Internet DRAFT - draft-moura-dnsop-negative-cache-loop
draft-moura-dnsop-negative-cache-loop
DNSOP Working Group G.C.M. Moura
Internet-Draft SIDN Labs/TU Delft
Intended status: Standards Track W. Hardaker
Expires: 12 May 2022 J. Heidemann
USC/Information Sciences Institute
S. Castro
IE Domain Registry
8 November 2021
Negative Caching of Looping NS records
draft-moura-dnsop-negative-cache-loop-00
Abstract
This document updates guidance about detecting DNS loops in recursive
resolver algorithms with new requirements to require recursive
resolvers to detect loops and to implement negative caches.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3
2. Past solutions . . . . . . . . . . . . . . . . . . . . . . . 3
3. Current Problem . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Root Causes of Traffic Surge . . . . . . . . . . . . . . 4
4. New requirement . . . . . . . . . . . . . . . . . . . . . . . 5
5. Operational considerations . . . . . . . . . . . . . . . . . 6
6. Security considerations . . . . . . . . . . . . . . . . . . . 6
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 6
8. IANA considerations . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 7
Appendix B. Current implemenations . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
Loops are a well-known configuration error in DNS zones. CNAME loops
were first documented in [RFC1034], and can occur when two domains
point to each other. For example:
.org zone file:
example.org CNAME example.com
.com zone file:
example.com CNAME example.org
[RFC1536] states that "a set of servers might form a loop wherein A
refers to B and B refers to A". Although RFC1536 did not explicitly
define other types of loops, others can also occur using NS records,
as shown in the example below:
.org zone file:
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example.org NS ns1.example.com
example.org NS ns2.example.com
.com zone file:
example.com NS ns1.example.org
example.com NS ns2.example.org
In both the CNAME and NS loop cases, recursive resolvers will not be
able to resolve these domain names, or any child domains underneath
these zones.
1.1. Requirements notation
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.
2. Past solutions
The first solution was proposed in RFC1034, which states that CNAME
loops should be "signalled (sic) as an error" (Section 3.6.2). To
avoid resolvers starting to loop infinitely in presence of
configuration errors, RFC1034 also recommends that resolvers limit
the number of queries it sends out when resolving an individual
domain name. [RFC1035] stipulates that resolvers should use counters
to implement these limits.
Later, [RFC1536] states that "a set of servers might form a loop
wherein A refers to B and B refers to A". It does not, however,
specify what type of records might create these loops. Additionally,
it offers no new solutions beyond what RFC1034 and RFC1035 suggested.
In short, [RFC1034], [RFC1035] and [RFC1536] describe the problem and
do provide guidance to resolver implementers to help avoid indefinite
loops in the presence of misconfigured zone files with NS or CNAME
loops. However, we continue to observe different forms of this
problem and so here we seek to clarify that guidance.
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3. Current Problem
Recent research[Moura21b] has shown how NS configuration loops can
lead to significant increases in traffic: New Zealand's .nz country-
code top-level domain (a ccTLD) experienced a 50% traffic surge when
two domains were misconfigured with NS loops. Another anonymous
European ccTLD saw its traffic grow by 10-fold when two subdomains
were also miscofigured with NS loops. [Moura21b] also reproduced the
experiments under multiple controlled scenarios.
If existing RFCs already provide solution for looping
misconfiguration (Section 2), how come recent research [Moura21b]
still showed that these loops exist in the wild and lead to such
traffic surges?
3.1. Root Causes of Traffic Surge
[Moura21b] documents two main sources of amplification in the
presence of NS loops:
* Looping recursive resolvers: these are resolvers that send non-
stop queries to authoritative servers after receiving a single
client query (Figure 1) targeting a domain with an NS loop. Such
recursive resolvers do not conform to the guidance in RFC1034 and
RFC1035, both of which set limits to the number queries a resolver
should send when resolving a name.
* Looping clients, stub-resolvers, and forwarders: another situation
occurs when parts of the DNS infrastructure, behind a recursive
resolver, send non-stop queries in the presence of NS loops.
These queries ultimately reach their upstream recursive resolvers,
which then send queries to authoritative servers (and which
themselves may further amplify the query stream).
