Internet Engineering Task Force (IETF) D. Lawrence
Request for Comments: 8767 Oracle
Updates: 1034, 1035, 2181 W. Kumari
Category: Standards Track P. Sood
ISSN: 2070-1721 Google
March 2020
Serving Stale Data to Improve DNS Resiliency
Abstract
This document defines a method (serve-stale) for recursive resolvers
to use stale DNS data to avoid outages when authoritative nameservers
cannot be reached to refresh expired data. One of the motivations
for serve-stale is to make the DNS more resilient to DoS attacks and
thereby make them less attractive as an attack vector. This document
updates the definitions of TTL from RFCs 1034 and 1035 so that data
can be kept in the cache beyond the TTL expiry; it also updates RFC
2181 by interpreting values with the high-order bit set as being
positive, rather than 0, and suggests a cap of 7 days.
Status of This Memo
This is an Internet Standards Track document.
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
Internet Standards 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/rfc8767.
Copyright Notice
Copyright (c) 2020 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
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described in the Simplified BSD License.
Table of Contents
1. Introduction
2. Terminology
3. Background
4. Standards Action
5. Example Method
6. Implementation Considerations
7. Implementation Caveats
8. Implementation Status
9. EDNS Option
10. Security Considerations
11. Privacy Considerations
12. NAT Considerations
13. IANA Considerations
14. References
14.1. Normative References
14.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
Traditionally, the Time To Live (TTL) of a DNS Resource Record (RR)
has been understood to represent the maximum number of seconds that a
record can be used before it must be discarded, based on its
description and usage in [RFC1035] and clarifications in [RFC2181].
This document expands the definition of the TTL to explicitly allow
for expired data to be used in the exceptional circumstance that a
recursive resolver is unable to refresh the information. It is
predicated on the observation that authoritative answer
unavailability can cause outages even when the underlying data those
servers would return is typically unchanged.
We describe a method below for this use of stale data, balancing the
competing needs of resiliency and freshness.
This document updates the definitions of TTL from [RFC1034] and
[RFC1035] so that data can be kept in the cache beyond the TTL
expiry; it also updates [RFC2181] by interpreting values with the
high-order bit set as being positive, rather than 0, and also
suggests a cap of 7 days.
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.
For a glossary of DNS terms, please see [RFC8499].
3. Background
There are a number of reasons why an authoritative server may become
unreachable, including Denial-of-Service (DoS) attacks, network
issues, and so on. If a recursive server is unable to contact the
authoritative servers for a query but still has relevant data that
has aged past its TTL, that information can still be useful for
generating an answer under the metaphorical assumption that "stale
bread is better than no bread."
[RFC1035], Section 3.2.1 says that the TTL "specifies the time
interval that the resource record may be cached before the source of
the information should again be consulted." [RFC1035], Section 4.1.3
further says that the TTL "specifies the time interval (in seconds)
that the resource record may be cached before it should be
discarded."
A natural English interpretation of these remarks would seem to be
clear enough that records past their TTL expiration must not be used.
However, [RFC1035] predates the more rigorous terminology of
[RFC2119], which softened the interpretation of "may" and "should".
[RFC2181] aimed to provide "the precise definition of the Time to
Live," but Section 8 of [RFC2181] was mostly concerned with the
numeric range of values rather than data expiration behavior. It
does, however, close that section by noting, "The TTL specifies a
maximum time to live, not a mandatory time to live." This wording
again does not contain BCP 14 key words [RFC2119], but it does convey
the natural language connotation that data becomes unusable past TTL
expiry.
As of the time of this writing, several large-scale operators use
stale data for answers in some way. A number of recursive resolver
packages, including BIND, Knot Resolver, OpenDNS, and Unbound,
provide options to use stale data. Apple macOS can also use stale
data as part of the Happy Eyeballs algorithms in mDNSResponder. The
collective operational experience is that using stale data can
provide significant benefit with minimal downside.
