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RFC 5684 - Unintended Consequences of NAT Deployments with Overl


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Independent Submission                                      P. Srisuresh
Request for Comments: 5684                               EMC Corporation
Category: Informational                                          B. Ford
ISSN: 2070-1721                                          Yale University
                                                           February 2010

               Unintended Consequences of NAT Deployments
                     with Overlapping Address Space

Abstract

   This document identifies two deployment scenarios that have arisen
   from the unconventional network topologies formed using Network
   Address Translator (NAT) devices.  First, the simplicity of
   administering networks through the combination of NAT and DHCP has
   increasingly lead to the deployment of multi-level inter-connected
   private networks involving overlapping private IP address spaces.
   Second, the proliferation of private networks in enterprises, hotels
   and conferences, and the wide-spread use of Virtual Private Networks
   (VPNs) to access an enterprise intranet from remote locations has
   increasingly lead to overlapping private IP address space between
   remote and corporate networks.  This document does not dismiss these
   unconventional scenarios as invalid, but recognizes them as real and
   offers recommendations to help ensure these deployments can
   function without a meltdown.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any
   other RFC stream.  The RFC Editor has chosen to publish this
   document at its discretion and makes no statement about its value
   for implementation or deployment.  Documents approved for
   publication by the RFC Editor are not a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any
   errata, and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5684.

Copyright

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http:trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1. Introduction and Scope ..........................................3
   2. Terminology and Conventions Used ................................4
   3. Multi-Level NAT Network Topologies ..............................4
      3.1. Operational Details of the Multi-Level NAT Network .........6
           3.1.1. Client/Server Communication .........................7
           3.1.2. Peer-to-Peer Communication ..........................7
      3.2. Anomalies of the Multi-Level NAT Network ...................8
           3.2.1. Plug-and-Play NAT Devices ..........................10
           3.2.2. Unconventional Addressing on NAT Devices ...........11
           3.2.3. Multi-Level NAT Translations .......................12
           3.2.4. Mistaken End Host Identity .........................13
   4. Remote Access VPN Network Topologies ...........................14
      4.1. Operational Details of the Remote Access VPN Network ......17
      4.2. Anomalies of the Remote Access VPNs .......................18
           4.2.1. Remote Router and DHCP Server Address Conflict .....18
           4.2.2. Simultaneous Connectivity Conflict .................20
           4.2.3. VIP Address Conflict ...............................21
           4.2.4. Mistaken End Host Identity .........................22
   5. Summary of Recommendations .....................................22
   6. Security Considerations ........................................24
   7. Acknowledgements ...............................................24
   8. References .....................................................25
      8.1. Normative References ......................................25
      8.2. Informative References ....................................25

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 host's
   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
      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 a 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 ([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.

   [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 3 and 4, arising out of
   unconventional NAT deployment and private address space overlap.
   Even though the scenarios appear unconventional, they are 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 terminology and conventions used in the
   document.  Section 3 describes the problem of private address space
   overlap in a multi-level NAT topology, the anomalies with the
   topology, and recommendations to address the anomalies.  Section 4
   describes the problem of private address space overlap with remote
   access Virtual Private Network (VPN) connections, the anomalies with
   the topology, and recommendations to address the anomalies.  Section
   5 describes the security considerations in these scenarios.

2.  Terminology and Conventions Used

   In this document, the IP addresses 192.0.2.1, 192.0.2.64,
   192.0.2.128, and 192.0.2.254 are used as example public IP addresses
   [RFC5735].  Although these addresses are all from the same /24
   network, this is a limitation of the example addresses available in
   [RFC5735].  In practice, these addresses would be on different
   networks.

   Readers are urged to refer to [NAT-TERM] for information on NAT
   taxonomy and terminology.  Unless prefixed with a NAT type or
   explicitly stated otherwise, the term NAT, used throughout this
   document, refers to Traditional NAT [NAT-TRAD].  Traditional NAT has
   two variations, namely, Basic NAT and Network Address Port Translator
   (NAPT).  Of these, NAPT is by far the most commonly deployed NAT
   device.  NAPT allows multiple private hosts to share a single public
   IP address simultaneously.

