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DNS Operations WG A. Durand
Internet-Draft SUN Microsystems, Inc.
Expires: January 17, 2006 J. Ihren
Autonomica
P. Savola
CSC/FUNET
July 16, 2005
Operational Considerations and Issues with IPv6 DNS
draft-ietf-dnsop-ipv6-dns-issues-11.txt
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on January 17, 2006.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This memo presents operational considerations and issues with IPv6
Domain Name System (DNS), including a summary of special IPv6
addresses, documentation of known DNS implementation misbehaviour,
recommendations and considerations on how to perform DNS naming for
service provisioning and for DNS resolver IPv6 support,
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Internet-Draft Considerations with IPv6 DNS July 2005
considerations for DNS updates for both the forward and reverse
trees, and miscellaneous issues. This memo is aimed to include a
summary of information about IPv6 DNS considerations for those who
have experience with IPv4 DNS.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Representing IPv6 Addresses in DNS Records . . . . . . . . 4
1.2 Independence of DNS Transport and DNS Records . . . . . . 4
1.3 Avoiding IPv4/IPv6 Name Space Fragmentation . . . . . . . 5
1.4 Query Type '*' and A/AAAA Records . . . . . . . . . . . . 5
2. DNS Considerations about Special IPv6 Addresses . . . . . . . 5
2.1 Limited-scope Addresses . . . . . . . . . . . . . . . . . 6
2.2 Temporary Addresses . . . . . . . . . . . . . . . . . . . 6
2.3 6to4 Addresses . . . . . . . . . . . . . . . . . . . . . . 6
2.4 Other Transition Mechanisms . . . . . . . . . . . . . . . 6
3. Observed DNS Implementation Misbehaviour . . . . . . . . . . . 7
3.1 Misbehaviour of DNS Servers and Load-balancers . . . . . . 7
3.2 Misbehaviour of DNS Resolvers . . . . . . . . . . . . . . 7
4. Recommendations for Service Provisioning using DNS . . . . . . 7
4.1 Use of Service Names instead of Node Names . . . . . . . . 8
4.2 Separate vs the Same Service Names for IPv4 and IPv6 . . . 8
4.3 Adding the Records Only when Fully IPv6-enabled . . . . . 9
4.4 The Use of TTL for IPv4 and IPv6 RRs . . . . . . . . . . . 10
4.4.1 TTL With Courtesy Additional Data . . . . . . . . . . 10
4.4.2 TTL With Critical Additional Data . . . . . . . . . . 10
4.5 IPv6 Transport Guidelines for DNS Servers . . . . . . . . 11
5. Recommendations for DNS Resolver IPv6 Support . . . . . . . . 11
5.1 DNS Lookups May Query IPv6 Records Prematurely . . . . . . 11
5.2 Obtaining a List of DNS Recursive Resolvers . . . . . . . 13
5.3 IPv6 Transport Guidelines for Resolvers . . . . . . . . . 13
6. Considerations about Forward DNS Updating . . . . . . . . . . 13
6.1 Manual or Custom DNS Updates . . . . . . . . . . . . . . . 14
6.2 Dynamic DNS . . . . . . . . . . . . . . . . . . . . . . . 14
7. Considerations about Reverse DNS Updating . . . . . . . . . . 15
7.1 Applicability of Reverse DNS . . . . . . . . . . . . . . . 15
7.2 Manual or Custom DNS Updates . . . . . . . . . . . . . . . 16
7.3 DDNS with Stateless Address Autoconfiguration . . . . . . 16
7.4 DDNS with DHCP . . . . . . . . . . . . . . . . . . . . . . 18
7.5 DDNS with Dynamic Prefix Delegation . . . . . . . . . . . 18
8. Miscellaneous DNS Considerations . . . . . . . . . . . . . . . 19
8.1 NAT-PT with DNS-ALG . . . . . . . . . . . . . . . . . . . 19
8.2 Renumbering Procedures and Applications' Use of DNS . . . 19
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
10. Security Considerations . . . . . . . . . . . . . . . . . . 20
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
11.1 Normative References . . . . . . . . . . . . . . . . . . . 20
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11.2 Informative References . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 24
A. Unique Local Addressing Considerations for DNS . . . . . . . . 25
B. Behaviour of Additional Data in IPv4/IPv6 Environments . . . . 25
B.1 Description of Additional Data Scenarios . . . . . . . . . 26
B.2 Which Additional Data to Keep, If Any? . . . . . . . . . . 27
B.3 Discussion of the Potential Problems . . . . . . . . . . . 28
Intellectual Property and Copyright Statements . . . . . . . . 30
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1. Introduction
This memo presents operational considerations and issues with IPv6
DNS; it is meant to be an extensive summary and a list of pointers
for more information about IPv6 DNS considerations for those with
experience with IPv4 DNS.
The purpose of this document is to give information about various
issues and considerations related to DNS operations with IPv6; it is
not meant to be a normative specification or standard for IPv6 DNS.
The first section gives a brief overview of how IPv6 addresses and
names are represented in the DNS, how transport protocols and
resource records (don't) relate, and what IPv4/IPv6 name space
fragmentation means and how to avoid it; all of these are described
at more length in other documents.
The second section summarizes the special IPv6 address types and how
they relate to DNS. The third section describes observed DNS
implementation misbehaviours which have a varying effect on the use
of IPv6 records with DNS. The fourth section lists recommendations
and considerations for provisioning services with DNS. The fifth
section in turn looks at recommendations and considerations about
providing IPv6 support in the resolvers. The sixth and seventh
sections describe considerations with forward and reverse DNS
updates, respectively. The eighth section introduces several
miscellaneous IPv6 issues relating to DNS for which no better place
has been found in this memo. Appendix A looks briefly at the
requirements for unique local addressing.
1.1 Representing IPv6 Addresses in DNS Records
In the forward zones, IPv6 addresses are represented using AAAA
records. In the reverse zones, IPv6 address are represented using
PTR records in the nibble format under the ip6.arpa. tree. See
[RFC3596] for more about IPv6 DNS usage, and [RFC3363] or [RFC3152]
for background information.
In particular one should note that the use of A6 records in the
forward tree or Bitlabels in the reverse tree is not recommended
[RFC3363]. Using DNAME records is not recommended in the reverse
tree in conjunction with A6 records; the document did not mean to
take a stance on any other use of DNAME records [RFC3364].
