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Versions: 00 01 02 03 04 05 06 07                                       
DNSOP WG                                                        T. Reddy
Internet-Draft                                                    McAfee
Intended status: Standards Track                                 N. Cook
Expires: July 18, 2021                                      Open-Xchange
                                                                 D. Wing
                                                                  Citrix
                                                            M. Boucadair
                                                                  Orange
                                                        January 14, 2021


                      DNS Access Denied Error Page
                    draft-reddy-dnsop-error-page-06

Abstract

   When a DNS server filters a query, the response to such query conveys
   no detailed explanation that explains why that query was blocked,
   leading thus to end-user confusion.  A solution to this problem is
   needed in order to enhance the user experience.

   This document defines a method to return an URI that explains the
   reason why a DNS query was filtered by a DNS server.

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 July 18, 2021.

Copyright Notice

   Copyright (c) 2021 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



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   (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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Error page URI EDNS0 Option Format  . . . . . . . . . . . . .   6
   4.  Error Page URI Processing . . . . . . . . . . . . . . . . . .   8
   5.  Sign and Verify . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Error Page  . . . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Usability Considerations  . . . . . . . . . . . . . . . . . .  11
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
     9.1.  A New Error Page URI EDNS Option  . . . . . . . . . . . .  12
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     11.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   DNS filters are deployed for a variety of reasons, including endpoint
   security, parental filtering, and filtering required by law
   enforcement.  Some of these reasons are discussed in more detail
   below:

   o  Various network security services are provided by Enterprise
      networks to protect endpoints (e.g., Hosts including IoT devices).
      Network-based security solutions such as firewalls and Intrusion
      Prevention Systems (IPS) rely upon network traffic inspection to
      implement perimeter-based security policies.  The network security
      services may, for example, prevent malware download, block known
      malicious domains, block phishing sites, etc.  These network
      security services act on DNS queries originating from endpoints.
      For example, DNS firewalls, a method of expressing DNS response
      policy information inside specially constructed DNS zones, known
      as Response Policy Zones (RPZs) allows DNS servers to modify their
      DNS responses in real time in order to stop access to malware and
      phishing domains.  Note that some of the commonly known types of
      malware are viruses, worms, trojans, bots, ransomware, backdoors,
      spyware, and adware.



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   o  Network devices in a home network offer network security to
      protect the devices within the home network by performing DNS-
      based content filtering.  The network security service may, for
      example, block access to specific domains to enforce parental
      control, block access to malware sites, etc.

   o  Internet Service Providers (ISPs) typically block access to some
      domains due to a requirement imposed by an external entity (e.g.,
      Law Enforcement Agency) by performing DNS-based content filtering.

   DNS responses can be filtered by sending a bogus (also called,
   "forged") A or AAAA response, NXDOMAIN error or empty answer, or an
   extended DNS error (EDE) 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
       point to an HTTP(S) server alerting the end user of the reason
       for blocking access to the requested domain (e.g., malware).
       When an HTTP(S) enabled domain name is blocked, the network
       security device (e.g., CPE, firewall) presents a block page
       instead of the HTTP response from the content provider hosting
       that domain.  If an HTTP enabled domain name is blocked, the
       network security device intercepts the HTTP request and returns a
       block page over HTTP.  If an HTTPS enabled domain is blocked, the
       block page is also served over HTTPS.  In order to return a block
       page over HTTPS, man in the middle (MITM) is enabled on endpoints
       by generating a local root certificate and an accompanying
       (local) public/private key pair.  The local root certificate is
       installed on the endpoint while the network security device(s)
       stores a copy of the private key.  During the TLS handshake, the
       network security device modifies the certificate provided by the
       server and (re)signs it with the private key from the local root
       certificate.

