draft-ietf-v6ops-natpt-to-exprmntl-02.txt   draft-ietf-v6ops-natpt-to-exprmntl-03.txt 
v6ops Working Group C. Aoun v6ops Working Group C. Aoun
Internet-Draft ZTE/ENST Paris Internet-Draft ZTE/ENST Paris
Updates: 2766 (if approved) E. Davies Updates: 2766 (if approved) E. Davies
Expires: October 3, 2005 Consultant Expires: April 23, 2006 Consultant
April 2005 October 20, 2005
Reasons to Move NAT-PT to Experimental Reasons to Move NAT-PT to Experimental
draft-ietf-v6ops-natpt-to-exprmntl-02 draft-ietf-v6ops-natpt-to-exprmntl-03
Status of this Memo Status of this Memo
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have been or will be disclosed, and any of which he or she becomes have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79. aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
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The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
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This Internet-Draft will expire on October 3, 2005. This Internet-Draft will expire on April 23, 2006.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). Copyright (C) The Internet Society (2005).
Abstract Abstract
This document discusses issues with the specific form of IPv6-IPv4 This document discusses issues with the specific form of IPv6-IPv4
protocol translation mechanism implemented by the Network Address protocol translation mechanism implemented by the Network Address
Translator - Protocol Translator (NAT-PT) defined in RFC 2766. These Translator - Protocol Translator (NAT-PT) defined in RFC 2766. These
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general purpose transition mechanism is no longer desirable, and this general purpose transition mechanism is no longer desirable, and this
document recommends that the IETF should reclassify RFC 2766 from document recommends that the IETF should reclassify RFC 2766 from
Standards Track to Experimental status. Standards Track to Experimental status.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Issues Unrelated to DNS-ALG . . . . . . . . . . . . . . . . . 6 2. Issues Unrelated to DNS-ALG . . . . . . . . . . . . . . . . . 6
2.1. Issues with Protocols Embedding IP Addresses . . . . . . . 6 2.1. Issues with Protocols Embedding IP Addresses . . . . . . . 6
2.2. NAPT-PT Redirection Issues . . . . . . . . . . . . . . . . 7 2.2. NAPT-PT Redirection Issues . . . . . . . . . . . . . . . . 7
2.3. NAT-PT Binding State Decay . . . . . . . . . . . . . . . . 7 2.3. NAT-PT Binding State Decay . . . . . . . . . . . . . . . . 8
2.4. Loss of Information through Incompatible Semantics . . . . 8 2.4. Loss of Information through Incompatible Semantics . . . . 8
2.5. NA(P)T-PT and Fragmentation . . . . . . . . . . . . . . . 9 2.5. NAT-PT and Fragmentation . . . . . . . . . . . . . . . . . 9
2.6. NAT-PT Interaction with SCTP and Multihoming . . . . . . . 10 2.6. NAT-PT Interaction with SCTP and Multihoming . . . . . . . 10
2.7. NAT-PT as a Proxy Correspondent Node for MIPv6 . . . . . . 10 2.7. NAT-PT as a Proxy Correspondent Node for MIPv6 . . . . . . 11
2.8. NAT-PT and Multicast . . . . . . . . . . . . . . . . . . . 11 2.8. NAT-PT and Multicast . . . . . . . . . . . . . . . . . . . 12
3. Issues exacerbated by the Use of DNS-ALG . . . . . . . . . . . 12 3. Issues exacerbated by the Use of DNS-ALG . . . . . . . . . . . 12
3.1. Network Topology Constraints Implied by NAT-PT . . . . . . 12 3.1. Network Topology Constraints Implied by NAT-PT . . . . . . 12
3.2. Scalability and Single Point of Failure Concerns . . . . . 13 3.2. Scalability and Single Point of Failure Concerns . . . . . 14
3.3. Issues with Lack of Address Persistence . . . . . . . . . 14 3.3. Issues with Lack of Address Persistence . . . . . . . . . 14
3.4. DOS Attacks on Memory and Address/Port Pools . . . . . . . 14 3.4. DoS Attacks on Memory and Address/Port Pools . . . . . . . 15
4. Issues Directly Related to use of DNS-ALG . . . . . . . . . . 15 4. Issues Directly Related to Use of DNS-ALG . . . . . . . . . . 16
4.1. Address Selection Issues when Communicating with 4.1. Address Selection Issues when Communicating with
Dual-Stack End-hosts . . . . . . . . . . . . . . . . . . . 15 Dual-Stack End-Hosts . . . . . . . . . . . . . . . . . . . 16
4.2. Non-global Validity of Translated RR Records . . . . . . . 17 4.2. Non-global Validity of Translated RR Records . . . . . . . 17
4.3. Inappropriate Translation of Responses to A Queries . . . 17 4.3. Inappropriate Translation of Responses to A Queries . . . 18
4.4. DNS-ALG and Multi-addressed Nodes . . . . . . . . . . . . 17 4.4. DNS-ALG and Multi-addressed Nodes . . . . . . . . . . . . 18
4.5. Limitations on Deployment of DNS Security Capabilities . . 18 4.5. Limitations on Deployment of DNS Security Capabilities . . 18
5. Impact on IPv6 Application Development . . . . . . . . . . . . 18 5. Impact on IPv6 Application Development . . . . . . . . . . . . 19
6. Security Considerations . . . . . . . . . . . . . . . . . . . 19 6. Security Considerations . . . . . . . . . . . . . . . . . . . 20
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 19 8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 20
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
10.1. Normative References . . . . . . . . . . . . . . . . . . . 20 10.1. Normative References . . . . . . . . . . . . . . . . . . . 21
10.2. Informative References . . . . . . . . . . . . . . . . . . 21 10.2. Informative References . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . . . . 25 Intellectual Property and Copyright Statements . . . . . . . . . . 25
1. Introduction 1. Introduction
The Network Address Translator - Protocol Translator NAT-PT) document The Network Address Translator - Protocol Translator (NAT-PT)
[RFC2766] defines a set of network layer translation mechanisms document [RFC2766] defines a set of network-layer translation
designed to allow nodes which only support IPv4 to communicate with mechanisms designed to allow nodes that only support IPv4 to
nodes which only support IPv6 during the transition to the use of communicate with nodes that only support IPv6 during the transition
IPv6 in the Internet. to the use of IPv6 in the Internet.
[RFC2766] specifies the basic NAT-PT in which only addresses are [RFC2766] specifies the basic NAT-PT in which only addresses are
translated and NAPT-PT (Network Address Port Translator - Protocol translated and Network Address Port Translator - Protocol Translator
Translator)which also translates transport identifiers, allowing for (NAPT-PT), which also translates transport identifiers, allowing for
greater economy of scarce IPv4 addresses. Protocol translation is greater economy of scarce IPv4 addresses. Protocol translation is
performed using the Stateless IP/ICMP Translation Algorithm (SIIT) performed using the Stateless IP/ICMP Translation Algorithm (SIIT)
defined in [RFC2765]. 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 A number of previous documents have raised issues with NAT-PT. This
document will summarize these issues, note several other issues document will summarize these issues, note several other issues
carried over from traditional IPv4 NATs and identify some additional carried over from traditional IPv4 NATs, and identify some additional
issues which have not been discussed elsewhere. Where solutions to issues that have not been discussed elsewhere. Where solutions to
the issues have been proposed these are mentioned and any resulting the issues have been proposed, these are mentioned and any resulting
need for changes to the specification identified. need for changes to the specification is identified.
Whereas NAT is seen as an on-going capability which is needed to Whereas NAT is seen as an ongoing capability that is needed to work
workaround the limited availability of globally unique IPv4 around the limited availability of globally unique IPv4 addresses,
addresses, NAT-PT has a different status as a transition mechanism NAT-PT has a different status as a transition mechanism for IPv6. As
for IPv6. As such, NAT-PT should not be allowed to constrain the such, NAT-PT should not be allowed to constrain the development of
development of IPv6 applications or impose limitations on future IPv6 applications or impose limitations on future developments of
developments of IPv6. IPv6.
This document draws the conclusion that the technical and operational This document draws the conclusion that the technical and operational
difficulties resulting from these issues, especially the possible difficulties resulting from these issues, especially the possible
future constraints on the development of IPv6 networks (see future constraints on the development of IPv6 networks (see
Section 5), make it undesirable to recommend NAT-PT as described in Section 5), make it undesirable to recommend NAT-PT as described in
[RFC2766] as a general purpose transition mechanism for [RFC2766] as a general purpose transition mechanism for
intercommunication between IPv6 networks and IPv4 networks. intercommunication between IPv6 networks and IPv4 networks.
Although the [RFC2766] form of packet translation is not generally Although the [RFC2766] form of packet translation is not generally
applicable, it is likely that in some circumstances a node which can applicable, it is likely that in some circumstances a node that can
only support IPv4 will need to communicate with a node which can only only support IPv4 will need to communicate with a node that can only
support IPv6: this is bound to need a translation mechanism of some support IPv6; this needs a translation mechanism of some kind.
kind. Although this may be better carried out by an application Although this may be better carried out by an application-level proxy
level proxy or transport layer translator, there may still be or transport-layer translator, there may still be scenarios in which
scenarios in which a (possibly restricted) version of NAT-PT can be a a (possibly restricted) version of NAT-PT can be a suitable solution;
suitable solution: accordingly this document recommends that the IETF accordingly, this document recommends that the IETF should reclassify
should reclassify RFC2766 from Standards Track to Experiemental RFC2766 from Standards Track to Experimental status.
status.
