DNSOP Working Group G. Moura
Internet-Draft SIDN Labs/TU Delft
Intended status: Informational W. Hardaker
Expires: August 23, 2021 J. Heidemann
USC/Information Sciences Institute
M. Davids
SIDN Labs
February 19, 2021
Considerations for Large Authoritative DNS Servers Operators
draft-moura-dnsop-authoritative-recommendations-08
Abstract
Recent research work has explored the deployment characteristics and
configuration of the Domain Name System (DNS). This document
summarizes the conclusions from these research efforts and offers
specific, tangible advice to operators when configuring authoritative
DNS servers.
It is possible that the results presented in this document could be
applicable in a wider context than just the DNS protocol, as some of
the results may generically apply to any stateless/short-duration,
anycasted service.
This document is not an IETF consensus document: it is published for
informational purposes.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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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 August 23, 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. C1: Deploy anycast in every authoritative server for better
load distribution . . . . . . . . . . . . . . . . . . . . . . 5
4. C2: Routing can matter more than locations . . . . . . . . . 6
5. C3: Collecting anycast catchment maps to improve design . . . 7
6. C4: When under stress, employ two strategies . . . . . . . . 9
7. C5: Consider longer time-to-live values whenever possible . . 10
8. Security considerations . . . . . . . . . . . . . . . . . . . 13
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 13
10. IANA considerations . . . . . . . . . . . . . . . . . . . . . 13
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
12.1. Normative References . . . . . . . . . . . . . . . . . . 14
12.2. Informative References . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
This document summarizes recent research work that explored the
deployed DNS configurations and offers derived, specific tangible
advice to DNS authoritative server operators (DNS operators
hereafter). The considerations (C1--C5) presented in this document
are backed by published research work, which used wide-scale Internet
measurements to draw their conclusions. This document summarizes the
research results and describes the resulting key engineering options.
In each section, it points readers to the pertinent publications
where additional details are presented.
These considerations are designed for operators of "large"
authoritative DNS servers. In this context, "large" authoritative
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servers refers to those with a significant global user population,
like top-level domain (TLD) operators, run by either a single or
multiple operators. Typically these networks are deployed on wide
anycast networks [RFC1546]. These considerations may not be
appropriate for smaller domains, such as those used by an
organization with users in one unicast network, or in one city or
region, where operational goals such as uniform, global low latency
are less required.
It is possible that the results presented in this document could be
applicable in a wider context than just the DNS protocol, as some of
the results may generically apply to any stateless/short-duration,
anycasted service. Because the conclusions of the reviewed studies
don't measure smaller networks, the wording in this document
concentrates solely on disusing large-scale DNS authoritative
services only.
This document is not an IETF consensus document: it is published for
informational purposes.
2. Background
The DNS has main two types of DNS servers: authoritative servers and
recursive resolvers, shown by a representational deployment model in
Figure 1. An authoritative server (shown as AT1--AT4 in Figure 1)
knows the content of a DNS zone, and is responsible for answering
queries about that zone. It runs using local (possibly automatically
updated) copies of the zone and does not need to query other servers
[RFC2181] in order to answer requests. A recursive resolver (Re1--
Re3) is a server that iteratively queries authoritative and other
servers to answer queries received from client requests [RFC1034]. A
client typically employs a software library called a stub resolver
(stub in Figure 1) to issue its query to the upstream recursive
resolvers [RFC1034].
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+-----+ +-----+ +-----+ +-----+
| AT1 | | AT2 | | AT3 | | AT4 |
+-----+ +-----+ +-----+ +-----+
^ ^ ^ ^
| | | |
| +-----+ | |
+------| Re1 |----+| |
| +-----+ |
| ^ |
| | |
| +----+ +----+ |
+------|Re2 | |Re3 |------+
+----+ +----+
^ ^
| |
| +------+ |
+-| stub |-+
+------+
Figure 1: Relationship between recursive resolvers (Re) and
authoritative name servers (ATn)
DNS queries issued by a client contribute to a user's perceived
perceived latency and affect user experience [Sigla2014] depending on
how long it takes for responses to be returned. The DNS system has
been subject to repeated Denial of Service (DoS) attacks (for
example, in November 2015 [Moura16b]) in order to specifically
degrade user experience.
