Internet Draft P. Srisuresh Document: draft-ford-behave-top-01.txt Consultant Expires: September 5, 2006 B. Ford M.I.T. March 5, 2006 Complications from Network Address Translator Deployment Topologies Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Abstract This document identifies two problems that have arisen from the the unconventional network topologies that are often constructed with the deployment of network address translator devices (NATs). This document also specifies best current practice recommendations for dealing with the issues identified with the two problems. First, the simplicity of administering networks through the combination of NAT and DHCP has increasingly lead to the deployment of multi-level hierarchies of inter-connected private networks involving overlapping private IP address spaces. Second, Srisuresh & Ford [Page 1]
Internet-Draft Complications from NAT Deployments March 2006 the proliferation of private networks in the corporates and the wide spread use of remote access virtual private networks (VPNs) can lead to conflict of private IP address space between the remote network where the VPN client is located and the corporate network. Table of Contents 1. Introduction and Scope........................................ 2. Multi-Level Private Address Space Topologies ................. 2.1. Operational Details of the Network ...................... 2.1.1. Client/Server Communication ........................... 2.1.2. Peer-to-Peer Communication ............................ 2.2. Anomalies and Caveats with the Network .................. 2.2.1. Anomalies with the Network ............................ 2.2.2. Caveats with the Network .............................. 2.3. Recommendations ......................................... 3. Remote Access VPN Topologies with Private Address Space ...... 3.1. Operational Details of the Network ...................... 3.2. Caveats with the Network ................................ 3.3. Recommendations ......................................... 4. Security Considerations ...................................... 5. Informational References ..................................... 1. Introduction and Scope The Internet was originally designed to use a single, global 32-bit IP address space to identify hosts on the network, allowing applications on one host to address and initiate communications with applications on any other host regardless of the respective hosts' topological locations or administrative domains. For a variety of pragmatic reasons, however, the Internet has gradually drifted away from strict conformance to this ideal of a single flat global address space, and towards a hierarchy of smaller "private" address spaces [RFC1918] clustered around a large central "public" address space. The most important pragmatic causes of this unintended evolution of the Internet's architecture appear to be: 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. Srisuresh & Ford [Page 2]
Internet-Draft Complications from NAT Deployments March 2006 In addition, the NAT's translation of a host's original IP addresses and port number in private network into an unrelated, external IP address and port number is perceived by some as a privacy benefit. 3. Ease-of-use: NAT vendors often combine the NAT function with a DHCP server function in the same device, which creates a compelling, effectively "plug-and-play" method of setting up small Internet-attached personal networks that is often much easier in practice for unsophisticated consumers than configuring an IP subnet. The many popular and inexpensive consumer NAT devices on the market are usually configured "out of the box" to obtain a single "public" IP address from an ISP or "upstream" network via DHCP, and the NAT device in turn acts as both a DHCP server and default router for any "downstream" hosts (and even other NATs) that the user plugs into it. Consumer NATs in this way effectively create and manage private home networks automatically without requiring any knowledge of network protocols or management on the part of the user. This ease-of-use benefit of NAT stems ultimately from the fact that DHCP is only capable of providing a single auto-configured IP address to each client. A DHCP client currently has no way to request a *block* of IP addresses from the server, from which it might form its own auto-configured "downstream" IP subnet for use with the DHCP service it offers. The fact that the DHCP function in the NAT devices is capable of auto-configuring client hosts makes NAT devices a compelling solution in this common scenario. The term NAT used throughout the document specifically refers to the traditional NAT, as defined in [NAT-TERM] and specified in [NAT-TRAD]. [NAT-PROT] identifies various complications with application protocols due to NAT devices. This document acts as an adjunct to [NAT-PROT]. The scope of the document is restricted specifically to two problems that were identified as arising out of private address space overlaps. For each of the problems, the document describes the problem statement, caveats, topologies in which the problem can occur, and offers recommendations on how to alleviate. Section 2 describes the problem of private address space overlap due to multi-level hierarchies of private networks and provides recommendations on how to alleviate them. Section 3 describes the problem of private address space conflict between the address space at remote access VPN client locations and the VPN server site, and makes recommendations on how to alleviate them. Section 4 refers the security considerations in these scenarios. Srisuresh & Ford [Page 3]
