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Versions: (draft-andrews-dns-no-response-issue) 00 01 02 03 04 05 06 07 08

Network Working Group                                         M. Andrews
Internet-Draft                                                       ISC
Intended status: Best Current Practice                     March 3, 2017
Expires: September 4, 2017


   A Common Operational Problem in DNS Servers - Failure To Respond.
                 draft-ietf-dnsop-no-response-issue-08

Abstract

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

   This document identifies a number of common kinds of queries to which
   some servers either fail to respond or else respond incorrectly.
   This document also suggests procedures for TLD and other zone
   operators to apply to help reduce / eliminate the problem.

   The document does not look at the DNS data itself, just the structure
   of the responses.

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 September 4, 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



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   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
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Consequences  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Common queries kinds that result in non responses.  . . . . .   5
     3.1.  Basic DNS Queries . . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Zone Existence  . . . . . . . . . . . . . . . . . . .   5
       3.1.2.  Unknown / Unsupported Type Queries  . . . . . . . . .   5
       3.1.3.  DNS Flags . . . . . . . . . . . . . . . . . . . . . .   6
       3.1.4.  Unknown DNS opcodes . . . . . . . . . . . . . . . . .   6
       3.1.5.  Recursive Queries . . . . . . . . . . . . . . . . . .   6
       3.1.6.  TCP Queries . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  EDNS Queries  . . . . . . . . . . . . . . . . . . . . . .   7
       3.2.1.  EDNS Queries - Version Independent  . . . . . . . . .   7
       3.2.2.  EDNS Queries - Version Specific . . . . . . . . . . .   7
       3.2.3.  EDNS Options  . . . . . . . . . . . . . . . . . . . .   7
       3.2.4.  EDNS Flags  . . . . . . . . . . . . . . . . . . . . .   7
       3.2.5.  Truncated EDNS Responses  . . . . . . . . . . . . . .   8
       3.2.6.  DNSSEC  . . . . . . . . . . . . . . . . . . . . . . .   8
       3.2.7.  EDNS over TCP . . . . . . . . . . . . . . . . . . . .   8
   4.  Firewalls and Load Balancers  . . . . . . . . . . . . . . . .   8
   5.  Scrubbing Services  . . . . . . . . . . . . . . . . . . . . .   9
   6.  Whole Answer Caches . . . . . . . . . . . . . . . . . . . . .   9
   7.  Response Code Selection . . . . . . . . . . . . . . . . . . .  10
   8.  Testing . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Testing - Basic DNS . . . . . . . . . . . . . . . . . . .  11
       8.1.1.  Is The Server Configured For The Zone?  . . . . . . .  11
       8.1.2.  Testing Unknown Types . . . . . . . . . . . . . . . .  11
       8.1.3.  Testing Header Bits . . . . . . . . . . . . . . . . .  12
       8.1.4.  Testing Unknown Opcodes . . . . . . . . . . . . . . .  14
       8.1.5.  Testing Recursive Queries . . . . . . . . . . . . . .  15
       8.1.6.  Testing TCP . . . . . . . . . . . . . . . . . . . . .  15
     8.2.  Testing - Extended DNS  . . . . . . . . . . . . . . . . .  16
       8.2.1.  Testing Minimal EDNS  . . . . . . . . . . . . . . . .  16
       8.2.2.  Testing EDNS Version Negotiation  . . . . . . . . . .  16
       8.2.3.  Testing Unknown EDNS Options  . . . . . . . . . . . .  17
       8.2.4.  Testing Unknown EDNS Flags  . . . . . . . . . . . . .  18
       8.2.5.  Testing EDNS Version Negotiation With Unknown EDNS
               Flags . . . . . . . . . . . . . . . . . . . . . . . .  18
       8.2.6.  Testing EDNS Version Negotiation With Unknown EDNS
               Options . . . . . . . . . . . . . . . . . . . . . . .  19



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       8.2.7.  Testing Truncated Responses . . . . . . . . . . . . .  20
       8.2.8.  Testing DNSSEC Queries  . . . . . . . . . . . . . . .  20
       8.2.9.  Testing EDNS Version Negotiation With DNSSEC  . . . .  21
       8.2.10. Testing With Multiple Defined EDNS Options  . . . . .  22
     8.3.  When EDNS Is Not Supported  . . . . . . . . . . . . . . .  22
   9.  Remediation . . . . . . . . . . . . . . . . . . . . . . . . .  22
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  24
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     12.2.  Informative References . . . . . . . . . . . . . . . . .  25
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Introduction

   The DNS [RFC1034], [RFC1035] is a query / response protocol.  Failure
   to respond to queries or to respond incorrectly 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.  Additionally
   failure to respond results in unnecessary queries being made by DNS
   clients, and delays being introduced to the resolution process.

   Due to the inability to distinguish 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.

   Servers which fail to respond to queries results in developers being
   hesitant to deploy new standards.  Such servers need to be
   identified.

   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.  There should be no need to drop queries because a
   nameserver does not understand them.

