Network Working Group C. Aoun
Request for Comments: 4966 Energize Urnet
Obsoletes: 2766 E. Davies
Category: Informational Folly Consulting
July 2007
Reasons to Move the Network Address Translator - Protocol Translator
(NAT-PT) to Historic Status
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.
Copyright Notice
Copyright (C) The IETF Trust (2007).
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
Proposed Standard to Historic status.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Issues Unrelated to an DNS-ALG . . . . . . . . . . . . . . . . 7
2.1. Issues with Protocols Embedding IP Addresses . . . . . . . 7
2.2. NAPT-PT Redirection Issues . . . . . . . . . . . . . . . . 8
2.3. NAT-PT Binding State Decay . . . . . . . . . . . . . . . . 8
2.4. Loss of Information through Incompatible Semantics . . . . 9
2.5. NAT-PT and Fragmentation . . . . . . . . . . . . . . . . . 10
2.6. NAT-PT Interaction with SCTP and Multihoming . . . . . . . 11
2.7. NAT-PT as a Proxy Correspondent Node for MIPv6 . . . . . . 12
2.8. NAT-PT and Multicast . . . . . . . . . . . . . . . . . . . 12
3. Issues Exacerbated by the Use of DNS-ALG . . . . . . . . . . . 13
3.1. Network Topology Constraints Implied by NAT-PT . . . . . . 13
3.2. Scalability and Single Point of Failure Concerns . . . . . 14
3.3. Issues with Lack of Address Persistence . . . . . . . . . 15
3.4. DoS Attacks on Memory and Address/Port Pools . . . . . . . 16
4. Issues Directly Related to Use of DNS-ALG . . . . . . . . . . 16
4.1. Address Selection Issues when Communicating with
Dual-Stack End-Hosts . . . . . . . . . . . . . . . . . . . 16
4.2. Non-Global Validity of Translated RR Records . . . . . . . 18
4.3. Inappropriate Translation of Responses to A Queries . . . 19
4.4. DNS-ALG and Multi-Addressed Nodes . . . . . . . . . . . . 19
4.5. Limitations on Deployment of DNS Security Capabilities . . 19
5. Impact on IPv6 Application Development . . . . . . . . . . . . 20
6. Security Considerations . . . . . . . . . . . . . . . . . . . 20
7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 21
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.1. Normative References . . . . . . . . . . . . . . . . . . . 22
9.2. Informative References . . . . . . . . . . . . . . . . . . 23
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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 that only support IPv4 to
communicate with nodes that 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 the Network Address Port Translator - Protocol
Translator (NAPT-PT), 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]. In the following discussion,
where the term "NAT-PT" is used unqualified, the discussion applies
to both basic NAT-PT and NAPT-PT. "Basic NAT-PT" will be used if
points apply to the basic address-only translator.
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 that have not been discussed elsewhere. Proposed solutions to
the issues are mentioned and any resulting need for changes to the
specification is identified.
Whereas NAT is seen as an ongoing capability that is needed to work
around 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 that can
only support IPv4 will need to communicate with a node that can only
support IPv6; this needs 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 revised, possibly restricted version of NAT-PT can be a suitable
solution; accordingly, this document recommends that the IETF should
reclassify RFC 2766 from Proposed Standard to Historic status to
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avoid it from being used in inappropriate scenarios while any
replacement is developed.
The following documents relating directly to NAT-PT have been
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 [NATP-APP]
o Issues with NAT-PT DNS ALG (Application Layer Gateway) in RFC 2766
[DNS-ALG-ISSUES]
o NAT-PT DNS ALG Solutions [DNS-ALG-SOL]
o NAT-PT Security Considerations [NATPT-SEC]
o Issues when Translating between IPv4 and IPv6 [TRANS-ISSUES]
o IPv6-IPv4 Translation Mechanism for SIP-Based Services in Third
Generation Partnership Project (3GPP) Networks [3GPP-TRANS]
o Analysis on IPv6 Transition in 3GPP Networks [RFC4215]
o Considerations for Mobile IP Support in NAT-PT [NATPT-MOB]
o An IPv6-IPv4 Multicast Translator based on Internet Group
Management Protocol / Multicast Listener Discovery (IGMP/MLD)
Proxying (mtp) [MTP]
o An IPv4-IPv6 Multicast Gateway [MCASTGW]
o Scalable mNAT-PT Solution [MUL-NATPT]
Because the majority of the documents containing discussions of the
issues are documents that 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:
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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
passed to the hosts in the absence of the DNS-ALG. Therefore, this
document 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 issues 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]. In the event that new forms
of a translator are developed as alternatives to NAT-PT, the generic
issues relevant to all IPv6-IPv4 translators should be borne in mind.
