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IETF RFC 5158
6to4 Reverse DNS Delegation Specification
Last modified on Wednesday, March 5th, 2008
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Network Working Group G. Huston
Request for Comments: 5158 APNIC
Category: Informational March 2008
6to4 Reverse DNS Delegation Specification
 Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Abstract
This memo describes the service mechanism for entering a delegation
of DNS servers that provide reverse lookup of 6to4 IPv6 addresses
into the 6to4 reverse zone file. The mechanism is based on a
conventional DNS delegation service interface, allowing the service
client to enter the details of a number of DNS servers for the
delegated domain. In the context of a 6to4 reverse delegation, the
client is primarily authenticated by its source address used in the
delegation request, and is authorized to use the function if its IPv6
address prefix corresponds to an address from within the requested
6to4 delegation address block.
Huston Informational PAGE 1 
RFC 5158 6to4 Reverse DNS March 2008
1. Introduction
6to4 [RFC 3056] defines a mechanism for allowing isolated IPv6 sites
to communicate using IPv6 over the public IPv4 Internet. This is
achieved through the use of a dedicated IPv6 global unicast address
prefix. A 6to4 'router' can use its IPv4 address value in
conjunction with this global prefix to create a local IPv6 site
prefix. Local IPv6 hosts use this site prefix to form their local
IPv6 address.
This address structure allows any site that is connected to the IPv4
Internet the ability to use IPv6 via automatically created IPv6 over
IPv4 tunnels. The advantage of this approach is that it allows the
piecemeal deployment of IPv6 using tunnels to traverse IPv4 network
segments. A local site can connect to an IPv6 network without
necessarily obtaining IPv6 services from its adjacent upstream
network provider.
As noted in [6to4-dns], the advantage of this approach is that: "it
decouples deployment of IPv6 by the core of the network (e.g.
Internet Service Providers or ISPs) from deployment of IPv6 at the
edges (e.g. customer sites), allowing each site or ISP to deploy IPv6
support in its own time frame according to its own priorities. With
6to4, the edges may communicate with one another using IPv6 even if
one or more of their ISPs do not yet provide native IPv6 service."
The particular question here is the task of setting up a set of
delegations that allows "reverse lookups" for this address space.
"[This] requires that there be a delegation path for the IP
address being queried, from the DNS root to the servers for the
[DNS] zone which provides the PTR records for that IP address.
For ordinary IPv6 addresses, the necessary DNS servers and records
for IPv6 reverse lookups would be maintained by the each
organization to which an address block is delegated; the
delegation path of DNS records reflects the delegation of address
blocks themselves. However, for IPv6 addresses beginning with the
6to4 address prefix, the DNS records would need to reflect IPv4
address delegation. Since the entire motivation of 6to4 is to
decouple site deployment of IPv6 from infrastructure deployment of
IPv6, such records cannot be expected to be present for a site
using 6to4 - especially for a site whose ISP did not yet support
IPv6 in any form." [6to4-dns]
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RFC 5158 6to4 Reverse DNS March 2008
The desired characteristics of a reverse lookup delegation mechanism
are that it:
* is deployable with minimal overhead or tool development
* has no impact on existing DNS software and existing DNS
operations
* performs name lookup efficiently
* does not compromise any DNS security functions
2. Potential Approaches
There are a number of approaches to this problem, ranging from a
conventional explicit delegation structure to various forms of
modified server behaviors that attempt to guess the location of non-
delegated servers for fragments of this address space. These
approaches have been explored in some detail in terms of their
advantages and drawbacks in [6to4-dns], so only a summary of these
approaches will be provided here.
2.1. Conventional Address Delegation
The problem with this form of delegation is the anticipated piecemeal
deployment of 6to4 sites. The reason why an end site would use 6to4
is commonly that the upstream Internet Service Provider does not
support an IPv6 transit service and the end site is using 6to4 to
tunnel through to IPv6 connectivity. A conventional end site
environment of this form would have the end site using provider-based
IPv4 addresses, where the end site's IPv4 address is a more specific
address block drawn from the upstream provider's address block, and
the end site would have an entry in the upstream provider's reverse
DNS zone file, or it would have authoritative local name servers that
are delegated from the upstream provider's DNS zone. In the case of
the 6to4 mapped IPv6 space, the upstream may not be providing any
IPv6-based services at all, and therefore would not be expected to
have a 6to4 reverse DNS delegation for its IPv4 address block. The
general observation is that 6to4 IPv6 reverse DNS delegations cannot
necessarily always precisely match the corresponding IPv4 reverse DNS
delegations.