The following documents relating directly to NAT-PT have been The following documents relating directly to NAT-PT have been
reviewed while drafting this document: reviewed while drafting this document:
o Network Address Translation - Protocol Translation (NAT-PT) o Network Address Translation - Protocol Translation (NAT-PT)
[RFC2766] [RFC2766]
o Stateless IP/ICMP Translation Algorithm (SIIT) [RFC2765] o Stateless IP/ICMP Translation Algorithm (SIIT) [RFC2765]
o NAT-PT applicability statement [I-D.satapati-v6ops-natpt- o NAT-PT applicability statement [I-D.satapati-v6ops-natpt-
applicability] applicability]
o Issues with NAT-PT DNS ALG in RFC2766 [I-D.durand-natpt-dns-alg- o Issues with NAT-PT DNS ALG in RFC2766 [I-D.durand-natpt-dns-alg-
issues] issues]
o NAT-PT DNS ALG solutions [I-D.hallin-natpt-dns-alg-solutions] 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 NAT-PT Security Considerations [I-D.okazaki-v6ops-natpt-security]
o Issues when translating between IPv4 and IPv6 [I-D.vanderpol- o Issues when translating between IPv4 and IPv6 [I-D.vanderpol-
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o NAT-PT Security Considerations [I-D.okazaki-v6ops-natpt-security] o NAT-PT Security Considerations [I-D.okazaki-v6ops-natpt-security]
o Issues when translating between IPv4 and IPv6 [I-D.vanderpol- o Issues when translating between IPv4 and IPv6 [I-D.vanderpol-
v6ops-translation-issues] v6ops-translation-issues]
o IPv6-IPv4 Translation mechanism for SIP-based services in Third o IPv6-IPv4 Translation mechanism for SIP-based services in Third
Generation Partnership Project (3GPP) Networks [I-D.elmalki- Generation Partnership Project (3GPP) Networks [I-D.elmalki-
sipping-3gpp-translator] sipping-3gpp-translator]
o Analysis on IPv6 Transition in 3GPP Networks [I-D.ietf-v6ops-3gpp- o Analysis on IPv6 Transition in 3GPP Networks [I-D.ietf-v6ops-3gpp-
analysis] analysis]
o Considerations for Mobile IP Support in NAT-PT [I-D.lee-v6ops- o Considerations for Mobile IP Support in NAT-PT [I-D.lee-v6ops-
natpt-mobility] natpt-mobility]
o An IPv6/IPv4 Multicast Translator based on IGMP/MLD Proxying (mtp) o An IPv6/IPv4 Multicast Translator based on Internet Group
[I-D.tsuchiya-mtp] Management Protocol / Multicast Listener Discovery (IGMP/MLD)
Proxying (mtp) [I-D.tsuchiya-mtp]
o An IPv4 - IPv6 multicast gateway [I-D.venaas-mboned-v4v6mcastgw] o An IPv4 - IPv6 multicast gateway [I-D.venaas-mboned-v4v6mcastgw]
o Scalable mNAT-PT Solution [I-D.park-scalable-multi-natpt] o Scalable mNAT-PT Solution [I-D.park-scalable-multi-natpt]
Because the majority of the documents containing discussions of the Because the majority of the documents containing discussions of the
issues are Internet Drafts which are unlikely to become RFCs, the issues are Internet Drafts which are unlikely to become RFCs, the
issues are summarized here to avoid the need for normative issues are summarized here to avoid the need for normative
references. references.
Some additional issues can be inferred from corresponding issues Some additional issues can be inferred from corresponding issues
known to exist in 'traditional' IPv4 NATs. The following documents known to exist in 'traditional' IPv4 NATs. The following documents
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o Scalable mNAT-PT Solution [I-D.park-scalable-multi-natpt] o Scalable mNAT-PT Solution [I-D.park-scalable-multi-natpt]
Because the majority of the documents containing discussions of the Because the majority of the documents containing discussions of the
issues are Internet Drafts which are unlikely to become RFCs, the issues are Internet Drafts which are unlikely to become RFCs, the
issues are summarized here to avoid the need for normative issues are summarized here to avoid the need for normative
references. references.
Some additional issues can be inferred from corresponding issues Some additional issues can be inferred from corresponding issues
known to exist in 'traditional' IPv4 NATs. The following documents known to exist in 'traditional' IPv4 NATs. The following documents
are relevant: are relevant:
o Protocol Complications with the IP Network Address Translator o Protocol Complications with the IP Network Address Translator
[RFC3027] [RFC3027]
o IP Network Address Translator (NAT) Terminology and Considerations o IP Network Address Translator (NAT) Terminology and Considerations
[RFC2663] [RFC2663]
There is some ambiguity in [RFC2766] about whether the Application There is some ambiguity in [RFC2766] about whether the Application
Layer Gateway (ALG) for DNS (referred to as DNS-ALG in this document) Layer Gateway (ALG) for DNS (referred to as DNS-ALG in this document)
is an integral and mandatory part of the specification. The is an integral and mandatory part of the specification. The
ambiguity arises mainly from the first section of the applicability ambiguity arises mainly from the first section of the applicability
section (Section 8) which appears to imply that 'simple' use of section (Section 8), which appears to imply that 'simple' use of
NAT-PT could avoid the use of the DNS-ALG. NAT-PT could avoid the use of the DNS-ALG.
This is important because a number of the major issues arise from the This is important because a number of the major issues arise from the
interactions between DNS and NAT-PT. However, detailed inspection of interactions between DNS and NAT-PT. However, detailed inspection of
[RFC2766] shows that the 'simple' case has not been worked out and it [RFC2766] shows that the 'simple' case has not been worked out and it
is unclear how information about the address translation could be is unclear how information about the address translation could be
passed to the hosts in the absence of the DNS-ALG. This document passed to the hosts in the absence of the DNS-ALG. Therefore, this
therefore assumes that the DNS-ALG is an integral part of NAT-PT: 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 accordingly, issues with the DNS-ALG must be considered as issues for
the whole specification. the whole specification.
Note that the issues which are not specifically related to the use of Note that issues not specifically related to the use of the DNS-ALG
the DNS-ALG will apply to any network layer translation scheme, will apply to any network-layer translation scheme, including any
including any based on the SIIT algorithm [RFC2765]. based on the SIIT algorithm [RFC2765]. In the event that new forms
of 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 NAT-PT can be categorized as follows: Issues raised with IPv6-IPv4 translators in general and NAT-PT in
o Issues which are independent of the use of a DNS-ALG: particular can be categorized as follows:
* Disruption of all protocols which embed IP addresses (and/or
ports) in packet payloads or which apply integrity mechanisms o Issues that are independent of the use of a DNS-ALG and are,
using IP addresses (and ports). therefore, applicable to any form of IPv6-IPv4 translator:
* Inability to re-direct traffic for protocols without any * Disruption of all protocols that embed IP addresses (and/or
demultiplexing capabilities or not built on top of specific ports) in packet payloads or apply integrity mechanisms using
transport layer protocols in situations where one NAPT-PT is IP addresses (and ports).
* Inability to re-direct 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. translating for multiple IPv6 hosts.
* Requirement for applications to use keep alive mechanisms to * Requirement for applications to use keep alive mechanisms to
workaround connectivity issues caused by premature NAT-PT state workaround connectivity issues caused by premature NAT-PT state
timeout. timeout.
* Loss of information due to incompatible semantics between IPv4 * Loss of information due to incompatible semantics between IPv4
and IPv6 versions of headers and protocols. and IPv6 versions of headers and protocols.
* Need for additional state and/or packet reconstruction in * Need for additional state and/or packet reconstruction in
NAPT-PT translators dealing with packet fragmentation. NAPT-PT translators dealing with packet fragmentation.
* Interaction with SCTP and multihoming. * Interaction with SCTP and multihoming.
* Need for NAT-PT to act as proxy for correspondent node when * Need for NAT-PT to act as proxy for correspondent node when
IPv6 node is mobile, with consequent restrictions on mobility. IPv6 node is mobile, with consequent restrictions on mobility.
* NAT-PT not being able to handle multicast traffic. * NAT-PT not being able to handle multicast traffic.
o Issues which are exacerbated by the use of a DNS-ALG:
o Issues that are exacerbated by the use of a DNS-ALG and are,
therefore, also applicable to any form of IPv6-IPv4 translator:
* Constraints on network topology. * Constraints on network topology.
* Scalability concerns together with introduction of single point * Scalability concerns together with introduction of single point
of failure and security attack nexus. of failure and security attack nexus.
* Lack of address mapping persistence: Some applications require * Lack of address mapping persistence: Some applications require
address retention between sessions. The user traffic will be address retention between sessions. The user traffic will be
disrupted if a different mapping is used. The use of the DNS- disrupted if a different mapping is used. The use of the DNS-
ALG to create address mappings with limited lifetimes means ALG to create address mappings with limited lifetimes means
that applications must start using the address shortly after that applications must start using the address shortly after
the mapping is created, as well as keeping it alive once they the mapping is created, as well as keeping it alive once they
start using it. start using it.
