Internet Engineering Task Force G. Montenegro INTERNET DRAFT V. Gupta Sun Microsystems, Inc. September 13, 1996 Firewall Support for Mobile IP draft-montenegro-firewall-sup-00.txt Status of This Memo This document is a submission to the Mobile IP Working Group of the Internet Engineering Task Force (IETF). Comments should be submitted either to the author, or 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). Abstract The Mobile IP specification establishes the mechanisms that enable a mobile host to maintain and use the same IP address as it changes its point of attachment to the network. Mobility implies higher security risks than static operation, because the traffic may at times take unforeseen network paths with unknown or unpredictable security characteristics. The Mobile IP specification uses cryptographic techniques to authenticate the parties involved in the registration protocol. However, it makes no provisions for securing data traffic. The mechanisms described in this document allow a mobile node out on a public sector of the network to negotiate access past a SKIP firewall, and construct a secure channel into its Montenegro Expires March 18, 1997 [Page 1] INTERNET DRAFT Firewall Support for Mobile IP September 1996 home network. 1. Introduction This document specifies what support is required at a firewall to allow Mobile IP [1] hosts access into a private network from the Internet. For example, a company employee could attach his/her laptop to some Internet access point by: a) Dialing into a PPP/SLIP account on an Internet service provider's network. b) Connecting into a 10Base-T or similar LAN network available at, for example, an IETF terminal room, a local university, or another company's premises. Notice that in these examples, the mobile node's relevant interface (PPP or 10Base-T) is configured with an IP address different from that which it uses "normally" (i.e. at the office). Furthermore, the IP address used is not necessarily a fixed assignment. It may be assigned temporarily and dynamically at the beginning of the session (e.g. by IPCP in the PPP case, or DHCP in the 10Base-T case). The following discussion assumes a network configuration consisting of a private network separated by a firewall from the general Internet or public network. The systems involved are: Private Network A protected network separated from the Internet by hosts enforcing access restrictions (firewalls). Public Network The Internet at large. Hosts are able to communicate with each other throughout the public network without firewall-imposed restrictions. Mobile Node (MN) Its permanent address falls within the range of the private network. The user may remove the system from its home network, and connects it to the Internet at another point. The mechanisms outlined in this discussion render this mobility transparent: the mobile node continues accessing its home network and its resources exactly as if it were still within it. Notice that when the mobile node leaves its Montenegro Expires March 18, 1997 [Page 2] INTERNET DRAFT Firewall Support for Mobile IP September 1996 home network, it may migrate both within and outside of the private network's boundaries. As defined by Mobile IP [1], a mobile node uses a care-of address while roaming. Pop-up A mobile node whose care-of address is an address associated to one of its own interfaces. This address may be a temporary address acquired dynamically (e.g. by means of DHCP or PPP's IPCP), or through manual intervention. It may also be a permanent address assigned to one of the mobile node's interfaces (e.g. a permanent PPP address). Since pop-ups do not require a separate foreign agent, they can operate in foreign nets that lack Mobile IP support. Home Agent (HA) for the mobile node Serves as a location registry and router as described in the Mobile IP IETF draft. Foreign Agent (FA) Serves as a registration relayer and care of address for the mobile node as described in the Mobile IP IETF draft. Correspondent Host (CH) A system that is exchanging data packets with the mobile node. Its communication with the mobile node does not change regardless of the latter's current point of attachment to the network. Firewall (FW) The system (or collection of systems) that enforces access control between the private network and the general Internet. It may do so by a combination of functions such as application gatewaying, packet filtering and cryptographic techniques. The mechanisms described in this document allow a mobile node out on a public sector of the network to negotiate access past a SKIP firewall, and construct a secure channel into its home network. This enables it to communicate with correspondent hosts that belong to the private network, and, if bi-directional tunnels are used, with external hosts that are reachable when the mobile node is at home. This document does not address the scenarion in which the mobile Montenegro Expires March 18, 1997 [Page 3] INTERNET DRAFT Firewall Support for Mobile IP September 1996 node attempts to access its private network, while within another private network. Sections 2 and 3 provide an overview of the environment being considered and the restrictions it imposes. Sections 4 examines firewall technologies. Section 5 discusses the best mode of operation of the participating entities from the point of view of Mobile IP. Section 6 discusses possible configuration for the secure channel. Finally, packet formats are the topic of of sections 7 and 8. 