WDIFF

draft-ietf-dnsop-ipv6-dns-issues-02.txt --> draft-ietf-dnsop-ipv6-dns-issues-04.txt

DNS Operations WG A. Durand
INTERNET-DRAFT
Internet-Draft SUN Microsystems,inc.
Feb, 27, 2003 Johan Microsystems, Inc.
Expires: July 1, 2004 J. Ihren
Expires August, 28, 2003
Autonomica
P. Savola
CSC/FUNET
Jan 2004

Operational Considerations and Issues with IPv6 DNS transition issues
<draft-ietf-dnsop-ipv6-dns-issues-02.txt>
draft-ietf-dnsop-ipv6-dns-issues-04.txt

Status of this memo Memo

This memo provides information to the Internet community. It does not
specify document is an Internet standard of any kind. This memo Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 RFC2026.

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 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/1id-abstracts.html 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
http://www.ietf.org/shadow.html.

This Internet-Draft will expire on July 1, 2004.

Abstract

This memo summarizes DNS related issues when transitioning a network
to IPv6. Consensus presents operational considerations and open issues are presented.

1. Representing IPv6 addresses in DNS records

In the direct zones, according to [RFC3363], IPv6 addresses are
represented using AAAA records [RFC1886]. In the reverse zone, with IPv6
addresses are represented using PTR records in nibble format under
the ip6.arpa. tree [RFC3152].

2. IPv4/IPv6 name space

2.1 Terminology

The phrase "IPv4 name server" indicates a name server available over
IPv4 transport. It does not imply anything about what DNS data is
served. Likewise, "IPv6 name server" indicates
Domain Name System (DNS), including a name server
available over summary of special IPv6 transport.

2.2. Introduction to the problem
addresses, documentation of name space fragmentation:
following the referral chain

The caching resolver that tries to lookup a name starts out at the
root, known DNS implementation misbehaviour,
recommendations and follows referrals until it is referred considerations on how to a nameserver that
is authoritative perform DNS naming for
service provisioning and for DNS resolver IPv6 support,
considerations for DNS updates for both the name. If somewhere down the chain of
referrals it is referred to a nameserver that is only accessible over
a type of transport that is unavailable, a traditional nameserver forward and reverse
trees, and miscellaneous issues. This memo is
unable to finish the task.

When the Internet moves from IPv4 aimed to include a mixture
summary of information about IPv6 DNS considerations for those who
have experience with IPv4 DNS.

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Internet-Draft Considerations and Issues with IPv6 it is
only a matter of time until this starts to happen and the complete DNS hierarchy starts to fragment into a graph where authoritative
nameservers for certain nodes are only accessible over a certain
transport. What is feared is that a node using only a particular
version of IP, querying information about another node using the same
version Jan 2004

Table of IP can not do it because, somewhere Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Representing IPv6 Addresses in the chain DNS Records . . . . . . . . . . 3
1.2 Independence of
servers accessed during the resolution process, one or more DNS Transport and DNS Records . . . . . . . . 3
1.3 Avoiding IPv4/IPv6 Name Space Fragmentation . . . . . . . . . 4
2. DNS Considerations about Special IPv6 Addresses . . . . . . . 4
2.1 Limited-scope Addresses . . . . . . . . . . . . . . . . . . . 4
2.2 Privacy (RFC3041) Address . . . . . . . . . . . . . . . . . . 4
2.3 6to4 Addresses . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Observed DNS Implementation Misbehaviour . . . . . . . . . . . 5
3.1 Misbehaviour of them
will only be accessible with the other version DNS Servers and Load-balancers . . . . . . . . 5
3.2 Misbehaviour of IP.

With all DNS data only available over IPv4 transport everything is
simple. Resolvers . . . . . . . . . . . . . . . . 6
4. Recommendations for Service Provisioning using DNS . . . . . . 6
4.1 Use of Service Names instead of Node Names . . . . . . . . . . 6
4.2 Separate vs the Same Service Names for IPv4 resolvers can use and IPv6 . . . . . 7
4.3 Adding the intended mechanism Records Only when Fully IPv6-enabled . . . . . . . 7
4.4 The Use of following
referrals from the root TTL for IPv4 and down while IPv6 resolvers have to work
through a "translator", i.e. they have to use a second name server on
a so-called "dual stack" host as a "forwarder" since they cannot
access the RRs . . . . . . . . . . . . . 8
4.5 Behaviour of Glue in Mixed IPv4/IPv6 Environments . . . . . . 8
4.6 IPv6 Transport Guidelines for DNS data directly.

