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DNS Error Reporting

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9567.
Authors Roy Arends , Matt Larson
Last updated 2024-04-26 (Latest revision 2024-03-04)
Replaces draft-arends-dns-error-reporting
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources DNS error reporting PoC in Unbound recursive name server
Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Benno Overeinder
Shepherd write-up Show Last changed 2023-12-20
IESG IESG state Became RFC 9567 (Proposed Standard)
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Warren "Ace" Kumari
Send notices to
IANA IANA review state Version Changed - Review Needed
IANA action state RFC-Ed-Ack
IANA expert review state Expert Reviews OK
IANA expert review comments Experts have approved the DNS EDNS0 Option Codes (OPT) and the Underscored and Globally Scoped DNS Node Names registrations.
dnsop                                                          R. Arends
Internet-Draft                                                 M. Larson
Intended status: Standards Track                                   ICANN
Expires: 5 September 2024                                   4 March 2024

                          DNS Error Reporting


   DNS error reporting is a lightweight reporting mechanism that
   provides the operator of an authoritative server with reports on DNS
   resource records that fail to resolve or validate.  A domain owner or
   DNS hosting organization can use these reports to improve domain
   hosting.  The reports are based on extended DNS errors as described
   in [RFC8914].

   When a domain name fails to resolve or validate due to a
   misconfiguration or an attack, the operator of the authoritative
   server may be unaware of this.  To mitigate this lack of feedback,
   this document describes a method for a validating resolver to
   automatically signal an error to a monitoring agent specified by the
   authoritative server.  The error is encoded in the QNAME, thus the
   very act of sending the query is to report the error.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 5 September 2024.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   license-info) 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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  EDNS0 Option Specification  . . . . . . . . . . . . . . . . .   5
   6.  DNS error reporting specification . . . . . . . . . . . . . .   6
     6.1.  Reporting Resolver Specification  . . . . . . . . . . . .   6
       6.1.1.  Constructing the Report Query . . . . . . . . . . . .   6
     6.2.  Authoritative server specification  . . . . . . . . . . .   7
     6.3.  Monitoring agent specification  . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Operational Considerations  . . . . . . . . . . . . . . . . .   8
     8.1.  Choosing an agent domain  . . . . . . . . . . . . . . . .   8
     8.2.  Managing Caching Optimizations  . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     11.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   When an authoritative server serves a stale DNSSEC-signed zone, the
   cryptographic signatures over the resource record sets (RRsets) may
   have lapsed.  A validating resolver will fail to validate these
   resource records.

   Similarly, when there is a mismatch between the Delegation Signer
   (DS) records at a parent zone and the key signing key at the child
   zone, a validating resolver will fail to authenticate records in the
   child zone.

   These are two of several failure scenarios that may go unnoticed for
   some time by the operator of a zone.

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   Today, there is no direct relationship between operators of
   validating resolvers and authoritative servers.  Outages are often
   noticed indirectly by end users, and reported via E-Mail or social
   media (if reported at all).

   When records fail to validate, there is no facility to report this
   failure in an automated way.  If there is any indication that an
   error or warning has happened, it may be buried in log files of the
   resolver or not logged at all.

   This document describes a method that can be used by validating
   resolvers to report DNSSEC validation errors in an automated way.

   It allows an authoritative server to announce a monitoring agent
   where validating resolvers can report issues if those resolvers are
   configured to do so.

   The burden to report a failure falls on the validating resolver.  It
   is important that the effort needed to report failure is low, with
   minimal impact to its main functions.  To accomplish this goal, the
   DNS itself is utilized to report the error.

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Terminology

   DNS Terminology used in this document is from BCP 219 [RFC8499], with
   these additions:

   Reporting resolver:  In the context of this document, "reporting
      resolver" is used as a shorthand for a validating resolver that
      supports DNS error reporting.

   Report query:  The DNS query used to report an error is called a
      report query.  A report query is for a DNS TXT resource record
      type.  The content of the error report is encoded in the QNAME of
      a DNS request to the monitoring agent.

   Monitoring agent:  An authoritative server that receives and responds
      to report queries.  This facility is indicated by a domain name,
      referred to as the agent domain.

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   Agent domain:  A domain name which is returned in the DNS Error
      Reporting EDNS0 option that indicates where DNS resolvers can send
      error reports.

