DNS Operations WG                                              A. Durand
Internet-Draft                                    SUN Microsystems, Inc.
Expires: November 10, 2004                                      J. Ihren
                                                               P. Savola
                                                            May 12, 2004

          Operational Considerations and Issues with IPv6 DNS

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   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,
   considerations for DNS updates for both the forward and reverse

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   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 'ANY' 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
   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 . . . . . .  8
     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   Behaviour of Additional Data in IPv4/IPv6 Environments . . 10
     4.5   The Use of TTL for IPv4 and IPv6 RRs . . . . . . . . . . . 11
     4.6   IPv6 Transport Guidelines for DNS Servers  . . . . . . . . 12
   5.  Recommendations for DNS Resolver IPv6 Support  . . . . . . . . 13
     5.1   DNS Lookups May Query IPv6 Records Prematurely . . . . . . 13
     5.2   Obtaining a List of DNS Recursive Resolvers  . . . . . . . 14
     5.3   IPv6 Transport Guidelines for Resolvers  . . . . . . . . . 15
   6.  Considerations about Forward DNS Updating  . . . . . . . . . . 15
     6.1   Manual or Custom DNS Updates . . . . . . . . . . . . . . . 15
     6.2   Dynamic DNS  . . . . . . . . . . . . . . . . . . . . . . . 15
   7.  Considerations about Reverse DNS Updating  . . . . . . . . . . 16
     7.1   Applicability of Reverse DNS . . . . . . . . . . . . . . . 17
     7.2   Manual or Custom DNS Updates . . . . . . . . . . . . . . . 17
     7.3   DDNS with Stateless Address Autoconfiguration  . . . . . . 18
     7.4   DDNS with DHCP . . . . . . . . . . . . . . . . . . . . . . 18
     7.5   DDNS with Dynamic Prefix Delegation  . . . . . . . . . . . 19
   8.  Miscellaneous DNS Considerations . . . . . . . . . . . . . . . 20
     8.1   NAT-PT with DNS-ALG  . . . . . . . . . . . . . . . . . . . 20
     8.2   Renumbering Procedures and Applications' Use of DNS  . . . 20
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
   10.   Security Considerations  . . . . . . . . . . . . . . . . . . 21
   11.   References . . . . . . . . . . . . . . . . . . . . . . . . . 21
   11.1  Normative References . . . . . . . . . . . . . . . . . . . . 21
   11.2  Informative References . . . . . . . . . . . . . . . . . . . 21
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 25
   A.  Site-local Addressing Considerations for DNS . . . . . . . . . 25

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       Intellectual Property and Copyright Statements . . . . . . . . 27

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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 site-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 [1]
   for more about IPv6 DNS usage, and [2] or [4] for background

   In particular one should note that the use of A6 records in the
   forward tree or Bitlabels in the reverse tree is not recommended [2].
   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 [5].

1.2  Independence of DNS Transport and DNS Records

   DNS has been designed to present a single, globally unique name space
   [7].  This property should be maintained, as described here and in

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   Section 1.3.

   In DNS, the 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 servers must not make
   any assumptions about what data to return for Answer and Authority

   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
   to begin with).

   As stated in [1]:

      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 [3] for more information.

1.4  Query Type 'ANY' and A/AAAA Records

   QTYPE=* is typically only used for debugging or management purposes;
   it is worth keeping in mind that QTYPE=* ("ANY" queries; note that
   QTYPE=* is the technically correct, though oxymoronic, term)
   literally return any available RRsets, not *all* the RRsets, as only
   some of these may be present in the caches.  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 [6] includes two kinds of local-use
   addresses: link-local (fe80::/10) and site-local (fec0::/10).  The
   site-local addresses are being deprecated [8], and are only discussed
   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 [9].

2.2  Temporary Addresses

   Temporary addresses defined in RFC3041 [10] (sometimes called
   "privacy addresses") use a random number as the interface identifier.
   Publishing DNS records relating to such addresses would defeat the
   purpose of the mechanism and is not recommended.  If absolutely
   necessary, a mapping could be made to some non-identifiable name, as
   described in [10].