   Unless a nameserver is under attack, it should respond to all queries
   directed to it.  Additionally, the nameserver should not assume that
   there isn't a delegation to the server even if it is not configured
   to serve the zone.  Broken nameservers are a common occurrence in the
   DNS and receiving queries for zones that the server is not configured
   for is not necessarily an indication that the server is under attack.
   Parent zone operators are supposed to regularly check that the
   delegating NS records are consistent with those of the delegated zone




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   and to correct them when they are not [RFC1034].  Doing this
   regularly should reduce the instances of broken delegations.

   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 occurring and the client should retry.  This
   however only works if legitimate clients are not being forced to
   guess whether EDNS queries are accepted or not.  While there is still
   a pool of servers that don't respond 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
   under attack.  Misclassification of server behaviour is unavoidable
   when rate limiting is used until the population of servers which fail
   to respond to well formed queries drops to near zero.

2.  Consequences

   Not following the relevant DNS RFCs has multiple adverse
   consequences.  Some resulting directly from the non-compliant
   behaviour and others as a result of work-arounds forced on recursive
   servers.  Addressing known issues now will reduce future
   interoperability issues as the DNS protocol continues to evolve and
   clients make use of newly introduced DNS features.

   Some examples of known consequences include:

   o  The AD flag bit in a response cannot be trusted to mean anything
      as many servers incorrectly copied the flag bit from the request
      to the response despite the prohibition.

   o  Widespread non response to EDNS queries has lead to recursive
      servers having to assume EDNS may not supported and that fallback
      to plain DNS is required.  Servers get incorrectly diagnosed as
      not supporting EDNS and when they also serve signed zones DNSSEC
      validation fails.

   o  Widespread non response to EDNS options, requires recursive
      servers to have to decide whether to probe to see if it is the
      EDNS option or just EDNS that is causing the non response.  In the
      limited amount of time required to resolve a query before the
      client times out this is not possible.

   o  Incorrectly returning FORMERR to a EDNS option being present,
      leads to the recursive server not being able to determine if the




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      server is just broken in the handling of the EDNS option or
      doesn't support EDNS at all.

   o  Mishandling of unknown query types has contributed to the
      abandoning of the transition of the SPF type.

   o  Mishandling of unknown query types has slowed up the development
      of DANE and and result in additional rules being specified to
      reduce the probability of interacting with a broken server when
      making TLSA queries.

   The consequences of servers not following the RFCs will only grow if
   measures are not put in place to remove non compliant servers from
   the ecosystem.  Working around issues due to non RFC compliance is
   not sustainable.

   Most, if not all, of these consequences could have been avoided if
   action had been taken to remove non compliant servers as soon as
   people were aware of them.  To actively seek out broken
   implementations and servers and inform their developers and operators
   that they need to fix their servers.

3.  Common queries kinds that result in non responses.

   There are a number common query kinds that fail to respond today.
   They are: EDNS queries with and without extensions; queries for
   unknown (unallocated) or unsupported types; and filtering of TCP
   queries.

3.1.  Basic DNS Queries

3.1.1.  Zone Existence

   Initially to test existence of the zone, an SOA query should be made.
   If the SOA record is not returned but some other response is
   returned, this is a indication of a bad delegation.  If the server
   fails to get a response to a SOA query, the Operator should make an A
   query for the zone, as some nameservers fail to respond to SOA
   queries but will respond to A queries.

3.1.2.  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, then making a
   query for an A record again.  IANA maintains a registry of allocated
   types.




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   If the server responds to the first and last queries but fails to
   respond to the queries for the unallocated type, 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.

3.1.3.  DNS Flags

   Some servers fail to respond to DNS queries with various DNS flags
   set, regardless of whether they are defined or still reserved.  At
   the time of writing there are servers that fail to respond to queries
   with the AD bit set to 1 and servers that fail to respond to queries
   with the last reserved flag bit set.

3.1.4.  Unknown DNS opcodes

   The use of previously undefined opcodes is to be expected.  Since the
   DNS was first defined two new opcodes have been added, UPDATE and
   NOTIFY.

   NOTIMP is the expected rcode to an unknown or unimplemented opcode.

   Note: while new opcodes will most probably use the current layout
   structure for the rest of the message there is no requirement that
   anything other than the DNS header match.

3.1.5.  Recursive Queries

   A non-recursive server is supposed to respond to recursive queries as
   if the RD bit is not set.

3.1.6.  TCP Queries

   All DNS servers are supposed to respond to queries over TCP
   [RFC7766].  Firewalls that drop TCP connection attempts, they should
   reset the connect attempt or send a ICMP/ICMPv6 administratively
   prohibited message.  Dropping TCP connections introduces excessive
   delays to the resolution process.

   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.








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3.2.  EDNS Queries

3.2.1.  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,
   followed by a series of otherwise identical queries 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 size.

   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 mis-
   configured.

3.2.2.  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.

   Some servers respond correctly to EDNS version 0 queries but fail to
   set QR=1 when responding to EDNS versions they do not support.  Such
   answers are discarded or treated as requests.

3.2.3.  EDNS Options

   Some servers fail to respond to EDNS queries with EDNS options set.
   Unknown EDNS options are supposed to be ignored by the server
   [RFC6891].

