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DNSOP WG                                                        T. Reddy
Internet-Draft                                                    McAfee
Intended status: Standards Track                                 N. Cook
Expires: January 9, 2021                                    Open-Xchange
                                                                 D. Wing
                                                                  Citrix
                                                            M. Boucadair
                                                                  Orange
                                                            July 8, 2020


                      DNS Access Denied Error page
                    draft-reddy-dnsop-error-page-00

Abstract

   When a DNS server filters a query the response conveys no detailed
   explanation of why the query was blocked, leading to end-user
   confusion.  This document defines a method to return an URL that
   explains the reason the DNS query was filtered.

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 January 9, 2021.

Copyright Notice

   Copyright (c) 2020 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



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   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.  Method to return the error page URL . . . . . . . . . . . . .   5
   4.  ERROR Page  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Usability Considerations  . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Error Page URL DNS Parameter  . . . . . . . . . . . . . .   8
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   DNS filters are deployed for a variety of reasons including endpoint
   security, parental filtering, and filtering required by law
   enforcement.  These 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 on 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 DNS
      responses in real time 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.

   o  Network devices in a home network offer network security to
      protect the devices connected to 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.



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   o  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 ("forged") A or AAAA
   response, NXDOMAIN error or empty answer, or an extended error code
   defined in [I-D.ietf-dnsop-extended-error].  Each of these have
   advantages and disadvantages, discussed below:

   1.  The DNS response is forged providing IP addresses that points to
       a HTTP(S) server alerting the end user of the reason for blocking
       access to the domain (e.g., malware).  When a HTTP(S) enabled
       domain name is blocked, the network security device presents a
       block page instead of the HTTP response from the content
       provider.  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, and the network security device(s) store 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 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),
          "Safari can't verify the identity of the website..." (Safari
          on MacOS)".

       *  Enterprise networks do not assume that all the devices
          connected to their network are managed by the IT team or
          Mobile Device Management (MDM) devices, especially in the
          quite common BYOD ("Bring Your Own Device") scenario.  In



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          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 unsuccessfully reach the
          domain.  Frustrated, the end user may use insecure interfaces
          to reach the domain, potentially compromising both security
          and privacy.  Furthermore, certificate errors train users to
          click through certificate errors, which is poor 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 [Chrome-Install-Cert] on a host device.
          Doing so, however, is also poor security practice as it
          creates a security vulnerability that may be exploited by a
          MITM attack.  When the 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 unsuccessfully reach the domain.  Frustrated,
       the end user may use insecure interfaces to reach the domain,
       potentially compromising both security and privacy.

   3.  The extended error codes Blocked, Censored, and Filtered defined
       in [I-D.ietf-dnsop-extended-error] can be returned by the DNS
       server to provide additional information about the cause of an
       DNS error.  If the extended error code "Forged answer" defined in
       [I-D.ietf-dnsop-extended-error] is returned by the DNS server,
       the client can identify the DNS response is forged and the reason
       for HTTPS certificate error.  These extended error codes do not
       suffer from the limitations discussed in (1) and (2) but the user
       still does not know the exact reason nor the user is aware of the
       exact entity blocking the access to the domain.  For example, a
       DNS server may block access domain based on the content category
       like "Adult Content" to enforce parental control, "Violence &
       Terrorism" due to an external requirement imposed by an external
       entity (e.g., Law Enforcement Agency), etc.  The content
       categories for domains 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.
       Further, the threat data used to categorize domains may sometimes
       mis-classify domains (e.g., Domains wrongly classified as DGA
       (Domain Generation Algorithm) by deep learning techniques, domain
       wrongly classified as phishing due to crowd sourcing, new domains
       not categorized by the threat data, etc.).  The end user needs to



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       know the contact details of the IT/InfoSec team to raise a
       complaint.

   No matter which type of response is generated (forged IP address,
   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 URL which, when accessed, provides such information to the
   user.

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.

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

   'DoH/DoT' refers to DNS-over-HTTPS and/or DNS-over-TLS.

3.  Method to return the error page URL

   The mechanism for providing additional information about the cause of
   blocking access to a domain is from the HTTPS DNS record
   [I-D.ietf-dnsop-svcb-https].  This HTTPS record in the "Service Form"
   provides the URL that gives additional information about the cause of
   blocking access to a domain.  In order to convey an error page URL,
   this HTTPS record SHOULD be returned along with the "Forged Answer"
   extended error code in Extended DNS Error (EDE) EDNS option and MUST
   contain the "eut" (Section 7) name.  The service parameter key "eut"
   MUST only be processed by the DNS client for a "Forged Answer"
   extended error code and MUST be ignored for any other type of DNS
   response.  When the "forged answer" extended error code is returned
   in conjunction with an HTTPS record containing the "eut" service
   parameter key, any other resource records in the answer MUST be
   ignored by clients supporting this specification.

