HomeDocsIETF standardsRFCRFC 3700 — 3799
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Network Working Group
Request for Comments: 3715
Category: Informational
B. Aboba
W. Dixon
Microsoft
March 2004

IPsec-Network Address Translation (NAT) Compatibility Requirements

Status of this Memo

This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.

Copyright Notice

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

Abstract

This document describes known incompatibilities between Network Address Translation (NAT) and IPsec, and describes the requirements for addressing them. Perhaps the most common use of IPsec is in providing virtual private networking capabilities. One very popular use of Virtual Private Networks (VPNs) is to provide telecommuter access to the corporate Intranet. Today, NATs are widely deployed in home gateways, as well as in other locations likely to be used by telecommuters, such as hotels. The result is that IPsec-NAT incompatibilities have become a major barrier in the deployment of IPsec in one of its principal uses.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
       1.1.  Requirements Language. . . . . . . . . . . . . . . . . .  2
   2.  Known Incompatibilities between NA(P)T and IPsec . . . . . . .  3
       2.1.  Intrinsic NA(P)T Issues. . . . . . . . . . . . . . . . .  3
       2.2.  NA(P)T Implementation Weaknesses . . . . . . . . . . . .  7
       2.3.  Helper Incompatibilities . . . . . . . . . . . . . . . .  8
   3.  Requirements for IPsec-NAT Compatibility . . . . . . . . . . .  8
   4.  Existing Solutions . . . . . . . . . . . . . . . . . . . . . . 12
       4.1.  IPsec Tunnel Mode. . . . . . . . . . . . . . . . . . . . 12
       4.2.  RSIP . . . . . . . . . . . . . . . . . . . . . . . . . . 13
       4.3.  6to4 . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   5.  Security Considerations. . . . . . . . . . . . . . . . . . . . 14
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
       6.1.  Normative References . . . . . . . . . . . . . . . . . . 15
       6.2.  Informative References . . . . . . . . . . . . . . . . . 16
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   8.  Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 17
   9 . Full Copyright Statement . . . . . . . . . . . . . . . . . . . 18

1. Introduction

Perhaps the most common use of IPsec [RFC2401] is in providing virtual private networking (VPN) capabilities. One very popular use of VPNs is to provide telecommuter access to the corporate Intranet. Today, Network Address Translations (NATs) as described in [RFC3022] and [RFC2663], are widely deployed in home gateways, as well as in other locations likely to be used by telecommuters, such as hotels. The result is that IPsec-NAT incompatibilities have become a major barrier in the deployment of IPsec in one of its principal uses. This document describes known incompatibilities between NAT and IPsec, and describes the requirements for addressing them.

1.1. Requirements Language

In this document, the key words "MAY", "MUST, "MUST NOT", "optional", "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as described in [RFC2119].

Please note that the requirements specified in this document are to be used in evaluating protocol submissions. As such, the requirements language refers to capabilities of these protocols; the protocol documents will specify whether these features are required, recommended, or optional. For example, requiring that a protocol support confidentiality is not the same thing as requiring that all protocol traffic be encrypted.

A protocol submission is not compliant if it fails to satisfy one or more of the MUST or MUST NOT requirements for the capabilities that it implements. A protocol submission that satisfies all the MUST, MUST NOT, SHOULD, and SHOULD NOT requirements for its capabilities is said to be "unconditionally compliant"; one that satisfies all the MUST and MUST NOT requirements, but not all the SHOULD or SHOULD NOT requirements for its protocols is said to be "conditionally compliant."

