Local root server copy: Running a Root Server Local to a Resolver

Internet Engineering Task Force (IETF)                         W. Kumari
Request for Comments: 8806                                        Google
Obsoletes: 7706                                               P. Hoffman
Category: Informational                                            ICANN
ISSN: 2070-1721                                                June 2020


               Running a Root Server Local to a Resolver

Abstract

   Some DNS recursive resolvers have longer-than-desired round-trip
   times to the closest DNS root server; those resolvers may have
   difficulty getting responses from the root servers, such as during a
   network attack.  Some DNS recursive resolver operators want to
   prevent snooping by third parties of requests sent to DNS root
   servers.  In both cases, resolvers can greatly decrease the round-
   trip time and prevent observation of requests by serving a copy of
   the full root zone on the same server, such as on a loopback address
   or in the resolver software.  This document shows how to start and
   maintain such a copy of the root zone that does not cause problems
   for other users of the DNS, at the cost of adding some operational
   fragility for the operator.

   This document obsoletes RFC 7706.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8806.

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
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   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Changes from RFC 7706
     1.2.  Requirements Notation
   2.  Requirements
   3.  Operation of the Root Zone on the Local Server
   4.  Security Considerations
   5.  IANA Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Appendix A.  Current Sources of the Root Zone
     A.1.  Root Zone Services
   Appendix B.  Example Configurations of Common Implementations
     B.1.  Example Configuration: BIND 9.12
     B.2.  Example Configuration: Unbound 1.8
     B.3.  Example Configuration: BIND 9.14
     B.4.  Example Configuration: Unbound 1.9
     B.5.  Example Configuration: Knot Resolver
     B.6.  Example Configuration: Microsoft Windows Server 2012
   Acknowledgements
   Authors' Addresses

1.  Introduction

   DNS recursive resolvers have to provide answers to all queries from
   their clients, even those for domain names that do not exist.  For
   each queried name that is within a top-level domain (TLD) that is not
   in the recursive resolver's cache, the resolver must send a query to
   a root server to get the information for that TLD or to find out that
   the TLD does not exist.  Research shows that the vast majority of
   queries going to the root are for names that do not exist in the root
   zone.

   Many of the queries from recursive resolvers to root servers get
   answers that are referrals to other servers.  Malicious third parties
   might be able to observe that traffic on the network between the
   recursive resolver and root servers.

   The primary goals of this design are to provide more reliable answers
   for queries to the root zone during network attacks that affect the
   root servers and to prevent queries and responses from being visible
   on the network.  This design will probably have little effect on
   getting faster responses to the stub resolver for good queries on
   TLDs, because the TTL for most TLDs is usually long-lived (on the
   order of a day or two) and is thus usually already in the cache of
   the recursive resolver; the same is true for the TTL for negative
   answers from the root servers.  (Although the primary goal of the
   design is for serving the root zone, the method can be used for any
   zone.)

   This document describes a method for the operator of a recursive
   resolver to have a complete root zone locally and to hide queries for
   the root zone from outsiders.  The basic idea is to create an up-to-
   date root zone service on the same host as the recursive server and
   use that service when the recursive resolver looks up root
   information.  The recursive resolver validates all responses from the
   root service on the same host, just as it would validate all
   responses from a remote root server.

   This design explicitly only allows the new root zone service to be
   run on the same server as the recursive resolver in order to prevent
   the server from serving authoritative answers to any other system.
   Specifically, the root service on the local system MUST be configured
   to only answer queries from resolvers on the same host and MUST NOT
   answer queries from any other resolver.

   At the time that RFC 7706 [RFC7706] was published, it was considered
   controversial, because there was not consensus on whether this was a
   "best practice".  In fact, many people felt that it is an excessively
   risky practice, because it introduced a new operational piece to
   local DNS operations where there was not one before.  Since then, the
   DNS operational community has largely shifted to believing that local
   serving of the root zone for an individual resolver is a reasonable
   practice.  The advantages listed above do not come free: if this new
   system does not work correctly, users can get bad data, or the entire
   recursive resolution system might fail in ways that are hard to
   diagnose.

