Structured Error Data for Filtered DNS
draft-ietf-dnsop-structured-dns-error-19

DNS Operations Working Group                                     D. Wing
Internet-Draft                                                    Citrix
Updates: 8914 (if approved)                                     T. Reddy
Intended status: Standards Track                                   Nokia
Expires: 8 October 2026                                          N. Cook
                                                            Open-Xchange
                                                            M. Boucadair
                                                                  Orange
                                                            6 April 2026

                 Structured Error Data for Filtered DNS
                draft-ietf-dnsop-structured-dns-error-19

Abstract

   DNS filtering is widely deployed for various reasons, including
   network security.  However, filtered DNS responses lack structured
   information for end users to understand the reason for the filtering.
   Existing mechanisms to provide explanatory details to end users cause
   harm especially if the blocked DNS response is for HTTPS resources.

   This document updates RFC 8914 by signaling client support for
   structuring the EXTRA-TEXT field of the Extended DNS Error to provide
   details on the DNS filtering.  Such details can be parsed by the
   client and displayed, logged, or used for other purposes.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at https://ietf-wg-
   dnsop.github.io/draft-ietf-dnsop-structured-dns-error/draft-ietf-
   dnsop-structured-dns-error.html.  Status information for this
   document may be found at https://datatracker.ietf.org/doc/draft-ietf-
   dnsop-structured-dns-error/.

   Discussion of this document takes place on the dnsop Working Group
   mailing list (mailto:dnsop@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/dnsop/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/dnsop/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-dnsop/draft-ietf-dnsop-structured-dns-
   error.

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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 https://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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 8 October 2026.

Copyright Notice

   Copyright (c) 2026 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 (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   4
   3.  DNS Filtering Techniques and Their Limitations  . . . . . . .   5
   4.  I-JSON in EXTRA-TEXT Field  . . . . . . . . . . . . . . . . .   7
   5.  Protocol Operation  . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Client Generating Request . . . . . . . . . . . . . . . .   9
     5.2.  Server Generating Response  . . . . . . . . . . . . . . .  10
     5.3.  Client Processing Response  . . . . . . . . . . . . . . .  11
     5.4.  Structured DNS Error (SDE) EDNS(0) Option Format  . . . .  12
   6.  New Sub-Error Codes Definition  . . . . . . . . . . . . . . .  13
     6.1.  Reserved  . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.2.  Network Operator Policy . . . . . . . . . . . . . . . . .  13
     6.3.  DNS Operator Policy . . . . . . . . . . . . . . . . . . .  13
   7.  New Extended DNS Errors . . . . . . . . . . . . . . . . . . .  14

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     7.1.  Extended DNS Error Code TBA1 - Blocked by Upstream DNS
           Server  . . . . . . . . . . . . . . . . . . . . . . . . .  14
   8.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   9.  Operational Considerations  . . . . . . . . . . . . . . . . .  15
     9.1.  Backward Compatibility  . . . . . . . . . . . . . . . . .  15
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  15
     10.1.  Authentication and Confidentiality . . . . . . . . . . .  15
     10.2.  Restrictions on Display of "c", "o", and "j" Fields  . .  15
     10.3.  Security Risks from Legacy DNS Forwarders  . . . . . . .  17
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
     11.1.  Structured DNS Error EDNS Option . . . . . . . . . . . .  17
     11.2.  New Registry for JSON Names  . . . . . . . . . . . . . .  17
     11.3.  New Registry for Contact URI Scheme  . . . . . . . . . .  19
     11.4.  New Registry for DNS Sub-Error Codes . . . . . . . . . .  19
     11.5.  New Extended DNS Error Code  . . . . . . . . . . . . . .  21
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     12.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Appendix A.  Interoperation with RPZ Servers  . . . . . . . . . .  24
   Appendix B.  Implementation Status  . . . . . . . . . . . . . . .  24
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Introduction

   DNS filters are deployed for a variety of reasons, e.g., endpoint
   security, parental filtering, and filtering required by law
   enforcement.  Network-based security solutions such as firewalls and
   Intrusion Prevention Systems (IPS) rely upon network traffic
   inspection to implement perimeter-based security policies and operate
   by filtering DNS responses.  In a home network, DNS filtering is used
   for the same reasons as above and additionally for parental control.
   Internet Service Providers (ISPs) typically block access to some DNS
   domains due to a requirement imposed by an external entity (e.g., law
   enforcement agency) also performed using DNS-based content filtering.

   End users or network administrators leveraging DNS services that
   perform filtering may wish to receive more explanatory information
   about such a filtering to resolve problems with the filter -- for
   example to contact the DNS service administrator to allowlist a DNS
   domain that was erroneously filtered or to understand the reason a
   particular domain was filtered.  With that information, they can
   choose to use another network, open a trouble ticket with the DNS
   service administrator to resolve erroneous filtering, log the
   information, etc.

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   For the DNS filtering mechanisms described in Section 3, the DNS
   server can return extended error codes Blocked, Filtered, Censored,
   or Forged Answer defined in Section 4 of [RFC8914].  However, these
   codes only explain that filtering occurred but lack detail for the
   user to diagnose erroneous filtering.

   No matter which type of response is generated (forged IP address(es),
   NXDOMAIN or empty answer, even with an extended error code), the end
   user who triggered the DNS query has little chance to understand
   which entity filtered the query, how to report a mistake in the
   filter, or why the entity filtered it at all.  This document
   describes a mechanism to provide such detail.