3.2.4.  EDNS Flags

   Some servers fail to respond to EDNS queries with EDNS flags set.
   Server should ignore EDNS flags they do not understand and should not
   add them to the response [RFC6891].






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3.2.5.  Truncated EDNS Responses

   Some EDNS aware servers fail to include a OPT record when a truncated
   response is sent.  A OPT record is supposed to be included in a
   truncated response [RFC6891].

   Some EDNS aware server fail to honour the advertised EDNS buffer size
   and send over sized responses.

3.2.6.  DNSSEC

   Servers should be checked to see if they support DNSSEC.  Servers
   should also be checked to see if they support DNSSEC with EDNS.

3.2.7.  EDNS over TCP

   Some EDNS aware servers incorrectly limit the TCP response sizes to
   the advertised UDP response size.

4.  Firewalls and Load Balancers

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

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

   Requests for unknown query types is normal client behaviour and
   should not be construed as an attack.  Nameservers have always been
   expected to be able to handle such queries.

   Requests for unknown query classes is normal client behaviour and
   should not be construed as an attack.  Nameservers have always been
   expected to be able to handle such queries.

   Requests with unknown opcodes is normal client behaviour and should
   not be construed as an attack.  Nameservers have always been expected
   to be able to handle such queries.

   Requests with unassigned flags set (DNS or EDNS) is expected client
   behaviour and should not be construed as an attack.  The behaviour
   for unassigned flags is to ignore them in the request and to not set
   them in the response.  Dropping DNS / EDNS packets with unassigned
   flags makes it difficult to deploy extensions that make use of them
   due to the need to reconfigure and update firewalls.




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   Requests with unknown EDNS options is expected client behaviour and
   should not be construed as an attack.  The correct behaviour for
   unknown EDNS options is to ignore there presence when constructing a
   reply.

   Requests with unknown EDNS versions is expected client behaviour and
   should not be construed as an attack.  The correct behaviour for
   unknown EDNS versions is to return BADVERS along with the highest
   EDNS version the server supports.  Dropping EDNS packet breaks EDNS
   version negotiation.

   Firewalls should not assume that there will only be a single response
   message to a requests.  There have been proposals to use EDNS to
   signal that multiple DNS messages be returned rather than a single
   UDP message that is fragmented at the IP layer.

   However, there may be times when a nameserver mishandles messages
   with a particular flag, EDNS option, EDNS version field, opcode, type
   or class field or combination there of to the point where the
   integrity of the nameserver is compromised.  Firewalls should offer
   the ability to selectively reject messages with an appropriately
   constructed response based on all these fields while awaiting a fix
   from the nameserver vendor.

   DNS and EDNS in particular is designed to allow clients to be able to
   use new features against older servers without having to validate
   every option.  Indiscriminate blocking of messages breaks that
   design.

5.  Scrubbing Services

   Scrubbing services, like firewalls, can affect the externally visible
   behaviour of a nameserver.  If a operator uses a scrubbing service,
   they should check that legitimate queries are not being blocked.

   Scrubbing services, unlike firewalls, are also turned on and off in
   response to denial of service attacks.  One needs to take care when
   choosing a scrubbing service and ask questions like mentioned above.

   Ideally, Operators should run these tests against a scrubbing service
   to ensure that these tests are not seen as attack vectors.

6.  Whole Answer Caches

   Whole answer caches take a previously constructed answer and return
   it to a subsequent query for the same qname, qtype and qclass, just
   updating the query id field and possibly the qname to match the
   incoming query to avoid constructing each response individually.



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   Whole answer caches can return the wrong response to a query if they
   do not take all of the attributes of the query into account, rather
   than just some of them e.g. qname, qtype and qclass.  This has
   implications when testing and with overall protocol compliance.

   Two current examples are:

   o  Whole answer caches that ignore the EDNS version field which
      results in incorrect answers to non EDNS version 0 queries being
      returned if they were preceded by a EDNS version 0 query for the
      same name and type.

   o  Whole answer caches that ignore the EDNS options in the query
      resulting in options only working some of the time and/or options
      being returned when not requested.

7.  Response Code Selection

   Choosing the correct response code when responding to DNS queries is
   important.  Just because a DNS qtype is not implemented does not mean
   that NOTIMP is the correct response code to return.  Response codes
   should be chosen considering how clients will handle them.

   For unimplemented opcodes NOTIMP is the expected response code.  For
   example, a new opcode could change the message format by extending
   the header or changing the structure of the records etc.  This may
   result in FORMERR being returned though NOTIMP would be more correct.

   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 the other
   authoritative servers for a zone looking for a server which will
   answer the query.

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

   If the server supports EDNS and receives a query with an unsupported
   EDNS version, the correct response is BADVERS [RFC6891].

   If the server does not support EDNS at all, FORMERR and NOTIMP are
   the expected error codes.  That said a minimal EDNS server
   implementation requires parsing the OPT records and responding with
   an empty OPT record.  There is no need to interpret any EDNS options
   present in the request as unsupported EDNS options are expected to be
   ignored [RFC6891].



