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INTERNET DRAFT                                                 Franck Le
File: draft-le-mip6-firewalls-01.txt                      Stefano Faccin
Expires: Januray 2005                                    Basavaraj Patil
                                                           H. Tschofenig
                                                               July 2004

                       Mobile IPv6 and Firewalls
                           Problem statement

Status of this Memo

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     Firewalls are an integral aspect of a majority of IP networks today
     given the state of security issues, threats and vulnerabilities to
     data networks. IP networks today are predominantly based on IPv4
     technology and hence firewalls have been designed for these
     networks. IPv6 networks are growing at a slow rate. Firewalls for
     IPv6 networks are still maturing and in development.

     The IETF has recently standardized Mobile IPv6 which adds mobility
     support to IPv6. Given the fact that Mobile IPv6 is a recent
     standard, most firewalls available for IPv6 networks today do not

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     support Mobile IPv6.

     Unless firewalls are aware of Mobile IPv6 protocol details, these
     security devices will hamper large-scale deployment of the
     protocol. This document presents in detail some of the issues that
     people deploying IPv6 networks which include firewalls should
     consider when expanding the scope to support Mobile IPv6 as well.

     The issues are not only applicable to firewalls protecting
     corporate networks, but are also applicable in 3G mobile networks
     such as GPRS/UMTS and cdma2000 networks where packet filters are
     implemented in the GGSN in GPRS/UMTS networks and the PDSN in
     cdma2000 networks.

     The goal of this Internet draft is to highlight the issues with
     firewalls and Mobile IPv6 and act as an enabler for further
     discussion. Issues identified here can be solved by developing
     appropriate solutions in the MIP6 WG.

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1. Introduction

     Mobile IPv6 enables IP mobility for IPv6 nodes. It allows a mobile
     IPv6 node to be reachable via its home IPv6 address irrespective of
     any link that the mobile attaches to. This is possible as a result
     of the extensions to IPv6 defined in the Mobile IPv6 specification

     Mobile IPv6 protocol design also incorporates a feature termed as
     Route Optimization.  This set of extensions is a fundamental part
     of the protocol that allows to optimize the routing of the packets
     between a Mobile Node and its correspondent node and therefore the
     performance of the communication.

     In most cases, current firewall technologies however do not support
     Mobile IPv6 or are even unaware of Mobile IPv6 headers and
     extensions. Since most networks in the current business environment
     deploy firewalls, this may prevent future large-scale deployment of
     the Mobile IPv6 protocol.

     This document presents in detail some of the issues that firewalls
     present for Mobile IPv6 deployment, as well as the impact of each

2. Background information

2.1 Overview of stateful inspection packet filters

     One set of issues is related to the way IP addresses are used in
     Mobile IP, and the way state information is created and maintained
     in stateful inspection packet filters. We refer to the internal
     node as the node connected to the network protected by the
     firewall, and to external node as the node outside the boundaries
     of the network protected by the firewall.

     Subsequently, we describe how stateful inspection packet filters

     When a MN connects to a TCP socket on another host in the Internet,
     it provides, at the connection setup, the socket (IP address and
     port) on which it expects to receive a response.

     When that SYN packet is routed through the firewall, the firewall
     makes an entry in its state table containing the destination socket
     and the response socket, and then forwards the packet to the

     When the response comes back, the filter looks up the packets

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     source and destination sockets in its state table: If they match an
     expected response, the firewall lets the packet pass. If no table
     entry exists, the packet is dropped since it was not requested from
     inside the network.

     The filter removes the state table entries when the TCP close
     session negotiation packets are routed through, or after some
     period of delay, usually a few minutes. This ensures that dropped
     connections do not leave table holes open.

     For UDP, similar state is created but since UDP is connectionless
     and the protocol does not have indication of the beginning nor the
     end of a session, the state is based only on timers.

2.2 Mobile IP6 issues with packet filtering in 3G networks

     In 3G networks, packet filtering functionalities may be implemented
     to prevent malicious nodes from flooding or launching other attacks
     against the 3G subscribers. The packet filtering functionality of
     3G networks are further described in [3GPP].

     In such case, packet filters are set up and applied to both uplink
     and downlink traffic: outgoing and incoming data not matching the
     packet filters is dropped.

     The issues described in the following sections thus also apply to
     3G networks.

3. Scenario when the external node is a Mobile Node

     Let's assume a communication between an internal node A, and an
     external Mobile Node B. The node A is in a network protected by a
     firewall, and node B may also be protected by a firewall but this
     section focuses on the issues related to the firewall protecting
     the node A. Issues related to the firewall protecting node B are
     further described in the following section.