2. Mobility without a Firewall Suppose the mobile node is roaming throughout the general Internet, but its home network is not protected by a firewall. This is typically found in academic environment as opposed to corporate networks. This works as prescribed by Mobile IP [1]. The only proviso is that the mobile node would most probably operate as a pop-up instead of using a separate foreign agent's care-of address. This is because, at least in the near term, it is far more likely to be able to secure a temporary care-of-address than it is to find a foreign agent already deployed at the site you are visiting. For example: - Internet Service Provider (ISP): pre-assigns customers IP addresses, or hands them out dynamically via PPP's address negotiation. - IETF terminal room: typically pre-assigns addresses for your use - other places would probably offer DHCP services 3. Restrictions imposed by a Firewall The firewall imposes restrictions on packets entering or leaving the private network. Packets are not allowed through unless they conform to a filtering specification, or unless there is a negotiation involving some sort of authentication. Another restriction is imposed by the separation between private addresses and general Internet addresses. Strictly speaking, this is not imposed by a firewall, but by the characteristics of the private network. For example, if a packet destined to an internal address originates in the general Internet, it will probably not be delivered. It is not that the firewall drops it. Rather, the Montenegro Expires March 18, 1997 [Page 4] INTERNET DRAFT Firewall Support for Mobile IP September 1996 Internet's routing fabric is unable to process it. This elicits an ICMP host unreachable packet sent back to the originating node. Because of this, the firewall must be explicitly targeted as the destination node by outside packets seeking to enter the private network. The routing fabric in the general Internet will only see the public address of the firewall and route accordingly. Once the packet arrives at the firewall, the real packet destined to a private address must be recovered. 4. Two Firewall Options: Application relay and IP Security Before delving into any details, lets examine two technologies which may provide firewall support for mobile nodes: - application relaying or proxying, or - IP Security To understand the implications, let's examine two specific schemes to accomplish the above: SOCKS version 5 and SKIP. 4.1 SOCKS version 5 [5] There is an effort within the authenticated firewall traversal WG (aft) of the IETF to provide a common interface for application relays. The solution being proposed is a revised specification of the SOCKS protocol. Version 5 has been extended to include UDP services as well. The SOCKS solution requires that the mobile node -- or another node on its behalf -- establish a TCP session to exchange UDP traffic with the FW. It also has to use the SOCKS library to encapsulate the traffic meant for the FW. The steps required by a SOCKS solution are: - TCP connection established to port 1080 (1.5 round trips) - version identifier/method selection negotiation (1 round trip) - method-dependent negotiation. For example, the Username/Password Authentication [6] requires 1 round trip: 1. client sends a Username/Password request 2. FW (server) responds The GSS-API negotiation [7] requires at least 3 round Montenegro Expires March 18, 1997 [Page 5] INTERNET DRAFT Firewall Support for Mobile IP September 1996 trips: 1. client context establishment (at least 1 round trip) 2. client initial token/server reply (1 round trip) 3. message protection subnegotiation (at least 1 round trip) - (finally) SOCKS request/reply (1 round trip) This is a minimum of 4 (6 with GSS-API) round-trips before the client is able to pass data through the FW using the following header: +----+------+------+----------+----------+----------+ |RSV | FRAG | ATYP | DST.ADDR | DST.PORT | DATA | +----+------+------+----------+----------+----------+ | 2 | 1 | 1 | Variable | 2 | Variable | +----+------+------+----------+----------+----------+ Bear in mind that the above must be done each time the mobile registers a new care-of address. In addition to this inefficiency, this scheme requires that we use UDP to encapsulate IP datagrams. There is at least one commercial network that does this, but it is not the best solution. This header contains the relay information needed by all parties involved to reach those not directly reachable. 