1.2 Independence of DNS Transport and DNS Records
DNS has been designed to present a single, globally unique name space
[RFC2826]. This property should be maintained, as described here and
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in Section 1.3.
The IP version used to transport the DNS queries and responses is
independent of the records being queried: AAAA records can be queried
over IPv4, and A records over IPv6. The DNS servers must not make
any assumptions about what data to return for Answer and Authority
sections based on the underlying transport used in a query.
However, there is some debate whether the addresses in Additional
section could be selected or filtered using hints obtained from which
transport was being used; this has some obvious problems because in
many cases the transport protocol does not correlate with the
requests, and because a "bad" answer is in a way worse than no answer
at all (consider the case where the client is led to believe that a
name received in the additional record does not have any AAAA records
at all).
As stated in [RFC3596]:
The IP protocol version used for querying resource records is
independent of the protocol version of the resource records; e.g.,
IPv4 transport can be used to query IPv6 records and vice versa.
1.3 Avoiding IPv4/IPv6 Name Space Fragmentation
To avoid the DNS name space from fragmenting into parts where some
parts of DNS are only visible using IPv4 (or IPv6) transport, the
recommendation is to always keep at least one authoritative server
IPv4-enabled, and to ensure that recursive DNS servers support IPv4.
See DNS IPv6 transport guidelines [RFC3901] for more information.
1.4 Query Type '*' and A/AAAA Records
QTYPE=* is typically only used for debugging or management purposes;
it is worth keeping in mind that QTYPE=* ("ANY" queries) only return
any available RRsets, not *all* the RRsets, because the caches do not
necessarily have all the RRsets and have no way of guaranteeing that
they have all the RRsets. Therefore, to get both A and AAAA records
reliably, two separate queries must be made.
2. DNS Considerations about Special IPv6 Addresses
There are a couple of IPv6 address types which are somewhat special;
these are considered here.
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2.1 Limited-scope Addresses
The IPv6 addressing architecture [RFC3513] includes two kinds of
local-use addresses: link-local (fe80::/10) and site-local
(fec0::/10). The site-local addresses have been deprecated [RFC3879]
but are discussed with unique local addresses in Appendix A.
Link-local addresses should never be published in DNS (whether in
forward or reverse tree), because they have only local (to the
connected link) significance [I-D.durand-dnsop-dont-publish].
2.2 Temporary Addresses
Temporary addresses defined in RFC3041 [RFC3041] (sometimes called
"privacy addresses") use a random number as the interface identifier.
Having DNS AAAA records that are updated to always contain the
current value of a node's temporary address would defeat the purpose
of the mechanism and is not recommended. However, it would still be
possible to return a non-identifiable name (e.g., the IPv6 address in
hexadecimal format), as described in [RFC3041].
2.3 6to4 Addresses
6to4 [RFC3056] specifies an automatic tunneling mechanism which maps
a public IPv4 address V4ADDR to an IPv6 prefix 2002:V4ADDR::/48.
If the reverse DNS population would be desirable (see Section 7.1 for
applicability), there are a number of possible ways to do so.
The main proposal [I-D.huston-6to4-reverse-dns] aims to design an
autonomous reverse-delegation system that anyone being capable of
communicating using a specific 6to4 address would be able to set up a
reverse delegation to the corresponding 6to4 prefix. This could be
deployed by e.g., Regional Internet Registries (RIRs). This is a
practical solution, but may have some scalability concerns.
2.4 Other Transition Mechanisms
6to4 is mentioned as a case of an IPv6 transition mechanism requiring
special considerations. In general, mechanisms which include a
special prefix may need a custom solution; otherwise, for example
when IPv4 address is embedded as the suffix or not embedded at all,
special solutions are likely not needed.
Note that it does not seem feasible to provide reverse DNS with
another automatic tunneling mechanism, Teredo [I-D.huitema-v6ops-
teredo]; this is because the IPv6 address is based on the IPv4
address and UDP port of the current NAT mapping which is likely to be
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relatively short-lived.
3. Observed DNS Implementation Misbehaviour
Several classes of misbehaviour in DNS servers, load-balancers and
resolvers have been observed. Most of these are rather generic, not
only applicable to IPv6 -- but in some cases, the consequences of
this misbehaviour are extremely severe in IPv6 environments and
deserve to be mentioned.
3.1 Misbehaviour of DNS Servers and Load-balancers
There are several classes of misbehaviour in certain DNS servers and
load-balancers which have been noticed and documented [RFC4074]: some
implementations silently drop queries for unimplemented DNS records
types, or provide wrong answers to such queries (instead of a proper
negative reply). While typically these issues are not limited to
AAAA records, the problems are aggravated by the fact that AAAA
records are being queried instead of (mainly) A records.
The problems are serious because when looking up a DNS name, typical
getaddrinfo() implementations, with AF_UNSPEC hint given, first try
to query the AAAA records of the name, and after receiving a
response, query the A records. This is done in a serial fashion --
if the first query is never responded to (instead of properly
returning a negative answer), significant timeouts will occur.
In consequence, this is an enormous problem for IPv6 deployments, and
in some cases, IPv6 support in the software has even been disabled
due to these problems.
The solution is to fix or retire those misbehaving implementations,
but that is likely not going to be effective. There are some
possible ways to mitigate the problem, e.g., by performing the
lookups somewhat in parallel and reducing the timeout as long as at
least one answer has been received; but such methods remain to be
investigated; slightly more on this is included in Section 5.
3.2 Misbehaviour of DNS Resolvers
Several classes of misbehaviour have also been noticed in DNS
resolvers [I-D.ietf-dnsop-bad-dns-res]. However, these do not seem
to directly impair IPv6 use, and are only referred to for
completeness.
4. Recommendations for Service Provisioning using DNS
When names are added in the DNS to facilitate a service, there are
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several general guidelines to consider to be able to do it as
smoothly as possible.
4.1 Use of Service Names instead of Node Names
It makes sense to keep information about separate services logically
separate in the DNS by using a different DNS hostname for each
service. There are several reasons for doing this, for example:
o It allows more flexibility and ease for migration of (only a part
of) services from one node to another,
o It allows configuring different properties (e.g., TTL) for each
service, and
o It allows deciding separately for each service whether to publish
the IPv6 addresses or not (in cases where some services are more
IPv6-ready than others).
