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Mobile IP Working Group                                         V. Gupta
INTERNET DRAFT                                          SUN Microsystems
                                                                S. Glass
                                                            FTP Software
                                                        January 20, 1997

        Firewall Traversal for Mobile IP: Goals and Requirements

Status of this Memo

   This document is a submission by the Mobile IP Working Group of the
   Internet Engineering Task Force (IETF). Comments should be submitted
   to the mobile-ip@SmallWorks.COM mailing list.

   Distribution of this memo is unlimited.

   This document is an Internet-Draft. Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   To learn the current status of any Internet-Draft, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).


   This document discusses problems arising out of the use of Mobile IP
   in a security conscious Internet and presents a list of solution
   requirements deemed important. These problems may need to be resolved
   in several stages. A priority order in which these problems should be
   solved is also proposed.

   All firewalls are assumed to implement standard mechanisms [RFCs
   1825,1826,1827] for authentication and encryption proposed by the
   IPSec working group of the IETF.

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   1. Introduction

   The IETF Mobile IP protocol [Per96a] allows a mobile node (MN) to
   continue sending and receiving IP datagrams using a fixed address,
   its home address, even when it is no longer connected to its home
   subnet. When a mobile node visits a foreign subnet, it obtains a
   care-of address on that network and registers that address with its
   home agent (HA), a special entity residing on its home subnet. The
   home agent, in turn, intercepts datagrams meant for the mobile node
   and tunnels them to the registered care-of address. Tunneling refers
   to the process of enclosing the original datagram, as data, inside
   another datagram with a new IP header [Per96b, Per96c]. The new
   header carries the mobile node's care-of address in the destination
   field. The care-of address may belong to a specially designated node
   -- the foreign agent (FA) -- or may be a temporary address assigned
   to one of the interfaces of the mobile node, e.g. through DHCP or
   PPP. In the latter case, the mobile node is said to have a co-located
   care-of address. This mode of operation obviates the need for
   explicit mobility support, in the form of a FA, on the foreign
   subnet.  The recipient of a tunneled packet recovers the original
   datagram before processing it further.

   Mobile IP assumes that routing of unicast traffic is based solely on
   the destination address. Many Internet routers now include other
   considerations in their forwarding decision, e.g. in an effort to
   guard against IP-spoofing attacks, source-filtering routers drop
   datagrams that arrive on an interface inconsistent with their source
   address [CA9621]. Such a router, if present on the foreign network,
   will block all datagrams generated by a visiting mobile node carrying
   its home address as source. A solution to this problem is to use a
   reverse tunnel directed from the mobile node's care-of address to its
   home agent [Mont96]. Under this arrangement, datagrams sent from MN
   to a correspondent node (CN) now carry the care-of address (rather
   than the home address) as source in the outermost IP header and pass
   unchallenged through source-filtering routers to the mobile node's
   home agent. The home agent strips the outer IP header and recovers
   the original packet. From then on, the packet is forwarded as if the
   mobile node were on its home subnet.

   Additionally, many organizations use firewalls to protect their
   networks from the general Internet. These firewalls impose additional
   constraints, e.g. they may drop unsolicited datagrams from untrusted
   external hosts [ChZw95]. Such a policy is enforced by carefully
   screening the source and destination addresses, as well as ports, on
   all transport level packets.  For TCP packets, the firewall may also
   monitor the ACK bit. In this situation, a mobile node's registration
   requests are likely to be dropped by a firewall  protecting its home
   network. Note that source-filtering is not an issue here because

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   registration requests arrive with a topologically correct source
   address - namely the current care-of address of the mobile node.

   To complicate matters, organizations often hide the topology of their
   internal network by using private addresses. These addresses are not
   advertised to the general Internet. Such addresses include, but are
   not restricted to, those defined in RFC 1918. The Internet's routing
   fabric is unable to route packets to these addresses (resulting in
   ICMP unreachables). To allow connections from the internal network to
   the general Internet, application relays (also called application
   gateways or proxies) are used. In a typical configuration, the
   internal network is separated from the general Internet by a
   "perimeter network" on which the firewall and proxies are located
   [ChZw95] (see Figure 1). Hosts on the peripheral network use public
   addresses, i.e. their addresses are advertised to the general
   Internet. When a host on the internal network wishes to connect to
   the Internet, two separate connections are set up: one between the
   internal host and the proxy and another between the proxy and the
   outside host. To the external host, the user at the other end appears
   to be on the proxy host.

