Initializing a DNS Resolver with Priming Queries
draft-klh-dnsop-rfc8109bis-03
Network Working Group P. Koch
Internet-Draft DENIC eG
Obsoletes: 8109 (if approved) M. Larson
Intended status: Best Current Practice P. Hoffman
Expires: 12 May 2022 ICANN
8 November 2021
Initializing a DNS Resolver with Priming Queries
draft-klh-dnsop-rfc8109bis-03
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.
Status of This Memo
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This Internet-Draft will expire on 12 May 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Changes from RFC 8109 . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Description of Priming . . . . . . . . . . . . . . . . . . . 4
2.1. Content of Priming Information . . . . . . . . . . . . . 4
3. Priming Queries . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Repeating Priming Queries . . . . . . . . . . . . . . . . 5
3.2. Target Selection . . . . . . . . . . . . . . . . . . . . 5
3.3. DNSSEC with Priming Queries . . . . . . . . . . . . . . . 6
4. Priming Responses . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Expected Properties of the Priming Response . . . . . . . 7
4.2. Completeness of the Response . . . . . . . . . . . . . . 7
5. Post-Priming Strategies . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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
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(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) 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.
* 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 informative references to RSSAC documents.
* Added short discussion about this document and private DNS.
* Future changes noted in the current text with [[ text like this
]].
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.
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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]. 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. 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 do priming instead of just going from a configured list to
get a full and accurate list of root servers.
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].
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 he 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 master file format,
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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 used to get the root server
information in a resolver. 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 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.)
[[ Need to discuss: when pre-fetching, does the resolver send the
queries to the addresses associated with the . / NS RRset in the
cache, or does the resolver go back to the addresses in the
configuration? ]]
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 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.
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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
[[ This section talks about sending the DO bit, but does not actually
talk about validating the response to the priming query. This became
important after the root KSK rollover in 2018 because some resolvers
apparently were validating and only had the old KSK, but were still
sending RFC 8145 telemetry even after failing to validate their
priming response. ]]
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 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 from rogue root name
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 could be a
denial of service, or the attacker seeing queries while returning
good answers, but 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.
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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.
If the Additional section is truncated, 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 on responses with a truncated Additional section.
5. Post-Priming Strategies
[[ Describe some common post-priming strategies for picking which RSI
to use for queries sent to the root, such as "always use the
fastest", "create buckets of fastness and pick randomly in the
buckets", and others. ]]
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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.
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>.
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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,
<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>.
[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>.
8.2. Informative References
[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.
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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