3.  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)
        ----+---------------+---------------+---------------+----
            |               |               |               |
            |               |               |               |
        192.0.2.1      192.0.2.64     192.0.2.128     192.0.2.254
        +-------+        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

   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 (192.0.2.1) 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
   NATed hosts" -- F, G, H, and I in this case -- all nominally function
   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.

3.1.  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.

3.1.1.  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 were
   initiated by the NAT itself.

   For example, if host C above initiates a connection to host A at IP
   address 192.0.2.64, 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 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 192.0.2.254.  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.

3.1.2.  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 session amongst
   themselves as well as with hosts on the public Internet.

3.2.  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
   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)
        ----+---------------+---------------+-----------
            |               |               |
            |               |               |
        192.0.2.1      192.0.2.64     192.0.2.128
        +-------+        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

                        Public Internet
                      (Public IP Addresses)
                ---+---------------+---------------+----
                   |               |               |
                   |               |               |
               192.0.2.64     192.0.2.128     192.0.2.254
                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

3.2.1.  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 can end up with a broken network.

   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 need to be aware 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 may even be from the same private address pool.  This could
   result in multiple end nodes on the same LAN ending up with identical
   IP addresses and breaking network connectivity.

   As it turns out, most consumer deployments have a single LAN where
   there they deploy a plug-and-play NAT device and the concerns raised
   above have not been an issue in reality.

3.2.2.  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 (e.g., 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
   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 Address Resolution Protocol (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 endpoints.  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 many of the NAT devices conform to REQ-7 of [BEH-UDP] and
   view the two interfaces as being on two disjoint routing domains and
   distinguish between sessions initiated from hosts on either interface
   (private or public).

3.2.3.  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 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 were not the cause of this.

   As described in sections 3.1.1 and 3.1.2, 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 to which private
   network the host is attached 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.

   When the deployed NAT devices conform to the hairpin translation
   requirements in [BEH-UDP], [BEH-TCP], and [BEH-ICMP], peer nodes are
   able to connect even in this type of multi-level NAT topologies.

3.2.4.  Mistaken End Host Identity

   Mistaken end host identity can result in accidental malfunction in
   some cases of multi-level NAT deployments.  Consider the scenario in
   figure 1.3.  Figure 1.3 depicts two levels of NATs between an end-
   user in private network 3 and the public Internet.

   Suppose CheapoISP assigns 10.1.1.11 to its DNS resolver, which it
   advertises through DHCP to NAT-3, the gateway for Ann's home.  NAT-3
   in turn advertises 10.1.1.11 as the DNS resolver to host F
   (10.1.1.10) and host G (10.1.1.11) on private network 3.  However,
   when host F sends a DNS query to 10.1.1.11, it will be delivered
   locally to host G on private network 3 rather than CheapoISP's DNS
   resolver.  This is clearly a case of mistaken identity due to
   CheapoISP advertising a server that could potentially overlap with
   its customers' IP addresses.

                  Public Internet
                (Public IP Addresses)
          ---+---------------+---------------+----
             |               |               |
             |               |               |
         192.0.2.64     192.0.2.128     192.0.2.254
          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
              | NAT-3 |          Host E
              | (Ann) |          (DNS Resolver)
              +-------+
               10.1.1.1
                   |    Private Network 3
                   |  (Private IP Addresses)
           ----+---+-----------+----------------
               |               |
               |               |
          10.1.1.10       10.1.1.11
            Host F          Host G

       Figure 1.3. Mistaken Server Identity in Multi-Level NAT Topology

   Recommendation-1: ISPs, using NAT devices to provide connectivity to
   customers, should assign non-overlapping addresses to servers
   advertised to customers.  One way to do this would be to assign
   global addresses to advertised servers.

4.  Remote Access VPN Network Topologies

   Enterprises use remote access VPN to allow secure access to 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 network.  While doing so, the employee should not
   jeopardize the integrity and confidentiality of the corporate network
   and the applications running on the network.

