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Versions: (draft-aoun-v6ops-natpt-deprecate)
00 01 02 03
draft-ietf-v6ops-natpt-to-historic
v6ops Working Group C. Aoun
Internet-Draft ZTE/ENST Paris
Updates: 2766 (if approved) E. Davies
Expires: October 3, 2005 Consultant
April 2005
Reasons to Move NAT-PT to Experimental
draft-ietf-v6ops-natpt-to-exprmntl-02
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document discusses issues with the specific form of IPv6-IPv4
protocol translation mechanism implemented by the Network Address
Translator - Protocol Translator (NAT-PT) defined in RFC 2766. These
issues are sufficiently serious that recommending RFC 2766 as a
general purpose transition mechanism is no longer desirable, and this
document recommends that the IETF should reclassify RFC 2766 from
Standards Track to Experimental status.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Issues Unrelated to DNS-ALG . . . . . . . . . . . . . . . . . 6
2.1. Issues with Protocols Embedding IP Addresses . . . . . . . 6
2.2. NAPT-PT Redirection Issues . . . . . . . . . . . . . . . . 7
2.3. NAT-PT Binding State Decay . . . . . . . . . . . . . . . . 7
2.4. Loss of Information through Incompatible Semantics . . . . 8
2.5. NA(P)T-PT and Fragmentation . . . . . . . . . . . . . . . 9
2.6. NAT-PT Interaction with SCTP and Multihoming . . . . . . . 10
2.7. NAT-PT as a Proxy Correspondent Node for MIPv6 . . . . . . 10
2.8. NAT-PT and Multicast . . . . . . . . . . . . . . . . . . . 11
3. Issues exacerbated by the Use of DNS-ALG . . . . . . . . . . . 12
3.1. Network Topology Constraints Implied by NAT-PT . . . . . . 12
3.2. Scalability and Single Point of Failure Concerns . . . . . 13
3.3. Issues with Lack of Address Persistence . . . . . . . . . 14
3.4. DOS Attacks on Memory and Address/Port Pools . . . . . . . 14
4. Issues Directly Related to use of DNS-ALG . . . . . . . . . . 15
4.1. Address Selection Issues when Communicating with
Dual-Stack End-hosts . . . . . . . . . . . . . . . . . . . 15
4.2. Non-global Validity of Translated RR Records . . . . . . . 17
4.3. Inappropriate Translation of Responses to A Queries . . . 17
4.4. DNS-ALG and Multi-addressed Nodes . . . . . . . . . . . . 17
4.5. Limitations on Deployment of DNS Security Capabilities . . 18
5. Impact on IPv6 Application Development . . . . . . . . . . . . 18
6. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 19
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
10.1. Normative References . . . . . . . . . . . . . . . . . . . 20
10.2. Informative References . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . . . . 25
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1. Introduction
The Network Address Translator - Protocol Translator NAT-PT) document
[RFC2766] defines a set of network layer translation mechanisms
designed to allow nodes which only support IPv4 to communicate with
nodes which only support IPv6 during the transition to the use of
IPv6 in the Internet.
[RFC2766] specifies the basic NAT-PT in which only addresses are
translated and NAPT-PT (Network Address Port Translator - Protocol
Translator)which also translates transport identifiers, allowing for
greater economy of scarce IPv4 addresses. Protocol translation is
performed using the Stateless IP/ICMP Translation Algorithm (SIIT)
defined in [RFC2765].
A number of previous documents have raised issues with NAT-PT. This
document will summarize these issues, note several other issues
carried over from traditional IPv4 NATs and identify some additional
issues which have not been discussed elsewhere. Where solutions to
the issues have been proposed these are mentioned and any resulting
need for changes to the specification identified.
Whereas NAT is seen as an on-going capability which is needed to
workaround the limited availability of globally unique IPv4
addresses, NAT-PT has a different status as a transition mechanism
for IPv6. As such, NAT-PT should not be allowed to constrain the
development of IPv6 applications or impose limitations on future
developments of IPv6.
This document draws the conclusion that the technical and operational
difficulties resulting from these issues, especially the possible
future constraints on the development of IPv6 networks (see
Section 5), make it undesirable to recommend NAT-PT as described in
[RFC2766] as a general purpose transition mechanism for
intercommunication between IPv6 networks and IPv4 networks.
Although the [RFC2766] form of packet translation is not generally
applicable, it is likely that in some circumstances a node which can
only support IPv4 will need to communicate with a node which can only
support IPv6: this is bound to need a translation mechanism of some
kind. Although this may be better carried out by an application
level proxy or transport layer translator, there may still be
scenarios in which a (possibly restricted) version of NAT-PT can be a
suitable solution: accordingly this document recommends that the IETF
should reclassify RFC2766 from Standards Track to Experiemental
status.
The following documents relating directly to NAT-PT have been
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reviewed while drafting this document:
o Network Address Translation - Protocol Translation (NAT-PT)
[RFC2766]
o Stateless IP/ICMP Translation Algorithm (SIIT) [RFC2765]
o NAT-PT applicability statement [I-D.satapati-v6ops-natpt-
applicability]
o Issues with NAT-PT DNS ALG in RFC2766 [I-D.durand-natpt-dns-alg-
issues]
o NAT-PT DNS ALG solutions [I-D.hallin-natpt-dns-alg-solutions]
o NAT-PT Security Considerations [I-D.okazaki-v6ops-natpt-security]
o Issues when translating between IPv4 and IPv6 [I-D.vanderpol-
v6ops-translation-issues]
o IPv6-IPv4 Translation mechanism for SIP-based services in Third
Generation Partnership Project (3GPP) Networks [I-D.elmalki-
sipping-3gpp-translator]
o Analysis on IPv6 Transition in 3GPP Networks [I-D.ietf-v6ops-3gpp-
analysis]
o Considerations for Mobile IP Support in NAT-PT [I-D.lee-v6ops-
natpt-mobility]
o An IPv6/IPv4 Multicast Translator based on IGMP/MLD Proxying (mtp)
[I-D.tsuchiya-mtp]
o An IPv4 - IPv6 multicast gateway [I-D.venaas-mboned-v4v6mcastgw]
o Scalable mNAT-PT Solution [I-D.park-scalable-multi-natpt]
Because the majority of the documents containing discussions of the
issues are Internet Drafts which are unlikely to become RFCs, the
issues are summarized here to avoid the need for normative
references.