2. Known Incompatibilities between NA(P)T and IPsec

The incompatibilities between NA(P)T and IPsec can be divided into three categories:

  1. Intrinsic NA(P)T issues. These incompatibilities derive directly from the NA(P)T functionality described in [RFC3022]. These incompatibilities will therefore be present in any NA(P)T device.
  1. NA(P)T implementation weaknesses. These incompatibilities are not intrinsic to NA(P)T, but are present in many NA(P)T implementations. Included in this category are problems in handling inbound or outbound fragments. Since these issues are not intrinsic to NA(P)T, they can, in principle, be addressed in future NA(P)T implementations. However, since the implementation problems appear to be wide spread, they need to be taken into account in a NA(P)T traversal solution.
  1. Helper issues. These incompatibilities are present in NA(P)T devices which attempt to provide for IPsec NA(P)T traversal. Ironically, this "helper" functionality creates further incompatibilities, making an already difficult problem harder to solve. While IPsec traversal "helper" functionality is not present in all NA(P)Ts, these features are becoming sufficiently popular that they also need to be taken into account in a NA(P)T traversal solution.

2.1. Intrinsic NA(P)T Issues

Incompatibilities that are intrinsic to NA(P)T include:

a) Incompatibility between IPsec AH [RFC2402] and NAT. Since the AH

header incorporates the IP source and destination addresses in the keyed message integrity check, NAT or reverse NAT devices making changes to address fields will invalidate the message integrity check. Since IPsec ESP [RFC2406] does not incorporate the IP source and destination addresses in its keyed message integrity check, this issue does not arise for ESP.

b) Incompatibility between checksums and NAT. TCP and UDP checksums

have a dependency on the IP source and destination addresses through inclusion of the "pseudo-header" in the calculation. As a result, where checksums are calculated and checked upon receipt, they will be invalidated by passage through a NAT or reverse NAT device.

As a result, IPsec Encapsulating Security Payload (ESP) will only pass through a NAT unimpeded if TCP/UDP protocols are not involved (as in IPsec tunnel mode or IPsec protected GRE), or checksums are not calculated (as is possible with IPv4 UDP). As described in [RFC793], TCP checksum calculation and verification is required in IPv4. UDP/TCP checksum calculation and verification is required in IPv6.

Stream Control Transmission Protocol (SCTP), as defined in [RFC2960] and [RFC3309], uses a CRC32C algorithm calculated only on the SCTP packet (common header + chunks), so that the IP header is not covered. As a result, NATs do not invalidate the SCTP CRC, and the problem does not arise.

Note that since transport mode IPsec traffic is integrity protected and authenticated using strong cryptography, modifications to the packet can be detected prior to checking UDP/TCP checksums. Thus, checksum verification only provides assurance against errors made in internal processing.

c) Incompatibility between IKE address identifiers and NAT. Where IP

addresses are used as identifiers in Internet Key Exchange Protocol (IKE) Phase 1 [RFC2409] or Phase 2, modification of the IP source or destination addresses by NATs or reverse NATs will result in a mismatch between the identifiers and the addresses in the IP header. As described in [RFC2409], IKE implementations are required to discard such packets.

In order to avoid use of IP addresses as IKE Phase 1 and Phase 2 identifiers, userIDs and FQDNs can be used instead. Where user authentication is desired, an ID type of ID_USER_FQDN can be used, as described in [RFC2407]. Where machine authentication is desired, an ID type of ID_FQDN can be used. In either case, it is necessary to verify that the proposed identifier is authenticated as a result of processing an end-entity certificate, if certificates are exchanged in Phase 1. While use of USER_FQDN or FQDN identity types is possible within IKE, there are usage scenarios (e.g. Security Policy Database (SPD) entries describing subnets) that cannot be accommodated this way.

Since the source address in the Phase 2 identifier is often used to form a full 5-tuple inbound SA selector, the destination address, protocol, source port and destination port can be used in the selector so as not to weaken inbound SA processing.

d) Incompatibility between fixed IKE source ports and NAPT. Where

multiple hosts behind the NAPT initiate IKE SAs to the same responder, a mechanism is needed to allow the NAPT to demultiplex the incoming IKE packets from the responder. This is typically accomplished by translating the IKE UDP source port on outbound packets from the initiator. Thus responders must be able to accept IKE traffic from a UDP source port other than 500, and must reply to that port. Care must be taken to avoid unpredictable behavior during re-keys. If the floated source port is not used as the destination port for the re-key, the NAT may not be able to send the re-key packets to the correct destination.