4. Standards Action
The definition of TTL in Sections 3.2.1 and 4.1.3 of [RFC1035] is
amended to read:
TTL a 32-bit unsigned integer number of seconds that specifies the
duration that the resource record MAY be cached before the
source of the information MUST again be consulted. Zero values
are interpreted to mean that the RR can only be used for the
transaction in progress, and should not be cached. Values
SHOULD be capped on the order of days to weeks, with a
recommended cap of 604,800 seconds (7 days). If the data is
unable to be authoritatively refreshed when the TTL expires, the
record MAY be used as though it is unexpired. See Sections 5
and 6 of [RFC8767] for details.
Interpreting values that have the high-order bit set as being
positive, rather than 0, is a change from [RFC2181], the rationale
for which is explained in Section 6. Suggesting a cap of 7 days,
rather than the 68 years allowed by the full 31 bits of Section 8 of
[RFC2181], reflects the current practice of major modern DNS
resolvers.
When returning a response containing stale records, a recursive
resolver MUST set the TTL of each expired record in the message to a
value greater than 0, with a RECOMMENDED value of 30 seconds. See
Section 6 for explanation.
Answers from authoritative servers that have a DNS response code of
either 0 (NoError) or 3 (NXDomain) and the Authoritative Answer (AA)
bit set MUST be considered to have refreshed the data at the
resolver. Answers from authoritative servers that have any other
response code SHOULD be considered a failure to refresh the data and
therefore leave any previous state intact. See Section 6 for a
discussion.
5. Example Method
There is more than one way a recursive resolver could responsibly
implement this resiliency feature while still respecting the intent
of the TTL as a signal for when data is to be refreshed.
In this example method, four notable timers drive considerations for
the use of stale data:
* A client response timer, which is the maximum amount of time a
recursive resolver should allow between the receipt of a
resolution request and sending its response.
* A query resolution timer, which caps the total amount of time a
recursive resolver spends processing the query.
* A failure recheck timer, which limits the frequency at which a
failed lookup will be attempted again.
* A maximum stale timer, which caps the amount of time that records
will be kept past their expiration.
Most recursive resolvers already have the query resolution timer and,
effectively, some kind of failure recheck timer. The client response
timer and maximum stale timer are new concepts for this mechanism.
When a recursive resolver receives a request, it should start the
client response timer. This timer is used to avoid client timeouts.
It should be configurable, with a recommended value of 1.8 seconds as
being just under a common timeout value of 2 seconds while still
giving the resolver a fair shot at resolving the name.
The resolver then checks its cache for any unexpired records that
satisfy the request and returns them if available. If it finds no
relevant unexpired data and the Recursion Desired flag is not set in
the request, it should immediately return the response without
consulting the cache for expired records. Typically, this response
would be a referral to authoritative nameservers covering the zone,
but the specifics are implementation dependent.
If iterative lookups will be done, then the failure recheck timer is
consulted. Attempts to refresh from non-responsive or otherwise
failing authoritative nameservers are recommended to be done no more
frequently than every 30 seconds. If this request was received
within this period, the cache may be immediately consulted for stale
data to satisfy the request.
Outside the period of the failure recheck timer, the resolver should
start the query resolution timer and begin the iterative resolution
process. This timer bounds the work done by the resolver when
contacting external authorities and is commonly around 10 to 30
seconds. If this timer expires on an attempted lookup that is still
being processed, the resolution effort is abandoned.
If the answer has not been completely determined by the time the
client response timer has elapsed, the resolver should then check its
cache to see whether there is expired data that would satisfy the
request. If so, it adds that data to the response message with a TTL
greater than 0 (as specified in Section 4). The response is then
sent to the client while the resolver continues its attempt to
refresh the data.
When no authorities are able to be reached during a resolution
attempt, the resolver should attempt to refresh the delegation and
restart the iterative lookup process with the remaining time on the
query resolution timer. This resumption should be done only once per
resolution effort.
Outside the resolution process, the maximum stale timer is used for
cache management and is independent of the query resolution process.
