NAT Working Group P. Srisuresh
INTERNET-DRAFT Lucent Technologies
Category: Informational February 1999
Expire in six months
Security Model for Network Address Translator (NAT) Domains
<draft-ietf-nat-security-01.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.
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Abstract
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 can offer network level 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 secure policies may be transparently communicated to IKE (for
automated KEY exchange) during Quick Mode. Applications that can
benefit from the secure NAT model are also outlined in the end.
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1. Introduction and Overview
NATs provide transparent routing to end hosts trying to communicate
from disparate routing 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 transport level payload security can be provided for
applications that do not embed realm-specific information that is
meaningless to one of the end-users. Applications that do embed
realm-specific information 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 transport level security.
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 point.
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 secure policies, session keys
and other Security Association (SA) attributes. This section
describes a method by which secure policies administered for
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.
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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. Clear-NAT
The term "Clear-NAT" is introduced to distinguish NAT processing on
the outer packet header from that of NAT processing used for secure
packets embedded within an IPsec secure tunnel, for which the NAT
device is a tunnel end point. The term "Clear-NAT" refers to NAT
processing on the outer packet header.
2.2. Secure-NAT
For lack of a better alternative, the term "Secure-NAT" is defined to
describe the manifestation of NAT applied to packets embedded
within an IPsec-secure IP tunnel, for which the NAT node is a tunnel
end point. A Secure-NAT device is essentially a tunnel-mode IPsec
gateway with NAT extensions applied to embedded packets.
Packets subject to secure-NAT processing are beneficiaries of IPsec
security between the NAT device and an external peer entity, be it a
host or a gateway node. Just as with Clear-NAT, Secure-NAT can
assume any of NAT flavors, including traditional NAT, bi-directional
NAT and Twice NAT.
3.0. Secure NAT operation
The IP security architecture document [Ref 2] describes how IP
network level security may be accomplished within a routing 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) Secure
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.
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+---------------+ 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.
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. In addition, the device would be required to perform
address translation for packets that adhere to secure policies.
A Secure-NAT performs address translation and secure processing
together, based on secure policies. The following diagrams
(figure 3 and figure 4) illustrate the operation of IPsec
tunneling in conjunction with NAT. Operation of Secure-NAT device
may be distinguished from that of an IPsec gateway that does not
support NAT as follows.
(1) Secure-NAT device has secure 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 a secure-NAT
device includes Secure-NAT address mapping and other NAT
parameter definitions in conjunction with Secure policies
and SA attributes. Fundamental elements of a traditional
IPsec gateway are limited only to Secure policies and SA
attributes.
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+---------------+ +-----------------+
| | No | Apply Clear-NAT | Forward
Outgoing |Does the packet| +--->| as appropriate. |-------->
-------->|match Outbound |-| +-----------------+
Packet |Security | | +--------+ +-------------+
(Private |Policies? | |Yes |Perform | |Perform | Forward
Domain) | | +--->|Outbound|->|Outbound |--------->
+---------------+ |NAT | |Security | IPsec Pkt
|(Secure)| |(Tunnel mode)|
+--------+ +-------------+
Figure 3. Tunnel-Mode IPsec on a Secure-NAT device for outgoing pkts
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)| |(Secure)| |Policies? |
Domain) +----------+ +--------+ +----------+
Figure 4. Tunnel-Mode IPsec on a Secure-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
secure policies and secure processing on a secure-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 Clear-NAT for packets that
do not require secure-NAT. Address mapping and other NAT parameter
definitions for clear-NAT and Secure-NAT are distinct. Figure 3
identifies how a NAT device distinguishes between outgoing packets
that need to be processed through clear-NAT vs. secure-NAT. As for
packets incoming from external realm, figure 4 outlines packets
that may be subject to secure-NAT. All other packets are subject
to clear-NAT processing only.
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4.0. Operation of IKE protocol on Secure-NAT device.
Secure-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 Secure-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 secure-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 a
routing 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 phases.
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 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.
A secure-NAT device, operating as IPsec gateway, has the secure
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. The
following diagram illustrates how an IKE-ALG process interfaces
with secure-NAT to take the secure policies and secure-NAT maps
and generates secure policies that IKE could communicate to
peers in the external realm.
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Depending on the nature of secure 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 secure 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
Secure-NAT gateway engine as described in the following diagram.
+---------+
| |
| |
| |
Negotiated Security Parameters | IKE |
+-------------------------------| Process |
|(including session Keys) | |
| | |
| | |
| +---------+
| ^ ^
| | |
| Translated| |
| Secure| |Security
| Policies| |Proposals
v | |
+---------+ Secure Policies, based +---------+
| |------------------------>| |
| | on Pvt. realm addressing| |
| Secure- | | |
| NAT | Secure-NAT MAPs | IKE-ALG |
| (IPsec |-----------------------> | |
| Gateway)| | |
| | Security Proposals | |
| |-----------------------> | |
| | | |
| | NAT Control exchange | |
| |<----------------------->| |
+---------+ +---------+
Figure 5. IKE-ALG translates Security policies, using NAT Maps.
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5.0. Secure NAT applications
Secure-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
Secure-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, secure-NAT device at the border of a
private realm can peer with an IPSec gateway of an external domain
to provide secure Extranet connectivity over the Internet.
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 secure-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,
secure-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 Secure-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.
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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 Secure-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 Secure-NAT gateway, however, is not within the scope of
this document.
6.0. 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.
Session 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 secure-NAT model described in this
document outlines an 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 Secure-NATs, ALGs and firewalls co-exist
to provide security at the border of a private network.
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REFERENCES
[1] P. Srisuresh, M. Holdrege, "The IP Network Address
Translator (NAT) terminology and considerations",
<draft-ietf-nat-terminology-01.txt> - Work in progress,
October 1998
[2] S. Kent, R. Atkinson, "Security Architecture for the Internet
Protocol", RFC 2401
[3] S. Kent, R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406
[4] S. Kent, R. Atkinson, "IP Authentication Header", RFC2402
[5] D. Harkins, D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409
[6] D. Piper, "The Internet IP Security Domain of Interpretation
for ISAKMP", RFC 2407
[7] Brian carpenter, Jon Crowcroft, Yakov Rekhter, "IPv4 Address
Behavior Today", RFC 2101
[8] Rekhter, Y., Moskowitz, B., Karrenberg, D., G. de Groot, and,
Lear, E. "Address Allocation for Private Internets", RFC 1918
Author's Address
Pyda Srisuresh
Lucent technologies
4464 Willow Road
Pleasanton, CA 94588-8519
U.S.A.
Voice: (925) 737-2153
Fax: (925) 737-2110
EMail: suresh@ra.lucent.com
Srisuresh [Page 10]