To reduce latency and improve resiliency against DoS attacks, the DNS
uses several types of service replication. Replication at the
authoritative server level can be achieved with (i) the deployment of
multiple servers for the same zone [RFC1035] (AT1---AT4 in Figure 1),
(ii) the use of IP anycast [RFC1546][RFC4786][RFC7094] that allows
the same IP address to be announced from multiple locations (each of
referred to as an "anycast instance" [RFC8499]) and (iii) the use of
load balancers to support multiple servers inside a single
(potentially anycasted) instance. As a consequence, there are many
possible ways an authoritative DNS provider can engineer its
production authoritative server network, with multiple viable choices
and no necessarily single optimal design.
In the next sections we cover the specific consideration (C1--C5) for
conclusions drawn within the academic papers about large
authoritative DNS server operators.
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3. C1: Deploy anycast in every authoritative server for better load
distribution
Authoritative DNS server operators announce their service using NS
records[RFC1034]. Different authoritative servers for a given zone
should return the same content; typically they stay synchronized
using DNS zone transfers (AXFR[RFC5936] and IXFR[RFC1995]),
coordinating the zone data they all return to their clients.
DNS heavily relies upon replication to support high reliability,
ensure capacity and to reduce latency [Moura16b]. DNS has two
complementary mechanisms for service replication. First, the DNs
protocol itself supports nameserver replication through the use of
multiple nameserver records (NS records), each operating on different
IP addresses. Second, each of these addresses can run at multiple
physical locations through the use of IP
anycast[RFC1546][RFC4786][RFC7094], by announcing the same IP address
from each instance at multiple locations -- Internet routing
(BGP[RFC4271]) associates the service's clients with their
topologically nearest anycast instance. Outside the DNS protocol,
replication can also be achieved by deploying load balancers at each
physical location. Nameserver replication is strongly recommended
for all zones (multiple NS records). IP anycast is used by many
large zones such as the DNS Root, most top-level domains[Moura16b]
and many large commercial enterprises, governments and other
organizations.
Most DNS operators strive to reduce service latency for users.
However, because they only have control over their authoritative
servers, and not over the client recursive resolvers, it is difficult
to ensure that recursives will be served by the closest authoritative
server. Server selection is up to the recursive resolver's software
implementation, and different vendors and even different releases
employ different criteria to chose the authoritative servers with
which to communicate.
Understanding how recursive resolvers choose authoritative servers is
a key step in improving the effectiveness of authoritative server
deployments. To measure and evaluate server deployments,
[Mueller17b] deployed seven unicast authoritative name servers in
different global locations and then queried them from more than 9000
RIPE authoritative server operators and their respective recursive
resolvers.
[Mueller17b] found that recursive resolvers in the wild query all
available authoritative servers, regardless of the observed latency.
But the distribution of queries tends to be skewed towards
authoritatives with lower latency: the lower the latency between a
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recursive resolver and an authoritative server, the more often the
recursive will send queries to that server. These results were
obtained by aggregating results from all of the vantage points and
were not specific to any specific vendor or version.
The authors believe this behavior is a consequence of combining the
two main criteria employed by resolvers when selecting authoritative
servers: resolvers regularly check all listed authoritative servers
in an NS set to determine which is closer (the least latent) and when
one isn't available selects one of the alternatives.
For an authoritative DNS operator, this result means that the latency
of all authoritative servers (NS records) matter, so they all must be
similarly capable -- all available authoritatives will be queried by
most recursive resolvers. Unicasted services, unfortunately, cannot
deliver good latency worldwide (a unicast authoritative server in
Europe will always have high latency to resolvers in California and
Australia, for example, given its geographical distance).
[Mueller17b] recommends that DNS operators deploy equally strong IP
anycast instances for every authoritative server (i.e., for each NS
record). Each large authoritative DNS server provider should phase
out their usage of unicast and deploy a well engineered number of
anycast instances with good peering strategies so they can provide
good latency to their global clients.
As a case study, the ".nl" TLD zone was originally served on seven
authoritative servers with a mixed unicast/anycast setup. In early
2018, .nl moved to a setup with 4 anycast authoritative servers.
[Mueller17b]'s contribution to DNS service engineering shows that
because unicast cannot deliver good latency worldwide, anycast needs
to be used to provide a low latency service worldwide.
