indicate fr (the
   apex) even while does exist (there is no zone cut between and fr).

   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.  It would require to address the problems
   mentioned in the first paragraph of this section.  A possible
   solution would be, when receiving a NXDOMAIN with a SOA which is more
   than one label up in the tree, to send requests for the domains which
   are between the QNAME and the owner name of the SOA.  (A resolver
   which does DNSSEC validation or QNAME minimisation will need to do
   it, anyway.)

Appendix B.  Related approaches

   The document [I-D.fujiwara-dnsop-nsec-aggressiveuse] describes
   another way to address some of the same concerns (decreasing the
   traffic for non-existing domain names).  Unlike NXDOMAIN cut, it
   requires DNSSEC but it is more powerful since it can synthetize
   NXDOMAINs for domains that were not queried.

Authors' Addresses

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

   Phone: +33 1 39 30 83 46

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   Shumon Huque
   Verisign Labs
   12061 Bluemont Way
   Reston  20190


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