Using SRV records [RFC2782] would avoid these problems.
Unfortunately, those are not sufficiently widely used to be
applicable in most cases. Hence an operation technique is to use
service names instead of node names (or, "hostnames"). This
operational technique is not specific to IPv6, but required to
understand the considerations described in Section 4.2 and
Section 4.3.
For example, assume a node named "pobox.example.com" provides both
SMTP and IMAP service. Instead of configuring the MX records to
point at "pobox.example.com", and configuring the mail clients to
look up the mail via IMAP from "pobox.example.com", one could use
e.g., "smtp.example.com" for SMTP (for both message submission and
mail relaying between SMTP servers) and "imap.example.com" for IMAP.
Note that in the specific case of SMTP relaying, the server itself
must typically also be configured to know all its names to ensure
loops do not occur. DNS can provide a layer of indirection between
service names and where the service actually is, and using which
addresses. (Obviously, when wanting to reach a specific node, one
should use the hostname rather than a service name.)
4.2 Separate vs the Same Service Names for IPv4 and IPv6
The service naming can be achieved in basically two ways: when a
service is named "service.example.com" for IPv4, the IPv6-enabled
service could either be added to "service.example.com", or added
separately under a different name, e.g., in a sub-domain, like,
"service.ipv6.example.com".
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These two methods have different characteristics. Using a different
name allows for easier service piloting, minimizing the disturbance
to the "regular" users of IPv4 service; however, the service would
not be used transparently, without the user/application explicitly
finding it and asking for it -- which would be a disadvantage in most
cases. When the different name is under a sub-domain, if the
services are deployed within a restricted network (e.g., inside an
enterprise), it's possible to prefer them transparently, at least to
a degree, by modifying the DNS search path; however, this is a
suboptimal solution. Using the same service name is the "long-term"
solution, but may degrade performance for those clients whose IPv6
performance is lower than IPv4, or does not work as well (see
Section 4.3 for more).
In most cases, it makes sense to pilot or test a service using
separate service names, and move to the use of the same name when
confident enough that the service level will not degrade for the
users unaware of IPv6.
4.3 Adding the Records Only when Fully IPv6-enabled
The recommendation is that AAAA records for a service should not be
added to the DNS until all of following are true:
1. The address is assigned to the interface on the node.
2. The address is configured on the interface.
3. The interface is on a link which is connected to the IPv6
infrastructure.
In addition, if the AAAA record is added for the node, instead of
service as recommended, all the services of the node should be IPv6-
enabled prior to adding the resource record.
For example, if an IPv6 node is isolated from an IPv6 perspective
(e.g., it is not connected to IPv6 Internet) constraint #3 would mean
that it should not have an address in the DNS.
Consider the case of two dual-stack nodes, which both have IPv6
enabled, but the server does not have (global) IPv6 connectivity. As
the client looks up the server's name, only A records are returned
(if the recommendations above are followed), and no IPv6
communication, which would have been unsuccessful, is even attempted.
The issues are not always so black-and-white. Usually it's important
that the service offered using both protocols is of roughly equal
quality, using the appropriate metrics for the service (e.g.,
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latency, throughput, low packet loss, general reliability, etc.) --
this is typically very important especially for interactive or real-
time services. In many cases, the quality of IPv6 connectivity may
not yet be equal to that of IPv4, at least globally -- this has to be
taken into consideration when enabling services.
4.4 The Use of TTL for IPv4 and IPv6 RRs
The behaviour of DNS caching when different TTL values are used for
different RRsets of the same name calls for explicit discussion. For
example, let's consider two unrelated zone fragments:
example.com. 300 IN MX foo.example.com.
foo.example.com. 300 IN A 192.0.2.1
foo.example.com. 100 IN AAAA 2001:db8::1
...
child.example.com. 300 IN NS ns.child.example.com.
ns.child.example.com. 300 IN A 192.0.2.1
ns.child.example.com. 100 IN AAAA 2001:db8::1
In the former case, we have "courtesy" additional data; in the
latter, we have "critical" additional data. See more extensive
background discussion of additional data handling in Appendix B.
4.4.1 TTL With Courtesy Additional Data
When a caching resolver asks for the MX record of example.com, it
gets back "foo.example.com". It may also get back either one or both
of the A and AAAA records in the additional section. The resolver
must explicitly query for both A and AAAA records [RFC2821].
After 100 seconds, the AAAA record is removed from the cache(s)
because its TTL expired. It could be argued to be useful for the
caching resolvers to discard the A record when the shorter TTL (in
this case, for the AAAA record) expires; this would avoid the
situation where there would be a window of 200 seconds when
incomplete information is returned from the cache. Further argument
for discarding is that in the normal operation, the TTL values are so
high that very likely the incurred additional queries would not be
noticeable, compared to the obtained performance optimization. The
behaviour in this scenario is unspecified.
4.4.2 TTL With Critical Additional Data
The difference to courtesy additional data is that the A/AAAA records
served by the parent zone cannot be queried explicitly. Therefore
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after 100 seconds the AAAA record is removed from the cache(s), but
the A record remains. Queries for the remaining 200 seconds
(provided that there are no further queries from the parent which
could refresh the caches) only return the A record, leading to a
potential opererational situation with unreachable servers.
Similar cache flushing strategies apply in this scenario; the record.
4.5 IPv6 Transport Guidelines for DNS Servers
As described in Section 1.3 and [RFC3901], there should continue to
be at least one authoritative IPv4 DNS server for every zone, even if
the zone has only IPv6 records. (Note that obviously, having more
servers with robust connectivity would be preferable, but this is the
minimum recommendation; also see [RFC2182].)
5. Recommendations for DNS Resolver IPv6 Support
When IPv6 is enabled on a node, there are several things to consider
to ensure that the process is as smooth as possible.
5.1 DNS Lookups May Query IPv6 Records Prematurely
The system library that implements the getaddrinfo() function for
looking up names is a critical piece when considering the robustness
of enabling IPv6; it may come in basically three flavours:
1. The system library does not know whether IPv6 has been enabled in
the kernel of the operating system: it may start looking up AAAA
records with getaddrinfo() and AF_UNSPEC hint when the system is
upgraded to a system library version which supports IPv6.
2. The system library might start to perform IPv6 queries with
getaddrinfo() only when IPv6 has been enabled in the kernel.