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8.  Testing

   Testing is divided into two sections.  Basic DNS which all servers
   should meet and Extended DNS which should be met by all servers that
   support EDNS (a server is deemed to support EDNS if it gives a valid
   EDNS response to any EDNS query).  If a server does not support EDNS
   it should still respond to all the tests.

   It is advisable to run all of the tests below in parallel so as to
   minimise the delays due to multiple timeouts when the servers do not
   respond.  There are 16 queries directed to each nameserver assuming
   no packet loss testing different aspects of Basic DNS and EDNS.

   The tests below use dig from BIND 9.11.0.

8.1.  Testing - Basic DNS

   This first set of tests cover basic DNS server behaviour and all
   servers should pass these tests.

8.1.1.  Is The Server Configured For The Zone?

   Ask for the SOA record of the zone the server is nominally configured
   to serve.  This query is made with no DNS flag bits set and without
   EDNS.

   We expect the SOA record for the zone to be returned in the answer
   section with the rcode set to NOERROR and the AA and QR bits to be
   set in the response, RA may also be set [RFC1034].  We do not expect
   a OPT record to be returned [RFC6891].

   Verify the server is configured for the zone:

   dig +noedns +noad +norec soa $zone @$server

   expect: status: NOERROR
   expect: the SOA record to be present in the answer section
   expect: flag: aa to be present
   expect: flag: rd to NOT be present
   expect: flag: ad to NOT be present
   expect: the OPT record to NOT be present

8.1.2.  Testing Unknown Types

   Ask for the TYPE1000 record at the zone's name.  This query is made
   with no DNS flag bits set and without EDNS.  TYPE1000 has been chosen
   for this purpose as IANA is unlikely to allocate this type in the




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   near future and it is not in type space reserved for end user
   allocation.

   We don't expect any records to be returned in the answer section with
   the rcode set to NOERROR and the AA and QR bits to be set in the
   response, RA may also be set [RFC1034].  We do not expect a OPT
   record to be returned [RFC6891].

   Check that queries for an unknown type work:

   dig +noedns +noad +norec type1000 $zone @$server

   expect: status: NOERROR
   expect: an empty answer section.
   expect: flag: aa to be present
   expect: flag: rd to NOT be present
   expect: flag: ad to NOT be present
   expect: the OPT record to NOT be present

   That new types are to be expected is specified in Section 3.6,
   [RFC1035].  Servers that don't support a new type are expected to
   reject a zone that contains a unsupported type as per Section 5.2,
   [RFC1035].  This means that a server that does load a zone can answer
   questions for unknown types with NOERROR or NXDOMAIN as per
   Section 4.3.2, [RFC1034].  [RFC6895] later reserved distinct ranges
   for meta and data types which allows servers to be definitive about
   whether a query should be answerable from zone content or not.

8.1.3.  Testing Header Bits

8.1.3.1.  Testing CD=1 Queries

   Ask for the SOA record of the zone the server is nominally configured
   to serve.  This query is made with only the CD DNS flag bit set and
   all other DNS bits clear and without EDNS.

   We expect the SOA record for the zone to be returned in the answer
   section with the rcode set to NOERROR and the AA and QR bits to be
   set in the response.  We do not expect a OPT record to be returned.

   If the server supports DNSSEC, CD should be set in the response
   [RFC4035] otherwise CD should be clear [RFC1034].









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   Check that queries with CD=1 work:

   dig +noedns +noad +norec +cd soa $zone @$server

   expect: status: NOERROR
   expect: the SOA record to be present in the answer section
   expect: flag: aa to be present
   expect: flag: rd to NOT be present
   expect: flag: ad to NOT be present
   expect: the OPT record to NOT be present

   CD use in queries is defined in [RFC4035].

8.1.3.2.  Testing AD=1 Queries

   Ask for the SOA record of the zone the server is nominally configured
   to serve.  This query is made with only the AD DNS flag bit set and
   all other DNS bits clear and without EDNS.

   We expect the SOA record for the zone to be returned in the answer
   section with the rcode set to NOERROR and the AA and QR bits to be
   set in the response.  We do not expect a OPT record to be returned.

   If the server supports DNSSEC, AD may be set in the response
   [RFC6840] otherwise AD should be clear [RFC1034].

   Check that queries with AD=1 work:

   dig +noedns +norec +ad soa $zone @$server

   expect: status: NOERROR
   expect: the SOA record to be present in the answer section
   expect: flag: aa to be present
   expect: flag: rd to NOT be present
   expect: the OPT record to NOT be present

   AD use in queries is defined in [RFC6840].

8.1.3.3.  Testing Reserved Bit

   Ask for the SOA record of the zone the server is nominally configured
   to serve.  This query is made with only the final reserved DNS flag
   bit set and all other DNS bits clear and without EDNS.

   We expect the SOA record for the zone to be returned in the answer
   section with the rcode set to NOERROR and the AA and QR bits to be
   set in the response, RA may be set.  The final reserved bit must not




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   be set [RFC1034].  We do not expect a OPT record to be returned
   [RFC6891].