2.3  6to4 Addresses

   6to4 [11] specifies an automatic tunneling mechanism which maps a
   public IPv4 address V4ADDR to an IPv6 prefix 2002:V4ADDR::/48.
   Providing reverse DNS delegation path for such addresses is not
   straightforward and practically impossible.

   Note that it does not seem feasible to provide reverse DNS with the
   other automatic tunneling mechanism, Teredo [12]; 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 relatively short-lived.

   If the reverse DNS population would be desirable (see Section 7.1 for
   applicability), there are a number of ways to tackle the delegation
   path problem [13], some more applicable than the others.

   The main proposal [14] has been to allocate  to
   Regional Internet Registries (RIRs) and let them do subdelegations in
   accordance to the delegations of the respective IPv4 address space.
   This has a major practical drawback: those ISPs and IPv4 address
   space holders where 6to4 is being used do not, in general, provide
   any IPv6 services -- as otherwise, most people would not have to use
   6to4 to begin with -- and it is improbable that the reverse
   delegation chain would be completed either.  In most cases, creating
   such delegation chains might just lead to latencies caused by lookups
   for (almost always) non-existent DNS records.

   Another proposal [15] aims to design an autonomous reverse-delegation

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   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., RIRs.
   This is a more practical solution, but may have some scalability

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 [16]: 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 [17].  However, these do not seem to directly impair IPv6
   use, and are only referred to for completeness.

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4.  Recommendations for Service Provisioning using DNS

   When names are added in the DNS to facilitate a 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 DNS.  This can be done by the use of
   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

   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 should 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.)

   This is a 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
   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

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   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

   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
   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 [18].

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4.4  Behaviour of Additional Data in IPv4/IPv6 Environments

   Consider the case where the query name is so long, the number of the
   additional records is so high, or for other reasons that the entire
   response would not fit in a single UDP packet.  In some cases, the
   responder truncates the response with the TC bit being set (leading
   to a retry with TCP), in order for the querier to get the entire
   response later.

   However, note that if too much additional information that is not
   strictly necessary would be added, one should remove unnecessary
   information instead of setting TC bit for this "courtesy" information

   Also notice that there are two kinds of additional data:

   1.  glue, i.e., "critical" additional data; this must be included in
       all scenarios, with all the RRsets as possible, 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 could lead to non-optimal results.

   Meanwhile, 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.  Notice that for
   the "critical" additional data getting all the RRsets can be

   An example of the "courtesy" additional data is A/AAAA records in
   conjunction of MX records as shown in the next section; an example of
   the "critical" additional data is shown below (where getting both the
   A and AAAA RRsets is critical):

   child.example.com.  IN NS ns.child.example.com.
   ns.child.example.com. IN A
   ns.child.example.com. IN AAAA 2001:db8::1

   In the case of too much additional data (whether courtesy or
   critical), 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 authoritative
   DNS server (or even a caching resolver).  So, whichever version the

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   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 [20]).

   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 to the users.  A
   protocol fix for this is using EDNS0 [40] 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.  An
   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.

   Additionally, to avoid the case where an application would not get an
   address at all due to non-critical additional data being omitted, the
   applications 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.

4.5  The Use of TTL for IPv4 and IPv6 RRs

   In the previous section, we discussed a danger with queries,
   potentially leading to omitting RRsets from the additional section;
   this could happen to both critical and "courtesy" additional data.
   This section discusses another problem with the latter, leading to
   omitting RRsets in cached data, highlighted in the IPv4/IPv6

   The behaviour of DNS caching when different TTL values are used for
   different RRsets 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
   foo.example.com.    100    IN    AAAA   2001:db8::1

   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.  So, there are
   three cases about returning records for the MX in the additional

   1.  We get back no A or AAAA RRsets: this is the simplest case,
       because then we have to query which information is required

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       explicitly, guaranteeing that we get all the information we're
       interested in.

   2.  We get back all the RRsets: this is an optimization as there is
       no need to perform more queries, causing lower latency.  However,
       it is impossible to guarantee that in fact we would always get
       back all the records (the only way to ensure that is to send a
       AAAA query for the name after getting the cached reply); however,
       one could try to work in the direction to try to ensure it as far
       as possible.

   3.  We only get back A or AAAA RRsets even if both existed: this is
       indistinguishable from the previous case, and problematic as
       described in the previous section.