e) Incompatibilities between overlapping SPD entries and NAT. Where

initiating hosts behind a NAT use their source IP addresses in Phase 2 identifiers, they can negotiate overlapping SPD entries with the same responder IP address. The responder could then send packets down the wrong IPsec SA. This occurs because to the responder, the IPsec SAs appear to be equivalent, since they exist between the same endpoints and can be used to pass the same traffic.

f) Incompatibilities between IPsec SPI selection and NAT. Since

IPsec ESP traffic is encrypted and thus opaque to the NAT, the NAT must use elements of the IP and IPsec header to demultiplex incoming IPsec traffic. The combination of the destination IP address, security protocol (AH/ESP), and IPsec SPI is typically used for this purpose.

However, since the outgoing and incoming SPIs are chosen independently, there is no way for the NAT to determine what incoming SPI corresponds to what destination host merely by inspecting outgoing traffic. Thus, were two hosts behind the NAT to attempt to create IPsec SAs at the same destination simultaneously, it is possible that the NAT will deliver the incoming IPsec packets to the wrong destination.

Note that this is not an incompatibility with IPsec per se, but rather with the way it is typically implemented. With both AH and ESP, the receiving host specifies the SPI to use for a given SA, a choice which is significant only to the receiver. At present, the combination of Destination IP, SPI, and Security Protocol (AH, ESP) uniquely identifies a Security Association. Also, SPI values in the range 1-255 are reserved to IANA and may be used in the 

      future.  This means that, when negotiating with the same external
      host or gateway, the internal hosts behind the same NAPT can
      select the same SPI value, such that one host inbound SA is
        (SPI=470, Internal Dest IP=192.168.0.4)
      and a different host inbound SA is
        (SPI=470, Internal Dest IP=192.168.0.5).
      The receiving NAPT will not be able to determine which internal
      host an inbound IPsec packet with SPI=470 should be forwarded to.

It is also possible for the receiving host to allocate a unique SPI to each unicast Security Association. In this case, the Destination IP Address need only be checked to see if it is "any valid unicast IP for this host", not checked to see if it is the specific Destination IP address used by the sending host. Using this technique, the NA(P)T can be assured of a low but non-zero chance of forwarding packets to the wrong internal host, even when two or more hosts establish SAs with the same external host.

This approach is completely backwards compatible, and only requires the particular receiving host to make a change to its SPI allocation and IPsec_esp_input() code. However, NA(P)T devices may not be able to detect this behavior without problems associated with parsing IKE payloads. And a host may still be required to use a SPI in the IANA reserved range for the assigned purpose.

g) Incompatibilities between embedded IP addresses and NAT. Since

the payload is integrity protected, any IP addresses enclosed within IPsec packets will not be translatable by a NAT. This renders ineffective Application Layer Gateways (ALGs) implemented within NATs. Protocols that utilize embedded IP addresses include FTP, IRC, SNMP, LDAP, H.323, SIP, SCTP (optionally), and many games. To address this issue, it is necessary to install ALGs on the host or security gateway that can operate on application traffic prior to IPsec encapsulation and after IPsec decapsulation. 

   h) Implicit directionality of NA(P)T.  NA(P)Ts often require an
   
      initial outbound packet to flow through them in order to create an
      inbound mapping state.  Directionality prohibits unsolicited
      establishment of IPsec SAs to hosts behind the NA(P)T.

i) Inbound SA selector verification. Assuming IKE negotiates phase 2

selectors, inbound SA processing will drop the decapsulated packet, since [RFC2401] requires a packet's source address match the SA selector value, which NA(P)T processing of an ESP packet would change.

