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Discovery of Designated Resolvers in the Presence of Legacy Forwarders
draft-schwartz-add-ddr-forwarders-00

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Author Benjamin M. Schwartz
Last updated 2021-09-21
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draft-schwartz-add-ddr-forwarders-00
dprive                                                       B. Schwartz
Internet-Draft                                                Google LLC
Intended status: Informational                         21 September 2021
Expires: 25 March 2022

 Discovery of Designated Resolvers in the Presence of Legacy Forwarders
                  draft-schwartz-add-ddr-forwarders-00

Abstract

   This draft describes how the Discovery of Designated Resolvers (DDR)
   standard interacts with legacy DNS forwarders, including potential
   incompatibilities and relevant mitigations.

Discussion Venues

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

   Discussion of this document takes place on the mailing list
   (add@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/add/.

   Source for this draft and an issue tracker can be found at
   https://github.com/bemasc/ddr-forwarders.

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 25 March 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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 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.  Conventions and Definitions . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Relaxed Validation client policy  . . . . . . . . . . . . . .   3
   4.  Naturally compatible behaviors  . . . . . . . . . . . . . . .   4
     4.1.  Malware and threat domain filtering . . . . . . . . . . .   4
     4.2.  Service category restrictions . . . . . . . . . . . . . .   4
     4.3.  Time of use restrictions  . . . . . . . . . . . . . . . .   4
   5.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
     6.1.  Transient attackers . . . . . . . . . . . . . . . . . . .   5
       6.1.1.  Solution: DNR . . . . . . . . . . . . . . . . . . . .   5
       6.1.2.  Mitigation: Frequent refresh  . . . . . . . . . . . .   5
       6.1.3.  Mitigation: Resolver reputation . . . . . . . . . . .   5
     6.2.  Forensic logging  . . . . . . . . . . . . . . . . . . . .   6
       6.2.1.  Network-layer logging . . . . . . . . . . . . . . . .   6
       6.2.2.  DNS-layer logging . . . . . . . . . . . . . . . . . .   6
   7.  Compatibility Considerations  . . . . . . . . . . . . . . . .   6
     7.1.  Split-horizon namespaces  . . . . . . . . . . . . . . . .   6
       7.1.1.  Mitigation: NXDOMAIN Fallback . . . . . . . . . . . .   7
     7.2.  Interposable domains  . . . . . . . . . . . . . . . . . .   7
       7.2.1.  Mitigation: Exemption list  . . . . . . . . . . . . .   7
     7.3.  Caching . . . . . . . . . . . . . . . . . . . . . . . . .   7
       7.3.1.  Mitigation: Stub caches . . . . . . . . . . . . . . .   8
     7.4.  General mitigation: User controls . . . . . . . . . . . .   8
   8.  Informative References  . . . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Conventions and Definitions

   Legacy DNS Forwarder - An apparent DNS resolver, known to the client
   only by a non-public IP address, that forwards the client's queries
   to an upstream resolver, and has not been updated with any knowledge
   of DDR.

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   Cross-Forwarder Upgrade - Establishment of a direct, encrypted
   connection between the client and the upstream resolver.

2.  Introduction

2.1.  Background

   The Discovery of Designated Resolvers specification [DDR] describes a
   mechanism for clients to learn about the encrypted protocols
   supported by a DNS server.  It also describes a conservative client
   validation policy that has strong security properties and is unlikely
   to create compatibility problems.

   On the topic of client validation of encrypted DNS transports, the
   DDR specification says:

      If the IP address of a Designated Resolver differs from that of an
      Unencrypted Resolver, clients MUST validate that the IP address of
      the Unencrypted Resolver is covered by the SubjectAlternativeName
      of the Encrypted Resolver's TLS certificate

   As TLS certificates cannot cover non-public IP addresses, this
   prevents clients that are behind a legacy DNS forwarder from
   connecting directly to the upstream resolver ("cross-forwarder
   upgrade").

   Recent estimates suggest that a large fraction, perhaps a majority,
   of residential internet users in the United States and Europe rely on
   local DNS forwarders that are not compatible with DDR.

2.2.  Scope

   This informational document describes the interaction between DDR and
   legacy DNS forwarders.  It discusses possible client policies,
   problems that might arise, and relevant mitigations.

   DNS forwarders and resolvers that are implemented with awareness of
   DDR are out of scope, as they are not affected by this discussion
   (although see Security Considerations, Section 6).

3.  Relaxed Validation client policy

   We define a "relaxed validation" client policy as a client behavior
   that removes the certificate validation requirement when the
   Unencrypted Resolver is identified by a non-public IP address,
   regardless of the Designated Resolver's IP address.  This client
   policy is otherwise identical to the one described in [DDR].

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4.  Naturally compatible behaviors

   The following network behaviors are naturally compatible with relaxed
   validation.

