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Initializing a DNS Resolver with Priming Queries

Document Type Active Internet-Draft (dnsop WG)
Authors Peter Koch , Matt Larson , Paul E. Hoffman
Last updated 2024-07-17 (Latest revision 2024-06-05)
Replaces draft-klh-dnsop-rfc8109bis
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Best Current Practice
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Tim Wicinski
Shepherd write-up Show Last changed 2024-06-19
IESG IESG state In Last Call (ends 2024-07-29)
Action Holder
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Warren "Ace" Kumari
Send notices to
IANA IANA review state IANA OK - No Actions Needed
Network Working Group                                            P. Koch
Internet-Draft                                                  DENIC eG
Obsoletes: 8109 (if approved)                                  M. Larson
Intended status: Best Current Practice                        P. Hoffman
Expires: 7 December 2024                                           ICANN
                                                             5 June 2024

            Initializing a DNS Resolver with Priming Queries


   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, when published, obsoletes RFC 8109.  See Section 1.1
   for the list of changes from RFC 8109.

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

   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 7 December 2024.

Copyright Notice

   Copyright (c) 2024 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
   Provisions Relating to IETF Documents (
   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

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   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Changes from RFC 8109 . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Description of Priming  . . . . . . . . . . . . . . . . . . .   4
     2.1.  Content of Priming Information  . . . . . . . . . . . . .   5
   3.  Priming Queries . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Repeating Priming Queries . . . . . . . . . . . . . . . .   5
     3.2.  Target Selection  . . . . . . . . . . . . . . . . . . . .   6
     3.3.  DNSSEC with Priming Queries . . . . . . . . . . . . . . .   6
   4.  Priming Responses . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Expected Properties of the Priming Response . . . . . . .   7
     4.2.  Completeness of the Response  . . . . . . . . . . . . . .   8
   5.  Post-Priming Strategies . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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.

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   This document only deals with recursive name servers (recursive
   resolvers, resolvers) for the IN class.

1.1.  Changes from RFC 8109

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

   *  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
      less sexist and more technically accurate.

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

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

   *  Added short discussion of post-priming strategies.

   *  Added informative references to 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 "Expected Properties of the Priming
      Response" to MUST-level.

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

   *  Added a note about priming and RFC 8806.

   *  Added a reference to research about discontinued root server

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1.2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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

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
   finishes with a full list of their names and their addresses in its

   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.  This list is usually correct and complete when shipped,
   but may become out of date over time.

   The domain names for the root servers are called the "root server
   identifiers".  This list has been stable since 1997, but the IPv4 and
   IPv6 addresses for the root server identifiers sometimes change.
   Research shows that after those addresses change, some resolvers
   never get the new addresses; for example, see [OLD-J].

   Therefore, it is important that resolvers be 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 also could
   apply to a private DNS as well.  These terms are defined in

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   Some systems serve a copy of the full root zone on the same server as
   the resolver, such as is described in [RFC8806].  In such a setup,
   the resolver primes its cache using the same methods as 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, sometimes as comments and sometimes as data consumed by
   the software.  For example, IANA's "Root Hints File" at
   <> 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 to adding such information, it is not
   useful in the root priming process.

3.  Priming Queries

   A priming query is a DNS query whose response provides root server
   names 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]).  The Recursion Desired
   (RD) bit MAY be set to 0 or 1, although the meaning of it being set
   to 1 is undefined for priming queries.

   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 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.)

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   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

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

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 listed above 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-" zone.  All root servers are also authoritative for the
   "" 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 "" 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 (TLD) NS addresses that are under the
   control of the attacker as being authoritative if the resolver:

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   *  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.

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

   If the "" 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

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.

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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 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 from [RFC1035].

   In the event of a response where the Additional section omits certain
   root server address information, re-issuing 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 that 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 [RFC1035] 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, 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 cache, and it gets 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 gives no preference to any particular
   strategy other than to suggest that resolvers not treat the root zone
   as special for this decision.

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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 Top-Level Domains (TLDs), because the root is
   the highest node of the DNS.  See Section 3.3 for more discussion.

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

7.  IANA Considerations

   This document does not require any 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,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3226]  Gudmundsson, O., "DNSSEC and IPv6 A6 aware server/resolver
              message size requirements", RFC 3226,
              DOI 10.17487/RFC3226, December 2001,

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   [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,

   [RFC5452]  Hubert, A. and R. van Mook, "Measures for Making DNS More
              Resilient against Forged Answers", RFC 5452,
              DOI 10.17487/RFC5452, January 2009,

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,

   [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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", RFC 8499, DOI 10.17487/RFC8499, January
              2019, <>.

   [RFC9471]  Andrews, M., Huque, S., Wouters, P., and D. Wessels, "DNS
              Glue Requirements in Referral Responses", RFC 9471,
              DOI 10.17487/RFC9471, September 2023,

8.2.  Informative References

   [OLD-J]    Wessels, D., "Thirteen Years of 'Old J Root'", 2015,

   [RFC8806]  Kumari, W. and P. Hoffman, "Running a Root Server Local to
              a Resolver", RFC 8806, DOI 10.17487/RFC8806, June 2020,

              "History of the Root Server System", 2016,

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              "RSSAC Lexicon", 2020,

Appendix A.  Acknowledgements

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

Authors' Addresses

   Peter Koch
   Kaiserstrasse 75-77
   60329 Frankfurt
   Phone: +49 69 27235 0
   Email: pk@DENIC.DE

   Matt Larson

   Paul Hoffman

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