This timer is conceptually different from the maximum cache TTL that
exists in many resolvers, the latter being a clamp on the value of
TTLs as received from authoritative servers and recommended to be
7 days in the TTL definition in Section 4. The maximum stale timer
should be configurable. It defines the length of time after a record
expires that it should be retained in the cache. The suggested value
is between 1 and 3 days.
6. Implementation Considerations
This document mainly describes the issues behind serving stale data
and intentionally does not provide a formal algorithm. The concept
is not overly complex, and the details are best left to resolver
authors to implement in their codebases. The processing of serve-
stale is a local operation, and consistent variables between
deployments are not needed for interoperability. However, we would
like to highlight the impact of various implementation choices,
starting with the timers involved.
The most obvious of these is the maximum stale timer. If this
variable is too large, it could cause excessive cache memory usage,
but if it is too small, the serve-stale technique becomes less
effective, as the record may not be in the cache to be used if
needed. Shorter values, even less than a day, can effectively handle
the vast majority of outages. Longer values, as much as a week, give
time for monitoring systems to notice a resolution problem and for
human intervention to fix it; operational experience has been that
sometimes the right people can be hard to track down and
unfortunately slow to remedy the situation.
Increased memory consumption could be mitigated by prioritizing
removal of stale records over non-expired records during cache
exhaustion. Eviction strategies could consider additional factors,
including the last time of use or the popularity of a record, to
retain active but stale records. A feature to manually flush only
stale records could also be useful.
The client response timer is another variable that deserves
consideration. If this value is too short, there exists the risk
that stale answers may be used even when the authoritative server is
actually reachable but slow; this may result in undesirable answers
being returned. Conversely, waiting too long will negatively impact
user experience.
The balance for the failure recheck timer is responsiveness in
detecting the renewed availability of authorities versus the extra
resource use for resolution. If this variable is set too large,
stale answers may continue to be returned even after the
authoritative server is reachable; per [RFC2308], Section 7, this
should be no more than 5 minutes. If this variable is too small,
authoritative servers may be targeted with a significant amount of
excess traffic.
Regarding the TTL to set on stale records in the response,
historically TTLs of 0 seconds have been problematic for some
implementations, and negative values can't effectively be
communicated to existing software. Other very short TTLs could lead
to congestive collapse as TTL-respecting clients rapidly try to
refresh. The recommended value of 30 seconds not only sidesteps
those potential problems with no practical negative consequences, it
also rate-limits further queries from any client that honors the TTL,
such as a forwarding resolver.
As for the change to treat a TTL with the high-order bit set as
positive and then clamping it, as opposed to [RFC2181] treating it as
zero, the rationale here is basically one of engineering simplicity
versus an inconsequential operational history. Negative TTLs had no
rational intentional meaning that wouldn't have been satisfied by
just sending 0 instead, and similarly there was realistically no
practical purpose for sending TTLs of 2^25 seconds (1 year) or more.
There's also no record of TTLs in the wild having the most
significant bit set in the DNS Operations, Analysis, and Research
Center's (DNS-OARC's) "Day in the Life" samples [DITL]. With no
apparent reason for operators to use them intentionally, that leaves
either errors or non-standard experiments as explanations as to why
such TTLs might be encountered, with neither providing an obviously
compelling reason as to why having the leading bit set should be
treated differently from having any of the next eleven bits set and
then capped per Section 4.
Another implementation consideration is the use of stale nameserver
addresses for lookups. This is mentioned explicitly because, in some
resolvers, getting the addresses for nameservers is a separate path
from a normal cache lookup. If authoritative server addresses are
not able to be refreshed, resolution can possibly still be successful
if the authoritative servers themselves are up. For instance,
consider an attack on a top-level domain that takes its nameservers
offline; serve-stale resolvers that had expired glue addresses for
subdomains within that top-level domain would still be able to
resolve names within those subdomains, even those it had not
previously looked up.
