Internet Draft P. Srisuresh
Document: draft-ford-behave-top-02.txt Consultant
Expires: January 29, 2007 B. Ford
M.I.T.
July 29, 2006
Complications from Network Address Translator Deployment Topologies
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Abstract
This document identifies two deployment scenarios that have arisen
from the unconventional network topologies formed using Network
Address Translator devices (NATs). First, the simplicity of
administering networks through the combination of NAT and DHCP has
increasingly lead to the deployment of multi-level hierarchies of
inter-connected private networks involving overlapping IP address
spaces. Second, the proliferation of private networks in enterprises,
hotels and conferences alike, and the wide spread use of remote
access Virtual Private Networks (VPNs) to access enterprise intranet
has increasingly lead to overlapping IP address space between remote
and corporate networks. The document does not dismiss these
unconventional scenarios as invalid, but recognizes them as real and
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offers recommendations to ensure these real scenarios do work.
Table of Contents
1. Introduction and Scope........................................
2. Multi-Level NAT Network Topologies ...........................
2.1 Operational Details of the Multi-Level NAT Network .......
2.1.1. Client/Server Communication .......................
2.1.2. Peer-to-Peer Communication ........................
2.2. Anomalies of the Multi-Level NAT Network ................
2.2.1. Plug-and-Play NAT Devices .........................
2.2.2. Unconventional Addressing on NAT Devices ..........
2.2.3. Multi-Level NAT Translations ......................
2.2.4. Mistaken End Host Identity ........................
2.3. Summary of Recommendations ..............................
3. Remote Access VPN Network Topologies .........................
3.1. Operational Details of the Remote Access VPN Network ....
3.2. Anomalies of the Remote Access VPN Network ..............
3.2.1. Remote Router and DHCP Server Address Conflict ...
3.2.2. Simultaneous Connectivity Conflict ...............
3.2.3. VIP Address Conflict .............................
3.2.4. Mistaken End Host Identity .......................
3.3. Summary of Recommendations ..............................
4. Security Considerations ......................................
5. Acknowledgements .............................................
6. Normative References .........................................
7. Informational References .....................................
1. Introduction and Scope
The Internet was originally designed to use a single, global 32-bit
IP address space to uniquely identify hosts on the network, allowing
applications on one host to address and initiate communications with
applications on any other host regardless of the respective hosts'
topological locations or administrative domains. For a variety of
pragmatic reasons, however, the Internet has gradually drifted away
from strict conformance to this ideal of a single flat global address
space, and towards a hierarchy of smaller "private" address spaces
[RFC1918] clustered around a large central "public" address space.
The most important pragmatic causes of this unintended evolution of
the Internet's architecture appear to be the following.
1. Depletion of the 32-bit IPv4 address space due to the exploding
total number of hosts on the Internet. Although IPv6 promises to
solve this problem, the uptake of IPv6 has in practice been slower
than expected.
2. Perceived Security and Privacy: Traditional NAT devices provide a
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filtering function that permits session flows to cross the NAT in
just one direction, from private hosts to public network hosts.
This filtering function is widely perceived as a security benefit.
In addition, the NAT's translation of a host's original IP
addresses and port number in private network into an unrelated,
external IP address and port number is perceived by some as a
privacy benefit.
3. Ease-of-use: NAT vendors often combine the NAT function with a
DHCP server function in the same device, which creates a
compelling, effectively "plug-and-play" method of setting up small
Internet-attached personal networks that is often much easier in
practice for unsophisticated consumers than configuring an
IP subnet. The many popular and inexpensive consumer NAT devices
on the market are usually configured "out of the box" to obtain a
single "public" IP address from an ISP or "upstream" network via
DHCP, and the NAT device in turn acts as both a DHCP server and
default router for any "downstream" hosts (and even other NATs)
that the user plugs into it. Consumer NATs in this way effectively
create and manage private home networks automatically without
requiring any knowledge of network protocols or management on the
part of the user. Auto-configuration of private hosts makes
NAT devices a compelling solution in this common scenario.
The term NAT used throughout the document refers to the traditional
NAT, as defined in [NAT-TERM] and specified in [NAT-TRAD].
[NAT-PROT] identifies various complications with application
protocols due to NAT devices. This document acts as an adjunct to
[NAT-PROT]. The scope of the document is restricted to the two
scenarios identified in sections 2 and 3, as arising out of
unconventional NAT deployment and private address space overlap.
Even though the scenarios appear unconventional, they are real and
and not uncommon to find. For each scenario, the document describes
the seeming anomalies and offers recommendations on how best to make
the topologies work.
Section 2 describes the problem of private address space overlap
due to multi-level NAT topology, the anomalies with the topology and
recommendations to address the anomalies. Section 3 describes the
problem of private address space overlap with remote access
Virtual Private Network (VPN) connection, the anomalies with
address overlap and recommendations to address the anomalies.
Section 4 describes the security considerations in these scenarios.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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2 Multi-Level NAT Network Topologies
Due to the pragmatic considerations discussed in the previous
section and perhaps others, NATs are increasingly, and often
unintentionally, used to create hierarchically interconnected
clusters of private networks as illustrated in figure 1 below. The
creation of multi-level hierarchies is often unintentional, since
each level of NAT is typically deployed by a separate
administrative entity such as an ISP, a corporation, or a home user.