4.  Overview

   An authoritative server indicates support for DNS error reporting by
   including an EDNS0 Report-Channel option with OPTION-CODE [TBD] and
   the agent domain in the response.  The agent domain is a fully
   qualified, uncompressed domain name in DNS wire format.  The
   authoritative server MUST NOT include this option in the response if
   the configured agent domain is empty, or is the null label (which
   would indicate the DNS root).

   The authoritative server includes the EDNS0 Report-Channel option
   unsolicited.  That is, the option is included in a response despite
   the EDNS0 Report-Channel option being absent in the request.

   If the authoritative server has indicated support for DNS error
   reporting and there is an issue that can be reported via extended DNS
   errors, the reporting resolver encodes the error report in the QNAME
   of the report query.  The reporting resolver builds this QNAME by
   concatenating the _er label, the QTYPE, the QNAME that resulted in
   failure, the extended error code (as described in [RFC8914]), the
   label "_er" again, and the agent domain.  See the example in
   Section 4.1.  Note that a regular RCODE is not included because the
   RCODE is not relevant to the extended error code.

   The resulting report query is sent as a standard DNS query for a TXT
   DNS resource record type by the reporting resolver.

   The report query will ultimately arrive at the monitoring agent.  A
   response is returned by the monitoring agent, which in turn can be
   cached by the reporting resolver.  This caching is essential.  It
   dampens the number of report queries sent by a reporting resolver for
   the same problem, that is, once per TTL.  However, certain
   optimizations such as [RFC8020] and [RFC8198] may reduce the number
   of error report queries as well.

   This document gives no guidance on the content of the TXT resource
   record RDATA for this record.

4.1.  Example

   A query for "broken.test.", type A, is sent by a reporting resolver.

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   The domain "test." is hosted on a set of authoritative servers.  One
   of these authoritative servers serves a stale version of the "test."
   zone.  This authoritative server has an agent domain configured:

   The authoritative server with the stale "test." zone receives the
   request for "broken.test".  It returns a response that includes the
   EDNS0 Report-Channel option with the domain name "a01.agent-

   The reporting resolver is unable to validate the "broken.test" RRset
   for type 1 (an A record), due to an RRSIG record with an expired

   The reporting resolver constructs the QNAME
   "_er.1.broken.test.7._er.a01.agent-domain.example." and resolves it.
   This QNAME indicates extended DNS error 7 occurred while trying to
   validate "broken.test." type 1 record.

   When this query is received at the monitoring agent (the operators of
   the authoritative server for a01.agent-domain.example), the agent can
   determine the "test." zone contained an expired signature record
   (extended error 7) for type A for the domain name "broken.test.".
   The monitoring agent can contact the operators of "test." to fix the

5.  EDNS0 Option Specification

   This method uses an EDNS0 [RFC6891] option to indicate the agent
   domain in DNS responses.  The option is structured as follows:

                        1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |        OPTION-CODE = TBD      |       OPTION-LENGTH           |
   /                         AGENT DOMAIN                          /

   Field definition details:

   OPTION-CODE:  2 octets; an EDNS0 code that is used in an EDNS0 option
      to indicate support for error reporting.  The name for this EDNS0
      option code is Report-Channel.

   OPTION-LENGTH:  2-octets; contains the length of the AGENT DOMAIN
      field in octets.

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   AGENT DOMAIN:  A fully qualified domain name [RFC8499] in
      uncompressed DNS wire format.

6.  DNS error reporting specification

   The various errors that a reporting resolver may encounter are listed
   in [RFC8914].  Note that not all listed errors may be supported by
   the reporting resolver.  This document does not specify what is or is
   not an error.

   The DNS class is not specified in the error report.

6.1.  Reporting Resolver Specification

   Care should be taken when more DNS resolving is needed to resolve the
   error reporting QNAME.  This resolving itself could trigger another
   error reporting to be created.  A maximum expense or depth limit MUST
   be used to prevent cascading errors.

   The EDNS0 Report-Channel option MUST NOT be included in queries.

   The reporting resolver MUST NOT use DNS error reporting if the
   authoritative server returned an empty agent domain field in the
   EDNS0 Report-Channel option.

   For the benefit of the monitoring agent to get more confidence that
   the report is not spoofed, the reporting resolver SHOULD send error
   reports over TCP [RFC7766] or other connection oriented protocols or
   SHOULD use DNS COOKIEs [RFC7873].  This makes it harder to falsify
   the source address.

