IPv6 Operations J. Livingood
Internet-Draft Comcast
Intended status: Informational May 29, 2011
Expires: November 30, 2011
IPv6 AAAA DNS Whitelisting Implications
draft-ietf-v6ops-v6-aaaa-whitelisting-implications-04
Abstract
The objective of this document is to describe the practice of
whitelisting of DNS recursive resolvers in order to limit AAAA
resource records responses, which contain IPv6 addresses, hereafter
referred to as DNS Whitelisting, as well as the implications of this
emerging practice and what alternatives or variations may exist.
This practice is a type of IPv6 transition mechanism used by domains,
as a method for incrementally transitioning inbound traffic to a
domain from IPv4 to IPv6 transport. The audience for this document
is the Internet community generally, particularly IPv6 implementers.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
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This Internet-Draft will expire on November 30, 2011.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. How DNS Whitelisting Works . . . . . . . . . . . . . . . . . . 5
2.1. Description of the Operation of DNS Whitelisting . . . . . 6
2.2. Comparison with Blacklisting . . . . . . . . . . . . . . . 8
3. What Problems Are Implementers Trying To Solve? . . . . . . . 8
3.1. IPv6-Related Impairment . . . . . . . . . . . . . . . . . 9
3.2. Volume-Based Concerns . . . . . . . . . . . . . . . . . . 10
3.3. Free Versus Subscription Services . . . . . . . . . . . . 10
4. Concerns Regarding DNS Whitelisting . . . . . . . . . . . . . 10
5. Similarities to Other DNS Operations . . . . . . . . . . . . . 13
5.1. Similarities to Split DNS . . . . . . . . . . . . . . . . 13
5.2. Similarities to DNS Load Balancing . . . . . . . . . . . . 13
6. Possible Deployment Scenarios . . . . . . . . . . . . . . . . 14
6.1. Deploying DNS Whitelisting On An Ad Hoc Basis . . . . . . 14
6.2. Deploying DNS Whitelisting Universally . . . . . . . . . . 15
7. Implications of DNS Whitelisting . . . . . . . . . . . . . . . 16
7.1. Architectural Implications . . . . . . . . . . . . . . . . 16
7.2. Public IPv6 Address Reachability Implications . . . . . . 17
7.3. Operational Implications . . . . . . . . . . . . . . . . . 18
7.3.1. De-Whitelisting May Occur . . . . . . . . . . . . . . 18
7.3.2. Authoritative DNS Server Operational Implications . . 18
7.3.3. DNS Recursive Resolver Server Operational
Implications . . . . . . . . . . . . . . . . . . . . . 19
7.3.4. Monitoring Implications . . . . . . . . . . . . . . . 20
7.3.5. Implications of Operational Momentum . . . . . . . . . 21
7.3.6. Troubleshooting Implications . . . . . . . . . . . . . 21
7.3.7. Additional Implications If Deployed On An Ad Hoc
Basis . . . . . . . . . . . . . . . . . . . . . . . . 21
7.4. Homogeneity May Be Encouraged . . . . . . . . . . . . . . 22
7.5. Technology Policy Implications . . . . . . . . . . . . . . 22
7.6. IPv6 Adoption Implications . . . . . . . . . . . . . . . . 24
7.7. Implications with Poor IPv4 and Good IPv6 Transport . . . 24
7.8. Implications for Users of Third-Party DNS Recursive
Resolvers . . . . . . . . . . . . . . . . . . . . . . . . 25
8. General Implementation Variations . . . . . . . . . . . . . . 26
8.1. Implement DNS Whitelisting Universally . . . . . . . . . . 26
8.2. Implement DNS Whitelisting On An Ad Hoc Basis . . . . . . 26
8.3. Do Not Implement DNS Whitelisting . . . . . . . . . . . . 26
8.3.1. Solve Current End User IPv6 Impairments . . . . . . . 26
8.3.2. Gain Experience Using IPv6 Transition Names . . . . . 27
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8.3.3. Implement DNS Blacklisting . . . . . . . . . . . . . . 27
9. Is DNS Whitelisting a Recommended Practice? . . . . . . . . . 28
10. Security Considerations . . . . . . . . . . . . . . . . . . . 28
10.1. DNSSEC Considerations . . . . . . . . . . . . . . . . . . 29
10.2. Authoritative DNS Response Consistency Considerations . . 29
11. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 30
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 30
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 31
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
15.1. Normative References . . . . . . . . . . . . . . . . . . . 32
15.2. Informative References . . . . . . . . . . . . . . . . . . 33
Appendix A. Document Change Log . . . . . . . . . . . . . . . . . 35
Appendix B. Open Issues . . . . . . . . . . . . . . . . . . . . . 37
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 39
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1. Introduction
This document describes the emerging practice of whitelisting of DNS
recursive resolvers in order to limit AAAA resource record (RR)
responses, which contain IPv6 addresses, hereafter referred to as DNS
Whitelisting. The document explores the implications of this
emerging practice are and what alternatives may exist. When
implemented, DNS Whitelisting in practice means that a domain's
authoritative DNS will return a AAAA resource record to DNS recursive
resolvers [RFC1035] on the whitelist, while returning no AAAA
resource records to DNS resolvers which are not on the whitelist.
This practice is a type of IPv6 transition mechanism used by domains,
as a method for incrementally transitioning inbound traffic to a
domain from IPv4 to IPv6 transport. The practice appears to have
first been used by major web content sites (sometimes described
herein as "high-traffic domains"), which have specific concerns
relating to maintain a high-quality user experience for all of their
users during their transition to IPv6.
Critics of the practice of DNS Whitelisting have articulated several
concerns. Among these are that:
o DNS Whitelisting is a very different behavior from the current
practice concerning the publishing of IPv4 address resource
records,
o that it may create a two-tiered Internet,
o that policies and decision-making for whitelisting and de-
whitelisting are opaque or likely to cause conflict,
o that DNS Whitelisting reduces interest in the deployment of IPv6,
o that new operational and management burdens are created,
o that the practice does not scale,
o that it violates a basic premise of cross-Internet
interoperability by requiring prior arrangements,
o and that the costs and negative implications of DNS Whitelisting
outweigh the perceived benefits.
This document explores the reasons and motivations for DNS
Whitelisting Section 3. It also explores the concerns regarding this
practice, and whether and when the practice is recommended Section 9.
Readers will hopefully better understand what DNS Whitelisting is,
why some parties are implementing it, and what criticisms of the
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practice exist.
2. How DNS Whitelisting Works
At a high level, using a whitelist means no traffic is permitted to
the destination host unless the originating host's IP address is
contained in the whitelist. In contrast, using a blacklist means
that all traffic is permitted to the destination host unless the
originating host's IP address is contained in the blacklist.
DNS Whitelisting is implemented in authoritative DNS servers, not in
DNS recursive resolvers. These authoritative servers implement IP
address-based restrictions on AAAA query responses. So far, DNS
Whitelisting has been primarily implemented by web server operators
deploying IPv6-enabled services, though this practice would affect
any protocols and services within a domain. For a given operator of
a website, such as www.example.com, the operator essentially applies
an access control list (ACL) on the authoritative DNS servers for the
domain example.com. The ACL is populated with the IPv4 and/or IPv6
addresses or prefix ranges of DNS recursive resolvers on the
Internet, which have been authorized to receive AAAA resource record
responses. These DNS recursive resolvers are operated by third
parties, such as ISPs, universities, governments, businesses, and
individual end users. If a DNS recursive resolver IS NOT matched in
the ACL, then AAAA resource records will NOT be sent in response to a
query for a hostname in the example.com domain. However, if a DNS
recursive resolver IS matched in the ACL, then AAAA resource records
will be sent in response to a query for a given hostname in the
example.com domain. While these are not network-layer access
controls they are nonetheless access controls that are a factor for
end users and other parties like network operators, especially as
networks and hosts transition from one network address family to
another (IPv4 to IPv6).
In practice, DNS Whitelisting generally means that a very small
fraction of the DNS recursive resolvers on the Internet (those in the
whitelist ACL) will receive AAAA responses. The large majority of
DNS resolvers on the Internet will therefore receive only A resource
records containing IPv4 addresses. Thus, quite simply, the
authoritative server hands out different answers depending upon who
is asking; with IPv4 and IPv6 resource records for all those the
authorized whitelist, and only IPv4 resource records for everyone
else. See Section 2.1 and Figure 1 for a description of how this
works.
DNS Whitelisting also works independently of whether an authoritative
DNS server, DNS recursive resolver, or end user host uses IPv4
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transport, IPv6, or both. So, for example, whitelisting may prevent
sending AAAA responses even in those cases where the DNS recursive
resolver has queried the authoritative server over IPv6 transport, or
where the end user host's original query to the DNS recursive
resolver was over IPv6 transport. One important reason for this is
that even though the DNS recursive resolver may have no IPv6-related
impairments, this is not a reliable predictor of whether the same is
true of the end user host. This also means that a DNS whitelist can
contain both IPv4 and IPv6 addresses.
Finally, DNS Whitelisting can be deployed in two primary ways:
universally on a global basis, or on an ad hoc basis. Deployment on
a universal deployment basis means that DNS Whitelisting is
implemented on all authoritative DNS servers, across the entire
Internet. In contrast, deployment on an ad hoc basis means that only
some authoritative DNS servers, and perhaps even only a few,
implement DNS Whitelisting. These two potential deployment models
are described in Section 6.