Some additional issues can be inferred from corresponding issues
known to exist in 'traditional' IPv4 NATs. The following documents
are relevant:
o Protocol Complications with the IP Network Address Translator
[RFC3027]
o IP Network Address Translator (NAT) Terminology and Considerations
[RFC2663]
There is some ambiguity in [RFC2766] about whether the Application
Layer Gateway (ALG) for DNS (referred to as DNS-ALG in this document)
is an integral and mandatory part of the specification. The
ambiguity arises mainly from the first section of the applicability
section (Section 8) which appears to imply that 'simple' use of
NAT-PT could avoid the use of the DNS-ALG.
This is important because a number of the major issues arise from the
interactions between DNS and NAT-PT. However, detailed inspection of
[RFC2766] shows that the 'simple' case has not been worked out and it
is unclear how information about the address translation could be
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passed to the hosts in the absence of the DNS-ALG. This document
therefore assumes that the DNS-ALG is an integral part of NAT-PT:
accordingly issues with the DNS-ALG must be considered as issues for
the whole specification.
Note that the issues which are not specifically related to the use of
the DNS-ALG will apply to any network layer translation scheme,
including any based on the SIIT algorithm [RFC2765].
Issues raised with NAT-PT can be categorized as follows:
o Issues which are independent of the use of a DNS-ALG:
* Disruption of all protocols which embed IP addresses (and/or
ports) in packet payloads or which apply integrity mechanisms
using IP addresses (and ports).
* Inability to re-direct traffic for protocols without any
demultiplexing capabilities or not built on top of specific
transport layer protocols in situations where one NAPT-PT is
translating for multiple IPv6 hosts.
* Requirement for applications to use keep alive mechanisms to
workaround connectivity issues caused by premature NAT-PT state
timeout.
* Loss of information due to incompatible semantics between IPv4
and IPv6 versions of headers and protocols.
* Need for additional state and/or packet reconstruction in
NAPT-PT translators dealing with packet fragmentation.
* Interaction with SCTP and multihoming.
* Need for NAT-PT to act as proxy for correspondent node when
IPv6 node is mobile, with consequent restrictions on mobility.
* NAT-PT not being able to handle multicast traffic.
o Issues which are exacerbated by the use of a DNS-ALG:
* Constraints on network topology.
* Scalability concerns together with introduction of single point
of failure and security attack nexus.
* Lack of address mapping persistence: Some applications require
address retention between sessions. The user traffic will be
disrupted if a different mapping is used. The use of the DNS-
ALG to create address mappings with limited lifetimes means
that applications must start using the address shortly after
the mapping is created, as well as keeping it alive once they
start using it.
* Creation of a DOS threat relating to exhaustion of memory and
address/port pool resources on the translator.
o Issues which result from the use of a DNS-ALG:
* Address selection issues when either the internal or external
hosts implement both IPv4 and IPv6.
* Restricted validity of translated DNS records: a translated
record may be forwarded to an application which cannot use it.
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* Inappropriate translation of responses to A queries from IPv6
nodes.
* Address selection issues and resource consumption in DNS-ALG
with multi-addressed nodes.
* Limitations on DNS security capabilities when using DNS-ALG.
Section 2, Section 3 and Section 4 discuss these groups of issues.
Section 5 examines the consequences of deploying NAT-PT for
application developers and the long term effects of NAT-PT on the
further development of IPv6.
The terminology used in this document is defined in [RFC2663],
[RFC2766] and [RFC3314].
2. Issues Unrelated to DNS-ALG
2.1. Issues with Protocols Embedding IP Addresses
It is well known from work on IPv4 NATs (see Section 8 of [RFC2663]
and [RFC3027]) that the large class of protocols which embed numeric
IP addresses in their payloads either cannot work through NATs or
require specific ALGs as helpers to translate the payloads in line
with the address and port translations. The same set of protocols
cannot pass through NAT-PT. The problem is exacerbated because the
IPv6 and IPv4 addresses are of different lengths so that packet
lengths as well as contents are altered. [RFC2766] describes the
consequences as part of the description of the FTP ALG: similar
workarounds are needed for all protocols with embedded IP addresses
that run over TCP transports.
The issues raised in Sections 2 and 3 of [RFC2663] relating to
authentication and encryption with NAT are also applicable to NAT-PT.
Implementing a suite of ALGs requires that NAT-PT equipment includes
the logic for each of the relevant protocols. Most of these
protocols are continuously evolving, requiring continual and
coordinated updates of the ALGs to keep them in step.
Assuming that the NAT-PT contains a co-located ALG for one of the
relevant protocols, the ALG could replace the embedded IP addresses
and ports. However this replacement can only happen if no
cryptographic integrity mechanism is used and the protocol messages
are sent in clear (i.e. not encrypted).