4. C2: Routing can matter more than locations
When selecting an anycast DNS provider or setting up an anycast
service, choosing the best number of anycast instances[RFC4786] to
deploy is a challenging problem. Selecting where and how many global
locations to announce from using BGP is tricky. Intuitively, one
could naively think that the more instances the better and simply
"more" will always lead to shorter response times.
This is not necessarily true, however. In fact, [Schmidt17a] found
that proper route engineering can matter more than the total number
of locations. They analyzed the relationship between the number of
anycast instances and service performance (measuring latency of the
round-trip time (RTT)), measuring the overall performance of four DNS
Root servers. The Root DNS servers are implemented by 12 separate
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organizations serving the DNS root zone at 13 different IPv4/IPv6
address pairs.
The results documented in [Schmidt17a] measured the performance of
the {c,f,k,l}.root-servers.net (hereafter, "C", "F", "K" and "L")
servers from more than 7.9k RIPE Atlas probes. RIPE Atlas is a
Internet measurement platform with more than 12000 global vantage
points called "Atlas Probes" -- it is used regularly by both
researchers and operators [RipeAtlas15a] [RipeAtlas19a].
[Schmidt17a] found that the C server, a smaller anycast deployment
consisting of only 8 instances, provided very similar overall
performance in comparison to the much larger deployments of K and L,
with 33 and 144 instances respectively. The median RTT for C, K and
L root server were all between 30-32ms.
Because RIPE Atlas is known to have better coverage in Europe than
other regions, the authors specifically analyzed the results per
region and per country (Figure 5 in [Schmidt17a]), and show that
known Atlas bias toward Europe does not change the conclusion that
properly selected anycast locations is more important to latency than
the number of sites.
The important conclusion of [Schmidt17a] is that when engineering
anycast services for performance, factors other than just the number
of instances (such as local routing connectivity) must be considered.
They showed that 12 instances can provide reasonable latency,
assuming they are globally distributed and have good local
interconnectivity. However, additional instances can still be useful
for other reasons, such as when handling Denial-of-service (DoS)
attacks [Moura16b].
5. C3: Collecting anycast catchment maps to improve design
An anycast DNS service may be deployed from anywhere from several
locations to hundreds of locations (for example, l.root-servers.net
has over 150 anycast instances at the time this was written).
Anycast leverages Internet routing to distribute incoming queries to
a service's hop-nearest distributed anycast locations. However,
usually queries are not evenly distributed across all anycast
locations, as found in the case of L-Root [IcannHedge18].
Adding locations to or removing locations from a deployed anycast
network changes the load distribution across all of its locations.
When a new location is announced by BGP, locations may receive more
or less traffic than it was engineered for, leading to suboptimal
service performance or even stressing some locations while leaving
others underutilized. Operators constantly face this scenario that
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when expanding an anycast service. Operators cannot easily directly
estimate future query distributions based on proposed anycast network
engineering decisions.
To address this need and estimate the query loads based on changing,
in particular expanding, anycast service changes [Vries17b] developed
a new technique enabling operators to carry out active measurements,
using an open-source tool called Verfploeter (available at
[VerfSrc]). The results allow the creation of detailed anycast maps
and catchment estimates. By running verfploeter combined with a
published IPv4 "hit list", DNS can precisely calculate which remote
prefixes will be matched to each anycast instance in a network. At
the moment of this writing, Verfploeter still does not support IPv6
as the IPv4 hit lists used are generated via frequent large scale
ICMP echo scans, which is not possible using IPv6.
As proof of concept, [Vries17b] documents how it verfploeter was used
to predict both the catchment and query load distribution for a new
anycast instance deployed for b.root-servers.net. Using two anycast
test instances in Miami (MIA) and Los Angeles (LAX), an ICMP echo
query was sent from an IP anycast addresses to each IPv4 /24 network
routing block on the Internet.
The ICMP echo responses were recorded at both sites and analyzed and
overlayed onto a graphical world map, resulting in an Internet scale
catchment map. To calculate expected load once the production
network was enabled, the quantity of traffic received by b.root-
servers.net's single site at LAX was recorded based on a single day's
traffic (2017-04-12, DITL datasets [Ditl17]). [Vries17b] predicted
that 81.6% of the traffic load would remain at the LAX site. This
estimate by verfploeter turned out to be very accurate; the actual
measured traffic volume when production service at MIA was enabled
was 81.4%.