2.1. Description of the Operation of DNS Whitelisting
The system logic of DNS Whitelisting is as follows:
1. The authoritative DNS server for example.com receives DNS queries
for the A (IPv4) and/or AAAA (IPv6) address resource records for
the FQDN www.example.com, for which AAAA (IPv6) resource records
exist.
2. The authoritative DNS server checks the IP address (IPv4, IPv6,
or both) of the DNS recursive resolver sending the AAAA (IPv6)
query against the access control list (ACL) that is the DNS
whitelist.
3. If the DNS recursive resolver's IP address IS matched in the ACL,
then the response to that specific DNS recursive resolver can
contain AAAA (IPv6) address resource records.
4. If the DNS recursive resolver's IP address IS NOT matched in the
ACL, then the response to that specific DNS recursive resolver
cannot contain AAAA (IPv6) address resource records. In this
case, the server should return a response with the response code
(RCODE) being set to 0 (No Error) with an empty answer section
for the AAAA record query.
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---------------------------------------------------------------------
A query is sent from a DNS recursive resolver that IS NOT on the DNS
whitelist:
Request Request
www.example.com www.example.com
AAAA +-------------+ AAAA +-----------------+
++--++ ---------> | RESOLVER | ---------> | www.example.com |
|| || A | **IS NOT** | A | IN A exists |
+-++--++-+ ---------> | ON | ---------> | IN AAAA exists |
+--------+ A | example.com | A | |
Host <--------- | WHITELIST | <--------- | |
Computer A Record +-------------+ A Record +-----------------+
Response DNS Recursive Response example.com
(only IPv4) Resolver (only IPv4) Authoritative
#1 Server
---------------------------------------------------------------------
A query is sent from a DNS recursive resolver that IS on the DNS
Whitelist:
Request Request
www.example.com www.example.com
AAAA +-------------+ AAAA +-----------------+
++--++ ---------> | RESOLVER | ---------> | www.example.com |
|| || A | **IS** | A | IN A exists |
+-++--++-+ ---------> | ON | ---------> | IN AAAA exists |
+--------+ AAAA | example.com | AAAA | |
Host <--------- | WHITELIST | <--------- | |
Computer A | | A | |
<--------- | | <--------- | |
A and AAAA +-------------+ A and AAAA +-----------------+
Record DNS Recursive Record example.com
Responses Resolver Responses Authoritative
(IPv4+IPv6) #2 (IPv4+IPv6) Server
---------------------------------------------------------------------
Figure 1: DNS Whitelisting - Functional Diagram
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---------------------------------------------------------------------
Resolver 1 - IS NOT ON the DNS Whitelist
Resolver 2 - IS ON the DNS Whitelist
---------------------------------------------------------------------
Host 1 Resolver 1 Authoritative Server
Query A and AAAA -----> Query A and AAAA ----> Receive A and
for www.example.com for www.example.com AAAA queries
A (IPv4) <------------- A (IPv4) <--------------- NOT on Whitelist
response response return only A (IPv4)
---------------------------------------------------------------------
Host 2 Resolver 2 Authoritative Server
Query A and AAAA -----> Query A and AAAA ----> Receive A and
for www.example.com for www.example.com AAAA queries
A (IPv4) <------------- A (IPv4) and <------------ IS on Whitelist
AAAA (IPv6) AAAA (IPv6) return A (IPv4)
responses responses and AAAA (IPv6)
---------------------------------------------------------------------
Figure 2: DNS Whitelisting - Request/Response Diagram
2.2. Comparison with Blacklisting
With DNS Whitelisting, DNS recursive resolvers can receive AAAA
resource records only if they are on the whitelist. In contrast,
blacklisting would be the opposite whereby all DNS recursive
resolvers can receive AAAA resource records unless they are on the
blacklist. So a whitelist contains a list of hosts allowed
something, whereby a blacklist contains a list of hosts disallowed
something. While the distinction between the concepts of
whitelisting and blacklisting are important, this is noted
specifically since some implementers of DNS Whitelisting may choose
to transition to DNS Blacklisting before returning to a state without
address-family-related ACLs in their authoritative DNS servers. It
is unclear when and if it would be appropriate to change from
whitelisting to blacklisting. Nor is it clear how implementers will
judge the network conditions to have changed sufficiently to justify
disabling AAAA resource record access controls.
3. What Problems Are Implementers Trying To Solve?
Implementers are attempting to protect users of their domain from
having a negative experience (poor performance) when they receive DNS
response containing AAAA resource records or when attempting to use
IPv6 transport. Therefore there are two concerns which relate to
this practice; one of which relates to IPv6-related impairment and
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the other which relates to the maturity or stability of IPv6
transport for high-traffic domains.
Finally, some domains, have run IPv6 experiments whereby they added
AAAA resource records and observed and measured errors [Heise Online
Experiment], which should be important reading for any domain
contemplating either the use of DNS Whitelisting or simply adding
IPv6 addressing to their site.
3.1. IPv6-Related Impairment
Some implementers have observed that when they added AAAA resource
records to their authoritative DNS servers in order to support IPv6
access to their content that a small fraction of end users had slow
or otherwise impaired access to a given web site with both AAAA and A
resource records. The fraction of users with such impaired access
has been estimated to be as high as 0.078% of total Internet users
[IETF-77-DNSOP] [NW-Article-DNSOP] [Evaluating IPv6 Adoption] [IPv6
Brokenness]. In these situations, DNS recursive resolvers are added
to the DNS Whitelist only when the measured level of impairment of
the hosts using that resolver declines to some level acceptable by
the implementer.
As a result of this impairment affecting end users of a given domain,
a few high-traffic domains have either implemented DNS Whitelisting
or are considering doing so [NW-Article-DNS-WL] [IPv6 Whitelist
Operations]. While it is outside the scope of this document to
explore the various reasons why a particular user's system (host) may
have impaired IPv6 access, for the users who experience this
impairment it has a very real performance impact. It would affect
access to all or most dual stack services to which the user attempts
to connect. This negative end user experience can range from
somewhat slower than usual (as compared to native IPv4-based access),
to extremely slow, to no access to the domain whatsoever. In
essence, whether the end user even has an IPv6 address or not (they
probably only have an IPv4 address), merely by receiving a AAAA
record response the user either cannot access a FQDN or it is so slow
that the user gives up and assumes the destination is unreachable.
In addition, at least one high-traffic domain has noted that they
have received requests to not send DNS responses with AAAA resource
records to particular resolvers. In this case, DNS recursive
resolvers operators have expressed a short-term concern that their
IPv6 network infrastructure is not yet ready to handle the large
traffic volume that may be associated with the hosts in their network
connecting to the websites of these domains. These end user networks
may also have other tools at their disposal in order to address this
concern, including applying rules to network equipment such as
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routers and firewalls (this will necessarily vary by the type of
network, as well as the technologies used and the design of a given
network), as well as configuration of their DNS recursive resolvers
(though modifying or suppressing AAAA resource records in a DNSSEC-
signed domain on a Security-Aware Resolver will be problematic
Section 10.1).
3.2. Volume-Based Concerns
Some implementers are trying to gradually add IPv6 traffic to their
domain since they may find that network operations, tools, processes
and procedures are less mature for IPv6 as compared to IPv4. While
for domains with small to moderate traffic volumes, whether by the
count of end users or count of bytes transferred, high-traffic
domains receive such a level of usage that it is prudent to undertake
any network changes gradually or in a manner which minimizes any risk
of disruption. For example, one can imagine for one of the top ten
sites globally that the idea of suddenly turning on a significant
amount of IPv6 traffic might be quite daunting. DNS Whitelisting may
therefore offer such high-traffic domains one potential method for
incrementally enabling IPv6. Thus, some implementers with high-
traffic domains plan to use DNS Whitelisting is a necessary, though
temporary, risk reduction tactic intended to ease their transition to
IPv6 and minimize any perceived risk in such a transition.
3.3. Free Versus Subscription Services
It is also worth noting the differences between domains containing
primarily subscription-based services compared to those containing
primarily free services. In the case of free services, such as
search engines, end users have no direct billing relationship with
the domain and can switch sites simply by changing the address they
enter into their browser (ignoring other value added services which
may tie a user?s preference to a given domain or otherwise create
switching costs). As a result, such domains explain that they
believe they are more sensitive to the quality of the services within
their domain since if the user has issues when they turn on IPv6,
then that user could switch to another domain that is not using IPv6.
4. Concerns Regarding DNS Whitelisting
The implications relating to DNS Whitelisting are further enumerated
here and in Section 7.