          Firewall w/    [FW]                                  E
          application   +---+          +------[R1]-----------  R
          proxies       |   |          |    Exterior/          N
                        |   |          |    Access             E
                        +-+-+          |    Router             T**
                          |            |
                 |  Perimeter Network**
     Interior/   |
     Choke      [R2]
     Router      |
                 |                       * = Uses Private Addresses
           Internal Network*            ** = Uses Public Addresses

           Figure 1: Screened-subnet firewall architecture.

   The use of private addresses on firewall-protected networks poses an
   additional challenge. A mobile node belonging to such a network can
   not use its home address (a private address) to communicate directly
   with correspondent nodes when it is outside the protected domain as
   replies from correspondent nodes to the private address are likely to
   generate a "host unreachable" ICMP message. If, somehow, a reverse
   tunnel can be established from the mobile node to its home agent, the

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   mobile node can continue using its private home address. Datagrams
   generated by the mobile node using its home address will appear to
   emerge from its home network and connections to external hosts will
   still involve an intermediate proxy.

   The presence of intermediate firewall(s) disrupts free flow of
   packets from a mobile node on the outside to its home agent on the
   inside. Appropriate security mechanisms are required to successfully
   traverse these firewalls to achieve required connectivity.

   2. Proposed Solution Framework

   The firewall traversal problem in its most general form (e.g.
   multiple firewalls under different administrative controls with
   limited mutual trust) does not lend itself directly to an easy
   solution. Therefore, it is reasonable to try solving this problem in
   several phases starting with the simplest (and still useful) variant
   described below.

   Consider Mobile IP operation for a mobile node (e.g. a notebook
   computer belonging to a corporate employee) when it is moved outside
   its firewall protected home domain into the general internet (e.g. a
   university or ISP environment) which imposes no access restrictions
   other than network ingress filtering. In order to reach it's home
   subnet, packets from a mobile node may need to traverse Multiple
   firewalls. However, they all belong to the mobile node's protected
   domain and their existence and relative placement (with respect to a
   mobile node's current location) is known apriori.

   The simplified problem does not address the case when some of the
   intermediate firewalls belong to administrative domains other than
   the mobile node's, nor does it address the case when a correspondent
   node (outside the mobile node's protected domain) is itself protected
   by a firewall. This latter issue, however, is orthogonal to mobility
   since the problem is unchanged even when the mobile node is at home.

    A reasonable aim is to provide a mobile node with the same
   connectivity (modulo performance penalties) outside its protected
   network that it enjoys at home. Additionally, it should be possible
   to encrypt all traffic to/from the mobile node in an effort to
   preserve the level of privacy that a mobile node enjoys at home.

   The problem of dynamically discovering intermediate firewalls should
   be treated separately at a later time (see item (e) in Section 3).

   3. Issues to Consider

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   This section discusses several important issues that should be
   considered while solving the firewall traversal problem for Mobile

   (a) Security processing of registrations requests sent on behalf of
   (or by) a Mobile node must not depend on its reachability at its home
   address. A violation of this assumption will create a circular
   dependency since a Mobile Node's reachability at its home address is
   at best uncertain until the registration request gets past the

   Solutions that involve a separate phase in which the mobile node
   negotiates access parameters with the firewall should attempt to
   minimize the number of round-trip delays.

   (b) Colocating Home Agent and Firewall functionality on the same host
   considerably simplifies the firewall traversal problem.  However,
   doing so effectively restricts the number of networks with mobility
   support. For example, in Figure 1, hosts on the perimeter network
   would enjoy mobility support but none on the internal network.
   Therefore, a solution must not require Home Agent and Firewall
   functionality to be colocated.

   (c) It is possible to configure intermediate firewalls such that UDP
   packets sent to port 434 of "well known" Home Agents are allowed to
   pass through without any IPSEC processing. This solves the firewall
   traversal problem for registration requests but not for reverse
   tunneled traffic. This approach also involves administrative
   maintenance of the list of "well known" home agents and assumes that
   all processes running on port 434 of these hosts are trusted.
   Ideally, a firewall should decide to let in a packet based solely on
   its IPSEC characteristics (e.g. is it authenticated, encrypted or
   both) rather than any higher level information. A solution that does
   not require Firewalls to understand Mobile IP is preferred.