A possible workaround relies on the NAT-PT being party to the
security association used to provide authentication and/or
encryption. The NAT-PT should then be aware of the cryptographic
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algorithms and keys used to secure the traffic and could modify and
re-secure the packets: this would certainly complicate network
operations and provides additional points of security vulnerability.
Unless UDP encapsulation is used for IPsec [RFC3498], traffic using
IPsec AH (in transport and tunnel mode) and IPsec ESP (in transport
mode) are unable to be carried through NAT-PT without terminating the
security associations on the NAT-PT, due to their usage of
cryptographic integrity protection.
A related issue with DNS security is discussed in Section 4.5.
2.2. NAPT-PT Redirection Issues
Section 4.2 of [RFC3027] discusses problems specific to RSVP and
NATs, one of which is actually a more generic problem for all port
translators. When several end-hosts are using a single NAPT-PT box,
protocols that do not have a demultiplexing capability similar to
transport layer port numbers may be unable to work through NAPT-PT
(and any other port translator) because there is nothing for NAPT-PT
to use to identify the correct binding.
This type of issue affects IPsec encrypted packets where the
transport port is not visible (although it might be possible to use
the SPI as an alternative demultiplexer) and protocols, such as RSVP,
which are carried directly in IP datagrams rather than using a
standard transport layer protocol such as TCP or UDP. In the case of
RSVP, packets going from the IPv4 domain to the IPv6 domain do not
necessarily carry a suitable demultiplexing field, because the port
fields in the flow identifer and traffic specifications are optional.
Several ad hoc workarounds could be used to solve the demultiplexing
issues, however in most cases these solutions are not documented
anywhere which could lead to non-deterministic, undesirable behavior
(for example, such workarounds often assume particular network
topologies etc in order to function correctly; if the assumptions are
not met in a deployment the workaround may not work as expected).
This issue is closely related to the fragmentation issue described in
Section 2.5.
2.3. NAT-PT Binding State Decay
NAT-PT will generally use dynamically created bindings to reduce the
need for IPv4 addresses both for NAT-PT and NAPT-PT. NA(P)T-PT uses
soft state mechanisms to manage the address and port pools used for
dynamically created address bindings. This allows the NA(P)T-PT to
operate autonomously without requiring clients to signal either
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implicitly or explicitly that a binding is no longer required. In
any case, without soft state timeouts network and application
unreliability would inevitably lead to leaks, eventually causing
address or port pool exhaustion.
For a dynamic binding to persist for longer than the soft state
timeout, packets must be sent periodically from one side of the
NAT-PT to the other (which direction is not specified by the NAT-PT
specification). If no packets are sent in the proper direction, the
NAT-PT binding will not be refreshed and the application connection
will be broken. Hence all applications need to maintain their NAT-PT
bindings during long idle periods by incorporating a keep-alive
mechanism, which may not be possible for legacy systems.
Also [RFC2766] does not specify how to choose timeouts for bindings:
as is discussed in [RFC2663] for traditional NATs, selecting suitable
values is a matter of heuristics and coordinating with application
expectations may be impossible.
2.4. Loss of Information through Incompatible Semantics
NAT-PT reuses the SIIT header and protocol translations defined in
[RFC2765]. Mismatches in semantics between IPv4 and IPv6 versions
can lead to loss of information when packets are translated. Three
issues arising from this are:
o There is no equivalent in IPv4 for the flow label field of the
IPv6 header. Hence any special treatment of packets based on flow
label patterns cannot be propagated into the IPv4 domain.
o IPv6 extension headers provide flexibility for improvements in the
IP protocol suite in future. New headers may be defined in future
which do not have equivalents in IPv4. In practice, some existing
extensions such as routing headers and mobility extensions are not
translatable.
o As described in section 2.2 of [I-D.satapati-v6ops-natpt-
applicability] there are no equivalents in IPv6 for some ICMP(v4)
messages, while for others (notably the 'Parameter Problem'
messages) the semantics are not equivalent. Translation of such
messages may lead to loss of information. However, this issue may
not be very severe because the error messages relate to packets
that have been translated by NAT-PT rather than arbitrary packets.
If the NAT-PT is functioning correctly, there is, for example, no
reason why IPv6 packets with unusual extension headers or options
should be generated. This case is cited in [I-D.satapati-v6ops-
natpt-applicability] as an example where the IPv6 error has no
equivalent in IPv4 resulting in lost information.
Loss of information in any of these cases could be a constraint to
certain applications.
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A related matter concerns the propagation of the Differentiated
Services Code Point (DSCP). NAT-PT and SIIT simply copy the DSCP
field when translating packets. Accordingly the IPv4 and IPv6
domains must have equivalent Per-Hop Behaviors for the same code
point or alternative means must be in place to translate the DSCP
between domains.
2.5. NA(P)T-PT and Fragmentation
As mentioned in [RFC3027], simple port translators are unable to
translate packet fragments other than the first from a fragmented
packet because subsequent fragments do not contain the port number
information.
This means that generally fragmentation cannot be allowed for any
traffic that traverses a NAPT-PT. One attempted workaround requires
the NAPT-PT to maintain state about fragmented packets in transit.
This is not a complete solution because fragment misordering could
lead to the first fragment appearing at the NAPT-PT after later
fragments. The NAPT-PT would then not have the information needed to
translate the fragments received before the first.
Although it would not be expected in normal operation, NAPT-PT needs
to be proofed against receiving short first fragments which don't
contain the transport port numbers. Note that such packets are a
problem for IPv6 stateful packet inspection. The current
specifications of IPv6 do not mandate any minimum packet size beyond
the need to carry the unfragmentable part (which doesn't include the
transport port numbers) or reassembly rules to minimise the effects
of overlapping fragments, leaving IPv6 open to the sort of attacks
described in [RFC1858] and [RFC3128].