   One of the other benefits of the approach described in this document
   is to eliminate the need to "spoof" block pages for HTTPS resources.
   This is achieved since clients implementing this approach would be
   able to display a meaningful error message, and would not need to
   connect to such a block page.  This approach thus avoids the need to
   install a local root certificate authority on those IT-managed
   devices.

   This document describes a format for machine-readable data in the
   EXTRA-TEXT field of [RFC8914].  It updates Section 2 of [RFC8914]
   which says the information in EXTRA-TEXT field is intended for human
   consumption (not automated parsing).

   This document does not recommend DNS filtering but provides a
   mechanism for better transparency to explain to the end users why
   some DNS queries are filtered.

2.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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.

   This document uses terms defined in DNS Terminology [RFC9499].

   "Encrypted DNS" refers to any encrypted scheme to convey DNS
   messages, for example, DNS-over-HTTPS [RFC8484], DNS-over-TLS
   [RFC7858], or DNS-over-QUIC [RFC9250].

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   The document refers to an Extended DNS Error (EDE) using its purpose,
   not its INFO-CODE as per Table 3 of [RFC8914].  "Forged Answer",
   "Blocked", "Censored", and "Filtered" are thus used to refer to
   "Forged Answer (4)", "Blocked (15)", "Censored (16)",and "Filtered
   (17)".

   In this document, "client security policy evaluation" refers to
   implementation-defined decision-making performed by the DNS client or
   consuming application (e.g., web browser) to determine how, or
   whether, structured error information is used, displayed, or acted
   upon.

   Structured DNS Error (SDE) is an EDNS(0) option indicating support
   for structured encoding of the EXTRA-TEXT field.

3.  DNS Filtering Techniques and Their Limitations

   DNS responses can be filtered by sending, e.g., a bogus (also called
   "forged") response, NXDOMAIN error, or empty answer.  Also, clients
   can be informed that filtering occurred by sending an Extended DNS
   Error code defined in [RFC8914].  Each of these methods have
   advantages and disadvantages that are discussed below:

   1.  The DNS response is forged to provide a list of IP addresses that
       points to an HTTP(S) server alerting the end user about the
       reason for blocking access to the requested domain (e.g.,
       malware).  If the host component [RFC3986] of an HTTP URL is
       blocked, the network security device (e.g., Customer Premises
       Equipment (CPE) or firewall) presents a block page instead of the
       HTTP response from the content provider hosting that domain.
       This works succesfully with HTTP.

       If this is an HTTPS URL, the network security device attempts to
       serve the block page over HTTPS.  In order to return a block page
       over HTTPS, the network security device uses a locally generated
       root certificate and corresponding key pair.  The local root
       certificate is installed on the endpoint while the network
       security device stores a copy of the private key.  During the TLS
       handshake, the on-path network security device modifies the
       certificate provided by the server and (re)signs it using the
       private key from the local root certificate.

       *  In deployments where DNSSEC is used, this approach becomes
          ineffective because DNSSEC ensures the integrity and
          authenticity of DNS responses, preventing forged DNS responses
          from being accepted.

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       *  The HTTPS server hosted on the network security device will
          have access to the client's IP address and the hostname being
          requested.  This information will be sensitive, as it will
          expose the end user's identity and the domain name that a end
          user attempted to access.

       *  Configuring a local root certificate on endpoints is not a
          viable option in several deployments like home networks,
          schools, Small Office/Home Office (SOHO), or Small/Medium
          Enterprise (SME).  In these cases, the typical behavior is
          that the filtered DNS response points to a server that will
          display the block page.  If the client is using HTTPS (via a
          web browser or another application) this results in a
          certificate validation error which gives no information to the
          end user about the reason for the DNS filtering.

       *  Enterprise networks do not always assume that all the
          connected devices are managed by the IT team or Mobile Device
          Management (MDM) devices, especially in the quite common Bring
          Your Own Device (BYOD) scenario.  In addition, the local root
          certificate cannot be installed on IoT devices without a
          device management tool.

       *  An end user does not know why the connection was prevented
          and, consequently, may repeatedly try to reach the domain but
          with no success.  Frustrated, the end user may switch to an
          alternate network that offers no DNS filtering against malware
          and phishing, potentially compromising both security and
          privacy.  Furthermore, certificate errors train end users to
          click through certificate errors, which is a bad security
          practice.  To eliminate the need for an end user to click
          through certificate errors, an end user may manually install a
          local root certificate on a host device.  Doing so, however,
          is also a bad security practice as it creates a security
          vulnerability that may be exploited by a MITM attack.  When a
          manually installed local root certificate expires, the end
          user has to (again) manually install the new local root
          certificate.

   2.  The DNS response is forged to provide an NXDOMAIN answer, causing
       the DNS lookup to fail.  This approach is incompatible with
       DNSSEC when the client performs validation, as the forged
       response will fail DNSSEC checks.  However, in deployments where
       the client relies on the DNS server to perform DNSSEC validation,
       a filtering DNS server can forge an NXDOMAIN response for a valid
       domain, and the client will trust it.  This undermines the
       integrity guarantees of DNSSEC, as the client has no way to
       distinguish between a genuine and a forged response.  Further,

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       the end user may not understand why a domain cannot be reached
       and may repeatedly attempt access without success.  Frustrated,
       the end user may resort to using insecure methods to reach the
       domain, potentially compromising both security and privacy.