   As the third case was considered in the previous section, we assume
   we get back both A and AAAA records of foo.example.com, or the stub
   resolver explicitly asks, in two separate queries, both A and AAAA

   After 100 seconds, the AAAA record is removed from the cache(s)
   because its TTL expired.  It would 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.  However, this is not mandated or mentioned
   by the specification(s).

   To simplify the situation, it might help to use the same TTL for all
   the resource record sets referring to the same name, unless there is
   a particular reason for not doing so.  However, there are some
   scenarios (e.g., when renumbering IPv6 but keeping IPv4 intact) where
   a different strategy is preferable.

   Thus, applications that use the response should not rely on a
   particular TTL configuration.  For example, even if an application
   gets a response that only has the A record in the example described
   above, it should not assume there is no AAAA record for
   "foo.example.com".  Instead, the application should try to fetch the
   missing records by itself if it needs the record.

4.6  IPv6 Transport Guidelines for DNS Servers

   As described in Section 1.3 and [3], 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 [21].)

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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
   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 [22].  In some cases, the
   implementation may attempt to connect or send a datagram on a

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   physical link [23], incurring very long protocol timeouts, instead of
   quickly failing back to IPv4.

   Now, we can consider the issues specific to each of the three

   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 [24].

   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 [25], 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  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 [29].  This option can be passed to a host through a subset of
   DHCPv6 [28].

   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 (May 2004).

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   In scenarios where DHCPv6 is unavailable or inappropriate, mechanisms
   under consideration for development of dnsop WG include the use of
   well-known addresses [26], the use of Router Advertisements to convey
   the information [27].

   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 DNS resolver
   discovery function.

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
   IPv6, it bears thinking about especially due to adding Stateless
   Address Autoconfiguration [30] 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
   relationship 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 into a
   DNS zone, instead of updating an existing name -- is considered out
   of scope for this memo.  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 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) [31][32] is a standardized mechanism for
   dynamically updating the DNS.  It works equally well with stateless

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   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 [32].  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
   host should not be able to state that its name is "www.example.com").
   DHCP-initiated DDNS updates have been extensively described in [33],
   [34] and [35].

   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
   autoconfigured 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 [37]; 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

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   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

   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 [38].

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.  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.

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7.3  DDNS with Stateless Address Autoconfiguration

   Dynamic DNS with SLAAC simpler than forward DNS updates in some
   regard, while being more difficult in another.

   The address space administrator decides whether the hosts are trusted
   to update their reverse DNS records or not.  If they are, 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 [32].  If they aren't
   allowed to update the reverses, no update can occur.

   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 knows the record it's
   updating, and 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 unfeasible.

   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

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

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   well; all such mappings would be pre-configured, and would require no

   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
   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

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   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.  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

   NAT-PT [36] 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 [39].

8.2  Renumbering Procedures and Applications' Use of DNS

   One of the most difficult problems of systematic IP address
   renumbering procedures [37] 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.

9.  Acknowledgements

   Some recommendations (Section 4.3, Section 5.1) about IPv6 service
   provisioning were moved here from [41] by Erik Nordmark and Bob

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   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, and Edward Lewis provided useful
   feedback during the WG last call.

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

   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.

11.  References

11.1  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-02 (work
        in progress), March 2004.

11.2  Informative References

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

   [5]   Austein, R., "Tradeoffs in Domain Name System (DNS) Support for
         Internet Protocol version 6 (IPv6)", RFC 3364, August 2002.

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

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

   [8]   Huitema, C. and B. Carpenter, "Deprecating Site Local
         Addresses", draft-ietf-ipv6-deprecate-site-local-03 (work in
         progress), March 2004.

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

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

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

   [12]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through NATs",
         draft-huitema-v6ops-teredo-01 (work in progress), February

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

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

   [15]  Huston, G., "6to4 Reverse DNS",
         draft-huston-6to4-reverse-dns-02 (work in progress), April

   [16]  Morishita, Y. and T. Jinmei, "Common Misbehavior against DNS
         Queries for IPv6 Addresses",
         draft-ietf-dnsop-misbehavior-against-aaaa-01 (work in
         progress), April 2004.