Huston Informational PAGE 3
RFC 5158 6to4 Reverse DNS March 2008
Sub-delegations of IPv4 provider address space are not consistently
recorded, and any 6to4 reverse zone operator would be required to
undertake reverse zone delegations in the absence of reliable current
address assignment information, undertaking a "hop over" of the
upstream provider's address block. Similarly, a delegated entity may
need to support the same "hop over" when undertaking further
delegations in their reverse zone.
2.2. Guessing a Non-Delegated 6to4 Reverse Server
One way to avoid such unreliable delegations is to alter server
behavior for reverse servers in this zone. Where no explicit
delegation information exists in the zone file, the server could look
up the in-addr.arpa domain for the servers for the equivalent IPv4
address root used in the 6to4 address. These servers could then be
queried for the IPv6 PTR query.
The issues with fielding altered server behaviors for this domain are
not to be taken lightly, and the delegation chain for IPv4 will not
be the same for 6to4 in any case. An isolated 6to4 site uses a
single IPv4 /32 address, and it is improbable that a single address
would have explicit in-addr.arpa delegation. In other words, it is
not likely that the delegation for IPv4 would parallel that of 6to4.
2.3. Locating Local Servers at Reserved Addresses
Another approach uses an altered server to resolve non-delegated 6to4
reverse queries. The 6to4 query is decoded to recover the original
6to4 IP address. The site-specific part of the address is rewritten
to a constant value, and this value is used as the target of a lookup
query. This requires that a 6to4 site should reserve local
addresses, and configure reverse servers on these addresses. Again,
this is a weak approach in that getting the DNS to query non-
delegated addresses is a case of generation of spurious traffic.
2.4. Synthesized Responses
The final approach considered here is to synthesize an answer when no
explicit delegation exists. This approach would construct a pseudo
host name using the IPv6 query address as the seed. Given that the
host name has no valid forward DNS mapping, then this becomes a case
of transforming one invalid DNS object into another.
Huston Informational PAGE 4
RFC 5158 6to4 Reverse DNS March 2008
2.5. Selecting a Reasonable Approach
It would appear that the most reasonable approach from this set of
potential candidates is to support a model of conventional standard
delegation. The consequent task is to reduce the administrative
overheads in managing the zone, supporting delegation of reverse zone
files on a basis of providing a delegation capability directly to
each 6to4 site.
3. 6to4 Networks Address Use
A 6to4 client network is an isolated IPv6 network composed as a set
of IPv6 hosts and a dual stack (IPv4 and IPv6) local router connected
to the local IPv6 network and the external IPv4 network.
An example of a 6to4 network is as follows:
+-------------+
IPv6-in-IPv4 packets (A)| | IPv6 packets
------------------------| 6to4router |--------------------------
| | | | | | |
+-------------+ local IPv6 clients
IPv4 network IPv6 network
Figure 1
The IPv4 address used as part of the generation of 6to4 addresses for
the local IPv6 network is that of the external IPv4 network interface
address (labelled '(A)' in the above diagram). For example, if the
interface (A) has the IPv4 address 192.0.2.1, then the local IPv6
clients will use a common IPv6 address prefix of the form 2002:
{192.0.2.1}::/48 (or (2002:C000:201::/48 in hex notation). All the
local IPv6 clients share this common /48 address prefix, irrespective
of any local IPv4 address that such host may use if they are
operating in a dual stack mode.
Huston Informational PAGE 5
RFC 5158 6to4 Reverse DNS March 2008
An example of a 6to4 network with addressing:
+-------------+
IPv4 network (A)| | IPv6 network
-------------------| 6to4router |-------------
192.0.2.1| | | | | interface identifier
+-------------+ 1A | | local IPv6 address
2002:C000:201::1A
| |
1B |
2002:C000:201::1B
|
1C
2002:C000:201::1C
Figure 2
4. Delegation Administration
This specification uses a single delegation level in the
2.0.0.2.ip6.arpa zone (the ip6.arpa zone is specified in [RFC 3596]),
delegating zones only at the 48th bit position. This corresponds
with individual delegations related to a single /32 IPv4 address, or
the equivalent of a single 6to4 local site.
