Internet Engineering Task Force (IETF)                         W. Kumari
Request for Comments: 7706                                        Google
Category: Informational                                       P. Hoffman
ISSN: 2070-1721                                                    ICANN
                                                           November 2015


   Decreasing Access Time to Root Servers by Running One on Loopback

Abstract

   Some DNS recursive resolvers have longer-than-desired round-trip
   times to the closest DNS root server.  Some DNS recursive resolver
   operators want to prevent snooping of requests sent to DNS root
   servers by third parties.  Such resolvers can greatly decrease the
   round-trip time and prevent observation of requests by running a copy
   of the full root zone on a loopback address (such as 127.0.0.1).
   This document shows how to start and maintain such a copy of the root
   zone that does not pose a threat to other users of the DNS, at the
   cost of adding some operational fragility for the operator.

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 a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7706.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Notation . . . . . . . . . . . . . . . . . .   4
   2.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Operation of the Root Zone on the Loopback Address  . . . . .   5
   4.  Using the Root Zone Server on the Loopback Address  . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Current Sources of the Root Zone . . . . . . . . . .   8
   Appendix B.  Example Configurations of Common Implementations . .   8
     B.1.  Example Configuration: BIND 9.9 . . . . . . . . . . . . .   9
     B.2.  Example Configuration: Unbound 1.4 and NSD 4  . . . . . .  10
     B.3.  Example Configuration: Microsoft Windows Server 2012  . .  11
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   DNS recursive resolvers have to provide answers to all queries from
   their customers, even those for domain names that do not exist.  For
   each queried name that has 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.  Typically, the vast majority of queries
   going to the root are for names that do not exist in the root zone,
   and the negative answers are cached for a much shorter period of
   time.  A slow path between the recursive resolver and the closest
   root server has a negative effect on the resolver's customers.

   Recursive resolvers currently send queries for all TLDs that are not
   in their caches to root servers, even though most of those queries
   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 one or more of the DNS roots.

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

   The primary goals of this design are to provide faster negative
   responses to stub resolver queries that contain junk queries, and to
   prevent queries and responses from being visible on the network.
   This design will probably have little effect on getting faster
   positive responses to stub resolver for good queries on TLDs, because
   the data for those zones is usually long-lived and already in the
   cache of the recursive resolver; thus, getting faster positive
   responses is a non-goal of this design.

   This design explicitly only allows the new root zone server to be run
   on a loopback address, in order to prevent the server from serving
   authoritative answers to any system other than the recursive
   resolver.

   It is important to note that the design being described here is not
   considered a "best practice".  In fact, many people feel that it is
   an excessively risky practice because it introduces a new operational
   piece to local DNS operations where there was not one before.  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 requires the addition of authoritative name server
   software running on the same machine as the recursive resolver.
   Thus, recursive resolver software such as BIND will not need to add
   much new functionality, but recursive resolver software such as
   Unbound will need to be able to talk to an authoritative server (such
   as NSD) running on the same host.

   Because of the significant operational risks described in this
   document, distributions of recursive DNS servers MUST NOT include
   configuration for the design described here.  It is acceptable to
   point to this document, but not to indicate that this configuration
   is something that should be considered without reading the entire
   document.

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

1.1.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  Requirements

   In order to implement the mechanism described in this document:

   o  The system MUST be able to validate a zone with DNSSEC [RFC4033].

   o  The system MUST have an up-to-date copy of the DNS root key.

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

   o  The system MUST be able to run an authoritative server on one of
      the IPv4 loopback addresses (that is, an address in the range
      127/8 for IPv4 or ::1 in IPv6).

   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 Loopback Address

   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.

   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 server with the root zone on a loopback
       address that is not in use.  For IPv4, this would typically be
       127.0.0.1, but if that address is in use, any address in 127/8 is
       acceptable.  For IPv6, this would be ::1.

   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.  If the
   contents of the zone cannot be refreshed before the expire time, the
   server MUST return a SERVFAIL error response for all queries until
   the zone can be successfully be set up again.