   This design uses an authoritative service running on the same machine
   as the recursive resolver.  Common open source recursive resolver
   software does not need to add new functionality to act as an
   authoritative server for some zones, but other recursive resolver
   software might need to be able to talk to an authoritative server
   running on the same host.  Some resolver software supports being both
   an authoritative server and a resolver but separated by logical
   "views", allowing a local root to be implemented within a single
   process; examples of this can be seen in Appendix B.

   A different approach to solving some of the problems discussed in
   this document is described in [RFC8198].

   Readers are expected to be familiar with [RFC8499].

1.1.  Changes from RFC 7706

   RFC 7706 explicitly required that a root server instance be run on
   the loopback interface of the host running the validating resolver.
   However, RFC 7706 also had examples of how to set up common software
   that did not use the loopback interface.  This document loosens the
   restriction on using the loopback interface and in fact allows the
   use of a local service, not necessarily an authoritative server.
   However, the document keeps the requirement that only systems running
   on that single host be able to query that authoritative root server
   or service.

   This document changes the use cases for running a local root service
   to be more consistent with the reasons operators said they had for
   using RFC 7706:

   *  Removed the prohibition on distribution of recursive DNS servers,
      including configurations for this design because some already do
      and others have expressed an interest in doing so.

   *  Added the idea that a recursive resolver using this design might
      switch to using the normal (remote) root servers if the local root
      server fails.

   *  Refreshed the list of where one can get copies of the root zone.

   *  Added examples of other resolvers and updated the existing
      examples.

1.2.  Requirements Notation

   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.

2.  Requirements

   In order to implement the mechanism described in this document:

   *  The system MUST be able to validate every signed record in a zone
      with DNSSEC [RFC4033].

   *  The system MUST have an up-to-date copy of the public part of the
      Key Signing Key (KSK) [RFC4033] used to sign the DNS root.

   *  The system MUST be able to retrieve a copy of the entire root zone
      (including all DNSSEC-related records).

   *  The system MUST be able to run an authoritative service for the
      root zone on the same host.  The authoritative root service MUST
      only respond to queries from the same host.  One way to ensure
      that the authoritative root service does not respond to queries
      from other hosts is to run an authoritative server for the root
      that responds only on one of the loopback addresses (that is, an
      address in the range 127/8 for IPv4 or ::1 in IPv6).  Another
      method is to have the resolver software also act as an
      authoritative server for the root zone, but only for answering
      queries from itself.

   A corollary of the above list is that authoritative data in the root
   zone used on the local authoritative server MUST be identical to the
   same data in the root zone for the DNS.  It is possible to change the
   unsigned data (the glue records) in the copy of the root zone, but
   such changes could cause problems for the recursive server that
   accesses the local root zone, and therefore any changes to the glue
   records SHOULD NOT be made.

3.  Operation of the Root Zone on the Local Server

   The operation of an authoritative server for the root in the system
   described here can be done separately from the operation of the
   recursive resolver, or it might be part of the configuration of the
   recursive resolver system.

   The steps to set up the root zone are:

   1.  Retrieve a copy of the root zone.  (See Appendix A for some
       current locations of sources.)

   2.  Start the authoritative service for the root zone in a manner
       that prevents any system other than a recursive resolver on the
       same host from accessing it.

   The contents of the root zone MUST be refreshed using the timers from
   the SOA record in the root zone, as described in [RFC1035].  This
   inherently means that the contents of the local root zone will likely
   be a little behind those of the global root servers, because those
   servers are updated when triggered by NOTIFY messages.

   There is a risk that a system using a local authoritative server for
   the root zone cannot refresh the contents of the root zone before the
   expire time in the SOA.  A system using a local authoritative server
   for the root zone MUST NOT serve stale data for the root zone.  To
   mitigate the risk that stale data is served, the local root server
   MUST immediately switch to using non-local root servers when it
   detects that it would be serving state data.