   3.  The extended error codes Blocked and Filtered defined in
       Section 4 of [RFC8914] can be returned by a DNS server to provide
       additional information about the cause of a DNS error.  These
       extended error codes do not suffer from the limitations discussed
       in bullets (1) and (2), but the user still does not know the
       exact reason nor is aware of the exact entity blocking the access
       to the domain.  For example, a DNS server may block access to a
       domain based on the content category such as "Malware" to protect
       the endpoint from malicious software, "Phishing" to prevent the
       end user from revealing sensitive information to the attacker,
       etc.  A end user may need to know the contact details of the IT/
       InfoSec team to raise a complaint.  Further, the information
       conveyed by [RFC8914] is intended for diagnostic purposes and is
       not structured for automated processing, localization, or
       extensibility.  This document defines a structured, machine-
       readable format for conveying such details in the EXTRA-TEXT
       field, enabling clients to process the information
       programmatically and present it to end users (e.g., with
       localization support), while allowing for extensibility and more
       granular, client security policy-driven handling of the
       information.  This specification requires that clients only act
       upon such information when it is received over an integrity-
       protected DNS response.

4.  I-JSON in EXTRA-TEXT Field

   DNS servers that are compliant with this specification and have
   received an indication that the client also supports this
   specification as per Section 5.1 send data in the EXTRA-TEXT field
   [RFC8914] as a JSON object encoded using the Internet JSON (I-JSON)
   message format [RFC7493].

   This document defines the following JSON names:

   c: (contact)  The contact details of the IT/InfoSec team to report
      misclassified DNS filtering.  This information is important for
      transparency and also to ease unblocking a legitimate domain name
      that got blocked due to wrong classification.

      The field is a JSON array of contact URIs.  When multiple contact
      details are provided, each contact URI is represented as a
      separate array element in the JSON array.

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      Contact URIs conveyed in the "c" field MUST use URI schemes
      registered in Section 11.3.

      This field is optional.

   j: (justification)  'UTF-8'-encoded [RFC5198] human-readable
      explanation for the DNS filtering decision.

      This field is particularly useful when no applicable sub-error
      code is defined or provided for the returned Extended DNS Error.

      The information conveyed in this field MUST NOT be used as input
      to automated processing that affects security policy enforcement
      or DNS protocol behavior.

      The DNS client determines, according to its client security
      policy, whether the contents of this field are displayed to the
      end user, logged, or ignored.

      Returning non-UTF-8 data, syntactically invalid content, or
      deliberately meaningless values (including empty strings)
      indicates that a DNS server is misbehaving.

      This field is optional.

   s: (sub-error)  An integer representing the sub-error code for this
      particular DNS filtering case.

      The integer values are defined in the IANA-managed registry for
      DNS Sub-Error Codes in Section 11.4.

      This field is optional.

   o: (organization)  'UTF-8'-encoded human-friendly name of the
      organization that filtered this particular DNS query.

      This field is optional.

   l: (language)  The "l" field indicates the language used for the
      JSON-encoded "j" and "o" fields.  The value of this field MUST
      conform to the language tag syntax specified in Section 2.1 of
      [RFC5646].

      This field is optional but RECOMMENDED to aid in localization.

   The text in the "j" and "o" names can include international
   characters.  The text will be in natural language, chosen by the DNS
   administrator to match its expected audience.

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   If the client supports diagnostic interfaces, it MAY use the "l"
   field to identify the language of the "j" text and optionally
   translate it.

   The "o" field MAY be displayed to end users, subject to the
   conditions described in Section 10.  If the text is in a language not
   understood by the end user, the "l" field can be used to identify the
   language and support translation into the end user's preferred
   language.

   To avoid exceeding the maximum EDNS0 size [RFC9715] the generated
   JSON values SHOULD be as short as possible: short domain names,
   concise text in the values for the "j" and "o" names, and minified
   JSON (that is, without spaces or line breaks between JSON elements).

   The JSON data can be parsed to display to the user, logged, or
   otherwise used to assist troubleshooting and diagnosis of DNS
   filtering.

   The sub-error codes provide a structured way to communicate more
   detailed and precise description of the cause of an error (e.g.,
   distinguishing between malware-related blocking and phishing-related
   blocking under the general blocked error).

      An alternate design for conveying the sub-error would be to define
      new EDE codes for these errors.  However, such design is
      suboptimal because it requires replicating an error code for each
      EDE code to which the sub-error applies (e.g., "Malware" sub-error
      in Table 3 would consume three EDE codes).

   New JSON names MUST consist only of lower-case ASCII characters,
   digits, and hyphen-minus (that is, Unicode characters U+0061 through
   007A, U+0030 through U+0039, and U+002D).  Also, these names MUST be
   63 characters or shorter and it is RECOMMENDED they be as short as
   possible to reduce contribution to exceeding maximum EDNS0 response
   size [RFC9715].

5.  Protocol Operation

5.1.  Client Generating Request

   When generating a DNS query, a client that supports this
   specification SHOULD include the Structured DNS Error (SDE) option
   defined in Section 5.4, unless instructed by local policy otherwise.

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   The presence of the SDE option indicates that the client desires the
   DNS server to include an EDE option in the DNS response when DNS
   filtering is performed, and that any data conveyed in the EXTRA-TEXT
   field of the EDE option is encoded and processed in accordance with
   this specification.

5.2.  Server Generating Response

   When the DNS server filters its DNS response to a query (e.g., A or
   AAAA resource record query), the DNS response MAY contain an empty
   answer, NXDOMAIN, or (less ideally) forged response, as desired by
   the DNS server.