The directive in Section 4 that only NoError and NXDomain responses
should invalidate any previously associated answer stems from the
fact that no other RCODEs that a resolver normally encounters make
any assertions regarding the name in the question or any data
associated with it. This comports with existing resolver behavior
where a failed lookup (say, during prefetching) doesn't impact the
existing cache state. Some authoritative server operators have said
that they would prefer stale answers to be used in the event that
their servers are responding with errors like ServFail instead of
giving true authoritative answers. Implementers MAY decide to return
stale answers in this situation.
Since the goal of serve-stale is to provide resiliency for all
obvious errors to refresh data, these other RCODEs are treated as
though they are equivalent to not getting an authoritative response.
Although NXDomain for a previously existing name might well be an
error, it is not handled that way because there is no effective way
to distinguish operator intent for legitimate cases versus error
cases.
During discussion in the IETF, it was suggested that, if all
authorities return responses with an RCODE of Refused, it may be an
explicit signal to take down the zone from servers that still have
the zone's delegation pointed to them. Refused, however, is also
overloaded to mean multiple possible failures that could represent
transient configuration failures. Operational experience has shown
that purposely returning Refused is a poor way to achieve an explicit
takedown of a zone compared to either updating the delegation or
returning NXDomain with a suitable SOA for extended negative caching.
Implementers MAY nonetheless consider whether to treat all
authorities returning Refused as preempting the use of stale data.
7. Implementation Caveats
Stale data is used only when refreshing has failed in order to adhere
to the original intent of the design of the DNS and the behavior
expected by operators. If stale data were to always be used
immediately and then a cache refresh attempted after the client
response has been sent, the resolver would frequently be sending data
that it would have had no trouble refreshing. Because modern
resolvers use techniques like prefetching and request coalescing for
efficiency, it is not necessary that every client request needs to
trigger a new lookup flow in the presence of stale data, but rather
that a good-faith effort has been recently made to refresh the stale
data before it is delivered to any client.
It is important to continue the resolution attempt after the stale
response has been sent, until the query resolution timeout, because
some pathological resolutions can take many seconds to succeed as
they cope with unavailable servers, bad networks, and other problems.
Stopping the resolution attempt when the response with expired data
has been sent would mean that answers in these pathological cases
would never be refreshed.
The continuing prohibition against using data with a 0-second TTL
beyond the current transaction explicitly extends to it being
unusable even for stale fallback, as it is not to be cached at all.
Be aware that Canonical Name (CNAME) and DNAME records [RFC6672]
mingled in the expired cache with other records at the same owner
name can cause surprising results. This was observed with an initial
implementation in BIND when a hostname changed from having an IPv4
Address (A) record to a CNAME. The version of BIND being used did
not evict other types in the cache when a CNAME was received, which
in normal operations is not a significant issue. However, after both
records expired and the authorities became unavailable, the fallback
to stale answers returned the older A instead of the newer CNAME.
8. Implementation Status
The algorithm described in Section 5 was originally implemented as a
patch to BIND 9.7.0. It has been in use on Akamai's production
network since 2011; it effectively smoothed over transient failures
and longer outages that would have resulted in major incidents. The
patch was contributed to the Internet Systems Consortium, and the
functionality is now available in BIND 9.12 and later via the options
stale-answer-enable, stale-answer-ttl, and max-stale-ttl.
Unbound has a similar feature for serving stale answers and will
respond with stale data immediately if it has recently tried and
failed to refresh the answer by prefetching. Starting from version
1.10.0, Unbound can also be configured to follow the algorithm
described in Section 5. Both behaviors can be configured and fine-
tuned with the available serve-expired-* options.
Knot Resolver has a demo module here: <https://knot-
resolver.readthedocs.io/en/stable/modules-serve_stale.html>.
Apple's system resolvers are also known to use stale answers, but the
details are not readily available.
In the research paper "When the Dike Breaks: Dissecting DNS Defenses
During DDoS" [DikeBreaks], the authors detected some use of stale
answers by resolvers when authorities came under attack. Their
research results suggest that more widespread adoption of the
technique would significantly improve resiliency for the large number
of requests that fail or experience abnormally long resolution times
during an attack.