However, this does not guarantee that there exists any useful
IPv6 connectivity (e.g., the node could be isolated from the
other IPv6 networks, only having link-local addresses).
3. The system library might implement a toggle which would apply
some heuristics to the "IPv6-readiness" of the node before
starting to perform queries; for example, it could check whether
only link-local IPv6 address(es) exists, or if at least one
global IPv6 address exists.
First, let us consider generic implications of unnecessary queries
for AAAA records: when looking up all the records in the DNS, AAAA
records are typically tried first, and then A records. These are
done in serial, and the A query is not performed until a response is
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received to the AAAA query. Considering the misbehaviour of DNS
servers and load-balancers, as described in Section 3.1, the look-up
delay for AAAA may incur additional unnecessary latency, and
introduce a component of unreliability.
One option here could be to do the queries partially in parallel; for
example, if the final response to the AAAA query is not received in
0.5 seconds, start performing the A query while waiting for the
result (immediate parallelism might be unoptimal, at least without
information sharing between the look-up threads, as that would
probably lead to duplicate non-cached delegation chain lookups).
An additional concern is the address selection, which may, in some
circumstances, prefer AAAA records over A records even when the node
does not have any IPv6 connectivity [I-D.ietf-v6ops-v6onbydefault].
In some cases, the implementation may attempt to connect or send a
datagram on a physical link [I-D.ietf-v6ops-onlinkassumption],
incurring very long protocol timeouts, instead of quickly failing
back to IPv4.
Now, we can consider the issues specific to each of the three
possibilities:
In the first case, the node performs a number of completely useless
DNS lookups as it will not be able to use the returned AAAA records
anyway. (The only exception is where the application desires to know
what's in the DNS, but not use the result for communication.) One
should be able to disable these unnecessary queries, for both latency
and reliability reasons. However, as IPv6 has not been enabled, the
connections to IPv6 addresses fail immediately, and if the
application is programmed properly, the application can fall
gracefully back to IPv4 [RFC4038].
The second case is similar to the first, except it happens to a
smaller set of nodes when IPv6 has been enabled but connectivity has
not been provided yet; similar considerations apply, with the
exception that IPv6 records, when returned, will be actually tried
first which may typically lead to long timeouts.
The third case is a bit more complex: optimizing away the DNS lookups
with only link-locals is probably safe (but may be desirable with
different lookup services which getaddrinfo() may support), as the
link-locals are typically automatically generated when IPv6 is
enabled, and do not indicate any form of IPv6 connectivity. That is,
performing DNS lookups only when a non-link-local address has been
configured on any interface could be beneficial -- this would be an
indication that either the address has been configured either from a
router advertisement, DHCPv6 [RFC3315], or manually. Each would
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indicate at least some form of IPv6 connectivity, even though there
would not be guarantees of it.
These issues should be analyzed at more depth, and the fixes found
consensus on, perhaps in a separate document.
5.2 Obtaining a List of DNS Recursive Resolvers
In scenarios where DHCPv6 is available, a host can discover a list of
DNS recursive resolvers through DHCPv6 "DNS Recursive Name Server"
option [RFC3646]. This option can be passed to a host through a
subset of DHCPv6 [RFC3736].
The IETF is considering the development of alternative mechanisms for
obtaining the list of DNS recursive name servers when DHCPv6 is
unavailable or inappropriate. No decision about taking on this
development work has been reached as of this writing (Aug 2004)
[I-D.ietf-dnsop-ipv6-dns-configuration].
In scenarios where DHCPv6 is unavailable or inappropriate, mechanisms
under consideration for development include the use of well-known
addresses [I-D.ohta-preconfigured-dns] and the use of Router
Advertisements to convey the information [I-D.jeong-dnsop-ipv6-dns-
discovery].
Note that even though IPv6 DNS resolver discovery is a recommended
procedure, it is not required for dual-stack nodes in dual-stack
networks as IPv6 DNS records can be queried over IPv4 as well as
IPv6. Obviously, nodes which are meant to function without manual
configuration in IPv6-only networks must implement the DNS resolver
discovery function.
5.3 IPv6 Transport Guidelines for Resolvers
As described in Section 1.3 and [RFC3901], the recursive resolvers
should be IPv4-only or dual-stack to be able to reach any IPv4-only
DNS server. Note that this requirement is also fulfilled by an IPv6-
only stub resolver pointing to a dual-stack recursive DNS resolver.
6. Considerations about Forward DNS Updating
While the topic of how to enable updating the forward DNS, i.e., the
mapping from names to the correct new addresses, is not specific to
IPv6, it should be considered especially due to the advent of
Stateless Address Autoconfiguration [RFC2462].
Typically forward DNS updates are more manageable than doing them in
the reverse DNS, because the updater can often be assumed to "own" a
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certain DNS name -- and we can create a form of security relationship
with the DNS name and the node which is allowed to update it to point
to a new address.
A more complex form of DNS updates -- adding a whole new name into a
DNS zone, instead of updating an existing name -- is considered out
of scope for this memo as it could require zone-wide authentication.
Adding a new name in the forward zone is a problem which is still
being explored with IPv4, and IPv6 does not seem to add much new in
that area.
6.1 Manual or Custom DNS Updates
The DNS mappings can also be maintained by hand, in a semi-automatic
fashion or by running non-standardized protocols. These are not
considered at more length in this memo.
6.2 Dynamic DNS
Dynamic DNS updates (DDNS) [RFC2136] [RFC3007] is a standardized
mechanism for dynamically updating the DNS. It works equally well
with stateless address autoconfiguration (SLAAC), DHCPv6 or manual
address configuration. It is important to consider how each of these
behave if IP address-based authentication, instead of stronger
mechanisms [RFC3007], was used in the updates.
1. manual addresses are static and can be configured
2. DHCPv6 addresses could be reasonably static or dynamic, depending
on the deployment, and could or could not be configured on the
DNS server for the long term
3. SLAAC addresses are typically stable for a long time, but could
require work to be configured and maintained.
As relying on IP addresses for Dynamic DNS is rather insecure at
best, stronger authentication should always be used; however, this
requires that the authorization keying will be explicitly configured
using unspecified operational methods.