Internet-Draft Complications from NAT Deployments March 2006 2 Multi-Level Private Address Space Topologies Due to the above pragmatic considerations and perhaps others, NATs are increasingly, and often unintentionally, used to create hierarchically interconnected clusters of private networks as illustrated in the following diagram. 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. Srisuresh & Ford [Page 4]
Internet-Draft Complications from NAT Deployments March 2006 Public Internet (Public IP addresses) ----+---------------+---------------+---------------+---- | | | | | | | | 66.39.3.7 18.181.0.31 138.76.29.7 155.99.25.1 +-------+ Host A Host B +-------------+ | NAT-1 | (Alice) (Jim) | NAT-2 | | (Bob) | | (CheapoISP) | +-------+ +-------------+ 10.1.1.1 10.1.1.1 | | | | Private Network 1 Private Network 2 (private IP addresses) (private IP addresses) ----+--------+---- ----+-----------------------+---- | | | | | | | | | | 10.1.1.10 10.1.1.11 10.1.1.10 10.1.1.11 10.1.1.12 Host C Host D +-------+ Host E +-------+ | NAT-3 | (Mary) | NAT-4 | | (Ann) | | (Lex) | +-------+ +-------+ 10.1.1.1 10.1.1.1 | | | | Private Network 3 | Private Network 4 (private IP addresses)| (private IP addresses) ----+-----------+---+ ----+-----------+---- | | | | | | | | 10.1.1.10 10.1.1.11 10.1.1.10 10.1.1.11 Host F Host G Host H Host I Figure 1: Multi-level NAT topology with private 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 (66.39.3.7) from Bob's ISP via DHCP, and automatically creates a private 10.1.1.x network for Bob's hosts C and D, acting as the DHCP server and default router for Srisuresh & Ford [Page 5]
Internet-Draft Complications from NAT Deployments March 2006 this private network. Bob probably does not even know anything about IP addresses; he merely knows that plugging the NAT into the Internet as instructed by the ISP, and then plugging his hosts into the NAT as the NAT's manual indicates, seems to work and gives all of his hosts access to Internet. CheapoISP, an inexpensive service provider, has allocated only one or a few globally routable IP addresses, and uses NAT to share these public IP addresses among its many customers. Such an arrangement is becoming increasingly common, especially in rapidly-developing countries where the exploding number of Internet-attached hosts greatly outstrips the ability of ISPs to obtain globally unique IP addresses for them. CheapoISP has chosen the popular 10.1.1.x address for its private network, since this is one of the three well-known private IP address blocks allocated in RFC1918 specifically for this purpose. Of the three incentives listed in section 1 for NAT deployment, the last two still apply even to customers of ISPs that use NAT, resulting in multi-level NAT topologies as illustrated in the right side of the above diagram. Even three-level NAT topologies are known to exist. CheapoISP's customers Ann, Mary, and Lex have each obtained a single IP address on CheapoISP's network (Private Network 2), via DHCP. Mary attaches only a single host at this point, but Ann and Lex each independently purchase and deploy consumer NATs in the same way that Bob did above. As it turns out, these consumer NATs also happen to use 10.1.1.x addresses for the private networks they create, since these are the configuration defaults hard-coded into the NATs by their vendors. Ann and Lex probably know nothing about IP addresses, and in particular they are probably unaware that the IP address spaces of their own private networks overlap not only with each other but also with the private IP address space used by their immediately upstream network. Nevertheless, despite this direct overlap, all of the "multi-level NATted hosts" - F, G, H, and I in this case - all nominally function and are able to initiate connections to any public server on the main 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 (3 or 4) into the IP address space of CheapoISP's Private Network 2, and then again by CheapoISP's NAT 2 into the main Internet's global IP address space. 2.1 Operational Details of the Network In the "de facto" Internet address architecture that has resulted from the above pragmatic and economic incentives, only the nodes on the main Internet have globally unique IP addresses assigned by the Srisuresh & Ford [Page 6]
Internet-Draft Complications from NAT Deployments March 2006 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. A given private IP address can be and often is reused across many different private networks, however. In the figure above, for example, private networks 1, 2, 3, and 4 all have a node with IP address 10.1.1.10. 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 private networks can use private IP addresses to refer to other nodes on the same private network, and nodes can use public IP addresses to refer to nodes on the main Internet. Nodes on private networks have no direct method of addressing nodes on other private networks, however, and nodes on the main Internet have no direct way to address nodes on any private network. For example, host F in the figure above 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 has no way even to address host E, even though 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 is "legitimate" in the NAT world because the two hosts are on different private networks. 2.1.1. Client/Server Communication When a host on a private network initiates a client/server-style communication session with a server on the main Internet, via the server's public IP address, the NAT intercepts the packets comprising that session (usually as a consequence of being the default router for the private network), and modifies the packets' IP and TCP/UDP headers so as to make the session appear externally as if it was initiated by the NAT itself. For example, if host C above initiates a connection to host A at IP address 18.181.0.31, NAT 1 modifies the packets comprising the session so as to appear on the main Internet as if the session originated from NAT 1. Similarly, if host F on private network 3 Srisuresh & Ford [Page 7]