4.2 SKIP [4] Alternatively, traffic from the mobile node to the firewall could be encrypted and authenticated using a session-less IP security mechanism like SKIP. This obviates the need to set up a TCP session just to exchange UDP traffic with the firewall. A solution based on SKIP is very attractive in this scenario, as no round trip times are involved before the mobile node and the firewall achieve mutual trust: the firewall can start relaying packets for the mobile node as soon as it receives the first one. This, of course, implies that SKIP is being used with AH [10] so that authentication information is contained in each packet. Encryption by using ESP [9] is also assumed in this scenario, since the Internet at large is considered a hostile environment. An ESP transform that provides both authentication and encryption could be used, in which case the AH header need not be included. The firewall and the mobile node must both be previously configured with the authenticated Diffie-Hellman public values for each other. Montenegro Expires March 18, 1997 [Page 6] INTERNET DRAFT Firewall Support for Mobile IP September 1996 Strictly speaking, they could obtain them in real-time, using any of the mechanisms defined by the SKIP protocol (online certificate directory service or certificate discovery protocol). However, in order to avoid round-trip times, it is preferable to manually distribute each other's public keys to the the mobile node and the SKIP firewall. This is part of the administrative process of enabling a mobile node to roam in the general Internet. Home agents and the firewall must also have access to each others public keys. There are other proposals besides SKIP to achieve IP layer security. However, they are session-oriented key management solutions, and typically imply negotiations spanning several round-trip times before cryptographically secure communications are possible. In this respect they raise similar concerns to those outlined previously in the discussion on SOCKS-based solutions. Others have arrived at similar conclusions regarding the importance of session-less key management for Mobile IP applications [12]. Another advantage of SKIP is its support for nomadic applications. Typically, two hosts communicating via a secure IP layer channel use the IP source and destination addresses on incoming packets to arrive at the appropriate security association. The SKIP header can easily supersede this default mechanism by including the key ID the recipient must use to obtain the right certificate. The access control entry for a nomadic host at a SKIP firewall is a so-called "nomadic" entry, which is filtered by key ID, instead of by IP source address, as is the usual case. It basically translates to "allow access from "any IP source address" if "keyID=". Incoming packets MUST have an AH header, so that after properly authenticating them, the firewall establishes a "current address" for the nomadic host. This information determines which key should be used when encrypting outgoing packets [13]. Notice that this supports Mobile IP, because the mobile node always initiates contact, so the SKIP firewall always has a chance to learn the "current address" to use when encrypting an outgoing packet. However, this precludes the use of simultaneous bindings by a mobile node. At the firewall, the last Registration Request sent by the mobile node replaces the association between its permanent address and any prior care-of address. In order to support simultaneous bindings the firewall must be able to interpret Mobile IP registration messages. If in addition to registration messages, the firewall understands the Mobile IP encapsulation process, it is possible to use Unsigned Diffie-Hellman public values [14]. Doing so greatly reduces SKIP's infrastructure requirements, because there is no need for a Montenegro Expires March 18, 1997 [Page 7] INTERNET DRAFT Firewall Support for Mobile IP September 1996 Certificate Authority. Of course, for this to be possible the principals' names MUST be securely communicated. Section 7.2.2 discusses another advantage of making the firewall understand Mobile IP packet formats. In what follows we assume a SKIP-based solution. 5. Supporting Mobile IP: Agents and Mobile Node Configurations The Mobile IP protocol specifies two ways that a mobile node can register a mobility binding with its home agent, depending on which address it uses as its tunnel endpoint: a) an address advertised for that purpose by the foreign agent b) an address belonging to one of the mobile node's interfaces (i.e. pop-up operation). From the firewall's point of view, the main difference between these two cases hinges on which node prepares the outermost encrypting encapsulation. The firewall must be able to obtain the public keys of the node that creates the outermost SKIP header in an incoming packet. This is only possible to guarantee in case "b", because the mobile node and the firewall both belong to the