



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


                 Structured Error Data for Filtered DNS
                draft-ietf-dnsop-structured-dns-error-16

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
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 6 August 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
   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  . . . . . . . . . . . . . . . . . . . . . . . .   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  . . . . . . . . . . . . . . .  10
   6.  Deployment Considerations . . . . . . . . . . . . . . . . . .  12
   7.  New Sub-Error Codes Definition  . . . . . . . . . . . . . . .  12
     7.1.  Reserved  . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.2.  Network Operator Policy . . . . . . . . . . . . . . . . .  13
     7.3.  DNS Operator Policy . . . . . . . . . . . . . . . . . . .  13
   8.  New Extended DNS Errors . . . . . . . . . . . . . . . . . . .  13




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     8.1.  Extended DNS Error Code TBA1 - Blocked by Upstream DNS
           Server  . . . . . . . . . . . . . . . . . . . . . . . . .  13
   9.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  14
     10.1.  Authentication and Confidentiality . . . . . . . . . . .  14
     10.2.  Restrictions on Display of "c", "o", and "j" Fields  . .  15
     10.3.  Security Risks from Legacy DNS Forwarders  . . . . . . .  16
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     11.1.  Structured DNS Error EDNS Option . . . . . . . . . . . .  16
     11.2.  New Registry for JSON Names  . . . . . . . . . . . . . .  16
     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 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 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].

   "Requestor" refers to the side that sends a request.  "Responder"
   refers to an authoritative, recursive resolver, or other DNS
   component that responds to questions.

   "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)".

   The term "DNS server" refers to a DNS recursive resolver or a DNS
   forwarder that generates DNS structured error responses.

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

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 authority component of an HTTP(S) 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.  If
       the authority component of an HTTP URL is blocked, the network
       security device intercepts the HTTP request and returns a block
       page over HTTP.  If the authority component of an HTTPS URL is
       blocked, the network security device serves 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.

       *  However, 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 user's identity and the domain name that a 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 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 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,
       the end user may not understand why a domain cannot be reached



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       and may repeatedly attempt access without success.  Frustrated,
       the 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
       user from revealing sensitive information to the attacker, etc.
       A user may need to know the contact details of the IT/InfoSec
       team to raise a complaint.

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] encoded using the Internet JSON (I-JSON) message format
   [RFC7493].

      Note that [RFC7493] was based on [RFC7159], but [RFC7159] was
      replaced by [RFC8259].

   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.

      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



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      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.

   New JSON names can be defined in the IANA registry introduced in
   Section 11.2.  Such 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.

   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 for IT administrators.

   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 reduce DNS message size the generated JSON 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 4 would consume three EDE codes).

5.  Protocol Operation

5.1.  Client Generating Request

   When generating a DNS query, a client that supports this
   specification MUST include the Structured DNS Error (SDE) option in
   the OPT pseudo-RR.

   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.








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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 indicating blocking or modification of the
   response, the DNS server MUST include additional detail in the EXTRA-
   TEXT field encoded as structured and machine-readable data.

   If the SDE option is not present, the DNS server MUST NOT include
   structured JSON data and MUST convey the EXTRA-TEXT field as human-
   readable text in accordance with [RFC8914].

   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.

   Because the DNS client explicitly signals support for structured
   error information using the SDE option (Section 5.1), and because the
   EDE option is carried in the non-cached OPT pseudo-RR (Section 6.2.1
   of [RFC6891]), the DNS server can tailor its filtered response to the
   capabilities of the client.

   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.

   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 responder,
   the following ordered actions are performed on the EXTRA-TEXT field:



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   1.   Servers which don't support this specification might use plain
        text in the EXTRA-TEXT field.  Requestors SHOULD properly handle
        both plaintext and JSON text in the EXTRA-TEXT field.  The
        requestor verifies that the field contains valid JSON.  If not,
        the requestor MUST consider the server does not support this
        specification and stop processing rest of the actions defined in
        this section, but may instead choose to treat EXTRA-TEXT as per
        [RFC8914].

   2.   If the DNS response is not received over an encrypted DNS
        channel, the requestor MUST NOT act upon data in the EXTRA-TEXT
        field, as there is no mechanism to verify the integrity of such
        data and it is vulnerable to modification by an on-path
        attacker.  The data MAY be retained for diagnostic or client
        security policy evaluation purposes.

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

   4.   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.

   5.   If the "c" field contains any contact URIs that use a scheme not
        registered in the Section 11.3 registry, only those URIs MUST be
        discarded.  Contact URIs using registered schemes can be
        processed.

   6.   If the DNS client has enabled the opportunistic privacy profile
        for DoT (Section 5 of [RFC8310]) and the identity of the DNS
        server cannot be verified, the DNS client MUST ignore the "c",
        "j", and "o" fields, as these fields may influence user behavior
        and are vulnerable to active attacks in the absence of resolver
        authentication.  The client MAY process the "s" field and other
        parts of the response.

   7.   In opportunistic discovery [RFC9462], where only the IP address
        of the DNS server is validated and the server identity is not
        authenticated, the DNS client MUST ignore the "c", "j", and "o"
        fields.  The DNS client MAY process the "s" field and other
        parts of the response.