Issues raised with IPv6-IPv4 translators in general and NAT-PT in
particular can be categorized as follows:
o Issues that are independent of the use of a DNS-ALG and are,
therefore, applicable to any form of an IPv6-IPv4 translator:
* Disruption of all protocols that embed IP addresses (and/or
ports) in packet payloads or apply integrity mechanisms using
IP addresses (and ports).
* Inability to redirect traffic for protocols that lack
demultiplexing capabilities or are 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 keepalive mechanisms to
workaround connectivity issues caused by premature NAT-PT state
timeout.
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* 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 that are exacerbated by the use of a DNS-ALG and are,
therefore, also applicable to any form of an IPv6-IPv4 translator:
* Constraints on network topology.
* Scalability concerns together with introduction of a single
point of failure and a 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 keep it alive once they
start using it.
* Creation of a DoS (Denial of Service) threat relating to
exhaustion of memory and address/port pool resources on the
translator.
o Issues that result from the use of a DNS-ALG and are, therefore,
specific to NAT-PT as defined in [RFC2766]:
* 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 that cannot use it.
* Inappropriate translation of responses to A queries from IPv6
nodes.
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* Address selection issues and resource consumption in a DNS-ALG
with multi-addressed nodes.
* Limitations on DNS security capabilities when using a 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 (or any
form of generally deployed IPv6-IPv4 translator) on the further
development of IPv6.
The terminology used in this document is defined in [RFC2663],
[RFC2766], and [RFC3314].
2. Issues Unrelated to an 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 that 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 packet 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 the
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 colocated 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 the 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. NAT-PT would then be aware of the cryptographic
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algorithms and keys used to secure the traffic. It could then modify
and re-secure the packets; this would certainly complicate network
operations and provide additional points of security vulnerability.
Unless UDP encapsulation is used for IPsec [RFC3498], traffic using
IPsec AH (Authentication Header), in transport and tunnel mode, and
IPsec ESP (Encapsulating Security Payload), in transport mode, is
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 Security Parameter Index (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 identifier
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 and 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 basic NAT-PT and NAPT-PT. Both
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basic NAT-PT and NAPT-PT use soft state mechanisms to manage the
address and, in the case of NAPT-PT, port pools are used for
dynamically created address bindings. This allows all types of
NAT-PT boxes to operate autonomously without requiring clients to
signal, either 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 (the 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 keepalive
mechanism, which may not be possible for legacy systems.
Also, [RFC2766] does not specify how to choose timeouts for bindings.
As 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 future improvements
in the IP protocol suite and new headers that do not have
equivalents in IPv4 may be defined. In practice, some existing
extensions such as routing headers and mobility extensions are not
translatable.
o As described in Section 2.2 of [NATP-APP], 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 the 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 by arbitrary packets. If the NAT-PT is
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functioning correctly, there is, for example, no reason why IPv6
packets with unusual extension headers or options should be
generated.
Loss of information in any of these cases could be a constraint to
certain applications.
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. NAT-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, in general, fragmentation cannot be allowed for any
traffic that traverses a NAPT-PT. One attempted workaround requires
the NAPT-PT to maintain state information derived from the first
fragment until all fragments of the packet have transited the
NAPT-PT. This is not a complete solution because fragment
misordering could lead to the first fragment appearing at the NAPT-PT
after later fragments. Consequently, the NAPT-PT would 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 that don't
contain the transport port numbers. Note that such packets are a
problem for many forms of stateful packet inspection applied to IPv6
packets. The current specifications of IPv6 do not mandate (1) any
minimum packet size beyond the need to carry the unfragmentable part
(which doesn't include the transport port numbers) or (2) reassembly
rules to minimize the effects of overlapping fragments. Thus, IPv6
is 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 any type of
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 a significant delay to
the packet, especially if it has to be re-fragmented before
transmission on the IPv6 side.