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          +----------------+                +----+
          |                |                | HA |
          |                |                +----+
          |                |              Home Agent
          |  +---+      +----+               of B
          |  | A |      | FW |
          |  +---+      +----+
          |                |                +---+
          |                |                | B |
          |                |                +---+
          +----------------+           External Mobile
          Network protected                  Node
            by a firewall

          Figure 1. Issues between MIP6 and firewalls
            when a firewall is protecting the CN

3.1 Return Routability Test

     As specified in Mobile IPv6 [MIP6], a MN should base its
     communication on the Home IP address of B, IP HoA B, and not on the
     care-of-address that B obtains while attached to a link other than
     its home link.

     The state created by the stateful inspection packet filter
     protecting A is therefore initially based on the IP address of A
     (IP A) and the home address of the node B (IP HoA B).

     If the Mobile Node B is connected to its home link, packets are
     directly exchanged between the nodes A and B. If the Mobile Node B
     is attached to any other link than its home link (in which case it
     has a care-of-address), the session can still be maintained by
     having the MN tunnel the traffic destined to the CN (Node A) via
     its home agent [MIP6]. Packets forwarded by the Home Agent to the
     node A will have the source IP address indicating the Home IP
     address of B and the destination IP address indicating the IP
     address of A. Such packets can thus pass the firewall functionality
     protecting A.

     However nodes A and B might be topologically close to each other
     while B's Home Agent may be far away, resulting in a trombone
     effect that can create delay and degrade the performance.

     Route Optimization is a feature that enables a MN to communicate
     directly with its CN, without involving the MN's Home Agent in the
     data path. So in the current scenario the MN B can initiate the
     route optimization procedure with Node A. Route optimization
     requires the MN B to send a Binding Update to Node A in order to

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     create an entry in its binding cache that maps the MNs home address
     to its current care-of-address. However, prior to sending the
     binding update, the Mobile Node must first execute a Return
     Routability Test:

     - the Mobile Node B has to send a Home Test Init message via its
       Home Agent and

     - a Care of Test Init message directly to its Correspondent Node A.

     The Care of Test Init message is sent using the new CoA of B as the
     source address. Such a packet does not match any entry in the
     firewall protecting A and as described in Section 2, the CoTi
     message will thus be dropped by the firewall. As a consequence, the
     RRT cannot be completed and route optimization cannot be applied.
     Every packet has to go through the node B's Home Agent and tunneled
     between B's Home Agent and B.

          |             +----+     HoTI (HoA)  +----+
          |             | FW |<----------------|HA B|
          |             +----X                 +----+
          |  +---+         | ^ (CoTI is dropped   ^
          |  | A |         | |      by FW)        |
          |  +---+         | |                    | HoTI
          |                | |                    |
          |                | |        CoTI (CoA)+---+
          |                | +------------------| B |
          +----------------+                    +---+
          Network protected                External Mobile
            by a firewall                        Node

          Figure 2. Issues with Return Routability Test

3.2 Issues with Firewall Status Update

     Even if firewalls are made MIPv6 aware (which might require a
     different Binding Update security solution) a firewall might still
     drop packets coming from the new CoA since these incoming packets
     do not match any existing entry.

     The packet filters in the firewall need to be updated with the COA
     of the MN in addition to its HoA.

     Requiring the stateful inspection filters to update the connection
     state upon detecting Binding Update messages from a node outside
     the network protected by the firewall does not appear feasible nor

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     desirable, since currently the firewall does not have any mean to
     verify the validity of Binding Update messages and to therefore
     securely modify the state information.  Changing the firewall
     states without verifying the validity of the Binding Update
     messages could lead to denial of service  attacks. Malicious nodes
     may send faked Binding Update forcing the firewall to change its
     state information, and therefore leading the firewall to drop
     packets from the connections that use the legitimate addresses. An
     adversary might also use an address update to enable its own
     traffic to enter the network.

4. Scenario when the inner node is a Mobile Node

     Let's assume a communication between an internal Mobile Node A,
     protected by a firewall, and an external node B. B can also be a
     Mobile Node protected by a firewall and issues raised in Section 3
     apply in such case.

          +----------------+       +----+
          |                |       | HA |
          |                |       +----+
          |                |      Home Agent
          |  +---+      +----+      of A               +---+
          |  | A |      | FW |                         | B |
          |  +---+      +----+                         +---+
          |Internal        |                         External
          |   MN           |                           Node
          |                |
          Network protected

            Figure 3. Issues between MIP6 and firewalls
            when a firewall is protecting the MN

4.1 Issues with Binding Updates and Acknowledgements between the Mobile
Nodes and their Home Agent

     As required by [MIP6], the Mobile Node and the Home Agent MUST use
     IPsec to protect the integrity and authenticity of the Binding
     Updates and Acknowledgements. Both the Mobile Nodes and the Home
     Agents SHOULD use the Encapsulating Security Payload (ESP).