       *  However, configuring the local root certificate on endpoints
          is not a viable option in several deployments like home
          networks, schools, Small Office/Home Office (SOHO), and Small/
          Medium Enterprise (SME).  In these cases, the typical behavior
          is that the forged DNS response directs the user towards a
          server hosted to display the block page which breaks the TLS
          connection.  For web-browsing this then results in an HTTPS
          certificate error message indicating that a secure connection
          could not be established, which gives no information to the
          end-user about the reason for the error.  The typical errors
          are "The security certificate presented by this website was
          not issued by a trusted certificate authority" (Internet
          Explorer/Edge"), "The site's security certificate is not
          trusted" (Chrome), "This Connection is Untrusted" (Firefox),



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          "Safari can't verify the identity of the website..." (Safari
          on MacOS)".

       *  Enterprise networks do not 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 reset 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-level protection 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 (e.g.
          [Chrome-Install-Cert]).  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 a NXDOMAIN response to
       cause the DNS lookup to terminate in failure.  In this case, an
       end user does not know why the domain cannot be reached and may
       repeatedly try to reach the domain but with no success.
       Frustrated, the end user may use insecure connections to reach
       the domain, potentially compromising both security and privacy.

   3.  The extended error codes Blocked, Censored, and Filtered defined
       in Section 4 of [RFC8914] can be returned by a DNS server to
       provide additional information about the cause of an DNS error.
       If the extended error code "Forged Answer" defined in Section 4.5
       of [RFC8914] is returned by the DNS server, the client can
       identify the DNS response is forged together with the reason for
       HTTPS certificate 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 he/she 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
       "Adult Content" to enforce parental control, "Violence &
       Terrorism" due to an external requirement imposed by an external



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       entity (e.g., Law Enforcement Agency), etc.  These content
       categories cannot be standardized because the classification of
       domains into content categories is vendor specific, typically
       ranges from 40 to 100 types of categories depending on the vendor
       and the categories keep evolving.  Furthermore, the threat data
       used to categorize domains may sometimes misclassify domains
       (e.g., domains wrongly classified as Domain Generation Algorithm
       (DGA) by deep learning techniques, domain wrongly classified as
       phishing due to crowd sourcing, new domains not categorized by
       the threat data).  A user needs to know the contact details of
       the IT/InfoSec team to raise a complaint.

   4.  The EXTRA-TEXT field of the EDE option defined in Section 2 of
       [RFC8914] can include additional textual information about the
       cause of the error, but the information could be provided in a
       language that is not understood by the user.  When a resolver or
       forwarder forwards the received EDE option, the EXTRA-TEXT field
       only conveys the source of the error (Section 3 of [RFC8914]) and
       does not provide additional textual information about the cause
       of the error.  Most importantly, EDE option does not offer
       authenticated information; it can thus be be spoofed by an
       attacker.  In addition, the additional textual information may
       not be able to convey all of the required information about the
       cause of the DNS error because lengthy EXTRA-TEXT content would
       be truncated to prevent fragmentation (Section 3 of [RFC8914]).

   No matter which type of response is generated (forged IP address(es),
   NXDOMAIN or empty answer, or an extended error code), the user who
   generated the 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 a URI which, when accessed, provides such information to the
   user.

   One of the other benefits of this approach is to eliminate the need
   to "spoof" block pages for HTTPS resources, as the block page no
   longer needs to create a signed certificate when blocking a
   destination.  Also, the approach avoids the need to install a local
   root certificate authority on those IT-managed devices.

2.  Terminology

   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.




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   This document makes use of the terms defined in [RFC8499] and
   [I-D.ietf-dnsop-terminology-ter].

   '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 [I-D.ietf-dprive-dnsoquic].

3.  Error page URI EDNS0 Option Format

   This document uses an EDNS0 [RFC6891] option to include the URI that
   gives additional information in a DNS response about the cause of
   blocking access to a domain.  This option is structured as depicted
   in Figure 1.