4.1.  Malware and threat domain filtering

   Certain DNS forwarders block access to domains associated with
   malware and other threats.  Such threats rely on frequently changing
   domains, so these forwarders necessarily maintain an actively curated
   list of domains to block.  To ensure that this service is not lost
   due to a cross-forwarder upgrade, the maintainers can simply add
   "resolver.arpa" to the list.

   This pattern has been deployed by Mozilla, with the domain "use-
   application-dns.net" [MOZILLA-CANARY].

4.2.  Service category restrictions

   Certain DNS forwarders may block access to domains based on the
   category of service provided by those domains, e.g. domains hosting
   services that are not appropriate for a work or school environment.
   As in the previous section, this requires an actively curated list of
   domains, because the set of domains that offer a given type of
   service is constantly changing.  An actively managed blocking list
   can easily be revised to include "resolver.arpa".

4.3.  Time of use restrictions

   Certain networks may impose restrictions on the time or duration of
   use by certain users.  This behavior is necessarily implemented below
   the DNS layer, because DNS-based blocking would be ineffective due to
   stub resolver caching, so it is not affected by changes in the DNS
   resolver.

5.  Privacy Considerations

   The conservative validation policy results in no encryption when a
   legacy DNS forwarder is present.  This leaves the user's query
   activity vulnerable to passive monitoring [RFC7258], either on the
   local network or between the user and the upstream resolver.

   The relaxed validation policy allows the use of encrypted transport
   in these configurations, reducing exposure to a passive surveillance
   adversary.

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6.  Security Considerations

   When the client uses the conservative validation policy described in
   [DDR], and a DDR-enabled resolver is identified by a non-public IP
   address, the client can establish a secure DDR connection only in the
   absence of an active attacker.  An on-path attacker can impersonate
   the resolver and intercept all queries, by preventing the DDR upgrade
   or advertising their own DDR endpoint.

   These basic security properties also apply if the client uses the
   relaxed validation policy described in Section 3.  Nonetheless, there
   are some subtle but important differences in the security properties
   of these two policies.

6.1.  Transient attackers

   With the conservative validation policy, a transient on-path attacker
   can only intercept queries for the duration of their active presence
   on the network, because the client will only send queries to the
   original (non-public) server IP address.

   With the relaxed validation behavior, a transient on-path attacker
   could implant a long-lived DDR response in the client's cache,
   directing its queries to an attacker-controlled server on the public
   internet.  This would allow the attack to continue long after the
   attacker has left the network.

6.1.1.  Solution: DNR

   This attack does not apply if the client and network implement
   support for Discovery of Network-designated Resolvers [DNR].

6.1.2.  Mitigation: Frequent refresh

   The client can choose to refresh the DDR record arbitrarily
   frequently, e.g. by limiting the TTL.  For example, by limiting the
   TTL to 5 minutes, a client could ensure that any attacker can
   continue to monitor queries for at most 5 minutes after they have
   left the local network.

6.1.3.  Mitigation: Resolver reputation

   A relaxed-validation client might choose to accept a potential cross-
   forwarder upgrade only if the designated encrypted resolver has
   sufficient reputation, according to some proprietary reputation
   scheme (e.g. a locally stored list of respectable resolvers).  This
   limits the ability of a DDR forgery attack to cause harm.

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   Major DoH client implementations already include lists of known
   resolvers [CHROME-DOH][MICROSOFT-DOH][MOZILLA-TRR].

6.2.  Forensic logging

6.2.1.  Network-layer logging

   With the conservative validation policy, a random sample of IP
   packets is likely sufficient for manual retrospective detection of an
   active attack.

   With the relaxed validation policy, forensic logs must capture a
   specific packet (the attacker's DDR designation response) to enable
   retrospective detection.

6.2.1.1.  Mitigation: Log all DDR responses

   Network-layer forensic logs that are not integrated with the resolver
   can enable detection of these attacks by logging all DDR responses,
   or more generally all DNS responses.  This makes retrospective attack
   detection straightforward, as the attacker's DDR response will
   indicate an unexpected server.

6.2.2.  DNS-layer logging

   DNS-layer forensic logging conducted by a legacy DNS forwarder would
   be lost in a cross-forwarder upgrade.

6.2.2.1.  Solution: Respond for resolver.arpa

   Forwarders that want to observe all queries from relaxed validation
   clients will have to synthesize their own response for resolver.arpa,
   either implementing DDR or disabling it.

7.  Compatibility Considerations

   Using DDR with legacy DNS forwarders also raises several potential
   concerns related to loss of existing network services.

7.1.  Split-horizon namespaces

   Some network resolvers contain additional names that are not
   resolvable in the global DNS.  If these local resolvers are also
   legacy DNS forwarders, a client that performs a cross-forwarder
   upgrade might lose access to these local names.

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7.1.1.  Mitigation: NXDOMAIN Fallback

   In "NXDOMAIN Fallback", the client repeats a query to the unencrypted
   resolver if the encrypted resolver returns NXDOMAIN.  This allows the
   resolution of local names, provided they do not collide with globally
   resolvable names (as required by [RFC2826]).