With all Servers . . . . . . . . . . 9
5. Recommendations for DNS data only available over Resolver IPv6 transport everything would
be equally simple, Support . . . . . . . . 9
5.1 DNS Lookups May Query IPv6 Records Prematurely . . . . . . . . 9
5.2 Recursive DNS Resolver Discovery . . . . . . . . . . . . . . . 11
5.3 IPv6 Transport Guidelines for Resolvers . . . . . . . . . . . 11
6. Considerations about Forward DNS Updating . . . . . . . . . . 11
6.1 Manual or Custom DNS Updates . . . . . . . . . . . . . . . . . 12
6.2 Dynamic DNS . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Considerations about Reverse DNS Updating . . . . . . . . . . 13
7.1 Applicability of Reverse DNS . . . . . . . . . . . . . . . . . 13
7.2 Manual or Custom DNS Updates . . . . . . . . . . . . . . . . . 14
7.3 DDNS with the exception Stateless Address Autoconfiguration . . . . . . . . 14
7.4 DDNS with DHCP . . . . . . . . . . . . . . . . . . . . . . . . 14
7.5 DDNS with Dynamic Prefix Delegation . . . . . . . . . . . . . 15
8. Miscellaneous DNS Considerations . . . . . . . . . . . . . . . 15
8.1 NAT-PT with DNS-ALG . . . . . . . . . . . . . . . . . . . . . 15
8.2 Renumbering Procedures and Applications' Use of old legacy IPv4 name servers
having to switch to a forwarding configuration.

However, the second situation will not arise in a foreseeable time.
Instead, DNS . . . . . 15
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
10. Security Considerations . . . . . . . . . . . . . . . . . . . 16
Normative References . . . . . . . . . . . . . . . . . . . . . 16
Informative References . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 19
A. Site-local Addressing Considerations for DNS . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . 21

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1. Introduction

This memo presents operational considerations and issues with IPv6
DNS; it is expected that the transition will be from IPv4 only meant to be an extensive summary and a mixture list of IPv4 and IPv6, with pointers
for more information about IPv6 DNS data of theoretically three
categories depending on whether it is available only over considerations for those with
experience with IPv4
transport, only over IPv6 or both. DNS.

The latter is the best situation, and first section gives a major question is brief overview of how to
ensure that it as quickly as possible becomes IPv6 addresses and
names are represented in the norm. However,
while it is obvious that some DNS data will only be available over v4 DNS, how transport for a long time it is also obvious that it is important to
avoid fragmenting the protocols and
resource records (don't) relate, and what IPv4/IPv6 name space available to IPv4 only hosts. I.e.
during transition it is not acceptable
fragmentation means and how to break the name space that
we presently have available for IPv4-only hosts.

2.3 Policy based avoidance avoid it; all of name space fragmentation.

Today there these are only a few DNS "zones" on described
at more length in other documents.

The second section summarizes the public Internet that
are available over special IPv6 transport, address types and how
they can mostly be regarded
as "experimental". However, as soon as there is a root name server
available over IPv6 transport it is reasonable to expect that it will
become more common relate to DNS. The third section describes observed DNS
implementation misbehaviours which have zones served by IPv6 servers over time.

Having those zones served only by IPv6-only name server would not be a good development, since this will fragment varying effect on the previously
unfragmented IPv4 name space use
of IPv6 records with DNS. The fourth section lists recommendations
and there are strong reasons to find a
mechanism to avoid it. considerations for provisioning services with DNS. The RECOMMENDED approach to maintain name space continuity is to use
administrative policies:
- every recursive DNS server SHOULD be either IPv4-only or dual
stack,
- every single DNS zone SHOULD be served by fifth
section in turn looks at least one IPv4
reachable DNS server.

This rules out IPv6-only recursive DNS servers recommendations and considerations about
providing IPv6 support in the resolvers. The sixth and seventh
sections describe considerations with forward and reverse DNS zones served
only by IPv6-only DNS servers. This approach could be revisited
if/when translation techniques between IPv4 and
updates, respectively. The eighth section introduces several
miscellaneous IPv6 were to be
widely deployed.

In order issues relating to enforce the second point, the zone validation process
SHOULD ensure that there is at least one IPv4 address record
available DNS for the name servers of any child delegations within the
zone.

3. Local Scope addresses.

[IPv6ADDRARCH] define three scopes of addresses, link local, site
local and global.

3.1 Link local addresses

Local addresses SHOULD NOT be published which no better place
has been found in this memo. Appendix A looks briefly at the DNS, neither
requirements for site-local addressing.