   If the query contained the SDE EDNS option (Section 5.1), and the DNS
   server returns an EDE code of "Blocked", "Filtered", "Censored", or
   "Blocked by Upstream DNS Server", the DNS server SHOULD include
   additional detail in the EXTRA-TEXT field encoded as structured and
   machine-readable data in accordance with the present specification,
   unless configured otherwise.  If including the additional detail
   would cause the response to exceed the EDNS0 size [RFC9715] (and thus
   setting TC=1), it SHOULD be omitted.  In deployments using DNS-over-
   TLS, DNS-over-HTTPS, or DNS-over-QUIC, transport size limitations are
   unlikely to necessitate omission of structured data in the EXTRA-TEXT
   field.

   If the SDE option was not present in the DNS request, the DNS server
   MUST process the request in accordance with [RFC8914] and MUST NOT
   assume that the client supports this specification.  This preserves
   compatibility with clients and servers that implement [RFC8914] but
   do not support this specification.

   Servers MAY decide to return small TTL values in filtered DNS
   responses (e.g., 10 seconds) to handle domain category and reputation
   updates.  Short TTLs allow for quick adaptation to dynamic changes in
   domain filtering decisions, but can result in increased query
   traffic.  In cases where updates are less frequent, TTL values of 30
   to 60 seconds MAY provide a better balance, reducing server load
   while still ensuring reasonable flexibility for updates.

   If the query includes the SDE option as per Section 5.1, the server
   MUST NOT return the "Forged Answer" extended error code because the
   client can take advantage of EDE's more sophisticated error reporting
   (e.g., "Filtered", "Blocked").  Continuing to send "Forged Answer"
   even to an EDE-supporting client will cause the persistence of the
   drawbacks described in Section 3.

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   When the "Censored" extended error code is included in the DNS
   response, the "c", "j", "o", and "l" fields may be conveyed in the
   EXTRA-TEXT field.  The sub-error codes defined in this specification
   are not applicable to the "Censored" extended error code and MUST NOT
   be used in conjunction with it.  Future specifications may update
   this behavior by defining sub-error codes applicable to "Censored".

5.3.  Client Processing Response

   On receipt of a DNS response with an EDE option from a DNS server,
   the following ordered actions are performed on the EXTRA-TEXT field:

   1.  If the integrity of the DNS response is not guaranteed, the DNS
       client MUST NOT act upon data in the EXTRA-TEXT field, as the
       data is vulnerable to modification by an on-path attacker.  An
       attacker can inject or modify a structured DNS error response in
       transit without detection, enabling fabrication of filtering
       information (e.g., misleading contact information or false
       resolver identity information) that appears to originate from the
       resolver.  The data MAY be retained for diagnostic or client
       security policy evaluation purposes.

   2.  Servers that do not support this specification might use plain
       text in the EXTRA-TEXT field.  DNS clients SHOULD handle both
       plaintext and structured content.  The client attempts to parse
       the EXTRA-TEXT field as I-JSON.  If parsing fails or the content
       is not valid I-JSON, the client MUST treat the data as invalid,
       MUST NOT process it according to this specification.  The client
       MAY instead process the EXTRA-TEXT field as unstructured text as
       specified in [RFC8914].

   3.  The DNS response MUST also contain an EDE code of "Blocked by
       Upstream DNS Server", "Blocked", "Censored", or "Filtered"
       [RFC8914], otherwise the EXTRA-TEXT field is discarded.

   4.  If the JSON object contains an "s" field and the sub-error code
       is not defined as applicable to the accompanying Extended DNS
       Error (EDE) code, the client MUST ignore the value of the "s"
       field and continue processing the remaining fields in accordance
       with this specification.

   5.  If the EXTRA-TEXT field does not contain at least one of the JSON
       names "c", "j", or "s", or if all of the fields that are present
       have empty values, the entire JSON object MUST be discarded.

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   6.  If the JSON object contains a "c" field any of its Contact URIs
       with schemes not registered in the Section 11.3 registry are
       ignored.  Remaining Contact URIs using registered schemes can be
       processed.

   7.  If the identity of the DNS server cannot be verified (e.g., when
       using opportunistic privacy such as Section 5 of [RFC8310] or
       opportunistic discovery [RFC9462]), the DNS client MUST ignore
       the "c", "j", and "o" fields, as these fields may influence end
       user behavior and are vulnerable to active attacks in the absence
       of resolver authentication.  If the DNS response was received
       over an encrypted connection without server authentication, the
       client MAY process the "s" field and other parts of the response,
       as the "s" field is a registry-defined, enumerated value and does
       not contain free-form text.

   8.  If the DNS client uses an authenticated connection to the DNS
       server (e.g., when using a strict privacy profile for DNS-over-
       TLS (Section 5 of [RFC8310]) or an authenticated DNS-over-HTTPS
       or DNS-over-QUIC connection), this mitigates both passive
       eavesdropping and client redirection (at the expense of providing
       no DNS service if such a connection is not available).  In such
       cases, the DNS client MAY process the EXTRA-TEXT field of the DNS
       response.

   9.  The DNS client MUST ignore any other JSON names that it does not
       support.

      Note that the strict and opportunistic privacy profiles as defined
      in [RFC8310] only apply to DoT; there has been no such distinction
      made for DoH.

