DNS priming queries: Initializing a DNS Resolver with Priming Queries

Internet Engineering Task Force (IETF)                           P. Koch
Request for Comments: 9609                                      DENIC eG
BCP: 209                                                       M. Larson
Obsoletes: 8109                                               P. Hoffman
Category: Best Current Practice                                    ICANN
ISSN: 2070-1721                                            February 2025


            Initializing a DNS Resolver with Priming Queries

Abstract

   This document describes the queries that a DNS resolver should emit
   to initialize its cache.  The result is that the resolver gets both a
   current NS resource record set (RRset) for the root zone and the
   necessary address information for reaching the root servers.

   This document obsoletes RFC 8109.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   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).  Further information on
   BCPs is available in 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/rfc9609.

Copyright Notice

   Copyright (c) 2025 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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   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Terminology
   2.  Description of Priming
     2.1.  Content of Priming Information
   3.  Priming Queries
     3.1.  Repeating Priming Queries
     3.2.  Target Selection
     3.3.  DNSSEC with Priming Queries
   4.  Priming Responses
     4.1.  Expected Properties of the Priming Response
     4.2.  Completeness of the Response
   5.  Post-Priming Strategies
   6.  Security Considerations
   7.  IANA Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Appendix A.  Changes from RFC 8109
   Acknowledgements
   Authors' Addresses

1.  Introduction

   Recursive DNS resolvers need a starting point to resolve queries.
   [RFC1034] describes a common scenario for recursive resolvers: They
   begin with an empty cache and some configuration for finding the
   names and addresses of the DNS root servers.  [RFC1034] describes
   that configuration as a list of servers that will give authoritative
   answers to queries about the root.  This has become a common
   implementation choice for recursive resolvers and is the topic of
   this document.

   This document describes the steps needed for this common
   implementation choice.  Note that this is not the only way to start a
   recursive name server with an empty cache, but it is the only one
   described in [RFC1034].  Some implementers have chosen other
   directions, some of which work well and others of which fail
   (sometimes disastrously) under different conditions.  For example, an
   implementation that only gets the addresses of the root name servers
   from configuration, not from the DNS as described in this document,
   will have stale data that could cause slower resolution.

   This document only deals with recursive name servers (also called
   "recursive resolvers" and just "resolvers") for the IN class.

   See Appendix A for the list of changes from [RFC8109].

1.1.  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.

   See [RSSAC026v2] for terminology that relates to the root server
   system.  See [RFC9499] for terminology that relates to the DNS in
   general.

2.  Description of Priming

   Priming is the act of finding the list of root servers from a
   configuration that lists some or all of the purported IP addresses of
   some or all of those root servers.  In priming, a recursive resolver
   starts with no cached information about the root servers, and it
   finishes with a full list of their names and addresses in its cache.

   Priming is described in Sections 5.3.2 and 5.3.3 of [RFC1034].  (It
   is called "SBELT", a "safety belt" structure, in that document.)  The
   scenario used in that description, that of a recursive server that is
   also authoritative, is no longer as common.

   The configured list of IP addresses for the root servers usually
   comes from the vendor or distributor of the recursive server
   software.  Although this list is generally accurate and complete at
   the time of distribution, it may become outdated over time.

   The domain names for the root servers are called the "root server
   identifiers".  Although this list has remained stable since 1997, the
   associated IPv4 and IPv6 addresses for these root server identifiers
   occasionally change.  Research indicates that, following such
   changes, certain resolvers fail to update to the new addresses; for
   further details, refer to [OLD-J].

   Therefore, it is important that resolvers are able to cope with
   change, even without relying upon configuration updates to be applied
   by their operator.  Root server identifier and address changes are
   the main reasons that resolvers need to use priming to get a full and
   accurate list of root servers, instead of just using a statically
   configured list.

   See [RSSAC023v2] for a history of the root server system.

   Although this document is targeted at the global DNS, it could apply
   to a private DNS as well.  These terms are defined in [RFC9499].

   Some systems serve a copy of the full root zone on the same server as
   the resolver, e.g., as described in [RFC8806].  In such a setup, the
   resolver primes its cache using the same methods as those described
   in the rest of this document.

2.1.  Content of Priming Information

   As described above, the configuration for priming is a list of IP
   addresses.  The priming information in software may be in any format
   that gives the software the addresses associated with at least some
   of the root server identifiers.

