wdiff draft-ietf-dnsop-serve-stale-02.txt draft-ietf-dnsop-serve-stale-03.txt

DNSOP Working Group D. Lawrence
Internet-Draft Oracle + Dyn
Updates: 1034, 1035 (if approved) W. Kumari
Intended status: Standards Track P. Sood
Expires: April 17, August 27, 2019 Google
October 14, 2018
February 23, 2019

Serving Stale Data to Improve DNS Resiliency
draft-ietf-dnsop-serve-stale-02
draft-ietf-dnsop-serve-stale-03

Abstract

This draft defines a method for recursive resolvers to use stale DNS
data to avoid outages when authoritative nameservers cannot be
reached to refresh expired data. It updates the definition of TTL
from [RFC1034] [RFC1034], [RFC1035], and [RFC1035] [RFC2181] to make it clear that data
can be kept in the cache beyond the TTL expiry and used for responses
when a refreshed answer is not readily available. 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.

Ed note

Text inside square brackets ([]) is additional background
information, answers to frequently asked questions, general musings,
etc. They will be removed before publication. This document is
being collaborated on in GitHub at <https://github.com/vttale/serve-
stale>. The most recent version of the document, open issues, etc
should all be available here. The authors gratefully accept pull
requests.

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
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This Internet-Draft will expire on April 17, August 27, 2019.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Notes to readers . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4.
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. 3
4. Standards Action . . . . . . . . . . . . . . . . . . . . . . 4
6. EDNS Option . . . . . . . . . . . . . . . . . . .
5. Example Method . . . . . . 5
6.1. Option Format Proposal 1 . . . . . . . . . . . . . . . . 5
6.2. Option Usage . . . 4
6. Implementation Caveats . . . . . . . . . . . . . . . . . . . 6
6.3. Option Format Proposal 2 . .
6.1. Implementation Considerations . . . . . . . . . . . . . . 7
7. Example Method . . . Implementation Status . . . . . . . . . . . . . . . . . . . . 7 8
8. Implementation Caveats . . . . . . . . . . . . . EDNS Option . . . . . . 9
9. Implementation Status . . . . . . . . . . . . . . . . . . . . 10
10. 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 11
11. 9
10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 11
12. 9
11. NAT Considerations . . . . . . . . . . . . . . . . . . . . . 11
13. 9
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
14. 10
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
15. 10
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
15.1. 10
14.1. Normative References . . . . . . . . . . . . . . . . . . 11
15.2. 10
14.2. Informative References . . . . . . . . . . . . . . . . . 12 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 11

1. Introduction

Traditionally the Time To Live (TTL) of a DNS resource record 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 proposes that the definition of the TTL be explicitly
expanded to 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
server 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. While this is intended
to be immediately useful to the installed base of DNS software, we
also propose an [RFC6891] EDNS option for enhanced signalling around
the use of stale data by implementations that understand it.

2. Notes to readers

[ RFC Editor, please remove this section before publication!
Readers: This is conversational text to describe what we've done, and
will be removed, please don't bother sending editorial nits. :-) ]

Due to circumstances, the authors of this document got sidetracked,
and we lost focus. We are now reviving it, and are trying to address
and incorporate comments. There has also been more deployment and
implementation recently, and so this document is now more describing
what is known to work instead of simply proposing a concept.

Open questions:

o The EDNS option that we propose for debugging is relatively full-
featured to identify which RRsets are stale. It could be
simplified to just indicate that an answer is stale, or even
removed entirely in favour of an Extended Error result that
indicates staleness.

o What TTL value to recommend be set in stale answers returned by
recursive resolvers.

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.

For a comprehensive treatment of DNS terms, please see [RFC7719].

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 the recursive server is unable to contact the
authoritative servers for a name 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, 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", and Section 4.1.3 further
says 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, 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"
Live", but in Section 8 was mostly concerned with the numeric range
of values and the possibility that very large values should be
capped. (It also has the curious suggestion that a value in the
range 2147483648 to 4294967295 should be treated as zero.) It closes
that section by noting, "The TTL specifies a maximum time to live,
not a mandatory time to live." This is again not [RFC2119]-normative
language, but does convey the natural language connotation that data
becomes unusable past TTL expiry.

Several major recursive resolver operators currently use stale data
for answers in some way, including Akamai (via both Nomimum and
Xerocole), (in three different
resolver implementations), BIND, Knot, OpenDNS, and Unbound. Apple
can also use stale data as part of the Happy Eyeballs algorithms in
mDNSResponder. The collective operational experience is that it
provides significant benefit with minimal downside.