1.1 Representing IPv6 Addresses in DNS Records

In the forward tree nor in zones, IPv6 addresses are represented using AAAA
records. In the reverse tree.

3.2 Site local addresses

Note: There is an ongoing discussion zones, IPv6 address are represented using
PTR records in the IPv6 wg on nibble format under the
usefulness of site local addresses that may end up deprecating ip6.arpa. -tree. See [1]
for more about IPv6 DNS usage, and [2] or
limiting [4] for background
information.

In particular one should note that the use of Site Local addresses.

Site local addresses are an evolution of private addresses [RFC1918] A6 records, DNAME
records in IPv4. The main difference is that, within a site, nodes are
expected to have several addresses with different scopes. [ADDRSELEC]
recommends to use the lowest possible scope possible for
communications. That is, if both site local & global addresses are
published reverse tree, or Bitlabels in the DNS for node B, and node A reverse tree is configured also with
both site local & global addresses, the communication between node A not
recommended [2].

1.2 Independence of DNS Transport and B DNS Records

DNS has been designed to use site local addresses.

For reasons illustrated in [DontPublish], site local addresses SHOULD
NOT present a single, globally unique name space
[6]. This property should be published maintained, as described here and in
Section 1.3.

In DNS, the public DNS. They MAY be published in a site
view IP version used to transport the queries and responses is
independent of the records being queried: AAAA records can be queried
over IPv4, and A records over IPv6. The DNS if two-face DNS is deployed.

For a related discussion on how servers must not make any
assumptions about what data to handle those "local" zones, see
[LOCAL].

3.3 Reverse path DNS return for site local addresses.

The main issue is that the view of a site may be different on a stub
resolver Answer and on a fully recursive resolver it points to. A simple
scenario to illustrate the issue is a home network deploying site
local addresses. Reverse DNS resolution for site local addresses has
to be done within the home network Authority

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sections.

However, there is some debate whether the addresses SHOULD NOT in Additional
section could be populated selected or filtered using hints obtained from which
transport was being used; this has some obvious problems because in
many cases the public reverse
tree. If two-face DNS transport protocol does not correlate with the
requests, and because a "bad" answer is deployed, site local addresses MAY be
populated in a way worse than no answer
at all (consider the local view of reverse tree.

4. Automatic population of the Reverse path DNS

Getting case where the reverse tree DNS populated correctly in IPv4 client is not an
easy exercise and very often led to believe that a
name received in the records are additional record does not really up have any AAAA records
to date or
simply are just not there. begin with).

As IPv6 addresses are much longer than
IPv4 addresses, stated in [1]:

The IP protocol version used for querying resource records is
independent of the situation protocol version of the reverse tree DNS will probably
be even worse.

A fairly common practice from resource records; e.g.,
IPv4 ISP is transport can be used to generate PTR query IPv6 records for
home customers automatically from 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 address itself. Something
like:

1.2.3.4.in-addr.arpa. IN PTR 4.3.2.1.local-ISP.net

It (or IPv6) transport, the
recommendation is not clear today if something similar need to be done in IPv6,
and, if yes, what is the best approach always keep at least one authoritative server
IPv4-enabled, and to this problem.

As the number of possible PTR records would be huge (2^80) ensure that recursive DNS servers support IPv4.
See DNS IPv6 transport guidelines [3] for more information.

2. DNS Considerations about Special IPv6 Addresses

There are a /48
prefix, a possible solution would be to use wildcards entries like:

*.0.1.2.3.4.5.6.7.8.9.a.b.c.ip6.arpa. IN PTR customer-42.local-
ISP.net

However, the use couple of wildcard is generally discouraged IPv6 address types which are somewhat special;
these are considered here.

2.1 Limited-scope Addresses

The IPv6 addressing architecture [5] includes two kinds of local-use
addresses: link-local (fe80::/10) and this may
not be an acceptable solution.

An alternative approach is to dynamically synthetize PTR records,
either on the server side or on the resolver side. This approach is site-local (fec0::/10). The
site-local addresses are being deprecated [7], and are only discussed at length
in [DYNREVERSE].

Other solutions like Appendix A.

Link-local addresses should never be published in DNS, because they
have only local (to the connected link) significance [8].