Public Internet
(Public IP addresses)
----+---------------+---------------+---------------+----
| | | |
| | | |
66.39.3.7 18.181.0.31 138.76.29.7 155.99.25.1
+-------+ Host A Host B +-------------+
| NAT-1 | (Alice) (Jim) | NAT-2 |
| (Bob) | | (CheapoISP) |
+-------+ +-------------+
10.1.1.1 10.1.1.1
| |
| |
Private Network 1 Private Network 2
(private IP addresses) (private IP addresses)
----+--------+---- ----+-----------------------+----
| | | | |
| | | | |
10.1.1.10 10.1.1.11 10.1.1.10 10.1.1.11 10.1.1.12
Host C Host D +-------+ Host E +-------+
| NAT-3 | (Mary) | NAT-4 |
| (Ann) | | (Lex) |
+-------+ +-------+
10.1.1.1 10.1.1.1
| |
| |
Private Network 3 | Private Network 4
(private IP addresses)| (private IP addresses)
----+-----------+---+ ----+-----------+----
| | | |
| | | |
10.1.1.10 10.1.1.11 10.1.1.10 10.1.1.11
Host F Host G Host H Host I
Figure 1. Multi-level NAT topology with Overlapping Address Space
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In the above scenario, Bob, Alice, Jim, and CheapoISP have each
obtained a "genuine", globally routable IP address from an upstream
service provider. Alice and Jim have chosen to attach only a single
machine at each of these public IP addresses, preserving the
originally intended architecture of the Internet and making their
hosts, A and B, globally addressable throughout the Internet. Bob,
in contrast, has purchased and attached a typical consumer NAT box.
Bob's NAT obtains its external IP address (66.39.3.7) from Bob's ISP
via DHCP, and automatically creates a private 10.1.1.x network for
Bob's hosts C and D, acting as the DHCP server and default router for
this private network. Bob probably does not even know anything about
IP addresses; he merely knows that plugging the NAT into the Internet
as instructed by the ISP, and then plugging his hosts into the NAT as
the NAT's manual indicates, seems to work and gives all of his hosts
access to Internet.
CheapoISP, an inexpensive service provider, has allocated only one or
a few globally routable IP addresses, and uses NAT to share these
public IP addresses among its many customers. Such an arrangement is
becoming increasingly common, especially in rapidly-developing
countries where the exploding number of Internet-attached hosts
greatly outstrips the ability of ISPs to obtain globally unique IP
addresses for them. CheapoISP has chosen the popular 10.1.1.x
address for its private network, since this is one of the three
well-known private IP address blocks allocated in [RFC1918]
specifically for this purpose.
Of the three incentives listed in section 1 for NAT deployment, the
last two still apply even to customers of ISPs that use NAT,
resulting in multi-level NAT topologies as illustrated in the right
side of the above diagram. Even three-level NAT topologies are known
to exist. CheapoISP's customers Ann, Mary, and Lex have each obtained
a single IP address on CheapoISP's network (Private Network 2), via
DHCP. Mary attaches only a single host at this point, but
Ann and Lex each independently purchase and deploy consumer NATs in
the same way that Bob did above. As it turns out, these consumer
NATs also happen to use 10.1.1.x addresses for the private networks
they create, since these are the configuration defaults hard-coded
into the NATs by their vendors. Ann and Lex probably know nothing
about IP addresses, and in particular they are probably unaware that
the IP address spaces of their own private networks overlap not only
with each other but also with the private IP address space used by
their immediately upstream network.
Nevertheless, despite this direct overlap, all of the "multi-level
NATted hosts" - F, G, H, and I in this case - all nominally function
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and are able to initiate connections to any public server on the
public Internet that has a globally routable IP address. Connections
made from these hosts to the main Internet are merely translated
twice. Once by the consumer NAT (NAT-3 or NAT-44) into the IP
address space of CheapoISP's Private Network 2, and then again by
CheapoISP's NAT-2 into the public Internet's global IP address
space.
2.1 Operational Details of the Multi-Level NAT Network
In the "de facto" Internet address architecture that has resulted
from the above pragmatic and economic incentives, only the nodes on
the public Internet have globally unique IP addresses assigned by
the official IP address registries. IP addresses on different
private networks are typically managed independently - either
manually by the administrator of the private network itself, or
automatically by the NAT through which the private network is
connected to its "upstream" service provider.
By convention, nodes on private networks are usually assigned IP
addresses in one of the private address space ranges specifically
allocated to this purpose in RFC 1918, ensuring that private IP
addresses are easily distinguishable and do not conflict with the
public IP addresses officially assigned to globally routable Internet
hosts. However, when "plug-and-play" NATs are used to create
hierarchically interconnected clusters of private networks, a given
private IP address can be and often is reused across many different
private networks. In figure 1 above, for example, private networks
1, 2, 3, and 4 all have a node with IP address 10.1.1.10.
2.1.1. Client/Server Communication
When a host on a private network initiates a client/server-style
communication session with a server on the public Internet, via the
server's public IP address, the NAT intercepts the packets comprising
that session (usually as a consequence of being the default router
for the private network), and modifies the packets' IP and TCP/UDP
headers so as to make the session appear externally as if it was
initiated by the NAT itself.