   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.  One way to do this is to have a separate
   process that periodically checks the SOA of the root zone from 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, 2015 was 2015010201.  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.  Using the Root Zone Server on the Loopback Address

   A recursive resolver that wants to use a root zone server operating
   as described in Section 3 simply specifies the local address as the
   place to look when it is looking for information from the root.  All
   responses from the root server must be validated using DNSSEC.

   Note that using this configuration will cause the recursive resolver
   to fail if the local root zone server fails.  See Appendix B for more
   discussion of this for specific software.

   To test the proper operation of the recursive resolver with the local
   root server, use a DNS client to send a query for the SOA of the root
   to the recursive server.  Make sure the response that comes back has
   the AA bit in the message header set to 0.

5.  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 only allows the new
   root zone server to be run on a loopback address, in order to prevent
   the server from serving authoritative answers to any system other
   than the recursive resolver.  This has the security property of
   limiting damage to any other system that might try to rely on an
   altered copy of the root.

6.  References

6.1.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc4033>.

6.2.  Informative References

   [AggressiveNSEC]
              Fujiwara, K. and A. Kato, "Aggressive use of NSEC/NSEC3",
              Work in Progress, draft-fujiwara-dnsop-nsec-
              aggressiveuse-02, October 2015.

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

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) over TCP from DNS
   servers at xfr.lax.dns.icann.org and xfr.cjr.dns.icann.org.

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

   o  b.root-servers.net

   o  c.root-servers.net

   o  f.root-servers.net

   o  g.root-servers.net

   o  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.

   To repeat the requirement from earlier in this document: if the
   contents of the zone cannot be refreshed before the expire time, the
   server MUST return a SERVFAIL error response for all queries until
   the zone can be successfully be set up again.

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

   The examples here use a loopback address of 127.12.12.12, but typical
   installations will use 127.0.0.1.  The different address is used in
   order to emphasize that the root server does not need to be on the
   device at "localhost".

B.1.  Example Configuration: BIND 9.9

   BIND 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.

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

   When slaving a zone, BIND 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 {
               192.228.79.201; # b.root-servers.net
               192.33.4.12;    # c.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
               2001:500:84::b; # b.root-servers.net
               2001:500:2f::f; # f.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 1.4 and NSD 4

   Unbound and NSD are separate software packages.  Because of this,
   there is no "fate-sharing" between the two servers in the following
   configurations.  That is, if the root server instance (NSD) dies, the
   recursive resolver instance (Unbound) will probably keep running but
   will not be able to resolve any queries for the root zone.
   Therefore, the administrator of this configuration might want to
   carefully monitor the NSD instance and restart it immediately if it
   dies.

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

   # Configuration for Unbound
   server:
       do-not-query-localhost: no
   stub-zone:
       name: "."
       stub-prime: no
       stub-addr: 127.12.12.12

   # Configuration for NSD
   server:
       ip-address: 127.12.12.12
   zone:
       name: "."
       request-xfr: 192.228.79.201 NOKEY # b.root-servers.net
       request-xfr: 192.33.4.12 NOKEY    # c.root-servers.net
       request-xfr: 192.5.5.241 NOKEY    # f.root-servers.net
       request-xfr: 192.112.36.4 NOKEY   # g.root-servers.net
       request-xfr: 193.0.14.129 NOKEY   # k.root-servers.net
       request-xfr: 192.0.47.132 NOKEY   # xfr.cjr.dns.icann.org
       request-xfr: 192.0.32.132 NOKEY   # xfr.lax.dns.icann.org
       request-xfr: 2001:500:84::b NOKEY # b.root-servers.net
       request-xfr: 2001:500:2f::f NOKEY # f.root-servers.net
       request-xfr: 2001:7fd::1 NOKEY    # k.root-servers.net
       request-xfr: 2620:0:2830:202::132 NOKEY  # xfr.cjr.dns.icann.org
       request-xfr: 2620:0:2d0:202::132 NOKEY  # xfr.lax.dns.icann.org

B.3.  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.  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 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 but to a very different problem
   (intermittent connectivity, instead of constant but slow
   connectivity) 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 localhost, and
   not a recommendation to do so.

Authors' Addresses

   Warren Kumari
   Google

   Email: Warren@kumari.net


   Paul Hoffman
   ICANN

   Email: paul.hoffman@icann.org