Internet-Draft Complications from NAT Deployments March 2006 initiates a connection to host A, NAT 3 modifies the session so as to appear on private network 2 as if it had originated from NAT 3 at IP address 10.1.1.10. The modified session then traverses private network 2 and arrives at NAT 2, which further modifies the session so as to appear on the main Internet as if it had originated from NAT 2 at public IP address 155.99.25.1. The NATs in effect serve as proxies that give their private "downstream" client nodes a temporary presence on "upstream" networks to support individual communication sessions. 2.1.2. Peer-to-Peer Communication While this network organization functions in practice for client/server-style communication, when the client is behind one or more levels of NAT and the server is on the main 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 main 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 in some fashion, 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. 2.2. Anomalies and Caveats with the Network 2.2.1 Anomalies with the network Even though conventional wisdom would suggest that the network described above is seriously broken, in practice it still works in many ways. Let us look at some anomalies here. For example, 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, and is also on subnet 10.1.1/24 through its private interface facing clients, Host-F and Host-G. Similarly the case with NAT-4. 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 are on a single connected LAN (bridged LAN or a single physical LAN). Clearly, that is not the case here. Even though both Srisuresh & Ford [Page 8]
Internet-Draft Complications from NAT Deployments March 2006 NAT-3 and NAT-4 have two interfaces on the same subnet 10.1.1/24, the two interfaces are on two disjoint subnets and LANs. So, the NATs are really not multi-homed. This is an anomaly. Both NAT-3 and NAT-4 are incapable of communicating reliably as a transport endpoint with other nodes on their adjacent networks (ex: private networks 2 and 3 in the case of NAT-3 and private Networks 2 and 4 in the case of NAT-4). This is because applications on either of the NAT devices cannot know to differentiate packets from hosts 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 ARP request on all of its physical interfaces bearing the same subnet, it may get a different response on each of its physical interfaces. This is another anomaly. Broken as it already is, it could have been worse and non-functional if the network layout wasn't carefully orchestrated. For example, all external interfaces for intermediate Nat devices in figure 1 are arranged hierarchically, so the outgoing path for all intermediate NAT devices are oriented towards the Internet facing NAT. Further, the NAT devices provide the DHCP-service on the private interfaces and the NAT service on the external interface. If the nodes are connected differently or the services were offered on the wrong interface, chaos could have ensued. Imagine NAT-3 device having its private interface on private network 2 and NAT interface on private network 3. Chaos would also ensue if there were multiple NAT devices on the same LAN. Multiple NAT devices providing DHCP service on the same LAN, from the same address pool is a recipe for chaos. Multiple nodes would end up having the same IP address. That would make the network broken. The network can also be broken if two NAT nodes attempted to provide NAT service without coordination between the two. The network will also be broken, if the address a NAT node (ex: NAT-3 or NAT-4) assumed on the DHCP-service providing interface is same as the address it is assigned on the external interface. This can easily happen when the vendors are different. Say, both interfaces being assigned 10.1.1.1 2.2.1 Caveats with the network Below are some known caveats with the network shown in figure 1. 1. The NAT boxes (NAT-3, NAT-4, NAT-1, NAT-2) themselves do not provide any service other than respond to ping. NATs are not Srisuresh & Ford [Page 9]
Internet-Draft Complications from NAT Deployments March 2006 managed. 2. Hosts on the private realm are all assumed to be on a single LAN subnet. 3. There can be a security threat. Suppose, an ISP unwisely used RFC1918 IP addresses for its mail servers and say these addresses overlapped with the customer's mail servers. In such a case, Customer mail messages could be hijacked by the ISPs mail server. 2.3. Recommendations Consumer-oriented "Plug and Play" NAT devices MUST, and all NATs SHOULD, be able to handle topologies such as the one described above, in which a private IP address space on one side of the NAT potentially conflicts with the private IP address space on the other side. This means that the NAT must be able to keep the two IP address spaces separate in its internal data structures, and base all packet processing decisions on the "side" or "port" from which the packet arrived and not just on the basis of the IP addresses it contains. NATs should individually conform to [BEHAVE-UDP] and [BEHAVE-TCP] guidelines, especially including hairpin translation support. Peer-to-peer apps should conform to [BEHAVE-APP] guidelines for middlebox traversal. Ideally, ISPs should not NAT their customers. If they do, any servers on the ISP's private network that need to be accessible to the ISP's customers (e.g., mail servers) should have global IP addresses, to ensure accessibility to customers who deploy NAT themselves. NAT boxes should provide an ability to use one of two DHCP address pools and automagically use an address pool that does not conflict with the external interface IP address. 3. Remote Access VPN Topologies with Private Address Space Remote Access Virtual Private Network (VPN) is popular with enterprises. Enterprises use Remote Access VPN to allow secure access to their employees working outside the enterprise premises. While outside the enterprise premises, the employee may be located in his/her home office, hotel, conference or a partner's office. In all cases, it is desirable for the employee at the remote site to have unhindered access to his/her corporate network and the applications running on the corporate networks. This is so the Srisuresh & Ford [Page 10]
