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Versions: (draft-koch-dnsop-resolver-priming) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 8109

Network Working Group                                            P. Koch
Internet-Draft                                                  DENIC eG
Intended status: Best Current Practice                         M. Larson
Expires: September 20, 2016                                    Dyn, Inc.
                                                              P. Hoffman
                                                          March 19, 2016

            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 RRSet for the root zone and the necessary address
   information for reaching the root servers.

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
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   This Internet-Draft will expire on September 20, 2016.

Copyright Notice

   Copyright (c) 2016 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
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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.

   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.

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 address 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 NS records for those root server operators, both for

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   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.  This is the main reason that one needs 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 MAY be sent over UDP or TCP.
   If the query is sent over UDP, the source port SHOULD be randomly
   selected (see [RFC5452]).  The 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

3.1.  Repeating Priming Queries

   The recursive resolver SHOULD send a priming query only when it is
   needed.  This would be when the resolver starts with an empty cache,
   and when the NS RRset for the root zone has expired.  The recursive
   resolver SHOULD expire the NS records of the root servers according
   to the TTL values given in the priming response.  (Note that a
   recursive resolver MAY pre-fetch the NS RRset before it expires.)

   If a priming query does not get a response within 2 seconds, the
   recursive resolver SHOULD retry with a different target address from
   the configuration.

3.2.  Target Selection

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

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   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 is the one that is recommended for

3.3.  DNSSEC with Priming Queries

   The resolver MAY set the DNSSEC OK [RFC4033] bit.  At the time this
   document is being published, there is little use to performing DNSSEC
   validation on the priming query because the "root-servers.net" zone
   is not signed, and so a man-in-the-middle attack on the priming query
   can result in malicious data in the responses.  However, if the
   "root-servers.net" zone is later signed, or if the root server
   operators choose a different zone to identify themselves 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 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 may be an Additional section with A
   and/or AAAA RRSets for the root name servers pointed at by the NS

   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.

4.2.  Completeness of the Response

   There are currently 13 root servers.  Of those 13, all have one IPv4
   address, and 11 have an IPv6 address.  The combined size of all the A
   and AAAA RRSets is (13 * 16) + (11 * 32), or 560 bytes.  Not even
   counting the NS RRSet, this value exceeds the original 512 octet
   payload limit from [RFC1035].

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   For an EDNS response, a resolver SHOULD consider the address
   information found in the Additional section complete for any
   particular server that appears at all.  Said another way: in an EDNS
   response, if the additional section only has an A RRSet for a server,
   the resolver SHOULD assume that no AAAA RRSet exists.

   It is important to note that if the recursive resolver did not
   announce a reassembly size larger than 512 octets, this assumption is
   invalid.  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.

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.

6.  IANA Considerations


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, <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,

   [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,

Appendix A.  Acknowledgements

   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
   Dyn, Inc.
   150 Dow St
   Manchester, NH  03101

   Email: mlarson@dyn.com

   Paul Hoffman

   Email: paul.hoffman@icann.org

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