Some parties in the Internet community, including ISPs, are concerned
that the practice of DNS Whitelisting for IPv6 address resource
records represents a departure from the generally accepted practices
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regarding IPv4 address resource records in the DNS on the Internet
[Whitelisting Concerns]. These parties explain their belief that
once an authoritative server operator adds an A record (IPv4) to the
DNS, then any DNS recursive resolver on the Internet can receive that
A record in response to a query. By extension, this means that any
of the hosts connected to any of these DNS recursive resolvers can
receive the IPv4 address resource records for a given FQDN. This
enables new server hosts which are connected to the Internet, and for
which a fully qualified domain name (FQDN) such as www.example.com
has been added to the DNS with an IPv4 address record, to be almost
immediately reachable by any host on the Internet. In this case,
these new servers hosts become more and more widely accessible as new
networks and new end user hosts connect to the Internet over time,
capitalizing on and increasing so-called "network effects" (also
called network externalities). It also means that the new server
hosts do not need to know about these new networks and new end user
hosts in order to make their content and applications available to
them, in essence that each end in this end-to-end model is
responsible for connecting to the Internet and once they have done so
they can connect to each other without additional impediments or
middle networks or intervening networks or servers knowing about
these end points and whether one is allowed to contact the other.
In contrast, the concern is that DNS Whitelisting may fundamentally
change this model. In the altered DNS Whitelisting end-to-end model,
one end (where the end user is located) cannot readily connect to the
other end (where the content is located), without parts of the middle
(DNS recursive resolvers) used by one end (the client, or end user
hosts) being known to an intermediary (authoritative nameservers) and
approved for access to the resource at the end. As new networks
connect to the Internet over time, those networks need to contact any
and all domains which have implemented DNS Whitelisting in order to
apply to be added to their DNS whitelist, in the hopes of making the
content and applications residing on named server hosts in those
domains accessible by the end user hosts on that new network.
Furthermore, this same need to contact all domains implementing DNS
Whitelisting also applies to all pre-existing (but not whitelisted)
networks connected to the Internet.
In the current IPv4 Internet when a new server host is added to the
Internet it is generally widely available to all end user hosts; when
DNS Whitelisting of IPv6 resource records is used, these new server
hosts are not accessible via a FQDN by any end user hosts until such
time as the operator of the authoritative DNS servers for those new
server hosts expressly authorizes access to those new server hosts by
adding DNS recursive resolvers around the Internet to the ACL. This
has the potential to be a significant change in reachability of
content and applications by end users and networks as these end user
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hosts and networks transition to IPv6, resulting in more (but
different) breakage. A concern expressed is that if much of the
content that end users are most interested in is not accessible as a
result, then end users and/or networks may resist adoption of IPv6 or
actively seek alternatives to it, such as using multi-layer network
address translation (NAT) techniques like NAT444
[I-D.shirasaki-nat444] on a long-term basis. There is also concern
that this practice could disrupt the continued increase in Internet
adoption by end users if they cannot simply access new content and
applications but must instead contact the operator of their DNS
recursive resolver, such as their ISP or another third party, to have
their DNS recursive resolver authorized for access to the content or
applications that interests them. Meanwhile, these parties say, over
99.9% of the other end users that are also using that same network or
DNS recursive resolver are unable to access the IPv6-based content,
despite their experience being a positive one.
While in Section 1 the level of IPv6-related impairment has been
estimated to be as high as 0.078% of Internet users, which is a
primary motivation cited for the practice of DNS Whitelisting, it is
not clear if the level of IPv4-related impairment is more or less
that this percentage (which in any case is likely to have declined
since its original citation). Indeed, as at least one document
reviewer has pointed out, it may simply be that websites are only
measuring IPv6 impairments and not IPv4 impairments, whether because
IPv6 is new or whether those websites are simply unable to or are
otherwise not in a position to be able to measure IPv4 impairment
(since this could result in no Internet access whatsoever). As a
result, it is worth considering that IPv4-related impairment could
exceed that of IPv6-related impairment and that such IPv4-related
impairment may have simply been accepted as "background noise" on the
Internet for a variety of reasons. Of course, this comparison of the
level of worldwide IPv6 impairments to IPv4 impairments is
speculation, as the author is not aware of any good measurement of
IPv4-related impairments which are comparable in nature to the IPv6-
related impairment measurements which have recently been conducted
around the world.
An additional concern is that the IP address of a DNS recursive
resolver is not a precise indicator of the IPv6 preparedness, or lack
of IPv6-related impairments, of end user hosts which query (use) a
particular DNS recursive resolver. While the DNS recursive resolver
may be an imperfect proxy for judging IPv6 preparedness, it is at
least one of the best available methods at the current time.
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5. Similarities to Other DNS Operations
Some aspects of DNS Whitelisting may be considered similar to other
common DNS operational techniques which are explored below.
5.1. Similarities to Split DNS
DNS Whitelisting has some similarities to so-called split DNS,
briefly described in Section 3.8 of [RFC2775]. When split DNS is
used, the authoritative DNS server returns different responses
depending upon what host has sent the query. While [RFC2775] notes
the typical use of split DNS is to provide one answer to hosts on an
Intranet and a different answer to hosts on the Internet, the essence
is that different answers are provided to hosts on different
networks. This is basically the way that DNS Whitelisting works,
whereby hosts on different networks which use different DNS recursive
resolvers, receive different answers if one DNS recursive resolver is
on the whitelist and the other is not.
In [RFC2956], Internet transparency and Internet fragmentation
concerns regarding split DNS are detailed in Section 2.1. [RFC2956]
further notes in Section 2.7, concerns regarding split DNS and that
it "makes the use of Fully Qualified Domain Names (FQDNs) as endpoint
identifiers more complex." Section 3.5 of [RFC2956] further
recommends that maintaining a stable approach to DNS operations is
key during transitions such as the one to IPv6 that is underway now,
stating that "Operational stability of DNS is paramount, especially
during a transition of the network layer, and both IPv6 and some
network address translation techniques place a heavier burden on
DNS."
5.2. Similarities to DNS Load Balancing
DNS Whitelisting also has some similarities to DNS load balancing.
There are of course many ways that DNS load balancing can be
performed. In one example, multiple IP address resource records (A
and/or AAAA) can be added to the DNS for a given FQDN. This approach
is referred to as DNS round robin [RFC1794]. DNS round robin may
also be employed where SRV resource records are used [RFC2782].
In another example, one or more of the IP address resource records in
the DNS will direct traffic to a load balancer. That load balancer,
in turn, may be application-aware, and pass the traffic on to one or
more hosts connected to the load balancer which have different IP
addresses. In cases where private IPv4 addresses are used [RFC1918],
as well as when public IP addresses are used, those end hosts may not
be directly reachable without passing through the load balancer first
. As such, while the IP address resource records have been added to
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the DNS, the end hosts are not necessarily directly reachable, which
is in a small way similar to one aspect of DNS Whitelisting.
Additionally, a geographically-aware authoritative DNS server may be
used, as is common with Content Delivery Networks (CDNs) or Global
Load Balancing (GLB, also referred to as Global Server Load
Balancing, or GSLB), whereby the IP address resource records returned
to a resolver in response to a query will vary based on the estimated
geographic location of the resolver [Resolvers in the Wild]. CDNs
perform this function in order to attempt to direct hosts to connect
to the nearest content cache. As a result, one can see some
similarities with DNS Whitelisting insofar as different IP address
resource records are selectively returned to resolvers based on the
IP address of each resolver (or other imputed factors related to that
IP address). However, what is different is that in this case the
resolvers are not deliberately blocked from receiving DNS responses
containing an entire class of addresses; this load balancing function
strives to perform a content location-improvement function and not an
access control function.
6. Possible Deployment Scenarios
In considering how DNS Whitelisting may emerge more widely, there are
two main deployment scenarios, which are explored below.
In either of these deployment scenarios, it is possible that
reputable third parties could create and maintain DNS whitelists, in
much the same way that blacklists are distributed and used for
reducing email spam. In the email context, a mail operator
subscribes to one or more of these lists and as such the operational
processes for additions and deletions to the list are managed by a
third party. A similar model could emerge for DNS Whitelisting,
whether deployment occurs universally or on an ad hoc basis.
An additional factor in either scenario is that a DNS recursive
resolver operator will have to determine whether or not DNS
Whitelisting has been implemented for a domain, since the absence of
AAAA resource records may simply be indicative that the domain has
not yet added IPv6 addressing for the domain, rather than that they
have done so but are using DNS Whitelisting. This will be
challenging at scale.
6.1. Deploying DNS Whitelisting On An Ad Hoc Basis
The most likely deployment scenario is where some authoritative DNS
server operators implement DNS Whitelisting but many or most others
do not do so. What can make this scenario challenging from the
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standpoint of a DNS recursive resolver operator is determining which
domains implement DNS Whitelisting, particularly since a domain may
not do so as they initially transition to IPv6, and may instead do so
later. Thus, a DNS recursive resolver operator may initially believe
that they can receive AAAA responses as a domain adopts IPv6, but
then notice via end user reports that they no longer receive AAAA
responses due to that domain adopting DNS Whitelisting. Of course, a
domain's IPv6 transition may be effectively invisible to DNS
recursive resolver operators due to the effect of DNS Whitelisting.
One benefit of DNS Whitelisting being deployed on an ad hoc basis is
that only the domains that are interested in doing so would have to
upgrade their authoritative DNS servers in order to implement the
ACLs necessary to perform DNS Whitelisting. Some domains have
proposed or are implementing this and are manually updating their
whitelist, while other such as CDNs have discussed the possibility of
an automated method for doing so.
