Internet DRAFT - draft-dwmtwc-dnsop-caching-resolution-failures
draft-dwmtwc-dnsop-caching-resolution-failures
Internet Engineering Task Force D. Wessels
Internet-Draft W. Carroll
Updates: 2308 (if approved) M. Thomas
Intended status: Standards Track Verisign
Expires: 27 January 2023 26 July 2022
Negative Caching of DNS Resolution Failures
draft-dwmtwc-dnsop-caching-resolution-failures-01
Abstract
In the DNS, resolvers employ caching to reduce both latency for end
users and load on authoritative name servers. The process of
resolution may result in one of three types of responses: (1) a
response containing the requested data; (2) a response indicating the
requested data does not exist; or (3) a non-response due to a
resolution failure in which the resolver does not receive any useful
information regarding the data's existence. This document concerns
itself only with the third type.
RFC 2308 specifies requirements for DNS negative caching. There,
caching of type (1) and (2) responses is mandatory and caching of
type (3) responses is optional. This document updates RFC 2308 to
require negative caching for DNS resolution failures.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 27 January 2023.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
Wessels, et al. Expires 27 January 2023 [Page 1]
Internet-Draft Caching Resolution Failures July 2022
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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Related Work . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
2. Types of DNS Resolution Failures . . . . . . . . . . . . . . 6
2.1. Server Failure . . . . . . . . . . . . . . . . . . . . . 6
2.2. Refused Response Code . . . . . . . . . . . . . . . . . . 6
2.3. Timeouts . . . . . . . . . . . . . . . . . . . . . . . . 7
2.4. Delegation Loops . . . . . . . . . . . . . . . . . . . . 7
2.5. Alias Loops . . . . . . . . . . . . . . . . . . . . . . . 8
2.6. DNSSEC Validation Failures . . . . . . . . . . . . . . . 8
3. DNS Negative Caching Requirements . . . . . . . . . . . . . . 8
3.1. Retries and Timeouts . . . . . . . . . . . . . . . . . . 8
3.2. TTLs . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.3. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.4. Requerying Delegation Information . . . . . . . . . . . . 9
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
8. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Caching has always been a fundamental component of DNS resolution on
the Internet. For example [RFC0882] states:
"The sheer size of the database and frequency of updates suggest that
it must be maintained in a distributed manner, with local caching to
improve performance."
Wessels, et al. Expires 27 January 2023 [Page 2]
Internet-Draft Caching Resolution Failures July 2022
The early DNS RFCs ([RFC0882], [RFC0883], [RFC1034], and [RFC1035])
primarily discuss caching in the context of what [RFC2308] calls
"positive" responses, that is, when the response includes the
requested data. In this case, a TTL is associated with each resource
record in the response. Resolvers can cache and reuse the data until
the TTL expires.
Section 4.3.4 of [RFC1034] describes negative response caching, but
notes it is optional and only talks about name errors (NXDOMAIN).
This is the origin of using the SOA MINIMUM field as a negative
caching TTL.
[RFC2308] updated [RFC1034] to specify new requirements for DNS
negative caching, including making it mandatory for name error
(NXDOMAIN) and no data responses. It further specified optional
negative caching for two DNS resolution failure cases: server failure
and dead / unreachable servers.
FOR DISCUSSION: RFC 2308 seems to use RFC 2119 keywords somewhat
inconsistently when in comes to requirements for negative caching of
type (1) and (2) responses. For example:
* Abstract: "negative caching should no longer be seen as an
optional part of..."
* Section 5: "A negative answer that resulted from a name error
(NXDOMAIN) should be cached..."
* Section 5: "A negative answer that resulted from a no data error
(NODATA) should be cached..."
* Section 8: "Negative caching in resolvers is no-longer optional,
if a resolver caches anything it must also cache negative
answers."
This document updates [RFC2308] to require negative caching of DNS
resolution failures, and provides additional examples of resolution
failures.
1.1. Motivation
Operators of DNS services have known for some time that recursive
resolvers become more aggressive when they experience resolution
failures. A number of different anecdotes, experiments, and
incidents support this claim.