5.4.  Structured DNS Error (SDE) EDNS(0) Option Format

   The Structured DNS Error (SDE) EDNS(0) option is used by a client to
   indicate support for I-JSON encoding in the EXTRA-TEXT field of an
   Extended DNS Error (EDE) option.

   The SDE option has no OPTION-DATA.  The OPTION-LENGTH field MUST be
   set to 0.  A server receiving an SDE option with a non-zero OPTION-
   LENGTH MUST ignore the option.

   The presence of the SDE option in a query indicates that the client
   supports processing the EXTRA-TEXT field in accordance with this
   specification.

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6.  New Sub-Error Codes Definition

   The document defines the following new IANA-registered Sub-Error
   codes.

6.1.  Reserved

   *  Number: 0

   *  Meaning: Reserved.  This sub-error code value MUST NOT be sent.
      If received, it has no meaning.

   *  Applicability: This code should never be used.

   *  Reference: This-Document

   *  Change Controller: IETF

6.2.  Network Operator Policy

   *  Number: 5

   *  Meaning: Network Operator Policy.  The code indicates that the
      request was filtered according to a policy imposed by the operator
      of the local network (where local network is a relative term,
      e.g., it may refer to a Local Area Network or to the network of
      the ISP selected by the end user).

   *  Applicability: Blocked

   *  Reference: This-Document

   *  Change Controller: IETF

6.3.  DNS Operator Policy

   *  Number: 6

   *  Meaning: DNS Operator Policy.  The code indicates that the request
      was filtered according to policy determined by the operator of the
      DNS server.  This is different from the "Network Operator Policy"
      code when a third-party DNS resolver is used.

   *  Applicability: Blocked

   *  Reference: This-Document

   *  Change Controller: IETF

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7.  New Extended DNS Errors

   This document defines an addition to the EDE codes defined in
   [RFC8914].

7.1.  Extended DNS Error Code TBA1 - Blocked by Upstream DNS Server

   The DNS server is unable to respond to the request because the domain
   is on a blocklist due to an internal security policy imposed by an
   upstream DNS server.  This error code is useful in deployments where
   a network-provided DNS forwarder is configured to use an external
   resolver that filters malicious domains.  When the DNS forwarder
   receives a Blocked (15) error code from the upstream DNS server, it
   can replace it with "Blocked by Upstream DNS Server" (TBA1) before
   forwarding the reply to the DNS client.  Additionally, the EXTRA-TEXT
   field may be forwarded to the DNS client.

   Implementations should ensure that the communication channel with the
   upstream DNS server provides adequate integrity protection to
   mitigate the threats described in step 1 of Section 5.3.

8.  Examples

   An example showing the nameserver at 'ns.example.net' that filtered a
   DNS "A" record query for 'example.org' is provided in Figure 1.

   {
     "c": [
       "tel:+358-555-1234567",
       "sips:bob@bobphone.example.com"
     ],
     "j": "malware present for 23 days",
     "s": 1,
     "o": "example.net Filtering Service",
     "l": "en"
   }

     Figure 1: JSON Returned in EXTRA-TEXT Field of Extended DNS Error
                                  Response

   In Figure 2 the same content is shown with minified JSON (no
   whitespace, no blank lines) with '\' line wrapping per [RFC8792].

   { "c":["tel:+358-555-1234567","sips:bob@bobphone.example.com"],\
   "j":"malware present for 23 days",\
   "s":1,\
   "o":"example.net Filtering Service",\
   "l":"en" }

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                        Figure 2: Minified Response

9.  Operational Considerations

   When a forwarder receives an EDE option, whether or not (and how) to
   pass along JSON information in the EXTRA-TEXT field to its client is
   implementation-dependent [RFC5625] and depends on operator policy.
   Implementations MAY choose not to forward the JSON information, or
   they MAY choose to create a new EDE option that conveys the
   information in the "c", "s", and "j" fields encoded in the JSON
   object.

   The application that triggered the DNS request may have a client
   security policy to override the contact information (e.g., redirect
   all complaint calls to a single contact point).  In such cases, the
   content of the "c" attribute MAY be ignored.

9.1.  Backward Compatibility

   Future extensions MUST NOT introduce mandatory JSON attributes, as
   existing implementations are required to ignore unknown JSON names
   (see Section 5.3).

10.  Security Considerations

10.1.  Authentication and Confidentiality

   Security considerations in Section 6 of [RFC8914] apply to this
   document.  [RFC8914] cautions against relying on EDE information
   because it may be unauthenticated and transmitted in cleartext.  This
   specification assumes the use of authenticated, integrity-protected
   DNS transports (e.g., DNS-over-TLS, DNS-over-HTTPS, or DNS-over-
   QUIC).  Such transports MUST be based on TLS 1.3 [RFC8446] or later.
   Under these conditions, EDE information is integrity-protected,
   reducing the risks associated with relying on structured EDE content.

   To minimize impact of active on-path attacks on the DNS channel, the
   client validates the response as described in Section 5.3.

10.2.  Restrictions on Display of "c", "o", and "j" Fields

   A client might choose to display the information in the "c" field to
   the end user if and only if the encrypted resolver has sufficient
   reputation, according to some client security policy (e.g.,
   administrative configuration, or a built-in list of respectable
   resolvers).  This limits the ability of a malicious encrypted
   resolver to cause harm.  For example, an end user can use the details
   in the "c" field to contact an attacker to solve the problem of being

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   unable to reach a domain.  The attacker can mislead the end user to
   install malware or spyware to compromise the device security posture
   or mislead the end user to reveal personal data.  If the client
   decides not to display all of the information in the EXTRA-TEXT
   field, it can be logged for diagnostics purpose and the client can
   only display the resolver hostname that blocked the domain, error
   description for the EDE code and the sub-error description for the
   "s" field to the end user.