In this potential deployment scenario, it is also possible that a
given domain will implement DNS Whitelisting temporarily. A domain,
particularly a high-traffic domain, may choose to do so in order to
ease their transition to IPv6 through a selective deployment and
minimize any perceived risk in such a transition. In addition, it is
possible that one or more DNS Whitelist clearinghouses may emerge,
providing implementers with a way to subscribe to a whitelist in a
manner similar to that used on email servers for blacklists.
6.2. Deploying DNS Whitelisting Universally
The less likely deployment scenario is one where DNS Whitelisting is
implemented on all authoritative DNS servers, across the entire
Internet. While this scenario seems less likely than ad hoc
deployment due to some parties not sharing the concerns that have so
far motivated the use of DNS Whitelisting, it is nonetheless
conceivable that it could be one of the ways in which DNS
Whitelisting is deployed, though such a universal deployment could be
considered harmful and problematic.
In order for this deployment scenario to occur, it is likely that DNS
Whitelisting functionality would need to be built into all
authoritative DNS server software, and that all operators of
authoritative DNS servers would have to upgrade their software and
enable this functionality. It is likely that new IETF Request for
Comment (RFC) documents would need to be developed which describe how
to properly configure, deploy, and maintain DNS Whitelisting. As a
result, it is unlikely that DNS Whitelisting would, at least in the
next several years, become universally deployed. Furthermore, these
DNS whitelists are likely to vary on a domain-by-domain basis,
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depending upon a variety of factors. Such factors may include the
motivation of each domain owner, the location of the DNS recursive
resolvers in relation to the source content, as well as various other
parameters that may be transitory in nature, or unique to a specific
end user host type. It is probably unlikely that a single
clearinghouse for managing whitelisting is possible; it will more
likely be unique to the source content owners and/or domains which
implement DNS whitelists (so multiple clearinghouses are certainly
possible).
7. Implications of DNS Whitelisting
There are many implications of DNS Whitelisting. The key
implications are detailed below.
7.1. Architectural Implications
DNS Whitelisting modifies the end-to-end model and the general notion
of spontaneous interoperability of the architecture that prevails on
the Internet today. This is because this approach moves additional
access control information and policies into the middle of the DNS
resolution path of the IPv6-addressed Internet, which generally did
not exist before on the IPv4-addressed Internet, and it requires some
type of prior registration with authoritative servers. This poses
some risks noted in [RFC3724]. In explaining the history of the end-
to-end principle [RFC1958] states that one of the goals is to
minimize the state, policies, and other functions needed in the
middle of the network in order to enable end-to-end communications on
the Internet. In this case, the middle network should be understood
to mean anything other than the end hosts involved in communicating
with one another. Some state, policies, and other functions have
always been necessary to enable such end-to-end communication, but
the goal of the approach has been to minimize this to the greatest
extent possible.
It is also possible that DNS Whitelisting could place at risk some of
the observed benefits of the end-to-end principle, as listed in
Section 4.1 of [RFC3724], such as protection of innovation.
[RFC3234] details issues and concerns regarding so-called
middleboxes, so there may also be parallel concerns with the DNS
Whitelisting approach, especially concerning modified DNS servers
noted in Section 2.16 of [RFC3234], as well as more general concerns
noted in Section 1.2 of [RFC3234] about the introduction of new
failure modes. In particular, there may be concerns that
configuration is no longer limited to two ends of a session, and that
diagnosis of failures and misconfigurations becomes more complex.
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Two additional sources worth considering as far as implications for
the end-to-end model are concerned are [Tussle in Cyberspace] and
[Rethinking the Internet]. In [Tussle in Cyberspace], the authors
note concerns regarding the introduction of new control points, as
well as "kludges" to the DNS, as risks to the goal of network
transparency in the end-to-end model. Some parties concerned with
the emerging use of DNS Whitelisting have shared similar concerns,
which may make [Tussle in Cyberspace] an interesting and relevant
document. In addition, [Rethinking the Internet] reviews similar
issues that may be of interest to readers of this document.
Also, it is somewhat possible that DNS Whitelisting could affect some
of the architectural assumptions which underlie parts of Section 2 of
[RFC4213] which outlines the dual stack approach to the IPv6
transition. DNS Whitelisting could modify the behavior of the DNS,
as described in Section 2.2 of [RFC4213] and could require different
sets of DNS servers to be used for hosts that are (using terms from
that document) IPv6/IPv4 nodes, IPv4-only nodes, and IPv6-only nodes.
As such, broad use of DNS Whitelisting may necessitate the review
and/or revision (though revision is unlikely to be neccessary) of
standards documents which describe dual-stack and IPv6 operating
modes, dual-stack architecture generally, and IPv6 transition
methods, including but not limited to [RFC4213].
7.2. Public IPv6 Address Reachability Implications
It is a critical to understand that the concept of reachability
described here depends upon a knowledge of an address in the DNS.
Thus, in order to establish reachability to an end point, a host is
dependent upon looking up an IP address in the DNS when a FQDN is
used. When DNS Whitelisting is used, it is quite likely that an
IPv6-enabled end user host could connect to an example server host
using the IPv6 address, even though the FQDN associated with that
server host is restricted via a DNS whitelist. Since most Internet
applications and hosts such as web servers depend upon the DNS, and
as end users connect to FQDNs such as www.example.com and do not
remember or wish to type in an IP address, the notion of reachability
described here should be understood to include knowledge of how to
associate a name with a network address.
The predominant experience of end user hosts and servers on the IPv4-
addressed Internet today is that when a new server with a public IPv4
address is added to the DNS, that a FQDN is immediately useful for
reaching it. This is a generalization and in Section 5 there are
examples of common cases where this may not necessarily be the case.
For the purposes of this argument, that concept of accessibility is
described as "pervasive reachability". It has so far been assumed
that the same expectations of pervasive reachability would exist in
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the IPv6-addressed Internet. However, if DNS Whitelisting is
deployed, this will not be the case since only end user hosts using
DNS recursive resolvers that are included in the ACL of a given
domain using DNS Whitelisting would be able to reach new servers in
that given domain via IPv6 addresses. The expectation of any end
user host being able to connect to any server (essentially both
hosts, just at either end of the network), defined here as "pervasive
reachability", will change to "restricted reachability" with IPv6.
Establishing DNS Whitelisting as an accepted practice in the early
phases of mass IPv6 deployment could establish it as an integral part
of how IPv6 DNS resource records are deployed globally. This risks
DNS Whitelisting living on for decades as a key foundational element
of domain name management on the Internet.
7.3. Operational Implications
This section explores some of the operational implications which may
occur as a result of, are related to, or become necessary when
engaging in the practice of DNS Whitelisting.
7.3.1. De-Whitelisting May Occur
It is possible for a DNS recursive resolver added to a whitelist to
then be removed from the whitelist, also known as de-whitelisting.
Since de-whitelisting can occur, through a decision by the
authoritative server operator, the domain owner, or even due to a
technical error, an operator of a DNS recursive resolver will have
new operational and monitoring requirements and/or needs as noted in
Section 7.3.3, Section 7.3.4, Section 7.3.6, and Section 7.5. One
particular risk is that, especially when a high-traffic domain de-
whitelists a large network, this may cause a sudden and dramatic
change to networks since a large volume of traffic will then switch
from IPv6 to IPv4. This can have dramatic effects on those being de-
whitelisted as well as on other interconnected networks. In some
cases, IPv4 network links may rapidly become congested and users of
affected networks will experience network access impairments well
beyond the domain which performed the de-whitelisting. Thus, once
"operational stability" has been achieved between a whitelisting and
whitelisted party, then de-whitelisting should generally not occur
except in cases of operational emergencies, and there should be
opportunities for joint troubleshooting or at least for advance
warning to affected parties.
7.3.2. Authoritative DNS Server Operational Implications
DNS Whitelisting serves as a critical infrastructure service; to be
useful it needs careful and extensive administration, monitoring and
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operation. Each new and essential mechanism creates substantial
follow-on support costs.
Operators of authoritative servers (which are frequently
authoritative for multiple domain names) will need to maintain an ACL
on a server-wide basis affecting all domains, or on a domain-by-
domain basis. As a result, operational practices and software
capabilities may need to be developed in order to support such
functionality. In addition, processes may need to be put in place to
protect against inadvertently adding or removing IP addresses, as
well as systems and/or processes to respond to such incidents if and
when they occur. For example, a system may be needed to record DNS
Whitelisting requests, report on their status along a workflow, add
IP addresses when whitelisting has been approved, remove IP addresses
when they have been de-whitelisted, log the personnel involved and
timing of changes, schedule changes to occur in the future, and to
roll back any inadvertent changes.
Operators may also need implement new forms of monitoring in order to
apply change control, as noted briefly in Section 7.3.4.
It is important for operators of authoritative servers to recognize
that the operational burden is likely to increase dramatically over
time, as more and more networks transition to IPv6. As a result, the
volume of new DNS Whitelisting requests will increase over time,
potentially at an extraordinary growth rate, which will place an
increasing burden on personnel, systems, and/or processes. Operators
should also consider that any supporting systems, including the
authoritative servers themselves, may experience reduced performance
when a DNS whitelist becomes quite large.