For example, if host C above initiates a connection to host A at IP
address 18.181.0.31, NAT-1 modifies the packets comprising the
session so as to appear on the public Internet as if the session
originated from NAT-1. Similarly, if host F on private network 3
initiates a connection to host A, NAT-3 modifies the outgoing packet
so the packet appears on private network 2 as if it had originated
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from NAT-3 at IP address 10.1.1.10. When the modified packet
traverses NAT-2 on private network 2, NAT-2 further modifies the
outgoing packet so as to appear on the public Internet as if it had
originated from NAT-2 at public IP address 155.99.25.1. The NATs in
effect serve as proxies that give their private "downstream" client
nodes a temporary presence on "upstream" networks to support
individual communication sessions.
In summary, all hosts on the private networks 1, 2, 3, and 4 in
figure 1 above are able to establish a client/server-style
communication sessions with servers on the public Internet.
2.1.2. Peer-to-Peer Communication
While this network organization functions in practice for
client/server-style communication, when the client is behind one or
more levels of NAT and the server is on the public Internet, the lack
of globally routable addresses for hosts on private networks makes
direct peer-to-peer communication between those hosts difficult. For
example, two private hosts F and H on the network shown above might
"meet" and learn of each other through a well-known server on the
public Internet, such as Host A, and desire to establish direct
communication between G and H without requiring A to forward each
packet. If G and H merely learn each other's (private) IP addresses
from a registry kept by A, their attempts to connect to each other
will fail because G and H reside on different private networks.
Worse, if their connection attempts are not properly authenticated,
they may appear to succeed but end up talking to the wrong host. For
example, G may end up talking to Host F, the host on private
network 3 that happens to have the same private IP address as Host H.
Host H might similarly end up unintentionally connecting to Host I.
In summary, peer-to-peer communication between hosts on disjoint
private networks 1, 2, 3, and 4 in figure 1 above is a challenge
without the assistance of a well known server on the public
Internet. However, with assistance from a node in the public
Internet, all hosts on the private networks 1, 2, 3, and 4 in
figure 1 above are able to establish a peer-to-peer style
communication sessions amongst themselves as well as with hosts
on the public Internet.
2.2. Anomalies of the Multi-Level NAT Network
Even though conventional wisdom would suggest that the network
described above is seriously broken, in practice it still works in
many ways. We break up figure 1 into two sub figures to better
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illustrate the anomalies. Figure 1.1 is the left half of figure 1
and reflects the conventional single NAT deployment that is widely
prevalent in many last-mile locations. The deployment in figure 1.1
is popularly viewed as a pragmatic solution to work around the
depletion of IPv4 address space and is not considered broken.
Figure 1.2 is the right half of figure-1 and is representative of
the anomalies we are about to discuss.
Public Internet
(Public IP addresses)
----+---------------+---------------+-----------
| | |
| | |
66.39.3.7 18.181.0.31 138.76.29.7
+-------+ Host A Host B
| NAT-1 | (Alice) (Jim)
| (Bob) |
+-------+
10.1.1.1
|
|
Private Network 1
(private IP addresses)
----+--------+----
| |
| |
10.1.1.10 10.1.1.11
Host C Host D
Figure 1.1. Conventional Single-level NAT Network topology
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Public Internet
(Public IP addresses)
---+---------------+---------------+----
| | |
| | |
18.181.0.31 138.76.29.7 155.99.25.1
Host A Host B +-------------+
(Alice) (Jim) | NAT-2 |
| (CheapoISP) |
+-------------+
10.1.1.1
|
|
Private Network 2
(private IP addresses)
----+---------------+-------------+--+-------
| | |
| | |
10.1.1.10 10.1.1.11 10.1.1.12
+-------+ Host E +-------+
| NAT-3 | (Mary) | NAT-4 |
| (Ann) | | (Lex) |
+-------+ +-------+
10.1.1.1 10.1.1.1
| |
| |
Private Network 3 Private Network 4
(private IP addresses) (private IP addresses)
----+-----------+------ ----+-----------+----
| | | |
| | | |
10.1.1.10 10.1.1.11 10.1.1.10 10.1.1.11
Host F Host G Host H Host I
Figure 1.2. Unconventional Multi-level NAT Network topology
2.2.1. Plug-and-Play NAT Devices
Consumer NAT devices are predominantly "plug-and-play" NAT devices,
and assume minimal user intervention during device setup. The
"plug-and-play" NAT devices provide DHCP service on one interface
and NAT function on another interface. Vendors of the consumer NAT
devices make assumptions about how their consumers configure and
hook up their PCs to the device. When consumers do not adhere to the
vendor assumptions, the consumers end up with a broken network.
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A "plug-and-play" NAT device provides DHCP service on the LAN
attached to the private interface, and assumes that all private
hosts at the consumer site have DHCP client enabled and are
connected to the single LAN. Consumers SHOULD be informed of the
assumption that all private hosts MUST be on a single LAN, with no
router in between.
A "Plug-and-Play" NAT device also assumes that there is no other
NAT device or DHCP server device on the same LAN at the customer
premises. When there are multiple "Plug-and-play" NAT devices on
the same LAN, each NAT device will offer DHCP service on the same
LAN, and likely from the same address pool. This could result
in multiple end nodes on the same LAN ending up with identical IP
addresses. That will break network connectivity to end hosts.