   In a resolver that is using an internal service for the root zone, if
   the contents of the root zone cannot be refreshed before the expire
   time in the SOA, the resolver MUST immediately switch to using non-
   local root servers.

   In the event that refreshing the contents of the root zone fails, the
   results can be disastrous.  For example, sometimes all the NS records
   for a TLD are changed in a short period of time (such as 2 days); if
   the refreshing of the local root zone is broken during that time, the
   recursive resolver will have bad data for the entire TLD zone.

   An administrator using the procedure in this document SHOULD have an
   automated method to check that the contents of the local root zone
   are being refreshed; this might be part of the resolver software.
   One way to do this is to have a separate process that periodically
   checks the SOA of the local root zone and makes sure that it is
   changing.  At the time that this document is published, the SOA for
   the root zone is the digital representation of the current date with
   a two-digit counter appended, and the SOA is changed every day even
   if the contents of the root zone are unchanged.  For example, the SOA
   of the root zone on January 2, 2019 was 2019010201.  A process can
   use this fact to create a check for the contents of the local root
   zone (using a program not specified in this document).

4.  Security Considerations

   A system that does not follow the DNSSEC-related requirements given
   in Section 2 can be fooled into giving bad responses in the same way
   as any recursive resolver that does not do DNSSEC validation on
   responses from a remote root server.  Anyone deploying the method
   described in this document should be familiar with the operational
   benefits and costs of deploying DNSSEC [RFC4033].

   As stated in Section 1, this design explicitly requires the local
   copy of the root zone information to be available only from resolvers
   on that host.  This has the security property of limiting damage to
   clients of any local resolver that might try to rely on an altered
   copy of the root.

5.  IANA Considerations

   This document has no IANA actions.

6.  References

6.1.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [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>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <https://www.rfc-editor.org/info/rfc4033>.

   [RFC7706]  Kumari, W. and P. Hoffman, "Decreasing Access Time to Root
              Servers by Running One on Loopback", RFC 7706,
              DOI 10.17487/RFC7706, November 2015,
              <https://www.rfc-editor.org/info/rfc7706>.

   [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>.

   [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>.

6.2.  Informative References

   [Manning2013]
              Manning, W., "Client Based Naming", May 2013,
              <http://www.sfc.wide.ad.jp/dissertation/bill_e.html>.

   [RFC5936]  Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
              (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
              <https://www.rfc-editor.org/info/rfc5936>.

   [RFC8198]  Fujiwara, K., Kato, A., and W. Kumari, "Aggressive Use of
              DNSSEC-Validated Cache", RFC 8198, DOI 10.17487/RFC8198,
              July 2017, <https://www.rfc-editor.org/info/rfc8198>.

Appendix A.  Current Sources of the Root Zone

   The root zone can be retrieved from anywhere as long as it comes with
   all the DNSSEC records needed for validation.  Currently, one can get
   the root zone from ICANN by zone transfer AXFR [RFC5936] over TCP
   from DNS servers at xfr.lax.dns.icann.org and xfr.cjr.dns.icann.org.
   The root zone file can be obtained using methods described at
   <https://www.iana.org/domains/root/files>.

   Currently, the root can also be retrieved by AXFR over TCP from the
   following root server operators:

   *  b.root-servers.net

   *  c.root-servers.net

   *  d.root-servers.net

   *  f.root-servers.net

   *  g.root-servers.net

   *  k.root-servers.net

   It is crucial to note that none of the above services are guaranteed
   to be available.  It is possible that ICANN or some of the root
   server operators will turn off the AXFR capability on the servers
   listed above.  Using AXFR over TCP to addresses that are likely to be
   anycast (as the ones above are) may conceivably have transfer
   problems due to anycast, but current practice shows that to be
   unlikely.

A.1.  Root Zone Services

   At the time that this document is published, there is one root zone
   service that is active and one that has been announced as in the
   planning stages.  This section describes all known active services.

   LocalRoot (<https://localroot.isi.edu/>) is an experimental service
   that embodies many of the ideas in this document.  It distributes the
   root zone by AXFR and also offers DNS NOTIFY messages when the
   LocalRoot system sees that the root zone has changed.