* Creation of a DOS threat relating to exhaustion of memory and * Creation of a DoS threat relating to exhaustion of memory and
address/port pool resources on the translator. address/port pool resources on the translator.
o Issues which result from the use of a DNS-ALG:
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 * Address selection issues when either the internal or external
hosts implement both IPv4 and IPv6. hosts implement both IPv4 and IPv6.
* Restricted validity of translated DNS records: a translated * Restricted validity of translated DNS records: a translated
record may be forwarded to an application which cannot use it. record may be forwarded to an application that cannot use it.
* Inappropriate translation of responses to A queries from IPv6 * Inappropriate translation of responses to A queries from IPv6
nodes. nodes.
* Address selection issues and resource consumption in DNS-ALG * Address selection issues and resource consumption in DNS-ALG
with multi-addressed nodes. with multi-addressed nodes.
* Limitations on DNS security capabilities when using DNS-ALG. * Limitations on DNS security capabilities when using DNS-ALG.
Section 2, Section 3 and Section 4 discuss these groups of issues. Section 2, Section 3 and Section 4 discuss these groups of issues.
Section 5 examines the consequences of deploying NAT-PT for Section 5 examines the consequences of deploying NAT-PT for
application developers and the long term effects of NAT-PT on the application developers and the long term effects of NAT-PT (or any
further development of IPv6. form of generally deployed IPv6-IPv4 translator) on the further
development of IPv6.
The terminology used in this document is defined in [RFC2663], The terminology used in this document is defined in [RFC2663],
[RFC2766] and [RFC3314]. [RFC2766], and [RFC3314].
2. Issues Unrelated to DNS-ALG 2. Issues Unrelated to DNS-ALG
2.1. Issues with Protocols Embedding IP Addresses 2.1. Issues with Protocols Embedding IP Addresses
It is well known from work on IPv4 NATs (see Section 8 of [RFC2663] 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 and [RFC3027]) that the large class of protocols that embed numeric
IP addresses in their payloads either cannot work through NATs or IP addresses in their payloads either cannot work through NATs or
require specific ALGs as helpers to translate the payloads in line require specific ALGs as helpers to translate the payloads in line
with the address and port translations. The same set of protocols with the address and port translations. The same set of protocols
cannot pass through NAT-PT. The problem is exacerbated because the cannot pass through NAT-PT. The problem is exacerbated because the
IPv6 and IPv4 addresses are of different lengths so that packet IPv6 and IPv4 addresses are of different lengths so that packet
lengths as well as contents are altered. [RFC2766] describes the lengths as well as contents are altered. [RFC2766] describes the
consequences as part of the description of the FTP ALG: similar consequences as part of the description of the FTP ALG: similar
workarounds are needed for all protocols with embedded IP addresses workarounds are needed for all protocols with embedded IP addresses
that run over TCP transports. that run over TCP transports.
The issues raised in Sections 2 and 3 of [RFC2663] relating to The issues raised in Sections 2 and 3 of [RFC2663], relating to
authentication and encryption with NAT are also applicable to NAT-PT. authentication and encryption with NAT, are also applicable to
NAT-PT.
Implementing a suite of ALGs requires that NAT-PT equipment includes Implementing a suite of ALGs requires that NAT-PT equipment includes
the logic for each of the relevant protocols. Most of these the logic for each of the relevant protocols. Most of these
protocols are continuously evolving, requiring continual and protocols are continuously evolving, requiring continual and
coordinated updates of the ALGs to keep them in step. coordinated updates of the ALGs to keep them in step.
Assuming that the NAT-PT contains a co-located ALG for one of the Assuming that the NAT-PT contains a co-located ALG for one of the
relevant protocols, the ALG could replace the embedded IP addresses relevant protocols, the ALG could replace the embedded IP addresses
and ports. However this replacement can only happen if no and ports. However, this replacement can only happen if no
cryptographic integrity mechanism is used and the protocol messages cryptographic integrity mechanism is used and the protocol messages
are sent in clear (i.e. not encrypted). are sent in the clear (i.e., not encrypted).
A possible workaround relies on the NAT-PT being party to the A possible workaround relies on the NAT-PT being party to the
security association used to provide authentication and/or security association used to provide authentication and/or
encryption. The NAT-PT should then be aware of the cryptographic encryption. NAT-PT would then be aware of the cryptographic
algorithms and keys used to secure the traffic and could modify and algorithms and keys used to secure the traffic. It could then modify
re-secure the packets: this would certainly complicate network and re-secure the packets; this would certainly complicate network
operations and provides additional points of security vulnerability. operations and provides additional points of security vulnerability.
Unless UDP encapsulation is used for IPsec [RFC3498], traffic using Unless UDP encapsulation is used for IPsec [RFC3498], traffic using
IPsec AH (in transport and tunnel mode) and IPsec ESP (in transport IPsec AH (in transport and tunnel mode) and IPsec ESP (in transport
mode) are unable to be carried through NAT-PT without terminating the mode) is unable to be carried through NAT-PT without terminating the
security associations on the NAT-PT, due to their usage of security associations on the NAT-PT, due to their usage of
cryptographic integrity protection. cryptographic integrity protection.
A related issue with DNS security is discussed in Section 4.5. A related issue with DNS security is discussed in Section 4.5.
2.2. NAPT-PT Redirection Issues 2.2. NAPT-PT Redirection Issues
Section 4.2 of [RFC3027] discusses problems specific to RSVP and Section 4.2 of [RFC3027] discusses problems specific to RSVP and
NATs, one of which is actually a more generic problem for all port 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, translators. When several end-hosts are using a single NAPT-PT box,
protocols that do not have a demultiplexing capability similar to protocols that do not have a demultiplexing capability similar to
transport layer port numbers may be unable to work through NAPT-PT transport-layer port numbers may be unable to work through NAPT-PT
(and any other port translator) because there is nothing for NAPT-PT (and any other port translator) because there is nothing for NAPT-PT
to use to identify the correct binding. to use to identify the correct binding.
This type of issue affects IPsec encrypted packets where the This type of issue affects IPsec encrypted packets where the
transport port is not visible (although it might be possible to use transport port is not visible (although it might be possible to use
the SPI as an alternative demultiplexer) and protocols, such as RSVP, the Security Parameter Index (SPI) as an alternative demultiplexer)
which are carried directly in IP datagrams rather than using a and protocols, such as RSVP, which are carried directly in IP
standard transport layer protocol such as TCP or UDP. In the case of datagrams rather than using a standard transport-layer protocol such
RSVP, packets going from the IPv4 domain to the IPv6 domain do not as TCP or UDP. In the case of RSVP, packets going from the IPv4
necessarily carry a suitable demultiplexing field, because the port domain to the IPv6 domain do not necessarily carry a suitable
fields in the flow identifer and traffic specifications are optional. 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 Several ad hoc workarounds could be used to solve the demultiplexing
issues, however in most cases these solutions are not documented issues, however in most cases these solutions are not documented
anywhere which could lead to non-deterministic, undesirable behavior anywhere, which could lead to non-deterministic, undesirable behavior
(for example, such workarounds often assume particular network (for example, such workarounds often assume particular network
topologies etc in order to function correctly; if the assumptions are topologies, etc., in order to function correctly; if the assumptions
not met in a deployment the workaround may not work as expected). are not met in a deployment, the workaround may not work as
expected).
This issue is closely related to the fragmentation issue described in This issue is closely related to the fragmentation issue described in
Section 2.5. Section 2.5.
2.3. NAT-PT Binding State Decay 2.3. NAT-PT Binding State Decay
NAT-PT will generally use dynamically created bindings to reduce the 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 need for IPv4 addresses both for basic NAT-PT and NAPT-PT. Both
soft state mechanisms to manage the address and port pools used for basic NAT-PT and NAPT-PT use soft state mechanisms to manage the
dynamically created address bindings. This allows the NA(P)T-PT to address and, in the case of NAPT-PT, port pools used for dynamically
operate autonomously without requiring clients to signal either created address bindings. This allows all types of NAT-PT box to
implicitly or explicitly that a binding is no longer required. In operate autonomously without requiring clients to signal, either
any case, without soft state timeouts network and application 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 unreliability would inevitably lead to leaks, eventually causing
address or port pool exhaustion. address or port pool exhaustion.
For a dynamic binding to persist for longer than the soft state For a dynamic binding to persist for longer than the soft state
timeout, packets must be sent periodically from one side of the 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 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 specification). If no packets are sent in the proper direction, the
NAT-PT binding will not be refreshed and the application connection NAT-PT binding will not be refreshed and the application connection
will be broken. Hence all applications need to maintain their NAT-PT will be broken. Hence, all applications need to maintain their
bindings during long idle periods by incorporating a keep-alive NAT-PT bindings during long idle periods by incorporating a keep-
mechanism, which may not be possible for legacy systems. alive mechanism, which may not be possible for legacy systems.