Note that with DHCP it is also possible that the DHCP server updates
the DNS, not the host. The host might only indicate in the DHCP
exchange which hostname it would prefer, and the DHCP server would
make the appropriate updates. Nonetheless, while this makes setting
up a secure channel between the updater and the DNS server easier, it
does not help much with "content" security, i.e., whether the
hostname was acceptable -- if the DNS server does not include
policies, they must be included in the DHCP server (e.g., a regular
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host should not be able to state that its name is "www.example.com").
DHCP-initiated DDNS updates have been extensively described in
[I-D.ietf-dhc-ddns-resolution], [I-D.ietf-dhc-fqdn-option] and
[I-D.ietf-dnsext-dhcid-rr].
The nodes must somehow be configured with the information about the
servers where they will attempt to update their addresses, sufficient
security material for authenticating themselves to the server, and
the hostname they will be updating. Unless otherwise configured, the
first could be obtained by looking up the authoritative name servers
for the hostname; the second must be configured explicitly unless one
chooses to trust the IP address-based authentication (not a good
idea); and lastly, the nodename is typically pre-configured somehow
on the node, e.g., at install time.
Care should be observed when updating the addresses not to use longer
TTLs for addresses than are preferred lifetimes for the addresses, so
that if the node is renumbered in a managed fashion, the amount of
stale DNS information is kept to the minimum. That is, if the
preferred lifetime of an address expires, the TTL of the record needs
be modified unless it was already done before the expiration. For
better flexibility, the DNS TTL should be much shorter (e.g., a half
or a third) than the lifetime of an address; that way, the node can
start lowering the DNS TTL if it seems like the address has not been
renewed/refreshed in a while. Some discussion on how an
administrator could manage the DNS TTL is included in [I-D.ietf-
v6ops-renumbering-procedure]; this could be applied to (smart) hosts
as well.
7. Considerations about Reverse DNS Updating
Updating the reverse DNS zone may be difficult because of the split
authority over an address. However, first we have to consider the
applicability of reverse DNS in the first place.
7.1 Applicability of Reverse DNS
Today, some applications use reverse DNS to either look up some hints
about the topological information associated with an address (e.g.
resolving web server access logs), or as a weak form of a security
check, to get a feel whether the user's network administrator has
"authorized" the use of the address (on the premises that adding a
reverse record for an address would signal some form of
authorization).
One additional, maybe slightly more useful usage is ensuring that the
reverse and forward DNS contents match (by looking up the pointer to
the name by the IP address from the reverse tree, and ensuring that a
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record under the name in the forward tree points to the IP address)
and correspond to a configured name or domain. As a security check,
it is typically accompanied by other mechanisms, such as a user/
password login; the main purpose of the reverse+forward DNS check is
to weed out the majority of unauthorized users, and if someone
managed to bypass the checks, he would still need to authenticate
"properly".
It may also be desirable to store IPsec keying material corresponding
to an IP address in the reverse DNS, as justified and described in
[RFC4025].
It is not clear whether it makes sense to require or recommend that
reverse DNS records be updated. In many cases, it would just make
more sense to use proper mechanisms for security (or topological
information lookup) in the first place. At minimum, the applications
which use it as a generic authorization (in the sense that a record
exists at all) should be modified as soon as possible to avoid such
lookups completely.
The applicability is discussed at more length in [I-D.ietf-dnsop-
inaddr-required].
7.2 Manual or Custom DNS Updates
Reverse DNS can of course be updated using manual or custom methods.
These are not further described here, except for one special case.
One way to deploy reverse DNS would be to use wildcard records, for
example, by configuring one name for a subnet (/64) or a site (/48).
As a concrete example, a site (or the site's ISP) could configure the
reverses of the prefix 2001:db8:f00::/48 to point to one name using a
wildcard record like "*.0.0.f.0.8.b.d.0.1.0.0.2.ip6.arpa. IN PTR
site.example.com." Naturally, such a name could not be verified from
the forward DNS, but would at least provide some form of "topological
information" or "weak authorization" if that is really considered to
be useful. Note that this is not actually updating the DNS as such,
as the whole point is to avoid DNS updates completely by manually
configuring a generic name.
7.3 DDNS with Stateless Address Autoconfiguration
Dynamic reverse DNS with SLAAC is simpler than forward DNS updates in
some regard, while being more difficult in another, as described
below.
The address space administrator decides whether the hosts are trusted
to update their reverse DNS records or not. If they are trusted and
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deployed at the same site (e.g., not across the Internet), a simple
address-based authorization is typically sufficient (i.e., check that
the DNS update is done from the same IP address as the record being
updated); stronger security can also be used [RFC3007]. If they
aren't allowed to update the reverses, no update can occur. However,
such address-based update authorization operationally requires that
ingress filtering [RFC3704] has been set up at the border of the site
where the updates occur, and as close to the updater as possible.
Address-based authorization is simpler with reverse DNS (as there is
a connection between the record and the address) than with forward
DNS. However, when a stronger form of security is used, forward DNS
updates are simpler to manage because the host can be assumed to have
an association with the domain. Note that the user may roam to
different networks, and does not necessarily have any association
with the owner of that address space -- so, assuming stronger form of
authorization for reverse DNS updates than an address association is
generally infeasible.
Moreover, the reverse zones must be cleaned up by an unspecified
janitorial process: the node does not typically know a priori that it
will be disconnected, and cannot send a DNS update using the correct
source address to remove a record.
A problem with defining the clean-up process is that it is difficult
to ensure that a specific IP address and the corresponding record are
no longer being used. Considering the huge address space, and the
unlikelihood of collision within 64 bits of the interface
identifiers, a process which would remove the record after no traffic
has been seen from a node in a long period of time (e.g., a month or
year) might be one possible approach.
To insert or update the record, the node must discover the DNS server
to send the update to somehow, similar to as discussed in
Section 6.2. One way to automate this is looking up the DNS server
authoritative (e.g., through SOA record) for the IP address being
updated, but the security material (unless the IP address-based
authorization is trusted) must also be established by some other
means.
One should note that Cryptographically Generated Addresses [RFC3972]
(CGAs) may require a slightly different kind of treatment. CGAs are
addresses where the interface identifier is calculated from a public
key, a modifier (used as a nonce), the subnet prefix, and other data.
Depending on the usage profile, CGAs might or might not be changed
periodically due to e.g., privacy reasons. As the CGA address is not
predicatable, a reverse record can only reasonably be inserted in the
DNS by the node which generates the address.