   The following example shows a record containing an error page URL:

   foo.example.com.  7200  IN HTTPS 1 (
      eut=https://block.example.net/block-page=ZXhhbXBsZS5jb20K )

                            Figure 1: Example 1




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   In the above example, if the URI template is
   "https://block.example.net/block-page={target-domain}" for the server
   returning the error page and access to the target domain
   "example.com" is blocked, the DNS server replaces the string
   "{target-domain}" in the template with the base64url-encoded target
   domain [RFC4648].

   The agent acting as HTTPS client on the endpoint uses the URL as
   given by the DNS server in a HTTP GET request to retrieve the error
   page.  HTTP/2 [RFC7540] is the minimum RECOMMENDED version of HTTP to
   use to retrieve the error page.

4.  ERROR Page

   The following text outlines the RECOMMENDED contents of an 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.

   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 MUST NOT include Ads.

   o  The blocked error page MUST NOT include dynamic content.

5.  Usability Considerations

   The error page SHOULD be returned in the user's preferred language as
   expressed by the Accept-Language header.  If the error page is
   displayed in a language not known to the end user and assuming
   Internationalization features failed, browser extensions to translate
   to user's native language can be used.  For example, "Google
   Translate" extension [Chrome-Translate] provided by Google on Chrome
   can be used by the user to translate the error page.  The "Google
   Translate" extension automatically detects whether the language of a
   page is different from the language the user has selected.  If it is
   in a different language, a banner appears at the top of the page.



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   The user can click on the Translate button in the banner to have all
   the text on the page appear in the language selected by the user.

6.  Security Considerations

   Security considerations in [I-D.ietf-dnsop-extended-error] need to be
   taken into consideration.  Unless the DNS response that conveys the
   URL that provides additional information about the cause of blocking
   access to a domain is sent over DNS-over-HTTPS (DoH) [RFC8484] or
   DNS-over-TLS (DoT) [RFC7858], the DNS response is susceptible to
   forgery.

   The agent acting as the HTTPS client on the endpoint MUST NOT fetch
   the URL unless DNS messages are exchanged over DoT/DoH.  Bad actors
   can host DoT/DoH servers, and claim the servers offer privacy and
   filtering capability to block malware domains but exactly do the
   opposite to invade the security and privacy of the end user.  For
   example, this attack can be mitigated if the endpoint selects DoT/DoH
   servers hosted by well-known organizations (e.g., ISPs, organization
   for which a user works, etc.) or the user selects DoT/DoH server with
   filtering capability pre-configured in the OS/Browser.  The DNS
   client can learn the filtering capability of a DoH/DoT server using
   [I-D.reddy-add-server-policy-selection].
   [I-D.reddy-add-server-policy-selection] also discusses how a DNS
   client can authenticate it is connecting to a DoH/DoT server hosted
   by a specific organization (e.g., ISP).  This information is
   cryptographically signed to attest its authenticity.  It is
   particularly useful when the DoH/DoT server is insecurely discovered
   and prevents the client from connecting to an attackers DoH/DoT
   server.

   The URL providing additional information about the cause of blocking
   access to a domain MUST only serve static content to limit the risk
   of security threats from dynamic content (e.g., malicious Java
   Script).  Most importantly, to deal with malicious servers, because
   the client knows that it is accessing a error page URL, it can know
   not to send cookies, not to send credentials, disable JavaScript,
   auto-enable private browsing mode or load the error page in a
   container isolated from other web activity, etc.  The client MUST
   reject the URL if the scheme is not "https".

   The DoH/DoT session provides transport security for the interaction
   between the DNS client and server, but DNSSEC signing and validation
   is not possible for the HTTPS record returning the error page URL
   along with the "Forged Answer" extended error.






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7.  IANA Considerations

7.1.  Error Page URL DNS Parameter

   This parameter indicates the URL that provides additional information
   about the cause of blocking access to a domain is designated for use
   with the "Forged answer" extended error code.  This is a string
   encoded as UTF-8 characters.  This is a string encoded as UTF-8
   characters.

   Name:  eut

   SvcParamKey:  TBD

   Meaning:  URL that provides additional information about the cause of
      blocking access to a domain.

   Reference:  This document.

8.  Acknowledgements

   TODO.

9.  References

9.1.  Normative References

   [I-D.ietf-dnsop-extended-error]
              Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
              Lawrence, "Extended DNS Errors", draft-ietf-dnsop-
              extended-error-16 (work in progress), May 2020.

   [I-D.ietf-dnsop-svcb-https]
              Schwartz, B., Bishop, M., and E. Nygren, "Service binding
              and parameter specification via the DNS (DNS SVCB and
              HTTPS RRs)", draft-ietf-dnsop-svcb-https-00 (work in
              progress), June 2020.

   [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>.

   [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>.





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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>.

9.2.  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->.

   [I-D.ietf-dnsop-terminology-ter]
              Hoffman, P., "Terminology for DNS Transports and
              Location", draft-ietf-dnsop-terminology-ter-01 (work in
              progress), February 2020.

   [I-D.reddy-add-server-policy-selection]
              Reddy.K, T., Wing, D., Richardson, M., and M. Boucadair,
              "DNS Server Selection: DNS Server Information with
              Assertion Token", draft-reddy-add-server-policy-
              selection-03 (work in progress), June 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>.

   [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>.





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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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