   The same client security policy considerations apply to the display
   of the "j" field, as it contains free-form, human-readable text that
   may influence end user behavior.

   When displaying the free-form text of "o", the client MUST NOT make
   any of those elements into actionable (clickable) links and these
   fields need to be rendered as text, not as HTML.  The contact details
   of "c" can be made into clickable links to provide a convenient way
   for end users to initiate, e.g., voice calls.  The client might
   choose to display the contact details only when the identity of the
   DNS server is verified.

   Clients MUST NOT automatically initiate connections to URIs derived
   from the EXTRA-TEXT field.  Doing so could allow a resolver to
   silently report client activity to third parties, enable denial-of-
   service reflection attacks, or be used to entrap a client.  The
   restriction of Contact URI schemes to "sips", "tel", and "mailto" is
   intentional, as these schemes do not result in automatic HTTP
   connections.

   Further, clients MUST NOT display the value of the "o" field to the
   end user unless one of the following conditions is met:

   *  The value matches a registered organization name listed in the
      [IANA-Enterprise] OR

   *  The value consists solely of an organization name and does not
      contain any additional free-form content such as instructions,
      URLs, or messaging intended to influence end user behavior, as
      determined by client security policy or heuristics.

   If the organization name cannot be verified through registry checks
   or heuristics, the client MUST NOT display the "o" field to the end
   user.

   DNS clients MAY keep all fields conveyed in the EXTRA-TEXT field for
   evaluation according to the client security policy.  Such data MUST
   NOT be automatically trusted, displayed to end users, or used to
   influence security decisions without appropriate validation.

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10.3.  Security Risks from Legacy DNS Forwarders

   An attacker might inject (or modify) the EDE EXTRA-TEXT field with a
   DNS proxy or DNS forwarder that is unaware of EDE.  Such a DNS proxy
   or DNS forwarder will forward that attacker-controlled EDE option.
   To prevent such an attack, clients can be configured to process EDE
   from explicitly configured DNS servers or utilize RESINFO [RFC9606].

11.  IANA Considerations

   This document requests five IANA actions as described in the
   following subsections.

      Note to the RFC Editor: Please replace RFCXXXX with the RFC number
      assigned to this document and "TBA1" with the value assigned by
      IANA.

11.1.  Structured DNS Error EDNS Option

   IANA is requested to register the following new EDNS(0) Option Code
   in the "DNS EDNS0 Option Codes (OPT)" registry under the "Domain Name
   System (DNS) Parameters" registry group [IANA-DNS]:

   Value:  TBD

   Name:  Structured DNS Error

   Status:  Standard

   Reference:  RFC XXXX

11.2.  New Registry for JSON Names

   This document requests IANA to create a new registry, entitled
   "EXTRA-TEXT JSON Names" under "Extended DNS Error Codes" registry,
   which is under the "Domain Name System (DNS) Parameters" registry
   group [IANA-DNS].  The registration request for a new JSON name must
   include the following fields:

   JSON Name:  Specifies the name of an attribute that is present in the
      JSON data enclosed in EXTRA-TEXT field.  The name must follow the
      guidelines in Section 4.

   Field Meaning:  Provides a brief, human-readable label summarizing
      the purpose of the JSON attribute.

   Short description:  Includes a short description of the requested
      JSON name.

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   Mandatory (Y/N?):  Indicates whether this attribute is mandatory or
      optional.

   Specification:  Provides a pointer to the reference document that
      specifies the attribute.

   The registry is initially populated with the following values:

   +======+===============+============================+===============+
   | JSON | Field Meaning | Description                | Specification |
   | Name |               |                            |               |
   +======+===============+============================+===============+
   |  c   | contact       | The contact details of     |  Section 4 of |
   |      |               | the IT/InfoSec team to     |    RFCXXXX    |
   |      |               | report misclassified       |               |
   |      |               | DNS filtering              |               |
   +------+---------------+----------------------------+---------------+
   |  j   | justification | UTF-8-encoded [RFC5198]    |  Section 4 of |
   |      |               | textual justification      |    RFCXXXX    |
   |      |               | for a particular DNS       |               |
   |      |               | filtering                  |               |
   +------+---------------+----------------------------+---------------+
   |  s   | sub-error     | Integer representing       |  Section 4 of |
   |      |               | the sub-error code for     |    RFCXXXX    |
   |      |               | this DNS filtering case    |               |
   +------+---------------+----------------------------+---------------+
   |  o   | organization  | UTF-8-encoded human-       |  Section 4 of |
   |      |               | friendly name of the       |    RFCXXXX    |
   |      |               | organization that          |               |
   |      |               | filtered this              |               |
   |      |               | particular DNS query       |               |
   +------+---------------+----------------------------+---------------+
   |  l   | language      | Indicates the language     |  Section 4 of |
   |      |               | of the "j" and "o"         |    RFCXXXX    |
   |      |               | fields as defined in       |               |
   |      |               | [RFC5646]                  |               |
   +------+---------------+----------------------------+---------------+

                    Table 1: Initial JSON Names Registry

   New JSON names are registered via IETF Review (Section 4.8 of
   [RFC8126]) and their formatting constraints are described in
   Section 4.