2.2. NA(P)T Implementation Weaknesses

Implementation problems present in many NA(P)Ts include: 

   j) Inability to handle non-UDP/TCP traffic.  Some NA(P)Ts discard
      non-UDP/TCP traffic or perform address-only translation when only
      one host is behind the NAT.  Such NAPTs are unable to enable SCTP,
      ESP (protocol 50), or AH (protocol 51) traffic.

k) NAT mapping timeouts. NA(P)Ts vary in the time for which a UDP

mapping will be maintained in the absence of traffic. Thus, even where IKE packets can be correctly translated, the translation state may be removed prematurely.

l) Inability to handle outgoing fragments. Most NA(P)Ts can properly

fragment outgoing IP packets in the case where the IP packet size exceeds the MTU on the outgoing interface. However, proper translation of outgoing packets that are already fragmented is difficult and most NAPTs do not handle this correctly. As noted in Section 6.3 of [RFC3022], where two hosts originate fragmented packets to the same destination, the fragment identifiers can overlap. Since the destination host relies on the fragmentation identifier and fragment offset for reassembly, the result will be data corruption. Few NA(P)Ts protect against identifier collisions by supporting identifier translation. Identifier collisions are not an issue when NATs perform the fragmentation, since the fragment identifier need only be unique within a source/destination IP address pair.

Since a fragment can be as small as 68 octets [RFC791], there is no guarantee that the first fragment will contain a complete TCP header. Thus, a NA(P)T looking to recalculate the TCP checksum may need to modify a subsequent fragment. Since fragments can be reordered, and IP addresses can be embedded and possibly even split between fragments, the NA(P)T will need to perform reassembly prior to completing the translation. Few NA(P)Ts support this.

m) Inability to handle incoming fragments. Since only the first

fragment will typically contain a complete IP/UDP/SCTP/TCP header, NAPTs need to be able to perform the translation based on the source/dest IP address and fragment identifier alone. Since fragments can be reordered, the headers to a given fragment identifier may not be known if a subsequent fragment arrives prior to the initial one, and the headers may be split between fragments. As a result, the NAPT may need to perform reassembly prior to completing the translation. Few NAPTs support this. Note that with NAT, the source/dest IP address is enough to determine the translation so that this does not arise. However, it is possible for the IPsec or IKE headers to be split between fragments, so that reassembly may still be required.

2.3. Helper Incompatibilities

Incompatibilities between IPsec and NAT "helper" functionality include:

n) Internet Security Association and Key Management Protocol (ISAKMP)

header inspection. Today some NAT implementations attempt to use IKE cookies to de-multiplex incoming IKE traffic. As with source-port de-multiplexing, IKE cookie de-multiplexing results in problems with re-keying, since Phase 1 re-keys typically will not use the same cookies as the earlier traffic.

o) Special treatment of port 500. Since some IKE implementations are

unable to handle non-500 UDP source ports, some NATs do not translate packets with a UDP source port of 500. This means that these NATs are limited to one IPsec client per destination gateway, unless they inspect details of the ISAKMP header to examine cookies which creates the problem noted above.

p) ISAKMP payload inspection. NA(P)T implementations that attempt to

parse ISAKMP payloads may not handle all payload ordering combinations, or support vendor_id payloads for IKE option negotiation.

3. Requirements for IPsec-NAT Compatibility

The goal of an IPsec-NAT compatibility solution is to expand the range of usable IPsec functionality beyond that available in the NAT-compatible IPsec tunnel mode solution described in Section 2.3.

In evaluating a solution to IPsec-NAT incompatibility, the following criteria should be kept in mind: 

   Deployment

Since IPv6 will address the address scarcity issues that frequently lead to use of NA(P)Ts with IPv4, the IPsec-NAT compatibility issue is a transitional problem that needs to be solved in the time frame prior to widespread deployment of IPv6. Therefore, to be useful, an IPsec-NAT compatibility solution MUST be deployable on a shorter time scale than IPv6.

Since IPv6 deployment requires changes to routers as well as hosts, a potential IPsec-NAT compatibility solution, which requires changes to both routers and hosts, will be deployable on approximately the same time scale as IPv6. Thus, an IPsec-NAT compatibility solution SHOULD require changes only to hosts, and not to routers.