same administrative domain. The problem is even more apparent when the mobile node attempts a registration request. Here, the foreign agent is not just a relayer, it actually examines the packet sent by the mobile node, and modifies its agent services accordingly. In short, assuming the current specification of Mobile IP and the current lack of trust in the internet at large, only case "b" is possible. Case "a" would require an extension (e.g. a "relay" registration request), and modifying code at the home agent, the firewall and the foreign agent. Assuming that the firewall offers a secure relay service (i.e. decapsulation and forwarding of packets), the mobile node can reach addresses internal to the private network by encapsulating the packets in a SKIP header and directing them to the firewall. Therefore, It is simplest to assume that the mobile node operates as a pop-up. 6. Supporting Mobile IP: Secure Channel Configurations The mobile node participates in two different types of traffic: Montenegro Expires March 18, 1997 [Page 8] INTERNET DRAFT Firewall Support for Mobile IP September 1996 Mobile IP registration protocol and regular data. For the sake of simplicity, the following discussion evaluates different secure channel configurations by examining the initial registration request sent by the mobile node to its home agent. Assuming the mobile node operates as a pop-up, it can talk directly to the firewall. The latter is able to reach the home agent in the private network. Also, the firewall must be able to authenticate the mobile node. The following channel configurations assume the mobile node is operating in pop-up mode. The region between the HA (home agent) and the FW (firewall) is a private network. The region between the FW and the MN (mobile node) is the outside or public network. 6.1 Option 1: Encryption only Outside of Private Network HA FW MN <=====================> SKIP (AH + ESP) <-----------------------------------> Data path The traffic is only encrypted between the mobile node out on the general Internet, and the firewall's external interface. This is minimum required. It is the most desirable configuration as the more expensive encrypted channel is only used where it is necessary: on the public network. 6.2 Option 2: End-to-End Encryption Another possible configuration extends the encrypted tunnel through the FW: HA FW MN <===================================> SKIP (AH + ESP) <-----------------------------------> Data path This limits the FW to perform a simple packet relay or gatewaying function. Even though this could be accomplished by using the proper destination NSID in the packet, in practice it is probably unrealizable. The reason is that this alternative is probably not very popular with computer security personnel, as the authentication functions are now being carried out by the HA, whose security is potentially much weaker than the FW operated by computer security personnel. Montenegro Expires March 18, 1997 [Page 9] INTERNET DRAFT Firewall Support for Mobile IP September 1996 6.3 Option 3: End-to-End Encryption, Intermediate Authentication A third alternative is to allow the FW to be party to the security association with the mobile node for *authentication* purposes only (AH header), and then forward the encrypted packet (ESP hdr) to the HA. HA FW MN <+++++++++++++++++++++> SKIP authentication <===================================> SKIP encryption <-----------------------------------> Data path The SKIP specification refers to this as "Intermediate Authentication with End-to-End security using SKIP" [4]: "Using SKIP, intermediate authentication of end-to-end protected IP traffic MAY be realized, if participating principals can share their long-term private keys with the intermediate node. This may not be desirable if the long-term keys belong to individual users, because of privacy related concerns,..." Whereas Option 2 above is probably not agreeable to security and system administration personnel, option 3 is unsavory to the end user. 6.4 Option 4: Encryption Inside and Outside HA FW MN <============><=====================> SKIP (AH + ESP) <-----------------------------------> Data path Traffic is encrypted on the public as well as on the private network. On the public network, encryption is dictated by a security association between the mobile node and the firewall. On the private network, it is dictated by a security association between the home agent and the firewall. 