An additional concern arises when a fragmented IPv4 UDP packet, which
does not have a transport layer checksum, traverses a NAT-PT box. As
described in [RFC2766], the NAT-PT has to reconstruct the whole
packet so that it can calculate the checksum needed for the
translated IPv6 packet. This can result in significant delay to the
packet, especially if it has to be re-fragmented before transmission
on the IPv6 side.
If NA(P)T-PT boxes reassembled all incoming fragmented packets (both
from the IPv4 and IPv6 directions) in the same way as they have to do
for unchecksummed IPv4 UDP packets, this would be a solution to the
first problem. The cost would be considerable: apart from the
potential delay problem if the outgoing packet has to be fragmented,
the NA(P)T-PT would consume extra memory and CPU resources, making
the NAT-PT even less scaleable (see Section 3.2).
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Packet reassembly in NA(P)T-PT also opens up the possibility of
various fragment-related security attacks. Some of these are
analagous to attacks identified for IPv4. Of particular concern is a
DOS attack based on sending large numbers of small fragments without
a terminating last fragment which would potentially overload the
reconstruction buffers and consume large amounts of CPU resources.
2.6. NAT-PT Interaction with SCTP and Multihoming
The Stream Control Transmission Protocol (SCTP) [RFC2960] is a
transport protocol which has been standardized since SIIT was
specified. SIIT does not explicitly cover translation of SCTP, but
SCTP uses transport port numbers in the same way as UDP and TCP so
that similar techniques could be used.
However, SCTP also supports multihoming. During connection setup
SCTP control packets carry embedded addresses which would have to be
translated. This would also require that the types of the options
fields in the SCTP control packets were changed with consequent
changes to packet length: the transport checksum would also have to
be recalculated. The ramifications of multihoming as it might
interact with NAT-PT have not been fully explored. Because of the
'chunked' nature of data transfer it does not appear that state would
have to be maintained to relate packets transmitted using the
different IP addresses associated with the connection. [Author's
Note: This needs to be considered by an SCTP expert].
Even if these technical issues can be overcome, using SCTP in a
NAT-PT environment may effectively nullify the multihoming advantages
of SCTP if all the connections run through the same NAT-PT. The
consequences of running a multihomed network with separate NAT-PT
boxes associated with each of the 'homes' have not been fully
explored, but one issue that will arise is described in Section 4.4.
SCTP will need an associated 'ALG' - actually a Transport Layer
Gateway - to handle the packet payload modifications. If it turns
out that state is required, the state would have to distributed and
synchronized across several NAT-PT boxes in a multihomed environment.
SCTP running through NAT-PT in a multihomed environment is also
incompatible with IPsec as described in Section 2.1.
2.7. NAT-PT as a Proxy Correspondent Node for MIPv6
As discussed in [I-D.lee-v6ops-natpt-mobility], it is not possible to
propagate Mobile IPv6 control messages into the IPv4 domain.
According to the IPv6 Node Requirements [I-D.ietf-ipv6-node-
requirements], IPv6 nodes should normally be prepared to support the
route optimization mechanisms needed in a correspondent node. If
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communications from an IPv6 mobile node is traversing a NAT-PT, the
destination IPv4 node is not going to support the correspondent node
features needed for route optimization.
This can be resolved in two ways:
o The NAT-PT can discard messages and headers relating to changes of
care-of addresses including reverse routing checks.
Communications with the mobile node will continue through the home
agent without route optimization. This is clearly sub-optimal but
communication should remain possible.
o Additional functionality could be implemented in the NAT-PT to
allow it to function as a proxy correspondent node for all IPv4
nodes for which it has bindings. This scheme adds considerably to
the complexity of NAT-PT. Depending on the routability of the
IPv6 PREFIX used for translated IPv4 addresses, it may also limit
the extent of mobility of the mobile node: all communications to
the IPv4 destination have to go through the same NAT-PT even if
the mobile node moves to a network which does not have direct IPv6
connectivity with the NAT-PT.
In both cases the existing NAT-PT specification would need to be
extended to deal with IPv6 mobile nodes and neither is a fully
satisfactory solution.
2.8. NAT-PT and Multicast
SIIT [RFC2765] cannot handle translation of multicast packets and
NAT-PT does not discuss a way to map multicast addresses between IPv4
and IPv6. Some separate work has been done to provide an alternative
mechanism to handle multicast. This uses a separate gateway which
understands some or all of the relevant multicast control and routing
protocols in each domain. This work has not been carried through
into standards as yet.
A basic mechanism which involves only IGMP on the IPv4 side and MLD
on the IPv6 side is described in 'An IPv6/IPv4 Multicast Translator
based on IGMP/MLD Proxying (mtp)' [I-D.tsuchiya-mtp]. A more
comprehensive approach which includes proxying of the multicast
routing protocols is described in 'An IPv4 - IPv6 multicast gateway'
[I-D.venaas-mboned-v4v6mcastgw]. Both approaches have several of the
issues described in this section, notably issues with embedded
addresses.
[I-D.okazaki-v6ops-natpt-security] identifies the possibility of a
multiplicative reflection attack if the NAT-PT can be spoofed into
creating a binding for a multicast address. This attack would be
very hard to mount because routers should not forward packets with
muticast addresses in the source address field. However, it points
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up that a naively implemented DNS-ALG could create such bindings from
spoofed DNS responses since [RFC2766] does not mention the need for
checks on the types of addresses in these responses.