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7.4 DDNS with DHCP
With DHCPv4, the reverse DNS name is typically already inserted to
the DNS that reflects to the name (e.g., "dhcp-67.example.com"). One
can assume similar practice may become commonplace with DHCPv6 as
well; all such mappings would be pre-configured, and would require no
updating.
If a more explicit control is required, similar considerations as
with SLAAC apply, except for the fact that typically one must update
a reverse DNS record instead of inserting one (if an address
assignment policy that reassigns disused addresses is adopted) and
updating a record seems like a slightly more difficult thing to
secure. However, it is yet uncertain how DHCPv6 is going to be used
for address assignment.
Note that when using DHCP, either the host or the DHCP server could
perform the DNS updates; see the implications in Section 6.2.
If disused addresses were to be reassigned, host-based DDNS reverse
updates would need policy considerations for DNS record modification,
as noted above. On the other hand, if disused address were not to be
assigned, host-based DNS reverse updates would have similar
considerations as SLAAC in Section 7.3. Server-based updates have
similar properties except that the janitorial process could be
integrated with DHCP address assignment.
7.5 DDNS with Dynamic Prefix Delegation
In cases where a prefix, instead of an address, is being used and
updated, one should consider what is the location of the server where
DDNS updates are made. That is, where the DNS server is located:
1. At the same organization as the prefix delegator.
2. At the site where the prefixes are delegated to. In this case,
the authority of the DNS reverse zone corresponding to the
delegated prefix is also delegated to the site.
3. Elsewhere; this implies a relationship between the site and where
DNS server is located, and such a relationship should be rather
straightforward to secure as well. Like in the previous case,
the authority of the DNS reverse zone is also delegated.
In the first case, managing the reverse DNS (delegation) is simpler
as the DNS server and the prefix delegator are in the same
administrative domain (as there is no need to delegate anything at
all); alternatively, the prefix delegator might forgo DDNS reverse
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capability altogether, and use e.g., wildcard records (as described
in Section 7.2). In the other cases, it can be slighly more
difficult, particularly as the site will have to configure the DNS
server to be authoritative for the delegated reverse zone, implying
automatic configuration of the DNS server -- as the prefix may be
dynamic.
Managing the DDNS reverse updates is typically simple in the second
case, as the updated server is located at the local site, and
arguably IP address-based authentication could be sufficient (or if
not, setting up security relationships would be simpler). As there
is an explicit (security) relationship between the parties in the
third case, setting up the security relationships to allow reverse
DDNS updates should be rather straightforward as well (but IP
address-based authentication might not be acceptable). In the first
case, however, setting up and managing such relationships might be a
lot more difficult.
8. Miscellaneous DNS Considerations
This section describes miscellaneous considerations about DNS which
seem related to IPv6, for which no better place has been found in
this document.
8.1 NAT-PT with DNS-ALG
The DNS-ALG component of NAT-PT mangles A records to look like AAAA
records to the IPv6-only nodes. Numerous problems have been
identified with DNS-ALG [I-D.ietf-v6ops-natpt-to-exprmntl]. This is
a strong reason not to use NAT-PT in the first place.
8.2 Renumbering Procedures and Applications' Use of DNS
One of the most difficult problems of systematic IP address
renumbering procedures [I-D.ietf-v6ops-renumbering-procedure] is that
an application which looks up a DNS name disregards information such
as TTL, and uses the result obtained from DNS as long as it happens
to be stored in the memory of the application. For applications
which run for a long time, this could be days, weeks or even months;
some applications may be clever enough to organize the data
structures and functions in such a manner that look-ups get refreshed
now and then.
While the issue appears to have a clear solution, "fix the
applications", practically this is not reasonable immediate advice;
the TTL information is not typically available in the APIs and
libraries (so, the advice becomes "fix the applications, APIs and
libraries"), and a lot more analysis is needed on how to practically
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go about to achieve the ultimate goal of avoiding using the names
longer than expected.
9. Acknowledgements
Some recommendations (Section 4.3, Section 5.1) about IPv6 service
provisioning were moved here from [I-D.ietf-v6ops-mech-v2] by Erik
Nordmark and Bob Gilligan. Havard Eidnes and Michael Patton provided
useful feedback and improvements. Scott Rose, Rob Austein, Masataka
Ohta, and Mark Andrews helped in clarifying the issues regarding
additional data and the use of TTL. Jefsey Morfin, Ralph Droms,
Peter Koch, Jinmei Tatuya, Iljitsch van Beijnum, Edward Lewis, and
Rob Austein provided useful feedback during the WG last call. Thomas
Narten provided extensive feedback during the IESG evaluation.
10. Security Considerations
This document reviews the operational procedures for IPv6 DNS
operations and does not have security considerations in itself.
However, it is worth noting that in particular with Dynamic DNS
Updates, security models based on the source address validation are
very weak and cannot be recommended -- they could only be considered
in the environments where ingress filtering [RFC3704] has been
deployed. On the other hand, it should be noted that setting up an
authorization mechanism (e.g., a shared secret, or public-private
keys) between a node and the DNS server has to be done manually, and
may require quite a bit of time and expertise.
To re-emphasize what was already stated, the reverse+forward DNS
check provides very weak security at best, and the only
(questionable) security-related use for them may be in conjunction
with other mechanisms when authenticating a user.
11. References
11.1 Normative References
[I-D.ietf-dnsop-ipv6-dns-configuration]
Jeong, J., "IPv6 Host Configuration of DNS Server
Information Approaches",
draft-ietf-dnsop-ipv6-dns-configuration-06 (work in
progress), May 2005.
[I-D.ietf-ipv6-unique-local-addr]
Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", draft-ietf-ipv6-unique-local-addr-09 (work in
progress), January 2005.
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[I-D.ietf-v6ops-renumbering-procedure]
Baker, F., "Procedures for Renumbering an IPv6 Network
without a Flag Day",
draft-ietf-v6ops-renumbering-procedure-05 (work in
progress), March 2005.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, April 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2182] Elz, R., Bush, R., Bradner, S., and M. Patton, "Selection
and Operation of Secondary DNS Servers", BCP 16, RFC 2182,
July 1997.
[RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC3007] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, November 2000.
[RFC3041] Narten, T. and R. Draves, "Privacy Extensions for
Stateless Address Autoconfiguration in IPv6", RFC 3041,
January 2001.
