DNSOP WG T. Reddy
Internet-Draft McAfee
Intended status: Standards Track N. Cook
Expires: January 28, 2021 Open-Xchange
D. Wing
Citrix
M. Boucadair
Orange
July 27, 2020
DNS Access Denied Error page
draft-reddy-dnsop-error-page-02
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
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Copyright Notice
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document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7.1. Error Page URL DNS Parameter . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
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.
One of the other benefits of this document is eliminating 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.
This avoids the need to install an 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.
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]. The "HTTPS" DNS resource record type
provides more information to the client before it attempts to
establish a HTTPS connection. This HTTPS record in the "ServiceMode"
(Section 2.4.2 of [I-D.ietf-dnsop-svcb-https]) 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) parameter. The value stored in the parameter is a URL.
The SvcParamKey "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. The HTTPS record MUST be placed as an
additional record in the additional data section of the DNS response.
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The name of the record MUST match the original QNAME in the DNS
request. When the "forged answer" extended error code is returned in
conjunction with an HTTPS record containing the "eut" SvcParamKey,
any other resource records in the answer MUST be ignored by clients
supporting this specification. The "eut" is a single-valued
SvcParamKey and the value MUST NOT be empty.
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=ZXhhbXBsZS5jb20 )
Figure 1: Example 1
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
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.
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.
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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.
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.
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 exchanged are cryptographically protected
using DoH/DoT. Bad actors can host DoH/DoT 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 DoH/DoT servers hosted by well-known
organizations (e.g., ISPs, organization for which a user works, etc.)
or the user selects DoH/DoT 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
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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.
In order 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 for the error page 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. The DNS resolver MUST
NOT forward the "Forged Answer" extended error and the HTTPS record
from an upstream resolver as an attacker (e.g., MiTM) could insert a
fake extended error response and HTTPS record into the DNS response.
As such, the HTTPS record returning the error page URL should be
treated only as diagnostic information and MUST NOT alter DNS
protocol processing.
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.
7. IANA Considerations
7.1. Error Page URL DNS Parameter
This document adds a parameter to the "Service Binding (SVCB)
Parameter" registry. If present, 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.
Name: eut
SvcParamKey: TBD
Meaning: URL that provides additional information about the cause of
blocking access to a domain.
Reference: This document.
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8. Acknowledgements
Thanks to Vittorio Bertola, Wes Hardaker and Bob Harold for the
comments.
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-01 (work in
progress), July 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>.
[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>.
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[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-04 (work in progress), July 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>.
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
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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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