7.3.3. DNS Recursive Resolver Server Operational Implications
For operators of DNS recursive resolvers, coping with DNS
Whitelisting becomes expensive in time and personnel as the practice
scales up. These operators include ISPs, enterprises, universities,
governments; a wide range of organization types with a range of DNS-
related expertise. They will need to implement new forms of
monitoring, as noted briefly in Section 7.3.4. But more critically,
such operators will need to add people, processes, and systems in
order to manage large numbers of DNS Whitelisting applications.
Since there is no common method for determining whether or not a
domain is engaged in DNS Whitelisting, operators will have to apply
to be whitelisted for a domain based upon one or more end user
requests, which means systems, processes, and personnel for handling
and responding to those requests will also be necessary.
When operators apply for DNS Whitelisting for all domains, that may
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mean doing so for all registered domains. Thus, some system would
have to be developed to discover whether each domain has been
whitelisted or not, which is touched on in Section 6 and may vary
depending upon whether DNS Whitelisting is universally deployed or is
deployed on an ad hoc basis.
These operators (of DNS recursive resolvers) will need to develop
processes and systems to track the status of all DNS Whitelisting
applications, respond to requests for additional information related
to these applications, determine when and if applications have been
denied, manage appeals, and track any de-whitelisting actions.
Given the large number of domains in existence, the ease with which a
new domain can be added, and the continued strong growth in the
numbers of new domains, readers should not underestimate the
potential significance in personnel and expense that this could
represent for such operators. In addition, it is likely that systems
and personnel may also be needed to handle new end user requests for
domains for which to apply for DNS Whitelisting, and/or inquiries
into the status of a whitelisting application, reports of de-
whitelisting incidents, general inquiries related to DNS
Whitelisting, and requests for DNS Whitelisting-related
troubleshooting by these end users.
7.3.4. Monitoring Implications
Once a DNS recursive resolver has been whitelisted for a particular
domain, then the operator of that DNS recursive resolver may need to
implement monitoring in order to detect the possible loss of DNS
Whitelisting in the future. This DNS recursive resolver operator
could configure a monitor to check for a AAAA response in the
whitelisted domain, as a check to validate continued status on the
DNS whitelist. The monitor could then trigger an alert if at some
point the AAAA responses were no longer received, so that operations
personnel could begin troubleshooting, as outlined in Section 7.3.6.
Also, authoritative DNS server operators are likely to need to
implement new forms of monitoring. In this case, they may desire to
monitor for significant changes in the size of the whitelist within a
certain period of time, which might be indicative of a technical
error such as the entire ACL being removed. Authoritative DNS server
operators may also wish to monitor their workflow process for
reviewing and acting upon DNS Whitelisting applications and appeals,
potentially measuring and reporting on service level commitments
regarding the time an application or appeal can remain at each step
of the process, regardless of whether or not such information is
shared with parties other than that authoritative DNS server
operator.
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7.3.5. Implications of Operational Momentum
It seems plausible that once DNS Whitelisting is implemented it will
be very difficult to deprecate such technical and operational
practices. This assumption is based on an understanding of human
nature, not to mention physics. For example, as Sir Isaac Newton
noted, "Every object in a state of uniform motion tends to remain in
that state of motion unless an external force is applied to it" [Laws
of Motion]. Thus, once DNS Whitelisting is implemented it is quite
likely that it would take considerable effort to deprecate the
practice and remove it everywhere on the Internet; it may otherwise
simply remain in place in perpetuity. To illustrate this point, one
could consider for example that there are many email servers
continuing to attempt to query anti-spam DNS blocklists which have
long ago ceased to exist.
7.3.6. Troubleshooting Implications
The implications of DNS whitelisted present many challenges, as
detailed throughout Section 7. These challenges may negatively
affect the end users' ability to troubleshoot, as well as that of DNS
recursive resolver operators, ISPs, content providers, domain owners
(where they may be different from the operator of the authoritative
DNS server for their domain), and other third parties. This may make
the process of determining why a server is not reachable via a FQDN
significantly more complex and time-consuming.
7.3.7. Additional Implications If Deployed On An Ad Hoc Basis
As more domains choose to implement DNS Whitelisting, and more
networks become IPv6-capable and request to be whitelisted, scaling
up operational processes, monitoring, and ACL updates will become
more difficult. The increased rate of change and increased size of
whitelists will increase the likelihood of configuration and other
operational errors.
It is unclear when and if it would be appropriate to change from
whitelisting to blacklisting. It is clear that trying to coordinate
this across the Internet is likely be be impossible, so such a change
to blacklisting would happen on a domain-by-domain basis (if at all).
Finally, some implementers consider DNS Whitelisting to be a
temporary measure. As such, it is not clear how these implementers
will judge the network conditions to have changed sufficiently to
justify disabling DNS Whitelisting (or Blacklisting, or other AAAA
resource record access controls) and/or what the process and timing
will be in order to discontinue this practice.
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One further implication is that an end user with only an IPv4
address, using a DNS resolver which has not been whitelisted by any
domains, would not be able to get any AAAA resource records. In such
a case, this could give that end user the incorrect impression that
there is no IPv6-based content on the Internet since they are unable
to discover any IPv6 addresses via the DNS.
7.4. Homogeneity May Be Encouraged
A broad trend on the Internet is a move toward more heterogeneity.
One manifestation of this is in an increasing number, variety, and
customization of end user hosts, including home networks, operating
systems, client software, home network devices, and personal
computing devices. This trend appears to have had a positive effect
on the development and growth of the Internet. This trend has
enabled end user to connect any technically compliant device or use
any technically compatible software to connect to the Internet. Not
only does this trend towards greater heterogeneity reduce the control
which is exerted in the middle of the network, described positively
in [Tussle in Cyberspace], [Rethinking the Internet], and [RFC3724],
but it also appears to help to enable greater and more rapid
innovation at the edges.
Some forms of so-called "network neutrality" principles around the
world include the notion that any IP-capable device should be able to
connect to a network, which seems to encourage heterogeneity. These
principles are often explicitly encouraged by application providers
given the reasons noted above, though some of these same providers
may also be implementing DNS Whitelisting. This is ironic, as one
implication of the adoption of DNS Whitelisting is that in encourages
a move back towards homogeneity. This is because some implementers
have expressed a preference for greater levels of control by networks
over end user hosts in order to attempt to enforce technical
requirements intended to reduce IPv6-related impairments. This
return to an environment of more homogenous and/or controlled end
user hosts could have unintended side effects on and counter-
productive implications for future innovation at the edges of the
network.
7.5. Technology Policy Implications
A key technology policy implication concerns the policies relating to
the process of reviewing an application for DNS Whitelisting, and the
decision-making process regarding whitelisting for a domain.
Important questions may include whether these policies have been
fully and transparently disclosed, are non-discriminatory, and are
not anti-competitive. A related implication is whether and what the
process for appeals is, when a domain decides against adding a DNS
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recursive resolver to the whitelist. Key questions here may include
whether appeals are allowed, what the process is, what the expected
turn around time is, and whether the appeal will be handled by an
independent third party or other entity/group.
Further implications arise when de-whitelisting occurs. Questions
that may naturally be raised in such a case include whether the
criteria for de-whitelisting have been fully and transparently
disclosed, are non-discriminatory, and are not anti-competitive.
Additionally, the question of whether or not there was a cure period
available prior to de-whitelisting, during which troubleshooting
activities, complaint response work, and corrective actions may be
attempted, and whether this cure period was a reasonable amount of
time.
It is also conceivable that whitelisting and de-whitelisting
decisions could be quite sensitive to concerned parties beyond the
operator of the domain which has implemented DNS Whitelisting and the
operator of the DNS recursive resolver, including end users,
application developers, content providers, advertisers, public policy
groups, governments, and other entities, which may also seek to
become involved in or express opinions concerning whitelisting and/or
de-whitelisting decisions. Lastly, it is conceivable that any of
these interested parties or other related stakeholders may seek
redress outside of the process a domain has establishing for DNS
Whitelisting and de-whitelisting.
A final concern is that decisions relating to whitelisting and de-
whitelisting may occur as an expression of other commercial,
governmental, and/or cultural conflicts, given the new control point
which has been established with DNS Whitelisting. For example, in
one imagined scenario, a domain could withhold adding a network to
their DNS Whitelisting unless that network agreed to some sort of
financial payment, legal agreement, agreement to sever a relationship
with a competitor of the domain, etc. In another example, a music-
oriented domain may be engaged in some sort of dispute with an
academic network concerning copyright infringement concerns within
that network, and may choose to de-whitelist that network as a
negotiating technique in some sort of commercial discussion. In a
final example, a major email domain may choose to de-whitelist a
network due to that network sending some large volume of spam. Thus,
it seems possible that DNS Whitelisting and de-whitelisting could
become a vehicle for adjudicating other disputes, and that this may
well have intended and unintended consequences for end users which
are affected by such decisions and are unlikely to be able to express
a strong voice in such decisions.