   IPsec, Layer 2 Tunneling Protocol (L2TP), and Secure Socket Layer
   (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 private address
   space as defined in RFC 1918 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 the
   Network File Systems (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 network.

   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      |
                          +--------+--------+
                             192.0.2.1
                                   |
                         {---------+------}
                       {                    }
                     {                        }
                   {      Public Internet       }
                   {   (Public IP Addresses)    }
                     {                        }
                       {                    }
                         {---------+------}
                                   |
                             192.0.2.254
                          +--------+--------+
                          | Remote Site     |
                          |  Plug-and-Play  |
                          | NAT Router      |
                          +--------+--------+
                               10.1.1.1
                                   |
      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
                               | VPN    |
                               | Client |
                               | on a PC|
                               +--------+

          Figure 2. Remote Access VPN with Overlapping Address Space

   In the above scenario, say an employee of the corporation is at a
   remote location and attempts to access the corporate network using
   the VPN client, the corporate network is laid out using the address
   pool of 10.0.0.0/8 as defined in RFC 1918, and 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 that also
   happens to be using a network based on the RFC 1918 address space 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.  That is, the NAT device at the remote site should not
   obstruct the VPN connection traversing it.  Additionally, 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.

4.1.  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.

   When an employee at a remote site launches his/her remote access VPN
   client, the VPN server at the corporate premises demands that the VPN
   client 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 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.

4.2.  Anomalies of the Remote Access VPNs

   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.

4.2.1.  Remote Router and DHCP Server Address Conflict

   Routing and DHCP service are bootstrap services essential for a
   remote host to establish a VPN connection.  Without DHCP lease, the
   remote host cannot communicate over the IP network.  Without a router
   to connect to the Internet, the remote host is unable to access past
   the local subnet to connect to the VPN server at the enterprise.  It
   is essential that neither of these bootstrap services be tampered
   with at the remote host in order for the VPN connection to stay
   operational.  Typically, a plug-and-play NAT device at the remote
   site provides both routing and DHCP services from the same IP
   address.

   When there is address overlap between hosts at the 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 lose connectivity to the enterprise entirely or lose
   connectivity to an arbitrary block of hosts at the enterprise.

   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 lose 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 a ping or some type of
   periodic keep-alive packets to the router (say, to test the liveness
   of the router), the packets would be intercepted by the VPN client
   and simply redirected to the VPN tunnel.  This type of unintended
   redirection has the twin effect of hijacking critical packets
   addressed 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 to route traffic directed to the local
   router or DHCP server to be redirected to the corporate intranet.  A
   VPN client on a remote PC should be configured such that IP packets
   whose target IP address matches any of the following are disallowed
   to be redirected over the VPN:

   a) IP address of the VPN client's next-hop router, used to access the
      VPN server.

   b) IP address of the DHCP server, providing address lease on the
      remote host network interface.

   Recommendation-2: A VPN client on a remote PC should be configured
   such that IP packets whose target IP address matches *any* of (a) or
   (b) are disallowed to be redirected over the VPN:

   a) IP address of the VPN client's next-hop router, used to access the
      VPN server.

   b) IP address of the DHCP server, providing address lease on the
      remote host network interface.

4.2.2.  Simultaneous Connectivity Conflict

   Ideally speaking, it is not desirable for the corporate intranet to
   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, it is advisable to disallow access to any
   corporate network resource that overlaps the client's subnet at the
   remote site.  By doing this, the remote user is able to connect to
   all local hosts simultaneously while the VPN connection is active.

   Some VPN clients allow the remote PC to access the corporate network
   over VPN and all other subnets directly without routing through the
   VPN.  Such a configuration is termed as "Split VPN" configuration.
   "Split VPN" configuration allows the remote user to run applications
   requiring communication with hosts at the remote site or the public
   Internet, as well as hosts at the corporate intranet, unless there is
   address overlap with the remote subnet.  Applications needing access
   to the hosts at the remote site or the public Internet do not
   traverse the VPN, and hence are likely to have better performance
   when compared to traversing the VPN.  This can be quite valuable for
   latency-sensitive applications such as Voice over IP (VoIP) and
   interactive gaming.  If there is no overriding security concern to
   directly accessing hosts at the remote site or the public Internet,
   the VPN client on remote PC should be configured in "Split VPN" mode.