2.2 Privacy (RFC3041) Address

Privacy addresses (RFC3041 [9]) use of ICMP name lookups [ICMPNL] have been
proposed but failed a random number as the interface
identifier. Publishing DNS records relating to reach consensus. It such addresses would work if and only the
remote host is reachable at
defeat the time purpose of the request mechanism and one can
somehow trust the value that would be returned by the remote host.
the

A more radical approach would be is not recommended. If
absolutely necessary, a mapping could be made to pre-populate the some
non-identifiable name, as described in [9].

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2.3 6to4 Addresses

6to4 [10] specifies an automatic tunneling mechanism which maps a
public IPv4 address V4ADDR to an IPv6 prefix 2002:V4ADDR::/48.
Providing reverse tree
at all. This approach claims that applications DNS delegation path for such addresses is a
challenge. Note that misuse similar difficulties don't surface with the
other automatic tunneling mechanisms (in particular, providing
reverse DNS information for any kind Teredo [11] hosts whose address includes
the UDP port of access control the NAT binding does not seem reasonable).

If the reverse DNS population would be desirable (see Section 7.1 for
applicability), there are fundamentally broken a number of ways to tackle the delegation
path problem [12], some more applicable than the others.

The main proposal [13] has been to allocate 2.0.0.2.ip6.arpa. to RIRs
and
should be fixed without introducing any kludge let them do subdelegations in accordance to the DNS. There is a
certain capital delegations of sympathy for this, however, ISP who who pre-
generate statically PTR records for their
the respective IPv4 customers do it for address space. This has a
reason, major practical
drawback: those ISPs and it is unlikely that this reason will disappear with the
introduction of IPv6.

5. Privacy extension addresses

[RFC3041] defines privacy extensions for IPv6 stateless
autoconfiguration IPv4 address space holders where the interface ID 6to4 is a random number. As those
addresses are designed to
being used do not, in general, provide privacy by making it more difficult any IPv6 services -- as
otherwise, most people would not have to log and trace back use 6to4 to the user, begin with --
and it makes no sense to in is improbable that the reverse tree DNS to have them pointing to a real name.

[RFC3041] type addresses SHOULD NOT delegation chain would be published
completed either. In most cases, creating such delegation chains
might just lead to latencies caused by lookups for (almost always)
non-existent DNS records.

3. Observed DNS Implementation Misbehaviour

Several classes of misbehaviour in the reverse tree DNS pointing to meaningful names. A servers, load-balancers and
resolvers have been observed. Most of these are rather generic, catch-all name MAY be
acceptable. An interesting alternative would be not
only applicable to use dynamic
synthesis as in [DYNREVERSE].

6. 6to4

6to4 addresses can be published in the forward DNS, however special
care is needed IPv6 -- but in some cases, the reverse tree. See [6to4ReverseDNS] for details.
The delegation consequences of 2.0.0.2.ip6.arpa. is suggested
this misbehaviour are extremely severe in [6to4ARPA],
however, delegations 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 [14]: 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 reverse zone under 2.0.0.2.ip6.arpa problems are aggravated by the core 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 problem. Delegating the next 32 bits AAAA records of the IPv4
address used in the 6to4 domain won't scale name, and delegating on less
may require cooperation from after receiving a
response, query the upstream IPSs. The problem here A records. This is
that, especially done in a serial fashion -- if
the case of home usage first query is never responded to (instead of 6to4, the entity being
delegated the x.y.z.t.2.0.0.2.ip6.arpa. zone (the ISP) may not be the
same as the one using 6to4 (the end customer). the

Another problem properly returning

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a negative answer), significant timeouts will occur.

In consequence, this is an enormous problem for 6to4 addresses 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 6to4
prefix may 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 transient. One investigated;
slightly more on this is included in Section 5.

3.2 Misbehaviour of the usage scenario DNS Resolvers

Several classes of 6to4 is to misbehaviour have
PCs connected via dial-up use 6to4 also been noticed in DNS
resolvers [15]. However, these do not seem to connect directly impair IPv6
use, and are only referred to for completeness.

4. Recommendations for Service Provisioning using DNS

When names are added in the IPv6 Internet.
In such DNS to facilitate a scenario, the lifetime service, there are
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

When a node includes multiple services, one should keep them
logically separate in the 6to4 prefix is the same as DNS. This can be done by the DHCP lease use of
service names instead of node names (or, "hostnames").

For example, assume a node named "pobox.example.com" provides both
SMTP and IMAP service. Instead of configuring the IPv4 address it is derived from. It means that
the reverse DNS delegation is only valid for MX records to
point at "pobox.example.com", and configuring the same duration.

A possible approach is not mail clients to populate
look up the reverse tree DNS for 6to4
addresses. Another mail via IMAP from "pobox.example.com", one is to should use dynamic synthesis as described
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
[DYNREVERSE].