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7.6. IPv6 Adoption Implications
As noted in Section 4, the implications of DNS Whitelisting may drive
end users and/or networks to delay, postpone, or cancel adoption of
IPv6, or to actively seek alternatives to it. Such alternatives may
include the use of multi-layer network address translation (NAT)
techniques like NAT444 [I-D.shirasaki-nat444], which these parties
may decide to pursue on a long-term basis to avoid the perceived
costs and aggravations related to DNS Whitelisting. This could of
course come at the very time that the Internet community is trying to
get these very same parties interested in IPv6 and motivated to begin
the transition to IPv6. As a result, parties that are likely to be
concerned over the negative implications of DNS Whitelisting could
logically be concerned of the negative effects that this practice
could have on the adoption of IPv6 if it became widespread or was
adopted by majors Internet domains or other major parties in the
Internet ecosystem.
At the same time, as noted in Section 3, some high-traffic domains
may find the prospect of transitioning to IPv6 daunting without
having some short-term ability to incrementally control the amount
and source of IPv6 traffic to their domains. Lacking such controls,
some domains may choose to substantially delay their transition to
IPv6.
7.7. Implications with Poor IPv4 and Good IPv6 Transport
It is possible that there could be situations where the differing
quality of the IPv4 and IPv6 connectivity of an end user could cause
complications in accessing content which is in a whitelisted domain,
when the end user's DNS recursive resolver is not on that whitelist.
While today most end users' IPv4 connectivity is typically superior
to IPv6 connectivity (if such connectivity exists at all), there
could be implications when the reverse is true and and end user has
markedly superior IPv6 connectivity as compared to IPv4. This is
admittedly theoretical but could become a factor as the transition to
IPv6 continues and IPv4 address availability within networks becomes
strained.
For example, in one possible scenario, a user is issued IPv6
addresses by their ISP and has a home network and devices or
operating systems which fully support IPv6. As a result this
theoretical user has very good IPv6 connectivity. However, this end
user's ISP may have exhausted their available pool of unique IPv4
address, and so that ISP uses NAT in order to reuse IPv4 addresses.
So for IPv4 content, the end user must send their IPv4 traffic
through some additional network element (e.g. NAT, proxy, tunnel
server). Use of this additional network element may cause the end
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user to experience sub-optimal IPv4 connectivity when certain
protocols or applications are used. This user then has good IPv6
connectivity but impaired IPv4 connectivity. Furthermore, this end
user's DNS recursive resolver is not whitelisted by the authoritative
server for a domain that the user is trying to access, meaning the
end user only gets A record responses for their impaired IPv4
transport rather than also AAAA record responses for their stable and
well-performing IPv6 transport. Thus, the user's poor IPv4
connectivity situation is potentially exacerbated by not having
access to a given domain's IPv6 content since they must use the
address family with relatively poor performance.
7.8. Implications for Users of Third-Party DNS Recursive Resolvers
In most cases it is assumed that end users will make use of DNS
recursive resolvers which are operated by their network provider,
whether that is an ISP, campus network, enterprise network, or some
other type of access network. However there are also cases where an
end user has changed their DNS server IP addresses in their device's
operating system to those of another party which operates DNS
recursive resolvers independently of end user access networks. In
these cases, an authoritative DNS server may receive a query from a
DNS recursive resolver in one network, though the end user sending
the original query to the DNS recursive resolver is in an entirely
different network. It may therefore be more challenging for a DNS
whitelist implementer to determine the level of IPv6-related
impairment when such third-party DNS recursive resolvers are used,
given the wide variety of end user access networks which may be used
and that this mix may change in unpredictable ways over time.
There may also be cases where end users are using a network where the
assigned DNS recursive resolver has not been whitelisted by a
particular authoritative DNS server, but where the end user knows
that a particular third-party DNS recursive resolver has been
whitelisted. While in most cases the end user will be able to switch
to use that third-party's servers, some access networks may prevent
switching to any DNS recursive resolvers other than those authorized
by or residing within a given access network. While the blocking of
third-party DNS recursive resolvers may be observed in many types of
networks such as ISPs, campus networks, and enterprise networks, this
may most often be observed in the specialized networks setup in
hotels, conference centers, coffee shops, and similar access
networks. In these cases, end users may be frustrated at their
inability to access content over IPv6 as a result of their access
network preventing them from using a whitelisted third-party DNS
recursive resolver. This may also result in complaints to both the
operator of the authoritative DNS server which has implemented
whitelisting as well as to the access network operator.
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8. General Implementation Variations
This section outlines several possible approaches which may be
followed when considering DNS Whitelisting and associated IPv6-
related issues.
8.1. Implement DNS Whitelisting Universally
One obvious approach is to implement DNS whitelisted universally, and
to do so using some sort of centralized registry of DNS Whitelisting
policies, contracts, processes, or other information. Such an
approach is also considered harmful and problematic, and almost
certain not to happen.
8.2. Implement DNS Whitelisting On An Ad Hoc Basis
DNS Whitelisting is now being adopted on an ad hoc, or domain-by-
domain basis. Therefore, only those domains interested in DNS
Whitelisting would need to adopt the practice, though as noted herein
discovering that a given domain has done so may be problematic. Also
in this scenario, ad hoc use by a particular domain may be a
temporary measure that has been adopted to ease the transition of the
domain to IPv6 over some short-term timeframe.
8.3. Do Not Implement DNS Whitelisting
As an alternative to adopting DNS Whitelisting, the Internet
community generally can choose not to implement DNS Whitelisting,
perpetuating the current predominant authoritative DNS operational
model on the Internet. As a result is is then up to end users with
IPv6-related impairments to discover and fix those impairments,
though clearly other parties including end user host operating system
developers can play a critical role [I-D.ietf-v6ops-happy-eyeballs].
8.3.1. Solve Current End User IPv6 Impairments
A further extension of not implementing DNS Whitelisting, is to also
endeavor to actually fix the underlying technical problems that have
prompted the consideration of DNS Whitelisting in the first place, as
an alternative to trying to apply temporary workarounds to avoid the
symptoms of underlying end user IPv6 impairments. A first step is
obviously to identify which users have such impairments, which would
appear to be possible, and then to communicate this information to
end users. Such end user communication is likely to be most helpful
if the end user is not only alerted to a potential problem but is
given careful and detailed advice on how to resolve this on their
own, or where they can seek help in doing so. Section 11 may also be
relevant in this case.
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One challenge with this option is the potential difficulty of
motivating members of the Internet community to work collectively
towards this goal, sharing the labor, time, and costs related to such
an effort. Of course, since just such a community effort is now
underway for IPv6, it is possible that this would call for only a
moderate amount of additional work [World IPv6 Day].
Despite any potential challenges, many in the Internet community are
already working towards this goal and/or have expressed a general
preference for this approach.
8.3.2. Gain Experience Using IPv6 Transition Names
Another alternative is for domains to gain experience using an FQDN
which has become common for domains beginning the transition to IPv6;
ipv6.example.com and www.ipv6.example.com. This can be a way for a
domain to gain IPv6 experience and increase IPv6 use on a relatively
controlled basis, and to inform any plans for DNS Whitelisting with
experience. While this is a good first step to functionally test and
prepare a domain for IPv6, the utility of the tactic is limited since
users must know the transition name, the traffic volume will be low,
and the traffic is unlikely to be representative of the general
population of end users, among other reasons.
8.3.3. Implement DNS Blacklisting
Some domains may wish to be more permissive than if they adopted DNS
Whitelisting Section 8.2, but not still have some level of control
over returning AAAA record responses Section 8.3. An alternative in
this case may be to employ DNS blacklisting, which would enable all
DNS recursive resolvers to receive AAAA record responses except for
the relatively small number that are listed in the blacklist. This
could enable an implementer to only prevent such responses where
there has been a relatively high level of IPv6-related impairments,
until such time as these impairments can be fixed or otherwise
meaningfully reduced to an acceptable level.
This approach is likely to be significantly less labor intensive for
an authoritative DNS server operator, as they would presumably focus
on a smaller number of DNS recursive resolvers than if they
implemented whitelisting. Thus, these authoritative DNS server
operators would only need to communicate with a few DNS recursive
resolver operators rather than potentially all such operators. This
should result in lower labor, systems, and process requirements,
which should be beneficial to all parties. This is not to say that
there will be no time required to work with those operators affected
by a blacklist, simply that there are likely to be fewer such
interactions and that each such interaction may be shorter in
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duration.
Of course one downside of this approach may be that the perception of
being blocked (blacklisted) is sometimes worse that not being
permitted to have access (whitelisted). However, the email industry
has a long experience with blacklists and, very generally speaking,
blacklists tend to be effective and well received when it is easy to
discover if a server is on a blacklist, if there is a transparent and
easily understood process for requesting removal from a blacklist,
and if the decision-making criteria for placing a server on a
blacklist is transparently disclosed and perceived as fair.
9. Is DNS Whitelisting a Recommended Practice?
Opinions in the Internet community concerning whether or not DNS
Whitelisting is a recommended practice are understandably quite
varied. However, there is clear consensus that DNS Whitelisting is
at best a useful tactic a domain may choose to use as they transition
to IPv6. In particular, some high-traffic domains view DNS
Whitelisting as one of the few practical and low-risk approaches
enabling them to transition to IPv6, without which their transition
may not take place for some time. However, there is also consensus
is that this practice is acceptable only in the short-term and that
it will not scale over the long-term. Thus, some domains may find
DNS Whitelisting a beneficial temporary tactic in their transition to
IPv6. Such temporary use during the transition to IPv6 is broadly
accepted within the community, so long as it does not become a long-
term practice.