                                                1   1   1   1   1   1
        0   1   2   3   4   5   6   7   8   9   0   1   2   3   4   5
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                         OPTION-CODE                           |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                        OPTION-LENGTH                          |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                ERROR-PAGE-URI-LENGTH  (fixed size)            |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      /                 ERROR-PAGE-URI (variable size)                /
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                 ERROR-PAGE-SIG-ALG (fixed size)               |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      /                 ERROR-PAGE-SIG  (variable size)               /
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

               Figure 1: Error page URI EDNS0 Option Format

   The description of the fields is as follows:

   o  OPTION-CODE: TBD, indicates the code assigned for Error page URI
      (Section 6.1.2 of [RFC6891]).  [RFC Editor: change TBD to the
      proper code once assigned by IANA.]

   o  OPTION-LENGTH: See Section 6.1.2 of [RFC6891].  This fields
      contains the length of the payload (everything after OPTION-
      LENGTH) in octets.  The variability of the option length stems
      from the variable-length ERROR-PAGE-URI and ERROR-PAGE-SIG fields.

   o  ERROR-PAGE-URI-LENGTH: This 16-bit field indicates the length of
      ERROR-PAGE-URI.  It MUST NOT be set to 0.

   o  ERROR-PAGE-URI: A variable length UTF-8 encoded [RFC5198] text
      field containing the URI Template [RFC6570] that gives additional



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      information about the cause of blocking access to a domain.  The
      ERROR-PAGE-URI field MUST NOT be zero octets in length.

   o  ERROR-PAGE-SIG-ALG: A 16-bits field that contains the algorithm
      used to generate the signature for the Error Page URI Template.
      The values are defined in the TLS SignatureScheme [TLS-SIG-SCHEME]
      with limitations described in Section 5.

   o  ERROR-PAGE-SIG, a variable length field containing the signature
      of the Error Page URI Template.  The signature generation process
      is discussed in Section 5.

   The Error page URI option can be included in any response (SERVFAIL,
   NXDOMAIN, REFUSED, and even NOERROR, etc) to a query that includes
   OPT Pseudo-RR [RFC6891].

   The URI Template defined in ERROR-PAGE-URI describes how to construct
   the URL to fetch the error page.  The agent acting as HTTPS client on
   the endpoint encodes a FQDN to which access is denied into an HTTP
   GET request to retrieve the error page.  The HTTPS server returning
   the error page defines the URI used by the HTTP GET request through
   the use of a URI Template.  The URI Template is processed with a
   defined variable "target-domain" whose value is set to the FQDN to
   which access is denied.  The FQDN is encoded using base64url
   [RFC4648] and then provided as the variable value for "target-domain"
   to expand the URI Template into a URI reference in the HTTP GET
   request.  Padding characters for base64url MUST NOT be included.

   An example is illustrated below:

      If the URI Template is "https://block.example.net/block-
      page{?target-domain}" for the HTTPS server returning the error
      page and access to the target domain "example.com" is blocked by
      the encrypted DNS server, the variable "target-domain" has the
      value "example.com" base64url encoded into an HTTP GET request.
      In the above example, the expansion of the above URI Template is
      "https://block.example.net/block-page?target-
      domain=ZXhhbXBsZS5jb20".

   HTTP/2 [RFC7540] is the minimum RECOMMENDED HTTP version to use to
   retrieve the error page.  The HTTPS client retrieving the error page
   MUST verify the entire certification path as per [RFC5280].  The
   HTTPS client additionally uses validation techniques described in
   [RFC6125] to compare the domain name in the error page URI to the
   server certificate provided in TLS handshake.  See [RFC7525] for
   additional TLS recommendations.





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4.  Error Page URI Processing

   The DNS client MUST follow the following rules to process the Error
   Page URI EDNS0 option:

   o  The Error Page URI EDNS0 option is susceptible to forgery.  In
      order to defend against this attack the DNS client MUST NOT
      process the DNS response with Error Page URI EDNS0 option unless
      DNS messages exchanged are cryptographically protected using
      encrypted DNS.

   o  If an DNS client has enabled opportunistic privacy profile
      (Section 5 of [RFC8310]) for DoT, the DNS client will either
      fallback to an encrypted connection without authenticating the DNS
      server provided by the local network or fallback to clear text
      DNS, and cannot exchange encrypted DNS messages.  The fallback
      adversely impacts security and privacy.  If the DNS client has
      enabled opportunistic privacy profile for DoT, the client MUST NOT
      process the DNS response with Error Page URI EDNS0 option.