   Some software has configuration that also contains the root server
   identifiers (such as "L.ROOT-SERVERS.NET"), sometimes as comments and
   sometimes as data consumed by the software.  For example, the "root
   hints file" published by IANA at <https://www.internic.net/domain/
   named.root> is derived directly from the root zone and contains all
   of the addresses of the root server identifiers found in the root
   zone.  It is in DNS zone file presentation format and includes the
   root server identifiers.  Although there is no harm in adding these
   names, they are not useful in the priming process.

3.  Priming Queries

   A priming query is a DNS query whose response provides root server
   identifiers and addresses.  It has a QNAME of ".", a QTYPE of NS, and
   a QCLASS of IN; it is sent to one of the addresses in the
   configuration for the recursive resolver.  The priming query can be
   sent over either UDP or TCP.  If the query is sent over UDP, the
   source port SHOULD be randomly selected (see [RFC5452]) to hamper on-
   path attacks.  DNS cookies [RFC7873] can also be used to hamper on-
   path attacks.  The Recursion Desired (RD) bit SHOULD be set to 0.
   The meaning when RD is set to 1 is undefined for priming queries and
   is outside the scope of this document.

   The recursive resolver SHOULD use EDNS0 [RFC6891] for priming queries
   and SHOULD announce and handle a reassembly size of at least 1024
   octets [RFC3226].  Doing so allows responses that cover the size of a
   full priming response (see Section 4.2) for the current set of root
   servers.  See Section 3.3 for discussion of setting the DNSSEC OK
   (DO) bit (defined in [RFC4033]).

3.1.  Repeating Priming Queries

   The recursive resolver SHOULD send a priming query only when it is
   needed, such as when the resolver starts with an empty cache or when
   the NS resource record set (RRset) for the root zone has expired.
   Because the NS records for the root zone are not special, the
   recursive resolver expires those NS records according to their TTL
   values.  (Note that a recursive resolver MAY pre-fetch the NS RRset
   before it expires.)

   If a resolver chooses to pre-fetch the root NS RRset before that
   RRset has expired in its cache, it needs to choose whether to use the
   addresses for the root NS RRset that it already has in its cache or
   to use the addresses it has in its configuration.  Such a resolver
   SHOULD send queries to the addresses in its cache in order to reduce
   the chance of delay due to out-of-date addresses in its
   configuration.

   If a priming query does not get a response, the recursive resolver
   MUST retry the query with a different target address from the
   configuration.

3.2.  Target Selection

   In order to spread the load across all the root server identifiers,
   the recursive resolver SHOULD select the target for a priming query
   randomly from the list of addresses.  The recursive resolver might
   choose either IPv4 or IPv6 addresses based on its knowledge of
   whether the system on which it is running has adequate connectivity
   on either type of address.

   Note that this recommended method is not the only way to choose from
   the list in a recursive resolver's configuration.  Two other common
   methods include picking the first from the list, and remembering
   which address in the list gave the fastest response earlier and using
   that one.  There are probably other methods in use today.  However,
   the random method SHOULD be used for priming.

3.3.  DNSSEC with Priming Queries

   The root NS RRset is signed and can be validated by a DNSSEC
   validating resolver.  At the time this document is published, the
   addresses for the names in the root NS RRset are in the "root-
   servers.net" zone.  All root servers are also authoritative for the
   "root-servers.net" zone, which allows priming responses to include
   the appropriate root name server A and AAAA RRsets.  However, because
   at the time this document is published the "root-servers.net" zone is
   not signed, the root name server A and AAAA RRsets cannot be
   validated.  An attacker that is able to provide a spoofed priming
   response can provide alternative A and AAAA RRsets and thus fool a
   resolver into considering addresses under the control of the attacker
   to be authoritative for the root zone.

   A rogue root name server can view all queries from the resolver to
   the root and alter all unsigned parts of responses, such as the
   parent-side NS RRsets and glue in referral responses.  A resolver can
   be fooled into trusting child (Top-Level Domain (TLD)) NS addresses
   that are under the control of the attacker as being authoritative if
   the resolver:

   *  follows referrals from a rogue root server,

   *  and does not explicitly query the authoritative NS RRset at the
      apex of the child (TLD) zone,

   *  and does not explicitly query for the authoritative A and AAAA
      RRsets for the child (TLD) NS RRsets.