   Check that queries with the last unassigned DNS header flag work and
   that the flag bit is not copied to the response:

   dig +noedns +noad +norec +zflag soa $zone @$server

   expect: status: NOERROR
   expect: the SOA record to be present in the answer section
   expect: MBZ to NOT be in the response
   expect: flag: aa to be present
   expect: flag: rd to NOT be present
   expect: flag: ad to NOT be present
   expect: the OPT record to NOT be present

   MBZ (Must Be Zero) presence indicates the flag bit has been
   incorrectly copied.  See Section 4.1.1, [RFC1035] "Z Reserved for
   future use.  Must be zero in all queries and responses."

8.1.4.  Testing Unknown Opcodes

   Construct a DNS message that consists of only a DNS header with
   opcode set to 15 (currently not allocated), no DNS header bits set
   and empty question, answer, authority and additional sections.

   Check that new opcodes are handled:

   dig +noedns +noad +opcode=15 +norec +header-only @$server

   expect: status: NOTIMP
   expect: SOA record to NOT be present
   expect: flag: aa to NOT be present
   expect: flag: rd to NOT be present
   expect: flag: ad to NOT be present
   expect: the OPT record to NOT be present

   As unknown opcodes have no definition, including packet format other
   than there must be a DNS header present (QR, OPCODE and RCODE are the
   only header fields that need to be common across all opcodes,
   everything else in the header can potentially be redefined), there is
   only one possible rcode that make sense to return to a request with a
   unknown opcode and that is NOTIMP.








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8.1.5.  Testing Recursive Queries

   Ask for the SOA record of the zone the server is nominally configured
   to serve.  This query is made with only the RD DNS flag bit set and
   without EDNS.

   We expect the SOA record for the zone to be returned in the answer
   section with the rcode set to NOERROR and the AA, QR and RD bits to
   be set in the response, RA may also be set [RFC1034].  We do not
   expect a OPT record to be returned [RFC6891].

   Check that recursive queries work:

   dig +noedns +noad +rec soa $zone @$server

   expect: status: NOERROR
   expect: the SOA record to be present in the answer section
   expect: flag: aa to be present
   expect: flag: rd to be present
   expect: flag: ad to NOT be present
   expect: the OPT record to NOT be present

8.1.6.  Testing TCP

   Ask for the SOA record of the zone the server is nominally configured
   to serve.  This query is made with no DNS flag bits set and without
   EDNS.  This query is to be sent using TCP.

   We expect the SOA record for the zone to be returned in the answer
   section with the rcode set to NOERROR and the AA and QR bits to be
   set in the response, RA may also be set [RFC1034].  We do not expect
   a OPT record to be returned [RFC6891].

   Check that TCP queries work:

   dig +noedns +noad +norec +tcp soa $zone @$server

   expect: status: NOERROR
   expect: the SOA record to be present in the answer section
   expect: flag: aa to be present
   expect: flag: rd to NOT be present
   expect: flag: ad to NOT be present
   expect: the OPT record to NOT be present

   The requirement that TCP be supported is defined in [RFC7766].






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8.2.  Testing - Extended DNS

   The next set of test cover various aspects of EDNS behaviour.  If any
   of these tests succeed, then all of them should succeed.  There are
   servers that support EDNS but fail to handle plain EDNS queries
   correctly so a plain EDNS query is not a good indicator of lack of
   EDNS support.

8.2.1.  Testing Minimal EDNS

   Ask for the SOA record of the zone the server is nominally configured
   to serve.  This query is made with no DNS flag bits set.  EDNS
   version 0 is used without any EDNS options or EDNS flags set.

   We expect the SOA record for the zone to be returned in the answer
   section with the rcode set to NOERROR and the AA and QR bits to be
   set in the response, RA may also be set [RFC1034].  We expect a OPT
   record to be returned.  There should be no EDNS flags present in the
   response.  The EDNS version field should be zero and there should be
   no EDNS options present [RFC6891].

   Check that plain EDNS queries work:

   dig +nocookie +edns=0 +noad +norec soa $zone @$server

   expect: status: NOERROR
   expect: the SOA record to be present in the answer section
   expect: a OPT record to be present in the additional section
   expect: EDNS Version 0 in response
   expect: flag: aa to be present
   expect: flag: ad to NOT be present

   +nocookie disables sending a EDNS COOKIE option in which is on by
   default in BIND 9.11.0.

8.2.2.  Testing EDNS Version Negotiation

   Ask for the SOA record of the zone the server is nominally configured
   to serve.  This query is made with no DNS flag bits set.  EDNS
   version 1 is used without any EDNS options or EDNS flags set.

   We expect the SOA record for the zone to NOT be returned in the
   answer section with the extended rcode set to BADVERS and the QR bit
   to be set in the response, RA may also be set [RFC1034].  We expect a
   OPT record to be returned.  There should be no EDNS flags present in
   the response.  The EDNS version field should be zero as EDNS versions
   other than 0 are yet to be specified and there should be no EDNS
   options present [RFC6891].