   o  If an 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; this
      mitigates both passive eavesdropping and client 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 client can process the
      DNS response with Error Page URI EDNS0 option.  Note that the
      strict and opportunistic privacy profiles as defined in [RFC8310]
      only applies to DoT protocol, there has been no such distinction
      made for DoH protocol.

   o  If the DNS response contains more than one Error Page URI EDNS0
      option, the DNS client MUST discard all Error Page URI EDNS0
      options in the DNS response.

   o  The Error Page URI EDNS0 option MUST be processed by the DNS
      client for a "Censored", "Blocked", "Filtered" or "Forged"
      extended error codes and MUST be ignored for any other type of
      extended DNS error code.  When "Censored", "Blocked", "Filtered"
      or "Forged" extended error code is returned in conjunction with an
      Error Page URI EDNS0 option, any other resource records in the
      answer MUST be ignored by clients supporting this specification.

   o  If the DNS client determines that the encrypted DNS server does
      not offer DNS filtering service, it MUST reject the Error Page URI
      EDNS0 option.  For example, the DNS client knows whether the pre-




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      configured encrypted DNS resolver performs DNS-based content
      filtering or not.

   o  The DNS client MUST reject the error page URI if the scheme is not
      "https".

   o  The DNS client verifies the signature in the ERROR-PAGE-SIG field
      (Figure 1) following the mechanism discussed in Section 5.  If the
      signature is valid, the client can positively identify that the
      Error Page URI EDNS0 option has been generated by the encrypted
      DNS server and the encrypted DNS server did not forward the Error
      Page URI EDNS0 option from an upstream resolver.  If signature
      validation fails, the DNS client MUST reject the Error Page URI
      EDNS0 option.

   A DNS resolver or forwarder MUST NOT propagate a received Error Page
   URI EDNS0 option over an unencrypted connection because an attacker
   can insert a bogus URI.  However, when a resolver or forwarder
   receives an Error Page URI EDNS0 option over an encrypted connection,
   whether or not to pass along Error Page URI EDNS0 option on to the
   original client is implementation dependent.  If the Implementation
   chooses to forward the Error Page URI EDNS0 option received over an
   encrypted connection, it MUST create a new Error Page URI EDNS0
   option that conveys the URI in the received Error Page URI EDNS0
   option after successful signature validation.  The signature for the
   new Error Page URI EDNS0 option MUST be generated using the private
   key of the DNS resolver or forwarder end-entity certificate used in
   the TLS connection to the original client.  If signature validation
   fails for the received Error Page URI EDNS0 option, the DNS resolver
   or forwarder end-entity certificate MUST reject the Error Page URI
   EDNS0 option.

   The DNS resolver or forwarder MUST NOT modify the ERROR-PAGE-URI
   field (Figure 1) in the forwarded Error Page URI EDNS0 option.

   If the resolver or forwarder simply forwards the received Error Page
   URI EDNS0 option without updating the signature in the ERROR-PAGE-SIG
   field (Figure 1), signature validation by the original client will
   fail and the forwarded Error Page URI EDNS0 option will be rejected.
   As a reminder, Section 3 of [RFC8914] discusses the source of the
   error should be attributed in the EXTRA-TEXT field, since an EDNS0
   option received by the original client will appear to have come from
   the resolver or forwarder sending it.  Because DNS forwarders (or DNS
   proxies) are supposed to propagate unknown EDNS0 options (Sections
   4.1 and 4.4.1 of [RFC5625]), the Error Page URI EDNS0 option may get
   propagated.  To detect this scenario, the Error Page URI Template is
   protected with an object signature as described in Section 5 to
   provide authenticated information.



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5.  Sign and Verify

   The algorithms for generating signature for DNS resource record sets
   (RRsets) are defined in [DNSKEY-IANA].  The "mandatory-to-implement"
   algorithms are RSA, Elliptic Curve Digital Signature Algorithm
   (ECDSA), and Edwards-curve Digital Security Algorithm (EdDSA)
   [RFC8624].  Along similar lines, the encrypted DNS server's end-
   entity certificate's public key and the signature algorithm with
   which the key can be used are RSA, ECDSA, and EdDSA [RFC8446].  If
   ECDSA is used, it is RECOMMENDED to use the deterministic digital
   signature generation procedure of the ECDSA, specified in [RFC6979].

