Internet DRAFT - draft-ietf-dnsop-rfc8109bis
draft-ietf-dnsop-rfc8109bis
Network Working Group P. Koch
Internet-Draft DENIC eG
Obsoletes: 8109 (if approved) M. Larson
Intended status: Best Current Practice P. Hoffman
Expires: 17 August 2024 ICANN
14 February 2024
Initializing a DNS Resolver with Priming Queries
draft-ietf-dnsop-rfc8109bis-04
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, when published, obsoletes RFC 8109. See Section 1.1
for the list of changes from RFC 8109.
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
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 17 August 2024.
Copyright Notice
Copyright (c) 2024 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 (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
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extracted from this document must include Revised BSD License text as
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Changes from RFC 8109 . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Description of Priming . . . . . . . . . . . . . . . . . . . 4
2.1. Content of Priming Information . . . . . . . . . . . . . 5
3. Priming Queries . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Repeating Priming Queries . . . . . . . . . . . . . . . . 5
3.2. Target Selection . . . . . . . . . . . . . . . . . . . . 6
3.3. DNSSEC with Priming Queries . . . . . . . . . . . . . . . 6
4. Priming Responses . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Expected Properties of the Priming Response . . . . . . . 7
4.2. Completeness of the Response . . . . . . . . . . . . . . 7
5. Post-Priming Strategies . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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.
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This document only deals with recursive name servers (recursive
resolvers, resolvers) for the IN class.
1.1. Changes from RFC 8109
This document obsoletes [RFC8109]. The significant changes from RFC
8109 are:
* 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
less sexist 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 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 "Expected Properties of the Priming
Response" to MUST-level.
* Clarified that "currently" means at the time that this document is
published.
* Added a note about priming and RFC 8806.
* Added a reference to research about discontinued root server
addresses.
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1.2. 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.
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
finishes with a full list of their names and their 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. This list is usually correct and complete when shipped,
but may become out of date over time.
The domain names for the root servers are called the "root server
identifiers". This list has been stable since 1997, but the IPv4 and
IPv6 addresses for the root server identifiers sometimes change.
Research shows that after those addresses change, some resolvers
never get the new addresses; for example, see [OLD-J].
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 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 also could
apply to a private DNS as well. These terms are defined in
[RFC8499].
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Some systems serve a copy of the full root zone on the same server as
the resolver, such as is described in [RFC8806]. In such a setup,
the resolver primes its cache using the same methods as 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, sometimes as comments and sometimes as data consumed by
the software. For example, IANA's "Root Hints File" 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 to adding such information, it is not
useful in the root priming process.
3. Priming Queries
A priming query is a DNS query whose response provides root server
names 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]). 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 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 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.)
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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 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. At the time this document is published, all root
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 signed at
the time this document is published, these RRsets cannot be
validated.
A machine-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 for signed TLDs from
rogue root servers if they are different from the real responses
because the resolver has a trust anchor for the root and the answers
from the root are signed. Thus, if there is a machine-in-the-middle
attack on the priming query, the results for a validating resolver
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for signed TLDs could be a denial of service, or the attacker seeing
queries while returning good answers, but not the resolver's
accepting the bad responses; however, for unsigned TLDs, the attack
would be successful.
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 is published, there are 13 root server
operators operating a total of more than 1500 root server instances.
Each 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 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. At the time this document is published, these
RRsets would be authoritatively available from the root name servers.
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If some root server addresses are omitted from the Additional
section, there is no expectation that the TC bit in the response will
be set to 1. At the time that this document is written, many of the
root servers are not setting the TC bit when omitting addresses from
the Additional section.
Note that [RFC9471] updates [RFC1035] with respect to the use of the
TC bit. It says "If message size constraints prevent the inclusion
of all glue records for in-domain name servers, the server must set
the TC (Truncated) flag to inform the client that the response is
incomplete and that the client should use another transport to
retrieve the full response." 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 cache, and it gets 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 gives no preference to 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 Top-Level Domains (TLDs), because the root is
the highest node of the DNS. See Section 3.3 for more discussion.
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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.
7. IANA Considerations
This document does not require any 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>.
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[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>.
[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.
[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>.
8.2. Informative References
[OLD-J] Wessels, D., "Thirteen Years of 'Old J Root'", 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", 2016,
<https://www.icann.org/en/system/files/files/rssac-
023-17jun20-en.pdf>.
[RSSAC026v2]
"RSSAC Lexicon", 2020,
<https://www.icann.org/en/system/files/files/rssac-026-
lexicon-12mar20-en.pdf>.
Appendix A. Acknowledgements
RFC 8109 was the product of the DNSOP WG and benefitted from the
reviews done there. This document also benefitted from review by
Duane Wessels.
Authors' Addresses
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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
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