Among other things, this implies that communication between the host and the NA(P)T SHOULD NOT be required by an IPsec-NAT compatibility solution, since that would require changes to the NA(P)Ts, and interoperability testing between the host and NA(P)T implementations. In order to enable deployment in the short term, it is necessary for the solution to work with existing router and NA(P)T products within the deployed infrastructure. 

   Protocol Compatibility

An IPsec NAT traversal solution is not expected to resolve issues with protocols that cannot traverse NA(P)T when unsecured with IPsec. Therefore, ALGs may still be needed for some protocols, even when an IPsec NAT traversal solution is available. 

   Security

Since NA(P)T directionality serves a security function, IPsec NA(P)T traversal solutions should not allow arbitrary incoming IPsec or IKE traffic from any IP address to be received by a host behind the NA(P)T, although mapping state should be maintained once bidirectional IKE and IPsec communication is established. 

   Telecommuter Scenario

Since one of the primary uses of IPsec is remote access to corporate Intranets, a NA(P)T traversal solution MUST support NA(P)T traversal, via either IPsec tunnel mode or L2TP over IPsec transport mode [RFC3193]. This includes support for traversal of more than one NA(P)T between the remote client and the VPN gateway.

The client may have a routable address and the VPN gateway may be behind at least one NA(P)T, or alternatively, both the client and the VPN gateway may be behind one or more NA(P)Ts. Telecommuters may use the same private IP address, each behind their own NA(P)T, or many telecommuters may reside on a private network behind the same NA(P)T, each with their own unique private address, connecting to the same VPN gateway. Since IKE uses UDP port 500 as the destination, it is not necessary to enable multiple VPN gateways operating behind the same external IP address. 

   Gateway-to-Gateway Scenario

In a gateway-gateway scenario, a privately addressed network (DMZ) may be inserted between the corporate network and the Internet. In this design, IPsec security gateways connecting portions of the corporate network may be resident in the DMZ and have private addresses on their external (DMZ) interfaces. A NA(P)T connects the DMZ network to the Internet. 

   End-to-End Scenario

A NAT-IPsec solution MUST enable secure host-host TCP/IP communication via IPsec, as well as host-gateway communications. A host on a private network MUST be able to bring up one or multiple IPsec-protected TCP connections or UDP sessions to another host with one or more NA(P)Ts between them. For example, NA(P)Ts may be deployed within branch offices connecting to the corporate network, with an additional NA(P)T connecting the corporate network to the Internet. Likewise, NA(P)Ts may be deployed within a corporate network LAN or WAN to connect wireless or remote location clients to the corporate network. This may require special processing of TCP and UDP traffic on the host.

Bringing up SCTP connections to another host with one or more NA(P)Ts between them may present special challenges. SCTP supports multi- homing. If more than one IP address is used, these addresses are transported as part of the SCTP packet during the association setup (in the INIT and INIT-ACK chunks). If only single homed SCTP end- points are used, [RFC2960] section 3.3.2.1 states:

Note that not using any IP address parameters in the INIT and INIT-ACK is an alternative to make an association more likely to work across a NAT box.

This implies that IP addresses should not be put into the SCTP packet unless necessary. If NATs are present and IP addresses are included, then association setup will fail. Recently [AddIP] has been proposed which allows the modification of the IP address once an association is established. The modification messages have also IP addresses in the SCTP packet, and so will be adversely affected by NATs. 

   Firewall Compatibility

Since firewalls are widely deployed, a NAT-IPsec compatibility solution MUST enable a firewall administrator to create simple, static access rule(s) to permit or deny IKE and IPsec NA(P)T traversal traffic. This implies, for example, that dynamic allocation of IKE or IPsec destination ports is to be avoided. 

   Scaling

An IPsec-NAT compatibility solution should be capable of being deployed within an installation consisting of thousands of telecommuters. In this situation, it is not possible to assume that only a single host is communicating with a given destination at a time. Thus, an IPsec-NAT compatibility solution MUST address the issue of overlapping SPD entries and de-multiplexing of incoming packets. 