   (d) If firewalls are unable to parse Mobile IP registrations,
   complications arise when the mobile node uses a care-of address
   belonging to a Foreign Agent (rather than a co-located address).  The
   main problem is that even if the Foreign Agent (FA) implements IPSEC
   and is able to establish a security association with the Mobile
   Node's firewall (FW), it can not win the Firewall's trust. A valid
   authentication header on a packet generated by a FA only tells a FW
   that the packet was indeed generated by the FA. However, the FW has
   no way of knowing whether it was generated on behalf of a trusted
   mobile node. The FA may have generated this packet on its own and the
   FW may not trust the FA. (Obviously, a mobile node must not divulge
   the secret it shares with the FW to its FA just so registrations can
   go through.)

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   Consider the case of a FA relaying a mobile node's (MN) registration
   request.  For this request to pass through FW, it must have an
   authenticator based on a security association between FW and MN. We
   could have MN precompute an "IP Authentication Header (AH)" [RFC
   1826] authenticator which, if inserted by the FA in the relayed
   datagram, will pass authentication at the firewall. The tricky part
   here is that the AH authenticator includes the Identifier field of
   the immediately preceding IP header and there is no easy way for MN
   to predetermine its value generated by FA for the relayed request.

   There is a questionable solution. The MN could generate its own
   Identifier and convey it to the FA with the understanding that the FA
   must use this value in the IP header of the relayed request. This
   value, the AH authenticator and the firewall address etc could all be
   bundled in a new MN to FA registration extension. With this
   mechanism, the FA need not even be an IPSEC node. For the case we are
   considering (FA belongs to a trusting university/ISP environment), it
   is quite likely that none of the mobility agents are capable of IPSEC
   processing.  However, this approach gets more convoluted when a
   mobile node must separately authenticate itself to multiple firewalls
   (see item (e) below).

   In contrast, Mobile nodes operating with a co-located care-of address
   are a lot simpler to deal with. In this case, requests seen by the
   firewall are generated directly by a mobile node for which a trusted
   relationship already exists.

   Solving the firewall traversal problem for mobile nodes registered
   through foreign agents may perhaps be postponed to a later phase.

   (e) When multiple firewalls within the home domain must be traversed,
   each firewall may require a different level of trust, e.g. a large
   corporation may have one firewall guarding its connection to the
   Internet and another guarding the finance department network from the
   rest of the company (Figure 2). Amongst all the employees that are
   allowed access through the main firewall, only a small subset may be
   allowed in through the finance firewall.

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                             [FW1]-------------------- INTERNET
                              / \
                             /   \
                 {Engineering}  [FW2]

        Figure 2: Nested firewalls requiring different levels
   of trust.

   In this scenario, a mobile node on the outside that wishes to
   communicate with its home agent in the finance network must be able
   to explicitly (separately) authenticate itself to each intervening

   (f) Many organizations use firewalls along with private addresses.
   If private addresses are used by the Mobile node's home domain, these
   addresses must not be exposed to routers on the outside. This may
   require additional tunneling e.g. a tunnel from the care-of address
   to at least the (outer most) Firewall may be needed to hide/bury the
   Home Agent's 'private' address as destination on reverse tunneled

   The problem is complicated when routers within the home domain are
   (by design) kept unaware of external destinations. Analogously, extra
   tunneling may be needed even on the inside, e.g.  in Figure 2, a
   tunnel may be needed between a home agent on the finance network and
   (the proxy) FW1 to hide the 'external' care-of destination address on
   encapsulated packets meant for a mobile node roaming outside.

   So far we've only considered the problem of exposing addresses as
   destinations to routers that are unaware of them. To make things
   worse, routers are likely to disallow unknown addresses even in the
   source field of the datagrams they forward.  For example, internal
   routers may disallow an external care-of address as source on reverse
   tunneled traffic. This may necessitate additional tunneling within
   the protected domain even for incoming packets e.g. an external
   care-of address may need to be hidden from R (Figure 2) on reverse-
   tunneled traffic.  Similarly, extra tunneling may be required to hide
   internal addresses (e.g. home agent's) from appearing as source on
   outgoing packets routed by external routers.

   Due to the widespread use of private addresses, solutions to the
   firewall traversal problem must accommodate private addresses.

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   (g) Proposals for dynamic discovery of intervening firewalls must not
   rely on firewalls generating ICMP "administratively unreachable"
   messages since many  firewalls are designed to hide as much of their
   presence as possible.

   According to Chapman and Zwicky [ChZw95, Chap 6, Section titled
   "Returning ICMP Error Codes"]:

   "Returning the new 'host administratively unreachable' or the fact
   that there is a packet filtering system at your site, which you may
   or may not want to do. These codes may also cause excessive reactions
   in faulty IP implementations."