Also [RFC2766] does not specify how to choose timeouts for bindings: Also, [RFC2766] does not specify how to choose timeouts for bindings.
as is discussed in [RFC2663] for traditional NATs, selecting suitable As is discussed in [RFC2663] for traditional NATs, selecting suitable
values is a matter of heuristics and coordinating with application values is a matter of heuristics, and coordinating with application
expectations may be impossible. expectations may be impossible.
2.4. Loss of Information through Incompatible Semantics 2.4. Loss of Information through Incompatible Semantics
NAT-PT reuses the SIIT header and protocol translations defined in NAT-PT reuses the SIIT header and protocol translations defined in
[RFC2765]. Mismatches in semantics between IPv4 and IPv6 versions [RFC2765]. Mismatches in semantics between IPv4 and IPv6 versions
can lead to loss of information when packets are translated. Three can lead to loss of information when packets are translated. Three
issues arising from this are: issues arising from this are:
o There is no equivalent in IPv4 for the flow label field of the 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 IPv6 header. Hence, any special treatment of packets based on
label patterns cannot be propagated into the IPv4 domain. flow label patterns cannot be propagated into the IPv4 domain.
o IPv6 extension headers provide flexibility for improvements in the o IPv6 extension headers provide flexibility for improvements in the
IP protocol suite in future. New headers may be defined in future IP protocol suite in future. In the future, new headers may be
which do not have equivalents in IPv4. In practice, some existing defined that do not have equivalents in IPv4. In practice, some
extensions such as routing headers and mobility extensions are not existing extensions such as routing headers and mobility
translatable. extensions are not translatable.
o As described in section 2.2 of [I-D.satapati-v6ops-natpt- o As described in Section 2.2 of [I-D.satapati-v6ops-natpt-
applicability] there are no equivalents in IPv6 for some ICMP(v4) applicability], there are no equivalents in IPv6 for some ICMP(v4)
messages, while for others (notably the 'Parameter Problem' messages, while for others (notably the 'Parameter Problem'
messages) the semantics are not equivalent. Translation of such messages) the semantics are not equivalent. Translation of such
messages may lead to loss of information. However, this issue may messages may lead to loss of information. However, this issue may
not be very severe because the error messages relate to packets not be very severe because the error messages relate to packets
that have been translated by NAT-PT rather than arbitrary packets. that have been translated by NAT-PT rather than arbitrary packets.
If the NAT-PT is functioning correctly, there is, for example, no If the NAT-PT is functioning correctly, there is, for example, no
reason why IPv6 packets with unusual extension headers or options reason why IPv6 packets with unusual extension headers or options
should be generated. This case is cited in [I-D.satapati-v6ops- should be generated. This case is cited in [I-D.satapati-v6ops-
natpt-applicability] as an example where the IPv6 error has no natpt-applicability] as an example where the IPv6 error has no
equivalent in IPv4 resulting in lost information. equivalent in IPv4 resulting in lost information.
Loss of information in any of these cases could be a constraint to Loss of information in any of these cases could be a constraint to
certain applications. certain applications.
A related matter concerns the propagation of the Differentiated A related matter concerns the propagation of the Differentiated
Services Code Point (DSCP). NAT-PT and SIIT simply copy the DSCP Services Code Point (DSCP). NAT-PT and SIIT simply copy the DSCP
field when translating packets. Accordingly the IPv4 and IPv6 field when translating packets. Accordingly, the IPv4 and IPv6
domains must have equivalent Per-Hop Behaviors for the same code domains must have equivalent Per-Hop Behaviors for the same code
point or alternative means must be in place to translate the DSCP point, or alternative means must be in place to translate the DSCP
between domains. between domains.
2.5. NA(P)T-PT and Fragmentation 2.5. NAT-PT and Fragmentation
As mentioned in [RFC3027], simple port translators are unable to As mentioned in [RFC3027], simple port translators are unable to
translate packet fragments other than the first from a fragmented translate packet fragments, other than the first, from a fragmented
packet because subsequent fragments do not contain the port number packet, because subsequent fragments do not contain the port number
information. information.
This means that generally fragmentation cannot be allowed for any This means that generally fragmentation cannot be allowed for any
traffic that traverses a NAPT-PT. One attempted workaround requires traffic that traverses a NAPT-PT. One attempted workaround requires
the NAPT-PT to maintain state about fragmented packets in transit. the NAPT-PT to maintain state about fragmented packets in transit.
This is not a complete solution because fragment misordering could This is not a complete solution because fragment misordering could
lead to the first fragment appearing at the NAPT-PT after later lead to the first fragment appearing at the NAPT-PT after later
fragments. The NAPT-PT would then not have the information needed to fragments. The NAPT-PT would then not have the information needed to
translate the fragments received before the first. translate the fragments received before the first.
Although it would not be expected in normal operation, NAPT-PT needs Although it would not be expected in normal operation, NAPT-PT needs
to be proofed against receiving short first fragments which don't to be proofed against receiving short first fragments that don't
contain the transport port numbers. Note that such packets are a contain the transport port numbers. Note that such packets are a
problem for IPv6 stateful packet inspection. The current problem for IPv6 stateful packet inspection. The current
specifications of IPv6 do not mandate any minimum packet size beyond specifications of IPv6 do not mandate (1) any minimum packet size
the need to carry the unfragmentable part (which doesn't include the beyond the need to carry the unfragmentable part (which doesn't
transport port numbers) or reassembly rules to minimise the effects include the transport port numbers) or (2) reassembly rules to
of overlapping fragments, leaving IPv6 open to the sort of attacks minimize the effects of overlapping fragments. Thus, IPv6 is open to
described in [RFC1858] and [RFC3128]. the sort of attacks described in [RFC1858] and [RFC3128].
An additional concern arises when a fragmented IPv4 UDP packet, which An additional concern arises when a fragmented IPv4 UDP packet, which
does not have a transport layer checksum, traverses a NAT-PT box. As does not have a transport-layer checksum, traverses any type of
described in [RFC2766], the NAT-PT has to reconstruct the whole NAT-PT box. As described in [RFC2766], the NAT-PT has to reconstruct
packet so that it can calculate the checksum needed for the the whole packet so that it can calculate the checksum needed for the
translated IPv6 packet. This can result in significant delay to the translated IPv6 packet. This can result in significant delay to the
packet, especially if it has to be re-fragmented before transmission packet, especially if it has to be re-fragmented before transmission
on the IPv6 side. on the IPv6 side.
If NA(P)T-PT boxes reassembled all incoming fragmented packets (both If NAT-PT boxes reassembled all incoming fragmented packets (both
from the IPv4 and IPv6 directions) in the same way as they have to do 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 for unchecksummed IPv4 UDP packets, this would be a solution to the
first problem. The cost would be considerable: apart from the first problem. The resource cost would be considerable apart from
potential delay problem if the outgoing packet has to be fragmented, the potential delay problem if the outgoing packet has to be re-
the NA(P)T-PT would consume extra memory and CPU resources, making fragmented. In any case, fragmentation would mean that the NAT-PT
the NAT-PT even less scaleable (see Section 3.2). would consume extra memory and CPU resources, making the NAT-PT even
less scalable (see Section 3.2).
Packet reassembly in NA(P)T-PT also opens up the possibility of Packet reassembly in a NAT-PT box also opens up the possibility of
various fragment-related security attacks. Some of these are various fragment-related security attacks. Some of these are
analagous to attacks identified for IPv4. Of particular concern is a analogous to attacks identified for IPv4. Of particular concern is a
DOS attack based on sending large numbers of small fragments without DoS attack based on sending large numbers of small fragments without
a terminating last fragment which would potentially overload the a terminating last fragment, which would potentially overload the
reconstruction buffers and consume large amounts of CPU resources. reconstruction buffers and consume large amounts of CPU resources.
2.6. NAT-PT Interaction with SCTP and Multihoming 2.6. NAT-PT Interaction with SCTP and Multihoming
The Stream Control Transmission Protocol (SCTP) [RFC2960] is a The Stream Control Transmission Protocol (SCTP) [RFC2960] is a
transport protocol which has been standardized since SIIT was transport protocol, which has been standardized since SIIT was
specified. SIIT does not explicitly cover translation of SCTP, but specified. SIIT does not explicitly cover translation of SCTP, but
SCTP uses transport port numbers in the same way as UDP and TCP so SCTP uses transport port numbers in the same way as UDP and TCP so
that similar techniques could be used. that similar techniques could be used.
However, SCTP also supports multihoming. During connection setup However, SCTP also supports multihoming. During connection setup,
SCTP control packets carry embedded addresses which would have to be SCTP control packets carry embedded addresses that would have to be
translated. This would also require that the types of the options translated. This would also require that the types of the options
fields in the SCTP control packets were changed with consequent fields in the SCTP control packets be changed with consequent changes
changes to packet length: the transport checksum would also have to to packet length; the transport checksum would also have to be
be recalculated. The ramifications of multihoming as it might recalculated. The ramifications of multihoming as it might interact
interact with NAT-PT have not been fully explored. Because of the with NAT-PT have not been fully explored. Because of the 'chunked'
'chunked' nature of data transfer it does not appear that state would nature of data transfer, it does not appear that state would have to
have to be maintained to relate packets transmitted using the be maintained to relate packets transmitted using the different IP
different IP addresses associated with the connection. [Author's addresses associated with the connection.