   Mode Support

At a minimum, an IPsec-NAT compatibility solution MUST support traversal of the IKE and IPsec modes required for support within [RFC2409] and [RFC2401]. For example, an IPsec gateway MUST support ESP tunnel mode NA(P)T traversal, and an IPsec host MUST support IPsec transport mode NA(P)T traversal. The purpose of AH is to protect immutable fields within the IP header (including addresses), and NA(P)T translates addresses, invalidating the AH integrity check. As a result, NA(P)T and AH are fundamentally incompatible and there is no requirement that an IPsec-NAT compatibility solution support AH transport or tunnel mode.

Backward Compatibility and Interoperability

An IPsec-NAT compatibility solution MUST be interoperable with existing IKE/IPsec implementations, so that they can communicate where no NA(P)T is present. This implies that an IPsec-NAT compatibility solution MUST be backwards-compatible with IPsec as defined in [RFC2401] and IKE as defined in [RFC2409]. In addition, it SHOULD be able to detect the presence of a NA(P)T, so that NA(P)T traversal support is only used when necessary. This implies that it MUST be possible to determine that an existing IKE implementation does not support NA(P)T traversal, so that a standard IKE conversation can occur, as described in [RFC2407], [RFC2408], and [RFC2409]. Note that while this implies initiation of IKE to port 500, there is no requirement for a specific source port, so that UDP source port 500 may or may not be used. 

   Security

An IPsec-NAT compatibility solution MUST NOT introduce additional IKE or IPsec security vulnerabilities. For example, an acceptable solution must demonstrate that it introduces no new denial of service or spoofing vulnerabilities. IKE MUST be allowed to re- key in a bi-directional manner as described in [RFC2408].

4. Existing Solutions

4.1. IPsec Tunnel Mode

In a limited set of circumstances, it is possible for an IPsec tunnel mode implementation, such as that described in [DHCP], to traverse NA(P)T successfully. However, the requirements for successful traversal are sufficiently limited so that a more general solution is needed:

  1. IPsec ESP. IPsec ESP tunnels do not cover the outer IP header within the message integrity check, and so will not suffer Authentication Data invalidation due to address translation. IPsec tunnels also need not be concerned about checksum invalidation.
  1. No address validation. Most current IPsec tunnel mode implementations do not perform source address validation so that incompatibilities between IKE identifiers and source addresses will not be detected. This introduces security vulnerabilities as described in Section 5.
  1. "Any to Any" SPD entries. IPsec tunnel mode clients can negotiate "any to any" SPDs, which are not invalidated by address translation. This effectively precludes use of SPDs for the filtering of allowed tunnel traffic.
  1. Single client operation. With only a single client behind a NAT, there is no risk of overlapping SPDs. Since the NAT will not need to arbitrate between competing clients, there is also no risk of re-key mis-translation, or improper incoming SPI or cookie de-multiplexing.
  1. No fragmentation. When certificate authentication is used, IKE fragmentation can be encountered. This can occur when certificate chains are used, or even when exchanging a single certificate if the key size, or the size of other certificate fields (such as the distinguished name and other extensions), is large enough. However, when pre-shared keys are used for authentication, fragmentation is less likely.
  1. Active sessions. Most VPN sessions typically maintain ongoing traffic flow during their lifetime so that UDP port mappings are less likely be removed due to inactivity.

4.2. RSIP

RSIP, described in [RSIP] and [RSIPFrame], includes mechanisms for IPsec traversal, as described in [RSIPsec]. By enabling host-NA(P)T communication, RSIP addresses issues of IPsec SPI de-multiplexing, as well as SPD overlap. It is thus suitable for use in enterprises, as well as home networking scenarios. By enabling hosts behind a NAT to share the external IP address of the NA(P)T (the RSIP gateway), this approach is compatible with protocols including embedded IP addresses.