4. Standards Action

The definition of TTL in [RFC1035] Sections 3.2.1 and 4.1.3 is
amended to read:

TTL a 32 bit 32-bit unsigned integer number of seconds in the range 0 -
2147483647 that specifies the time interval
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 with the high order bit set SHOULD
be capped at no more than 2147483647. on the orders of days to weeks, with a recommended cap
of 604,800 seconds. If the authority for the data is unavailable
when attempting to refresh the data past the given
interval, refresh, the record MAY be used as though it is
unexpired.

[ Discussion point: capping

Interpreting values with which have the high order bit set as being
max
positive, rather than 0, is a change from [RFC2181]. Also, we
could use this opportunity to recommend Suggesting a much more sane maximum
value like 604800 seconds, one week, instead
cap of seven days, rather than the literal maximum
of 68 years. ]

6. EDNS Option

While the basic behaviour of this answer-of-last-resort can be
achieved with changes only to resolvers, explicit signalling about
the use of stale data can be done with an EDNS [RFC6891] option.
This option can be included from a stub or forwarding resolver to a
recursive resolver, explicitly signalling that it does not want stale
answers, or for learning that stale data was in use. It is expected
that this could be useful for debugging.

[ NOTE: This section will be fleshed out a bit more thoroughly if
there is interest in pursuing the option. Here are two potential
options that could be used, one more fully-featured to indicate which
RRsets are stale and one much more simple to indicate that stale data
is present. These are proposed as mutually exclusive; the final
document will have one or zero such options. We're especially
soliciting feedback on this from the working group. ]

6.1. Option Format Proposal 1

The option is structured as follows:

+0 (MSB) +1 (LSB)
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
0: | OPTION-CODE |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
2: | OPTION-LENGTH |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
4: | STALE-RRSET-INDEX 1 |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
6: | |
8: | TTL-EXPIRY 1 |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
: ... additional STALE-RRSET-INDEX / TTL-EXPIRY pairs ... :
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

OPTION-CODE: 2 octets per [RFC6891]. For Serve-Stale the code is
TBD years allowed by IANA.

OPTION-LENGTH: 2 octets per [RFC6891]. Contains [RFC2181],
reflects the length of the
payload following OPTION-LENGTH, in octets.

STALE-RRSET-INDEX: Two octets as a signed integer, indicating the
first RRSet in the message which is beyond its TTL, with RRSet
counting starting at 1 and spanning message sections.

TTL-EXPIRY: Four octets as an unsigned integer, representing the
number current practice of seconds that have passed since the TTL for the RRset
expired.

6.2. Option Usage

Software making a major modern DNS request can signal that it understands Serve-
Stale by including the option with one STALE-RRSET-INDEX initialized
to any negative value and TTY-EXPIRY initialized to 0. The index is
set to a negative value to detect mere reflection of the option by
responders that don't really understand it.

If the request is made to a recursive resolver which used any stale
RRsets in its reply, it then fills in the corresponding indices and
staleness values. If no records are stale, STALE-RRSET-INDEX and
TTL-EXPIRY are set to 0.

If the request is made to an authoritative nameserver, it can use the
option in the reply to indicate how the resolver should treat the
records in the reply if they are unable to be refreshed later. A
default for all RRsets in the message is established by setting the
first STALE-RRSET-INDEX to 0, with optional additional STALE-RRSET-
INDEX values overriding the default. A TTL-EXPIRY value of 0 means
to never serve the RRset as stale, while non-zero values represent
the maximum amount of time it can be used before it MUST be evicted.
[ Does anyone really want to do this? It adds more state into resolvers. Is the idea only for purists, or is there a practical
application? ]

No facility is made for a client of a resolver to signal that it
doesn't want stale answers, because if a client has knowledge of
Serve-Stale as an option, it also has enough knowledge to just ignore
any records that come back stale. [ There is admittedly the issue
that the client might just want to wait out the whole attempted
resolution, which there's currently no way to indicate. The absolute
value of STALE-RRSET-INDEX could be taken as a timer the requester is
willing to wait for an answer, but that's kind of gross overloading
it like that. Shame to burn another field on that, but on the other
hand it might also be nice if a client could always signal its
impatience level - "I must have an answer within 900 milliseconds!" ]

6.3. Option Format Proposal 2

The option is structured as follows:

OPTION-CODE: 2 octets per [RFC6891]. For Serve-Stale the code is
TBD by IANA.

OPTION-LENGTH: 2 octets per [RFC6891]. Contains the length of the
payload following OPTION-LENGTH, in octets.

D Flag: If set, the client explicitly does NOT want stale answers.
If clear, the client would like an indication of whether any data
in the response is stale.