Consumers SHOULD be cautioned against using more than one
"plug-and-play" NAT device on the same LAN.
Recommendation-1. Consumers SHOULD be informed that all private
hosts behind a "Plug-and-play" NAT must be on a single LAN subnet,
and that there SHOULD be no more than one "Plug-and-play" NAT device
on the same LAN.
2.2.2. Unconventional Addressing on NAT Devices
Let us consider the unconventional addressing with NAT-3 and
NAT-4. NAT-3 and NAT-4 are apparently multi-homed on the same
subnet through both their interfaces. NAT-3 is on subnet 10.1.1/24
through its external interface facing NAT-2, as well as through its
private interface facing clients Host-F and Host-G. Likewise, NAT-4
also has two interfaces on the same subnet 10.1.1/24.
In a traditional network, when a node has multiple interfaces with
IP addresses on the same subnet, it is natural to assume that all
interfaces with addresses on the same subnet must be on a single
connected LAN (bridged LAN or a single physical LAN). Clearly, that
is not the case here. Even though both NAT-3 and NAT-4 have two
interfaces on the same subnet 10.1.1/24, the NAT devices view the
two interfaces as being on two disjoint subnets and routing realms.
The "plug-and-play" NAT devices are really not multi-homed on the
same subnet as in a traditional sense.
In a traditional network, both NAT-3 and NAT-4 in figure 1.2 should
be incapable of communicating reliably as a transport endpoint with
other nodes on their adjacent networks (ex: private networks 2 and
3 in the case of NAT-3 and private Networks 2 and 4 in the case of
NAT-4). This is because applications on either of the NAT devices
cannot know to differentiate packets from hosts on either of the
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subnets bearing the same IP address. If NAT-3 attempts to resolve
the IP address of a neighboring host in the conventional manner by
broadcasting an ARP request on all of its physical interfaces
bearing the same subnet, it may get a different response on each
of its physical interfaces.
Even though both NAT-3 and NAT-4 have hosts bearing the same IP
address on the adjacent networks, the NAT devices do communicate
effectively as end points. Many of the "plug-and-play" NAT devices
offer a limited number of services on them. For example, many of
the NAT devices respond to pings from hosts on either of the
interfaces. Even though a NAT device is often not actively
managed, many of the NAT devices are equipped to be managed from
the private interface. This unconventional communication with
NAT devices is achievable because NAT devices view the
two interfaces as being on two disjoint routing domains and
distinguish between sessions with hosts on either interface
(private or public).
Consumer oriented "Plug-and-Play" NAT devices MUST and all NATs
SHOULD be able to handle multi-level NAT topologies such as the one
described in figure 1.2, in which a private IP address space on one
side of the NAT potentially conflicts with the private IP address
space on the other side. Essentially NAT must be able to keep the
two IP address spaces separate in its internal data structures, and
base all packet processing decisions on the "side" or "port" from
which the packet arrived and not just on the basis of the IP
addresses it contains.
Recommendation-2. NAT devices SHOULD support IP address space
overlap between the address space on its private interface and the
address space on its public interface. Essentially, a NAT device
SHOULD keep the two IP address spaces separate and base all packet
processing decisions on the "side" or "port" from which the packet
arrived and not just on the basis of the IP addresses it contains.
2.2.3. Multi-Level NAT Translations
Use of a single NAT to connect private hosts to the public Internet
as in figure 1.1 is a fairly common practice. Many consumer NATs are
deployed this way. However, use of multi-level NAT translations as
in figure 1.2 is not a common practice and is not well understood.
Let us consider the conventional single NAT translation in
figure 1.1. Because the public and private IP address ranges are
numerically disjoint, nodes on private networks can make use of both
public and private IP addresses when initiating network
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communication sessions. Nodes on a private network can use private
IP addresses to refer to other nodes on the same private network,
and public IP addresses to refer to nodes on the public Internet.
For example, host C in figure 1.1 is on private network 1 and can
directly address hosts A, B and D using their assigned IP addresses.
This is in spite of the fact that hosts A and B are on the public
Internet and host D alone is on the private network.
Next, let us consider the unconventional multi-level NAT topology in
figure 1.2. In this scenario, private hosts are able to connect to
hosts on the public Internet. But, private hosts are not able to
connect with all other private hosts. For example, host F in
figure 1.2 can directly address hosts A, B, and G using their
assigned IP addresses, but F has no way to address any of the
other hosts in the diagram. Host F in particular cannot address
host E by its assigned IP address, even though host E is located on
the immediately "upstream" private network through which F is
connected to the Internet. Host E has the same IP address as
host G. Yet, this addressing is "legitimate" in the NAT world
because the two hosts are on different private networks.
It would seem that the topology in figure 1.2 with multiple
NAT translations is broken because private hosts are not able to
address each other directly. However, the network is not broken.
Nodes on any private network have no direct method of addressing
nodes on other private networks. The private networks 1, 2, 3 and 4
are all disjoint. Hosts on private network 1 are unable to directly
address nodes on private networks 2, 3 or 4 and vice versa. Multiple
NAT translations was not the cause of this.