The zone files containing the end site delegations are to be operated
with a low TTL (configured to be a time value in the scale of hours
rather than days or weeks), and updates for delegation requests in
the 2.0.0.2.ipv6.arpa zone are to be made using dynamic DNS updates
[RFC 2136].
The delegation system is to be self-driven by clients residing within
6to4 networks. The 6to4 reverse DNS delegation function is to be
accessible only by clients using 6to4 IPv6 source addresses, and the
only delegation that can be managed is that corresponding to the /48
prefix of the IPv6 source address of the client.
This service is to operate the delegation management service using
web-based server access using Transport Layer Security (TLS)
[RFC 4346] (accessible via a "https:" URL). This is intended to
ensure that the source address-driven delegation selection function
cannot be disrupted through proxy caching of the web server's
responses, and also to ensure that the delegation service cannot be
readily mimicked.
The service is to be found at https://6to4.nro.net
Huston Informational PAGE 6
RFC 5158 6to4 Reverse DNS March 2008
This service is implemented by web servers that are operated on a
dual-stack IPv4 / IPv6 server, accessible via SSL. The web server's
actions will be determined by the source address of the client. If
the client uses a 6to4 source address, the server will present a
delegation interface for the corresponding 6to4 reverse zone.
Otherwise, the server will provide a description of the delegation
process.
When accessed by a 6to4 source address, the interface presented by
the delegation service is a conventional DNS delegation interface,
allowing the client to enter the details of a number of DNS servers
for the corresponding reverse domain. The targets of the DNS
delegation are checked by the delegation manager using IPv4 and IPv6,
according to the addresses of the targets, to ensure that they are
responding, that they are configured consistently, and are
authoritative for the delegated domain. If these conditions are met,
the delegation details are entered into the zone at the primary
master. In order to avoid the server being used as a denial of
service platform, the server should throttle the number of DNS
delegation requests made to any single IP address, and also throttle
the number of redelegation requests for any single 6to4 zone.
In other cases the system provides diagnostic information to the
client.
The benefits of this structure include a fully automated mode of
operation. The service delivery is on demand and the system only
permits self-operation of the delegation function.
The potential issues with this structure include:
o Clients inside a 6to4 site could alter the delegation details
without the knowledge of the site administrator. It is noted that
this is intended for small-scale sites. Where there are potential
issues of unauthorized access to this delegation function, the
local site administrator could take appropriate access control
measures.
o IPv4 DHCP-based 6to4 sites, or any 6to4 site that uses
dynamically-assigned external IPv4 interface addresses, could
inherit nonsense reverse entries created by previous users of the
dynamically assigned address. In this case, the client site could
request delegation of the reverse zone as required. More
generally, given the potentially for inheritance of 'stale'
reverse DNS information in this context, in those cases where the
issues of potential inheritance of 'stale' reverse DNS information
is a concern, it is recommended that a 6to4 site either use a
static IPv4 address in preference to a dynamically-assigned
Huston Informational PAGE 7
RFC 5158 6to4 Reverse DNS March 2008
address, or ensure that the reverse delegation information is
updated using the service mechanism described here upon each
dynamic address assignment event.
o The approach does not scale efficiently, as there is the potential
that the flat zone file may grow considerably. However, it is
noted that 6to4 is intended to be a transition mechanism useful
for a limited period of time in a limited context of an isolated
network where other forms of a tunnelled connection is not
feasible. It is envisaged that at some point the density of IPv6
adoption in stub network would provide adequate drivers for
widespread adoption of native IPv6 services, obviating the need
for continued scaling of 6to4 support services. An estimate of
the upper bound of the size of the 6to4 reverse delegation zone
would be of the order of millions of entries. It is also noted
that the value of a reverse delegation is a questionable
proposition and many deployment environments have no form of
reverse delegation.
o It is also conceivable that an enterprise network could decide to
use 6to4 internally in some form of private context, with the
hosts only visible in internal DNS servers. In this mechanism the
reverse delegation, if desired, would need to be implemented in an
internal private (non-delegated) corresponding zone of the 6to4
reverse domain space.
There may be circumstances where an IPv4 address controller wishes to
"block" the ability for users of these addresses to use this 6to4
scheme. Scenarios that would motivate this concern would include
situations when the IPv4 provider is also offering an IPv6 service,
and native IPv6 should be deployed instead of 6to4. In such
circumstances the 6to4 reverse zone operator should allow for such a
6to4 reverse delegation blocking function upon application to the
delegation zone operator.