6.5 Choosing a Secure Channel Configuration A potential problem in both options 2 and 3 is that their end-to-end channel components assume that the mobile node and the home agent have reachability to each other. This is generally not the case, as the Internet routing fabric may not have routes to addresses that belong to private networks, and the private routing fabric may ignore how to route to public addresses -- or doing so may be administratively restricted. Therefore, it is necessary for packets Montenegro Expires March 18, 1997 [Page 10] INTERNET DRAFT Firewall Support for Mobile IP September 1996 to be addressed directly to the firewall, and indirectly -- via some tunneling or relaying capability -- to the real destination on the other side of the firewall. Options 1 and 4 are essentially equivalent. The latter may be considered overkill, because it uses encryption even within the private network, and this is generally not necessary. What is necessary even within the private network is for the home agent to add an encapsulation (not necessarily encrypted) so as to direct datagrams to the mobile node via the firewall. How this encapsulation is achieved is the difference between options 1 and 4. Option 4 uses SKIP, while option 1 uses a cleartext encapsulation mechanism. This is obtainable by, for example, using IP in IP encapsulation [2], or by use of a currently undefined null transform in the SKIP header. Options 1 and 4 are mostly interchangeable, except in pathologically paranoid private networks. For example, option 1 allows a malicious node operation from within the private network to launch a chosen plaintext attack, by sending data through the firewall. In the interest of being conservative, in what follows we assume option 4 (i.e. traffic is encrypted on the general Internet, as well as within the private network. Since the firewall is party to the security associations governing encryption on both the public and private networks, it is always able to inspect the traffic being exchanged by the home agent and the mobile node. If this is of any concern, the home agent and mobile node could set up a bi-directional tunnel [8] and encrypt it. 7. Mobile IP Registration Procedure with a SKIP Firewall When roaming within a private network, a mobile node sends registration requests directly to its home agent. On the public Internet, it MUST encapsulates the original registration request in a SKIP packet destined to the firewall. The mobile node MUST distinguish between "inside" and "outside" addresses. This could be accomplished by a set of rules defining the address ranges. Nevertheless, actual installations may present serious difficulties in defining exactly what is a private address and what is not. Because of this, errors in judgement are to be expected. Accordingly, the firewall SHOULD be configured such that it will still perform its relaying duties even if they are unnecessarily required by a mobile node with an inside care-of address. Montenegro Expires March 18, 1997 [Page 11] INTERNET DRAFT Firewall Support for Mobile IP September 1996 Upon arriving at a foreign net and acquiring a care-of address, the mobile node must first -- before any data transfer is possible -- initiate a registration procedure. This consists of an authenticated exchange by which the mobile node informs its home agent of its current whereabouts (i.e. its current care-of address), and receives an acknowledgement. This first step of the protocol is very convenient, because the SKIP firewall can use it to dynamically configure its packet filter. The remainder if this section shows the packet formats used. Section 7.1 discusses how a mobile node sends a Registration Request to its home agent via the SKIP firewall. Section 7.2 discusses how the home agent send the corresponding Registration Reply to the mobile node. 7.1. Registration Request from the Mobile Node to the Home Agent via the SKIP Firewall The mobile node arrives at a foreign net, and using mechanisms defined by Mobile IP, discovers it has moved away from home. It acquires a local address at the foreign site, and composes a registration request meant for its HA. It must decide whether this packet needs to be processed by SKIP or not. This is not a simple rule triggered by a given destination address. It must be applied whenever the following conditions are met: a) the mobile node is using a care-of address that does not belong to the private network (i.e. the mobile node is "outside" its private network), and b) either of: b1) the source address of the packet is the mobile node's home address, or b2) the source address of the packet is the care-of address and the destination address belongs to the private network Since the above conditions are mobility related, it is best for the Mobile IP function in the node to evaluate them, and then -- using a special API -- request the appropriate security services from SKIP. The SKIP module must use the firewall destination address and the firewall's certificate in order to address and encrypt the packet. It encrypts it using SKIP combined with the ESP [9] protocol and Montenegro Expires March 18, 1997 [Page 12] INTERNET DRAFT Firewall Support for Mobile IP September 1996 possibly the AH [10] protocol. The SKIP header's NSID fields indicate that the Master Key-ID for the source is that of the mobile node's home address, even though the packet's source address corresponds to the care-of address -- an address whose corresponding public key is unknown to the firewall. The