Wessels, et al. Expires 27 January 2023 [Page 3]
Internet-Draft Caching Resolution Failures July 2022
[The authors vaguely recall stories of a moderately popular DNSBL
that wanted to shut down, but found that not responding or REFUSED
caused an overwhelming amount of traffic. Are there any citable
references to this happening?]
In December 2009, a secondary server for a number of in-addr.arpa
subdomains saw its traffic suddenly double, and queries of type
DNSKEY in particular increase by approximately two orders of
magnitude, coinciding with a DNSSEC key rollover by the zone operator
[roll-over-and-die]. This predated a signed root zone and an
operating system vendor was providing non-root trust anchors to the
recursive resolver, which became out-of-date following the rollover.
Unable to validate responses for the affected in-addr.arpa zones,
recursive resolvers aggressively retried their queries.
In 2016, the internet infrastructure company Dyn experienced a large
attack that impacted many high-profile customers. As documented in a
technical presentation detailing the attack [dyn-attack], Dyn staff
wrote: "At this point we are now experiencing botnet attack traffic
and what is best classified as a 'retry storm'. Looking at certain
large recursive platforms > 10x normal volume."
In 2018 the root zone key signing key (KSK) was rolled over
[root-ksk-roll]. Throughout the rollover period, the root servers
experienced a significant increase in DNSKEY queries. Before the
rollover, a.root-servers.net and j.root-servers.net together received
about 15 million DNSKEY queries per day. At the end of the
revocation period, they received 1.2 billion per day -- an 80x
increase. Removal of the revoked key from the zone caused DNSKEY
queries to drop to post-rollover but pre-revoke levels, indicating
there is still a population of recursive resolvers using the previous
root trust anchor and aggressively retrying DNSKEY queries.
In 2021, Verisign researchers used botnet query traffic to
demonstrate that certain large, public recursive DNS services exhibit
very high query rates when all authoritative name servers for a zone
return REFUSED or SERVFAIL [botnet]. When configured normally, query
rates for a single botnet domain averaged approximately 50 queries
per second. However, when configured to return SERVFAIL, the query
rate increased to 60,000 per second. Furthermore, increases were
also observed at the Root and TLD levels, even though delegations at
those levels were unchanged and continued operating normally.
Wessels, et al. Expires 27 January 2023 [Page 4]
Internet-Draft Caching Resolution Failures July 2022
Later that same year, on October 4, Facebook experienced a widespread
and well-publicized outage [fb-outage]. During the 6-hour outage,
none of Facebook's authoritative name servers were reachable and did
not respond to queries. Recursive name servers attempting to resolve
Facebook domains experienced timeouts. During this time query
traffic on the .COM/.NET infrastructure increased from 7,000 to
900,000 queries per second [CITATION NEEDED].
1.2. Related Work
[RFC2308] describes negative caching for four types of DNS queries
and responses: Name errors, no data, server failures, and dead /
unreachable servers. It places the strongest requirements on
negative caching for name errors and no data responses, while server
failures and dead servers are left as optional.
[RFC4697] is a Best Current Practice that documents observed
resolution misbehaviors. It describes a number of situations that
can lead to excessive queries from recursive resolvers, including:
requerying for delegation data, lame servers, responses blocked by
firewalls, and records with zero TTL. [RFC4697] makes a number of
recommendations, varying from "SHOULD" to "MUST."
An expired Internet Draft describes "The DNS thundering herd problem"
[thundering-herd] as a situation arising when cached data expires at
the same time for a large number of users. Although that document is
not focused on negative caching, it does describe the benefits of
combining multiple, identical queries to upstream name servers. That
is, when a recursive resolver receives multiple queries for the same
name, class, and type that cannot be answered from cached data, it
should combine or join them into a single upstream query, rather than
emit repeated, identical upstream queries.
[RFC5452], "Measures for Making DNS More Resilient against Forged
Answers," includes a section that describes the phenomenon known as
birthday attacks. Here, again, the problem arises when a recursive
resolver emits multiple, identical upstream queries. Multiple
outstanding queries makes it easier for an attacker to guess and
correctly match some of the DNS message parameters, such as the port
number and ID field. This situation is only exacerbated in the case
of timeout-based resolution failures. DNSSEC, of course, is a
suitable defense to spoofing attacks.