U Flag: This indicates that the server understands the Serve-Stale
EDNS option, and more information is communicated via the S flag.
It exists to get around the issue of some authorative servers
simply echoing back ENDS options it does not understand.

S Flag: If set, this indicates that the response contains stale
data. If clear, no data in the response has reached its TTL
expiry.

RESERVED: Reserved for future use. Should be set to zero on send
and ignored on receipt.

5. Example Method

There is conceivably 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 three four notable timers drive considerations for
the use of stale data, as follows:

o 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.

o A query resolution timer, which caps the total amount of time a
recursive resolver spends processing the query.

o A resolution recheck timer, which limits the frequency at which a
failed lookup will be attempted.

o A maximum stale timer, which caps the amount of time that records
will be kept past their expiration.

Recursive

Most recursive resolvers already have the second timer; the first query resolution timer, and
third timers
effectively some kind of resolution recheck timer. The client
response timer and maximum stale timer are new concepts for this
mechanism.

When a request is received by the recursive resolver, 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.
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 an any unexpired answer. data that
satisfies the request and of course returns them if available. If it
finds none 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.

If iterative lookups will be, done then the resolution recheck timer
is consulted. Attempts to refresh from the authorities are
recommended to be done, it 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 resolution recheck timer, the resolver
SHOULD start the query resolution timer. timer and begin the iterative
resolution process. This timer bounds the work done by the resolver, resolver
when contacting external authorities, and is commonly around 10 to 30
seconds.

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 and SHOULD message; it MUST
set the TTL of each expired record in the message to greater than 0,
with 30 seconds. seconds recommended. The response is then sent to the client
while the resolver continues its attempt to refresh the data. 30 second was chosen because
historically 0 second TTLs have been problematic for some
implementations, and similarly very short TTLs could lead

When no authorities are able to
congestive collapse as TTL-respecting clients rapidly try be reached during a resolution
attempt, the resolver SHOULD attempt to refresh.
30 seconds not only sidesteps those potential problems with no
practical negative consequence, it would also rate limit further
queries from any client that is honoring refresh the delegation.

Outside the resolution process, the TTL, such as a
forwarding resolver. [ NOTE: we're looking for further working group
feedback on this value. ]

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. servers and recommended to be 7
days in the TTL definition above. The maximum stale timer SHOULD be
configurable, and defines the length of time after a record expires
that it SHOULD be retained in the cache. The suggested value is 7
days, which gives time for monitoring to notice the resolution
problem and for human intervention to fix it.

This same basic technique MAY be used to handle stale data associated
with delegations. If authoritative server addresses are not able to
be refreshed, resolution can possibly still be successful if the
authoritative servers themselves are still up.

6. Implementation Caveats

Answers from authoritative servers that have a DNS Response Code of
either 0 (NOERROR) or 3 (NXDOMAIN) MUST be considered to have
refreshed the data at the resolver. In particular, this means that
this method is not meant to protect against operator error at the
authoritative server that turns a name that is intended to be valid
into one that is non-existent, because there is no way for a resolver
to know intent.

Resolution is given a chance to succeed before stale

Stale data is used only when refreshing has failed, in order to
adhere to the original intent of the design of the DNS. This
mechanism is only intended to add robustness to failures, DNS and to be
enabled all the time.
behaviour 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. As modern resolvers
use techniques like pre-
fetching pre-fetching 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 than
that a good-faith effort have has been recently made to refresh the stale
data before it is delivered to any client. The recommended period
between attempting refreshes is 30 seconds.

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.

Canonical Name (CNAME) records 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.

[

6.1. Implementation Considerations

This probably applies 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 other occluding types, so more thought
should be given resolver
authors to the overall issue. ]

Keeping records around after implement in their normal expiration will codebases. The processing of course
cause caches serve-
stale is a local operation, and consistent variables between
deployments are not needed for interoperability. However, we would
like to grow larger than if records were removed at their
TTL. Specific guidance on managing highlight the impact of various variables.

The most obvious of these is the maximum stale timer. If this
variable is too large it could cause excessive cache sizes memory usage,
but if it is outside too small, the serve-stale technique becomes less
effective, as the record may not be in the scope cache to be used if
needed. Memory consumption could be mitigated by prioritizing
removal of this document. Some areas for consideration include stale records over non-expired records during cache
exhaustion. Implementations may also wish to consider whether to
track the popularity of names in client requests for their last time of use or their
popularity, using that as an additional factor when considering cache
eviction. A feature to manually flush only stale records could also
be useful.

The client response timer is another variable which 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 sub-optimal answers
being returned. Conversely, waiting too long will negatively impact
user experience.

