wdiff rfc9609v1.txt rfc9609.txt

Internet Engineering Task Force (IETF) P. Koch
Request for Comments: 9609 DENIC eG
BCP: 209 M. Larson
Obsoletes: 8109 P. Hoffman
Category: Best Current Practice ICANN
ISSN: 2070-1721 November December 2024

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.

This document obsoletes RFC 8109.

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
https://www.rfc-editor.org/info/rfc9609.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://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 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
1.1. Terminology
2. Description of Priming
2.1. Content of Priming Information
3. Priming Queries
3.1. Repeating Priming Queries
3.2. Target Selection
3.3. DNSSEC with Priming Queries
4. Priming Responses
4.1. Expected Properties of the Priming Response
4.2. Completeness of the Response
5. Post-Priming Strategies
6. Security Considerations
7. IANA Considerations
8. References
8.1. Normative References
8.2. Informative References
Appendix A. Changes from RFC 8109
Acknowledgements
Authors' Addresses

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.

This document only deals with recursive name servers (recursive
resolvers, resolvers) (also called
"recursive resolvers" and just "resolvers") for the IN class.

See Appendix A for the list of changes from [RFC8109].

1.1. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"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
system. See [RFC9499] for terminology that relates to the DNS in
general.

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

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. Although this list is generally accurate and complete at
the time of distribution, it may become outdated over time.

The domain names for the root servers are called the "root server
identifiers". Although this list has remained stable since 1997, the
associated IPv4 and IPv6 addresses for these root server identifiers
occasionally change. Research indicates that, following such
changes, certain resolvers fail to update to the new addresses; for
further details, refer to [OLD-J].

Therefore, it is important that resolvers are 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 could apply
to a private DNS as well. These terms are defined in [RFC9499].

Some systems serve a copy of the full root zone on the same server as
the resolver, e.g., as described in [RFC8806]. In such a setup, the
resolver primes its cache using the same methods as those 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 (such as "L.ROOT-SERVERS.NET"), sometimes as comments and
sometimes as data consumed by the software. For example, the "root
hints file" published by IANA at <https://www.internic.net/domain/
named.root> 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 in adding these
names, they are not useful in the priming process.

3. Priming Queries

A priming query is a DNS query whose response provides root server
identifiers 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]) to hamper on-
path attacks. DNS Cookies cookies [RFC7873] can also be used to hamper on-
path attacks. The Recursion Desired (RD) bit SHOULD be set to 0.
The meaning when RD is set to 1 is undefined for priming queries and
is outside the scope of this document.

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 resource record set (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.)

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
configuration.

If a priming query does not get a response, the recursive resolver
MUST 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 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 was published, the
addresses for the names in the root NS RRset are in the "root-
servers.net" zone. All root servers are also authoritative for the
"root-servers.net" zone, which allows priming responses to include
the appropriate root name server A and AAAA RRsets. However, because
at the time this document was published the "root-servers.net" 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 (Top-Level Domain (TLD)) NS addresses
that are under the control of the attacker as being authoritative if
the resolver:

* 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

* 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 unless other
protections are configured at the resolver.

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

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. 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
response.

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.

4.2. Completeness of the Response

At the time this document was published, there are 13 root server
operators operating a total of more than 1500 root server instances.
Each instance 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 specified in [RFC1035].

In the event of a response where the Additional section omits certain
root server address information, reissuing 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 was 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 (Truncated) bit in the response will
be set to 1. At the time this document was published, many of the
root servers are not setting the TC bit when omitting addresses from
the Additional section.

Note that [RFC9471] updates [RFC1035] [RFC1034] with respect to the use of the
TC bit. It says

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 its cache and it receives 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 does not specify a
preference for any particular strategy other than to suggest that
resolvers not treat the root zone as special for this decision.

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 TLDs, because the root is the highest node of
the DNS. See Section 3.3 for more discussion.

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

7. IANA Considerations

This document has no 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,
<https://www.rfc-editor.org/info/rfc1034>.

[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://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,
<https://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,
<https://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,
<https://www.rfc-editor.org/info/rfc4033>.

[RFC5452] Hubert, A. and R. van Mook, "Measures for Making DNS More
Resilient against Forged Answers", RFC 5452,
DOI 10.17487/RFC5452, January 2009,
<https://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,
<https://www.rfc-editor.org/info/rfc6891>.

[RFC7873] Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
Cookies", RFC 7873, DOI 10.17487/RFC7873, May 2016,
<https://www.rfc-editor.org/info/rfc7873>.

[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,
<https://www.rfc-editor.org/info/rfc8109>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC9471] Andrews, M., Huque, S., Wouters, P., and D. Wessels, "DNS
Glue Requirements in Referral Responses", RFC 9471,
DOI 10.17487/RFC9471, September 2023,
<https://www.rfc-editor.org/info/rfc9471>.

[RFC9499] Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
RFC 9499, DOI 10.17487/RFC9499, March 2024,
<https://www.rfc-editor.org/info/rfc9499>.

8.2. Informative References

[OLD-J] Wessels, D., Castonguay, J., and P. Barber, "Thirteen
Years of 'Old J Root'", DNS-OARC Fall 2015 Workshop,
October 2015,
<https://indico.dns-oarc.net/event/24/contributions/378/>.

[RFC8806] Kumari, W. and P. Hoffman, "Running a Root Server Local to
a Resolver", RFC 8806, DOI 10.17487/RFC8806, June 2020,
<https://www.rfc-editor.org/info/rfc8806>.

[RSSAC023v2]
"History of the Root Server System", A Report from the
ICANN Root Server System Advisory Committee (RSSAC),
RSSAC023v2, June 2020,
<https://www.icann.org/en/system/files/files/rssac-
023-17jun20-en.pdf>.

[RSSAC026v2]
"RSSAC Lexicon", An Advisory from the ICANN Root Server
System Advisory Committee (RSSAC), RSSAC026v2, March 2020,
<https://www.icann.org/en/system/files/files/rssac-026-
lexicon-12mar20-en.pdf>.

Appendix A. Changes from RFC 8109

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

* 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
more inclusive and more technically accurate.

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

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

* Added short discussion of post-priming strategies.

* Added informative references to Root Server System Advisory
Committee (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 Section 4.1 ("Expected Properties of
the Priming Response") to MUST-level.

* Clarified that "currently" means "at the time this document was
published".

* Added a note about priming and RFC 8806.

* Added a reference to research about discontinued root server
addresses.

Acknowledgements

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

Authors' Addresses

Peter Koch
DENIC eG
Kaiserstrasse 75-77
60329 Frankfurt
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