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If NAT-PT boxes reassembled all incoming fragmented packets (both
from the IPv4 and IPv6 directions) in the same way they have to for
unchecksummed IPv4 UDP packets, this would be a solution to the first
problem. The resource cost would be considerable apart from the
potential delay problem if the outgoing packet has to be re-
fragmented. In any case, fragmentation would mean that the NAT-PT
would consume extra memory and CPU resources, making the NAT-PT even
less scalable (see Section 3.2).
Packet reassembly in a NAT-PT box also opens up the possibility of
various fragment-related security attacks. Some of these are
analogous 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 the translation of SCTP,
but SCTP uses transport port numbers in the same way that UDP and TCP
do, so similar techniques can be used when translating SCTP packets.
However, SCTP also supports multihoming. During connection setup,
SCTP control packets carry embedded addresses that would have to be
translated. This would also require that the types of the options
fields in the SCTP control packets be 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 that state would
have to be maintained to relate packets transmitted using the
different IP addresses associated with the connection.
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 that state is required, the state would have to be
distributed and synchronized across several NAT-PT boxes in a
multihomed environment.
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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 [NATPT-MOB], it is not possible to propagate Mobile
IPv6 (MIPv6) control messages into the IPv4 domain. According to the
IPv6 Node Requirements [RFC4294], IPv6 nodes should normally be
prepared to support the route optimization mechanisms needed in a
correspondent node. If communications from an IPv6 mobile node are
traversing a NAT-PT, the destination IPv4 node will certainly not be
able 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 that 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 the 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 work uses a separate gateway
that understands some or all of the relevant multicast control and
routing protocols in each domain. It has not yet been carried
through into standards.
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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)' [MTP]. A more comprehensive
approach, which includes proxying of the multicast routing protocols,
is described in 'An IPv4 - IPv6 multicast gateway' [MCASTGW]. Both
approaches have several of the issues described in this section,
notably issues with embedded addresses.
[NATPT-SEC] 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 multicast addresses in the
source address field. However, it highlights the possibility 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 will not be
appropriate.
The implication is that the NAT-PT box also has to be the default
IPv6 router for the site so 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 [DNS-ALG-ISSUES].
[DNS-ALG-SOL] proposes a solution for flows initiated from the IPv6
domain, but it appears that this solution still has issues.
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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.
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 the DNS RR (Resource Record) [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. Furthermore, the anchoring of
flows to a particular NAT-PT makes the NAT-PT a potential single
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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 [MUL-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.
3.3. Issues with Lack of Address Persistence
Using the DNS-ALG to create address bindings requires that the
translated address returned by the DNS query is used for
communications 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 to diagnose" problems
with applications.
Additionally, the DNS-ALG needs to determine the initial lifetime of
bindings that 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 that might open up additional DoS attack
possibilities if very long binding lifetimes 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.
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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 [NATPT-SEC] discuss
the possible denial of service (DoS) attacks on basic NAT-PT and
NAPT-PT that would result in a resource depletion associated with
address and port pools. NAT-PT does not specify any authentication
mechanisms; thus, an attacker may be able to create spurious bindings
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).
The use of the DNS-ALG in NAT-PT introduces another vulnerability
that can result in resource depletion. The attack identified in
[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 basic NAT-PT or NAPT-PT binding
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.
[DNS-ALG-SOL] proposes to mitigate the DoS attack by using Access
Control Lists (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
[DNS-ALG-ISSUES] discusses NAT-PT DNS-ALG issues with regard to
address selection. As specified in [RFC2766], the DNS-ALG returns
AAAA Resource Records (RRs) from two possible sources, to the IPv6
host that has made an AAAA DNS query.
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
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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; this is obviously
undesirable.
[DNS-ALG-SOL] proposes a solution that involves modification to the
NAT-PT specification intended to return only the most appropriate
address(es) to an IPv6 capable host as described below:
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. The following two results
can occur:
* 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 succeeds.
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 than for queries not
passing through a DNS ALG because the NAT-PT may have to make two
separate, sequential queries of which the client is not aware. 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.
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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 use the
NAT-PT PREFIX [RFC2766] within its address selection policies and to
assign it a low selection priority. This solution requires an
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 a
configuration file download (in case the host or Dynamic Host
Configuration Protocol for IPv6 (DHCPv6) server did not support
vendor options and to avoid a 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 that is identified in [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 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 that 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 that generated the
translation.
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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 that induced
the A request on the IPv4 side. The default action is to translate
the response.
The specification of NAT-PT could be modified to maintain a 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 basic NAT-PT
and NAPT-PT, as bindings will have to be created for each address.