For a delegation to be undertaken the following conditions should
hold:
o The 6to4 site must have configured a minimum of one primary and
one secondary server for the 6to4 IPv6 reverse address zone.
o At the time of the delegation request, the primary and secondary
servers must be online, reachable, correctly configured, and in a
mutually consistent state with respect to the 6to4 reverse zone.
In this context, "mutually consistent" implies the same SOA RR and
identical NS RRSets.
Huston Informational PAGE 8
RFC 5158 6to4 Reverse DNS March 2008
o The 6to4 reverse delegation service will only accept delegation
requests associated with the 6to4 source address of the requesting
client.
The approach described here, of a fully automated system driven by
the site administrators of the 6to4 client networks, appears to
represent an appropriate match of the operational requirements of
managing reverse DNS domains for 6to4 addresses.
For maintenance of the reverse delegation zones the service maintains
an email contact point for each active delegation, derived from the
zone's SOA contact address (SOA RNAME), or explicitly entered in the
delegation interface. This contact point would be informed upon any
subsequent change of delegation details.
The 6to4 reverse DNS management system also undertakes a periodic
sweep of all active delegations, so that each delegation is checked
every 30 days. If the delegation fails this integrity check the
email contact point is informed of the problem, and a further check
is scheduled for 14 days later. If this second check fails, the
delegation is automatically removed, and a further notice is issued
to the contact point.
5. Security Considerations
This system offers a rudimentary level of assurance in attempting to
ensure that delegation requests from a 6to4 site can only direct the
delegation of the corresponding 6to4 reverse DNS domain and no other.
Address-based authentication is not a very robust method from a
security perspective, as addresses can be readily spoofed.
Accordingly, reverse delegation information does not provide reliable
information in either validating a domain name or in validating an IP
address, and no conclusions should be drawn from the presence or
otherwise of a reverse DNS mapping for any IP address.
The service management interface allows a 6to4 client to insert any
server name as a DNS server, potentially directing the delegation
test system to make a DNS query to any nominated system. The server
throttles the number of requests made to any single IP address to
mitigate the attendant risk of a high volume of bogus DNS queries
being generated by the server. For similar reasons, the server also
throttles the number of redelegation requests for any single 6to4
zone.
For a general threat analysis of 6to4, especially the additional risk
of address spoofing in 2002::/16, see [RFC 3964].
Huston Informational PAGE 9
RFC 5158 6to4 Reverse DNS March 2008
Section 4 notes that the local site administrator could take
appropriate access control measures to prevent clients inside a 6to4
site performing unauthorized changes to the delegation details. This
may be in the form of a firewall configuration, regarding control of
access to the service from the interior of a 6to4 site, or a similar
mechanism that enforces local access policies.
6. IANA Considerations
The IANA has delegated the 2.0.0.2.ip6.arpa domain according to
delegation instructions provided by the Internet Architecture Board.
7. Acknowledgements
The author acknowledges the prior work of Keith Moore in preparing a
document that enumerated a number of possible approaches to undertake
the delegation and discovery of reverse zones. The author
acknowledges the assistance of George Michaelson and Andrei
Robachevsky in preparing this document, and Peter Koch, Pekka Savola,
Jun-ichiro Itojun Hagino, and Catherine Meadows for their helpful
review comments.
8. References
8.1. Normative References
[RFC 2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, April 1997.
[RFC 3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, February 2001.
[RFC 3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", RFC 3596,
October 2003.
[RFC 4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
8.2. Informative References
[6to4-dns] Moore, K., "6to4 and DNS", Work in Progress, April 2003.
[RFC 3964] Savola, P. and C. Patel, "Security Considerations for
6to4", RFC 3964, December 2004.
Huston Informational PAGE 10
RFC 5158 6to4 Reverse DNS March 2008
Author's Address
Geoff Huston
APNIC
EMail: gih@apnic.net
URI: http://www.apnic.net
Huston Informational PAGE 11
RFC 5158 6to4 Reverse DNS March 2008
Full Copyright Statement
Copyright © The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
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Huston Informational PAGE 12
6to4 Reverse DNS Delegation Specification
RFC TOTAL SIZE: 25362 bytes
PUBLICATION DATE: Wednesday, March 5th, 2008
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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