     Many firewalls would however drop ESP packets (default behavior).
     This may cause the Binding Updates and Acknowledgements between the
     Mobile Nodes and their Home Agent to be dropped.

4.2 Issues with Reachability

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     One of the main advantages of Mobile IPv6 is that it allows the
     Mobile Node to be always reachable thanks to the Home Agent. A node
     desiring to establish a communication will send a packet to the
     Home Address of the MN which causes the packet to be routed to the
     home link of the MN. The Home agent intercepts the packet destined
     for the MN and forwards it to the MNs current point of attachment
     which is indicated by its care-of-address.

     When considering firewalls, (e.g. when the Mobile Node roams to a
     network protected by a firewall), the packet forwarded from the
     Home Agent to the Mobile Node CoA may be dropped at the firewall
     since it does not match any existing entry. The following further
     describes the problem that might occur:

     When entering the visited network, the MN first acquires a Care of
     Address and then sends a Binding Update to its Home Agent. This
     message creates a state in the firewall:

     - it may be a state for the IPsec packet (in the case, the Binding
       Update message is protected by IPsec)

     - or it may be a state for a mobility header in case IPsec is not
       used, but the security of the Binding Update is being provided by
       some other means such as an authentication option as specified in
       [AUTH] to solve the issue described in Section 4.1

     The Binding Acknowledgement response can pass the firewall due to
     the created state, and be delivered to the Mobile Node.

     Some firewalls may leave the created state open for a while
     (implementation dependent), whereas other firewalls may delete the
     state upon receiving the Binding Acknowledgement message.

     Let's assume a Correspondent Node tries to initiate a communication
     with a Mobile Node. The Correspondent Node sends a packet to the
     Mobile Node's home address. The packet is intercepted by the MN's
     Home Agent which tunnels it to the MN's CoA.

     As described in Section 2, the lifetime corresponding to the state
     in the firewall may have been expired and the state may have been
     removed. In such case, the incoming packet sent from the CN does
     not match any existing entry and is therefore dropped at the

     Even if the state created above has not expired yet, the state
     created is for the Binding Update message (IPsec or Mobility
     Header) whereas the packet sent from the CN is received under the
     form of an IP in IP packet. The latter does not match any existing

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     entry and is also dropped.

4.3 Return Routability Test

     Security of Mobile IPv6 Binding Update between the MN and the CN is
     based on the RRT mechanism, the routing infrastructure and secret
     sharing (see [MIP6]). Since some RRT messages are routed via the
     home network, the strong trust relationship between the mobile node
     and the home agent (and the usage of IPsec ESP) is important. As
     specified in Mobile IPv6 [MIP6] in Section 5.2.5:

     "For improved security, the data passed between the Home Agent and
     the Mobile Node is made immune to inspection and passive attacks.
     Such protection is gained by encrypting the home keygen token as it
     is tunneled from the Home Agent to the Mobile Node as specified in
     Section 10.4.6."

     Section 10.4.6 furthermore specifies:

     "The return routability procedure described in Section 5.2.5
     assumes that the confidentiality of the Home Test Init and Home
     Test messages is protected as they are tunneled between the Home
     Agent to the Mobile Node.  Therefore, the Home Agent MUST support
     tunnel mode IPsec ESP for the protection of packets belonging to
     the return routability procedure.".

     This assumption is valid in some environments, however for networks
     protected by a firewall, the requirement can be an issue.

     Typically firewalls need to filter the packets based on the
     source/destination IP addresses and the TCP/UDP source/destination
     ports numbers. When a packet is encrypted using IPsec ESP, such
     information is not available (in particular the port numbers),
     therefore firewalls may drop the Home Test messages forwarded by
     the HA to the MNs CoA. The result is that the MN cannot complete
     the RRT procedure, and consequently cannot perform route
     optimization by sending any Binding Update messages.

     When ESP is applied, the firewall cannot differentiate packets
     containing the Mobility Header defined by MIPv6, i.e., packets for
     which Mobile IPv6 is used, from other packets. In order to support
     RRT, one possible idea could be to let the firewall pass all ESP
     packets coming from the MNs Home Agent. This may, however, not be
     desirable since it would allow several types of attacks (e.g.
     flooding) to be carried out against the MN. In cellular networks
     such flooding may result in attacks such as overbilling since the
     user is required to pay for all air-interface utilization.

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     A common approach, which is also used for NAT traversal, is to
     apply UDP encapsulation of IPsec packets. Unlike with NAT traversal
     it is not possible to detect the presence of a Firewall
     automatically in the same fashion as with a NAT. A NAT modifies the
     source IP address when an IP packet travels from the private to the
     public addressing space. For a Firewall this is not true. Hence,
     UDP encapsulation needs to be enabled proactively.