The balance for the resolution recheck timer is responsiveness in
detecting the renewed availability of authorities versus evicting by
maximum age, and whether the extra
resource use of resolution. If this variable is set too large, stale
answers may continue to provide be returned even after the authoritative
server is reachable. If this variable is too small, authoritative
servers may be rapidly hit with a feature significant amount of traffic when
they become reachable again.

Regarding the TTL to set on stale records in the response,
historically TTLs of zero seconds have been problematic for manually flushing 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 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.

Apart from timers, one more implementation consideration is the use
of stale records.

9. 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
toplevel domain that takes its nameservers offline; serve-stale
resolvers that had expired glue addresses for subdomains within that
TLD would still be able to resolve names within those subdomains,
even those it had not previously looked up.

7. Implementation Status

[RFC Editor: per RFC 6982 this section should be removed prior to
publication.]

The algorithm described in the Section 7 5 section was originally
implemented as a patch to BIND 9.7.0. It has been in production on
Akamai's production network since 2011, and effectively smoothed over
transient failures and longer outages that would have resulted in
major incidents. The patch was contributed to Internet Systems
Consortium and the functionality is now available in BIND 9.12 via
the options stale-answer-enable, stale-answer-ttl, and max-stale-ttl.

Unbound has a similar feature for serving stale answers, but it works
in a very different way by returning whatever cached answer will
respond with stale data immediately if it has
before trying recently tried and
failed to refresh expired records. This is unfortunately not
faithful to the ideal that data past expiry should attempt to be
refreshed before being served. answer by pre-fetching.

Knot Resolver has an a demo module here: https://knot-
resolver.readthedocs.io/en/stable/modules.html#serve-stale

Details of Apple's implementation are not currently known.

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.

10.

8. EDNS Option

During the discussion of serve-stale in the IETF dnsop working group,
it was suggested that an EDNS option should be available to either
explicitly opt-in to getting data that is possibly stale, or at least
as a debugging tool to indicate 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 as working group
participants determined that even the simpler version of a proposed
option was still too much bother to implement for too little
perceived value.

9. 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. This would only be an issue
if the authoritative servers are unreachable, the only time the
techniques in this document are used, and thus 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. This
potentially makes that easier, although without introducing a new
risk.

11.

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 such as to get
domain-validated certificates fraudulently issued to an attacker.
While this RFC does not create a new vulnerability in this area, it
does potentially enlarge the window in which such an attack could be
made. An obvious mitigation is that not only should a certificate
authority not use a resolver that has this feature enabled, it should
probably not use a caching resolver at all and instead fully look up
each name freshly from the root.

10. Privacy Considerations

This document does not add any practical new privacy issues.

12.

11. NAT Considerations

The method described here is not affected by the use of NAT devices.

13.

12. IANA Considerations

This document contains

There are no actions for IANA. This section will be
removed during conversion into an RFC by the RFC editor.

14. IANA considerations.

13. Acknowledgements

The authors wish to thank Robert Edmonds, Tony Finch, Bob Harold,
Matti Klock, Mukund Sivaraman, Jean Roy,
and Jason Moreau for initial review. Feedback from Robert Edmonds, Moreau, Giovane Moura, Jean Roy, Mukund Sivaraman,
Davey Song, and Paul Vixie, Ralf Weber has also been incorporated.

15. and Paul Wouters for their review
and feedback.

14. References

15.1.

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>.

[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75,

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 6891, 8174, DOI 10.17487/RFC6891, April 2013,
<https://www.rfc-editor.org/info/rfc6891>.

15.2. 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", ACM 2018 Applied
Networking Research Workshop, DOI 10.1145/3232755.3232859,
July 2018, <https://www.ndss-symposium.org/wp-
content/uploads/2018/02/
ndss2018_06A-4_Borgolte_paper.pdf>.

[DikeBreaks]
Moura, G., Heidemann, J., Mueller, M., Schmidt, R., and M.
Davids, "When the Dike Breaks: Dissecting DNS Defenses
During DDos", ACM 2018 Internet Measurement Conference,
DOI 10.1145/3278532.3278534, October 2018,
<https://www.isi.edu/~johnh/PAPERS/Moura18b.pdf>.

[RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 7719, DOI 10.17487/RFC7719, December
2015, <https://www.rfc-editor.org/info/rfc7719>.

Authors' Addresses

David C Lawrence
Oracle + Dyn

Email: tale@dd.org

Warren "Ace" Kumari
Google
1600 Amphitheatre Parkway
Mountain View CA 94043
USA

Email: warren@kumari.net

Puneet Sood
Google

Email: puneets@google.com