The issues for NAT-PT and multicast reflect the fact that NAT-PT is
at best a partial solution. Completing the translation solution to
cater for multicast traffic is likely to carry a similar set of
issues to the current unicast NAT-PT and may open up significant
additional security risks.
3. Issues exacerbated by the Use of DNS-ALG
3.1. Network Topology Constraints Implied by NAT-PT
Traffic flow initiators in a NAT-PT environment are dependent on the
DNS-ALG in the NAT-PT to provide the mapped address needed to
communicate with the flow destination on the other side of the
NAT-PT. Whether used for flows initiated in the IPv4 domain or the
IPv6 domain, the NAT-PT has to be on the path taken by the DNS query
sent by the flow initiator to the relevant DNS server; otherwise the
DNS query will not be modified and the response type would not be
appropriate.
The implication is that the NAT-PT box also has to be the default
IPv6 router for the site in order that the DNS-ALG is able to examine
all DNS requests made over IPv6. On sites with both IPv6 and dual-
stack nodes, this will result in all traffic flowing through the
NAT-PT with consequent scalability concerns.
These constraints are described in more detail in [I-D.durand-natpt-
dns-alg-issues].
[I-D.hallin-natpt-dns-alg-solutions] proposes a solution for flows
initiated from the IPv6 domain but it appears that this solution
still has issues.
For IPv6-only clients the solution requires the use of a DNS server
in the IPv4 domain accessed via an IPv6 address which uses the NAT-PT
PREFIX (see [RFC2766]). Queries to this server would necessarily
pass through the NAT-PT. Dual-stack hosts would use a separate DNS
server accessed through a normal IPv6 address. This removes the need
for the NAT-PT box to be the default IPv6 gateway for the domain.
The primary proposal suggests that the IPv6-only clients should use
this DNS server for all queries. This is expensive on NAT-PT
resources because requests relating to hosts with native IPv6
addresses would also use the NAT-PT DNS-ALG.
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The alternate suggestion to reduce this burden appears to be flawed:
if IPv6-only clients are provided with a list of DNS servers
including both the server accessed via NAT-PT and server(s) accessed
natively via IPv6, the proposal suggests that the client could avoid
using NAT-PT for hosts that have native IPv6 addresses.
Unfortunately for the alternate suggestion, there is no a priori way
in which the initiator can decide which DNS server to use for a
particular query. In the event that the initiator makes the wrong
choice, the DNS query will return an empty list rather than failing
to respond. With standard DNS logic, the initiator will not try
alternative DNS servers because it has received a response. This
means that the solution would consist of always using DNS servers
having the NAT-PT prefix. This imposes the burden of always
requiring DNS RR [RFC1035] translation.
For flows initiated from the IPv4 network, the proposal recommends
that the advertised DNS servers for the IPv6 network would have the
IPv4 address of the NAT-PT. Again there is no deterministic way to
choose the correct DNS server for each query resulting in the same
issues as were raised for flows initiated from the IPv6 domain.
Although the engineering workaround, just described, provides a
partial solution to the topology constraints issue it mandates that
DNS queries and responses should still go through a NAT-PT even if
there would normally be no reason to do so. This mandatory passage
through the NAT-PT for all DNS requests will exacerbate the other DNS
related issues discussed in Section 3.4 and Section 4.1.
3.2. Scalability and Single Point of Failure Concerns
As with traditional NAT, NAT-PT is a bottleneck in the network with
significant scalability concerns and the anchoring of flows to a
particular NAT-PT makes the NAT-PT a potential single point of
failure in the network. The addition of the DNS-ALG in NAT-PT
further increases the scalability concerns.
Solutions to both problems have been envisaged using collections of
cooperating NAT-PT boxes, but such solutions require coordination and
state synchronization which has not yet been standardized and again
adds to the functional and operational complexity of NAT-PT. One
such solution is described in [I-D.park-scalable-multi-natpt].
As with traditional NAT, the concentration of flows through NAT-PT
and the legitimate modification of packets in the NAT-PT make NAT-PTs
enticing targets for security attacks.
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3.3. Issues with Lack of Address Persistence
Using the DNS-ALG to create address bindings requires that the
application uses the translated address returned by the DNS query
before the NAT-PT binding state is timed out (see Section 2.3).
Applications will not normally be aware of this constraint, which may
be different from the existing lifetime of DNS query responses. This
could lead to difficult diagnosis problems with applications.
Additionally, the DNS-ALG needs to determine the initial lifetime of
bindings which it creates. As noted in Section 2.3, this may need to
be determined heuristically. The DNS-ALG does not know which
protocol the mapping is to be used for, and so needs another way to
determine the initial lifetime. This could be tied to the DNS
response lifetime but this might open up additional DOS attack
possibilities if very long validities are allowed. Also the lifetime
should be adjusted once the NAT-PT determines which protocol is being
used with the binding.
As with traditional NATs (see Section 2.5 of [RFC3027], NAT-PT will
most likely break applications that require address mapping to be
retained across contiguous sessions. These applications require the
IPv4 to IPv6 address mapping to be retained between sessions so the
same mapped address may be reused for subsequent session
interactions. NAT-PT cannot know this requirement and may reassign
the previously used mapped address to different hosts between
sessions.
Trying to keep NAT-PT from discarding an address mapping would
require either a NAT-PT extension protocol that would allow the
application to request the NAT-PT device to retain the mappings, or
an extended ALG (which has all the issues discussed in Section 2.1)
that can interact with NAT-PT to keep the address mapping from being
discarded after a session.