[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, February 2001.
[RFC3152] Bush, R., "Delegation of IP6.ARPA", BCP 49, RFC 3152,
August 2001.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3363] Bush, R., Durand, A., Fink, B., Gudmundsson, O., and T.
Hain, "Representing Internet Protocol version 6 (IPv6)
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Addresses in the Domain Name System (DNS)", RFC 3363,
August 2002.
[RFC3364] Austein, R., "Tradeoffs in Domain Name System (DNS)
Support for Internet Protocol version 6 (IPv6)", RFC 3364,
August 2002.
[RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6
(IPv6) Addressing Architecture", RFC 3513, April 2003.
[RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", RFC 3596,
October 2003.
[RFC3646] Droms, R., "DNS Configuration options for Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
December 2003.
[RFC3736] Droms, R., "Stateless Dynamic Host Configuration Protocol
(DHCP) Service for IPv6", RFC 3736, April 2004.
[RFC3879] Huitema, C. and B. Carpenter, "Deprecating Site Local
Addresses", RFC 3879, September 2004.
[RFC3901] Durand, A. and J. Ihren, "DNS IPv6 Transport Operational
Guidelines", BCP 91, RFC 3901, September 2004.
[RFC4038] Shin, M-K., Hong, Y-G., Hagino, J., Savola, P., and E.
Castro, "Application Aspects of IPv6 Transition",
RFC 4038, March 2005.
[RFC4074] Morishita, Y. and T. Jinmei, "Common Misbehavior Against
DNS Queries for IPv6 Addresses", RFC 4074, May 2005.
11.2 Informative References
[I-D.durand-dnsop-dont-publish]
Durand, A. and T. Chown, "To publish, or not to publish,
that is the question.", draft-durand-dnsop-dont-publish-00
(work in progress), February 2005.
[I-D.huitema-v6ops-teredo]
Huitema, C., "Teredo: Tunneling IPv6 over UDP through
NATs", draft-huitema-v6ops-teredo-05 (work in progress),
April 2005.
[I-D.huston-6to4-reverse-dns]
Huston, G., "6to4 Reverse DNS Delegation",
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draft-huston-6to4-reverse-dns-03 (work in progress),
October 2004.
[I-D.ietf-dhc-ddns-resolution]
Stapp, M. and B. Volz, "Resolution of FQDN Conflicts among
DHCP Clients", draft-ietf-dhc-ddns-resolution-09 (work in
progress), June 2005.
[I-D.ietf-dhc-fqdn-option]
Stapp, M. and Y. Rekhter, "The DHCP Client FQDN Option",
draft-ietf-dhc-fqdn-option-10 (work in progress),
February 2005.
[I-D.ietf-dnsext-dhcid-rr]
Stapp, M., Lemon, T., and A. Gustafsson, "A DNS RR for
encoding DHCP information (DHCID RR)",
draft-ietf-dnsext-dhcid-rr-09 (work in progress),
February 2005.
[I-D.ietf-dnsop-bad-dns-res]
Larson, M. and P. Barber, "Observed DNS Resolution
Misbehavior", draft-ietf-dnsop-bad-dns-res-03 (work in
progress), October 2004.
[I-D.ietf-dnsop-inaddr-required]
Senie, D., "Encouraging the use of DNS IN-ADDR Mapping",
draft-ietf-dnsop-inaddr-required-06 (work in progress),
February 2005.
[I-D.ietf-v6ops-3gpp-analysis]
Wiljakka, J., "Analysis on IPv6 Transition in 3GPP
Networks", draft-ietf-v6ops-3gpp-analysis-11 (work in
progress), October 2004.
[I-D.ietf-v6ops-mech-v2]
Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", draft-ietf-v6ops-mech-v2-07
(work in progress), March 2005.
[I-D.ietf-v6ops-natpt-to-exprmntl]
Aoun, C. and E. Davies, "Reasons to Move NAT-PT to
Experimental", draft-ietf-v6ops-natpt-to-exprmntl-01 (work
in progress), July 2005.
[I-D.ietf-v6ops-onlinkassumption]
Roy, S., "IPv6 Neighbor Discovery On-Link Assumption
Considered Harmful", draft-ietf-v6ops-onlinkassumption-03
(work in progress), May 2005.
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[I-D.ietf-v6ops-v6onbydefault]
Roy, S., Durand, A., and J. Paugh, "Issues with Dual Stack
IPv6 on by Default", draft-ietf-v6ops-v6onbydefault-03
(work in progress), July 2004.
[I-D.jeong-dnsop-ipv6-dns-discovery]
Jeong, J., "IPv6 DNS Configuration based on Router
Advertisement", draft-jeong-dnsop-ipv6-dns-discovery-04
(work in progress), February 2005.
[I-D.ohta-preconfigured-dns]
Ohta, M., "Preconfigured DNS Server Addresses",
draft-ohta-preconfigured-dns-01 (work in progress),
February 2004.
[RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address
Translation - Protocol Translation (NAT-PT)", RFC 2766,
February 2000.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC2826] Internet Architecture Board, "IAB Technical Comment on the
Unique DNS Root", RFC 2826, May 2000.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
[RFC4025] Richardson, M., "A Method for Storing IPsec Keying
Material in DNS", RFC 4025, March 2005.
Authors' Addresses
Alain Durand
SUN Microsystems, Inc.
17 Network circle UMPL17-202
Menlo Park, CA 94025
USA
Email: Alain.Durand@sun.com
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Johan Ihren
Autonomica
Bellmansgatan 30
SE-118 47 Stockholm
Sweden
Email: johani@autonomica.se
Pekka Savola
CSC/FUNET
Espoo
Finland
Email: psavola@funet.fi
Appendix A. Unique Local Addressing Considerations for DNS
Unique local addresses [I-D.ietf-ipv6-unique-local-addr] have
replaced the now-deprecated site-local addresses [RFC3879]. From the
perspective of the DNS, the locally generated unique local addresses
(LUL) and site-local addresses have similar properties.
The interactions with DNS come in two flavors: forward and reverse
DNS.
To actually use local addresses within a site, this implies the
deployment of a "split-faced" or a fragmented DNS name space, for the
zones internal to the site, and the outsiders' view to it. The
procedures to achieve this are not elaborated here. The implication
is that local addresses must not be published in the public DNS.