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   Check that EDNS version 1 queries work (EDNS supported):

   dig +nocookie +edns=1 +noednsneg +noad +norec soa $zone @$server

   expect: status: BADVERS
   expect: the SOA record to NOT be present in the answer section
   expect: a OPT record to be present in the additional section
   expect: EDNS Version 0 in response
   expect: flag: aa to NOT be present
   expect: flag: ad to NOT be present

   Only EDNS Version 0 is currently defined so the response should
   always be a 0 version.  This will change when EDNS version 1 is
   defined.  BADVERS is the expected rcode if EDNS is supported as per
   Section 6.1.3, [RFC6891].

8.2.3.  Testing Unknown EDNS Options

   Ask for the SOA record of the zone the server is nominally configured
   to serve.  This query is made with no DNS flag bits set.  EDNS
   version 0 is used without any EDNS flags.  A EDNS option is present
   with a value from the yet to be assigned range.  The unassigned value
   chosen is 100 and will need to be adjusted when IANA assigns this
   value formally.

   We expect the SOA record for the zone to be returned in the answer
   section with the rcode set to NOERROR and the AA and QR bits to be
   set in the response, RA may also be set [RFC1034].  We expect a OPT
   record to be returned.  There should be no EDNS flags present in the
   response.  The EDNS version field should be zero as EDNS versions
   other than 0 are yet to be specified and there should be no EDNS
   options present as unknown EDNS options are supposed to be ignored by
   the server [RFC6891].

   Check that EDNS queries with an unknown option work (EDNS supported):

   dig +nocookie +edns=0 +noad +norec +ednsopt=100 soa $zone @$server

   expect: status: NOERROR
   expect: the SOA record to be present in the answer section
   expect: a OPT record to be present in the additional section
   expect: OPT=100 to NOT be present
   expect: EDNS Version 0 in response
   expect: flag: aa to be present
   expect: flag: ad to NOT be present

   Unknown EDNS options are supposed to be ignored, Section 6.1.2,
   [RFC6891].



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8.2.4.  Testing Unknown EDNS Flags

   Ask for the SOA record of the zone the server is nominally configured
   to serve.  This query is made with no DNS flag bits set.  EDNS
   version 0 is used without any EDNS options.  A unassigned EDNS flag
   bit is set (0x40 in this case).

   We expect the SOA record for the zone to be returned in the answer
   section with the rcode set to NOERROR and the AA and QR bits to be
   set in the response, RA may also be set [RFC1034].  We expect a OPT
   record to be returned.  There should be no EDNS flags present in the
   response as unknown EDNS flags are supposed to be ignored.  The EDNS
   version field should be zero and there should be no EDNS options
   present [RFC6891].

   Check that EDNS queries with unknown flags work (EDNS supported):

   dig +nocookie +edns=0 +noad +norec +ednsflags=0x40 soa $zone @$server

   expect: status: NOERROR
   expect: the SOA record to be present in the answer section
   expect: a OPT record to be present in the additional section
   expect: MBZ not to be present
   expect: EDNS Version 0 in response
   expect: flag: aa to be present
   expect: flag: ad to NOT be present

   MBZ (Must Be Zero) presence indicates the flag bit has been
   incorrectly copied as per Section 6.1.4, [RFC6891].

8.2.5.  Testing EDNS Version Negotiation With Unknown EDNS Flags

   Ask for the SOA record of the zone the server is nominally configured
   to serve.  This query is made with no DNS flag bits set.  EDNS
   version 1 is used without any EDNS options.  A unassigned EDNS flag
   bit is set (0x40 in this case).

   We expect the SOA record for the zone to NOT be returned in the
   answer section with the extended rcode set to BADVERS and the QR bit
   to be set in the response, RA may also be set [RFC1034].  We expect a
   OPT record to be returned.  There should be no EDNS flags present in
   the response as unknown EDNS flags are supposed to be ignored.  The
   EDNS version field should be zero as EDNS versions other than 0 are
   yet to be specified and there should be no EDNS options present
   [RFC6891].






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   Check that EDNS version 1 queries with unknown flags work (EDNS
   supported):

   dig +nocookie +edns=1 +noednsneg +noad +norec +ednsflags=0x40 soa \
       $zone @$server

   expect: status: BADVERS
   expect: SOA record to NOT be present
   expect: a OPT record to be present in the additional section
   expect: MBZ not to be present
   expect: EDNS Version 0 in response
   expect: flag: aa to NOT be present
   expect: flag: ad to NOT be present

   +noednsneg disables EDNS version negotiation in DiG; MBZ (Must Be
   Zero) presence indicates the flag bit has been incorrectly copied.

8.2.6.  Testing EDNS Version Negotiation With Unknown EDNS Options

   Ask for the SOA record of the zone the server is nominally configured
   to serve.  This query is made with no DNS flag bits set.  EDNS
   version 1 is used.  A unknown EDNS option is present (option code 100
   has been chosen).

   We expect the SOA record for the zone to NOT be returned in the
   answer section with the extended rcode set to BADVERS and the QR bit
   to be set in the response, RA may also be set [RFC1034].  We expect a
   OPT record to be returned.  There should be no EDNS flags present in
   the response.  The EDNS version field should be zero as EDNS versions
   other than 0 are yet to be specified and there should be no EDNS
   options present [RFC6891].