3.4. DOS Attacks on Memory and Address/Port Pools
As discussed in Section 2.3 a NAT-PT may create dynamic NAT bindings
each of which consumes memory resources as well as an address (or
port if NAPT-PT is used) from an address (or port) pool. A number of
documents, including [RFC2766] and [I-D.okazaki-v6ops-natpt-security]
discuss possible denial of service (DOS) attacks on NA(P)T-PT which
result in resource depletion associated with address and port pools.
NAT-PT does not specify any authentication mechanisms so that an
attacker may be able to create spurious binds by spoofing addresses
in packets sent through NAT-PT. The attack is more damaging if the
attacker is able to spoof protocols with long binding timeouts
(typically used for TCP).
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The use of the DNS-ALG in NAT-PT introduces another vulnerability
which can result in resource depletion. The attack identified in
[I-D.durand-natpt-dns-alg-issues] exploits the use of DNS queries
traversing NAT-PT to create dynamic bindings. Every time a DNS query
is sent through the NAT-PT the NAT-PT may create a new NA(P)T-PT bind
without any end-host authentication or authorization mechanisms.
This behavior could lead to a serious DOS attack on both memory and
address or port pools. Address spoofing is not required for this
attack to be successful.
[I-D.hallin-natpt-dns-alg-solutions] proposes to mitigate the DOS
attack by using ACLs and static binds which increases the operational
cost and may not always be practical.
The ideal mitigation solution would be to disallow dynamically
created binds until authentication and authorization of the end-host
needing the protocol translation has been carried out. This would
require that the proper security infrastructure be in place to
support the authentication and authorization, which increases the
network operational complexity.
4. Issues Directly Related to use of DNS-ALG
4.1. Address Selection Issues when Communicating with Dual-Stack End-
hosts
[I-D.durand-natpt-dns-alg-issues] discusses NAT-PT DNS-ALG issues
with regard to address selection. As specified in [RFC2766], the
DNS-ALG returns AAAA RRs from two possible sources to the IPv6 host
which has made an AAAA DNS query AAAA.
If the query relates to a dual-stack host, the query will return both
the native IPv6 address(es) and the translated IPv4 address(es) in
AAAA RRs. Without additional information, the IPv6 host address
selection may pick a translated IPv4 address instead of selecting the
more appropriate native IPv6 address. Under some circumstances, the
address selection algorithms [RFC3484] will always prefer the
translated address over the native IPv6 address which is obviously
undesirable.
[I-D.hallin-natpt-dns-alg-solutions] proposes a solution which
involves modification to the NAT-PT specification intended to return
only the most appropriate address(es) to an IPv6 capable host:
o When a DNS AAAA query traverses the NAT-PT DNS-ALG, the NAT-PT
will forward the query to the DNS server in the IPv4 domain
unchanged but using IPv4 transport:
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* If the authoritative DNS server has one or more AAAA records,
it returns them. The DNS-ALG then forwards this response to
the IPv6 host and does not send an A query as the standard
NAT-PT would do.
* Otherwise, if the DNS server does not understand the AAAA query
or has no AAAA entry for the host, it will return an error.
The NAT-PT DNS-ALG will intercept the error or empty return and
send an A query for the same host. If this query returns an
IPv4 address, the ALG creates a binding and synthesizes a
corresponding AAAA record which it sends back to the IPv6 host.
o The NAT-PT thus forwards the result of the first successful DNS
response back to the end-host or an error if neither succeeeds.
Consequently only AAAA RRs from one source will be provided
instead of two as specified in [RFC2766] and it will contain the
most appropriate address for a dual-stack or IPv6-only querier.
There is, however, still an issue with the proposed solution:
o The DNS client may timeout the query if it doesn't receive a
response in time. This is more likely because the NAT-PT may have
to make two separate queries sequentially which the client is not
aware of. It may be possible to reduce the response time by
sending the two queries in parallel and ignoring the result of the
A query if the AAAA returns one or more addresses. However it is
still necessary to delay after receiving the first response to
determine if a second is coming, which may still trigger the DNS
client timeout.
Unfortunately, the two queries cannot be combined in a single DNS
request (all known DNS servers only process a single DNS query per
request message because of difficulties expressing authoritativeness
for arbitrary combinations of requests).
An alternative solution would be to allow the IPv6 host to have
within its address selection policies the NAT-PT PREFIX [RFC2766]
used and assign to it a low selection priority. This solution
requires a automatic configuration of the NAT-PT PREFIX as well as
its integration within the address selection policies. The simplest
way to integrate this automatic configuration would be through
configuration file download (in case the host or DHCPv6 server did
not support vendor options - to avoid standardization effort on the
NAT-PT PREFIX option). This solution does not require any
modification to the NAT-PT specification.
Neither of these solutions resolves a second issue related to address
selection identified in [I-D.durand-natpt-dns-alg-issues].
Applications have no way of knowing that the IPv6 address returned
from the DNS-ALG is not a 'real' IPv6 address, but a translated IPv4
address. The application may therefore be led to believe that it has
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end-to-end IPv6 connectivity with the destination. As a result, the
application may use IPv6 specific options that are not supported by
NAT-PT. This issue is closely related to the issue described in
Section 4.2 and the discussion in Section 5.
4.2. Non-global Validity of Translated RR Records
Some applications propagate information records retrieved from DNS to
other applications. The published semantics of DNS imply that the
results will be consistent to any user for the duration of the
attached lifetime. RR records translated by NAT-PT violate these
semantics because the retrieved addresses are only usable for
communications through the translating NAT-PT.
Applications which pass on retrieved DNS records to other
applications will generally assume that they can rely on the passed
on addresses to be usable by the receiving application. This may not
be the case if the receiving application is on another node
especially if it is not in the domain served by the NAT-PT which
generated the translation.