Note: This needs to be considered by an SCTP expert].
Even if these technical issues can be overcome, using SCTP in a Even if these technical issues can be overcome, using SCTP in a
NAT-PT environment may effectively nullify the multihoming advantages NAT-PT environment may effectively nullify the multihoming advantages
of SCTP if all the connections run through the same NAT-PT. The of SCTP if all the connections run through the same NAT-PT. The
consequences of running a multihomed network with separate NAT-PT consequences of running a multihomed network with separate NAT-PT
boxes associated with each of the 'homes' have not been fully boxes associated with each of the 'homes' have not been fully
explored, but one issue that will arise is described in Section 4.4. explored, but one issue that will arise is described in Section 4.4.
SCTP will need an associated 'ALG' - actually a Transport Layer SCTP will need an associated 'ALG' -- actually a Transport Layer
Gateway - to handle the packet payload modifications. If it turns Gateway -- to handle the packet payload modifications. If it turns
out that state is required, the state would have to distributed and out that state is required, the state would have to distributed and
synchronized across several NAT-PT boxes in a multihomed environment. synchronized across several NAT-PT boxes in a multihomed environment.
SCTP running through NAT-PT in a multihomed environment is also SCTP running through NAT-PT in a multihomed environment is also
incompatible with IPsec as described in Section 2.1. incompatible with IPsec as described in Section 2.1.
2.7. NAT-PT as a Proxy Correspondent Node for MIPv6 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 As discussed in [I-D.lee-v6ops-natpt-mobility], it is not possible to
propagate Mobile IPv6 control messages into the IPv4 domain. propagate Mobile IPv6 control messages into the IPv4 domain.
According to the IPv6 Node Requirements [I-D.ietf-ipv6-node- According to the IPv6 Node Requirements [I-D.ietf-ipv6-node-
requirements], IPv6 nodes should normally be prepared to support the requirements], IPv6 nodes should normally be prepared to support the
route optimization mechanisms needed in a correspondent node. If route optimization mechanisms needed in a correspondent node. If
communications from an IPv6 mobile node is traversing a NAT-PT, the communications from an IPv6 mobile node are traversing a NAT-PT, the
destination IPv4 node is not going to support the correspondent node destination IPv4 node will certainly not be able to support the
features needed for route optimization. correspondent node features needed for route optimization.
This can be resolved in two ways: This can be resolved in two ways:
o The NAT-PT can discard messages and headers relating to changes of o The NAT-PT can discard messages and headers relating to changes of
care-of addresses including reverse routing checks. care-of addresses, including reverse routing checks.
Communications with the mobile node will continue through the home Communications with the mobile node will continue through the home
agent without route optimization. This is clearly sub-optimal but agent without route optimization. This is clearly sub-optimal,
communication should remain possible. but communication should remain possible.
o Additional functionality could be implemented in the NAT-PT to o Additional functionality could be implemented in the NAT-PT to
allow it to function as a proxy correspondent node for all IPv4 allow it to function as a proxy correspondent node for all IPv4
nodes for which it has bindings. This scheme adds considerably to nodes for which it has bindings. This scheme adds considerably to
the complexity of NAT-PT. Depending on the routability of the the complexity of NAT-PT. Depending on the routability of the
IPv6 PREFIX used for translated IPv4 addresses, it may also limit IPv6 PREFIX used for translated IPv4 addresses, it may also limit
the extent of mobility of the mobile node: all communications to 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 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 the mobile node moves to a network that does not have direct IPv6
connectivity with the NAT-PT. connectivity with the NAT-PT.
In both cases the existing NAT-PT specification would need to be In both cases, the existing NAT-PT specification would need to be
extended to deal with IPv6 mobile nodes and neither is a fully extended to deal with IPv6 mobile nodes, and neither is a fully
satisfactory solution. satisfactory solution.
2.8. NAT-PT and Multicast 2.8. NAT-PT and Multicast
SIIT [RFC2765] cannot handle translation of multicast packets and SIIT [RFC2765] cannot handle translation of multicast packets and
NAT-PT does not discuss a way to map multicast addresses between IPv4 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 and IPv6. Some separate work has been done to provide an alternative
mechanism to handle multicast. This uses a separate gateway which mechanism to handle multicast. This uses a separate gateway that
understands some or all of the relevant multicast control and routing understands some or all of the relevant multicast control and routing
protocols in each domain. This work has not been carried through protocols in each domain. This work has not been carried through
into standards as yet. into standards as yet.
A basic mechanism which involves only IGMP on the IPv4 side and MLD 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 on the IPv6 side, is described in 'An IPv6/IPv4 Multicast Translator
based on IGMP/MLD Proxying (mtp)' [I-D.tsuchiya-mtp]. A more based on IGMP/MLD Proxying (mtp)' [I-D.tsuchiya-mtp]. A more
comprehensive approach which includes proxying of the multicast comprehensive approach, which includes proxying of the multicast
routing protocols is described in 'An IPv4 - IPv6 multicast gateway' routing protocols, is described in 'An IPv4 - IPv6 multicast gateway'
[I-D.venaas-mboned-v4v6mcastgw]. Both approaches have several of the [I-D.venaas-mboned-v4v6mcastgw]. Both approaches have several of the
issues described in this section, notably issues with embedded issues described in this section, notably issues with embedded
addresses. addresses.
[I-D.okazaki-v6ops-natpt-security] identifies the possibility of a [I-D.okazaki-v6ops-natpt-security] identifies the possibility of a
multiplicative reflection attack if the NAT-PT can be spoofed into multiplicative reflection attack if the NAT-PT can be spoofed into
creating a binding for a multicast address. This attack would be creating a binding for a multicast address. This attack would be
very hard to mount because routers should not forward packets with very hard to mount because routers should not forward packets with
muticast addresses in the source address field. However, it points multicast addresses in the source address field. However, it
up that a naively implemented DNS-ALG could create such bindings from highlights the possibility that a naively implemented DNS-ALG could
spoofed DNS responses since [RFC2766] does not mention the need for create such bindings from spoofed DNS responses since [RFC2766] does
checks on the types of addresses in these responses. 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 The issues for NAT-PT and multicast reflect the fact that NAT-PT is
at best a partial solution. Completing the translation solution to at best a partial solution. Completing the translation solution to
cater for multicast traffic is likely to carry a similar set of cater for multicast traffic is likely to carry a similar set of
issues to the current unicast NAT-PT and may open up significant issues to the current unicast NAT-PT and may open up significant
additional security risks. additional security risks.
3. Issues exacerbated by the Use of DNS-ALG 3. Issues exacerbated by the Use of DNS-ALG
3.1. Network Topology Constraints Implied by NAT-PT 3.1. Network Topology Constraints Implied by NAT-PT
Traffic flow initiators in a NAT-PT environment are dependent on the 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 DNS-ALG in the NAT-PT to provide the mapped address needed to
communicate with the flow destination on the other side of the communicate with the flow destination on the other side of the
NAT-PT. Whether used for flows initiated in the IPv4 domain or 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 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 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 DNS query will not be modified and the response type would not be
appropriate. appropriate.
The implication is that the NAT-PT box also has to be the default 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 IPv6 router for the site so that the DNS-ALG is able to examine all
all DNS requests made over IPv6. On sites with both IPv6 and dual- DNS requests made over IPv6. On sites with both IPv6 and dual-stack
stack nodes, this will result in all traffic flowing through the nodes, this will result in all traffic flowing through the NAT-PT
NAT-PT with consequent scalability concerns. with consequent scalability concerns.
These constraints are described in more detail in [I-D.durand-natpt- These constraints are described in more detail in [I-D.durand-natpt-
dns-alg-issues]. dns-alg-issues].
[I-D.hallin-natpt-dns-alg-solutions] proposes a solution for flows [I-D.hallin-natpt-dns-alg-solutions] proposes a solution for flows
initiated from the IPv6 domain but it appears that this solution initiated from the IPv6 domain, but it appears that this solution
still has issues. still has issues.
For IPv6-only clients the solution requires the use of a DNS server 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 in the IPv4 domain accessed via an IPv6 address which uses the NAT-PT
PREFIX (see [RFC2766]). Queries to this server would necessarily PREFIX (see [RFC2766]). Queries to this server would necessarily
pass through the NAT-PT. Dual-stack hosts would use a separate DNS pass through the NAT-PT. Dual-stack hosts would use a separate DNS
server accessed through a normal IPv6 address. This removes the need 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. 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 The primary proposal suggests that the IPv6-only clients should use
this DNS server for all queries. This is expensive on NAT-PT this DNS server for all queries. This is expensive on NAT-PT
resources because requests relating to hosts with native IPv6 resources because requests relating to hosts with native IPv6
addresses would also use the NAT-PT DNS-ALG. addresses would also use the NAT-PT DNS-ALG.
skipping to change at page 13, line 28 skipping to change at page 14, line 4
having the NAT-PT prefix. This imposes the burden of always having the NAT-PT prefix. This imposes the burden of always
requiring DNS RR [RFC1035] translation. requiring DNS RR [RFC1035] translation.