   Check that EDNS version 1 queries with unknown options work (EDNS
   supported):

   dig +nocookie +edns=1 +noednsneg +noad +norec +ednsopt=100 soa \
       $zone @$server

   expect: status: BADVERS
   expect: SOA record to NOT be present
   expect: a OPT record to be present in the additional section
   expect: OPT=100 to NOT be present
   expect: EDNS Version 0 in response
   expect: flag: aa to be present
   expect: flag: ad to NOT be present

   +noednsneg disables EDNS version negotiation in DiG.




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8.2.7.  Testing Truncated Responses

   Ask for the DNSKEY records of the zone the server is nominally
   configured to serve.  This query is made with no DNS flag bits set.
   EDNS version 0 is used without any EDNS options.  The only EDNS flag
   set is DO.  The EDNS UDP buffer size is set to 512.  The intention of
   this query is elicit a truncated response from the server.  Most
   signed DNSKEY responses are bigger than 512 bytes.

   We expect a response with the rcode set to NOERROR and the AA and QR
   bits to be set, AD may be set in the response if the server supports
   DNSSEC otherwise it should be clear.  TC and RA may also be set
   [RFC1034].  We expect a OPT record to be present in the response.
   There should be no EDNS flags other than DO present in the response.
   The EDNS version field should be zero and there should be no EDNS
   options present [RFC6891].

   If TC is not set it is not possible to confirm that the server
   correctly adds the OPT record to the truncated responses or not.

   dig +norec +dnssec +bufsize=512 +ignore dnskey $zone @$server
   expect: NOERROR
   expect: OPT record with version set to 0

8.2.8.  Testing DNSSEC Queries

   Ask for the SOA record of the zone the server is nominally configured
   to serve.  This query is made with no DNS flag bits set.  EDNS
   version 0 is used without any EDNS options.  The only EDNS flag set
   is DO.

   We expect the SOA record for the zone to be returned in the answer
   section with the rcode set to NOERROR and the AA and QR bits to be
   set in the response, AD may be set in the response if the server
   supports DNSSEC otherwise it should be clear.  RA may also be set
   [RFC1034].  We expect a OPT record to be returned.  There should be
   no EDNS flags other than DO present in the response which should be
   present if the server supports DNSSEC.  The EDNS version field should
   be zero and there should be no EDNS options present [RFC6891].












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   Check that a DNSSEC queries work (EDNS supported):

   dig +nocookie +edns=0 +noad +norec +dnssec soa $zone @$server

   expect: status: NOERROR
   expect: the SOA record to be present in the answer section
   expect: a OPT record to be present in the additional section
   expect: DO=1 to be present if a RRSIG is in the response
   expect: EDNS Version 0 in response
   expect: flag: aa to be present

   DO=1 should be present if RRSIGs are returned as they indicate that
   the server supports DNSSEC.  Servers that support DNSSEC are supposed
   to copy the DO bit from the request to the response as per [RFC3225].

8.2.9.  Testing EDNS Version Negotiation With DNSSEC

   Ask for the SOA record of the zone the server is nominally configured
   to serve.  This query is made with no DNS flag bits set.  EDNS
   version 1 is used without any EDNS options.  The only EDNS flag set
   is DO.

   We expect the SOA record for the zone to NOT be returned in the
   answer section with the rcode set to BADVERS and the only the QR bit
   and possibly the RA bit to be set [RFC1034].  We expect a OPT record
   to be returned.  There should be no EDNS flags other than DO present
   in the response which should be present if the server supports
   DNSSEC.  The EDNS version field should be zero and there should be no
   EDNS options present [RFC6891].

   Check that EDNS version 1 DNSSEC queries work (EDNS supported):

   dig +nocookie +edns=1 +noednsneg +noad +norec +dnssec soa \
       $zone @$server

   expect: status: BADVERS
   expect: SOA record to NOT be present
   expect: a OPT record to be present in the additional section
   expect: DO=1 to be present if the EDNS version 0 DNSSEC query test
           returned DO=1
   expect: EDNS Version 0 in response
   expect: flag: aa to NOT be present

   +noednsneg disables EDNS version negotiation in DiG.







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8.2.10.  Testing With Multiple Defined EDNS Options

   Ask for the SOA record of the zone the server is nominally configured
   to serve.  This query is made with no DNS flag bits set.  EDNS
   version 0 is used.  A number of defined EDNS options are present
   (NSID [RFC5001], DNS COOKIE [RFC7873], EDNS Client Subnet [RFC7871]
   and EDNS Expire [RFC7314]).

   We expect the SOA record for the zone to be returned in the answer
   section with the rcode set to NOERROR and the AA and QR bits to be
   set in the response, RA may also be set [RFC1034].  We expect a OPT
   record to be returned.  There should be no EDNS flags present in the
   response.  The EDNS version field should be zero.  Any of the
   requested EDNS options supported by the server and permitted server
   configuration may be returned [RFC6891].

   Check that EDNS queries with multiple defined EDNS options work:

   dig +edns=0 +noad +norec +cookie +nsid +expire +subnet=0.0.0.0/0 \
       soa $zone @$server

   expect: status: NOERROR
   expect: the SOA record to be present in the answer section
   expect: a OPT record to be present in the additional section
   expect: EDNS Version 0 in response
   expect: flag: aa to be present
   expect: flag: ad to NOT be present

8.3.  When EDNS Is Not Supported

   If EDNS is not supported by the nameserver, we expect a response to
   all the above queries.  That response may be a FORMERR or NOTIMP
   error response or the OPT record may just be ignored.