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11.3.  New Registry for Contact URI Scheme

   This document requests IANA to create a new registry, entitled
   "Contact URI Schemes" under "Extended DNS Error Codes" registry,
   which is under the "Domain Name System (DNS) Parameters" registry
   group [IANA-DNS].  The registration request for a new Contact URI
   scheme has to include the following fields:

   *  Name: URI scheme name.

   *  Meaning: Provides a short description of the scheme.

   *  Reference: Provides a pointer to an IETF-approved specification
      that defines the URI scheme.

   The Contact URI scheme registry is initially populated with the
   following schemes:

                 +========+==================+===========+
                 |  Name  | Meaning          | Reference |
                 +========+==================+===========+
                 |  sips  | SIP Call         | [RFC5630] |
                 +--------+------------------+-----------+
                 |  tel   | Telephone Number | [RFC3966] |
                 +--------+------------------+-----------+
                 | mailto | Internet mail    | [RFC6068] |
                 +--------+------------------+-----------+

                                  Table 2

   The registration procedure for adding new Contact URI schemes to the
   "Contact URI Schemes" registry is "IETF Review" as defined in
   Section 4.8 of [RFC8126].

11.4.  New Registry for DNS Sub-Error Codes

   This document requests IANA to create a new registry, entitled "Sub-
   Error Codes" under "Extended DNS Error Codes" registry, which is
   under the "Domain Name System (DNS) Parameters" registry group
   [IANA-DNS].  The registration request for a new sub-error code must
   include the following fields:

   *  Number: Is the wire format sub-error code (range 0-255).

   *  Meaning: Provides a short description of the sub-error.

   *  EDE Codes Applicability: Indicates which Extended DNS Error (EDE)
      Codes apply to this sub-error code.

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   *  Reference: Provides a pointer to an IETF-approved specification
      that registered the code and/or an authoritative specification
      that describes the meaning of this code.

   The Sub-Error Code registry is initially populated with the following
   values:

       +========+==========+=====================+================+
       | Number | Meaning  | EDE Codes           | Reference      |
       |        |          | Applicability       |                |
       +========+==========+=====================+================+
       |   0    | Reserved | Not used            | Section 6.1 of |
       |        |          |                     | this document  |
       +--------+----------+---------------------+----------------+
       |   1    | Malware  | "Blocked", "Blocked | Section 5.5 of |
       |        |          | by Upstream DNS     | [RFC5901]      |
       |        |          | Server", "Filtered" |                |
       +--------+----------+---------------------+----------------+
       |   2    | Phishing | "Blocked", "Blocked | Section 5.5 of |
       |        |          | by Upstream DNS     | [RFC5901]      |
       |        |          | Server", "Filtered" |                |
       +--------+----------+---------------------+----------------+
       |   3    | Spam     | "Blocked", "Blocked | Page 289 of    |
       |        |          | by Upstream DNS     | [RFC4949]      |
       |        |          | Server", "Filtered" |                |
       +--------+----------+---------------------+----------------+
       |   4    | Spyware  | "Blocked", "Blocked | Page 291 of    |
       |        |          | by Upstream DNS     | [RFC4949]      |
       |        |          | Server", "Filtered" |                |
       +--------+----------+---------------------+----------------+
       |   5    | Network  | "Blocked"           | Section 6.2 of |
       |        | operator |                     | this document  |
       |        | policy   |                     |                |
       +--------+----------+---------------------+----------------+
       |   6    | DNS      | "Blocked"           | Section 6.3 of |
       |        | operator |                     | this document  |
       |        | policy   |                     |                |
       +--------+----------+---------------------+----------------+

                 Table 3: Initial Sub-Error Code Registry

   The registration procedure to add New Sub-Error Codes is IETF Review
   as defined in Section 4.8 of [RFC8126].

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11.5.  New Extended DNS Error Code

   IANA is requested to assign the following Extended DNS Error code
   from the "Extended DNS Error Codes" registry under the "Domain Name
   System (DNS) Parameters" registry group [IANA-DNS]:

        +===========+================================+===========+
        | INFO-CODE | Purpose                        | Reference |
        +===========+================================+===========+
        |    TBA1   | Blocked by Upstream DNS Server |  RFCXXXX  |
        +-----------+--------------------------------+-----------+

                       Table 4: New DNS Error Code

12.  References

12.1.  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,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers",
              RFC 3966, DOI 10.17487/RFC3966, December 2004,
              <https://www.rfc-editor.org/rfc/rfc3966>.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/rfc/rfc4949>.

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008,
              <https://www.rfc-editor.org/rfc/rfc5198>.

   [RFC5630]  Audet, F., "The Use of the SIPS URI Scheme in the Session
              Initiation Protocol (SIP)", RFC 5630,
              DOI 10.17487/RFC5630, October 2009,
              <https://www.rfc-editor.org/rfc/rfc5630>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <https://www.rfc-editor.org/rfc/rfc5646>.

   [RFC5901]  Cain, P. and D. Jevans, "Extensions to the IODEF-Document
              Class for Reporting Phishing", RFC 5901,
              DOI 10.17487/RFC5901, July 2010,
              <https://www.rfc-editor.org/rfc/rfc5901>.

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   [RFC6068]  Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto'
              URI Scheme", RFC 6068, DOI 10.17487/RFC6068, October 2010,
              <https://www.rfc-editor.org/rfc/rfc6068>.