For flows initiated from the IPv4 network, the proposal recommends For flows initiated from the IPv4 network, the proposal recommends
that the advertised DNS servers for the IPv6 network would have the 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 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 choose the correct DNS server for each query resulting in the same
issues as were raised for flows initiated from the IPv6 domain. issues as were raised for flows initiated from the IPv6 domain.
Although the engineering workaround, just described, provides a Although the engineering workaround, just described, provides a
partial solution to the topology constraints issue it mandates that partial solution to the topology constraints issue, it mandates that
DNS queries and responses should still go through a NAT-PT even if 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 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 through the NAT-PT for all DNS requests will exacerbate the other
related issues discussed in Section 3.4 and Section 4.1. DNS-related issues discussed in Section 3.4 and Section 4.1.
3.2. Scalability and Single Point of Failure Concerns 3.2. Scalability and Single Point of Failure Concerns
As with traditional NAT, NAT-PT is a bottleneck in the network with As with traditional NAT, NAT-PT is a bottleneck in the network with
significant scalability concerns and the anchoring of flows to a significant scalability concerns and the anchoring of flows to a
particular NAT-PT makes the NAT-PT a potential single point of 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 failure in the network. The addition of the DNS-ALG in NAT-PT
further increases the scalability concerns. further increases the scalability concerns.
Solutions to both problems have been envisaged using collections of Solutions to both problems have been envisaged using collections of
cooperating NAT-PT boxes, but such solutions require coordination and cooperating NAT-PT boxes, but such solutions require coordination and
state synchronization which has not yet been standardized and again state synchronization, which has not yet been standardized and again
adds to the functional and operational complexity of NAT-PT. One adds to the functional and operational complexity of NAT-PT. One
such solution is described in [I-D.park-scalable-multi-natpt]. such solution is described in [I-D.park-scalable-multi-natpt].
As with traditional NAT, the concentration of flows through NAT-PT As with traditional NAT, the concentration of flows through NAT-PT
and the legitimate modification of packets in the NAT-PT make NAT-PTs and the legitimate modification of packets in the NAT-PT make NAT-PTs
enticing targets for security attacks. enticing targets for security attacks.
3.3. Issues with Lack of Address Persistence 3.3. Issues with Lack of Address Persistence
Using the DNS-ALG to create address bindings requires that the Using the DNS-ALG to create address bindings requires that the
application uses the translated address returned by the DNS query application uses the translated address returned by the DNS query
before the NAT-PT binding state is timed out (see Section 2.3). before the NAT-PT binding state is timed out (see Section 2.3).
Applications will not normally be aware of this constraint, which may Applications will not normally be aware of this constraint, which may
be different from the existing lifetime of DNS query responses. This be different from the existing lifetime of DNS query responses. This
could lead to difficult diagnosis problems with applications. could lead to "difficult to diagnose" problems with applications.
Additionally, the DNS-ALG needs to determine the initial lifetime of 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 bindings that it creates. As noted in Section 2.3, this may need to
be determined heuristically. The DNS-ALG does not know which be determined heuristically. The DNS-ALG does not know which
protocol the mapping is to be used for, and so needs another way to 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 determine the initial lifetime. This could be tied to the DNS
response lifetime but this might open up additional DOS attack response lifetime, but that might open up additional DoS attack
possibilities if very long validities are allowed. Also the lifetime possibilities if very long validities are allowed. Also, the
should be adjusted once the NAT-PT determines which protocol is being lifetime should be adjusted once the NAT-PT determines which protocol
used with the binding. is being used with the binding.
As with traditional NATs (see Section 2.5 of [RFC3027], NAT-PT will As with traditional NATs (see Section 2.5 of [RFC3027], NAT-PT will
most likely break applications that require address mapping to be most likely break applications that require address mapping to be
retained across contiguous sessions. These applications require the retained across contiguous sessions. These applications require the
IPv4 to IPv6 address mapping to be retained between sessions so the IPv4 to IPv6 address mapping to be retained between sessions so the
same mapped address may be reused for subsequent session same mapped address may be reused for subsequent session
interactions. NAT-PT cannot know this requirement and may reassign interactions. NAT-PT cannot know this requirement and may reassign
the previously used mapped address to different hosts between the previously used mapped address to different hosts between
sessions. sessions.
Trying to keep NAT-PT from discarding an address mapping would Trying to keep NAT-PT from discarding an address mapping would
require either a NAT-PT extension protocol that would allow the require either a NAT-PT extension protocol that would allow the
application to request the NAT-PT device to retain the mappings, or 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) 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 that can interact with NAT-PT to keep the address mapping from being
discarded after a session. discarded after a session.
3.4. DOS Attacks on Memory and Address/Port Pools 3.4. DoS Attacks on Memory and Address/Port Pools
As discussed in Section 2.3 a NAT-PT may create dynamic NAT bindings As discussed in Section 2.3, a NAT-PT may create dynamic NAT
each of which consumes memory resources as well as an address (or bindings, each of which consumes memory resources as well as an
port if NAPT-PT is used) from an address (or port) pool. A number of address (or port if NAPT-PT is used) from an address (or port) pool.
documents, including [RFC2766] and [I-D.okazaki-v6ops-natpt-security] A number of documents, including [RFC2766] and [I-D.okazaki-v6ops-
discuss possible denial of service (DOS) attacks on NA(P)T-PT which natpt-security] discuss possible denial of service (DoS) attacks on
result in resource depletion associated with address and port pools. basic NAT-PT and NAPT-PT that result in resource depletion associated
NAT-PT does not specify any authentication mechanisms so that an with address and port pools. NAT-PT does not specify any
attacker may be able to create spurious binds by spoofing addresses authentication mechanisms; thus, an attacker may be able to create
in packets sent through NAT-PT. The attack is more damaging if the spurious bindings by spoofing addresses in packets sent through
attacker is able to spoof protocols with long binding timeouts NAT-PT. The attack is more damaging if the attacker is able to spoof
(typically used for TCP). protocols with long binding timeouts (typically used for TCP).
The use of the DNS-ALG in NAT-PT introduces another vulnerability The use of the DNS-ALG in NAT-PT introduces another vulnerability
which can result in resource depletion. The attack identified in that can result in resource depletion. The attack identified in
[I-D.durand-natpt-dns-alg-issues] exploits the use of DNS queries [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 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 is sent through the NAT-PT, the NAT-PT may create a new basic NAT-PT
without any end-host authentication or authorization mechanisms. or NAPT-PT binding without any end-host authentication or
This behavior could lead to a serious DOS attack on both memory and authorization mechanisms. This behavior could lead to a serious DoS
address or port pools. Address spoofing is not required for this attack on both memory and address or port pools. Address spoofing is
attack to be successful. not required for this attack to be successful.
[I-D.hallin-natpt-dns-alg-solutions] proposes to mitigate the DOS [I-D.hallin-natpt-dns-alg-solutions] proposes to mitigate the DoS
attack by using ACLs and static binds which increases the operational attack by using Access Control Lists (ACLs) and static binds, which
cost and may not always be practical. increases the operational cost and may not always be practical.
The ideal mitigation solution would be to disallow dynamically The ideal mitigation solution would be to disallow dynamically
created binds until authentication and authorization of the end-host created binds until authentication and authorization of the end-host
needing the protocol translation has been carried out. This would needing the protocol translation has been carried out. This would
require that the proper security infrastructure be in place to require that the proper security infrastructure be in place to
support the authentication and authorization, which increases the support the authentication and authorization, which increases the
network operational complexity. network operational complexity.
4. Issues Directly Related to use of DNS-ALG 4. Issues Directly Related to Use of DNS-ALG
4.1. Address Selection Issues when Communicating with Dual-Stack End- 4.1. Address Selection Issues when Communicating with Dual-Stack End-
hosts Hosts
[I-D.durand-natpt-dns-alg-issues] discusses NAT-PT DNS-ALG issues [I-D.durand-natpt-dns-alg-issues] discusses NAT-PT DNS-ALG issues
with regard to address selection. As specified in [RFC2766], the with regard to address selection. As specified in [RFC2766], the
DNS-ALG returns AAAA RRs from two possible sources to the IPv6 host DNS-ALG returns AAAA resource records (RRs) from two possible sources
which has made an AAAA DNS query AAAA. 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 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 the native IPv6 address(es) and the translated IPv4 address(es) in
AAAA RRs. Without additional information, the IPv6 host address AAAA RRs. Without additional information, the IPv6 host address
selection may pick a translated IPv4 address instead of selecting the selection may pick a translated IPv4 address instead of selecting the
more appropriate native IPv6 address. Under some circumstances, the more appropriate native IPv6 address. Under some circumstances, the
address selection algorithms [RFC3484] will always prefer the address selection algorithms [RFC3484] will always prefer the
translated address over the native IPv6 address which is obviously translated address over the native IPv6 address; this is obviously
undesirable. undesirable.
[I-D.hallin-natpt-dns-alg-solutions] proposes a solution which [I-D.hallin-natpt-dns-alg-solutions] proposes a solution that
involves modification to the NAT-PT specification intended to return involves modification to the NAT-PT specification intended to return
only the most appropriate address(es) to an IPv6 capable host: 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 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 will forward the query to the DNS server in the IPv4 domain
unchanged but using IPv4 transport: unchanged, but using IPv4 transport:
* If the authoritative DNS server has one or more AAAA records, * If the authoritative DNS server has one or more AAAA records,
it returns them. The DNS-ALG then forwards this response to it returns them. The DNS-ALG then forwards this response to
the IPv6 host and does not send an A query as the standard the IPv6 host and does not send an A query as the standard
NAT-PT would do. NAT-PT would do.