   Some nameservers only return a EDNS response when a particular EDNS
   option or flag (e.g.  DO=1) is present in the request.  This
   behaviour is not compliant behaviour and may hide other incorrect
   behaviour from the above tests.  Re-testing with the triggering
   option / flag present will expose this misbehaviour.

9.  Remediation

   Name server operators are generally expected to test their own
   infrastructure for compliance to standards.  The above tests should
   be run when new systems are brought online, and should be repeated
   periodically to ensure continued interoperability.





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   Domain registrants who do not maintain their own DNS infrastructure
   are entitled to a DNS service that conforms to standards and
   interoperates well.  Registrants who become aware that their DNS
   operator does not have a well maintained or compliant infrastructure
   should insist that their service provider correct issues, and switch
   providers if they do not.

   In the event that an operator experiences problems due to the
   behaviour of name servers outside their control, the above tests will
   help in narrowing down the precise issue(s) which can then be
   reported to the relevant party.

   If contact information for the operator of a misbehaving name server
   is not already known, the following methods of communication could be
   considered:

   o  the RNAME of the zone authoritative for the name of the
      misbehaving server

   o  the RNAME of zones for which the offending server is authoritative

   o  administrative or technical contacts listed in the registration
      information for the parent domain of the name of the misbehaving
      server, or for zones   for which the name server is authoritative

   o  the registrar or registry for such zones

   o  DNS-specific operational fora (e.g. mailing lists)

   Operators of parent zones may wish to regularly test the
   authoritative name servers of their child zones.   However, parent
   operators can have widely varying capabilities in terms of
   notification or remediation depending on whether they have a direct
   relationship with the child operator.   Many TLD registries, for
   example, cannot directly contact their registrants and may instead
   need to communicate through the relevant registrar.   In such cases
   it may be most efficient for registrars to take on the responsibility
   for testing the name servers of their registrants, since they have a
   direct relationship.

   When notification is not effective at correcting problems with a
   misbehaving name server, parent operators can choose to remove NS
   record sets (and glue records below) that refer to the faulty server.
   This should only be done as a last resort and with due consideration,
   as removal of a delegation can have unanticipated side effects.  For
   example, other parts of the DNS tree may depend on names below the
   removed zone cut, and the parent operator may find themselves
   responsible for causing new DNS failures to occur.



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10.  Security Considerations

   Testing protocol compliance can potentially result in false reports
   of attempts to break services from Intrusion Detection Services and
   firewalls.  None of the tests listed above should break nominally
   EDNS compliant servers.  None of the tests above should break non
   EDNS servers.  All the tests above are well formed, though not
   necessarily common, DNS queries.

   Relaxing firewall settings to ensure EDNS compliance could
   potentially expose a critical implementation flaw in the nameserver.
   Nameservers should be tested for conformance before relaxing firewall
   settings.

   When removing delegations for non-compliant servers there can be a
   knock on effect on other zones that require these zones to be
   operational for the nameservers addresses to be resolved.

11.  IANA Considerations

   There are no actions for IANA.

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>.

   [RFC3225]  Conrad, D., "Indicating Resolver Support of DNSSEC",
              RFC 3225, DOI 10.17487/RFC3225, December 2001,
              <http://www.rfc-editor.org/info/rfc3225>.

   [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,
              <http://www.rfc-editor.org/info/rfc4035>.

   [RFC6840]  Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and
              Implementation Notes for DNS Security (DNSSEC)", RFC 6840,
              DOI 10.17487/RFC6840, February 2013,
              <http://www.rfc-editor.org/info/rfc6840>.




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   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,
              <http://www.rfc-editor.org/info/rfc6891>.

   [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>.

   [RFC7766]  Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
              D. Wessels, "DNS Transport over TCP - Implementation
              Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,
              <http://www.rfc-editor.org/info/rfc7766>.

12.2.  Informative References

   [RFC5001]  Austein, R., "DNS Name Server Identifier (NSID) Option",
              RFC 5001, DOI 10.17487/RFC5001, August 2007,
              <http://www.rfc-editor.org/info/rfc5001>.

   [RFC7314]  Andrews, M., "Extension Mechanisms for DNS (EDNS) EXPIRE
              Option", RFC 7314, DOI 10.17487/RFC7314, July 2014,
              <http://www.rfc-editor.org/info/rfc7314>.

   [RFC7871]  Contavalli, C., van der Gaast, W., Lawrence, D., and W.
              Kumari, "Client Subnet in DNS Queries", RFC 7871,
              DOI 10.17487/RFC7871, May 2016,
              <http://www.rfc-editor.org/info/rfc7871>.

   [RFC7873]  Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
              Cookies", RFC 7873, DOI 10.17487/RFC7873, May 2016,
              <http://www.rfc-editor.org/info/rfc7873>.

Author's Address

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

   Email: marka@isc.org









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