By tunneling IKE and IPsec packets, RSIP avoids changes to the IKE and IPsec protocols, although major changes are required to host IKE and IPsec implementations to retrofit them for RSIP-compatibility. It is thus compatible with all existing protocols (AH/ESP) and modes (transport and tunnel).

In order to handle de-multiplexing of IKE re-keys, RSIP requires floating of the IKE source port, as well as re-keying to the floated port. As a result, interoperability with existing IPsec implementations is not assured.

RSIP does not satisfy the deployment requirements for an IPsec-NAT compatibility solution because an RSIP-enabled host requires a corresponding RSIP-enabled gateway in order to establish an IPsec SA with another host. Since RSIP requires changes only to clients and routers and not to servers, it is less difficult to deploy than IPv6. However, for vendors, implementation of RSIP requires a substantial fraction of the resources required for IPv6 support. Thus, RSIP solves a "transitional" problem on a long-term time scale, which is not useful.

4.3. 6to4

6to4, as described in [RFC3056] can form the basis for an IPsec-NAT traversal solution. In this approach, the NAT provides IPv6 hosts with an IPv6 prefix derived from the NAT external IPv4 address, and encapsulates IPv6 packets in IPv4 for transmission to other 6to4 hosts or 6to4 relays. This enables an IPv6 host using IPsec to communicate freely to other hosts within the IPv6 or 6to4 clouds.

While 6to4 is an elegant and robust solution where a single NA(P)T separates a client and VPN gateway, it is not universally applicable. Since 6to4 requires the assignment of a routable IPv4 address to the NA(P)T in order to allow formation of an IPv6 prefix, it is not usable where multiple NA(P)Ts exist between the client and VPN gateway. For example, an NA(P)T with a private address on its external interface cannot be used by clients behind it to obtain an IPv6 prefix via 6to4.

While 6to4 requires little additional support from hosts that already support IPv6, it does require changes to NATs, which need to be upgraded to support 6to4. As a result, 6to4 may not be suitable for deployment in the short term.

5. Security Considerations

By definition, IPsec-NAT compatibility requires that hosts and routers implementing IPsec be capable of securely processing packets whose IP headers are not cryptographically protected. A number of issues arise from this that are worth discussing.

Since IPsec AH cannot pass through a NAT, one of the side effects of providing an IPsec-NAT compatibility solution may be for IPsec ESP with null encryption to be used in place of AH where a NAT exists between the source and destination. However, it should be noted that ESP with null encryption does not provide the same security properties as AH. For example, there are security risks relating to IPv6 source routing that are precluded by AH, but not by ESP with null encryption.

In addition, since ESP with any transform does not protect against source address spoofing, some sort of source IP address sanity checking needs to be performed. The importance of the anti-spoofing check is not widely understood. There is normally an anti-spoofing check on the Source IP Address as part of IPsec_{esp,ah}_input(). This ensures that the packet originates from the same address as that claimed within the original IKE Phase 1 and Phase 2 security associations. When a receiving host is behind a NAT, this check might not strictly be meaningful for unicast sessions, whereas in the Global Internet this check is important for tunnel-mode unicast sessions to prevent a spoofing attack described in [AuthSource], which can occur when access controls on the receiver depend upon the source IP address of verified ESP packets after decapsulation. IPsec-NAT compatibility schemes should provide anti-spoofing protection if it uses source addresses for access controls.

Let us consider two hosts, A and C, both behind (different) NATs, who negotiate IPsec tunnel mode SAs to router B. Hosts A and C may have different privileges; for example, host A might belong to an employee trusted to access much of the corporate Intranet, while C might be a contractor only authorized to access a specific web site.

If host C sends a tunnel mode packet spoofing A's IP address as the source, it is important that this packet not be accorded the privileges corresponding to A. If authentication and integrity checking is performed, but no anti-spoofing check (verifying that the originating IP address corresponds to the SPI) then host C may be allowed to reach parts of the network that are off limits. As a result, an IPsec-NAT compatibility scheme MUST provide some degree of anti-spoofing protection.