As described in sections 2.1.1 and 2.1.2, client-server and
peer-to-peer communication can and should be possible even with
multi-level NAT topology deployment. A host on any private network
MUST be able to communicate with any other host, no matter which
private network the host is attached to or where the private network
is located. Host F should be able to communicate with host E and
carry out both client-server communication and peer-to-peer
communication, and vice versa. Host F and host E form a hairpin
session through NAT-2 to communicate with each other. Each host
uses the public endpoint assigned by the Internet facing NAT (NAT-2)
to address its peer. NAT devices SHOULD support hairpin translation
([P2P-STATE]) for session flows that originate from a host on one
attached private network and targeted to a host on another private
network also attached to the same NAT device. Hairpin translation
is explained in detail in section 4.4 of [P2P-STATE].
Ideally, ISPs SHOULD not use NAT devices to connect their customers,
so the customers do not get caught up in a multi-level NAT scenario
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and hairpin session flows. NAT devices MUST support hairpin
translations for all protocol sessions the NAT device supports.
Hairpin translation support is a requirement for peer node
connectivity in multi-level NAT topologies.
Recommendation-3. NAT devices MUST support hairpin translation for
all protocol sessions the NAT device supports.
2.2.4. Mistaken End Host Identity
There can be a potential security threat due to mistaken identity
in figure 1.2. Suppose, the CheapoISP in figure 1.2 used host E as
the ISP mail server. Host E is assigned an RFC 1918 address of
10.1.1.11. This address can potentially overlap with addresses on
private networks 3 and 4. So, if a customer of CheapoISP had a
mail server installed on his/her private network, bearing an IP
address exactly the same as host E, this could pose a severe
security threat to customer's mail messages. Potentially, the
customer mail messages could be hijacked by the ISP's mail server.
Ideally, ISPs should not use NAT devices to provide connectivity to
their customers. If they do, any servers on the ISP's private
network that need to be accessible to the ISP's customers
(e.g., mail servers) SHOULD have global IP addresses, to ensure
accessibility to customers who deploy NAT devices themselves.
Recommendation-4. Ideally, ISPs SHOULD not use NAT devices to
provide connectivity to their customers. If they do, any
servers on the ISP's private network that need to be accessible to
the ISP's customers (e.g., mail servers) SHOULD have global IP
addresses, to ensure accessibility to customers who deploy NAT
devices themselves.
2.3. Summary of Recommendations
The following is a summary of recommendations identified in section
2.2 to support the unconventional multi-level NAT topologies, such
as the one identified in figure 1. The recommendations are addressed
to NAT vendors, ISPs and the consumers.
Recommendation-1. Consumers SHOULD be informed that all private
hosts behind a "Plug-and-play" NAT must be on a single LAN subnet,
and that there SHOULD be no more than one "Plug-and-play" NAT device
on the same LAN.
Recommendation-2. NAT devices SHOULD support IP address space
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overlap between the address space on its private interface and the
address space on its public interface. Essentially, a NAT device
SHOULD keep the two IP address spaces separate and base all packet
processing decisions on the "side" or "port" from which the packet
arrived and not just on the basis of the IP addresses it contains.
Recommendation-3. NAT devices MUST support hairpin translation for
all protocol sessions the NAT device supports.
Recommendation-4. Ideally, ISPs SHOULD not use NAT devices to
provide connectivity to their customers. If they do, any
servers on the ISP's private network that need to be accessible to
the ISP's customers (e.g., mail servers) SHOULD have global IP
addresses, to ensure accessibility to customers who deploy NAT
devices themselves.
3. Remote Access VPN Network Topologies
Enterprises use remote access VPN to allow secure access to the
employees working outside the enterprise premises. While outside
the enterprise premises, an employee may be located in his/her
home office, hotel, conference or a partner's office. In all cases,
it is desirable for the employee at the remote site to have
unhindered access to his/her corporate network and the applications
running on the corporate networks. This is so the employee can get
his/her work done seamlessly without jeopardizing the integrity and
confidentiality of the corporate network and the applications
running on the network.
IPsec, L2TP and SSL are some of the well known secure VPN
technologies used by the remote access vendors. Besides
authenticating employees for granting access, remote access VPN
servers often enforce different forms of security (e.g. IPsec, SSL)
to protect the integrity and confidentiality of the run-time
traffic between the VPN client and the VPN server.
Many enterprises deploy their internal networks using RFC-1918
private address space and use NAT devices to connect to the public
Internet. Further, many of the applications in the corporate network
refer to information (such as URLs) and services using private
addresses in the corporate network. Applications such as NFS rely on
simple source IP address based filtering to restrict access to
corporate users. These are some reasons why the remote access VPN
servers are configured with a block of IP addresses from the
corporate private network to assign to remote access clients. VPN
clients use the virtual IP (VIP) address assigned to them (by the
corporate VPN server) to access applications inside the corporate.
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Consider the remote access VPN scenario in figure 2 below.