   If simultaneous connectivity to hosts at the remote site is not
   important, the VPN client may be configured to direct all
   communication traffic from the remote user to the VPN.  Such a
   configuration is termed as "Non-Split VPN" configuration.  "Non-Split
   VPN" configuration ensures that all communication from the remote
   user's PC traverses the VPN link and is routed through the VPN
   server, with the exception of traffic directed to the router and DHCP
   server at the remote site.  No other communication takes place with
   hosts at the remote site.  Applications needing access to the public
   Internet also traverse the VPN.  If the goal is to maximize the
   security and reliability of connectivity to the corporate network,
   the VPN client on remote PC should be configured in "Non-Split VPN"
   mode.  "Non-Split VPN" configuration will minimize the likelihood of
   access loss to corporate hosts.

   Recommendation-3: A VPN client on a remote PC should be configured in
   "Non-Split VPN" mode if the deployment goal is (a), or in "Split VPN"
   mode if the deployment goal is (b):

   a) If the goal is to maximize the security and reliability of
      connectivity to the corporate network, the VPN client on the
      remote PC should be configured in "Non-Split VPN" mode.  "Non-
      Split VPN" mode ensures that the VPN client directs all traffic

      from the remote user to the VPN server (at the corporate site),
      with the exception of traffic directed to the router and DHCP
      server at the remote site.

   b) If there is no overriding security concern to directly accessing
      hosts at the remote site or the public Internet, the VPN client on
      the remote PC should be configured in "Split VPN" mode.  "Split
      VPN" mode ensures that only the corporate traffic is directed over
      the VPN.  All other traffic does not have the overhead of
      traversing the VPN.

4.2.3.  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 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 can 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.

   Some VPN vendors offer provisions to detect conflict of VIP addresses
   with remote site address space and switch between two or more address
   pools with different subnets so the VIP address assigned is not in
   conflict with the address space at remote site.  Enterprises
   deploying VPNs that support this type of vendor provisioning are
   advised to configure the VPN server with a minimum of two distinct IP
   address pools.  However, this is not universally the case.

   Alternately, enterprises may deploy two or more VPN servers with
   different address pools.  By doing so, a VPN client that detects
   conflict of a VIP address with the subnet at the remote site will
   have the ability to switch to an alternate VPN server that will not
   conflict.

   Recommendation-4: Enterprises deploying remote access VPN solutions
   are advised to adapt a strategy of (a) or (b) to avoid VIP address
   conflict with the subnet at the remote site.

   a) If the VPN server being deployed has been provisioned to configure
      two or more address pools, configure the VPN server with a minimum
      of two distinct IP address pools.

   b) Deploy two or more VPN servers with distinct IP address pools.  By
      doing so, a VPN client that detects conflicts of VIP addresses
      with the subnet at the remote site will have the ability to switch
      to an alternate VPN server that will not conflict.

4.2.4.  Mistaken End Host Identity

   When "Split VPN" is configured on the VPN client on a remote PC,
   there can be a potential security threat due to mistaken identity.
   Say, a certain service (e.g., 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.  This is akin to Recommendation-1 provided in conjunction with
   multi-level NAT deployments.

   Recommendation-5: When "Split VPN" is configured on a VPN client of a
   remote PC, enterprises deploying remote access VPN servers are
   advised to assign global IP addresses for the critical servers the
   remote VPN clients are likely to access.

5.  Summary of Recommendations

   NAT vendors are advised to refer to the BEHAVE protocol documents
   ([BEH-UDP], [BEH-TCP], and [BEH-ICMP]) for a comprehensive list of
   conformance requirements for NAT devices.

   The following is a summary of recommendations to support the
   unconventional NAT topologies identified in this document.  The
   recommendations are deployment-specific and addressed to the
   personnel responsible for the deployments.  These personnel include
   ISP administrators and enterprise IT administrators.