Appendix B.  Example Configurations of Common Implementations

   This section shows fragments of configurations for some popular
   recursive server software that is believed to correctly implement the
   requirements given in this document.  The examples have been updated
   since the publication of [RFC7706].

   The IPv4 and IPv6 addresses in this section were checked in March
   2020 by testing for AXFR over TCP from each address for the known
   single-letter names in the root-servers.net zone.

B.1.  Example Configuration: BIND 9.12

   BIND 9.12 acts both as a recursive resolver and an authoritative
   server.  Because of this, there is "fate-sharing" between the two
   servers in the following configuration.  That is, if the root server
   dies, it is likely that all of BIND is dead.

   Note that a future version of BIND will support a much more robust
   method for creating a local mirror of the root or other zones; see
   Appendix B.3.

   Using this configuration, queries for information in the root zone
   are returned with the Authoritative Answer (AA) bit not set.

   When slaving a zone, BIND 9.12 will treat zone data differently if
   the zone is slaved into a separate view (or a separate instance of
   the software) versus slaved into the same view or instance that is
   also performing the recursion.

   Validation:  When using separate views or separate instances, the DS
      records in the slaved zone will be validated as the zone data is
      accessed by the recursive server.  When using the same view, this
      validation does not occur for the slaved zone.

   Caching:  When using separate views or instances, the recursive
      server will cache all of the queries for the slaved zone, just as
      it would using the traditional "root hints" method.  Thus, as the
      zone in the other view or instance is refreshed or updated,
      changed information will not appear in the recursive server until
      the TTL of the old record times out.  Currently, the TTL for DS
      and delegation NS records is two days.  When using the same view,
      all zone data in the recursive server will be updated as soon as
      it receives its copy of the zone.

   view root {
       match-destinations { 127.12.12.12; };
       zone "." {
           type slave;
           file "rootzone.db";
           notify no;
           masters {
               199.9.14.201;         # b.root-servers.net
               192.33.4.12;          # c.root-servers.net
               199.7.91.13;          # d.root-servers.net
               192.5.5.241;          # f.root-servers.net
               192.112.36.4;         # g.root-servers.net
               193.0.14.129;         # k.root-servers.net
               192.0.47.132;         # xfr.cjr.dns.icann.org
               192.0.32.132;         # xfr.lax.dns.icann.org
               2001:500:200::b;      # b.root-servers.net
               2001:500:2::c;        # c.root-servers.net
               2001:500:2d::d;       # d.root-servers.net
               2001:500:2f::f;       # f.root-servers.net
               2001:500:12::d0d;     # g.root-servers.net
               2001:7fd::1;          # k.root-servers.net
               2620:0:2830:202::132; # xfr.cjr.dns.icann.org
               2620:0:2d0:202::132;  # xfr.lax.dns.icann.org
           };
       };
   };

   view recursive {
       dnssec-validation auto;
       allow-recursion { any; };
       recursion yes;
       zone "." {
           type static-stub;
           server-addresses { 127.12.12.12; };
       };
   };

B.2.  Example Configuration: Unbound 1.8

   Similar to BIND, Unbound, starting with version 1.8, can act both as
   a recursive resolver and an authoritative server.

   auth-zone:
       name: "."
       master: 199.9.14.201         # b.root-servers.net
       master: 192.33.4.12          # c.root-servers.net
       master: 199.7.91.13          # d.root-servers.net
       master: 192.5.5.241          # f.root-servers.net
       master: 192.112.36.4         # g.root-servers.net
       master: 193.0.14.129         # k.root-servers.net
       master: 192.0.47.132         # xfr.cjr.dns.icann.org
       master: 192.0.32.132         # xfr.lax.dns.icann.org
       master: 2001:500:200::b      # b.root-servers.net
       master: 2001:500:2::c        # c.root-servers.net
       master: 2001:500:2d::d       # d.root-servers.net
       master: 2001:500:2f::f       # f.root-servers.net
       master: 2001:500:12::d0d     # g.root-servers.net
       master: 2001:7fd::1          # k.root-servers.net
       master: 2620:0:2830:202::132 # xfr.cjr.dns.icann.org
       master: 2620:0:2d0:202::132  # xfr.lax.dns.icann.org
       fallback-enabled: yes
       for-downstream: no
       for-upstream: yes