   The signature is generated by the encrypted DNS server using the
   Error Page URI Template, private key of the encrypted DNS server's
   end-entity certificate as inputs to the signature algorithm.  The
   signature algorithm in the ERROR-PAGE-SIG-ALG field MUST be
   compatible with the key in the DNS server's end-entity certificate.
   The implementation MUST support the same set of algorithms in the TLS
   client for validating the signature in the CertificateVerify message
   from the server in the TLS handshake and in the DNS client to
   validate the signature for the Error Page URI Template.  As a
   reminder, the server's end-entity certificate's public key will be
   compatible with the selected authentication algorithm from the
   client's "signature_algorithms" TLS extension (Section 4.4.2.2 of
   [RFC8446]).

   If the signature algorithm in the ERROR-PAGE-SIG-ALG field is not
   compatible with the key in the DNS server's end-entity certificate,
   the DNS client MUST reject the Error Page URI EDNS0 option.  The DNS
   client verifies the signature using the signature in the ERROR-PAGE-
   SIG field, Error Page URI Template and DNS server's end-entity
   certificate's public key as inputs to the signature algorithm.  For
   example, if Ed25519 is used, Ed25519 signature algorithm and
   verification of the Ed25519 signature are described in Sections 5.1.6
   and 5.1.7 of [RFC8032], respectively.

6.  Error Page

   The following outlines the RECOMMENDED contents of an error page to
   assist the operator developing the error page:

   o  The exact reason for blocking access to the domain.  If the domain
      is blocked based on some threat data, the threat type associated
      with the blocked domain can be provided/displayed to the end user.
      For example, the reason can indicate the type of malware blocked
      like spyware and the damage it can do the security and privacy of
      the user.




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   o  The domain name blocked.

   o  If query was blocked by regulation, a pointer to a regulatory text
      that mandates this query block.

   o  The entity (or organization) blocking the access to the domain and
      contact details of the IT/InfoSec team to raise a complaint.

   o  The blocked error page to not include Ads and dynamic content.

   The content of the error page discussed above is non-normative, the
   above text only provides the guidelines and template for the error
   page and:

   o  does not attempt to offer an exhaustive list for the contents of
      an error page.

   o  it is not intended to form the basis of any legal/compliance for
      developing the error page.

7.  Usability Considerations

   The error page SHOULD be returned in the user's preferred language as
   expressed by the Accept-Language HTTP header.

8.  Security Considerations

   Security considerations in Section 6 of [RFC8914] and [RFC8624] need
   to be taken into consideration.

   The Error Page URI EDNS0 option causes an HTTPS retrieval by the
   client.  To prevent forgery of the Error Page URI EDNS0 option, this
   specification requires it only be sent only over an encrypted DNS
   channel with an authorized DNS server.

   The client knows it is connecting to a HTTPS server returning the
   error page.  To reduce threat surface the client can retrieve the
   Error page URL using, for example, an isolated environment and take
   other precautions such as clearly labeling the page as untrusted or
   prevent user interaction with the page.  Such isolation should
   prevent transmitting cookies, block JavaScript, block auto-fill of
   credentials or personal information, and be isolated from the user's
   normal environment.

   Browsers perform some of the above restrictions when accessing
   captive portals (Section 5 of [RFC8910] or [Safari-Cookie]), during
   private browsing, or using containerization [Facebook-Container].




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   Note that the means to use a sandbox environment and a user interface
   presenting the error page are not covered in this document.  By its
   nature, these aspects are implementation specific and best left to
   the application and user interface designers.

   The encrypted DNS session provides transport security for the
   interaction between the DNS client and server, but DNSSEC signing and
   validation is not possible for the Error Page URI EDNS0 option
   returning the Error Page URI Template.  However, the signature in the
   Error Page URI EDNS0 option provides authentication for the Error
   Page URI EDNS0 option.

   By design, the object referenced by the error page URL potentially
   exposes additional information about the DNS resolution process that
   may leak information.  An example of this is the reason for blocking
   the access to the domain name and the entity blocking access to the
   domain.