Verfploeter can also be used to estimate traffic shifts based on
other BGP route engineering techniques (for example, AS path
prepending or BGP community use) in advance of operational
deployment. [Vries17b] studied this using prepending with 1-3 hops
at each instance and compared the results against real operational
changes to validate the techniques accuracy.
An important operational takeaway [Vries17b] provides is how DNS
operators can make informed engineering choices when changing DNS
anycast network deployments by using Verfploeter in advance.
Operators can identify sub-optimal routing situations in advance with
significantly better coverage than using other active measurement
platforms such as RIPE Atlas. To date, Verfploeter has been deployed
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on a operational testbed (Anycast testbed) [AnyTest], on a large
unnamed operator and is run daily at b.root-servers.net[Vries17b].
Operators are encouraged to use active measurement techniques like
Verfploeter in advance of potential anycast network changes to
accurately measure the benefits and potential issues ahead of time.
6. C4: When under stress, employ two strategies
DDoS attacks are becoming bigger, cheaper, and more frequent
[Moura16b]. The most powerful recorded DDoS attack against DNS
servers to date reached 1.2 Tbps by using IoT devices [Perlroth16].
How should a DNS operator engineer its anycast authoritative DNS
server react to such a DDoS attack? [Moura16b] investigates this
question using empirical observations grounded with theoretical
option evaluations.
An authoritative DNS server deployed using anycast will have many
server instances distributed over many networks. Ultimately, the
relationship between the DNS provider's network and a client's ISP
will determine which anycast instance will answer queries for a given
client, given that BGP is the protocol that maps clients to specific
anycast instances by using routing information [RF:KDar02]. As a
consequence, when an anycast authoritative server is under attack,
the load that each anycast instance receives is likely to be unevenly
distributed (a function of the source of the attacks), thus some
instances may be more overloaded than others which is what was
observed analyzing the Root DNS events of Nov. 2015 [Moura16b].
Given the fact that different instances may have different capacity
(bandwidth, CPU, etc.), making a decision about how to react to
stress becomes even more difficult.
In practice, an anycast instance is overloaded with incoming traffic,
operators have two options:
o They can withdraw its routes, pre-prepend its AS route to some or
all of its neighbors, perform other traffic shifting tricks (such
as reducing route announcement propagation using BGP
communities[RFC1997]), or by communicating with its upstream
network providers to apply filtering (potentially using FlowSpec
[RFC5575]). These techniques shift both legitimate and attack
traffic to other anycast instances (with hopefully greater
capacity) or to block traffic entirely.
o Alternatively, operators can be become a degraded absorber by
continuing to operate, knowing dropping incoming legitimate
requests due to queue overflow. However, this approach will also
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absorb attack traffic directed toward its catchment, hopefully
protecting the other anycast instances.
[Moura16b] saw both of these behaviors deployed in practice by
studying instance reachability and route-trip time (RTTs) in the DNS
root events. When withdraw strategies were deployed, the stress of
increased query loads were displaced from one instance to multiple
other sites. In other observed events, one site was left to absorb
the brunt of an attack leaving the other sites to remain relatively
less affected.
Operators should consider having both a anycast site withdraw
strategy and a absorption strategy ready to be used before a network
overload occurs. Ideally, these should be encoded into operating
playbooks with defined site measurement guidelines for which strategy
to employ based on measured data from past events.
[Moura16b] speculates that careful, explicit, and automated
management policies may provide stronger defenses to overload events.
DNS operators should be ready to employ both traditional filtering
approaches and other routing load balancing techniques
(withdraw/prepend/communities or isolate instances), where the best
choice depends on the specifics of the attack.
Note that this consideration refers to the operation of just one
anycast service point, i.e., just one anycasted IP address block
covering one NS record. However, DNS zones with multiple
authoritative anycast servers may also expect loads to shift from one
anycasted server to another, as resolvers switch from on
authoritative service point to another when attempting to resolve a
name [Mueller17b].
7. C5: Consider longer time-to-live values whenever possible
Caching is the cornerstone of good DNS performance and reliability.
A 50 ms response to a new DNS query may be considered fast, but a
less than 1 ms response to a cached entry is far faster. [Moura18b]
showed that caching also protects users from short outages and even
significant DDoS attacks.