   8.   If a DNS client has enabled strict privacy profile (Section 5 of
        [RFC8310]) for DoT, the DNS client requires an encrypted
        connection and successful authentication of the DNS server.  In
        doing so, this mitigates both passive eavesdropping and client



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        redirection (at the expense of providing no DNS service if an
        encrypted, authenticated connection is not available).  If the
        DNS client has enabled strict privacy profile for DoT, 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.

   10.  If the EXTRA-TEXT field does not conform to the I-JSON
        requirements [RFC7493], the client MUST treat the data as
        invalid and MUST NOT process it according to this specification.
        The client MAY process the EXTRA-TEXT field as unstructured text
        as specified in [RFC8914].

      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.

6.  Deployment 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].  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.

7.  New Sub-Error Codes Definition

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

7.1.  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




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   *  Change Controller: IETF

7.2.  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 user).

   *  Applicability: Blocked

   *  Reference: This-Document

   *  Change Controller: IETF

7.3.  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

8.  New Extended DNS Errors

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

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

   The DNS server (e.g., a DNS forwarder) 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.  Typically, when the DNS forwarder receives a Blocked (15)
   error code from the upstream DNS server, it will replace it with
   "Blocked by Upstream DNS Server" (TBA1) before forwarding the reply



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   to the DNS client.  Additionally, the EXTRA-TEXT field is forwarded
   to the DNS client.

9.  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" }

                        Figure 2: Minified Response

10.  Security Considerations

10.1.  Authentication and Confidentiality

   Security considerations in Section 6 of [RFC8914] apply to this
   document, except the guard against using EDE content to alter DNS
   protocol processing.  The guard is relaxed in the current
   specification as it mandates encryption and recommends the use of an
   authenticated connection to the DNS server, while [RFC8914] assumes
   that EDE information is unauthenticated and sent over clear text.

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





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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., user
   configuration, 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 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 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.

   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.

   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 assign a new EDNS(0) Option Code from the "DNS
   EDNS0 Option Codes" registry with the following properties:

        +============+=======+========================+===========+
        | Name       | Value | Description            | Reference |
        +============+=======+========================+===========+
        | Structured | TBD   | Signals client support | RFCXXXX   |
        | DNS Error  |       | for structured JSON in |           |
        |            |       | EDE EXTRA-TEXT         |           |
        +------------+-------+------------------------+-----------+

                                  Table 1

11.2.  New Registry for JSON Names

   This document requests IANA to create a new registry, entitled
   "EXTRA-TEXT JSON Names" under "Domain Name System (DNS) Parameters,
   Extended DNS Error Codes" registry [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:












































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

                    Table 2: Initial JSON Names Registry

   New JSON names are registered via IETF Review (Section 4.8 of
   [RFC8126]).





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   The "Mandatory" column is informational only.  This specification
   does not define any mandatory JSON names.

11.3.  New Registry for Contact URI Scheme

   This document requests IANA to create a new registry, entitled
   "Contact URI Schemes" under "Domain Name System (DNS) Parameters,
   Extended DNS Error Codes" registry [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 be populated with the
   following schemes:

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

                                  Table 3

   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 "Domain Name System (DNS) Parameters, Extended DNS
   Error Codes" registry [IANA-DNS].  The registration request for a new
   sub-error codes 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.




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

   *  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 be populated with the
   following values:

   +========+==========+==============================+================+
   | Number | Meaning  | EDE Codes                    | Reference      |
   |        |          | Applicability                |                |
   +========+==========+==============================+================+
   |   0    | Reserved | Not used                     | Section 7.1 of |
   |        |          |                              | this document  |
   +--------+----------+------------------------------+----------------+
   |   1    | Malware  | "Blocked", "Blocked          | Section 5.5 of |
   |        |          | by Upstream                  | [RFC5901]      |
   |        |          | Server", "Filtered"          |                |
   +--------+----------+------------------------------+----------------+
   |   2    | Phishing | "Blocked", "Blocked          | Section 5.5 of |
   |        |          | by Upstream                  | [RFC5901]      |
   |        |          | Server", "Filtered"          |                |
   +--------+----------+------------------------------+----------------+
   |   3    | Spam     | "Blocked", "Blocked          | Page 289 of    |
   |        |          | by Upstream                  | [RFC4949]      |
   |        |          | Server", "Filtered"          |                |
   +--------+----------+------------------------------+----------------+
   |   4    | Spyware  | "Blocked", "Blocked          | Page 291 of    |
   |        |          | by Upstream                  | [RFC4949]      |
   |        |          | Server", "Filtered"          |                |
   +--------+----------+------------------------------+----------------+
   |   5    | Network  | "Blocked"                    | Section 7.2 of |
   |        | operator |                              | this document  |
   |        | policy   |                              |                |
   +--------+----------+------------------------------+----------------+
   |   6    | DNS      | "Blocked"                    | Section 7.3 of |
   |        | operator |                              | this document  |
   |        | policy   |                              |                |
   +--------+----------+------------------------------+----------------+

                  Table 4: Initial Sub-Error Code Registry

   The registration procedure to add New Sub-Error Codes are registered
   via 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 "Domain Name System (DNS) Parameters, Extended DNS Error
   Codes" registry [IANA-DNS]:

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

                       Table 5: 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>.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <https://www.rfc-editor.org/rfc/rfc7159>.

   [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>.

   [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>.

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/>.

   [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>.





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

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

   [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>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/rfc/rfc8259>.

   [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>.

   [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>.

   [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>.

   [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, and Daniel Migault for the comments.

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

   Thanks 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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