9. EDNS Option
During the discussion of serve-stale in the IETF, it was suggested
that an EDNS option [RFC6891] should be available. One proposal was
to use it to opt in to getting data that is possibly stale, and
another was to signal when stale data has been used for a response.
The opt-in use case was rejected, as the technique was meant to be
immediately useful in improving DNS resiliency for all clients.
The reporting case was ultimately also rejected because even the
simpler version of a proposed option was still too much bother to
implement for too little perceived value.
10. Security Considerations
The most obvious security issue is the increased likelihood of DNSSEC
validation failures when using stale data because signatures could be
returned outside their validity period. Stale negative records can
increase the time window where newly published TLSA or DS RRs may not
be used due to cached NSEC or NSEC3 records. These scenarios would
only be an issue if the authoritative servers are unreachable (the
only time the techniques in this document are used), and thus serve-
stale does not introduce a new failure in place of what would have
otherwise been success.
Additionally, bad actors have been known to use DNS caches to keep
records alive even after their authorities have gone away. The
serve-stale feature potentially makes the attack easier, although
without introducing a new risk. In addition, attackers could combine
this with a DDoS attack on authoritative servers with the explicit
intent of having stale information cached for a longer period of
time. But if attackers have this capacity, they probably could do
much worse than prolonging the life of old data.
In [CloudStrife], it was demonstrated how stale DNS data, namely
hostnames pointing to addresses that are no longer in use by the
owner of the name, can be used to co-opt security -- for example, to
get domain-validated certificates fraudulently issued to an attacker.
While this document does not create a new vulnerability in this area,
it does potentially enlarge the window in which such an attack could
be made. A proposed mitigation is that certificate authorities
should fully look up each name starting at the DNS root for every
name lookup. Alternatively, certificate authorities should use a
resolver that is not serving stale data.
11. Privacy Considerations
This document does not add any practical new privacy issues.
12. NAT Considerations
The method described here is not affected by the use of NAT devices.
13. IANA Considerations
This document has no IANA actions.
14. References
14.1. 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>.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<https://www.rfc-editor.org/info/rfc2181>.
[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>.
[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>.
14.2. Informative References
[CloudStrife]
Borgolte, K., Fiebig, T., Hao, S., Kruegel, C., and G.
Vigna, "Cloud Strife: Mitigating the Security Risks of
Domain-Validated Certificates",
DOI 10.1145/3232755.3232859, ACM 2018 Applied Networking
Research Workshop, July 2018, <https://www.ndss-
symposium.org/wp-content/uploads/2018/02/ndss2018_06A-
4_Borgolte_paper.pdf>.
[DikeBreaks]
Moura, G.C.M., Heidemann, J., Müller, M., Schmidt, R. de
O., and M. Davids, "When the Dike Breaks: Dissecting DNS
Defenses During DDoS", DOI 10.1145/3278532.3278534,
ACM 2018 Internet Measurement Conference, October 2018,
<https://www.isi.edu/~johnh/PAPERS/Moura18b.pdf>.
[DITL] DNS-OARC, "DITL Traces and Analysis", January 2018,
<https://www.dns-oarc.net/oarc/data/ditl>.
[RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the
DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,
<https://www.rfc-editor.org/info/rfc6672>.
[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>.
[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>.
Acknowledgements
The authors wish to thank Brian Carpenter, Vladimir Cunat, Robert
Edmonds, Tony Finch, Bob Harold, Tatuya Jinmei, Matti Klock, Jason
Moreau, Giovane Moura, Jean Roy, Mukund Sivaraman, Davey Song, Paul
Vixie, Ralf Weber, and Paul Wouters for their review and feedback.
Paul Hoffman deserves special thanks for submitting a number of Pull
Requests.
Thank you also to the following members of the IESG for their final
review: Roman Danyliw, Benjamin Kaduk, Suresh Krishnan, Mirja
Kühlewind, and Adam Roach.
Authors' Addresses
David C Lawrence
Oracle
Email: tale@dd.org
Warren "Ace" Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States of America
Email: warren@kumari.net
Puneet Sood
Google
Email: puneets@google.com
