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Internet Engineering Task Force (IETF)                           P. KochRequest for Comments: 8109                                      DENIC eGBCP: 209                                                       M. LarsonCategory: Best Current Practice                               P. HoffmanISSN: 2070-1721                                                    ICANN                                                              March 2017Initializing a DNS Resolver with Priming QueriesAbstract   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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc8109.Copyright Notice   Copyright (c) 2017 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/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 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.Koch et al.               Best Current Practice                 [Page 1]

RFC 8109                   DNS Priming Queries                March 2017Table of Contents1. Introduction ....................................................22. Description of Priming ..........................................33. Priming Queries .................................................33.1. Repeating Priming Queries ..................................43.2. Target Selection ...........................................43.3. DNSSEC with Priming Queries ................................44. Priming Responses ...............................................54.1. Expected Properties of the Priming Response ................54.2. Completeness of the Response ...............................55. Security Considerations .........................................66. IANA Considerations .............................................67. Normative References ............................................6   Acknowledgements ...................................................7   Authors' Addresses .................................................71.  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",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [RFC2119].   This document only deals with recursive name servers (recursive   resolvers, resolvers) for the IN class.Koch et al.               Best Current Practice                 [Page 2]

RFC 8109                   DNS Priming Queries                March 20172.  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 Sections5.3.2 and5.3.3 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 (seeSection 4.2) for the current set of   root servers.  SeeSection 3.3 for discussion of setting the DNSSEC   OK (DO) bit (defined in [RFC4033]).Koch et al.               Best Current Practice                 [Page 3]

RFC 8109                   DNS Priming Queries                March 20173.1.  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   configuration.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   "root-servers.net" and the AAAA and A RRsets for the root server   names reside in the "root-servers.net" 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 "root-servers.net" 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 theKoch et al.               Best Current Practice                 [Page 4]

RFC 8109                   DNS Priming Queries                March 2017   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 "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.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   response.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.Koch et al.               Best Current Practice                 [Page 5]

RFC 8109                   DNS Priming Queries                March 20175.  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.  SeeSection 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, <http://www.rfc-editor.org/info/rfc1034>.   [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>.   [RFC3226]   Gudmundsson, O., "DNSSEC and IPv6 A6 aware               server/resolver message size requirements",RFC 3226,               DOI 10.17487/RFC3226, December 2001,               <http://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,               <http://www.rfc-editor.org/info/rfc4033>.Koch et al.               Best Current Practice                 [Page 6]

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,               <http://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,               <http://www.rfc-editor.org/info/rfc6891>.Acknowledgements   This document is the product of the DNSOP WG and benefitted from the   reviews done there.Authors' Addresses   Peter Koch   DENIC eG   Kaiserstrasse 75-77   Frankfurt  60329   Germany   Phone: +49 69 27235 0   Email: pk@DENIC.DE   Matt Larson   ICANN   Email: matt.larson@icann.org   Paul Hoffman   ICANN   Email: paul.hoffman@icann.orgKoch et al.               Best Current Practice                 [Page 7]