To faciliate reverse DNS (if desired) with local addresses, the stub
resolvers must look for DNS information from the local DNS servers,
not e.g. starting from the root servers, so that the local
information may be provided locally. Note that the experience of
private addresses in IPv4 has shown that the root servers get loaded
for requests for private address lookups in any case. This
requirement is discussed in [I-D.ietf-ipv6-unique-local-addr].
Appendix B. Behaviour of Additional Data in IPv4/IPv6 Environments
DNS responses do not always fit in a single UDP packet. We'll
examine the cases which happen when this is due to too much data in
the Additional Section.
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B.1 Description of Additional Data Scenarios
There are two kinds of additional data:
1. "critical" additional data; this must be included in all
scenarios, with all the RRsets, and
2. "courtesy" additional data; this could be sent in full, with only
a few RRsets, or with no RRsets, and can be fetched separately as
well, but at the cost of additional queries.
The responding server can algorithmically determine which type the
additional data is by checking whether it's at or below a zone cut.
Only those additional data records (even if sometimes carelessly
termed "glue") are considered "critical" or real "glue" if and only
if they meet the abovementioned condition, as specified in Section
4.2.1 of [RFC1034].
Remember that resource record sets (RRsets) are never "broken up", so
if a name has 4 A records and 5 AAAA records, you can either return
all 9, all 4 A records, all 5 AAAA records or nothing. In
particular, notice that for the "critical" additional data getting
all the RRsets can be critical.
In particular, [RFC2181] specifies (in Section 9) that:
a. if all the "critical" RRsets do not fit, the sender should set
the TC bit, and the recipient should discard the whole response
and retry using mechanism allowing larger responses such as TCP.
b. "courtesy" additional data should not cause the setting of TC
bit, but instead all the non-fitting additional data RRsets
should be removed.
An example of the "courtesy" additional data is A/AAAA records in
conjunction with MX records as shown in Section 4.4; an example of
the "critical" additional data is shown below (where getting both the
A and AAAA RRsets is critical w.r.t. to the NS RR):
child.example.com. IN NS ns.child.example.com.
ns.child.example.com. IN A 192.0.2.1
ns.child.example.com. IN AAAA 2001:db8::1
When there is too much "courtesy" additional data, at least the non-
fitting RRsets should be removed [RFC2181]; however, as the
additional data is not critical, even all of it could be safely
removed.
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When there is too much "critical" additional data, TC bit will have
to be set, and the recipient should ignore the response and retry
using TCP; if some data were to be left in the UDP response, the
issue is which data could be retained.
Failing to discard the response with TC bit or omitting critical
information but not setting TC bit lead to an unrecoverable problem.
Omitting only some of the RRsets if all would not fit (but not
setting TC bit) leads to a performance problem. These are discussed
in the next two subsections.
B.2 Which Additional Data to Keep, If Any?
If the implementation decides to keep as much data (whether
"critical" or "courtesy") as possible in the UDP responses, it might
be tempting to use the transport of the DNS query as a hint in either
of these cases: return the AAAA records if the query was done over
IPv6, or return the A records if the query was done over IPv4.
However, this breaks the model of independence of DNS transport and
resource records, as noted in Section 1.2.
With courtesy additional data, as long as enough RRsets will be
removed so that TC will not be set, it is allowed to send as many
complete RRsets as the implementations prefers. However, the
implementations are also free to omit all such RRsets, even if
complete. Omitting all the RRsets (when removing only some would
suffice) may create a performance penalty, whereby the client may
need to issue one or more additional queries to obtain necessary
and/or consistent information.
With critical additional data, the alternatives are either returning
nothing (and absolutely requiring a retry with TCP) or returning
something (working also in the case if the recipient does not discard
the response and retry using TCP) in addition to setting the TC bit.
If the process for selecting "something" from the critical data would
otherwise be practically "flipping the coin" between A and AAAA
records, it could be argued that if one looked at the transport of
the query, it would have a larger possibility of being right than
just 50/50. In other words, if the returned critical additional data
would have to be selected somehow, using something more sophisticated
than a random process would seem justifiable.
That is, leaving in some intelligently selected critical additional
data is a tradeoff between creating an optimization for those
resolvers which ignore the "should discard" recommendation, and
causing a protocol problem by propagating inconsistent information
about "critical" records in the caches.
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Similarly, leaving in the complete courtesy additional data RRsets
instead of removing all the RRsets is a performance tradeoff as
described in the next section.
B.3 Discussion of the Potential Problems
As noted above, the temptation for omitting only some of the
additional data could be problematic. This is discussed more below.
For courtesy additional data, this causes a potential performance
problem as this requires that the clients issue re-queries for the
potentially omitted RRsets. For critical additional data, this
causes a potential unrecoverable problem if the response is not
discarded and the query not re-tried with TCP, as the nameservers
might be reachable only through the omitted RRsets.
If an implementation would look at the transport used for the query,
it is worth remembering that often the host using the records is
different from the node requesting them from the authoritative DNS
server (or even a caching resolver). So, whichever version the
requestor (e.g., a recursive server in the middle) uses makes no
difference to the ultimate user of the records, whose transport
capabilities might differ from those of the requestor. This might
result in e.g., inappropriately returning A records to an IPv6-only
node, going through a translation, or opening up another IP-level
session (e.g., a PDP context [I-D.ietf-v6ops-3gpp-analysis]).
Therefore, at least in many scenarios, it would be very useful if the
information returned would be consistent and complete -- or if that
is not feasible, return no misleading information but rather leave it
to the client to query again.
The problem of too much additional data seems to be an operational
one: the zone administrator entering too many records which will be
returned either truncated (or missing some RRsets, depending on
implementations) to the users. A protocol fix for this is using
EDNS0 [RFC2671] to signal the capacity for larger UDP packet sizes,
pushing up the relevant threshold. Further, DNS server
implementations should rather omit courtesy additional data
completely rather than including only some RRsets [RFC2181]. An
operational fix for this is having the DNS server implementations
return a warning when the administrators create zones which would
result in too much additional data being returned. Further, DNS
server implementations should warn of or disallow such zone
configurations which are recursive or otherwise difficult to manage
by the protocol.
Additionally, to avoid the case where an application would not get an
address at all due to some of courtesy additional data being omitted,
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the resolvers should be able to query the specific records of the
desired protocol, not just rely on getting all the required RRsets in
the additional section.
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