(Corporate Private network 10.0.0.0/8)
---------------+----------------------
|
10.1.1.10
+---------+-------+
| Enterprise Site |
| Remote Access |
| VPN Server |
+--------+--------+
129.32.34.18
|
{---------+------}
{ }
{ }
{ Public Internet }
{ (Public IP addresses) }
{ }
{ }
{---------+------}
|
155.99.25.1
+--------+--------+
| Remote Site NAT |
|(in a Hotel, | +--------+
| remote | | Remote |
| Conference, or | | Site |
| any remote site)| | DHCP |
| | | Server |
+--------+--------+ +----+---+
10.1.1.1 10.1.1.2
| |
Remote Site Private Network | |
-----+-----------+-----------+-------------+----------+--
| | | |
10.1.1.10 10.1.1.11 10.1.1.12 10.1.1.13
Host A Host B +--------+ Host C
| Remote |
| User |
| PC |
|(VPN |
| Client)|
+--------+
Figure 2. Remote Access VPN with Overlapping Address Space
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In the above scenario, say an employee of the corporate is at a
remote location and attempts to access the corporate network using
the VPN client. The corporate network is laid out using RFC-1918
address pool of 10.0.0.0/8 and say the VPN server is configured with
an address block of 10.1.1.0/24 to assign virtual IP addresses to
remote access VPN clients. Now, say the employee at the remote site
is attached to a network on the remote site which also happens to be
using a RFC-1918 address space based network and coincidentally
overlaps the corporate network. In this scenario, it is
conventionally problematic for the VPN client to connect to the
server(s) and other hosts at the enterprise.
Nevertheless, despite the direct address overlap, the remote access
VPN connection between the VPN client at the remote site and the
VPN server at the enterprise should remain connected and should be
made to work. I.e., the NAT device at the remote site should not
obstruct the VPN connection traversing it. And, the remote user
should be able to connect to any host at the enterprise through the
VPN from the remote desktop.
The following subsections describe the operational details of the
VPN, anomalies with the address overlap and recommendations on
how best to address the situation.
3.1. Operational Details of Remote Access VPN Network
As mentioned earlier, in the "de facto" Internet address
architecture, only the nodes on the public Internet have globally
unique IP addresses assigned by the official IP address registries.
Many of the networks in the edges use private IP addresses from
RFC 1918 and use NAT devices to connect their private networks
to the public Internet. Many enterprises adapted the approach of
using private IP addresses internally. Employees within the
enterprise's Intranet private network are "trusted" and may connect
to any of the internal hosts with minimal administrative or policy
enforcement overhead. When an employee leaves the enterprise
premises, remote access VPN provides the same level of intranet
connectivity to the remote user.
The objective of this section is to provide an overview of the
operational details of a remote access VPN application so the reader
has an appreciation for the problem of remote address space overlap.
This is not a tutorial or specification of remote access VPN
products, per se.
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When an employee at a remote site launches his/her remote access VPN
client, the VPN server at the corporate premises demands the VPN
client to authenticate itself. When the authentication succeeds,
the VPN server assigns a Virtual IP (VIP) address to the client for
connecting with the corporate Intranet. From this point onwards,
while the VPN is active, outgoing IP packets directed to the hosts
in the corporate Intranet are tunneled through the VPN, in that the
the VPN server serves as the next-hop and the VPN connection as the
next-hop link for these packets. Within the corporate Intranet, the
outbound IP packets appear as arriving from the VIP address. so,
IP packets from the corporate hosts to the remote user are sent to
the remote user's VIP address and the IP packets are tunneled
inbound to the remote user's PC through the VPN tunnel.
This works well so long as the subnets in the corporate network
do not conflict with subnets at the remote site where the remote
user's PC is located. However, when the corporate network is built
using RFC 1918 private address space and the remote location where
the VPN client is launched is also using an overlapping network from
RFC 1918 address space, there can be addressing conflicts. The
remote user's PC will have a conflict in accessing nodes on the
corporate site and nodes at the remote site bearing the same IP
address simultaneously. Consequently, the VPN client may be unable
to have full access to the employee's corporate network and the
local network at the remote site simultaneously.
In spite of address overlap, remote access VPN clients should be
able to successfully establish connections with Intranet hosts in
the enterprise.
3.2. Anomalies of the Remote Access VPN network
Even though conventional wisdom would suggest that the remote access
VPN scenario with overlapping address space would be seriously
broken, in practice it still works in many ways. Let us look at some
anomalies where there might be a problem and identify solutions
through which the remote access VPN application could be made to
work even under the problem situations.
3.2.1. Remote Router and DHCP Server Address Conflict
When there is address overlap between hosts at corporate Intranet
and hosts at the remote site, the remote VPN user is often unaware
of the address conflict. Address overlap could potentially cause the
remote user to loose connectivity to the enterprise entirely or
loose connectivity to an arbitrary block of hosts at the enterprise.
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Consider, for example, a scenario where the IP address of the DHCP
server at the remote site matched the IP address of a host at
the enterprise network. When the remote user's PC is ready to
renew the lease of the locally assigned IP address, the remote
user's VPN client would incorrectly identify the IP packet
as being addressed to an enterprise host and tunnel the DHCP
renewal packet over the VPN to the remote VPN server. The DHCP
renewal requests simply do not reach the DHCP server at the
remote site. As a result, the remote PC would eventually loose the
lease on the IP address and the VPN connection to the enterprise
would be broken.
Consider another scenario where the IP address of the remote user's
router overlapped with the IP address of a host in the enterprise
network. If the remote user's PC were to send ping or some type of
periodic keep-alive packets to the router (say, to test the liveness
of the router), the packets are intercepted by the VPN client and
simply redirected to the VPN tunnel. This type of unintended
redirection has the twin effect of hijackng critical packets
addresed to the router as well as the host in the enterprise
network (bearing the same IP address as the remote router) being
bombarded with unintended keep-alive packets. Loss of connectivity
to the router can result in the VPN connection being broken.