SKIP Firewall's dynamic packet filtering uses this information to establish a dynamic mapping between the care-of address and the mobile node's Master Key-ID. The destination NSID field is zero, prompting the firewall to process the SKIP header and recover the internal packet. It then delivers the original packet to another outbound interface, because it is addressed to the home agent (an address within the private network). Assuming secure channel configuration number 4, the firewall will encrypt the packet using SKIP before forwarding to the home agent. PACKET FORMAT 1: +---------------+----------+----+-----+--------------+--------------+ | IP Hdr (SKIP) | SKIP Hdr | AH | ESP | Inner IP Hdr | Reg. Request | +---------------+----------+----+-----+--------------+--------------+ IP Hdr (SKIP): Source mobile node's care-of address Destination public (outside) address on the firewall SKIP Hdr: Source NSID = 1 Master Key-ID = IPv4 address of the mobile node Destination NSID = 0 Master Key-ID = none Inner IP Hdr: Source mobile node's care-of address Destination home agent's address 7.2. Registration Reply from the Home Agent to the Mobile Node via the SKIP Firewall The home agent processes the registration request, and composes a registration reply. Before responding, it examines the care-of address reported by the mobile node, and determines whether or not it corresponds to an outside address. If so, the home agent needs to send all traffic back through the firewall. The home agent can accomplish this by encapsulating the original registration reply in a SKIP packet destined to the firewall (i.e. we assume secure channel configuration number 4). Montenegro Expires March 18, 1997 [Page 13] INTERNET DRAFT Firewall Support for Mobile IP September 1996 7.2.1. On the Inside (Private) Network The packet from the home agent to the mobile node via the SKIP Firewall has the same format as shown above. The relevant fields are: PACKET FORMAT 2: +---------------+----------+----+-----+--------------+------------+ | IP Hdr (SKIP) | SKIP Hdr | AH | ESP | Inner IP Hdr | Reg. Reply | +---------------+----------+----+-----+--------------+------------+ IP Hdr (SKIP): Source home agent's address Destination private (inside) address on the firewall SKIP Hdr: Source NSID = 0 Master Key-ID = none Destination NSID = 0 Master Key-ID = none Inner IP Hdr: Source home agent's address Destination mobile node's care-of address 7.2.2. On the Outside (Public) Network The SKIP Firewall recovers the original registration reply packet and looks at the destination address: the mobile node's care-of address. The SKIP Firewall's dynamic packet filtering used the initial registration request (Secton 7.1) to establish a dynamic mapping between the care-of address and the mobile node's Master Key-ID. Hence, before forwarding the registration reply, it encrypts it using the mobile node's public key. This dynamic binding capability and the use of tunneling mode ESP obviate the need to extend the Mobile IP protocol with a "relay registration request". However, it requires that the Registration Reply exit the private network through the same firewall that forwarded the corresponding Registration Request. Instead of obtaining the mobile node's permanent address from the dynamic binding, a Mobile IP aware firewall could also obtain it from the Registration Reply itself. This renders the firewall stateless, and lets Registration Requests and Replies traverse the periphery of the private network through different firewalls. Montenegro Expires March 18, 1997 [Page 14] INTERNET DRAFT Firewall Support for Mobile IP September 1996 PACKET FORMAT 3: +---------------+----------+----+-----+--------------+------------+ | IP Hdr (SKIP) | SKIP Hdr | AH | ESP | Inner IP Hdr | Reg. Reply | +---------------+----------+----+-----+--------------+------------+ IP Hdr (SKIP): Source firewall's public (outside) address Destination mobile node's care-of address SKIP Hdr: Source NSID = 0 Master Key-ID = none Destination NSID = 1 Master Key-ID = IPv4 addr of the mobile node Inner IP Hdr: Source home agent's address Destination mobile node's care-of address 8. Data Transfer Data transfer proceeds along lines similar to the registration request outlined above. Section 8.1 discusses data traffic sent by a mobile node to a correspondent host. Sections 8.2, 8.3 and 8.4 show three alternative packet formats for the reverse traffic from the home agent to the mobile node. 8.1. Data Packet From the Mobile Node to a Correspondent Host The mobile node composes a packet destined to a correspondent host located within the private network. The Mobile IP function in the mobile node examines the Inner IP header, and determines that it satisfies conditions "a" and "b1" from Section 7.1. The mobile node requests the proper encryption and encapsulation services from SKIP. Thus, the mobile node in pop-up mode sends encrypted traffic to the firewall, using the following format: Montenegro Expires March 18, 1997 [Page 