7. Recursive DNS the specific case of SMTP relaying, the server discovery

[DNSdiscovery] has been proposed itself
must typically also be configured to reserve a well known site local
unicast address know all its names to configure the ensure
loops do not occur. DNS resolver as can provide a last resort
mechanism, when no other information is available. Another approach layer of indirection between
service names and where the service actually is, and using which
addresses.

This is to use a DHCPv6 extensions [DHCPv6DNS].

8. DNSsec

There is nothing specific good practice with IPv4 as well, because it provides more
flexibility and enables easier migration of services from one host to
another. A specific reason why this is relevant for IPv6 is that the
different services may have a different level of IPv6 support -- that
is, one node providing multiple services might want to enable just

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one service to be IPv6-visible while keeping some others as
IPv4-only. Using service names enables more flexibility with
different IP versions as well.

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 be either added to "service.example.com", or added
separately to a sub-domain, like, "service.ipv6.example.com".

Both methods have different characteristics. Using a sub-domain
allows for easier service piloting, minimizing the disturbance to the
"regular" users of IPv4 service; however, the service would not be
used without explicitly asking for it (or, within a restricted
network, modifying the DNS search path) -- so it will not actually be
used that much. 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 the
next subsection 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

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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
if the service offered using both protocols is of roughly equal
quality, using the appropriate metrics for the service (e.g.,
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
is not yet equal to that of IPv4, at least globally -- this has to be
taken into consideration when enabling services [16].

4.4 The Use of TTL for IPv4 and IPv6 RRs

The behaviour of DNS caching when different TTL values are used for
different records of the same name requires explicit discussion. For
example, let's consider a part of a zone:

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

Now, when a caching resolver asks for the MX record of example.com,
it gets both A and AAAA records of foo.example.com. Then, after 100
seconds, the AAAA record is removed from the cache because its TTL
expired. Now, subsequent queries only result in the cache returning
the A record; after 200 seconds the A record is purged as well. So,
in this particular case, there is a window of 200 seconds when
incomplete information is returned from the cache.

Therefore, when the same name refers to both A and AAAA records,
these records should have the same TTL. Otherwise, the caches may
return incomplete information about the queried names. More issues
with caching and A/AAAA records is presented in the next section.

4.5 Behaviour of Glue in Mixed IPv4/IPv6 Environments

In the previous section, we discussed the effect of impartial data
returned from the caches when the TTLs are not kept the same. Now,
we present another problem highlighted in the mixed IPv4/IPv6
environments.

Consider the case where the query is so long or the number of the
additional ("glue") records is so high that the response must either
be truncated (leading to a retry with TCP) or some of the additional
data removed from the reply. Further, resource record sets are never
"broken up", so if a name has 4 A records and 5 AAAA records, you can

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either return all 9, all 4 A records, all 5 AAAA records or nothing.

In the case of too much additional data, it might be tempting to not
return the AAAA records if the transport for DNS query was IPv4, or
not return the A records, if the transport was IPv6. However, this
breaks the model of independence of DNS transport and resource
records, as noted in Section 1.2.

This temptation would have significant problems in multiple areas.
Remember that often the end-node, which will be using the records, is
not the same one as the node requesting them from the authorative DNS
server (or even a caching resolver). So, whichever version the
requestor ("the middleman") uses makes no difference to the ultimate
user of the records. 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 [31]).

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 impartial to the users. A protocol fix
for this is using EDNS0 [32] to signal the capacity for larger UDP
packet sizes, pushing up the relevant threshold. The operational fix
for this is having the DNS server implementations return a warning
when the administrators create the zones which would result in too
much additional data being returned.

4.6 IPv6 Transport Guidelines for DNS Servers

As described in Section 1.3 and [3], there should continue to be at
least one authorative 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 [17].)

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

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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
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 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 [18]. In some cases, the
implementation may attempt to connect or send a datagram on a
physical link [19], 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

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application is programmed properly, the application can fall
gracefully back to IPv4 [20].

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, or manually. Each would 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 Recursive DNS Resolver Discovery

Recursive IPv6 DNS resolver discovery is a subject of active debate
at the moment: the main proposed mechanisms include the use of
well-known addresses [21], the use of Router Advertisements to convey
the information [22], and using DHCPv6 (or the stateless subset of it
[23]) for DNS resolver configuration. No consensus has been reached
yet.

