Independent Submission                                         R. Arends
Internet-Draft                                                 M. Larson
Intended status: Standards Track                                   ICANN
Expires: May 1, 2021                                    October 28, 2020

                          DNS Error Reporting


   DNS Error Reporting is a lightweight error reporting mechanism that
   provides the operator of an authoritative server with reports on DNS
   resource records that fail to resolve or validate, that a Domain
   Owner or DNS Hosting organization can use to improve domain hosting.
   The reports are based on Extended DNS Errors [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 recursive resolver
   to automatically signal an error to an agent specified by the
   authoritative server.  DNS Error Reporting uses the DNS to report

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on May 1, 2021.

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Table of Contents

   1.  Introduction
   2.  Requirements Notation
   3.  Terminology
   4.  Overview
     4.1.  Managing Caching Optimizations
     4.2.  Example
   5.  EDNS0 Option Specification
   6.  DNS Error Reporting Specification
     6.1.  Reporting Resolver Specification
       6.1.1.  Constructing the Reporting Query
     6.2.  Authoritative Server Specification
     6.3.  Reporting Agent Specification
     6.4.  Choosing a Reporting Agent Domain
   7.  Limitations
   8.  IANA Considerations
   9.  Security Considerations
   10. Acknowledgements
   11. 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 recursive resolver will fail to validate
   these resource records.

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

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

   There is no direct relationship between operators of validating
   recursive resolvers and authoritative servers.  Outages are often
   noticed indirectly, by end users, and reported via 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 is buried in log files of the
   validating resolver, if these errors are logged at all.

   This document describes a facility that can be used by validating
   recursive resolvers to report errors in an automated way.

   It allows an authoritative server to signal a reporting agent where
   the validating recursive resolver can report issues if it is
   configured to do so.

   The burden of reporting a failure falls on the validating recursive
   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",
   document are to be interpreted as described in [RFC2119].

3.  Terminology

   Reporting Resolver: In the context of this document, the term
   reporting resolver is used as a shorthand for a validating recursive
   resolver that supports DNS Error Reporting.

   Reporting Query: The DNS query used to report an error is called a
   reporting query.  A reporting query is for DNS resource record type
   NULL.  The details of the error report are encoded in the QNAME of
   the reporting query.

   Reporting Agent: A facility responsible for receiving error reports
   on behalf of authoritative servers.  This facility is indicated by a
   domain name.

   Reporting Agent Domain: a domain name which the reporting resolver
   includes in the QNAME of the reporting query.

4.  Overview

   In a query-response exchange, a reporting resolver indicates support
   for DNS Error Reporting by including an EDNS option with OPTION-CODE
   [TBD] [RFC Editor: change TBD to the proper code when assigned by
   the EDNS option is absent in a query.

   An authoritative server indicates support for DNS Error Reporting by
   including an EDNS0 option with OPTION-CODE [TBD] [RFC Editor: change
   TBD to the proper code when assigned by IANA.] and the REPORTING
   AGENT DOMAIN in the option's payload.  The authoritative server MUST
   NOT include this option if the reporting resolver has not signalled
   support for DNS Error Reporting.  The authoritative server MUST NOT
   include this option in the response if the configured reporting agent
   domain is empty or the null label (the root).

   When a reporting resolver sends a reporting query to report an error,
   it MUST NOT include the EDNS0 Error Reporting option in the reporting
   query.  This avoids additional compounding error reporting when the
   reporting agent server is misconfigured.

   To report an error, the reporting resolver encodes the error report
   in the QNAME of the reporting query.  The reporting resolver builds
   this QNAME by concatenating the extended error code [RFC8914], the
   QTYPE and QNAME that resulted in failure, the label "_er", and the
   reporting agent domain.  See the example in section 4.2.  Note that a
   regular RCODE is not included, as the RCODE is not relevant to the
   extended error code.

   The resulting concatenated domain name is sent as a standard DNS
   query for DNS resource record type NULL by the reporting resolver.
   This query MUST NOT have the EDNS0 option code [TBD] set to avoid
   compounding error notifications.

   The query will ultimately arrive at an authoritative server of the
   reporting agent.  A NODATA negative response is returned by the
   authoritative server of the reporting agent domain, which in turn can
   be cached by the reporting resolver.

   This caching is essential.  It ensures that the number of reports
   sent by a reporting resolver for the same problem is dampened, i.e.
   once per TTL, however, certain optimizations such as [RFC8020] and
   [RFC8198] may reduce the error reporting.

