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RFC 2709

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Security Model with Tunnel-mode IPsec for NAT Domains


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Network Working Group                                       P. Srisuresh
Request for Comments: 2709                           Lucent Technologies
Category: Informational                                     October 1999

         Security Model with Tunnel-mode IPsec for NAT Domains

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.


   There are a variety of NAT flavors, as described in [Ref 1]. Of the
   domains supported by NATs, only Realm-Specific IP clients are able to
   pursue end-to-end IPsec secure sessions. However, all flavors of NAT
   are capable of offering tunnel-mode IPsec security to private domain
   hosts peering with nodes in external realm. This document describes a
   security model by which tunnel-mode IPsec security can be architected
   on NAT devices. A section is devoted to describing how security
   policies may be transparently communicated to IKE (for automated KEY
   exchange) during Quick Mode. Also outlined are applications that can
   benefit from the Security Model described.

1. Introduction and Overview

   NAT devices provide transparent routing to end hosts trying to
   communicate from disparate address realms, by modifying IP and
   transport headers en-route. This solution works best when the end
   user identifier (such as host name) is different from the address
   used to locate end user.

   End-to-end application level payload security can be provided for
   applications that do not embed realm-specific information in payloads
   that is meaningless to one of the end-users. Applications that do
   embed realm-specific information in payload will require an
   application level gateway (ALG) to make the payload meaningful in
   both realms. However, applications that require assistance of an ALG
   en-route cannot pursue end-to-end application level security.

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   All applications traversing a NAT device, irrespective of whether
   they require assistance of an ALG or not, can benefit from IPsec
   tunnel-mode security, when NAT device acts as the IPsec tunnel end

   Section 2 below defines terms specific to this document.

   Section 3 describes how tunnel mode IPsec security can be recognized
   on NAT devices. IPsec Security architecture, format and operation of
   various types of security mechanisms may be found in [Ref 2], [Ref 3]
   and [Ref 4].  This section does not address how session keys and
   policies are exchanged between a NAT device acting as IPsec gateway
   and external peering nodes. The exchange could have taken place
   manually or using any of known automatic exchange techniques.

   Section 4 assumes that Public Key based IKE protocol [Ref 5] may be
   used to automate exchange of security policies, session keys and
   other Security Association (SA) attributes. This section describes a
   method by which security policies administered for a private domain
   may be translated for communicating with external nodes. Detailed
   description of IKE protocol operation may be found in [Ref 5] and
   [Ref 6].

   Section 5 describes applications of the security model described in
   the document. Applications listed include secure external realm
   connectivity for private domain hosts and secure remote access to
   enterprise mobile hosts.

2. Terminology

   Definitions for majority of terms used in this document may be found
   in one of (a) NAT Terminology and Considerations document [Ref 1],
   (b) IP security Architecture document [Ref 2], or (c) Internet Key
   Enchange (IKE) document [Ref 5]. Below are terms defined specifically
   for this document.

2.1. Normal-NAT

   The term "Normal-NAT" is introduced to distinguish normal NAT
   processing from the NAT processing used for secure packets embedded
   within an IPsec secure tunnel. "Normal-NAT" is the normal NAT
   processing as described in [Ref 1].

2.2. IPsec Policy Controlled NAT (IPC-NAT)

   The term "IPsec Policy Controlled NAT" (IPC-NAT, for short) is
   defined to describe the NAT transformation applied as an extension of
   IPsec transformation to packets embedded within an IP-IP tunnel, for

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   which the NAT node is a tunnel end point. IPC-NAT function is
   essentially an adaptation of NAT extensions to embedded packets of
   tunnel-mode IPsec. Packets subject to IPC-NAT processing are
   beneficiaries of IPsec security between the NAT device and an
   external peer entity, be it a host or a gateway node.

   IPsec policies place restrictions on what NAT mappings are used.  For
   example, IPsec access control security policies to a peer gateway
   will likely restrict communication to only certain addresses and/or
   port numbers. Thus, when NAT performs translations, it must insure
   that the translations it performs are consist with the security

   Just as with Normal-NAT, IPC-NAT function can assume any of NAT
   flavors, including Traditional-NAT, Bi-directional-NAT and Twice-NAT.
   An IPC-NAT device would support both IPC-NAT and normal-NAT

3. Security model of IPC-NAT

   The IP security architecture document [Ref 2] describes how IP
   network level security may be accomplished within a globally unique
   address realm. Transport and tunnel mode security are discussed. For
   purposes of this document, we will assume IPsec security to mean
   tunnel mode IPsec security, unless specified otherwise. Elements
   fundamental to this security architecture are (a) Security Policies,
   that determine which packets are permitted to be subject to Security
   processing, and (b) Security Association Attributes that identify the
   parameters for security processing, including IPsec protocols,
   algorithms and session keys to be applied.