   With such resolvers, an attacker that controls a rogue root server
   effectively controls the entire domain name space and can view all
   queries and alter all unsigned data undetected unless other
   protections are configured at the resolver.

   An attacker controlling a rogue root name server also has complete
   control over all unsigned delegations and over the entire domain name
   space in the case of non-DNSSEC validating resolvers.

   If the "root-servers.net" zone is later signed or if the root servers
   are named in a different zone and that zone is signed, having DNSSEC
   validation for the priming queries might be valuable.  The benefits
   and costs of resolvers validating the responses will depend heavily
   on the naming scheme used.

4.  Priming Responses

   A priming query is a normal DNS query.  Thus, a root server cannot
   distinguish a priming query from any other query for the root NS
   RRset.  Thus, the root server's response will also be a normal DNS
   response.

4.1.  Expected Properties of the Priming Response

   The priming response MUST have an RCODE of NOERROR and MUST have the
   Authoritative Answer (AA) bit set.  Also, it MUST have an NS RRset in
   the Answer section (because the NS RRset originates from the root
   zone) and an empty Authority section (because the NS RRset already
   appears in the Answer section).  There will also be an Additional
   section with A and/or AAAA RRsets for the root servers pointed at by
   the NS RRset.

   Resolver software SHOULD treat the response to the priming query as a
   normal DNS response, just as it would use any other data fed to its
   cache.  Resolver software SHOULD NOT expect 13 NS RRs because,
   historically, some root servers have returned fewer.

4.2.  Completeness of the Response

   At the time this document is published, there are 13 root server
   operators operating a total of more than 1500 root server instances.
   Each instance has one IPv4 address and one IPv6 address.  The
   combined size of all the A and AAAA RRsets exceeds the original
   512-octet payload limit specified in [RFC1035].

   In the event of a response where the Additional section omits certain
   root server address information, reissuing of the priming query does
   not help with those root name servers that respond with a fixed order
   of addresses in the Additional section.  Instead, the recursive
   resolver needs to issue direct queries for A and AAAA RRsets for the
   remaining names.  At the time this document is published, these
   RRsets would be authoritatively available from the root name servers.

   If some root server addresses are omitted from the Additional
   section, there is no expectation that the TC bit in the response will
   be set to 1.  At the time this document is written, many of the root
   servers are not setting the TC bit when omitting addresses from the
   Additional section.

   Note that [RFC9471] updates [RFC1034] with respect to the use of the
   TC bit.  It says

   |  If message size constraints prevent the inclusion of all glue
   |  records for in-domain name servers over the chosen transport, the
   |  server MUST set the TC (Truncated) flag to inform the client that
   |  the response is incomplete and that the client SHOULD use another
   |  transport to retrieve the full response.

   Because the priming response is not a referral, root server addresses
   in the priming response are not considered glue records.  Thus,
   [RFC9471] does not apply to the priming response and root servers are
   not required to set the TC bit if not all root server addresses fit
   within message size constraints.  There are no requirements on the
   number of root server addresses that a root server must include in a
   priming response.

5.  Post-Priming Strategies

   When a resolver has a zone's NS RRset in its cache and it receives a
   query for a domain in that zone that cannot be answered from its
   cache, the resolver has to choose which NS to send queries to.  (This
   statement is as true for the root zone as for any other zone in the
   DNS.)  Two common strategies for choosing are "determine the fastest
   name server and always use it" and "create buckets of fastness and
   pick randomly in the buckets".  This document does not specify a
   preference for any particular strategy other than to suggest that
   resolvers not treat the root zone as special for this decision.

6.  Security Considerations

   Spoofing a response to a priming query can be used to redirect all of
   the queries originating from a victim recursive resolver to one or
   more servers for the attacker.  Until the responses to priming
   queries are protected with DNSSEC, there is no definitive way to
   prevent such redirection.

   An on-path attacker who sees a priming query coming from a resolver
   can inject false answers before a root server can give correct
   answers.  If the attacker's answers are accepted, this can set up the
   ability to give further false answers for future queries to the
   resolver.  False answers for root servers are more dangerous than,
   say, false answers for TLDs, because the root is the highest node of
   the DNS.  See Section 3.3 for more discussion.

   In both of the scenarios listed here, a validating resolver will be
   able to detect the attack if its chain of queries comes for a zone
   that is signed, but not for those that are unsigned.

7.  IANA Considerations

   This document has no IANA actions.

8.  References

8.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [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>.