   A reporting resolver MUST validate responses received from the
   monitoring agent.  There is no special treatment for responses to
   error-reporting queries.  The Security Considerations section
   contains the rationale behind this.

6.1.1.  Constructing the Report Query

   The QNAME for the report query is constructed by concatenating the
   following elements:

   *  A label containing the string "_er".

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   *  The QTYPE that was used in the query that resulted in the extended
      DNS error, presented as a decimal value, in a single DNS label.
      If additional QTYPEs were present in the query, such as described
      in [I-D.ietf-dnssd-multi-qtypes], they are represented as unique,
      ordered decimal values separated by a hyphen.  As an example, if
      both QTYPE A and AAAA were present in the query, they are
      presented as the label "1-28".

   *  The list of non-null labels representing the query name which is
      the subject of the DNS error report.

   *  The extended DNS error, presented as a decimal value, in a single
      DNS label.

   *  A label containing the string "_er".

   *  The agent domain.  The agent domain as received in the EDNS0
      Report-Channel option set by the authoritative server.

   If the report query QNAME exceeds 255 octets, it MUST NOT be sent.

   The "_er" labels allow the monitoring agent to differentiate between
   the agent domain and the faulty query name.  When the specified agent
   domain is empty, or a null label (despite being not allowed in this
   specification), the report query will have "_er" as a top-level
   domain as a result and not the original query.  The purpose of the
   first "_er" label is to indicate that a complete report query has
   been received, instead of a shorter report query due to query

6.2.  Authoritative server specification

   The authoritative server MUST NOT include more than one EDNS0 Report-
   Channel option in a response.

   The authoritative server includes the EDNS0 Report-Channel option
   unsolicited in responses.  There is no requirement that the EDNS0
   Report-Channel option is present in queries.

6.3.  Monitoring agent specification

   It is RECOMMENDED that the authoritative server for the agent domain
   replies with a positive response (i.e., not NODATA or NXDOMAIN)
   containing a TXT record.

   The monitoring agent SHOULD respond to queries received over UDP that
   have no DNS COOKIE set with a response that has the truncation bit
   (TC bit) set to challenge the resolver to re-query over TCP.

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7.  IANA Considerations

   IANA is requested to assign the following DNS EDNS0 option code

         Value Name           Status   Reference
         ----- -------------- -------- ---------------
         TBD   Report-Channel Standard [this document]

   IANA is requested to assign the following Underscored and Globally
   Scoped DNS Node Name registry:

         RR Type  _NODE NAME  Reference
         -----    ----------  ---------
         TXT      _er         [this document]

8.  Operational Considerations

8.1.  Choosing an agent domain

   It is RECOMMENDED that the agent domain be kept relatively short to
   allow for a longer QNAME in the report query.  The agent domain MUST
   NOT be a subdomain of the domain it is reporting on.  That is, if the
   authoritative server hosts the foo.example domain, then its agent
   domain MUST NOT end in foo.example.

8.2.  Managing Caching Optimizations

   The reporting resolver may utilize various caching optimizations that
   inhibit subsequent error reporting to the same monitoring agent.

   If the monitoring agent were to respond with NXDOMAIN (name error),
   [RFC8020] states that any name at or below that domain should be
   considered unreachable, and negative caching would prohibit
   subsequent queries for anything at or below that domain for a period
   of time, depending on the negative TTL [RFC2308].

   Since the monitoring agent may not know the contents of all the zones
   for which it acts as a monitoring agent, the monitoring agent MUST
   NOT respond with NXDOMAIN for domains it is monitoring because that
   could inhibit subsequent queries.  One method to avoid NXDOMAIN is to
   use a wildcard domain name [RFC4592] in the zone for the agent

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   When the agent domain is signed, a resolver may use aggressive
   negative caching (described in [RFC8198]).  This optimization makes
   use of NSEC and NSEC3 (without opt-out) records and allows the
   resolver to do the wildcard synthesis.  When this happens, the
   resolver does not send subsequent queries because it will be able to
   synthesize a response from previously cached material.

   A solution is to avoid DNSSEC for the agent domain.  Signing the
   agent domain will incur an additional burden on the reporting
   resolver, as it has to validate the response.  However, this response
   has no utility to the reporting resolver other than dampening the
   query load for error reports.