Wessels, et al. Expires 27 January 2023 [Page 5]
Internet-Draft Caching Resolution Failures July 2022
[RFC8767] describes "Serving Stale Data to Improve DNS Resiliency."
This permits a recursive resolver to return possibly stale data when
it is unable to refresh cached, expired data. It introduces the idea
of a failure recheck timer and says: "Attempts to refresh from non-
responsive or otherwise failing authoritative nameservers are
recommended to be done no more frequently than every 30 seconds."
1.3. 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.
The terms Private Use, Reserved, Unassigned, and Specification
Required are to be interpreted as defined in [RFC8126].
2. Types of DNS Resolution Failures
This section describes a number of different resolution failure
conditions.
2.1. Server Failure
Server failure is defined in [RFC1035] as: "The name server was
unable to process this query due to a problem with the name server."
A server failure is signaled by setting the RCODE field to SERVFAIL.
Authoritative servers, and more specifically secondary servers,
return server failure responses when they don't have any valid data
for a zone. That is, a secondary server has been configured to serve
a particular zone, but is unable to retrieve or refresh the zone data
from the primary server.
Recursive servers return server failure in response to a number of
different conditions, including many described below.
2.2. Refused Response Code
A name server returns a message with the RCODE field set to REFUSED
when it refuses to process the query for policy reasons.
Authoritative servers generally return REFUSED when processing a
query for which they are not authoritative. For example, a server
that is configured to be authoritative for only the EXAMPLE.NET zone,
may return REFUSED in response to a query for EXAMPLE.COM.
Wessels, et al. Expires 27 January 2023 [Page 6]
Internet-Draft Caching Resolution Failures July 2022
Recursive servers generally return REFUSED for query sources that do
not match configured access control lists. For example, a server
that is configured to allow queries from only 2001:DB8:1::/48 may
return REFUSED in response to a query from 2001:DB8:5::1.
2.3. Timeouts
A timeout occurs when a resolver fails to receive any response from a
server within a reasonable amount of time. [RFC2308] refers to this
as a "dead / unreachable server."
Note that resolver implementations may have two types of timeouts: a
smaller timeout which might trigger a query retry and a larger
timeout after which the server is considered unresponsive.
Timeouts can present a particular problem for negative caching,
depending on how the resolver handles multiple, outstanding queries
for the same <query name, type, class> tuple. For example, consider
a very popular website in a zone whose name servers are all
unresponsive. A recursive resolver might receive tens or hundreds of
queries per second for the popular website. If the recursive server
implementation "joins" these outstanding queries together, then it
only sends one recursive-to-authoritative query for the numerous
pending stub-to-recursive queries. If, however, the implementation
does not join outstanding queries together, then it sends one
recursive-to-authoritative query for each stub-to-recursive query.
If the incoming query rate is high and the timeout is large, this
might result in hundreds or thousands of recursive-to-authoritative
queries while waiting for an authoritative server to time out.
2.4. Delegation Loops
A delegation loop, or cycle, can occur when one domain utilizes name
servers in a second domain, and the second domain uses name servers
in the first. For example:
FOO.EXAMPLE. NS NS1.EXAMPLE.COM.
FOO.EXAMPLE. NS NS2.EXAMPLE.COM.
EXAMPLE.COM. NS NS1.FOO.EXAMPLE.
EXAMPLE.COM. NS NS2.FOO.EXAMPLE.
In this example, no names under FOO.EXAMPLE or EXAMPLE.COM can be
resolved because of the delegation loop. Note that delegation loop
may involve more than two domains. A resolver that does not detect
delegation loops may generate DDoS-levels of attack traffic to
authoritative name servers, as documented in the TsuNAME
vulnerability [TsuNAME].
Wessels, et al. Expires 27 January 2023 [Page 7]
Internet-Draft Caching Resolution Failures July 2022
2.5. Alias Loops
An alias loop, or cycle, can occur when one CNAME or DNAME RR refers
to a second name, which in turn is specified as an alias for the
first. For example:
APP.FOO.EXAMPLE. CNAME APP.EXAMPLE.NET.