   Operation of tunnel mode IPsec security on a device that does not
   support Network Address Translation may be described as below in
   figures 1 and 2.

            +---------------+  No  +---------------------------+
            |               | +--->|Forward packet in the Clear|
   Outgoing |Does the packet| |    |Or Drop, as appropriate.   |
   -------->|match Outbound |-|    +---------------------------+
   Packet   |Security       | |    +-------------+
            |Policies?      | |Yes |Perform      | Forward
            |               | +--->|Outbound     |--------->
            +---------------+      |Security     | IPsec Pkt
                                   |(Tunnel Mode)|

   Figure 1. Operation of Tunnel-Mode IPsec on outgoing packets.

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   IPsec packet +----------+          +----------+
   destined to  |Perform   | Embedded |Does the  | No(Drop)
   ------------>|Inbound   |--------->|Pkt match |-------->
   the device   |Security  | Packet   |Inbound SA| Yes(Forward)
                |(Detunnel)|          |Policies? |
                +----------+          +----------+

   Figure 2. Operation of Tunnel-Mode IPsec on Incoming packets

   A NAT device that provides tunnel-mode IPsec security would be
   required to administer security policies based on private realm
   addressing. Further, the security policies determine the IPsec tunnel
   end-point peer. As a result, a packet may be required to undergo
   different type of NAT translation depending upon the tunnel end-point
   the IPsec node peers with. In other words, IPC-NAT will need a unique
   set of NAT maps for each security policy configured. IPC-NAT will
   perform address translation in conjunction with IPsec processing
   differently with each peer, based on security policies.  The
   following diagrams (figure 3 and figure 4) illustrate the operation
   of IPsec tunneling in conjunction with NAT. Operation of an IPC-NAT
   device may be distinguished from that of an IPsec gateway that does
   not support NAT as follows.

        (1) IPC-NAT device has security policies administered using
            private realm addressing. A traditional IPsec gateway will
            have its security policies administered using a single realm
            (say, external realm) addressing.

        (2) Elements fundamental to the security model of an IPC-NAT
            device includes IPC-NAT address mapping  (and other NAT
            parameter definitions) in conjunction with Security policies
            and SA attributes. Fundamental elements of a traditional
            IPsec gateway are limited only to Security policies and SA

            +---------------+      +-------------------------+
            |               |  No  | Apply Normal-NAT or Drop|
   Outgoing |Does the packet| +--->| as appropriate          |
   -------->|match Outbound |-|    +-------------------------+
   Packet   |Security       | |    +---------+  +-------------+
   (Private |Policies?      | |Yes |Perform  |  |Perform      |Forward
    Domain) |               | +--->|Outbound |->|Outbound     |-------->
            +---------------+      |NAT      |  |Security     |IPsec Pkt
                                   |(IPC-NAT)|  |(Tunnel mode)|
                                   +---------+  +-------------+

   Figure 3. Tunnel-Mode IPsec on an IPC-NAT device for outgoing pkts

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   IPsec Pkt +----------+          +---------+  +----------+
   destined  |Perform   | Embedded |Perform  |  |Does the  |No(Drop)
   --------->|Inbound   |--------->|Inbound  |->|Pkt match |-------->
   to device |Security  | Packet   |NAT      |  |Inbound SA|Yes(Forward)
   (External |(Detunnel)|          |(IPC-NAT)|  |Policies? |
    Domain)  +----------+          +---------+  +----------+

   Figure 4. Tunnel-Mode IPsec on an IPC-NAT device for Incoming pkts

   Traditional NAT is session oriented, allowing outbound-only sessions
   to be translated. All other flavors of NAT are Bi-directional.  Any
   and all flavors of NAT mapping may be used in conjunction with the
   security policies and secure processing on an IPC-NAT device. For
   illustration purposes in this document, we will assume tunnel mode
   IPsec on a Bi-directional NAT device.

