Network Working Group                                         M. Andrews
Internet-Draft                                                       ISC
Expires: November 7, 2014                                    May 6, 2014

   A Common Operational Problem in DNS Servers - Failure To Respond.


   The DNS is a query / response protocol.  Failure to respond to
   queries causes both immediate operational problems and long term
   problems with protocol development.

   This document identifies a number of common classes of queries that
   some servers fail to respond too.  This document also suggests
   procedures for TLD and other similar zone operators to apply to
   reduce / eliminate the problem.

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Common queries class that result in non responses.  . . . . . . 4
     2.1.  EDNS Queries - Version Independent  . . . . . . . . . . . . 4
     2.2.  EDNS Queries - Version Specific . . . . . . . . . . . . . . 4
     2.3.  Unknown / Unsupported Type Queries  . . . . . . . . . . . . 4
     2.4.  TCP Queries . . . . . . . . . . . . . . . . . . . . . . . . 5
   3.  Remediating . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   4.  Firewalls and Load Balancers  . . . . . . . . . . . . . . . . . 6
   5.  Response Code Selection . . . . . . . . . . . . . . . . . . . . 7
   6.  Normative References  . . . . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   The DNS [RFC1034], [RFC1035] is a query / response protocol.  Failure
   to respond to queries causes both immediate operational problems and
   long term problems with protocol development.

   Failure to respond to a query is indistinguishable from a packet loss
   without doing a analysis of query response patterns and results in
   unnecessary additional queries being made by DNS clients and
   unnecessary delays being introduced to the resolution process.

   Due to the inability to distingish between packet loss and
   nameservers dropping EDNS [RFC6891] queries, packet loss is sometimes
   misclassified as lack of EDNS support which can lead to DNSSEC
   validation failures.

   Allowing servers which fail to respond to queries to remain results
   in developers being afraid to deploy implementations of recent
   standards.  Such servers need to be identified and corrected /

   The DNS has response codes that cover almost any conceivable query
   response.  A nameserver should be able to respond to any conceivable
   query using them.

   Unless a nameserver is under attack, it should respond to all queries
   directed to it as a result of following delegations.  Additionally
   code should not assume that there isn't a delegation to the server
   even if it is not configured to serve the zone.  Broken delegation
   are a common occurrence in the DNS and receiving queries for zones
   that you are not configured for is not a necessarily a indication
   that you are under attack.  Parent zone operators are supposed to
   regularly check that the delegating NS records are consistent with
   those of the delegated zone and to correct them when they are not
   [RFC1034].  If this was being done regularly the instances of broke
   delegations would be much lower.

   When a nameserver is under attack it may wish to drop packets.  A
   common attack is to use a nameserver as a amplifier by sending
   spoofed packets.  This is done because response packets are bigger
   than the queries and big amplification factors are available
   especially if EDNS is supported.  Limiting the rate of responses is
   reasonable when this is occuring and the client should retry.  This
   however only works if legitimate clients are not being forced to
   guess whether EDNS queries are accept or not.  While there is still a
   pool of servers that don't repsond to EDNS requests, clients have no
   way to know if the lack of response is due to packet loss, EDNS
   packets not being supported or rate limiting due to the server being

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   under attack.  Mis-classifications of server characteristics are
   unavoidable when rate limiting is done.

2.  Common queries class that result in non responses.

   There are three common query classes that result in non responses
   today.  These are EDNS queries, queries for unknown (unallocated) or
   unsupported types and filtering of TCP queries.

2.1.  EDNS Queries - Version Independent

   Identifying servers that fail to respond to EDNS queries can be done
   by first identifying that the server responds to regular DNS queries
   then making a series of otherwise identical responses using EDNS,
   then making the original query again.  A series of EDNS queries is
   needed as at least one DNS implementation responds to the first EDNS
   query with FORMERR but fails to respond to subsequent queries from
   the same address for a period until a regular DNS query is made.  The
   EDNS query should specify a UDP buffer size of 512 bytes to avoid
   false classification of not supporting EDNS due to response packet

   If the server responds to the first and last queries but fails to
   respond to most or all of the EDNS queries it is probably faulty.
   The test should be repeated a number of times to eliminate the
   likelihood of a false positive due to packet loss.

   Firewalls may also block larger EDNS responses but there is no easy
   way to check authoritative servers to see if the firewall is

2.2.  EDNS Queries - Version Specific

   Some servers respond correctly to EDNS version 0 queries but fail to
   respond to EDNS queries with version numbers that are higher than
   zero.  Servers should respond with BADVERS to EDNS queries with
   version numbers that they do not support.

2.3.  Unknown / Unsupported Type Queries

   Identifying servers that fail to respond to unknown or unsupported
   types can be done by making an initial DNS query for an A record,
   making a number of queries for an unallocated type, them making a
   query for an A record again.  IANA maintains a registry of allocated

   If the server responds to the first and last queries but fails to

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   respond to the queries for the unallocated type it is probably
   faulty.  The test should be repeated a number of times to eliminate
   the likely hood of a false positive due to packet loss.