4.3. Inappropriate Translation of Responses to A Queries
Some applications running on dual-stack nodes may wish to query the
IPv4 address of a destination. If the resulting A query passes
through the NAT-PT DNS-ALG, the DNS-ALG will translate the response
inappropriately into a AAAA record using a translated address. This
happens because the DNS-ALG specified in [RFC2766] operates
statelessly and hence has no memory of the IPv6 query which induced
the A request on IPv4 side. The default action is to translate the
response.
The specification of NAT-PT could be modified to maintain minimal
state about queries passed through the DNS-ALG, and hence to respond
correctly to A queries as well as AAAA queries.
4.4. DNS-ALG and Multi-addressed Nodes
Many IPv6 nodes, especially in multihomed situations but also in
single homed deployments, can expect to have multiple global
addresses. The same may be true for multihomed IPv4 nodes.
Responses to DNS queries for these nodes will normally contain all
these addresses. Since the DNS-ALG in the NAT-PT has no knowledge
which of the addresses can or will be used by the application issuing
the query, it is obliged to translate all of them.
This could be a significant drain on resources in both NAT-PT and
NAPT-PT, as bindings will have to be created for each address.
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When using SCTP in a multihomed network, the problem is exacerbated
if multiple NAT-PTs translate mutiple addresses: also it is not clear
that SCTP will actually look up all the destination IP addresses via
DNS so that bindings may not be in place when packets arrive.
4.5. Limitations on Deployment of DNS Security Capabilities
Secure DNS (DNSSEC) [I-D.ietf-dnsext-dnssec-intro] uses public key
cryptographic signing to authenticate DNS responses. The DNS-ALG
modifies DNS query responses traversing the NAT-PT in both directions
which would invalidate the signatures as (partially) described in
Section 7.5 of [RFC2766].
Workarounds have been proposed, such as making the DNS-ALG behave
like a secure DNS server. This would need to be done separately for
both the IPv6 and IPv4 domains. This is operationally very complex
and there is a risk that the server could be accessed in mistake for
a conventional DNS server. The NAT-PT specification would have to be
altered to implement any such workaround.
Hence DNSSEC is not deployable in domains that use NAT-PT as
currently specified. Widespread deployment of NAT-PT would become a
serious obstacle to the large scale deployment of DNSSEC.
5. Impact on IPv6 Application Development
One of the major design goals for IPv6 is to restore the end-to-end
transparency of the Internet. Because IPv6 may be expected to remove
the need for NATs and similar impediments to transparency, developers
creating applications to work with IPv6 may be tempted to assume that
the complex and (development) time-consuming expedients that might
have been needed to make the application work in an 'NATted' IPv4
environment are not required.
Consequently some classes of applications (e.g., peer-to-peer) that
would need special measures to manage NAT traversal, including
special encapsulations, attention to binding lifetime and provision
of keepalives, may build in assumptions on whether IPv6 is being used
or not. Developers would also like to exploit additional
capabilities of IPv6 not available in IPv4.
NAT-PT as specified in [RFC2766] is intended to work autonomously and
be transparent to applications. There is therefore no way for
application developers to discover that a path contains a NAT-PT.
If NAT-PT is deployed, applications that have assumed a NAT-free IPv6
environment may break when the traffic passes through a NAT-PT. This
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is bad enough, but requiring developers to include special
capabilities to workaround what is supposed to be a temporary
transition 'aid' is even worse. Finally, deployment of NAT-PT is
likely to inhibit the development and use of additional IPv6
capabilities enabled by the flexible extension header system in IPv6
packets.
Some of these deleterious effects could possibly be alleviated if
applications could discover the presence of NAT-PT boxes on paths in
use, allowing them to take steps to workaround the problems. However
requiring applications to incorporate extra code to workaround
problems with a transition aid still seems to be a very bad idea: the
behavior of the application in native IPv6 and NAT-PT environments
would be likely to be significantly different.
6. Security Considerations
This document summarizes security issues related to the NAT-PT
[RFC2766] specification. Security issues are discussed in various
sections:
o Section 2.1 discusses how IPsec AH (transport and tunnel mode) and
IPsec ESP transport mode are broken by NAT-PT (when IPSEC UDP
encapsulation is not used [RFC3498]); and authentication and
encryption are generally incompatible with NAT-PT.
o Section 2.5 discusses possible fragmentation related security
attacks on NAT-PT.
o Section 2.8 discusses security issues related to multicast
addresses and NAT-PT.
o Section 3.3 highlights that NAT-PT is an enticing nexus for
security attacks.
o Section 3.4 discusses possible NAT-PT DOS attacks on both memory
and address/port pools.
o Section 4.5 discusses why NAT-PT is incompatible with DNSSEC
[RFC2535] and how deployment of NAT-PT may inhibit deployment of
DNSSEC.
7. IANA Considerations
There are no IANA considerations defined in this document.
8. Conclusion
This document has discussed a number of significant issues with
NAT-PT as defined in [RFC2766]. From a deployment perspective 3GPP
networks are currently the only 'standardised' scenario where an IPv6
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only host communicates with an IPv4 only host using NAT-PT as
described in the 3GPP IPv6 transition analysis [I-D.ietf-v6ops-3gpp-
analysis] but NAT-PT has seen some limited usage for other purposes.