   Recommendation-1: ISPs, using NAT devices to provide connectivity to
   customers, should assign non-overlapping addresses to servers
   advertised to customers.  One way to do this would be to assign
   global addresses to advertised servers.

   Recommendation-2: A VPN client on a remote PC should be configured
   such that IP packets whose target IP address matches *any* of (a) or
   (b) are disallowed to be redirected over the VPN:

   a) IP address of the VPN client's next-hop router, used to access the
      VPN server.

   b) IP address of the DHCP server, providing address lease on the
      remote host network interface.

   Recommendation-3: A VPN client on a remote PC should be configured in
   "Non-Split VPN" mode if the deployment goal is (a), or in "Split VPN"
   mode if the deployment goal is (b):

   a) If the goal is to maximize the security and reliability of
      connectivity to the corporate network, the VPN client on the
      remote PC should be configured in "Non-Split VPN" mode.  "Non-
      Split VPN" mode ensures that the VPN client directs all traffic
      from the remote user to the VPN server (at the corporate site),
      with the exception of traffic directed to the router and DHCP
      server at the remote site.

   b) If there is no overriding security concern to directly accessing
      hosts at the remote site or the public Internet, the VPN client on
      the remote PC should be configured in "Split VPN" mode.  "Split
      VPN" mode ensures that only the corporate traffic is directed over
      the VPN.  All other traffic does not have the overhead of
      traversing the VPN.

   Recommendation-4: Enterprises deploying remote access VPN solutions
   are advised to adapt a strategy of (a) or (b) to avoid VIP address
   conflict with the subnet at the remote site.

   a) If the VPN server being deployed has been provisioned to configure
      two or more address pools, configure the VPN server with a minimum
      of two distinct IP address pools.

   b) Deploy two or more VPN servers with distinct IP address pools.  By
      doing so, a VPN client that detects conflicts of VIP addresses
      with the subnet at the remote site will have the ability to switch
      to an alternate VPN server that will not conflict.

   Recommendation-5: When "Split VPN" is configured on a VPN client of a
   remote PC, enterprises deploying remote access VPN servers are
   advised to assign global IP addresses for the critical servers the
   remote VPN clients are likely to access.

6.  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 applicable 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, specifically 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 (e.g., 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 public 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.

7.  Acknowledgements

   The authors wish to thank Dan Wing for reviewing the document in
   detail and making helpful suggestions in reorganizing the document
   format.  The authors also wish to thank Ralph Droms for helping with
   rewording the text and Recommendation-1 in section 3.2.4 and Cullen
   Jennings for helping with rewording the text and Recommendation-3 in
   section 4.2.2.

8.  References

8.1.  Normative References

   [BEH-ICMP]  Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
               Behavioral Requirements for ICMP", BCP 148, RFC 5508,
               April 2009.

   [BEH-TCP]   Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and
               P. Srisuresh, "NAT Behavioral Requirements for TCP", BCP
               142, RFC 5382, October 2008.

   [BEH-UDP]   Audet, F., Ed., and C. Jennings, "Network Address
               Translation (NAT) Behavioral Requirements for Unicast
               UDP", BCP 127, RFC 4787, January 2007.

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

   [NAT-TRAD]  Srisuresh, P. 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 E. Lear, "Address Allocation for Private Internets",
               BCP 5, RFC 1918, February 1996.

8.2.  Informative References

   [DHCP]      Droms, R., "Dynamic Host Configuration Protocol", RFC
               2131, March 1997.

   [NAT-PROT]  Holdrege, M. and P. Srisuresh, "Protocol Complications
               with the IP Network Address Translator", RFC 3027,
               January 2001.

   [RFC5735]   Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
               BCP 153, RFC 5735, January 2010.

Authors' Addresses

   Pyda Srisuresh
   EMC Corporation
   1161 San Antonio Rd.
   Mountain View, CA  94043
   U.S.A.

   Phone: +1 408 836 4773
   EMail: srisuresh@yahoo.com

   Bryan Ford
   Department of Computer Science
   Yale University
   51 Prospect St.
   New Haven, CT 06511

   Phone: +1-203-432-1055
   EMail: bryan.ford@yale.edu

 

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