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DNS Error Reporting
RFC 9567

Document Type RFC - Proposed Standard (April 2024)
Authors Roy Arends , Matt Larson
Last updated 2024-04-26
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Warren "Ace" Kumari
Send notices to (None)
RFC 9567

Internet Engineering Task Force (IETF)                         R. Arends
Request for Comments: 9567                                     M. Larson
Category: Standards Track                                          ICANN
ISSN: 2070-1721                                               April 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 RFC 8914.

   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 is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2024 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
   ( 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.  Requirements Notation
   3.  Terminology
   4.  Overview
     4.1.  Example
   5.  EDNS0 Option Specification
   6.  DNS Error Reporting Specification
     6.1.  Reporting Resolver Specification
       6.1.1.  Constructing the Report Query
     6.2.  Authoritative Server Specification
     6.3.  Monitoring Agent Specification
   7.  IANA Considerations
   8.  Operational Considerations
     8.1.  Choosing an Agent Domain
     8.2.  Managing Caching Optimizations
   9.  Security Considerations
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Authors' Addresses

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.

   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 email 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 to
   which 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

   This document uses DNS terminology defined in BCP 219 [RFC9499].
   This document also defines and uses the following terms:

   Reporting resolver:  A validating resolver that supports DNS error

   Report query:  The DNS query used to report an error.  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

   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".

   Agent domain:  A domain name that is returned in the EDNS0 Report-
      Channel option and indicates where DNS resolvers can send error

4.  Overview

   An authoritative server indicates support for DNS error reporting by
   including an EDNS0 Report-Channel option with OPTION-CODE 18 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 DNS error code (as described in [RFC8914]), the
   label "_er" again, and the agent domain.  See the example in
   Section 4.1 and the specification in Section 6.1.1.  Note that a
   regular RCODE is not included because the RCODE is not relevant to
   the extended DNS 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, with caching, one report query per TTL is
   sent).  However, certain optimizations, such as those described in
   [RFC8020] and [RFC8198], may reduce the number of error report
   queries as well.

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

4.1.  Example

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

   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 as

   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 A (an RR type with value 1), due to an RRSIG record
   with an expired signature.

   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." for a type A (an RR type with value 1)

   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 DNS 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 = 18       |       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.

   AGENT DOMAIN:  A fully qualified domain name [RFC9499] 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 additional DNS resolution is needed to
   resolve the QNAME that contains the error report.  This resolution
   itself could trigger another error report 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 monitoring agent to gain 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

   A reporting resolver MUST validate responses received from the
   monitoring agent.  There is no special treatment for responses to
   error-reporting queries.  Section 9 ("Security Considerations")
   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".

   *  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 [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

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

   *  The extended DNS error code, 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 QNAME of the report query exceeds 255 octets, it MUST NOT be

   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 is a null label (despite being not allowed in
   this specification), the report query will have "_er" as a top-level
   domain, and not the top-level domain from the query name that was the
   subject of this error report.  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 minimization.

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 be present in queries.

6.3.  Monitoring Agent Specification

   It is RECOMMENDED that the authoritative server for the agent domain
   reply with a positive response (i.e., not with 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 requery over TCP.

7.  IANA Considerations

   IANA has assigned the following in the "DNS EDNS0 Option Codes (OPT)"

             | Value | Name           | Status   | Reference |
             | 18    | Report-Channel | Standard | RFC 9567  |

                                  Table 1

   IANA has assigned the following in the "Underscored and Globally
   Scoped DNS Node Names" registry:

                   | RR Type | _NODE NAME | Reference |
                   | TXT     | _er        | RFC 9567  |

                                 Table 2

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

   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 them 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
   requery 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 themselves may be

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

   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 [RFC9499] 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 RDATA in
   the TXT resource record, if the RDATA content is logged, the
   monitoring agent MUST assume the content can be malicious and take
   appropriate measures to avoid exploitation.  One such method could be
   to log in hexadecimal.  This would avoid remote code execution
   through logging string attacks, such as the vulnerability described
   in [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.  References

10.1.  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,

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

10.2.  Informative References

              CVE, "CVE-2021-44228", 26 November 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, <>.

   [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,

   [RFC9499]  Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
              RFC 9499, DOI 10.17487/RFC9499, March 2024,


   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

Authors' Addresses

   Roy Arends

   Matt Larson