Note that IPv6 DNS resolver discovery, while an important topic, is
not required for dual-stack nodes in dual-stack networks: IPv6 DNS
records can very well be queried over IPv4 as well.

5.3 IPv6 Transport Guidelines for Resolvers

As described in Section 1.3 and [3], 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 how to enable updating the forward DNS, i.e., the
mapping from names to the correct new addresses, is not specific to

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IPv6, it bears thinking about especially due to adding Stateless
Address Autoconfiguration [24] to the mix.

Typically forward DNS updates are more manageable than doing them in
the reverse DNS, because the updater can, typically, be assumed to
"own" a certain DNS name -- and we can create a form of security
association with the DNS name and the node allowed to update it to
point to a new address.

A more complex form of DNS updates -- adding a whole new name to a
DNS zone, instead of updating an existing name -- is considered
out-of-scope: this is not an IPv6-specific problem, and one still
being explored.

6.1 Manual or Custom DNS Updates

The DNS mappings can 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) [25][26] is a standardized mechanism for
dynamically updating the DNS. It works equally well with stateless
address autoconfiguration (SLAAC), DHCPv6 or manual address
configuration. The only (minor) twist is that with SLAAC, the DNS
server cannot tie the authentication of the user to the IP address,
and stronger mechanisms must be used. Actually, relying on IP
addresses for Dynamic DNS is rather insecure at best, so this is
probably not a significant problem (but requires that the
authorization keying will be explicitly configured).

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
host should not be able to state that its name is "www.example.com").

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 authorative name servers
for the hostname; the second must be configured explicitly unless one

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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
autoconfigured addresses, so that if the node is renumbered in a
managed fashion, the amount of stale DNS information is kept to the
minimum. Actually, 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 be
renewed/refreshed in a while. Some discussion on how to manage the
DNS TTL is included in [28].

7. Considerations about Reverse DNS Updating

Forward DNS updating is rather straightforward; reverse DNS is
significantly trickier especially with certain mechanisms. However,
first it makes sense to look at 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 the
reverse and forward DNS contents match 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 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 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 [29].

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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).
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 DNS with SLAAC is a bit complicated, but manageable with a
rather low form of security with some implementation.

Every node on a link must then be allowed to insert its own reverse
DNS record in the reverse zone. However, in the typical case, there
can be no stronger form of authentication between the nodes and the
server than the source IP address (the user may roam to other
administrative domains as well, requiring updates to foreign DNS
servers), which might make attacks more lucrative.

Moreover, the reverse zones must be cleaned up by some 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.

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 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.

7.4 DDNS with DHCP

With DHCP, the reverse DNS name is typically already inserted to the
DNS that reflects to the name (e.g., "dhcp-67.example.com"). This is
pre-configured, and requires 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 -- due to a denser
address assignment policy -- and updating a record seems like a

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slightly more difficult thing to secure.

Note that when using DHCP, either the host or the DHCP server could
perform the DNS updates; see the implications in Section 6.2.

7.5 DDNS with Dynamic Prefix Delegation

In cases where more than one address is being used and updated, one
should consider where the updated server resides. That is, whether
the prefixes have been delegated to a node in the local site, or
whether they reside elsewhere, e.g., at the ISP. The reverse DNS
updates are typically easier to manage if they can be done within a
single administrative entity -- and therefore, if a reverse DNS
delegation has been made, it may be easier to enable reverse DNS at
the site, e.g. by a wildcard record, or by some DNS update mechanism.

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

NAT-PT [27] DNS-ALG is a critical component (unless something
replacing that functionality is specified) which mangles A records to
look like AAAA records to the IPv6-only nodes. Numerous problems have
been identified with DNS-ALG [30].

8.2 Renumbering Procedures and Applications' Use of DNS

One of the most difficult problems of systematic IP address
renumbering procedures [28] 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
go about to achieve the ultimate goal of avoiding using the names
longer than expected.

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9. Acknowledgements

Some recommendations (Section 4.3, Section 5.1) about IPv6 service
provisioning were moved here from [33] 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.

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. 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 which was already stated, reverse DNS checks provide
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.

Normative References

[1] Thomson, S., Huitema, C., Ksinant, V. and M. Souissi, "DNS
Extensions to Support IP Version 6", RFC 3596, October 2003.

[2] Bush, R., Durand, A., Fink, B., Gudmundsson, O. and T. Hain,
"Representing Internet Protocol version 6 (IPv6) Addresses in
the Domain Name System (DNS)", RFC 3363, August 2002.