DNS record TTLs (time-to-live values) [RFC1034][RFC1035] directly
control cache durations and affect latency, resilience, and the role
of DNS in CDN server selection. Some early work modeled caches as a
function of their TTLs [Jung03a], and recent work has examined their
interaction with DNS[Moura18b], but until [Moura19a] no research
provided considerations about the benefits of various TTL value
choices. To study this, Moura et. al. [Moura19a] carried out a
measurement study investigating TTL choices and their impact on user
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experiences in the wild. They performed this study independent of
specific resolvers (and their caching architectures), vendors, or
setups.
First, they identified several reasons why operators and zone-owners
may want to choose longer or shorter TTLs:
o As discussed, longer TTLs lead to a longer cache life, resulting
in faster responses. [Moura19a] measured this in the wild and
showed that by increasing the TTL for .uy TLD from 5 minutes
(300s) to 1 day (86400s) the latency measured from 15k Atlas
vantage points changed significantly: the median RTT decreased
from 28.7ms to 8ms, and the 75%ile decreased from 183ms to 21ms.
o Longer caching times also results in lower DNS traffic:
authoritative servers will experience less traffic with extended
TTLs, as repeated queries are answered by resolver caches.
o Consequently, longer caching results in a lower overall cost if
DNS is metered: some DNS-As-A-Service providers charge a per query
(metered) cost (often in addition to a fixed monthly cost).
o Longer caching is more robust to DDoS attacks on DNS
infrastructure. [Moura18b] also measured and show that DNS
caching can greatly reduce the effects of a DDoS on DNS, provided
that caches last longer than the attack.
o However, shorter caching supports deployments that may require
rapid operational changes: An easy way to transition from an old
server to a new one is to simply change the DNS records. Since
there is no method to remotely remove cached DNS records, the TTL
duration represents a necessary transition delay to fully shift
from one server to another. Thus, low TTLs allow for more rapid
transitions. However, when deployments are planned in advance
(that is, longer than the TTL), it is possible to lower the TTLs
just-before a major operational change and raise them again
afterward.
o Shorter caching can also help with a DNS-based response to DDoS
attacks. Specifically, some DDoS-scrubbing services use the DNS
to redirect traffic during an attack. Since DDoS attacks arrive
unannounced, DNS-based traffic redirection requires the TTL be
kept quite low at all times to allow operators to suddenly have
their zone served by a DDoS-scrubbing service.
o Shorter caching helps DNS-based load balancing. Many large
services are known to rotate traffic among their servers using
DNS-based load balancing. Each arriving DNS request provides an
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opportunity to adjust service load by rotating IP address records
(A and AAAA) to the lowest unused server. Shorter TTLs may be
desired in these architectures to react more quickly to traffic
dynamics. Many recursive resolvers, however, have minimum caching
times of tens of seconds, placing a limit on this form of agility.
Given these considerations, the proper choice for a TTL depends in
part on multiple external factors -- no single recommendation is
appropriate for all scenarios. Organizations must weigh these trade-
offs and find a good balance for their situation. Still, some
guidelines can be reached when choosing TTLs:
o For general DNS zone owners, [Moura19a] recommends a longer TTL of
at least one hour, and ideally 8, 12, or 24 hours. Assuming
planned maintenance can be scheduled at least a day in advance,
long TTLs have little cost and may, even, literally provide a cost
savings.
o For registry operators: TLD and other public registration
operators (for example most ccTLDs and .com, .net, .org) that host
many delegations (NS records, DS records and "glue" records),
[Moura19a] demonstrates that most resolvers will use the TTL
values provided by the child delegations while the others some
will choose the TTL provided by the parent's copy of the record.
As such, [Moura19a] recommends longer TTLs (at least an hour or
more) for registry operators as well for child NS and other
records.
o Users of DNS-based load balancing or DDoS-prevention services may
require shorter TTLs: TTLs may even need to be as short as 5
minutes, although 15 minutes may provide sufficient agility for
many operators. There is always a tussle between shorter TTLs
providing more agility against all the benefits listed above for
using longer TTLs.
o Use of A/AAAA and NS records: The TTLs for A/AAAA records should
be shorter to or equal to the TTL for the corresponding NS records
for in-bailiwick authoritative DNS servers, since [Moura19a] finds
that once an NS record expires, their associated A/AAAA will also
be re-queried when glue is required to be sent by the parents.
