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: June 1, 2016                                  November 29, 2015

           NXDOMAIN really means there is nothing underneath


   This document states clearly that when a DNS resolver receives a
   response with response 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 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
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   This Internet-Draft will expire on June 1, 2016.

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   document authors.  All rights reserved.

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   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction and background . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Terminology  . . . . . . . . . . . . . . . .   3
   2.  Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Benefits  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Possible issues . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Future work . . . . . . . . . . . . . . . . . . . . . . . . .   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
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction and background

   The DNS protocol [RFC1035] defines response code 3 as "Name Error",
   or "NXDOMAIN", i.e. 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 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 ([RFC4035], section, but the implication on non-DNSSEC resolvers wasn't fully

   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 NXDOMAIN
   responses in this manner.  Since the domain names are organized in a

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   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.  Requirements Terminology

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

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.

   TODO The next paragraph is challenged because too implementation-
   oriented.  Should we just keep the first paragraph?  My concern is
   that some resolvers may have an implementation of the cache made of a
   tree plus a hashed index and therefore won't "search downward" if
   they have a cached answer.

   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.  "Deleted" means that
   subsequent requests for these names will yield NXDOMAIN.  [TODO:
   currently under discussion, some people find it dangerous.  MAY
   instead of SHOULD?  Only if the NXDOMAIN is DNSSEC-validated?
   Perhaps the resolver could be configured to not "bulk delete" TLDs
   (or root).  See Section 7.]  [TODO: currently under discussion, some
   people find it costly for the resolver.  A large purge also causes
   the resolver to do a lot of (possibly CPU intensive) work, and could
   also affect the traffic levels between a recursive and
   authoritatives.  Perhaps a cache may want to limit the number of
   nodes/records that it deletes per NXDOMAIN response.]

   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.

   These rules replace the second paragraph of section 5 of [RFC2308].
   Otherwise, [RFC2308] applies unchanged, and the fact that a subtree
   does not exist is not forever: the NXDOMAIN is cached only for the
   duration of the "negative TTL" (section 3 of xref

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   Validating resolvers need to select the necessary NSEC or NSEC3
   records and include them in the AUTHORITY section of the response to
   provide authenticated denial of existence for names underneath the
   NXDOMAIN boundary.

   Warning: because of [RFC6604], the name whose existence is denied by
   the NXDOMAIN is not always the QNAME.  If there is a chain of CNAME
   (or DNAME), the name which does not exist is the last of the chain.
   TODO: find a dedicated terminology such as "NXDOMAINed name" or
   "denied domain" instead of "QNAME".

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 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, 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

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

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   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

   TODO: drop this section entirely?  Or just downgrade it to an
   appendix "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 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, 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).

   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.  It would require to address
   the problems mentioned in the previous paragraph.  A possible
   solution would be, when receiving a NXDOMAIN with a SOA which is more

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   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.)

   TODO a mention of [I-D.fujiwara-dnsop-nsec-aggressiveuse]?  Unlike
   NXDOMAIN cut, it requires DNSSEC but it is more powerful since it can
   synthetize NXDOMAINs.

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, 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".

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   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, Rodney Joffe and Frederico Neves.

   Thanks to Duane Wessels, Tony Finch and Jinmei Tatuya for fact
   checking and explanations.

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, <>.

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

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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,

              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.

              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,

Authors' Addresses

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

   Phone: +33 1 39 30 83 46

   Shumon Huque
   Verisign labs
   12061 Bluemont Way
   Reston  20190


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