Clearly, it is not desirable for the corporate intranet to conflict
with the IP addresses of the router and DHCP server at the remote
site. VPN servers should, at a minimum, disallow access to corporate
hosts that are using an IP address that might match any of the
following entities at the remote site - a) client's next-hop router
IP address used to access the VPN server, and b) DHCP server
providing address lease on the remote host network interface. By
doing this, VPN client on the remote PC will not intercept IP
packets whose target IP addresses are not in the authorized list
of enterprise hosts. And, the VPN connection remains. This however
has the downside that the VPN client looses connectivity to a
potentially mission critical host at the corporate site.
Recommendation-1. When there is conflict of address space between
corporate Intranet and the subnet on remote site, the VPN server
server SHOULD disallow access from the VPN client to corporate
hosts bearing the same IP address as the router or DHCP server at
the remote site.
3.2.2. Simultaneous Connectivity Conflict
Ideally speaking, it is not desirable for the corporate intranet to
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conflict with any of the hosts at the remote site. As a general
practice, if simultaneous communication with end hosts at the remote
location is important to the remote user, it is advisable to
disallow access to any corporate network resource that overlaps
the client's external IP subnet. By doing this, the remote user
is able to connect to all local hosts simultaneously while the VPN
connection is active. For example, if the PC's external network
interface is configured with 10.1.1.1/24, the VPN server may be
configured to disallow access to the corporate network that
overlaps this subnet from the remote access VPN client. Such a
configuration on the VPN server is also termed sometimes as
"Split VPN" configuration. With "Split VPN" configuration set,
the remote user is able to carry out simultaneous communication
with hosts at the remote site and the hosts at the corporate
intranet, with the exception of the hosts that overlap the remote
subnet.
If simultaneous connectivity to local hosts is not important, the
VPN server may be configured to require the VPN client to direct
all communication traffic from the remote user to the VPN server
across the VPN tunnel. This essentially ensures that all
communication from the remote user's PC traverses the VPN link and
no communication takes place with hosts on the local subnet. This
configuration on the VPN server is also termed sometimes as
"Non-split VPN", as all traffic from the remote user's PC is
directed to the VPN server, with the exception of traffic directed
to the local router and DHCP server.
Recommendation-2. If simultaneous communication with end hosts at
the remote location is important to the remote user, enterprises
SHOULD configure the VPN server in "Split VPN" mode and disallow
access to any corporate network resource that overlaps the client's
external IP subnet. If simultaneous connectivity to local hosts is
not important, enterprises SHOULD configure the VPN server in
"Non-split VPN" mode, so the VPN client directs to the VPN tunnel
all traffic from the remote user, with the exception of traffic to
the local router and DHCP server.
3.2.3. VIP Address Conflict
When the VIP address assigned to the VPN client at the remote site
is in direct conflict with the IP address of the existing network
interface, the VPN client might be unable to establish the VPN
connection.
Consider a scenario where the VIP address assigned by the
VPN server directly matched the IP address of the networking
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interface at the remote site. When the VPN client on the remote
host attempts to set the VIP address on a virtual adapter (specific
to the remote access application), the VIP address configuration
will simply fail due to conflict with the IP address of the existing
network interface. The configuration failure in turn will result in
the remote access VPN tunnel not being established.
Clearly, it is not advisable to have the VIP address overlap
the IP address of the remote user's existing network interface. As a
general rule, it is not advisable for the VIP address to overlap
any IP address in the remote user's local subnet, as the VPN client
on the remote PC might be forced to respond to ARP requests on the
remote site and the VPN client might not process the handling of ARP
requests gracefully.
We RECOMMEND that VPN vendors offer provision to detect conflict of
VIP address with remote site address space and switch between a
minimum two VIP address pools on the VPN server. We also RECOMMEND
enterprises deploying the VPN solution to use this vendor provision
and configure the VPN server with a minimum of two distinct IP
address pools. Alternately, the enterprises SHOULD deploy a minimum
of two VPN servers with different address pools. By doing this, a
VPN client that detected the conflict of VIP address with the local
subnet is able to reconnect with the alternate VPN server using
the alternate address pool that will not conflict.
Recommendation-3. We RECOMMEND that VPN vendors offer provision to
detect conflict of VIP address with remote site address space and
switch between a minimum two VIP address pools with different
subnets on the VPN server to ensure that the VIP address is not in
conflict with the subnet on the remote PC location.
Recommendation-4. We RECOMMEND that enterprises deploying the VPN
solution SHOULD adapt one of the following strategies to avoid VIP
address conflict with the subnet on remote PC location.
a) If the VPN device being deployed has provision to configure two
address pools (as in recommendation 3 above), configure the VPN
server with a minimum of two distinct IP address pools.
b) Deploy a minimum of two VPN servers with different address pools.
By doing this, a VPN client that detected the conflict of VIP
address with the local subnet is able to switch to alternate VPN
server and obviate VIP address conflict with the local subnet.
3.2.4. Mistaken End Host Identity
When "Split VPN" configuration is set on the VPN server, there can
be a potential security threat due to mistaken identity.