2.4.  TCP Queries

   All DNS servers are supposed to respond to queries over TCP
   [RFC5966].  Firewalls that drop TCP connection attempts rather that
   resetting the connect attempt or send a ICMP/ICMPv6 administratively
   prohibited message introduce excessive delays to the resolution

   Whether a server accepts TCP connections can be tested by first
   checking that it responds to UDP queries to confirm that it is up and
   operating then attempting the same query over TCP.  An additional
   query should be made over UDP if the TCP connection attempt fails to
   confirm that the server under test is still operating.

3.  Remediating

   While the first step in remediating this problem is to get the
   offending nameserver code corrected, there is a very long tail
   problem with DNS servers in that it can often take over a decade
   between the code being corrected and a nameserver being upgraded with
   corrected code.  With that in mind it is requested that TLD, and
   other similar zone operators, take steps to identify and inform their
   customers, directly or indirectly through registrars, that they are
   running such servers and that the customers need to correct the

   TLD operators should construct a list of servers child zones are
   delegated to along with a delegated zone name.  This name shall be
   the query name used to test the server as it is supposed to exist.

   For each server the TLD operator shall make an SOA query the
   delegated zone name.  This should result in the SOA record being
   returned in the answer section.  If the SOA record is not return but
   some other response is returned this is a indication of a bad
   delegation and the TLD operator should take whatever steps it
   normally takes to rectify a bad delegation.  If more that one zone is
   delegated to the server it should choose another zone until it finds
   a zone which responds correctly or it exhausts the list of zones
   delegated to the server.

   If the server fails to get a response to a SOA query the TLD operator
   should make a A query as some nameservers fail to respond to SOA
   queries but respond to A queries.  If it gets no response to the A

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   query another delegated zone should be queried for as some
   nameservers fail to respond to zones they are not configured for.  If
   subsequent queries find a responding zone all delegation to this
   server need to be checked and rectified using the TLD's normal

   Having identified a working <server, query name> tuple the TLD
   operator should now check that the server responds to EDNS, Unknown
   Query Type and TCP tests as described above.  If the TLD operator
   finds that server fails any of the tests, the TLD operator shall take
   steps to inform the operator of the server that they are running a
   faulty nameserver and that they need to take steps to correct the
   matter.  The TLD operator shall also record the <server, query name>
   for followup testing.

   If repeated attempts to inform and get the customer to correct /
   replace the faulty server are unsuccessful the TLD operator shall
   remove all delegations to said server from the zone.

   It will also be necessary for TLD operators to repeat the scans
   periodically.  It is recommended that this be performed monthly
   backing off to bi-annually once the numbers of faulty servers found
   drops off to less than 1 in 100000 servers tested.  Follow up tests
   for faulty servers still need to be performed monthly.

   Some operators claim that they can't perform checks at registration
   time.  If a check is not performed at registration time it needs to
   be performed within a week of registration in order to detect faulty
   servers swiftly.

   Checking of delegations by TLD operators should be nothing new as
   they have been required from the very beginings of DNS to do this
   [RFC1034].  Checking for compliance of nameserver operations should
   just be a extension of such testing.

   It is recommended that TLD operators setup a test web page which
   performs the tests the TLD operator performs as part of their regular
   audits to allow nameserver operators to test that they have correctly
   fixed their servers.  Such tests should be rate limited to avoid
   these pages being a denial of service vector.

4.  Firewalls and Load Balancers

   Firewalls and load balancers can affect the externally visible
   behaviour of a nameserver.  Tests for conformance need to be done
   from outside of any firewall so that the system as a whole is tested.

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   Firewalls and load balancers should not drop DNS packets that they
   don't understand.  They should either pass through the packets or
   generate a appropriate error response.

   Requests for unknown query types are not attacks and should not be
   treated as such.

5.  Response Code Selection

   Choosing the correct response code when fixing a nameserver is
   important.  Just because a type is not implemented does not mean that
   NOTIMP is the correct response code to return.  Response codes need
   to be choosen considering how clients will handle them.

   For unimplemented opcodes NOTIMP is the expected response code.

   In general, for unimplemented type codes Name Error (NXDOMAIN) and
   NOERROR (no data) are the expected response codes.  A server is not
   supposed to serve a zone which contains unsupported types ([RFC1034])
   so the only thing left is return if the QNAME exists or not.  NOTIMP
   and REFUSED are not useful responses as they force the clients to try
   all the authoritative servers for a zone looking for a server which
   will answer the query.

   Meta queries type may be the exception but these need to be thought
   about on a case by case basis.

   If you support EDNS and get a query with a unsupported EDNS version
   the correct response is BADVERS [RFC6891].

   If you do not support EDNS at all FORMERR and NOTIMP are the expected
   error codes.  That said a mimimal EDNS server implementation just
   requires parsing the OPT records and responding with a empty OPT
   record.  There is no need to interpret any EDNS options present in
   the request as unsupported options are expected to be ignored

6.  Normative References

              STD 13, RFC 1034, November 1987.

              SPECIFICATION", STD 13, RFC 1035, November 1987.

   [RFC5966]  Bellis, R., "DNS Transport over TCP - Implementation

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              Requirements", RFC 5966, August 2010.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.

Author's Address

   M. Andrews
   Internet Systems Consortium
   950 Charter Street
   Redwood City, CA  94063


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