   Notice however that a NAT device capable of providing security across
   IPsec tunnels can continue to support Normal-NAT for packets that do
   not require IPC-NAT. Address mapping and other NAT parameter
   definitions for Normal-NAT and IPC-NAT are distinct. Figure 3
   identifies how a NAT device distinguishes between outgoing packets
   that need to be processed through Normal-NAT vs. IPC-NAT. As for
   packets incoming from external realm, figure 4 outlines packets that
   may be subject to IPC-NAT. All other packets are subject to Normal-
   NAT processing only.

4. Operation of IKE protocol on IPC-NAT device.

   IPC-NAT operation described in the previous section can be
   accomplished based on manual session key exchange or using an
   automated key Exchange protocol between peering entities. In this
   section, we will consider adapting IETF recommended Internet Key
   Exchange (IKE) protocol on a IPC-NAT device for automatic exchange of
   security policies and SA parameters. In other words, we will focus on
   the operation of IKE in conjunction with tunnel mode IPsec on NAT
   devices. For the reminder of this section, we will refer NAT device
   to mean IPC-NAT device, unless specified otherwise.

   IKE is based on UDP protocol and uses public-key encryption to
   exchange session keys and other attributes securely across an address
   realm. The detailed protocol and operation of IKE in the context of
   IP may be found in [Ref 3] and [Ref 4]. Essentially, IKE has 2

   In the first phase, IKE peers operate in main or aggressive mode to
   authenticate each other and set up a secure channel between
   themselves. A NAT device  has an interface to the external realm and
   is no different from any other node in the realm to negotiate phase I

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   with peer external nodes. The NAT device may assume any of the valid
   Identity types and authentication methodologies necessary to
   communicate and authenticate with peers in the realm. The NAT device
   may also interface with a Certification Authority (CA) in the realm
   to retrieve certificates  and perform signature validation.

   In the second phase, IKE peers operate in Quick Mode to exchange
   policies and IPsec security proposals to negotiate and agree upon
   security transformation algorithms, policies, keys, lifetime and
   other security attributes. During this phase, IKE process must
   communicate with IPsec Engine to (a) collect secure session
   attributes and other parameters  to negotiate with peer IKE nodes,
   and to (b) notify security parameters agreed upon (with peer) during
   the negotiation.

   An IPC-NAT device, operating as IPsec gateway, has the security
   policies administered based on private realm addressing. An ALG will
   be required to translate policies from private realm addressing into
   external addressing, as the IKE process needs to communicate these
   policies to peers in external realm. Note, IKE datagrams are not
   subject to any NAT processing. IKE-ALG simply translates select
   portions of IKE payload as per the NAT map defined for the policy
   match. The following diagram illustrates how an IKE-ALG process
   interfaces with IPC-NAT to take the security policies and IPC-NAT
   maps and generates security policies that IKE could communicate
   during quick mode to peers in the external realm.

   Policies in quick mode are exchanged with a peer as a combination of
   IDci and IDcr payloads. The combination of IDs (policies) exchanged
   by each peer must match in order for the SA parameters on either end
   to be applied uniformly. If the IDs are not exchanged, the assumption
   would be that the Quick mode negotiated SA parameters are applicable
   between the IP addresses assumed by the main mode.

   Depending on the nature of security policies in place(ex: end-to-end
   sessions between a pair of nodes vs. sessions with an address range),
   IKE-ALG may need to request NAT to set up address bindings and/or
   transport bindings for the lifetime (in seconds or Kilo-Bytes) the
   sessions are negotiated. In the case the ALG is unable to setup the
   necessary address bindings or transport bindings, IKE-ALG will not be
   able to translate security policies and that will result in IKE not
   pursuing phase II negotiation for the effected policies.

   When the Negotiation is complete and successful, IKE will communicate
   the negotiated security parameters directly to the IPC-NAT gateway
   engine as described in the following diagram.

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                                        |         |
        Negotiated Security Parameters  |  IKE    |
       +--------------------------------| Process |
       |(including session Keys)        |         |
       |                                +---------+
       |                                   ^   ^
       |                         Translated|   |
       |                             Secure|   |Security
       |                           Policies|   |Proposals
       v                                   |   |
   +---------+ Security Policies, based +---------+
   |         |------------------------->|         |
   |         | on Pvt. realm addressing |         |
   | IPC-NAT |                          |         |
   | (IPsec  | IPC-NAT MAPs             | IKE-ALG |
   | Gateway)|------------------------->|         |
   |         |                          |         |
   |         | Security Proposals       |         |
   |         |------------------------->|         |
   |         |                          |         |
   |         |  NAT Control exchange    |         |
   |         |<------------------------>|         |
   +---------+                          +---------+

   Figure 5. IKE-ALG translates Security policies, using NAT Maps.