Although some of issues identified with NAT-PT appear to have
solutions, many of the solutions required significant alterations to
the existing specification and would be likely to increase
operational complexity. Even if these solutions were applied, we
have shown that NAT-PT still has significant irresolvable issues and
appears to have limited applicability. The potential constraints on
the development of IPv6 applications described in Section 5 are
particularly un desirable. It appears that alternatives to NAT-PT
exist to cover the circumstances where NAT-PT has been suggested as a
solution, such as the use of tunneling and header compression in 3GPP
scenarios.
However it is clear that in some circumstances a IPv6/IPv4 protocol
translation solution may be a useful transitional solution,
particularly in more constrained situations where the translator is
not required to deal with traffic for a wide variety of protocols
which are not determined in advance. It is therefore possible that a
more limited form of NAT-PT could be defined for use in specific
situations.
Accordingly we recommend that the IETF no longer suggest its usage as
a general IPv4/IPv6 transition mechanism in the Internet but retain
it as an experimental mechanism while further experience is gained
and any future replacement is defined and deployed. Consequently we
recommend moving RFC2766 to experimental status.
9. Acknowledgments
This work builds on a large body of existing work examining the
issues and applicability of NAT-PT: the work of the authors of the
documents referred in Section 1 has been extremely useful in creating
this document. Particular thanks are due to Pekka Savola for rapid
and thorough review of the document.
10. References
10.1. Normative References
[RFC2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm
(SIIT)", RFC 2765, February 2000.
[RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address
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Translation - Protocol Translation (NAT-PT)", RFC 2766,
February 2000.
[RFC2535] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999.
[RFC3027] Holdrege, M. and P. Srisuresh, "Protocol Complications
with the IP Network Address Translator", RFC 3027,
January 2001.
[RFC3314] Wasserman, M., "Recommendations for IPv6 in Third
Generation Partnership Project (3GPP) Standards",
RFC 3314, September 2002.
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[I-D.ietf-ipv6-node-requirements]
Loughney, J., "IPv6 Node Requirements",
draft-ietf-ipv6-node-requirements-11 (work in progress),
August 2004.
[I-D.ietf-v6ops-3gpp-analysis]
Wiljakka, J., "Analysis on IPv6 Transition in 3GPP
Networks", draft-ietf-v6ops-3gpp-analysis-11 (work in
progress), October 2004.
[I-D.ietf-dnsext-dnssec-intro]
Arends, R., Austein, R., Massey, D., Larson, M., and S.
Rose, "DNS Security Introduction and Requirements",
draft-ietf-dnsext-dnssec-intro-13 (work in progress),
October 2004.
10.2. Informative References
[RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security
Considerations for IP Fragment Filtering", RFC 1858,
October 1995.
[RFC3128] Miller, I., "Protection Against a Variant of the Tiny
Fragment Attack (RFC 1858)", RFC 3128, June 2001.
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[RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
Zhang, L., and V. Paxson, "Stream Control Transmission
Protocol", RFC 2960, October 2000.
[RFC3498] Kuhfeld, J., Johnson, J., and M. Thatcher, "Definitions of
Managed Objects for Synchronous Optical Network (SONET)
Linear Automatic Protection Switching (APS)
Architectures", RFC 3498, March 2003.
[I-D.satapati-v6ops-natpt-applicability]
Satapati, S., "NAT-PT Applicability",
draft-satapati-v6ops-natpt-applicability-00 (work in
progress), October 2003.
[I-D.durand-natpt-dns-alg-issues]
Durand, A., "Issues with NAT-PT DNS ALG in RFC2766",
draft-durand-natpt-dns-alg-issues-00 (work in progress),
February 2002.
[I-D.hallin-natpt-dns-alg-solutions]
Hallingby, P. and S. Satapati, "NAT-PT DNS ALG solutions",
draft-hallin-natpt-dns-alg-solutions-01 (work in
progress), July 2002.
[I-D.lee-v6ops-natpt-mobility]
Shin, M. and J. Lee, "Considerations for Mobility Support
in NAT-PT", draft-lee-v6ops-natpt-mobility-01 (work in
progress), July 2005.
[I-D.okazaki-v6ops-natpt-security]
Okazaki, S. and A. Desai, "NAT-PT Security
Considerations", draft-okazaki-v6ops-natpt-security-00
(work in progress), June 2003.
[I-D.vanderpol-v6ops-translation-issues]
Pol, R., Satapati, S., and S. Sivakumar, "Issues when
translating between IPv4 and IPv6",
draft-vanderpol-v6ops-translation-issues-00 (work in
progress), January 2003.
[I-D.elmalki-sipping-3gpp-translator]
Malki, K., "IPv6-IPv4 Translation mechanism for SIP-based
services in Third Generation Partnership Project (3GPP)
Networks", draft-elmalki-sipping-3gpp-translator-00 (work
in progress), December 2003.
[I-D.tsuchiya-mtp]
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Tsuchiya, K., Higuchi, H., Sawada, S., and S. Nozaki, "An
IPv6/IPv4 Multicast Translator based on IGMP/MLD Proxying
(mtp)", draft-tsuchiya-mtp-01 (work in progress),
February 2003.
[I-D.venaas-mboned-v4v6mcastgw]
Venaas, S., "An IPv4 - IPv6 multicast gateway",
draft-venaas-mboned-v4v6mcastgw-00 (work in progress),
February 2003.
[I-D.park-scalable-multi-natpt]
Park, S., "Scalable mNAT-PT Solution",
draft-park-scalable-multi-natpt-00 (work in progress),
May 2003.
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Authors' Addresses
Cedric Aoun
ZTE Corporation/ENST Paris
France
Email: aoun.cedric@zte.com.fr
Elwyn B. Davies
Consultant
Soham, Cambs
UK
Phone: +44 7889 488 335
Email: elwynd@dial.pipex.com
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