   "If your router returns an ICMP error code for every packet that
   violates your filtering policy, you're also giving an attacker a way
   to probe your filtering system. By observing which packets evoke an
   ICMP error response, attackers can discover what types of packets do
   and don't violate your policy. You should not give this information
   away, because it greatly simplifies the attacker's job. The attacker
   knows that packets that don't get the ICMP error are going somewhere,
   and can concentrate on those protocols, where you actually have
   vulnerabilities. You'd rather that the attacker spent plenty of time
   sending you packets you happily throw away."

   "All in all, the safest thing to do seems to be to drop packets
   without returning any ICMP error codes. If your router offers
   flexibility, it might make sense to configure it to return ICMP error
   codes to internal systems (which would like to know immediately that
   something is going to fail, rather than wait for a timeout), but not
   to external systems ... "

   (h) The Authenticated Firewall Traversal (AFT) working group of the
   IETF has developed the SOCKS protocol [LGLKKJ96] as a general
   mechanism for firewall traversal. The protocol is conceptually a
   "shim-layer" between the application layer and the transport layer,
   and as such does not provide network layer gateway services, such as
   forwarding of ICMP (or even IPinIP) messages. IPinIP messages (from
   HA to MN) would have to be encapsulated within UDP or TCP in order to
   use SOCKS.  Beside the resulting inefficiency, there are other
   concerns regarding its appropriateness for the problem at hand

   The SOCKS solution requires that the mobile node -- or another node
   on its behalf -- establish a TCP session to exchange UDP traffic with
   each firewall. SOCKS incurs a minimum delay of four round-trips (six
   with GSS-API) before a client is able to pass data through the
   firewall. It seems unlikely that this negotiation will be efficient
   enough to allow a mobile node to change its point of attachment as

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   often as once per second, as specified in [Per96a].

   Given the likelihood that most firewalls in the future will be based
   on IPSEC mechanisms, it is imperative that a solution be based on
   standard IPSEC protocols, e.g. ISAKMP/Oakley.

   4. Security Considerations

   Mobile IP assumes that routing of unicast datagrams is based solely
   on the destination address. Packet filtering routers and Firewalls
   include additional considerations in their routing decisions. Special
   mechanisms are needed to ensure Mobile IP operation in the presence
   of these and related (e.g. use of private addresses) security
   measures currently becoming popular on the Internet.


   Ideas in this document have benefited from discussions with at least
   the following people: Gabriel Montenegro, Rich Skrenta and Tsutomo


      [CA9621] CERT Advisory CA-96.21, "TCP SYN Flooding and IP
           Spoofing Attacks", available at ftp://info.cert.org/pub/

      [ChZw95] D. B. Chapman and E. Zwicky, "Building Internet
           Firewalls", O'Reilly & Associates, Inc., Sept. 1995.

      [LGLK96] M. Leech, M. Ganis, Y. Lee, R. Kuris, D. Koblas and
           L. Jones, "SOCKS Protocol Version 5", RFC 1928, March 1996.

      [Leec96] M. Leech, "Username/Password Authentication for SOCKS
           V5", RFC 1929, March 1996.

      [McMa96] P. McMahon, "GSS-API Authentication Method for SOCKS
           Version 5", RFC 1961, June 1996.

      [Mont96] G. Montenegro, "Bi-directional Tunneling for Mobile IP",
           Draft <draft-montenegro-tunneling-01.txt>-- work in progress,
           Sept. 1996.

      [MoGu96] G. Montenegro and V. Gupta, "Firewall Support for Mobile
           IP". Draft <draft-montenegro-firewall-sup-00.txt>, work
           in progress, Sept. 1996.

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      [Per96a] C. Perkins, "IP Mobility Support", RFC 2002.

      [Per96b] C. Perkins, "IP Encapsulation within IP", RFC 2003.

      [Per96c] C. Perkins, "Minimal Encapsulation within IP", RFC 2004.

Author's Address

   Vipul Gupta
   Sun Microsystems, Inc.
   2550 Garcia Avenue
   Mailstop UMPK 15-214
   Mountain View, CA 94043-1100

   Tel: (415) 786 3614
   Fax: (415) 786 6445

   EMail: vipul.gupta@eng.sun.com

   Steven M. Glass
   FTP Software
   2 High Street
   North Andover, MA 01949

   Tel: (508) 685 4000
   Fax: (508) 684 6105

   EMail: glass@ftp.com

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