4.1.  Managing Caching Optimizations

   The reporting resolver may utilize various caching optimizations that
   inhibit subsequent error reporting by the reporting resolver to the
   authoritative server for an agent domain.

   If the authoritative server for the agent domain were to respond with
   NXDOMAIN (name error), [RFC8020] rules state 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

   Since the authoritative server for an agent domain may not know the
   contents of all the zones it acts as an agent for, it is crucial that
   the authoritative does not respond with NXDOMAIN, as that may inhibit
   subsequent queries.  The use of a wildcard domain name [RFC4592] in
   the zone for the agent domain will ensure the RCODE is consistently

   Considering the Resource Record type for this wildcard record, type
   NULL is prohibited in master zone files [RFC1035].  However, any type
   that is not special according to [RFC4592] section 4 will do, such as
   a TXT record with an email address for the reporting agent in the

   Wildcard expansion occurs, even if the QTYPE is not for the type
   owned by the wildcard domain name.  The response is a "no error, but
   no data" response ([RFC4592], section 2.2.1.) that contains a NOERROR
   RCODE and empty answer section.  Note that reporting resolvers are
   not expected to query for this TXT record, since reporting queries
   use type NULL.  This record is solely present to ensure a NODATA
   response is returned in response to reporting queries.

   When the zone for the reporting agent domain is signed, a resolver
   may utilize aggressive negative caching, discussed 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 may not send subsequent queries as it will
   be able to synthesize a response from previously cached material.

   A solution is to avoid DNSSEC for the reporting agent domain's zone.
   Signing the agent domain's zone 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.

   If an operator does sign a reporting agent domain's zone for whatever
   reason, one option is to use NSEC3 with opt-out, as that
   configuration precludes wildcard synthesis on the resolver.

4.2.  Example

   The domain broken.test is hosted on a set of authoritative servers.
   One of these serves a stale version.  This authoritative server has a
   reporting agent configured: a01.reporting-agent.example.

   The reporting resolver is unable to validate the broken.test RRSet
   for type A, due to an RRSIG record with an expired signature.

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

   After this query is received at one of the authoritative servers for
   the reporting agent domain (a01.reporting-agent.example), the
   reporting agent (the operators of the authoritative server for
   a01.reporting-agent.example) determines that the authoritative server
   for the broken.test zone suffers from an expired signature record
   (extended error 7) for type A for the domain name broken.test.  The
   reporting agent can contact the operators of broken.test to fix the

5.  EDNS0 Option Specification

   This method uses an EDNS0 [RFC6891] option to indicate support for
   sending DNS error reports and responding with the Reporting Agent
   Domain in DNS messages.  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           |
   /                    REPORTING AGENT DOMAIN                     /

   Field definition details:

   o  OPTION-CODE, 2-octets/16-bits (defined in [RFC6891]), for
      indicating error reporting support is TBD.  [RFC Editor: change
      TBD to the proper code when assigned by IANA.]

   o  OPTION-LENGTH, 2-octets/16-bits ((defined in [RFC6891]) contains
      the length of the REPORTING AGENT DOMAIN field in octets.

   o  REPORTING AGENT DOMAIN, a Domain name [RFC8499].

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 an
   error and what is not.

   The DNS class is not specified in the error report.

6.1.  Reporting Resolver Specification

   Reporting Resolvers may have a configuration that allows the

   o  DNS Error Reporting level: warning and / or errors

   o  Do nothing: the reporting resolver does not indicate support for
      DNS Error Reporting.

   o  Report to Reporting Agent: Indicate DNS Error Reporting in queries
      and use the reporting agent specified in the EDNS0 option received
      from the authoritative server.

   o  Report to Configured Agent: Use the reporting agent specified in
      local configuration.  This may override or supplement "Reporting
      Agent Domain".  The use for such an option could be to allow a
      recursive resolver to report all errors to a reporting agent of
      its choosing, not just in zones with DNS Error Reporting enabled.

   The reporting resolver MUST NOT use DNS error reporting to report a
   failure in resolving the reporting query.

   The reporting resolver MUST NOT use DNS error reporting if the
   authoritative server has an empty Reporting Agent Domain field in the
   EDNS Error Reporting option.

6.1.1.  Constructing the Reporting Query

   The QNAME for the reporting query is constructed by concatenating the
   following elements, appending each successive element in the list to
   the right-hand side of the QNAME:

   o  The Extended DNS error, presented as a decimal value, in a single
      DNS label.

   o  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.

   o  The QNAME that was used in the query that resulted in the extended
      DNS error.  The QNAME may consist of multiple labels and is
      concatenated as-is.

   o  A label containing the string "_er".

   o  The reporting agent domain.  The reporting agent domain consists
      of multiple labels and is concatenated exactly as received in the
      EDNS option sent by the authoritative server.