5. Applications of IPC-NAT security model

   IPC-NAT operational model described thus far illustrates how a NAT
   device can be used as an IPsec tunnel end point to provide secure
   transfer of data in external realm. This section will attempt to
   illustrate two applications of such a model.

5.1. Secure Extranet Connectivity

   IPC-NAT Model has a direct application of being able to provide clear
   as well as secure connectivity with external realm using a NAT
   device. In particular, IPC-NAT device at the border of a private
   realm can peer with an IPsec gateway of an external domain to secure
   the Extranet connection. Extranet refers to the portion of the path
   that crosses the Internet between peering gateway nodes.

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5.2. Secure Remote Access to Mobile Users of an Enterprise

   Say, a node from an enterprise moves out of the enterprise, and
   attempts to connect to the enterprise from remote site, using a
   temporary service provider assigned address (Care-of-Address). In
   such a case, the mobile user could setup an IPsec tunnel session with
   the corporate IPC-NAT device using a user-ID and authentication
   mechanism that is agreed upon. Further, the user may be configured
   with enterprise DNS server, as an extension of authentication
   following IKE Phase I. This would allow the user to access enterprise
   resources by name.

   However, many enterprise servers and applications rely on source IP
   address for authentication and deny access for packets that do not
   originate from the enterprise address space. In these scenarios,
   IPC-NAT has the ability (unlike a traditional IPsec gateway) to
   perform Network Address Translation (NAT) for remote access users, so
   their temporary address in external realm is translated into a
   enterprise domain address, while the packets are within private
   realm. The flavor of IPC-NAT performed would be traditional NAT
   (i.e., assuming mobile-user address space to be private realm and
   Enterprise address space to be external realm), which can either be
   Basic NAT (using a block of enterprise addresses for translation) or
   NAPT(using a single enterprise address for translation).

   The secure remote access application described is pertinent to all
   enterprises, irrespective of whether an enterprise uses IANA
   registered addresses or not.

   The secure remote access application described is different from
   Mobile-IP in that, the mobile node (described in this application)
   does not retain the Home-Network address and simply uses the Care-
   Of-address for communication purposes. It is conceivable for the
   IPC-NAT Gateway to transparently provide Mobile-IP type connectivity
   to the Mobile node by binding the mobile node's Care-of-Address with
   its Home Address. Provision of such an address mapping to IPC-NAT
   gateway, however, is not within the scope of this document.

6. Security Considerations

   If NATs and ALGs are not in a trusted boundary, that is a major
   security problem, as ALGs snoop end user traffic payload.
   Application level payload could be encrypted end-to-end, so long as
   the payload does not contain IP addresses and/or transport
   identifiers that are valid in only one of the realms. With the
   exception of Realm-Specific IP, end-to-end IP network level security
   assured by current IPsec techniques is not attainable with NATs in
   between. The IPC-NAT model described in this document outlines an

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   approach by which network level security may be obtained within
   external realm.

   NATs, when combined with ALGs, can ensure that the datagrams injected
   into Internet have no private addresses in headers or payload.
   Applications that do not meet these requirements may be dropped using
   firewall filters. For this reason, it is not uncommon to find that
   IPC-NATs, ALGs and firewalls co-exist to provide security at the
   border of a private network.


   [1]  Srisuresh, P. and M. Holdrege, "IP Network Address Translator
        (NAT) Terminology and Considerations", RFC 2663, August 1999.

   [2]  Kent, S. and R. Atkinson, "Security Architecture for the
        Internet Protocol", RFC 2401, November 1998

   [3]  Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
        (ESP)", RFC 2406, November 1998

   [4]  Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
        November 1998.

   [5]  Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
        RFC 2409, November 1998.

   [6]  Piper, D., "The Internet IP Security Domain of Interpretation
        for ISAKMP", RFC 2407, November 1998.

   [7]  Carpenter, B., Crowcroft, J. and Y. Rekhter, "IPv4 Address
        Behavior Today", RFC 2101, February 1997.

   [8]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot G. and E.
        Lear, "Address Allocation for Private Internets", BCP 5, RFC
        1918, February 1996.

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Author's Address

   Pyda Srisuresh
   Lucent technologies
   4464 Willow Road
   Pleasanton, CA 94588-8519

   Phone: (925) 737-2153
   Fax:   (925) 737-2110

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