When using SCTP in a multihomed network, the problem is exacerbated
if multiple NAT-PTs translate multiple 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) [RFC4033] 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 mistaken for a
conventional DNS server. The NAT-PT specification would have to be
altered to implement any such workaround.
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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. Therefore, 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 expedients that might have been
needed to make the application work in a '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. Therefore, there is 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
is bad enough, but requiring developers to include special
capabilities to work around 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 the applications 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:
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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
[RFC4033] and how deployment of NAT-PT may inhibit deployment of
DNSSEC.
7. 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 NAT-PT
is envisaged as a potential mechanism to allow communication between
an IPv6-only host and an IPv4-only host as discussed in the 3GPP IPv6
transition analysis [RFC4215]. But RFC 4215 recommends that the
generic form of NAT-PT should not be used and that modified forms
should only be used under strict conditions. Moreover, it documents
a number of caveats and security issues specific to 3GPP. In
addition, NAT-PT has seen some limited usage for other purposes.
Although some of the issues identified with NAT-PT appear to have
solutions, many of the solutions proposed require significant
alterations to the existing specification and would likely 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 undesirable. 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 application proxies in 3GPP scenarios
[RFC4215]
However, it is clear that in some circumstances an IPv6-IPv4 protocol
translation solution may be a useful transitional solution,
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particularly in more constrained situations where the translator is
not required to deal with traffic for a wide variety of protocols
that are not determined in advance. Therefore, it is possible that a
more limited form of NAT-PT could be defined for use in specific
situations.
Accordingly, we recommend that:
o the IETF no longer suggest its usage as a general IPv4-IPv6
transition mechanism in the Internet, and
o RFC 2766 is moved to Historic status to limit the possibility of
it being deployed inappropriately.
8. 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 to 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.
9. References
9.1. Normative References
[RFC2765] Nordmark, E., "Stateless IP/ICMP Translation
Algorithm (SIIT)", RFC 2765, February 2000.
[RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address
Translation - Protocol Translation (NAT-PT)",
RFC 2766, February 2000.
[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.
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[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC4294] Loughney, J., "IPv6 Node Requirements", RFC 4294,
April 2006.
[RFC4215] Wiljakka, J., "Analysis on IPv6 Transition in Third
Generation Partnership Project (3GPP) Networks",
RFC 4215, October 2005.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D.,
and S. Rose, "DNS Security Introduction and
Requirements", RFC 4033, March 2005.
9.2. 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.
[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.
[NATP-APP] Satapati, S., "NAT-PT Applicability", Work
in Progress, October 2003.
[DNS-ALG-ISSUES] Durand, A., "Issues with NAT-PT DNS ALG in
RFC2766", Work in Progress, February 2002.
[DNS-ALG-SOL] Hallingby, P. and S. Satapati, "NAT-PT DNS ALG
solutions", Work in Progress, July 2002.
[NATPT-MOB] Shin, M. and J. Lee, "Considerations for Mobility
Support in NAT-PT", Work in Progress, July 2005.
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RFC 4966 NAT-PT Issues Analysis July 2007
[NATPT-SEC] Okazaki, S. and A. Desai, "NAT-PT Security
Considerations", Work in Progress, June 2003.
[TRANS-ISSUES] Pol, R., Satapati, S., and S. Sivakumar, "Issues
when translating between IPv4 and IPv6", Work
in Progress, January 2003.
[3GPP-TRANS] Malki, K., "IPv6-IPv4 Translation mechanism for
SIP-based services in Third Generation Partnership
Project (3GPP) Networks", Work in Progress,
December 2003.
[MTP] Tsuchiya, K., Higuchi, H., Sawada, S., and S.
Nozaki, "An IPv6/IPv4 Multicast Translator based on
IGMP/MLD Proxying (mtp)", Work in Progress,
February 2003.
[MCASTGW] Venaas, S., "An IPv4 - IPv6 multicast gateway",
Work in Progress, February 2003.
[MUL-NATPT] Park, S., "Scalable mNAT-PT Solution", Work
in Progress, May 2003.
Authors' Addresses
Cedric Aoun
Energize Urnet
Paris
France
EMail: ietf@energizeurnet.com
Elwyn B. Davies
Folly Consulting
Soham, Cambs
UK
Phone: +44 7889 488 335
EMail: elwynd@dial.pipex.com
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