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Versions: 00 01 02 RFC 2709
NAT Working Group P. Srisuresh
INTERNET-DRAFT Lucent Technologies
Category: Informational August 1999
Expire in six months
Security Model with Tunnel-mode IPsec for NAT Domains
<draft-ietf-nat-security-02.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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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 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.
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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.
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 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.
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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. 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 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 policies.
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
functions.
3.0. 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
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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.
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
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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
attributes.
+---------------+ +-------------------------+
| | 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
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.
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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.
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4.0. 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 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.
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.
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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.
+---------+
| |
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.
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5.0. 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.
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
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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.
6.0. 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.
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 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.
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REFERENCES
[1] P. Srisuresh, M. Holdrege, "The IP Network Address
Translator (NAT) terminology and considerations", RFC xxxx
[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: srisuresh@lucent.com
Srisuresh [Page 11]
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