[3] Durand, A. and J. Ihren, "DNS IPv6 transport operational
guidelines", draft-ietf-dnsop-ipv6-transport-guidelines-01 (work
in progress), October 2003.

Informative References

[4] Bush, R., "Delegation of IP6.ARPA", BCP 49, RFC 3152, August
2001.

[5] Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
Addressing Architecture", RFC 3513, April 2003.

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[6] Internet Architecture Board, "IAB Technical Comment on the
Unique DNS Root", RFC 2826, May 2000.

[7] Huitema, C. and B. Carpenter, "Deprecating Site Local
Addresses", draft-ietf-ipv6-deprecate-site-local-02 (work in
progress), November 2003.

[8] Hazel, P., "IP Addresses that should never appear in the public
DNS", draft-ietf-dnsop-dontpublish-unreachable-03 (work in
progress), February 2002.

[9] Narten, T. and R. Draves, "Privacy Extensions for Stateless
Address Autoconfiguration in IPv6", RFC 3041, January 2001.

[10] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via
IPv4 Clouds", RFC 3056, February 2001.

[11] Huitema, C., "Teredo: Tunneling IPv6 over UDP through NATs",
draft-huitema-v6ops-teredo-00 (work in progress), June 2003.

[12] Moore, K., "6to4 and DNS", draft-moore-6to4-dns-03 (work in
progress), October 2002.

[13] Bush, R. and J. Damas, "Delegation of 2.0.0.2.ip6.arpa",
draft-ymbk-6to4-arpa-delegation-00 (work in progress), February
2003.

[14] Morishita, Y. and T. Jinmei, "Common Misbehavior against DNS
Queries for IPv6 Addresses",
draft-morishita-dnsop-misbehavior-against-aaaa-00 (work in
progress), June 2003.

[15] Larson, M. and P. Barber, "Observed DNS Resolution
Misbehavior", draft-ietf-dnsop-bad-dns-res-01 (work in
progress), June 2003.

[16] Savola, P., "Moving from 6bone to IPv6 or IPv4 Internet",
draft-savola-v6ops-6bone-mess-01 (work in DNSsec. However,
translation tools such as NAT-PT [RFC2766] introduce a DNS-ALG that
will break DNSsec progress), November
2002.

[17] Elz, R., Bush, R., Bradner, S. and M. Patton, "Selection and
Operation of Secondary DNS Servers", BCP 16, RFC 2182, July
1997.

[18] Roy, S., "Dual Stack IPv6 on by imposing a change Default",
draft-ietf-v6ops-v6onbydefault-00 (work in progress), October
2003.

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[19] Roy, S., "IPv6 Neighbor Discovery On-Link Assumption Considered
Harmful", draft-ietf-v6ops-onlinkassumption-00 (work in
progress), October 2003.

[20] Shin, M., "Application Aspects of IPv6 Transition",
draft-ietf-v6ops-application-transition-00 (work in progress),
December 2003.

[21] Ohta, M., "Preconfigured DNS Server Addresses",
draft-ohta-preconfigured-dns-00 (work in progress), July 2003.

[22] Jeong, J., "IPv6 DNS Discovery based on Router Advertisement",
draft-jeong-dnsop-ipv6-dns-discovery-00 (work in progress),
July 2003.

[23] Droms, R., "Stateless DHCP Service for IPv6",
draft-ietf-dhc-dhcpv6-stateless-04 (work in progress), January
2004.

[24] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.

[25] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic
Updates in the trust model. See [DNS-
ALG] Domain Name System (DNS UPDATE)", RFC 2136,
April 1997.

[26] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, November 2000.

[27] Tsirtsis, G. and P. Srisuresh, "Network Address Translation -
Protocol Translation (NAT-PT)", RFC 2766, February 2000.

[28] Baker, F., "Procedures for details.

9. Security considerations

Using wildcard Renumbering an IPv6 Network without
a Flag Day", draft-baker-ipv6-renumber-procedure-01 (work in
progress), October 2003.

[29] Senie, D., "Requiring DNS IN-ADDR Mapping",
draft-ietf-dnsop-inaddr-required-03 (work in progress), March
2002.

[30] Durand, A., "Issues with NAT-PT DNS records ALG in the reverse path tree may have some
implication when used RFC2766",
draft-durand-v6ops-natpt-dns-alg-issues-00 (work in conjunction progress),
February 2003.

[31] Wiljakka, J., "Analysis on IPv6 Transition in 3GPP Networks",
draft-ietf-v6ops-3gpp-analysis-07 (work in progress), October
2003.

