Internet Engineering Task Force (IETF)                           P. Koch
Request for Comments: 8109                                      DENIC eG
BCP: 209                                                       M. Larson
Category: Best Current Practice                               P. Hoffman
ISSN: 2070-1721                                                    ICANN
                                                              March 2017

            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.

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

Copyright Notice

   Copyright (c) 2017 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
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1. Introduction ....................................................2
   2. Description of Priming ..........................................3
   3. Priming Queries .................................................3
      3.1. Repeating Priming Queries ..................................4
      3.2. Target Selection ...........................................4
      3.3. DNSSEC with Priming Queries ................................4
   4. Priming Responses ...............................................5
      4.1. Expected Properties of the Priming Response ................5
      4.2. Completeness of the Response ...............................5
   5. Security Considerations .........................................6
   6. IANA Considerations .............................................6
   7. Normative References ............................................6
   Acknowledgements ...................................................7
   Authors' Addresses .................................................7

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.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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

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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.  A recursive resolver starts with
   no information about the root servers, and ends up with a list of
   their names and their addresses.

   Priming is described in Sections 5.3.2 and 5.3.3 of [RFC1034].  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 list of root server operators and the domain name associated with
   each one has been stable since 1997.  However, there are address
   changes for the root server domain names, both for IPv4 and IPv6
   addresses.  However, research shows that after those addresses
   change, some resolvers never get the new addresses.  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 change is the main reason that resolvers need to do
   priming instead of just going from a configured list to get a full
   and accurate list of root servers.

3.  Priming Queries

   A priming query is a DNS query used to get the root server
   information in a resolver.  It has a QNAME of "." and a QTYPE of NS,
   and 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 EDNS(0) [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]).

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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 and when
   the NS RRset for the root zone has expired.  Because the NS records
   for the root 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 priming query does not get a response, the recursive resolver
   needs to 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 domain names,
   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 resolver MAY set the DNSSEC OK (DO) bit.  At the time of
   publication, there is little use to performing DNSSEC validation on
   the priming query.  Currently, all root name server names end in
   "" and the AAAA and A RRsets for the root server
   names reside in the "" zone.  All root servers are
   also authoritative for this zone, allowing priming responses to
   include the appropriate root name server A and AAAA RRsets.  But,
   because the "" zone is not currently signed, these
   RRsets cannot be validated.

   A man-in-the-middle attack on the priming query could direct a
   resolver to a rogue root name server.  Note, however, that a
   validating resolver will not accept responses from rogue root name
   servers if they are different from the real responses because the

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   resolver has a trust anchor for the root and the answers from the
   root are signed.  Thus, if there is a man-in-the-middle attack on the
   priming query, the only result for a validating resolver will be a
   denial of service, not the resolver's accepting the bad responses.

   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.

4.  Priming Responses

   A priming query is a normal DNS query.  Thus, a root name 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 is expected to have an RCODE of NOERROR, and to
   have the Authoritative Answer (AA) bit set.  Also, it is expected to
   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 name 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 exactly 13 NS RRs
   because, historically, some root servers have returned fewer.

4.2.  Completeness of the Response

   There are currently 13 root servers.  All have one IPv4 address and
   one IPv6 address.  Not even counting the NS RRset, 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.  Currently, these RRsets would be authoritatively
   available from the root name servers.

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5.  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.

6.  IANA Considerations

   This document does not require any IANA actions.

7.  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,

   [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,

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RFC 8109                   DNS Priming Queries                March 2017

   [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,


   This document is the product of the DNSOP WG and benefitted from the
   reviews done there.

Authors' Addresses

   Peter Koch
   Kaiserstrasse 75-77
   Frankfurt  60329

   Phone: +49 69 27235 0
   Email: pk@DENIC.DE

   Matt Larson


   Paul Hoffman


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