   This is similar to the fallback behavior currently deployed in
   Mozilla Firefox [FIREFOX-FALLBACK].

   NXDOMAIN Fallback results in slight changes to the security and
   privacy properties of encrypted DNS.  Queries for nonexistent names
   no longer have protection against a local passive adversary, and
   local names are revealed to the upstream resolver.

   NXDOMAIN Fallback is only applicable when a legacy DNS forwarder
   might be present, i.e. the unencrypted resolver has a non-public IP
   address, and the encrypted resolver has a different IP address.  In
   the other DDR configurations, any local names are expected to resolve
   similarly on both resolvers.

7.2.  Interposable domains

   An "interposable domain" is a domain whose owner deliberately allows
   resolvers to forge certain responses.  This arrangement is most
   common for search engines, which often support a configuration where
   resolvers forge a CNAME record to direct all clients to a child-
   appropriate instance of the search engine
   [DUCK-CNAME][BING-CNAME][GOOGLE-CNAME].

   Future deployments of interposable domains can instruct
   administrators to enable or disable DDR when adding the forged
   record, but forged records in legacy DNS forwarders could be lost due
   to a cross-forwarder upgrade.

7.2.1.  Mitigation: Exemption list

   There are a small number of pre-existing interposable domains,
   largely of interest only to web browsers.  Clients can maintain a
   list of relevant interposable domains and resolve them only via the
   network's resolver.

7.3.  Caching

   Some legacy DNS forwarders also provide a shared cache for all
   network users.  Cross-forwarder upgrades will bypass this cache,
   resulting in slower DNS resolution.

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7.3.1.  Mitigation: Stub caches

   Clients can compensate partially for any loss of shared caching by
   implementing local DNS caches.  This mitigation is already widely
   deployed in browsers and operating systems.

7.4.  General mitigation: User controls

   For these and other compatibility concerns, a possible mitigation is
   to provide users or administrators with the ability to control
   whether DDR is used with legacy forwarders.  For example, this
   control could be provided via a general preference, or via a
   notification when connecting to a new network.

8.  Informative References

   [BING-CNAME]
              "Block adult content with SafeSearch - Map at a network
              level", n.d., <https://help.bing.microsoft.com/#apex/bing/
              en-us/10003/0>.

   [CHROME-DOH]
              "DoH providers: criteria, process for Chrome", n.d.,
              <https://docs.google.com/document/d/128i2YTV2C7T6Gr3I-
              81zlQ-_Lprnsp24qzy_20Z1Psw/edit>.

   [DDR]      Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T.
              Jensen, "Discovery of Designated Resolvers", Work in
              Progress, Internet-Draft, draft-ietf-add-ddr-02, 8 July
              2021, <https://datatracker.ietf.org/doc/html/draft-ietf-
              add-ddr-02>.

   [DNR]      Boucadair, M., Reddy, T., Wing, D., Cook, N., and T.
              Jensen, "DHCP and Router Advertisement Options for the
              Discovery of Network-designated Resolvers (DNR)", Work in
              Progress, Internet-Draft, draft-ietf-add-dnr-02, 17 May
              2021, <https://datatracker.ietf.org/doc/html/draft-ietf-
              add-dnr-02>.

   [DUCK-CNAME]
              "Force Safe Search at a Network Level", n.d.,
              <https://help.duckduckgo.com/duckduckgo-help-
              pages/features/safe-search/>.

   [FIREFOX-FALLBACK]
              "About our rollout of DNS over HTTPS", n.d.,
              <https://support.mozilla.org/en-US/kb/firefox-dns-over-
              https#w_about-our-rollout-of-dns-over-https>.

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   [GOOGLE-CNAME]
              "Keep SafeSearch turned on for your school, workplace, or
              home network", n.d.,
              <https://support.google.com/websearch/
              answer/186669?hl=en>.

   [MICROSOFT-DOH]
              "Determine which DoH servers are on the known server
              list", n.d., <https://docs.microsoft.com/en-us/windows-
              server/networking/dns/doh-client-support#determine-which-
              doh-servers-are-on-the-known-server-list>.

   [MOZILLA-CANARY]
              "Canary domain - use-application-dns.net", n.d.,
              <https://support.mozilla.org/en-US/kb/canary-domain-use-
              application-dnsnet>.

   [MOZILLA-TRR]
              "Mozilla Policy Requirements for DNS over HTTPs Partners",
              n.d., <https://wiki.mozilla.org/Security/DOH-resolver-poli
              cy#Mozilla_Policy_Requirements_for_DNS_over_HTTPs_Partners
              >.

   [RFC2826]  Internet Architecture Board, "IAB Technical Comment on the
              Unique DNS Root", RFC 2826, DOI 10.17487/RFC2826, May
              2000, <https://www.rfc-editor.org/rfc/rfc2826>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/rfc/rfc7258>.

Acknowledgments

   Thanks to Anthony Lieuallen and Eric Orth for early reviews.

Author's Address

   Benjamin Schwartz
   Google LLC

   Email: bemasc@google.com

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