For out-of-bailiwick servers, A, AAAA and NS records are usually
all cached independently, so different TTLs can be used
effectively if desired. In either case, short A and AAAA records
may still be desired if DDoS-mitigation services are required.
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8. Security considerations
This document discusses applying measured research results to
operational deployments. Most of the considerations affect mostly
operational practice, though a few do have security related impacts.
Specifically, C4 discusses a couple of strategies to employ when a
service is under stress from DDoS attacks and offers operators
additional guidance when handling excess traffic.
Similarly, C5 identifies the trade-offs with respect to the
operational and security benefits of using longer time-to-live
values.
9. Privacy Considerations
This document does not add any practical new privacy issues, aside
from possible benefits in deploying longer TTLs as suggested in C5.
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.
10. IANA considerations
This document has no IANA actions.
11. Acknowledgements
This document is a summary of the main considerations of six research
works performed by the authors and others. This document would not
have been possible without the hard work of these authors and co-
authors:
o Ricardo de O. Schmidt
o Wouter B de Vries
o Moritz Mueller
o Lan Wei
o Cristian Hesselman
o Jan Harm Kuipers
o Pieter-Tjerk de Boer
o Aiko Pras
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We would like also to thank the reviewers of this draft that offered
valuable suggestions: Duane Wessels, Joe Abley, Toema Gavrichenkov,
John Levine, Michael StJohns, Kristof Tuyteleers, Stefan Ubbink,
Klaus Darilion and Samir Jafferali, and comments provided at the IETF
DNSOP session (IETF104).
Besides those, we would like thank those acknowledged in the papers
this document summarizes for helping produce the results: RIPE NCC
and DNS OARC for their tools and datasets used in this research, as
well as the funding agencies sponsoring the individual research
works.
12. References
12.1. 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>.
[RFC1546] Partridge, C., Mendez, T., and W. Milliken, "Host
Anycasting Service", RFC 1546, DOI 10.17487/RFC1546,
November 1993, <https://www.rfc-editor.org/info/rfc1546>.
[RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
DOI 10.17487/RFC1995, August 1996,
<https://www.rfc-editor.org/info/rfc1995>.
[RFC1997] Chandra, R., Traina, P., and T. Li, "BGP Communities
Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,
<https://www.rfc-editor.org/info/rfc1997>.
[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>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast
Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
December 2006, <https://www.rfc-editor.org/info/rfc4786>.
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[RFC5575] Marques, P., Sheth, N., Raszuk, R., Greene, B., Mauch, J.,
and D. McPherson, "Dissemination of Flow Specification
Rules", RFC 5575, DOI 10.17487/RFC5575, August 2009,
<https://www.rfc-editor.org/info/rfc5575>.
[RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
(AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
<https://www.rfc-editor.org/info/rfc5936>.
[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>.
[RFC7094] McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
"Architectural Considerations of IP Anycast", RFC 7094,
DOI 10.17487/RFC7094, January 2014,
<https://www.rfc-editor.org/info/rfc7094>.
[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>.
12.2. Informative References
[AnyTest] Schmidt, R., "Anycast Testbed", December 2018,
<http://www.anycast-testbed.com/>.
[Ditl17] OARC, D., "2017 DITL data", October 2018,
<https://www.dns-oarc.net/oarc/data/ditl/2017>.
[IcannHedge18]
ICANN, ., "DNS-STATS - Hedgehog 2.4.1", October 2018,
<http://stats.dns.icann.org/hedgehog/>.
[Jung03a] Jung, J., Berger, A., and H. Balakrishnan, "Modeling TTL-
based Internet caches", ACM 2003 IEEE INFOCOM,
DOI 10.1109/INFCOM.2003.1208693, July 2003,
<http://www.ieee-infocom.org/2003/papers/11_01.PDF>.
[Moura16b]
Moura, G., Schmidt, R., Heidemann, J., Mueller, M., Wei,
L., and C. Hesselman, "Anycast vs DDoS Evaluating the
November 2015 Root DNS Events.", ACM 2016 Internet
Measurement Conference, DOI /10.1145/2987443.2987446,
October 2016,
<https://www.isi.edu/~johnh/PAPERS/Moura16b.pdf>.