9.  IANA Considerations

9.1.  A New Error Page URI EDNS Option

   This document defines a new EDNS(0) option, entitled "Error Page
   URI", assigned a value of TBD from the "DNS EDNS0 Option Codes (OPT)"
   registry [to be removed upon publication:
   [http://www.iana.org/assignments/dns-parameters/dns-
   parameters.xhtml#dns-parameters-11]

   Value  Name                 Status    Reference
   -----  ----------------     ------    ------------------
    TBD   Error Page URI      Standard       [ This document ]

10.  Acknowledgements

   Thanks to Vittorio Bertola, Wes Hardaker, Ben Schwartz, Erid Orth,
   Viktor Dukhovni, Warren Kumari and Bob Harold for the comments.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.






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   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

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

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

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

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,
              <https://www.rfc-editor.org/info/rfc6570>.

   [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/info/rfc6891>.

   [RFC6979]  Pornin, T., "Deterministic Usage of the Digital Signature
              Algorithm (DSA) and Elliptic Curve Digital Signature
              Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
              2013, <https://www.rfc-editor.org/info/rfc6979>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.






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   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [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/info/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/info/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/info/rfc8446>.

   [RFC8624]  Wouters, P. and O. Sury, "Algorithm Implementation
              Requirements and Usage Guidance for DNSSEC", RFC 8624,
              DOI 10.17487/RFC8624, June 2019,
              <https://www.rfc-editor.org/info/rfc8624>.

   [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/info/rfc8914>.

   [TLS-SIG-SCHEME]
              "IANA, TLS SignatureScheme",
              <https://www.iana.org/assignments/tls-parameters/tls-
              parameters.xhtml#tls-signaturescheme>.

11.2.  Informative References

   [Chrome-Install-Cert]
              "How to manually install the Securly SSL certificate in
              Chrome", <support.securly.com/hc/en-us/articles/206081828-
              How-to-manually-install-the-Securly-SSL-certificate-in-
              Chrome>.

   [Chrome-Translate]
              "Google Translate",
              <https://chrome.google.com/webstore/detail/google-
              translate/aapbdbdomjkkjkaonfhkkikfgjllcleb/RK%3D2/
              RS%3DBBFW_pnWkPY0xPMYsAZI5xOgQEE->.





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   [DNSKEY-IANA]
              "IANA, Domain Name System Security (DNSSEC) Algorithm
              Numbers",
              <http://www.iana.org/assignments/dns-sec-alg-numbers>.

   [Facebook-Container]
              "Facebook container for Firefox",
              <https://www.mozilla.org/en-US/firefox/
              facebookcontainer/>.

   [I-D.ietf-dnsop-terminology-ter]
              Hoffman, P., "Terminology for DNS Transports and
              Location", draft-ietf-dnsop-terminology-ter-02 (work in
              progress), August 2020.

   [I-D.ietf-dprive-dnsoquic]
              Huitema, C., Mankin, A., and S. Dickinson, "Specification
              of DNS over Dedicated QUIC Connections", draft-ietf-
              dprive-dnsoquic-01 (work in progress), October 2020.

   [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/info/rfc7858>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

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

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

   [RFC8910]  Kumari, W. and E. Kline, "Captive-Portal Identification in
              DHCP and Router Advertisements (RAs)", RFC 8910,
              DOI 10.17487/RFC8910, September 2020,
              <https://www.rfc-editor.org/info/rfc8910>.

   [Safari-Cookie]
              "Isolated cookie store (CVE-2016-1730)",
              <https://support.apple.com/en-us/HT205732>.





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Authors' Addresses

   Tirumaleswar Reddy
   McAfee, Inc.
   Embassy Golf Link Business Park
   Bangalore, Karnataka  560071
   India

   Email: kondtir@gmail.com


   Neil Cook
   Open-Xchange
   UK

   Email: neil.cook@noware.co.uk


   Dan Wing
   Citrix Systems, Inc.
   USA

   Email: dwing-ietf@fuggles.com


   Mohamed Boucadair
   Orange
   Rennes  35000
   France

   Email: mohamed.boucadair@orange.com




















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