   [RFC3226]  Gudmundsson, O., "DNSSEC and IPv6 A6 aware server/resolver
              message size requirements", RFC 3226,
              DOI 10.17487/RFC3226, December 2001,
              <https://www.rfc-editor.org/info/rfc3226>.

   [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>.

   [RFC5452]  Hubert, A. and R. van Mook, "Measures for Making DNS More
              Resilient against Forged Answers", RFC 5452,
              DOI 10.17487/RFC5452, January 2009,
              <https://www.rfc-editor.org/info/rfc5452>.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,
              <https://www.rfc-editor.org/info/rfc6891>.

   [RFC7873]  Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
              Cookies", RFC 7873, DOI 10.17487/RFC7873, May 2016,
              <https://www.rfc-editor.org/info/rfc7873>.

   [RFC8109]  Koch, P., Larson, M., and P. Hoffman, "Initializing a DNS
              Resolver with Priming Queries", BCP 209, RFC 8109,
              DOI 10.17487/RFC8109, March 2017,
              <https://www.rfc-editor.org/info/rfc8109>.

   [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>.

   [RFC9471]  Andrews, M., Huque, S., Wouters, P., and D. Wessels, "DNS
              Glue Requirements in Referral Responses", RFC 9471,
              DOI 10.17487/RFC9471, September 2023,
              <https://www.rfc-editor.org/info/rfc9471>.

   [RFC9499]  Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
              RFC 9499, DOI 10.17487/RFC9499, March 2024,
              <https://www.rfc-editor.org/info/rfc9499>.

8.2.  Informative References

   [OLD-J]    Wessels, D., Castonguay, J., and P. Barber, "Thirteen
              Years of 'Old J Root'", DNS-OARC Fall 2015 Workshop,
              October 2015,
              <https://indico.dns-oarc.net/event/24/contributions/378/>.

   [RFC8806]  Kumari, W. and P. Hoffman, "Running a Root Server Local to
              a Resolver", RFC 8806, DOI 10.17487/RFC8806, June 2020,
              <https://www.rfc-editor.org/info/rfc8806>.

   [RSSAC023v2]
              "History of the Root Server System", A Report from the
              ICANN Root Server System Advisory Committee (RSSAC),
              RSSAC023v2, June 2020,
              <https://www.icann.org/en/system/files/files/rssac-
              023-17jun20-en.pdf>.

   [RSSAC026v2]
              "RSSAC Lexicon", An Advisory from the ICANN Root Server
              System Advisory Committee (RSSAC), RSSAC026v2, March 2020,
              <https://www.icann.org/en/system/files/files/rssac-026-
              lexicon-12mar20-en.pdf>.

Appendix A.  Changes from RFC 8109

   This document obsoletes [RFC8109].  The significant changes from RFC
   8109 are as follows:

   *  Added section on the content of priming information.

   *  Added paragraph about no expectation that the TC bit in responses
      will be set.

   *  Added paragraph about RFC 9471 and requirements on authoritative
      servers and the TC bit.  This clarified the role of glue records
      and truncation for responses from the root zone.

   *  Changed "man-in-the-middle" to "machine-in-the-middle" to be both
      more inclusive and more technically accurate.

   *  Clarified that there are other effects of machine-in-the-middle
      attacks.

   *  Clarified language for root server domain names as "root server
      identifiers".

   *  Added short discussion of post-priming strategies.

   *  Added informative references to Root Server System Advisory
      Committee (RSSAC) documents.

   *  Added short discussion about this document and private DNS.

   *  Clarified that machine-in-the-middle attacks could be successful
      for non-signed TLDs.

   *  Added discussion of where resolvers that pre-fetch should get the
      root NS addresses.

   *  Elevated the expectations in Section 4.1 ("Expected Properties of
      the Priming Response") to MUST-level.

   *  Clarified that "currently" means "at the time this document is
      published".

   *  Added a note about priming and RFC 8806.

   *  Added a reference to research about discontinued root server
      addresses.

Acknowledgements

   RFC 8109 was the product of the DNSOP WG and benefited from the
   reviews done there.  This document also benefited from review by
   Duane Wessels.

Authors' Addresses

   Peter Koch
   DENIC eG
   Email: pk@DENIC.DE


   Matt Larson
   ICANN
   Email: matt.larson@icann.org


   Paul Hoffman
   ICANN
   Email: paul.hoffman@icann.org