   [RFC7493]  Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
              DOI 10.17487/RFC7493, March 2015,
              <https://www.rfc-editor.org/rfc/rfc7493>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/rfc/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [RFC8310]  Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
              for DNS over TLS and DNS over DTLS", RFC 8310,
              DOI 10.17487/RFC8310, March 2018,
              <https://www.rfc-editor.org/rfc/rfc8310>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

   [RFC8914]  Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
              Lawrence, "Extended DNS Errors", RFC 8914,
              DOI 10.17487/RFC8914, October 2020,
              <https://www.rfc-editor.org/rfc/rfc8914>.

   [RFC9499]  Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
              RFC 9499, DOI 10.17487/RFC9499, March 2024,
              <https://www.rfc-editor.org/rfc/rfc9499>.

12.2.  Informative References

   [IANA-DNS] IANA, "Domain Name System (DNS) Parameters, Extended DNS
              Error Codes", <https://www.iana.org/assignments/dns-
              parameters/dns-parameters.xhtml#extended-dns-error-codes>.

   [IANA-Enterprise]
              "Private Enterprise Numbers (PENs)",
              <https://www.iana.org/assignments/enterprise-numbers/>.

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   [Impl-1]   "Use of DNS Errors To improve Browsing User Experience
              With network based malware protection", March 2023,
              <https://datatracker.ietf.org/meeting/116/materials/
              slides-116-dnsop-dns-errors-implementation-proposal-
              slides-116-dnsop-update-on-dns-errors-implementation-00>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/rfc/rfc3986>.

   [RFC5625]  Bellis, R., "DNS Proxy Implementation Guidelines",
              BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009,
              <https://www.rfc-editor.org/rfc/rfc5625>.

   [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, <https://www.rfc-editor.org/rfc/rfc7858>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/rfc/rfc8484>.

   [RFC8792]  Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,
              "Handling Long Lines in Content of Internet-Drafts and
              RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020,
              <https://www.rfc-editor.org/rfc/rfc8792>.

   [RFC9250]  Huitema, C., Dickinson, S., and A. Mankin, "DNS over
              Dedicated QUIC Connections", RFC 9250,
              DOI 10.17487/RFC9250, May 2022,
              <https://www.rfc-editor.org/rfc/rfc9250>.

   [RFC9462]  Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T.
              Jensen, "Discovery of Designated Resolvers", RFC 9462,
              DOI 10.17487/RFC9462, November 2023,
              <https://www.rfc-editor.org/rfc/rfc9462>.

   [RFC9606]  Reddy.K, T. and M. Boucadair, "DNS Resolver Information",
              RFC 9606, DOI 10.17487/RFC9606, June 2024,
              <https://www.rfc-editor.org/rfc/rfc9606>.

   [RFC9715]  Fujiwara, K. and P. Vixie, "IP Fragmentation Avoidance in
              DNS over UDP", RFC 9715, DOI 10.17487/RFC9715, January
              2025, <https://www.rfc-editor.org/rfc/rfc9715>.

   [RPZ]      "Response Policy Zone", <https://dnsrpz.info>.

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Appendix A.  Interoperation with RPZ Servers

   This appendix provides a non-normative guidance for operation with a
   Response Policy Zones (RPZ) server [RPZ] that indicates filtering
   with a NXDOMAIN response with the Recursion Available bit cleared
   (RA=0).  This guidance is provided to ease interoperation with RPZ.

   When a DNS client supports this specification, it includes the SDE
   option in its DNS query.

   If the server does not support this specification and is performing
   RPZ filtering, the server ignores the SDE option in the DNS query and
   replies with NXDOMAIN and RA=0.  The DNS client can continue to
   accept such responses.

   If the server does support this specification and is performing RPZ
   filtering, the server can use the SDE option in the query to identify
   an SDE-aware client and respond appropriately (that is, by generating
   a response described in Section 5.2) as NXDOMAIN and RA=0 are not
   necessary when generating a response to such a client.

Appendix B.  Implementation Status

      Note to the RFC Editor: please remove this appendix prior
      publication.

   At IETF#116, Gianpaolo Scalone (Vodafone) and Ralf Weber (Akamai)
   presented an implementation of this specification.  More details can
   be found at [Impl-1].

Acknowledgements

   Thanks to Vittorio Bertola, Wes Hardaker, Ben Schwartz, Erid Orth,
   Viktor Dukhovni, Warren Kumari, Paul Wouters, John Levine, Bob
   Harold, Mukund Sivaraman, Gianpaolo Angelo Scalone, Mark Nottingham,
   Stephane Bortzmeyer, Daniel Migault, and Stephen Farrell for the
   comments.

   Thanks to Ralf Weber and Gianpaolo Scalone for sharing details about
   their implementation.

   Thanks Petr Špaček, Di Ma and Matt Brown for the DNS directorate
   reviews, and Joseph Salowey for the Security directorate review.

   Thanks Paul Kyzivat for the Art review.

   Thanks to Éric Vyncke for the AD review.

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Authors' Addresses

   Dan Wing
   Citrix Systems, Inc.
   United States of America
   Email: danwing@gmail.com

   Tirumaleswar Reddy
   Nokia
   Bangalore
   Karnataka
   India
   Email: kondtir@gmail.com

   Neil Cook
   Open-Xchange
   United Kingdom
   Email: neil.cook@noware.co.uk

   Mohamed Boucadair
   Orange
   Rennes
   35000
   France
   Email: mohamed.boucadair@orange.com

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