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[Moura18b]
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>.
[Moura19a]
Moura, G., Heidemann, J., Schmidt, R., and W. Hardaker,
"Cache Me If You Can: Effects of DNS Time-to-Live",
ACM 2019 Internet Measurement Conference,
DOI 10.1145/3355369.3355568, October 2019,
<https://www.isi.edu/~johnh/PAPERS/Moura19b.pdf>.
[Moura20a]
Moura, G., Heidemann, J., Hardaker, W., Bulten, J., Ceron,
J., and C. Hesselman, "Old but Gold: Prospecting TCP to
Engineer DNS Anycast (extended)", Technical Report ISI-
TR-740 USC/Information Sciences Institute. , June 2020,
<https://www.isi.edu/~johnh/PAPERS/Moura20a.pdf>.
[Moura20b]
Moura, G., Castro, S., Hardaker, W., Wullink, M., and C.
Hesselman, "Clouding up the Internet: how centralized is
DNS traffic becoming?", ACM 2020 Internet Measurement
Conference, DOI 10.1145/3419394.3423625, October 2020,
<http://giovane-moura.nl/resources/paper/Moura20b.pdf>.
[Mueller17b]
Mueller, M., Moura, G., Schmidt, R., and J. Heidemann,
"Recursives in the Wild- Engineering Authoritative DNS
Servers.", ACM 2017 Internet Measurement Conference,
DOI 10.1145/3131365.3131366, October 2017,
<https://www.isi.edu/%7ejohnh/PAPERS/Mueller17b.pdf>.
[Perlroth16]
Perlroth, N., "Hackers Used New Weapons to Disrupt Major
Websites Across U.S.", October 2016,
<https://www.nytimes.com/2016/10/22/business/internet-
problems-attack.html>.
[RipeAtlas15a]
Staff, R., "RIPE Atlas A Global Internet Measurement
Network", September 2015, <http://ipj.dreamhosters.com/wp-
content/uploads/issues/2015/ipj18-3.pdf>.
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[RipeAtlas19a]
NCC, R., "Ripe Atlas - RIPE Network Coordination Centre",
September 2019, <https://atlas.ripe.net/>.
[Schmidt17a]
Schmidt, R., Heidemann, J., and J. Kuipers, "Anycast
Latency - How Many Sites Are Enough. In Proceedings of the
Passive and Active Measurement Workshop", PAM Passive and
Active Measurement Conference, March 2017,
<https://www.isi.edu/%7ejohnh/PAPERS/Schmidt17a.pdf>.
[Sigla2014]
Singla, A., Chandrasekaran, B., Godfrey, P., and B. Maggs,
"The Internet at the speed of light. In Proceedings of the
13th ACM Workshop on Hot Topics in Networks (Oct 2014)",
ACM Workshop on Hot Topics in Networks, October 2014,
<http://speedierweb.web.engr.illinois.edu/cspeed/papers/
hotnets14.pdf>.
[VerfSrc] Vries, W., "Verfploeter source code", November 2018,
<https://github.com/Woutifier/verfploeter>.
[Vries17b]
Vries, W., Schmidt, R., Hardaker, W., Heidemann, J., Boer,
P., and A. Pras, "Verfploeter - Broad and Load-Aware
Anycast Mapping", ACM 2017 Internet Measurement
Conference, DOI 10.1145/3131365.3131371, October 2017,
<https://www.isi.edu/%7ejohnh/PAPERS/Vries17b.pdf>.
Authors' Addresses
Giovane C. M. Moura
SIDN Labs/TU Delft
Meander 501
Arnhem 6825 MD
The Netherlands
Phone: +31 26 352 5500
Email: giovane.moura@sidn.nl
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Wes Hardaker
USC/Information Sciences Institute
PO Box 382
Davis 95617-0382
U.S.A.
Phone: +1 (530) 404-0099
Email: ietf@hardakers.net
John Heidemann
USC/Information Sciences Institute
4676 Admiralty Way
Marina Del Rey 90292-6695
U.S.A.
Phone: +1 (310) 448-8708
Email: johnh@isi.edu
Marco Davids
SIDN Labs
Meander 501
Arnhem 6825 MD
The Netherlands
Phone: +31 26 352 5500
Email: marco.davids@sidn.nl
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