World IPv6 Day, sponsored by the Internet Society [World IPv6 Day],
is scheduled to occur on June 8, 2011. This will be an opportunity
for domains to add AAAA resource records to the DNS without using DNS
Whitelisting. As a result, this is likely an excellent opportunity
for domains to evaluate the utility or necessity of DNS Whitelisting,
even in the short-term. A major German news website, Heise Online,
also ran a similar IPv6 experiment whereby they added AAAA resource
records and observed and measured any errors [Heise Online
Experiment], which is important reading for any domain contemplating
either the use of DNS Whitelisting or simply adding IPv6 addressing
to their site.
10. Security Considerations
If DNS Whitelisting is adopted, then organizations which apply DNS
Whitelisting policies in their authoritative servers should have
procedures and systems which do not allow unauthorized parties to
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either remove whitelisted DNS resolvers from the whitelist or add
non-whitelisted DNS resolvers to the whitelist, just as all
configuration settings for name servers should be protected by
appropriate procedures and systems. Should such unauthorized
additions or removals from the whitelist can be quite damaging, and
result in content providers and/or ISPs to incur substantial support
costs resulting from end user and/or customer contacts. As such,
great care must be taken to control access to the whitelist for an
authoritative server.
In addition, two other key security-related issues should be taken
into consideration:
10.1. DNSSEC Considerations
DNS security extensions defined in [RFC4033], [RFC4034], and
[RFC4035] use cryptographic digital signatures to provide origin
authentication and integrity assurance for DNS data. This is done by
creating signatures for DNS data on a Security-Aware Authoritative
Name Server that can be used by Security-Aware Resolvers to verify
the answers. Since DNS Whitelisting is implemented on an
authoritative server, which provides different answers depending upon
which resolver server has sent a query, the DNSSEC chain of trust is
not altered. Even though the authoritative server will not always
return a AAAA resource record when one exists, respective A resource
records and AAAA resource records can and should both be signed.
Therefore there are no DNSSEC implications per se. However, any
implementer of DNS Whitelisting should be careful if they implement
both DNSSEC signing of their domain and also DNS Whitelisting of that
same domain. Specifically, those domains should ensure that resource
records are being appropriately and reliably signed, which may
present incremental operational and/or technical challenges.
However, as noted in Section 3, end user networks may also choose to
implement tools at their disposal in order to address IPv6-related
impairments. One of those possible tools could involve unspecified
changes to the configuration of their DNS recursive resolvers. If it
is a Security-Aware Resolver, modifying or suppressing AAAA resource
records for a DNSSEC-signed domain will be problematic and could
break the chain of trust established with DNSSEC.
10.2. Authoritative DNS Response Consistency Considerations
In addition to the considerations raised in Section 10.1, it is
conceivable that security concerns may arise when end users or other
parties notice that the responses sent from an authoritative DNS
server appear to vary from one network or one DNS recursive resolver
to another. This may give rise to concerns that, since the
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authoritative responses vary that there is some sort of security
issue and/or some or none of the responses can be trusted. While
this may seem a somewhat obscure concern, domains nonetheless may
wish to consider this when contemplating whether or not to pursue DNS
Whitelisting.
11. Privacy Considerations
As noted in Section 8.3.1, there may be methods to detect IPv6-
related impairments for a particular end user. For example, this may
be possible when an end user visits the website of a particular
domain. In that example, there are likely no privacy considerations
in automatically communicating to that end user that the domain has
detected a particular impairment. However, if that domain decided to
share information concerning that particular end user with their
network operator or another party, then the visited domain may wish
to in some manner advise the end user of this or otherwise seek to
obtain the user's consent to such information sharing. This may be
achieved in a wide variety of ways, from presenting a message asking
the user for consent (which will of course help them solve a
technical problem of which they are likely unaware) to adding this to
a domain's website terms of use / service. Such information sharing
and communication of such sharing to end users may well vary by
geographic area and/or legal jurisdiction. Thus, a domain should
consider any potential privacy issues these sorts of scenarios.
To the extent that domains or network operators decide to publish
impairment statistics, they should not identify individual hosts,
host identifiers, or users.
12. IANA Considerations
There are no IANA considerations in this document.
13. Contributors
The following people made significant textual contributions to this
document and/or played an important role in the development and
evolution of this document:
- John Brzozowski
- Chris Griffiths
- Tom Klieber
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- Yiu Lee
- Rich Woundy
14. Acknowledgements
The author and contributors also wish to acknowledge the assistance
of the following individuals or groups. Some of these people
provided helpful and important guidance in the development of this
document and/or in the development of the concepts covered in this
document. Other people assisted by performing a detailed review of
this document, and then providing feedback and constructive criticism
for revisions to this document. All of this was helpful and
therefore the following individuals merit acknowledgement:
- Bernard Aboba
- Jari Arkko
- Frank Bulk
- Brian Carpenter
- Dave Crocker
- Ralph Droms
- Wesley Eddy
- Adrian Farrel
- Stephen Farrell
- Tony Finch
- Karsten Fleischhauer
- Wesley George
- Philip Homburg
- Jerry Huang
- Ray Hunter
- Joel Jaeggli
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- Erik Kline
- Suresh Krishnan
- Victor Kuarsingh
- John Leslie
- John Mann
- Danny McPherson
- Martin Millnert
- Russ Mundy
- Thomas Narten
- Pekka Savola
- Robert Sparks
- Barbara Stark
- Joe Touch
- Hannes Tschofenig
- Tina Tsou
- Members of the Broadband Internet Technical Advisory Group (BITAG)
15. References
15.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC1958] Carpenter, B., "Architectural Principles of the Internet",
RFC 1958, June 1996.
[RFC2775] Carpenter, B., "Internet Transparency", RFC 2775,
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February 2000.
[RFC2956] Kaat, M., "Overview of 1999 IAB Network Layer Workshop",
RFC 2956, October 2000.
[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
Issues", RFC 3234, February 2002.
[RFC3724] Kempf, J., Austein, R., and IAB, "The Rise of the Middle
and the Future of End-to-End: Reflections on the Evolution
of the Internet Architecture", RFC 3724, March 2004.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, October 2005.
15.2. Informative References
[Evaluating IPv6 Adoption]
Colitti, L., Gunderson, S., Kline, E., and T. Refice,
"Evaluating IPv6 adoption in the Internet", Passive and
Active Management (PAM) Conference 2010, April 2010,
<http://www.google.com/research/pubs/archive/36240.pdf>.
[Heise Online Experiment]
Heise.de, "World IPv6 Day - June 8, 2011", Heise.de
Website http://www.h-online.com, January 2011, <http://
www.h-online.com/features/
The-big-IPv6-experiment-1165042.html>.
[I-D.ietf-v6ops-happy-eyeballs]
Wing, D. and A. Yourtchenko, "Happy Eyeballs: Trending
Towards Success with Dual-Stack Hosts",
draft-ietf-v6ops-happy-eyeballs-02 (work in progress),
May 2011.
[I-D.shirasaki-nat444]
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Yamagata, I., Shirasaki, Y., Nakagawa, A., Yamaguchi, J.,
and H. Ashida, "NAT444", draft-shirasaki-nat444-03 (work
in progress), January 2011.
[IETF-77-DNSOP]
Gashinsky, I., "IPv6 & recursive resolvers: How do we make
the transition less painful?", IETF 77 DNS Operations
Working Group, March 2010,
<http://www.ietf.org/proceedings/77/slides/dnsop-7.pdf>.
[IPv6 Brokenness]
Anderson, T., "Measuring and Combating IPv6 Brokenness",
Reseaux IP Europeens (RIPE) 61st Meeting, November 2010,
<http://ripe61.ripe.net/presentations/162-ripe61.pdf>.
[IPv6 Whitelist Operations]
Kline, E., "IPv6 Whitelist Operations", Google Google IPv6
Implementors Conference, June 2010, <http://
sites.google.com/site/ipv6implementors/2010/agenda/
IPv6_Whitelist_Operations.pdf>.
[Laws of Motion]
Newton, I., "Mathematical Principles of Natural Philosophy
(Philosophiae Naturalis Principia Mathematica)",
Principia Mathematical Principles of Natural Philosophy
(Philosophiae Naturalis Principia Mathematica), July 1687,
<http://en.wikipedia.org/wiki/Newton's_laws_of_motion>.
[NW-Article-DNS-WL]
Marsan, C., "Google, Microsoft, Netflix in talks to create
shared list of IPv6 users", Network World , March 2010, <h
ttp://www.networkworld.com/news/2010/
032610-dns-ipv6-whitelist.html>.
[NW-Article-DNSOP]
Marsan, C., "Yahoo proposes 'really ugly hack' to DNS",
Network World , March 2010, <http://www.networkworld.com/
news/2010/032610-yahoo-dns.html>.
[RFC1794] Brisco, T., "DNS Support for Load Balancing", RFC 1794,
April 1995.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[Resolvers in the Wild]
Ager, B., Smaragdakis, G., Muhlbauer, W., and S. Uhlig,
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"Comparing DNS Resolvers in the Wild", ACM Sigcomm
Internet Measurement Conference 2010, November 2010,
<http://conferences.sigcomm.org/imc/2010/papers/p15.pdf>.