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Internet-Draft Considerations and Issues with DNSsec. Security
considerations IPv6 DNS Jan 2004

[32] Vixie, P., "Extension Mechanisms for referenced documents are described in those memos DNS (EDNS0)", RFC 2671,
August 1999.

[33] Nordmark, E. and are not replicated here.

10. Author addresses R. Gilligan, "Basic Transition Mechanisms for
IPv6 Hosts and Routers", draft-ietf-v6ops-mech-v2-01 (work in
progress), October 2003.

Authors' Addresses

Alain Durand
SUN Microsystems, Inc Inc.
17 Network circle UMPK17-202 UMPL17-202
Menlo Park, CA, CA 94025
USA
Mail:

EMail: Alain.Durand@sun.com

Johan Ihren
Autonomica
Bellmansgatan 30
SE-118 47 Stockholm, Stockholm
Sweden
Mail:

EMail: johani@autonomica.se

11. References

[RFC1918] Address Allocation

Pekka Savola
CSC/FUNET

Espoo
Finland

EMail: psavola@funet.fi

Appendix A. Site-local Addressing Considerations for Private Internets. Y. Rekhter, B.
Moskowitz, D. Karrenberg, G. J. de Groot, E. Lear. February
1996.

[RFC2766] Network Address Translation - Protocol Translation (NAT-
PT).
G. Tsirtsis, P. Srisuresh. February 2000.

[RFC3041] Privacy Extensions DNS

As site-local addressing is being deprecated, and it is not yet clear
whether an addressing-based replacement (and which kind) is devised,
the considerations for Stateless Address Autoconfiguration
in IPv6,
T. Narten, R. Draves, January 2001.

[RFC3152] Delegation of ip6.arpa, R. Bush, August 2001.

[RFC3363] Representing Internet Protocol version 6 (IPv6) Addresses site-local addressing are discussed briefly
here.

The interactions with DNS come in the Domain Name System (DNS), R. Bush, A. Durand, B.
Fink, O. Gudmundsson, T. Hain. August 2002.

[DYNREVERSE] Dynamic two flavors: forward and reverse
DNS.

To actually use site-local addresses within a site, this implies the
deployment of a "split-faced" or a fragmented DNS for IPv6, A. name space, for the

Durand,
draft-durand-dnsops-dynreverse-00.txt, work in progress.

[DNS-ALG] et al. Expires July 1, 2004 [Page 19]

Internet-Draft Considerations and Issues with NAT-PT IPv6 DNS ALG in RFC2766, A. Durand,
draft-durand-v6ops-natpt-dns-alg-issues-00.txt, work in
progress.

[LOCAL] Operational Guidelines for "local" Jan 2004

zones in internal to the DNS,
Kato, A., Vixie, P., draft-kato-dnsop-local-zones-00.txt,
work site, and the outsiders' view to it. The
procedures to achieve this are not elaborated here. The implication
is that site-local addresses must not be published in progress.

[ICMPNL] Use of ICMPv6 node information query for the public DNS.

To faciliate reverse DNS lookup,
Jun-ichiro itojun Hagino, draft-itojun-ipv6-nodeinfo-
revlookup-00.txt, work (if desired) with site-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
site-local information may be provided locally. Note that the
experience private addresses in progress.

[IPv6ADDRARCH] IP Version 6 Addressing Architecture, R. Hinden,
draft-ipngwg-addr-arch-v3-11.txt, work IPv4 has shown that the root servers
get loaded for requests for private address lookups in progress.

[6to4ARPA] Delegation any.

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Intellectual Property Statement

The IETF takes no position regarding the validity or scope of 2.0.0.2.ip6.arpa, Bush, R., Damas, J.,
draft-ymbk-6to4-arpa-delegation-00.txt, work any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in progress.

[6to4ReverseDNS] 6to4
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and DNS, K. Moore, draft-moore-6to4-dns-03.txt,
work
standards-related documentation can be found in progress.

[DNSdiscovery] Well known site local unicast addresses BCP-11. Copies of
claims of rights made available for DNS
resolver,
A. Durand, J. hagano, D. Thaler, draft-ietf-ipv6-dns-
discovery-07.txt, work in progress.

[DHCPv6DNS] DNS Configuration options publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for DHCPv6, Droms, R.
draft-ietf-dhc-dhcpv6-opt-dnsconfig-02.txt, work in
progress.

12. the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.

Full Copyright Statement

"Copyright

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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgment

Funding for the RFC Editor function is currently provided by the
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Durand, et al. Expires July 1, 2004 [Page 22]

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