15] INTERNET DRAFT Firewall Support for Mobile IP September 1996 PACKET FORMAT 4: +---------------+----------+----+-----+--------------+------+ | IP Hdr (SKIP) | SKIP Hdr | AH | ESP | Inner IP Hdr | ULP | +---------------+----------+----+-----+--------------+------+ IP Hdr (SKIP): Source mobile node's care-of address Destination public (outside) address on the firewall SKIP Hdr: Source NSID = 1 Master Key-ID = IPv4 address of the mobile node Destination NSID = 0 Master Key-ID = none Inner IP Hdr: Source mobile node's home address Destination correspondent host's address The SKIP Firewall intercepts this packet, decrypts the Inner IP Hdr and upper-layer packet (ULP) and checks the destination address. Since the packet is destined to a correspondent host in the private network, the "Inner" IP datagram is delivered internally. Once the SKIP firewall injects this packet into the private network, it is routed independently of its source address. As this last assumption is not always true, the mobile node may construct a bi-directional tunnel [8] with its home agent. Doing so, guarantees that the "Inner IP Hdr" is: Inner IP Hdr: Source mobile node's home address Destination home agent address When at home, communication between the the mobile node and certain external correspondent hosts might need to go through application-specific firewalls or proxies, different from the SKIP firewall. When on the public network, the mobile node's communication with these hosts, MUST use a bi-directional tunnel. 8.2. Data Packet Tunneled by the Home Agent to the Mobile Node The home agent intercepts a packet from a correspondent host to the mobile node. It encapsulates it such that the Mobile IP header's source and destination addresses are the home agent and care-of addresses, respectively. This would suffice for delivery within the private network. Since the current care-of address of the mobile node is not within the private network, this packet must be sent via Montenegro Expires March 18, 1997 [Page 16] INTERNET DRAFT Firewall Support for Mobile IP September 1996 the firewall. The home agent can accomplish this by encapsulating the datagram in a SKIP packet destined to the firewall (i.e. we assume secure channel configuration number 4). 8.2.1 Within the Inside (Private) Network From the home agent to the private (inside) address of the firewall the packet format is: PACKET FORMAT 5: +--------+------+----+-----+--------+--------+-----+ | IP Hdr | SKIP | AH | ESP | mobip | Inner | ULP | | (SKIP) | Hdr | | | IP Hdr | IP Hdr | | +--------+------+----+-----+--------+--------+-----+ IP Hdr (SKIP): Source home agent's address Destination private (inside) address on the firewall SKIP Hdr: Source NSID = 0 Master Key-ID = none Destination NSID = 0 Master Key-ID = none Mobile-IP IP Hdr: Source home agent's address Destination care-of address Inner IP Hdr: Source correspondent host's address Destination mobile node's address ULP: upper-layer packet The SKIP firewall intercepts the packet and recovers the Mobile IP encapsulated packet. Before sending out this packet, the dynamic packet filter configured by the original registration request above triggers encryption of this packet, this time by the SKIP firewall for consumption by the mobile node. The packet format above does not require the firewall to have a dynamic entry. The association between the mobile node's permanent address and it care-of address can be deduced from the contents of the "Mobile-IP IP Hdr" and the "Inner IP Hdr". Montenegro Expires March 18, 1997 [Page 17] INTERNET DRAFT Firewall Support for Mobile IP September 1996 The home agent MAY eliminate the Mobile IP header if it discriminates between mobile nodes that are registered within the private network, and those that are outside. In this case, the resultant packet is: PACKET FORMAT 6: +---------------+----------+----+-----+--------------+-----+ | IP Hdr (SKIP) | SKIP Hdr | AH | ESP | Inner IP Hdr | ULP | +---------------+----------+----+-----+--------------+-----+ IP Hdr (SKIP): Source home agent's address Destination private (inside) address on the firewall SKIP Hdr: Source NSID = 0 Master Key-ID = none Destination NSID = 0 Master Key-ID = none Inner IP Hdr: Source correspondent host's address Destination mobile node's address The SKIP firewall decrypts the packet, and recovers the original datagram. Before forwarding it, it examines its ruleset. It finds a dynamic rule which was added in response to the Registration Request sent by the mobile node. Accordingly, the SKIP firewall encrypts the packet again before forwarding to the mobile node's care-of address. This optimization requires that the firewall keep some state in the form of the aforementioned dynamic entry for the mobile node. 8.2.2. On the Outside (Public) Network The SKIP firewall intercepts the packet, and--assuming packet format 5--recovers the Mobile IP encapsulated datagram. Before sending it out, the dynamic packet filter configured by the original Registration Request