* Otherwise, if the DNS server does not understand the AAAA query * Otherwise, if the DNS server does not understand the AAAA query
or has no AAAA entry for the host, it will return an error. 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 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 send an A query for the same host. If this query returns an
IPv4 address, the ALG creates a binding and synthesizes a IPv4 address, the ALG creates a binding and synthesizes a
corresponding AAAA record which it sends back to the IPv6 host. corresponding AAAA record, which it sends back to the IPv6
host.
o The NAT-PT thus forwards the result of the first successful DNS 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. response back to the end-host or an error if neither succeeds.
Consequently only AAAA RRs from one source will be provided Consequently, only AAAA RRs from one source will be provided
instead of two as specified in [RFC2766] and it will contain the instead of two as specified in [RFC2766], and it will contain the
most appropriate address for a dual-stack or IPv6-only querier. most appropriate address for a dual-stack or IPv6-only querier.
There is, however, still an issue with the proposed solution: There is, however, still an issue with the proposed solution:
o The DNS client may timeout the query if it doesn't receive a 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 response in time. This is more likely because the NAT-PT may have
to make two separate queries sequentially which the client is not to make two separate, sequential queries of which the client is
aware of. It may be possible to reduce the response time by not aware. It may be possible to reduce the response time by
sending the two queries in parallel and ignoring the result of the 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 A query if the AAAA returns one or more addresses. However, it is
still necessary to delay after receiving the first response to still necessary to delay after receiving the first response to
determine if a second is coming, which may still trigger the DNS determine if a second is coming, which may still trigger the DNS
client timeout. client timeout.
Unfortunately, the two queries cannot be combined in a single DNS Unfortunately, the two queries cannot be combined in a single DNS
request (all known DNS servers only process a single DNS query per request (all known DNS servers only process a single DNS query per
request message because of difficulties expressing authoritativeness request message because of difficulties expressing authoritativeness
for arbitrary combinations of requests). for arbitrary combinations of requests).
An alternative solution would be to allow the IPv6 host to have An alternative solution would be to allow the IPv6 host to have,
within its address selection policies the NAT-PT PREFIX [RFC2766] within its address selection policies, the NAT-PT PREFIX [RFC2766]
used and assign to it a low selection priority. This solution used and to assign it a low selection priority. This solution
requires a automatic configuration of the NAT-PT PREFIX as well as requires an automatic configuration of the NAT-PT PREFIX as well as
its integration within the address selection policies. The simplest its integration within the address selection policies. The simplest
way to integrate this automatic configuration would be through way to integrate this automatic configuration would be through
configuration file download (in case the host or DHCPv6 server did configuration file download (in case the host or Dynamic Host
not support vendor options - to avoid standardization effort on the Configuration Protocol for IPv6 (DHCPv6) server did not support
NAT-PT PREFIX option). This solution does not require any vendor options, to avoid standardization effort on the NAT-PT PREFIX
modification to the NAT-PT specification. option). This solution does not require any modification to the
NAT-PT specification.
Neither of these solutions resolves a second issue related to address Neither of these solutions resolves a second issue related to address
selection identified in [I-D.durand-natpt-dns-alg-issues]. selection that is identified in [I-D.durand-natpt-dns-alg-issues].
Applications have no way of knowing that the IPv6 address returned 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 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 address. The application may therefore be led to believe that it has
end-to-end IPv6 connectivity with the destination. As a result, the end-to-end IPv6 connectivity with the destination. As a result, the
application may use IPv6 specific options that are not supported by application may use IPv6-specific options that are not supported by
NAT-PT. This issue is closely related to the issue described in NAT-PT. This issue is closely related to the issue described in
Section 4.2 and the discussion in Section 5. Section 4.2 and the discussion in Section 5.
4.2. Non-global Validity of Translated RR Records 4.2. Non-global Validity of Translated RR Records
Some applications propagate information records retrieved from DNS to Some applications propagate information records retrieved from DNS to
other applications. The published semantics of DNS imply that the other applications. The published semantics of DNS imply that the
results will be consistent to any user for the duration of the results will be consistent to any user for the duration of the
attached lifetime. RR records translated by NAT-PT violate these attached lifetime. RR records translated by NAT-PT violate these
semantics because the retrieved addresses are only usable for semantics because the retrieved addresses are only usable for
communications through the translating NAT-PT. communications through the translating NAT-PT.
Applications which pass on retrieved DNS records to other Applications that pass on retrieved DNS records to other applications
applications will generally assume that they can rely on the passed will generally assume that they can rely on the passed on addresses
on addresses to be usable by the receiving application. This may not to be usable by the receiving application. This may not be the case
be the case if the receiving application is on another node if the receiving application is on another node, especially if it is
especially if it is not in the domain served by the NAT-PT which not in the domain served by the NAT-PT that generated the
generated the translation. translation.
4.3. Inappropriate Translation of Responses to A Queries 4.3. Inappropriate Translation of Responses to A Queries
Some applications running on dual-stack nodes may wish to query the Some applications running on dual-stack nodes may wish to query the
IPv4 address of a destination. If the resulting A query passes IPv4 address of a destination. If the resulting A query passes
through the NAT-PT DNS-ALG, the DNS-ALG will translate the response through the NAT-PT DNS-ALG, the DNS-ALG will translate the response
inappropriately into a AAAA record using a translated address. This inappropriately into a AAAA record using a translated address. This
happens because the DNS-ALG specified in [RFC2766] operates happens because the DNS-ALG specified in [RFC2766] operates
statelessly and hence has no memory of the IPv6 query which induced statelessly and hence has no memory of the IPv6 query that induced
the A request on IPv4 side. The default action is to translate the the A request on IPv4 side. The default action is to translate the
response. response.
The specification of NAT-PT could be modified to maintain minimal The specification of NAT-PT could be modified to maintain minimal
state about queries passed through the DNS-ALG, and hence to respond state about queries passed through the DNS-ALG, and hence to respond
correctly to A queries as well as AAAA queries. correctly to A queries as well as AAAA queries.
4.4. DNS-ALG and Multi-addressed Nodes 4.4. DNS-ALG and Multi-addressed Nodes
Many IPv6 nodes, especially in multihomed situations but also in Many IPv6 nodes, especially in multihomed situations but also in
single homed deployments, can expect to have multiple global single homed deployments, can expect to have multiple global
addresses. The same may be true for multihomed IPv4 nodes. addresses. The same may be true for multihomed IPv4 nodes.
Responses to DNS queries for these nodes will normally contain all Responses to DNS queries for these nodes will normally contain all
these addresses. Since the DNS-ALG in the NAT-PT has no knowledge 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 which of the addresses can or will be used by the application issuing
the query, it is obliged to translate all of them. the query, it is obliged to translate all of them.
This could be a significant drain on resources in both NAT-PT and This could be a significant drain on resources in both basic NAT-PT
NAPT-PT, as bindings will have to be created for each address. and NAPT-PT, as bindings will have to be created for each address.
When using SCTP in a multihomed network, the problem is exacerbated When using SCTP in a multihomed network, the problem is exacerbated
if multiple NAT-PTs translate mutiple addresses: also it is not clear if multiple NAT-PTs translate multiple addresses. Also, it is not
that SCTP will actually look up all the destination IP addresses via clear that SCTP will actually look up all the destination IP
DNS so that bindings may not be in place when packets arrive. addresses via DNS so that bindings may not be in place when packets
arrive.
4.5. Limitations on Deployment of DNS Security Capabilities 4.5. Limitations on Deployment of DNS Security Capabilities
Secure DNS (DNSSEC) [I-D.ietf-dnsext-dnssec-intro] uses public key Secure DNS (DNSSEC) [I-D.ietf-dnsext-dnssec-intro] uses public key
cryptographic signing to authenticate DNS responses. The DNS-ALG cryptographic signing to authenticate DNS responses. The DNS-ALG
modifies DNS query responses traversing the NAT-PT in both directions modifies DNS query responses traversing the NAT-PT in both directions
which would invalidate the signatures as (partially) described in which would invalidate the signatures as (partially) described in
Section 7.5 of [RFC2766]. Section 7.5 of [RFC2766].
Workarounds have been proposed, such as making the DNS-ALG behave Workarounds have been proposed, such as making the DNS-ALG behave
like a secure DNS server. This would need to be done separately for like a secure DNS server. This would need to be done separately for
both the IPv6 and IPv4 domains. This is operationally very complex 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 and there is a risk that the server could be mistaken for a
a conventional DNS server. The NAT-PT specification would have to be conventional DNS server. The NAT-PT specification would have to be
altered to implement any such workaround. altered to implement any such workaround.
Hence DNSSEC is not deployable in domains that use NAT-PT as Hence DNSSEC is not deployable in domains that use NAT-PT as
currently specified. Widespread deployment of NAT-PT would become a currently specified. Widespread deployment of NAT-PT would become a
serious obstacle to the large scale deployment of DNSSEC. serious obstacle to the large scale deployment of DNSSEC.