6. References

6.1. Normative References

   [RFC791]     Postel, J., "Internet Protocol", STD 5, RFC 791,
                September 1981.
   
   [RFC793]     Postel, J., "Transmission Control Protocol", STD 7, RFC
                793, September 1981.
   
   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC2401]    Atkinson, R. and S. Kent, "Security Architecture for the
                Internet Protocol", RFC 2401, November 1998.
   
   [RFC2402]    Kent, S. and R. Atkinson, "IP Authentication Header",
                RFC 2402, November 1998.
   
   [RFC2406]    Kent,S. and R. Atkinson, "IP Encapsulating Security
                Payload (ESP)", RFC 2406, November 1998.
   
   [RFC2407]    Piper, D., "The Internet IP Security Domain of
                Interpretation for ISAKMP", RFC 2407, November 1998.
   
   [RFC2409]    Harkins, D. and D. Carrel, "The Internet Key Exchange
                (IKE)", RFC 2409, November 1998.
   
   [RFC2663]    Srisuresh, P. and M. Holdredge, "IP Network Address
                Translator (NAT) Terminology and Considerations", RFC
                2663, August 1999.
   
   [RFC3022]    Srisuresh, P. and K. Egevang, "Traditional IP Network
                Address Translator (Traditional NAT)", RFC 3022, January
                2001.

6.2. Informative References

   [RFC2408]    Maughan, D., Schertler, M., Schneider, M. and J. Turner,
                "Internet Security Association and Key Management
                Protocol (ISAKMP)", RFC 2408, November 1998.
   
   [RFC2960]    Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
                Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
                Zhang, M. and V. Paxson, "Stream Control Transmission
                Protocol", RFC 2960, October 2000.
   
   [RFC3056]    Carpenter, B. and K. Moore, "Connection of IPv6 Domains
                via IPv4 Clouds", RFC 3056, February 2001.
   
   [RFC3193]    Patel, B., Aboba, B., Dixon, W., Zorn, G. and S. Booth,
                "Securing L2TP using IPsec", RFC 3193, November 2001.
   
   [RFC3309]    Stone, J., Stewart, R. and D. Otis, "Stream Control
                Transmission Protocol (SCTP) Checksum Change", RFC 3309,
                September 2002.
   
   [RSIPFrame]  Borella, M., Lo, J., Grabelsky, D. and G. Montenegro,
                "Realm Specific IP: Framework", RFC 3102, October 2001.
   
   [RSIP]       Borella, M., Grabelsky, D., Lo, J. and K. Taniguchi,
                "Realm Specific IP: Protocol Specification", RFC 3103,
                October 2001.
   
   [RSIPsec]    Montenegro, G. and M. Borella, "RSIP Support for End-
                to-End IPsec", RFC 3104, October 2001.
   
   [DHCP]       Patel, B., Aboba, B., Kelly, S. and V. Gupta, "Dynamic
                Host Configuration Protocol (DHCPv4) Configuration of
                IPsec Tunnel Mode", RFC 3456, January 2003.
   
   [AuthSource] Kent, S., "Authenticated Source Addresses", IPsec WG
                Archive (ftp://ftp.ans.net/pub/archive/IPsec), Message-
                Id:  <v02130517ad121773c8ed@[128.89.0.110]>, January 5,
                1996.
   
   [AddIP]      Stewart, R., et al., "Stream Control Transmission
                Protocol (SCTP) Dynamic Address Reconfiguration", Work
                in Progress.

7. Acknowledgments

Thanks to Steve Bellovin of AT&T Research, Michael Tuexen of Siemens, Peter Ford of Microsoft, Ran Atkinson of Extreme Networks, and Daniel Senie for useful discussions of this problem space.

8. Authors' Addresses

Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052 

   Phone: +1 425 706 6605
   Fax:   +1 425 936 7329
   EMail: bernarda@microsoft.com

William Dixon
V6 Security, Inc.
601 Union Square, Suite #4200-300
Seattle, WA 98101 

   EMail: ietf-wd@v6security.com

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