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IETF RFC 8109
Last modified on Thursday, March 16th, 2017
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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
 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.
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
http://www.rfc-editor.org/info/RFC 8109.
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
(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 2017
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.
[RFC 1034] 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. [RFC 1034] 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 [RFC 1034]. 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 [RFC 2119].
This document only deals with recursive name servers (recursive
resolvers, resolvers) for the IN class.
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RFC 8109 DNS Priming Queries March 2017
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 [RFC 1034]. 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 [RFC 5452]). 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) [RFC 6891] for priming
queries and SHOULD announce and handle a reassembly size of at least
1024 octets [RFC 3226]. 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 [RFC 4033]).
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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
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 the
Koch 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 [RFC 1035].
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 2017
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
[RFC 1034] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, RFC 1034, DOI 10.17487/RFC 1034,
November 1987, <http://www.rfc-editor.org/info/RFC 1034>.
[RFC 1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC 1035,
November 1987, <http://www.rfc-editor.org/info/RFC 1035>.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 3226] Gudmundsson, O., "DNSSEC and IPv6 A6 aware
server/resolver message size requirements", RFC 3226,
DOI 10.17487/RFC 3226, December 2001,
<http://www.rfc-editor.org/info/RFC 3226>.
[RFC 4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC 4033, March 2005,
<http://www.rfc-editor.org/info/RFC 4033>.
Koch et al. Best Current Practice PAGE 6
RFC 8109 DNS Priming Queries March 2017
[RFC 5452] Hubert, A. and R. van Mook, "Measures for Making DNS More
Resilient against Forged Answers", RFC 5452,
DOI 10.17487/RFC 5452, January 2009,
<http://www.rfc-editor.org/info/RFC 5452>.
[RFC 6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC 6891, April 2013,
<http://www.rfc-editor.org/info/RFC 6891>.
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.org
Koch et al. Best Current Practice PAGE 7
RFC TOTAL SIZE: 15645 bytes
PUBLICATION DATE: Thursday, March 16th, 2017
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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