   If the resulting reporting query QNAME would exceed 255 octets, it
   MUST NOT be sent.

   The purpose of the "_er" label is twofold.  First, it allows the
   reporting agent to quickly differentiate between the agent domain and
   the faulty query name.  Second, if the specified agent domain is
   empty, or a NULL label (even if it is not allowed in this
   specification), the reporting query will have "_er" as a top-level
   domain as a result and not the original query.

6.2.  Authoritative Server Specification

   The Authoritative Server MUST NOT have multiple reporting agent
   domains configured for a single zone.  To support multiple reporting
   agents, a single agent can act as a syndicate to subsequently inform
   additional agents.

   An authoritative server for a zone with DNS error reporting enabled
   MUST NOT also be authoritative for that zone's reporting agent
   domain's zone.

6.3.  Reporting Agent Specification

   While there are many zone configurations possible for the reporting
   agent domain, such as DNAME, CNAME or special delegation structures
   to redistribute errors, please note that the burden of reporting is
   on the reporting resolvers and that creating complicated
   configurations that cause additional work for the reporting resolver
   on behalf of misconfigured servers is NOT RECOMMENDED.

   It is RECOMMENDED that the reporting agent zone uses a wildcard DNS
   record of type TXT with an arbitrary string in the RDATA and a TTL of
   at least one hour.

6.4.  Choosing a Reporting Agent Domain

   Each authoritative server SHOULD be configured with a unique
   reporting agent domain.  When different authoritative servers share
   the same reporting agent domain, it is not possible to determine
   which authoritative server the reported error relates to.

   It is RECOMMENDED that the reporting agent domain be kept relatively
   short to allow for a longer QNAME in the reporting query.

   While it may be obvious to use the hostname of the authoritative
   server as the reporting agent domain, it is not a requirement, as
   long as the reporting agent is able to map the reporting agent domain
   to the proper authoritative server.  Using the hostname of the
   authoritative server as the reporting agent domain is NOT RECOMMENDED
   when the hostname has multiple addresses, or when addresses are

7.  Limitations

   The length of the owner name for which errors can be reported is
   limited due to the requirement to append the reporting agent domain
   and prepend the Extended Error value and the QTYPE to the reporting
   query's QNAME.

8.  IANA Considerations

   IANA is requested to assign the following DNS EDNS0 option code

         Value    Name              Status      Reference
         -----    ----------------  --------    ---------------
         TBD      DNS ERROR REPORT  Standard    [this document]

   [RFC Editor: change TBD to the proper code when assigned by IANA.]

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

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

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
   reporting agent.

   DNS Error reporting SHOULD be done using DNS Query Name Minimization
   [RFC7816] to improve privacy.

   DNS Error Reporting is done without any authentication between the
   reporting resolver and the authoritative server of the agent domain.
   Authentication significantly increases the burden on the reporting
   resolver without any benefit to the reporting agent, authoritative
   server or reporting resolver.

   The reporting resolver MUST NOT report about queries and responses
   from an encrypted channel (such as DNS over TLS [RFC7858] and DNS
   over HTTPS [RFC8484]).

   The reporting resolver MUST NOT report about responses that did not
   match the qname/qtype/qclass and query-id in the original query
   [RFC5452], section 4.2.

   The method described in this document will cause additional queries
   by the reporting resolver to authoritative servers in order to
   resolve the reporting query.  This additional load is equivalent to
   the additional load when a resolver resolves the canonical name in a
   CNAME record.

   This method can be abused by deploying broken zones with agent
   domains that are delegated to servers operated by the intended victim
   in combination with open resolvers [RFC8499].  This method MUST NOT
   be deployed by default on reporting resolvers and authoritative
   servers without requiring an explicit configuration element.

10.  Acknowledgements

   This document is based on an idea by Roy Arends and David Conrad.

11.  Informative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

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

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

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

   [RFC5452]  Hubert, A. and R. van Mook, "Measures for Making DNS More
              Resilient against Forged Answers", RFC 5452,
              DOI 10.17487/RFC5452, January 2009,

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

   [RFC7816]  Bortzmeyer, S., "DNS Query Name Minimisation to Improve
              Privacy", RFC 7816, DOI 10.17487/RFC7816, March 2016,

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, 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, <>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,

   [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,

Authors' Addresses

   Roy Arends


   Matt Larson