     The Mobile Node would have to send UDP packets to the Home Agent to
     create the corresponding necessary state in the firewall. The Home
     Agent should also encapsulate the HoT message in a UDP datagram.

     As other possible solutions, the home keygen token could be
     encrypted not using IPsec ESP but specific MIP6 fields within the
     HoT message so that the packet still appears as a Mobility Header
     one to the firewall as specified in [AUTH].

4.4 Issues with Change of CoA

     The internal node A may change its CoA within the network which is
     protected by a firewall. Node A updates its mobility binding at the
     Home Agent by sending a Binding Update. Node A may also send
     Binding Update to its correspondent nodes.

     However, even if firewalls are made MIPv6 aware to address the
     issues described in sections 4.1, 4.2 and 4.3, a firewall might
     still drop incoming packets sent to the new CoA since these
     incoming packets do not match any existing entry.

     The packet filters in the firewall needs to be updated with the new
     COA of the MN.

4.5 Change of firewall

     When the MN A moves, it may move to a link that is served by a
     different firewall. Node A might be sending BU to its CN, however
     incoming packets may be dropped at the firewall since the firewall
     on the new link that the MN attaches to does not have any state
     that is associated with the MN.

5. Conclusion

     Current firewalls may not only prevent route optimization but may
     also prevent communications to be established in some cases. This
     document describes some of the issues between the Mobile IP
     protocol and current firewall technologies.

     The authors consider the problems presented in this document quite

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     relevant to deployments of Mobile IPv6 and intend this information
     be available to the community, so that network administrators who
     intend to deploy Mobile IPv6 can consider them, and so that
     solutions/tools can also be developed to solve the identified

6. Security Considerations

     This document describes several issues that exist between the
     Mobile IPv6 protocol and firewalls.

     Firewalls may prevent Mobile IP6 traffic and drop incoming/outgoing

     If the firewall configuration is modified in order to support the
     Mobile IPv6 protocol but not properly configured, many attacks may
     be possible as outlined above: malicious nodes may be able to
     launch different types of denial of service attacks.

7. References

     [3GPP]     X. Chen, M. Watson, J. Wiljakka and J. Rinne
                Problem Statement for MIPv6 Interactions with GPRS/UMTS
                Packet Filtering IETF internet draft <draft-chen-
                mip6-gprs-01.txt>, July 2004

     [AUTH]     A. Patel, K. Leung, M. Khalil, H. Akhtar and
                K. Chowdhury, Authentication Protocol for Mobile IPv6,
                IETF internet draft <draft-patel-mipv6-auth-
                protocol-01.txt>, May 24, 2004

     [CHES]     William R. Cheswick and Steve M. Bellovin
                Firewalls and Internet Security, Repelling the Wily

     [MIP6]     D. Johnson, C. Perkins, J. Arkko, "Mobility Support
                in IPv6", RFC 3775, June 2004

     [STUN]     Rosenberg, J., Weinberger, J., Huitema, C. and
                R. Mahy, "STUN - Simple Traversal of User Datagram
                Protocol (UDP) Through Network Address Translators
                (NATs)", RFC 3489, March 2003.

8. Author's Addresses:

      Franck Le

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      Nokia Research Center, Dallas
      6000 Connection Drive
      Irving, TX-75039, USA.

      E-Mail: franck.le@nokia.com
      Phone : +1 972 374 1256

      Stefano Faccin
      Nokia Research Center, Dallas
      6000 Connection Drive
      Irving, TX-75039. USA.

      E-Mail: stefano.faccin@nokia.com
      Phone : +1 972 894 4994

      Basavaraj Patil
      Nokia, Dallas
      6000 Connection Drive
      Irving, TX-75039, USA.

      EMail: Basavaraj.Patil@nokia.com
      Phone: +1 972-894-6709

      Hannes Tschofenig
      Otto-Hahn-Ring 6
      Munich, Bayern  81739

      EMail: Hannes.Tschofenig@siemens.com

9. IPR Statement

     Intellectual Property Statement

     The IETF takes no position regarding the validity or scope of any
     Intel- lectual Property Rights or other rights that might be
     claimed to pertain to the implementation or use of the technology
     described in this docu- ment or the extent to which any license
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     Copies of IPR disclosures made to the IETF Secretariat and any

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     assurances of licenses to be made available, or the result of an
     attempt made to obtain a general license or permission for the use
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     specification can be obtained from the IETF on-line IPR repository
     at http://www.ietf.org/ipr.

     The IETF invites any interested party to bring to its attention any
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     Disclaimer of Warranty

     This document and the information contained herein are provided on

     Copyright Statement

     Copyright (C) The Internet Society (2004).  This document is
     subject to the rights, licenses and restrictions contained in BCP
     78, and except as set forth therein, the authors retain all their

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