9.  Security Considerations

   Use of DNS error reporting may expose local configuration mistakes in
   the reporting resolver, such as stale DNSSEC trust anchors to the
   monitoring agent.

   DNS error reporting SHOULD be done using DNS query name minimization
   [RFC9156] to improve privacy.

   DNS error reporting is done without any authentication between the
   reporting resolver and the authoritative server of the agent domain.

   Resolvers that send error reports SHOULD send these over TCP
   [RFC7766] or SHOULD use DNS COOKIEs [RFC7873].  This makes it hard to
   falsify the source address.  The monitoring agent SHOULD respond to
   queries received over UDP that have no DNS COOKIE set with a response
   that has the truncation bit (TC bit) set to challenge the resolver to
   re-query over TCP.

   Well-known addresses of reporting resolvers can provide a higher
   level of confidence in the error reports, and potentially enable more
   automated processing of these reports.

   Monitoring agents that receive error reports over UDP should consider
   that the source of the reports and the reports itself may be false.

   The method described in this document will cause additional queries
   by the reporting resolver to authoritative servers in order to
   resolve the report query.

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   This method can be abused by intentionally deploying broken zones
   with agent domains that are delegated to victims.  This is
   particularly effective when DNS requests that trigger error messages
   are sent through open resolvers [RFC8499] or widely distributed
   network monitoring systems that perform distributed queries from
   around the globe.

   An adversary may create massive error report flooding to camouflage
   an attack.

   Though this document gives no guidance on the content of the TXT
   resource record RDATA for this record, if the RDATA content is
   logged, it MUST assume the content can be malicious and take
   appropriate meassures to avoid exploitation.  One such method could
   be to log in hexadecimal.  This would avoid remote code execution
   through logging string attacks such as [CVE-2021-44228].

   The rationale behind mandating DNSSEC validation for responses from a
   reporting agent, even if the agent domain is proposed to remain
   unsigned, is to mitigate the risk of a downgrade attack orchestrated
   by adversaries.  In such an attack, a victim's legitimately signed
   domain could be deceptively advertised as an agent domain by
   malicious actors.  Consequently, if the validating resolver treats it
   as unsigned, it is exposed to potential cache poisoning attacks.  By
   enforcing DNSSEC validation, this vulnerability is preemptively

10.  Acknowledgements

   This document is based on an idea by Roy Arends and David Conrad.
   The authors would like to thank Peter van Dijk, Stephane Bortzmeyer,
   Shane Kerr, Vladimir Cunat, Paul Hoffman, Philip Homburg, Mark
   Andrews, Libor Peltan, Matthijs Mekking, Willem Toorop, Tom Carpay,
   Dick Franks, Ben Schwartz, Yaron Sheffer, Viktor Dukhovni, Wes
   Hardaker, James Gannon, Tim Wicinski, Warren Kumari, Gorry Fairhurst,
   Benno Overeinder, Paul Wouters and Petr Spacek for their

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

11.2.  Informative References

              Common Vulnerabilities and Exposures, "CVE-2021-44228", 10
              December 2021, <

              Bellis, R., "DNS Multiple QTYPEs", Work in Progress,
              Internet-Draft, draft-ietf-dnssd-multi-qtypes-00, 4
              December 2023, <

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

   [RFC4592]  Lewis, E., "The Role of Wildcards in the Domain Name
              System", RFC 4592, DOI 10.17487/RFC4592, July 2006,

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,

   [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,

   [RFC7873]  Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
              Cookies", RFC 7873, DOI 10.17487/RFC7873, May 2016,

   [RFC8020]  Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is
              Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020,
              November 2016, <>.

   [RFC8198]  Fujiwara, K., Kato, A., and W. Kumari, "Aggressive Use of
              DNSSEC-Validated Cache", RFC 8198, DOI 10.17487/RFC8198,
              July 2017, <>.

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   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <>.

   [RFC8914]  Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
              Lawrence, "Extended DNS Errors", RFC 8914,
              DOI 10.17487/RFC8914, October 2020,

   [RFC9156]  Bortzmeyer, S., Dolmans, R., and P. Hoffman, "DNS Query
              Name Minimisation to Improve Privacy", RFC 9156,
              DOI 10.17487/RFC9156, November 2021,

Authors' Addresses

   Roy Arends

   Matt Larson

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