triggers encryption of this packet, this time by the SKIP firewall for consumption by the mobile node. The resultant packet is: Montenegro Expires March 18, 1997 [Page 18] INTERNET DRAFT Firewall Support for Mobile IP September 1996 PACKET FORMAT 7: +--------+------+----+-----+--------+--------+-----+ | IP Hdr | SKIP | AH | ESP | mobip | Inner | ULP | | (SKIP) | Hdr | | | IP Hdr | IP Hdr | | +--------+------+----+-----+--------+--------+-----+ IP Hdr (SKIP): Source firewall's public (outside) address Destination mobile node's care-of address SKIP Hdr: Source NSID = 0 Master Key-ID = none Destination NSID = 1 Master Key-ID = IPv4 address of the mobile node Mobile-IP IP Hdr: Source home agent's address Destination care-of address Inner IP Hdr: Source correspondent host's address Destination mobile node's address If the firewall receives a packet in format 6, the outgoing datagram to the mobile node is: PACKET FORMAT 8: +---------------+----------+----+-----+--------------+-----+ | IP Hdr (SKIP) | SKIP Hdr | AH | ESP | Inner IP Hdr | ULP | +---------------+----------+----+-----+--------------+-----+ IP Hdr (SKIP): Source public (outside) address on the firewall Destination mobile node's care-of address SKIP Hdr: Source NSID = 0 Master Key-ID = none Destination NSID = 1 Master Key-ID = IPv4 address of the mobile node Inner IP Hdr: Source correspondent host's address Destination mobile node's address As an optimization, the firewall MAY produce packet format 8 even if the packet it receives from the home agent is in format 5. This is Montenegro Expires March 18, 1997 [Page 19] INTERNET DRAFT Firewall Support for Mobile IP September 1996 possible even if the firewall lacks state in the form of a dynamic binding. At the mobile node, SKIP processes the packets sent by the firewall. Eventually, the inner IP header and the upper-layer packet (ULP) are retrieved and passed on. 9. Security Considerations The topic of this document is security. Nevertheless, it is imperative to point out the perils involved in allowing a flow of IP packets through a firewall. In essence, the mobile host itself MUST also take on responsibility for securing the private network, because it extends its periphery. This does not mean it stops exchanging unencrypted IP packets with hosts on the public network. For example, it MAY have to do so in order to satisfy billing requirements imposed by the foreign site, or to renew its DHCP lease. In the latter case it might filter not only on IP source address, but also on protocol and port numbers. Therefore, it MUST have some firewall capabilities, otherwise, any malicious individual that gains access to it will have gained access to the private network as well. Acknowledgements Ideas in this document have benefited from discussions with at least the following people: Bill Danielson, Martin Patterson, Tom Markson, Rich Skrenta, Atsushi Shimbo, Behfar Razavi, Avinash Agrawal, Tsutomu Shimomura and Don Hoffman. References [1] C. Perkins. IP Mobility Support. Internet Draft -- work in progress, May 1996. [2] C. Perkins. IP Encapsulation within IP. Internet Draft -- work in progress, May 1996. [3] C. Perkins. Minimal Encapsulation within IP. Internet Draft -- work in progress, May 1996. [4] A. Aziz, T. Markson, H. Prafullchandra. Simple Key-Management For Internet Protocols (SKIP). Internet Draft -- work in progress, August 14, 1996. Montenegro Expires March 18, 1997 [Page 20] INTERNET DRAFT Firewall Support for Mobile IP September 1996 [5] M. Leech, M. Ganis, Y. Lee, R. Kuris, D. Koblas and . Jones. SOCKS Protocol Version 5. RFC 1928, March 1996. [6] M. Leech. Username/Password Authentication for SOCKS V5. RFC 1929, March 1996. [7] P V McMahon. GSS-API Authentication Method for SOCKS Version 5. Internet Draft -- work in progress, July 1995. [8] G. Montenegro. Bi-directional Tunneling for Mobile IP. Internet Draft -- work in progress, September 1996. [9] R. Atkinson. IP Encapsulating Payload. RFC 1827, August 1995 [10] R. Atkinson. IP Authentication Header. RFC 1826, August 1995. [11] A. Aziz, M. Patterson. "Design and Implementation of SKIP". Internet Commerce Group white paper ICG-95-004, June 28, 1995. [12] Stephen Kent, message to the IETF's IPSEC mailing list, Message-Id: , September 6, 1996. [13] Tom Markson, private communication, June 12, 1996. [14] A. Aziz, T. Markson, H. Prafullchandra. Encoding of an Unsigned Diffie-Hellman Public Value. Internet Draft -- work in progress, August 14, 1996. Montenegro Expires March 18, 1997 [Page 21] INTERNET DRAFT Firewall Support for Mobile IP September 1996 Author's Address Gabriel E. Montenegro Sun Microsystems, Inc. 2550 Garcia Avenue Mailstop UMPK 15-214 Mountain View, California 94043-1100 Tel: (415)786-6288 Fax: (415)786-6445 gabriel.montenegro@Eng.Sun.COM Vipul Gupta Sun Microsystems, Inc. 2550 Garcia Avenue Mailstop UMPK 15-214 Mountain View, California 94043-1100 Tel: (415)786-3614 Fax: (415)786-6445 vipul.gupta@Eng.Sun.COM Montenegro Expires March 18, 1997 [Page 22]