APP.EXAMPLE.NET. CNAME APP.FOO.EXAMPLE.
The need to detect CNAME loops has been known since at least
[RFC1034] which states in Section 3.6.2:
"Of course, by the robustness principle, domain software should not
fail when presented with CNAME chains or loops; CNAME chains should
be followed and CNAME loops signaled as an error."
2.6. DNSSEC Validation Failures
Negative caching of DNSSEC validation errors is described in section
4.7 of [RFC4035].
FOR DISCUSSION: RFC4035 says "resolvers MAY cache data with invalid
signatures" while in this document all resolution failures MUST be
negatively cached. The focus of 4035 seems to be on caching bad
*data* rather than caching a more general resolution failure (e.g.
inability to retrieve keys).
3. DNS Negative Caching Requirements
3.1. Retries and Timeouts
A resolver MUST NOT retry more than twice (i.e., three queries in
total) before considering a server unresponsive.
This document does not place any requirements on timeout values,
which may be implementation- or configuration-dependent. It is
generally expected that typical timeout values range from 3 to 30
seconds.
3.2. TTLs
Resolvers MUST cache resolution failures for at least 5 seconds.
Resolvers SHOULD employ an exponential backoff algorithm to increase
the amount of time for subsequent resolution failures. For example,
the initial TTL for negatively caching a resolution failure is set to
5 seconds. The TTL is doubled after each retry that results in
another resolution failure. Consistent with [RFC2308], resolution
failures MUST NOT be cached for longer than 5 minutes.
Wessels, et al. Expires 27 January 2023 [Page 8]
Internet-Draft Caching Resolution Failures July 2022
3.3. Scope
Resolution failures MUST be cached against the specific query tuple
<query name, type, class, server IP address>.
It is common for resolvers to have multiple servers from which to
choose for a particular query. For example, in the case of stub-to-
recursive, the stub resolver may be configured with multiple resolver
addresses. In the case of recursive-to-authoritative, a given zone
usually has more than one name server (NS record), each of which can
have multiple IP addresses.
Nothing in this document prevents a resolver from retrying a query at
a different server. However, if all known servers for a query tuple
<query name, type, class> result in resolution failure, the resolver
MUST NOT send further queries for the tuple until the corresponding
negative cache entries expire.
3.4. Requerying Delegation Information
Quoting from [RFC4697]:
There can be times when every name server in a zone's NS RRSet is
unreachable (e.g., during a network outage), unavailable (e.g., the
name server process is not running on the server host), or
misconfigured (e.g., the name server is not authoritative for the
given zone, also known as "lame").
This document reiterates the requirement from Section 2.1.1 of
[RFC4697]:
An iterative resolver MUST NOT send a query for the NS RRSet of a
non-responsive zone to any of the name servers for that zone's parent
zone. For the purposes of this injunction, a non-responsive zone is
defined as a zone for which every name server listed in the zone's NS
RRSet:
1. is not authoritative for the zone (i.e., lame), or
2. returns a server failure response (RCODE=2), or
3. is dead or unreachable according to Section 7.2 of [RFC2308].
FOR DISCUSSION: the requirement quoted above may be problematic
today. e.g., focusing on NS as the query type (a) probably goes
against qname minimization, and (b) is not the real problem. Also
RFC 4697 doesn't place any time restriction (TTL) on this.
Wessels, et al. Expires 27 January 2023 [Page 9]
Internet-Draft Caching Resolution Failures July 2022
4. IANA Considerations
None
5. Security Considerations
This is intended to improve security.
Future work: Think about if/how new requirements could be abused,
used for DoS.
6. Privacy Considerations
This specification has no impact on user privacy.
7. Acknowledgments
The authors wish to thank ...
8. Change Log
RFC Editor: Please remove this section before publication.
This section lists substantial changes to the document as it is being
worked on.
From -00 to -01:
* use phrase "the initial TTL for negatively caching a resolution
failure" instead of "negative cache TTL"
* typos, etc
9. References
9.1. Normative References
[RFC1034] Mockapetris, P V., "Domain names - concepts and
facilities", DOI 10.17487/RFC1034, STD 13, RFC 1034,
November 1987, <https://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P V., "Domain names - implementation and
specification", DOI 10.17487/RFC1035, RFC 1035, STD 13,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
Wessels, et al. Expires 27 January 2023 [Page 10]
Internet-Draft Caching Resolution Failures July 2022
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", DOI 10.17487/RFC2119, BCP 14,
RFC 2119, 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>.