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

   [18]  Savola, P., "Moving from 6bone to IPv6 Internet",
         draft-savola-v6ops-6bone-mess-01 (work in progress), November

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   [19]  Elz, R. and R. Bush, "Clarifications to the DNS Specification",
         RFC 2181, July 1997.

   [20]  Wiljakka, J., "Analysis on IPv6 Transition in 3GPP Networks",
         draft-ietf-v6ops-3gpp-analysis-09 (work in progress), March

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

   [22]  Roy, S., Durand, A. and J. Paugh, "Issues with Dual Stack IPv6
         on by Default", draft-ietf-v6ops-v6onbydefault-02 (work in
         progress), May 2004.

   [23]  Roy, S., Durand, A. and J. Paugh, "IPv6 Neighbor Discovery
         On-Link Assumption Considered Harmful",
         draft-ietf-v6ops-onlinkassumption-02 (work in progress), May

   [24]  Shin, M., "Application Aspects of IPv6 Transition",
         draft-ietf-v6ops-application-transition-02 (work in progress),
         March 2004.

   [25]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M.
         Carney, "Dynamic Host Configuration Protocol for IPv6
         (DHCPv6)", RFC 3315, July 2003.

   [26]  Ohta, M., "Preconfigured DNS Server Addresses",
         draft-ohta-preconfigured-dns-01 (work in progress), February

   [27]  Jeong, J., "IPv6 DNS Discovery based on Router Advertisement",
         draft-jeong-dnsop-ipv6-dns-discovery-01 (work in progress),
         February 2004.

   [28]  Droms, R., "Stateless Dynamic Host Configuration Protocol
         (DHCP) Service for IPv6", RFC 3736, April 2004.

   [29]  Droms, R., "DNS Configuration options for Dynamic Host
         Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, December

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

   [31]  Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic
         Updates in the Domain Name System (DNS UPDATE)", RFC 2136,

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         April 1997.

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

   [33]  Stapp, M., "Resolution of DNS Name Conflicts Among DHCP
         Clients", draft-ietf-dhc-ddns-resolution-06 (work in progress),
         October 2003.

   [34]  Stapp, M. and Y. Rekhter, "The DHCP Client FQDN Option",
         draft-ietf-dhc-fqdn-option-06 (work in progress), October 2003.

   [35]  Stapp, M., Lemon, T. and A. Gustafsson, "A DNS RR for encoding
         DHCP information (DHCID RR)", draft-ietf-dnsext-dhcid-rr-07
         (work in progress), October 2003.

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

   [37]  Baker, F., Lear, E. and R. Droms, "Procedures for Renumbering
         an IPv6 Network without a Flag Day",
         draft-ietf-v6ops-renumbering-procedure-00 (work in progress),
         February 2004.

   [38]  Senie, D., "Requiring DNS IN-ADDR Mapping",
         draft-ietf-dnsop-inaddr-required-05 (work in progress), April

   [39]  Durand, A., "Issues with NAT-PT DNS ALG in RFC2766",
         draft-durand-v6ops-natpt-dns-alg-issues-00 (work in progress),
         February 2003.

   [40]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671,
         August 1999.

   [41]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for
         IPv6 Hosts and Routers", draft-ietf-v6ops-mech-v2-02 (work in
         progress), February 2004.

   [42]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
         Addresses", draft-ietf-ipv6-unique-local-addr-03 (work in
         progress), February 2004.

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Authors' Addresses

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

   EMail: Alain.Durand@sun.com

   Johan Ihren
   Bellmansgatan 30
   SE-118 47 Stockholm

   EMail: johani@autonomica.se

   Pekka Savola


   EMail: psavola@funet.fi

Appendix A.  Site-local Addressing Considerations for DNS

   As site-local addressing is being deprecated, the considerations for
   site-local addressing are discussed briefly here.  Unique local
   addressing format [42] has been proposed as a replacement, but being
   work-in-progress, it is not considered further.

   The interactions with DNS come in two flavors: forward and reverse

   To actually use site-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 site-local addresses must not be published in the public DNS.

   To faciliate reverse DNS (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

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   experience of private addresses in IPv4 has shown that the root
   servers get loaded for requests for private address lookups in any

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