[Rethinking the Internet]
Blumenthal, M. and D. Clark, "Rethinking the design of the
Internet: The end to end arguments vs. the brave new
world", ACM Transactions on Internet Technology Volume 1,
Number 1, Pages 70-109, August 2001, <http://
dspace.mit.edu/bitstream/handle/1721.1/1519/
TPRC_Clark_Blumenthal.pdf>.
[Tussle in Cyberspace]
Braden, R., Clark, D., Sollins, K., and J. Wroclawski,
"Tussle in Cyberspace: Defining Tomorrow's Internet",
Proceedings of ACM Sigcomm 2002, August 2002, <http://
groups.csail.mit.edu/ana/Publications/PubPDFs/
Tussle2002.pdf>.
[Whitelisting Concerns]
Brzozowski, J., Griffiths, C., Klieber, T., Lee, Y.,
Livingood, J., and R. Woundy, "IPv6 DNS Whitelisting -
Could It Hinder IPv6 Adoption?", ISOC Internet Society
IPv6 Deployment Workshop, April 2010, <http://
www.comcast6.net/
IPv6_DNS_Whitelisting_Concerns_20100416.pdf>.
[World IPv6 Day]
The Internet Society, "World IPv6 Day - June 8, 2011",
Internet Society Website http://www.isoc.org,
January 2011, <http://isoc.org/wp/worldipv6day/>.
Appendix A. Document Change Log
[RFC Editor: This section is to be removed before publication]
-04: Made changed based on feedback received during IESG review.
This does NOT include updated from the more general IETF last call -
that will be in a -05 version of the document. Per Ralph Droms,
change the title of 6.2 from "Likely Deployment Scenarios" to
"Possible Deployment Scenarios", as well as changes to improve the
understanding of sentences in Sections 2, 3, 4, and 8.2. Per Adrian
Farrel, made a minor change to Section 3. Per Robert Sparks, to make
clear in Section 2 that whitelisting is done on authoritative servers
and not DNS recursive resolvers, and to improve Section 8.3 and add a
reference to I-D.ietf?v6ops?happy?eyeballs. Per Wesley Eddy, updated
Section 7.3.2 to address operational concerns and re-titled Section 8
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from "Solutions" to "General Implementation Variations". Per Stephen
Farrell, added text to Section 8.1 and Section 6.2, with a reference
to 8.1 in the Introduction, to say that universal deployment is
considered harmful. Added text to Section 2 per the v6ops list
discussion to indicate that whitelisting is independent of the IP
address family of the end user host or resolver. There was also
discussion with the IESG to change the name of the draft to IPv6 DNS
Resolver Whitelisting or IPv6 AAAA DNS Resolver Whitelisting (as
suggested originally by John Mann) but there was not a strong
consensus to do so. Added a new section 7.7, at the suggestion of
Philip Homburg. Per Joe Touch, added a new Section 8.4 on
blacklisting as an alternative, mentioned blacklisting in Section 2,
added a new Section 7.8 on the use of 3rd party resolvers, and
updated section 6.2 to change Internet Draft documents to RFCs.
Minor changes from Barbara Stark. Changes to the Privacy
Considerations section based on feedback from Alissa Cooper. Changed
"highly-trafficked" domains to "high-traffic" domains. Per Bernard
Aboba, added text noting that a whitelist may be manually or
automatically updated, contrasting whitelisting with blacklisting,
reorganized Section 3, added a note on multiple clearinghouses being
possible. Per Pekka Savola, added a note regarding multiple
clearinghouses to the Ad Hoc section, corrected grammar in Section
7.5, reworded Section 7.3.7, corrected the year in a RIPE reference
citation. Also incorporated general feedback from the Broadband
Internet Technical Advisory Group. Per Jari Arkko, simplified the
introduction to the Implications section, played down possible
impacts on RFC 4213, added caveats to Section 8.3.2 on the utility of
transition names, re-wrote Section 9. Updated the Abstract and
Introduction, per errors noted by Tony Finch. Updated the Security
Considerations based on feedback from Russ Mundy. Per Ray Hunter,
added some text to the De-Whitelisting implications section regarding
effects on networks of switching from IPv6 to IPv4. Updated 7.3.1
per additional feedback from Karsten Fleischhauer. Per Dave Crocker,
added a complete description of the practice to the Abstract, added a
note to the Introduction that the operational impacts are
particularly acute at scale, added text to Intro to make clear this
practice affects all protocols and not just HTTP, added a new query/
response diagram, added text to the Abstract and Introduction noting
that this is an IPv6 transition mechanism, and too many other changes
to list.
-03: Several changes suggested by Joel Jaeggli at the end of WGLC.
This involved swapping the order of Section 6.1 and 6.2, among other
changes to make the document more readable, understandable, and
tonally balanced. As suggested by Karsten Fleischhauer, added a
reference to RFC 4213 in Section 7.1, as well as other suggestions to
that section. As suggested by Tina Tsou, made some changes to the
DNSSEC section regarding signing. As suggested by Suresh Krishnan,
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made several changes to improve various sections of the document,
such as adding an alternative concerning the use of ipv6.domain,
improving the system logic section, and shortening the reference
titles. As suggested by Thomas Narten, added some text regarding the
imperfection of making judgements as to end user host impairments
based upon the DNS recursive resolver's IP and/or network. Finally,
made sure that variations in the use of 'records' and 'resource
records' was updated to 'resource records' for uniformity and to
avoid confusion.
-02: Called for and closed out feedback on dnsop and v6ops mailing
lists. Closed out open feedback items from IETF 79. Cleared I-D
nits issues, added a section on whether or not this is recommended,
made language less company-specific based on feedback from Martin
Millnert, Wes George, and Victor Kuarsingh. Also mentioned World
IPv6 Day per Wes George's suggestion. Added references to the ISOC
World IPv6 Day and the Heise.de test at the suggestion of Jerry
Huang, as well as an additional implication in 7.3.7. Made any
speculation on IPv4 impairment noted explicitly as such, per feedback
from Martin Millnert. Added a reference to DNS SRV in the load
balancing section. Added various other references. Numerous changes
suggested by John Brzozowski in several sections, to clean up the
document. Moved up the section on why whitelisting is performed to
make the document flow more logically. Added a note in the ad hoc
deployment scenario explaining that a deployment may be temporary,
and including more of the perceived benefits of this tactic. Added a
Privacy Considerations section to address end-user detection and
communication.
-01: Incorporated feedback received from Brian Carpenter (from 10/19/
2010), Frank Bulk (from 11/8/2010), and Erik Kline (from 10/1/2010).
Also added an informative reference at the suggestion of Wes George
(from from 10/22/2010). Closed out numerous editorial notes, and
made a variety of other changes.
-00: First version published as a v6ops WG draft. The preceding
individual draft was
draft-livingood-dns-whitelisting-implications-01. IMPORTANT TO NOTE
that no changes have been made yet based on WG and list feedback.
These are in queue for a -01 update.
Appendix B. Open Issues
[RFC Editor: This section is to be removed before publication]
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1. RFC Editor: Please consider shortening the names of the non-RFC
references.
2. Ensure references are in the proper section (normative/
informative)
3. JD Falk question - should I add a sub-section to 9 to explain how
best to implement if you did it? (transparent/published policies,
SLA on decision making,etc.)
4. Per Ray Hunter - "Which is why my original posting suggested that
the WG might wish to express a concern about anyone making
assumptions about any undocumented architectural links between
DNS and transport, and also suggest that operators of aaaa
whitelists (party X) should disclose their aaaa methodology and
data, and that they should not share whitelist data with other
aaaa operators in an uncontrolled manner, so that at least Party
Y could see what was happening and why, and also have a chance of
correcting it."
5. Per Joe Touch, consider moving section 8 to just after section 3.
(only do so after -04)
6. Per Dave Crocker, this is a bit long-winded and should be edited
down - in particular removing mentions of 'parties' in various
parts ("I suggest merely noting that there are concerns and then
listing and discussing the concerns, rather than adding text to
attribute the concerns to others, even if the conclusion of your
text is that a particular concern is not valid.") (and ""These
parties explain their belief" is an example of personalization
that is not needed. This isn't about the believers. It is about
possible problems."). John Leslie concurs - feels it should be
simplified.
7. Per Pekka Savola and Stephen Farrell, should universal deployment
be removed completely (consider after -04).
8. Per Jari Arkko, review the document again after -04 to ensure the
right tone and that all motivations are properly accounted for.
9. Per Dave Crocker - do we need to change the name from
whitelisting to an alternate term? Dave suggests "IPv6 Resolver
Whitelisting" or "IPv6 DNS Response Preference List" or "DNS
Response Content Preference List". Tony Finch suggests: So I
suggest retitling the document "IPv6 DNS resolver whitelisting"
and revising the terminology throughout to match. Mark Andrews
also suggests "DNS resolver whitelisting for AAAA resolution".
John Leslie suggests "AAAA-blocking".
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Author's Address
Jason Livingood
Comcast Cable Communications
One Comcast Center
1701 John F. Kennedy Boulevard
Philadelphia, PA 19103
US
Email: jason_livingood@cable.comcast.com
URI: http://www.comcast.com
Livingood Expires November 30, 2011 [Page 39]