To illustrate this, consider Figure 1. The Current RFCs provide
solutions to prevent recursive resolvers from looping. Assume a
client sends a query to its stub resolver, which they will forward to
one of its locally configured recursive resovlers (Re1 and Re2).
Assuming Re2 receives the query, it will then carry out the recursive
recursion tasks. The current solutions limit the number of queries
that Re2 will send to authoritative servers (AT2) when resolving the
domain -- so the recursive resolver itself prevents looping. The
recursive resolver should answer the client with a SERVFAIL error
code in response.
However, this does not protect clients, stubs, or DNS forwarders (as
Re1, which forwards to Re3) to start to repeatedly asking the same
query. If, for example, Re2 sends up to 20 queries when resolving a
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domain name, every new incoming client query can trigger up to new 20
queries. This was exactly the problem the researchers found in
Google Public DNS' implementation.
+-----+ +-----+ +-----+ +-----+
| AT1 | | AT2 | | AT3 | | AT4 |
+-----+ +-----+ +-----+ +-----+
^ ^ ^ ^
| | | |
| +-----+ | |
+------| Re3 |----+| |
+-----+ |
^ |
| |
+----+ +----+ |
|Re1 | |Re2 |---------+
+----+ +----+
^ ^
| |
| +------+
+-|stub|
+------+
^
|
| +--------+
+-| client |
+--------+
Figure 1: Relationship between clients, stub, recursive resolvers
(Re) and authoritative name servers (ATn)
4. New requirement
Besides following the recommendations from RFC1034, RFC1035 and
RFC2181 for handling loops, this memo requires that recursive
resolvers MUST detect loop and MUST cache these records (negative
caching)[RFC2308]. Recursive resolvers need to refrain from
forwarding queries from clients/stub/forwarders to misconfigured
domain names when a negative answer can be answered from its cache.
How long these loops should be cached for is an implementation
choice; however, recursive results MUST answer from it's cache for at
least 15 minutes, given that most looping NS/CNAME record situations
will require human intervention.
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5. Operational considerations
TBD
6. Security considerations
TBD
7. Privacy Considerations
This document does not add any practical new privacy issues, aside
from possible benefits in deploying longer TTLs which in turn
requires less traffic to be sent and thus preserves privacy by query
omission: longer TTLs may help preserve a user's privacy by reducing
the number of requests that get transmitted in both the client-to-
resolver and resolver-to-authoritative cases.
8. IANA considerations
This document has no IANA actions.
9. References
9.1. 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>.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
<https://www.rfc-editor.org/info/rfc2308>.
9.2. Informative References
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[Moura21b] Moura, G.C.M., Castro, S., Heidemann, J., and W. Hardaker,
"TsuNAME - exploiting misconfiguration and vulnerability
to DDoS DNS", ACM 2021 Internet Measurement Conference,
DOI 10.1145/3487552.3487824, 2 November 2016,
<https://www.isi.edu/%7ejohnh/PAPERS/Moura21b.pdf>.
[RFC1536] Kumar, A., Postel, J., Neuman, C., Danzig, P., and S.
Miller, "Common DNS Implementation Errors and Suggested
Fixes", RFC 1536, DOI 10.17487/RFC1536, October 1993,
<https://www.rfc-editor.org/info/rfc1536>.
[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>.
Appendix A. Acknowledgements
TBD
Appendix B. Current implemenations
The requirements in this document have been implemented and deployed
by:
* Google Public DNS
* Cisco OpenDNS
Authors' Addresses
Giovane C. M. Moura
SIDN Labs/TU Delft
Meander 501
6825 MD Arnhem
Netherlands
Phone: +31 26 352 5500
Email: giovane.moura@sidn.nl
Wes Hardaker
USC/Information Sciences Institute
PO Box 382
Davis, 95617-0382
United States of America
Phone: +1 (530) 404-0099
Email: ietf@hardakers.net
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John Heidemann
USC/Information Sciences Institute
4676 Admiralty Way
Marina Del Rey, 90292-6695
United States of America
Phone: +1 (310) 448-8708
Email: johnh@isi.edu
Sebastian Castro
IE Domain Registry
2 Harbour Square, Dun Laoghaire
Dublin
A96 D6R0
Ireland
Phone: +353 1 2365400
Email: scastro@weare.ie
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