B.3.  Example Configuration: BIND 9.14

   BIND 9.14 can set up a local mirror of the root zone with a small
   configuration option:

   zone "." {
       type mirror;
   };

   The simple "type mirror" configuration for the root zone works for
   the root zone because a default list of primary servers for the IANA
   root zone is built into BIND 9.14.  In order to set up mirroring of
   any other zone, an explicit list of primary servers needs to be
   provided.

   See the documentation for BIND 9.14 for more detail about how to use
   this simplified configuration.

B.4.  Example Configuration: Unbound 1.9

   Recent versions of Unbound have an "auth-zone" feature that allows
   local mirroring of the root zone.  Configuration looks as follows:

   auth-zone:
       name: "."
       master: "b.root-servers.net"
       master: "c.root-servers.net"
       master: "d.root-servers.net"
       master: "f.root-servers.net"
       master: "g.root-servers.net"
       master: "k.root-servers.net"
           fallback-enabled: yes
       for-downstream: no
       for-upstream: yes
       zonefile: "root.zone"

B.5.  Example Configuration: Knot Resolver

   Knot Resolver uses its "prefill" module to load the root zone
   information.  This is described at <https://knot-
   resolver.readthedocs.io/en/v5.0.1/modules-rfc7706.html>.

B.6.  Example Configuration: Microsoft Windows Server 2012

   Windows Server 2012 contains a DNS server in the "DNS Manager"
   component.  When activated, that component acts as a recursive
   server.  The DNS Manager can also act as an authoritative server.

   Using this configuration, queries for information in the root zone
   are returned with the AA bit set.

   The steps to configure the DNS Manager to implement the requirements
   in this document are:

   1.  Launch the DNS Manager GUI.  This can be done from the command
       line ("dnsmgmt.msc") or from the Service Manager (the "DNS"
       command in the "Tools" menu).

   2.  In the hierarchy under the server on which the service is
       running, right-click on the "Forward Lookup Zones", and select
       "New Zone".  This brings up a succession of dialog boxes.

   3.  In the "Zone Type" dialog box, select "Secondary zone".

   4.  In the "Zone Name" dialog box, enter ".".

   5.  In the "Master DNS Servers" dialog box, enter
       "b.root-servers.net".  The system validates that it can do a zone
       transfer from that server.  (After this configuration is
       completed, the DNS Manager will attempt to transfer from all of
       the root zone servers.)

   6.  In the "Completing the New Zone Wizard" dialog box, click
       "Finish".

   7.  Verify that the DNS Manager is acting as a recursive resolver.
       Right-click on the server name in the hierarchy, choosing the
       "Advanced" tab in the dialog box.  See that "Disable recursion
       (also disables forwarders)" is not selected and that "Enable
       DNSSEC validation for remote responses" is selected.

Acknowledgements

   The authors fully acknowledge that running a copy of the root zone on
   the loopback address is not a new concept and that we have chatted
   with many people about that idea over time.  For example, Bill
   Manning described a similar solution to the problems in his doctoral
   dissertation in 2013 [Manning2013].

   Evan Hunt contributed greatly to the logic in the requirements.
   Other significant contributors include Wouter Wijngaards, Tony Hain,
   Doug Barton, Greg Lindsay, and Akira Kato.  The authors also received
   many offline comments about making the document clear that this is
   just a description of a way to operate a root zone on the same host
   and not a recommendation to do so.

   People who contributed to this update to [RFC7706] include Florian
   Obser, nusenu, Wouter Wijngaards, Mukund Sivaraman, Bob Harold, and
   Leo Vegoda.

Authors' Addresses

   Warren Kumari
   Google

   Email: Warren@kumari.net


   Paul Hoffman
   ICANN

   Email: paul.hoffman@icann.org