5. Impact on IPv6 Application Development 5. Impact on IPv6 Application Development
One of the major design goals for IPv6 is to restore the end-to-end 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 transparency of the Internet. Therefore, because IPv6 may be
the need for NATs and similar impediments to transparency, developers expected to remove the need for NATs and similar impediments to
creating applications to work with IPv6 may be tempted to assume that transparency, developers creating applications to work with IPv6 may
the complex and (development) time-consuming expedients that might be tempted to assume that the complex expedients that might have been
have been needed to make the application work in an 'NATted' IPv4 needed to make the application work in a 'NATted' IPv4 environment
environment are not required. are not required.
Consequently some classes of applications (e.g., peer-to-peer) that Consequently, some classes of applications (e.g., peer-to-peer) that
would need special measures to manage NAT traversal, including would need special measures to manage NAT traversal, including
special encapsulations, attention to binding lifetime and provision special encapsulations, attention to binding lifetime, and provision
of keepalives, may build in assumptions on whether IPv6 is being used of keepalives, may build in assumptions on whether IPv6 is being used
or not. Developers would also like to exploit additional or not. Developers would also like to exploit additional
capabilities of IPv6 not available in IPv4. capabilities of IPv6 not available in IPv4.
NAT-PT as specified in [RFC2766] is intended to work autonomously and NAT-PT as specified in [RFC2766] is intended to work autonomously and
be transparent to applications. There is therefore no way for be transparent to applications. Therefore, there is no way for
application developers to discover that a path contains a NAT-PT. application developers to discover that a path contains a NAT-PT.
If NAT-PT is deployed, applications that have assumed a NAT-free IPv6 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 environment may break when the traffic passes through a NAT-PT. This
is bad enough, but requiring developers to include special is bad enough, but requiring developers to include special
capabilities to workaround what is supposed to be a temporary capabilities to workaround what is supposed to be a temporary
transition 'aid' is even worse. Finally, deployment of NAT-PT is transition 'aid' is even worse. Finally, deployment of NAT-PT is
likely to inhibit the development and use of additional IPv6 likely to inhibit the development and use of additional IPv6
capabilities enabled by the flexible extension header system in IPv6 capabilities enabled by the flexible extension header system in IPv6
packets. packets.
Some of these deleterious effects could possibly be alleviated if Some of these deleterious effects could possibly be alleviated if
applications could discover the presence of NAT-PT boxes on paths in applications could discover the presence of NAT-PT boxes on paths in
use, allowing them to take steps to workaround the problems. However use, allowing the applications to take steps to workaround the
requiring applications to incorporate extra code to workaround problems. However, requiring applications to incorporate extra code
problems with a transition aid still seems to be a very bad idea: the to workaround problems with a transition aid still seems to be a very
behavior of the application in native IPv6 and NAT-PT environments bad idea: the behavior of the application in native IPv6 and NAT-PT
would be likely to be significantly different. environments would be likely to be significantly different.
6. Security Considerations 6. Security Considerations
This document summarizes security issues related to the NAT-PT This document summarizes security issues related to the NAT-PT
[RFC2766] specification. Security issues are discussed in various [RFC2766] specification. Security issues are discussed in various
sections: sections:
o Section 2.1 discusses how IPsec AH (transport and tunnel mode) and 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 IPsec ESP transport mode are broken by NAT-PT (when IPSEC UDP
encapsulation is not used [RFC3498]); and authentication and encapsulation is not used [RFC3498]); and authentication and
encryption are generally incompatible with NAT-PT. encryption are generally incompatible with NAT-PT.
o Section 2.5 discusses possible fragmentation related security o Section 2.5 discusses possible fragmentation related security
attacks on NAT-PT. attacks on NAT-PT.
o Section 2.8 discusses security issues related to multicast o Section 2.8 discusses security issues related to multicast
addresses and NAT-PT. addresses and NAT-PT.
o Section 3.3 highlights that NAT-PT is an enticing nexus for o Section 3.3 highlights that NAT-PT is an enticing nexus for
security attacks. security attacks.
skipping to change at page 19, line 34 skipping to change at page 20, line 21
o Section 2.1 discusses how IPsec AH (transport and tunnel mode) and 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 IPsec ESP transport mode are broken by NAT-PT (when IPSEC UDP
encapsulation is not used [RFC3498]); and authentication and encapsulation is not used [RFC3498]); and authentication and
encryption are generally incompatible with NAT-PT. encryption are generally incompatible with NAT-PT.
o Section 2.5 discusses possible fragmentation related security o Section 2.5 discusses possible fragmentation related security
attacks on NAT-PT. attacks on NAT-PT.
o Section 2.8 discusses security issues related to multicast o Section 2.8 discusses security issues related to multicast
addresses and NAT-PT. addresses and NAT-PT.
o Section 3.3 highlights that NAT-PT is an enticing nexus for o Section 3.3 highlights that NAT-PT is an enticing nexus for
security attacks. security attacks.
o Section 3.4 discusses possible NAT-PT DOS attacks on both memory o Section 3.4 discusses possible NAT-PT DoS attacks on both memory
and address/port pools. and address/port pools.
o Section 4.5 discusses why NAT-PT is incompatible with DNSSEC o Section 4.5 discusses why NAT-PT is incompatible with DNSSEC
[RFC2535] and how deployment of NAT-PT may inhibit deployment of [RFC2535] and how deployment of NAT-PT may inhibit deployment of
DNSSEC. DNSSEC.
7. IANA Considerations 7. IANA Considerations
There are no IANA considerations defined in this document. There are no IANA considerations defined in this document.
8. Conclusion 8. Conclusion
This document has discussed a number of significant issues with This document has discussed a number of significant issues with
NAT-PT as defined in [RFC2766]. From a deployment perspective 3GPP NAT-PT as defined in [RFC2766]. From a deployment perspective, 3GPP
networks are currently the only 'standardised' scenario where an IPv6 networks are currently the only 'standardised' scenario where an
only host communicates with an IPv4 only host using NAT-PT as IPv6-only host communicates with an IPv4-only host using NAT-PT as
described in the 3GPP IPv6 transition analysis [I-D.ietf-v6ops-3gpp- described in the 3GPP IPv6 transition analysis [I-D.ietf-v6ops-3gpp-
analysis] but NAT-PT has seen some limited usage for other purposes. analysis], but NAT-PT has seen some limited usage for other purposes.
Although some of issues identified with NAT-PT appear to have Although some of the issues identified with NAT-PT appear to have
solutions, many of the solutions required significant alterations to solutions, many of the solutions required significant alterations to
the existing specification and would be likely to increase the existing specification and would be likely to increase
operational complexity. Even if these solutions were applied, we operational complexity. Even if these solutions were applied, we
have shown that NAT-PT still has significant irresolvable issues and have shown that NAT-PT still has significant, irresolvable issues and
appears to have limited applicability. The potential constraints on appears to have limited applicability. The potential constraints on
the development of IPv6 applications described in Section 5 are the development of IPv6 applications described in Section 5 are
particularly un desirable. It appears that alternatives to NAT-PT particularly un desirable. It appears that alternatives to NAT-PT
exist to cover the circumstances where NAT-PT has been suggested as a 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 solution, such as the use of tunneling and header compression in 3GPP
scenarios. scenarios.
However it is clear that in some circumstances a IPv6/IPv4 protocol However, it is clear that in some circumstances an IPv6/IPv4 protocol
translation solution may be a useful transitional solution, translation solution may be a useful transitional solution,
particularly in more constrained situations where the translator is particularly in more constrained situations where the translator is
not required to deal with traffic for a wide variety of protocols not required to deal with traffic for a wide variety of protocols
which are not determined in advance. It is therefore possible that a 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 more limited form of NAT-PT could be defined for use in specific
situations. situations.
Accordingly we recommend that the IETF no longer suggest its usage as Accordingly, we recommend that the IETF no longer suggest its usage
a general IPv4/IPv6 transition mechanism in the Internet but retain as a general IPv4/IPv6 transition mechanism in the Internet, but
it as an experimental mechanism while further experience is gained retain it as an experimental mechanism while further experience is
and any future replacement is defined and deployed. Consequently we gained and any future replacement is defined and deployed.
recommend moving RFC2766 to experimental status. Consequently, we recommend moving RFC2766 to experimental status.
9. Acknowledgments 9. Acknowledgments
This work builds on a large body of existing work examining the 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 issues and applicability of NAT-PT: the work of the authors of the
documents referred in Section 1 has been extremely useful in creating documents referred to in Section 1 has been extremely useful in
this document. Particular thanks are due to Pekka Savola for rapid creating this document. Particular thanks are due to Pekka Savola
and thorough review of the document. for rapid and thorough review of the document.
10. References 10. References
10.1. Normative References 10.1. Normative References
[RFC2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm [RFC2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm
(SIIT)", RFC 2765, February 2000. (SIIT)", RFC 2765, February 2000.
[RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address [RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address
Translation - Protocol Translation (NAT-PT)", RFC 2766, Translation - Protocol Translation (NAT-PT)", RFC 2766,
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