[RFC4697] Larson, M. and P. Barber, "Observed DNS Resolution
Misbehavior", BCP 123, DOI 10.17487/RFC4697, RFC 4697,
October 2006, <https://www.rfc-editor.org/info/rfc4697>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", RFC 8126,
DOI 10.17487/RFC8126, BCP 26, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", DOI 10.17487/RFC8174, RFC 8174, BCP 14,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References
[botnet] Wessels, D. and M. Thomas, "Botnet Traffic Observed at
Various Levels of the DNS Hierarchy", May 2021,
<https://indico.dns-oarc.net/event/38/contributions/841/>.
[dyn-attack]
Sullivan, A., "Dyn, DDoS, and DNS", March 2017,
<https://ccnso.icann.org/sites/default/files/file/field-
file-attach/2017-04/presentation-oracle-dyn-ddos-dns-
13mar17-en.pdf>.
[fb-outage]
Janardhan, S., "More details about the October 4 outage",
October 2021, <https://engineering.fb.com/2021/10/05/
networking-traffic/outage-details/>.
[RFC0882] Mockapetris, P V., "Domain names: Concepts and
facilities", DOI 10.17487/RFC0882, RFC 882, November 1983,
<https://www.rfc-editor.org/info/rfc882>.
[RFC0883] Mockapetris, P V., "Domain names: Implementation
specification", DOI 10.17487/RFC0883, RFC 883, November
1983, <https://www.rfc-editor.org/info/rfc883>.
Wessels, et al. Expires 27 January 2023 [Page 11]
Internet-Draft Caching Resolution Failures July 2022
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", DOI 10.17487/RFC4035, RFC 4035, March 2005,
<https://www.rfc-editor.org/info/rfc4035>.
[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>.
[RFC8767] Lawrence, D., Kumari, W., and P. Sood, "Serving Stale Data
to Improve DNS Resiliency", DOI 10.17487/RFC8767,
RFC 8767, March 2020,
<https://www.rfc-editor.org/info/rfc8767>.
[roll-over-and-die]
Michaleson, G., Wallström, P., Arends, R., and G. Huston,
"Roll Over and Die?", February 2010,
<https://www.potaroo.net/ispcol/2010-02/rollover.html>.
[root-ksk-roll]
Müller, M., Thomas, M., Wessels, D., Hardaker, W., Chung,
T., Toorop, W., and R.v. Rijswijk-Deij, "Roll, Roll, Roll
Your Root: A Comprehensive Analysis of the First Ever
DNSSEC Root KSK Rollover", October 2019,
<https://dl.acm.org/doi/10.1145/3355369.3355570>.
[thundering-herd]
Sivaraman, M. and C. Liu, "The DNS thundering herd problem
(expired Internet Draft)", June 2020,
<https://datatracker.ietf.org/doc/draft-muks-dnsop-dns-
thundering-herd/>.
[TsuNAME] Moura, G. C. M., Castro, S., Heidemann, J., and W.
Hardaker, "TsuNAME: exploiting misconfiguration and
vulnerability to DDoS DNS", November 2021,
<https://dl.acm.org/doi/10.1145/3487552.3487824>.
Authors' Addresses
Duane Wessels
Verisign
12061 Bluemont Way
Reston
Phone: +1 703 948-3200
Email: dwessels@verisign.com
URI: https://verisign.com
Wessels, et al. Expires 27 January 2023 [Page 12]
Internet-Draft Caching Resolution Failures July 2022
William Carroll
Verisign
12061 Bluemont Way
Reston
Phone: +1 703 948-3200
Email: wicarroll@verisign.com
URI: https://verisign.com
Matthew Thomas
Verisign
12061 Bluemont Way
Reston
Phone: +1 703 948-3200
Email: mthomas@verisign.com
URI: https://verisign.com
Wessels, et al. Expires 27 January 2023 [Page 13]