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Say, a certain service (ex: SMTP mail service) is configured on
exactly the same IP address on both the corporate site and the
remote site. The user could unknowingly be using the service on the
remote site, thereby violating the integrity and confidentiality of
the contents relating to that application. Potentially, remote
user mail messages could be hijacked by the ISP's mail server.
Enterprises deploying remote access VPN servers SHOULD allocate
global IP addresses for the critical servers the remote VPN clients
typically need to access. By doing this, even if most of the private
corporate network uses RFC 1918 address space, this will ensure that
the remote VPN clients can always access the critical servers
regardless of the private address space used at the remote
attachment point.
Recommendation-5. When "Split VPN" is configured on the VPN server,
enterprises deploying remote access VPN servers SHOULD allocate
global IP addresses for the critical servers the remote VPN clients
typically need to access.
3.3. Summary of Recommendations
The following is a summary of recommendations identified in section
3.2 to support the address overlap in remote access VPN networks,
such as the one identified in figure 2. The recommendations are
addressed to remote access VPN vendors, enterprises deploying the
VPN servers and finally, the remote access VPN consumers. Following
the recommendations will help ensure that a complete "network
meltdown" is prevented.
Recommendation-1. When there is conflict of address space between
corporate Intranet and the subnet on remote site, the VPN server
server SHOULD disallow access from the VPN client to corporate
hosts bearing the same IP address as the router or DHCP server at
the remote site.
Recommendation-2. If simultaneous communication with end hosts at
the remote location is important to the remote user, enterprises
SHOULD configure the VPN server in "Split VPN" mode and disallow
access to any corporate network resource that overlaps the client's
external IP subnet. If simultaneous connectivity to local hosts is
not important, enterprises SHOULD configure the VPN server in
"Non-split VPN" mode, so the VPN client directs to the VPN tunnel
all traffic from the remote user, with the exception of traffic to
the local router and DHCP server.
Recommendation-3. We RECOMMEND that VPN vendors offer provision to
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detect conflict of VIP address with remote site address space and
switch between a minimum two VIP address pools with different
subnets on the VPN server to ensure that the VIP address is not in
conflict with the subnet on the remote PC location.
Recommendation-4. We RECOMMEND that enterprises deploying the VPN
solution SHOULD adapt one of the following strategies to avoid VIP
address conflict with the subnet on remote PC location.
a) If the VPN device being deployed has provision to configure two
address pools (as in recommendation 3 above), configure the VPN
server with a minimum of two distinct IP address pools.
b) Deploy a minimum of two VPN servers with different address pools.
By doing this, a VPN client that detected the conflict of VIP
address with the local subnet is able to switch to alternate VPN
server and obviate VIP address conflict with the local subnet.
Recommendation-5. When an enterprise configures the VPN server in
"Split VPN" mode, the enterprise SHOULD allocate global IP addresses
for the critical servers the remote VPN clients typically need to
access.
4. Security Considerations
This document does not inherently create new security issues.
Security issues known to DHCP servers and NAT devices are
applicable, but not within the scope of this document. Likewise,
security issues specific to remote access VPN devices are also
appliable to the remote access VPN topology, but not within the
scope of this document. The security issues reviewed here only
those relevant to the topologies described in sections 2 and 3,
specifcally as they apply to private address space overlap in the
topologies described.
Mistaken end host identity is a security concern present in both
topologies discussed. Mistaken end host identity, described in
sections 2.2.4 and 3.2.4 for each of the topologies reviewed,
essentially points the possibility of application services being
hijacked by the wrong application server (ex: Mail server). Security
violation due to mistaken end host identity arises principally due
to critical servers being assigned RFC 1918 private addresses. The
recommendation suggested for both scenarios is to assign globally
unique pulic IP addresses for the critical servers.
It is also recommended in section 2.1.2 that applications adapt
end-to-end authentication and not depend on source IP address for
authentication. Doing this will thwart connection hijacking and
denial of service attacks.
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5. Acknowledgements
The authors wish to thank Dan Wing for reviewing the document in
detail and making helpful suggestions in reorganizing the
document format.
6. Normative References
[NAT-TERM] P. Srisuresh and M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations", RFC 2663, August
1999.
[NAT-TRAD] P. Srisuresh and K. Egevang, "Traditional IP Network Address
Translator (Traditional NAT)", RFC 3022, January 2001.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot,G. and
Lear, E., "Address Allocation for Private Internets", BCP 5,
RFC 1918, February 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
7. Informational References
[NAT-PROT] Holdrege, M., and Srisuresh, P., "Protocol Complications
with the IP Network Address Translator", RFC 3027,
January 2001.
[P2P-STATE] Srisuresh, P., Ford, B., and Kegel, D., "State of Peer-to-
Peer(P2P) Communication Across Network Address Translators
(NATs)", draft-srisuresh-behave-p2p-state-03.txt,
June 2006, Work in Progress.
Authors' Addresses:
Pyda Srisuresh
Consultant
20072 Pacifica Dr.
Cupertino, CA 95014
U.S.A.
Phone: (408) 836-4773
E-mail: srisuresh@yahoo.com
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Bryan Ford
Computer Science and Artificial Intelligence Laboratory
Massachusetts Institute of Technology
77 Massachusetts Ave.
Cambridge, MA 02139
U.S.A.
Phone: (617) 253-5261
E-mail: baford@mit.edu
Web: http://www.brynosaurus.com/
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