Internet-Draft Complications from NAT Deployments March 2006 employee can get his/her work done seamlessly without jeopardizing the integrity and confidentiality of the corporate network and the applications running on the network. Besides authenticating employees for granting access, remote access VPN servers often enforce different forms of security to protect the integrity and confidentiality of the run-time traffic between the VPN client and the remote access server. IPsec, L2TP and SSL are some of the well known secure VPN technologies used by the remote access vendors. Many small enterprises deploy their internal networks using RFC-1918 private address space. The enterprises use NAT devices to connect to the public Internet. Further, many of the applications in the corporate network refer to information (such as URLs) and services using private addresses in the corporate network. Applications such as NFS rely on simple source IP address based filtering to restrict access to corporate users. These are some reasons why the remote access VPN servers are configured with a block of IP addresses from the corporate private network to assign to remote access clients. VPN clients use the virtual IP address assigned to them (by the corporate VPN server) to access applications inside the corporate. Consider the following remote access scenario. Srisuresh & Ford [Page 11]
Internet-Draft Complications from NAT Deployments March 2006 (Corporate Private network 10.0.0.0/8) --------+---------------+------------+---------- | | 10.1.1.1 +---------------+ | | | Secure | | Remote Access | | VPN Server | +------+--------+ 129.32.34.18 | {--------------------+---------------} { } { Public Internet } { (Public IP addresses) } { } {--------------------+---------------} | 155.99.25.1 +----------------+ | NAT | | (in a Hotel | +--------+ | or home office | | | | or a partner's | | DHCP | | corp. office | | Server | | | | | +----------------+ +--------+ 10.1.1.1 10.1.1.2 | | Remote Private Network | | ----+-----------+-----------+-------------+----------+------ | | | | | | | | 10.1.1.10 10.1.1.11 10.1.1.12 10.1.1.13 Host A Host B +--------+ Host C | RA-VPN | | Client | | PC | +--------+ Figure 2: Remote access VPN to access enterprise from private LAN In the above scenario, say an employee of the corporate is at a remote site and trying to access the corporate network using the Srisuresh & Ford [Page 12]
Internet-Draft Complications from NAT Deployments March 2006 VPN client. The corporate network is laid out using 10.0.0.0/8 and say the VPN server is configured with an address block of 10.1.1.0/24 to assign virtual IP addresses to its remote access clients. Now. say the employee at the remote site is attached to a network on the remote site which also happens to be using a RFC-1918 address space based network and coincidentally overlaps the corporate network. In such a situation, there can be several problems with using the VPN client to connect to the remote access server at the enterprise. The following subsections describe the operational of VPNs, caveats with the address overlap scenario and potential remedies to correct the situation. 3.1. Operational Details of the Network Below is a high level description of how a remote access VPN typically works. The specifics may vary from vendor to vendor. The intent is to provide a high level understanding of the operation to gain appreciation for the problem at hand. Typically, when an employee at the remote site launches his/her VPN client, the VPN client is required to authenticate with the VPN server at the corporate premises. Once the authentication succeeds, the VPN server assigns a Virtual IP (VIP) address for use by the VPN client in all its transactions with the corporate network and applications. The VPN client in turn installs a Virtual adapter (VA) on the PC and configures the VA with the VIP address it was assigned by the VPN server. Further, the VPN client adds new routes to the PC such that all the subnets in the corporate are accessible via the Virtual Adapter. By doing this, all traffic directed to and from the corporate networks is redirected to the secure VPN, while leaving all other routes unchanged on the PC. This works well so long as there is no conflict of routes on the PC when new routes to the corporate network are added. This becomes tricky when the corporate intranet network is built using RFC1918 address space and the remote location where the VPN client is launched is also using an overlapping network from RFC1918 address space. In such a situation, the routing table on the PC will have a conflict in accessing nodes on the corporate site and nodes in 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. 3.2. Caveats with the Network Srisuresh & Ford [Page 13]
Internet-Draft Complications from NAT Deployments March 2006 When there is address overlap between corporate network and the remote site, the VPN user may be unaware of this and may loose connectivity to an arbitrary block of services on the corporate network. Worse yet, when a certain service (ex: SMTP mail service) is configured on exactly the same IP address on both the corporate site and the remote site, the user could unknowingly be using the service on the wrong node at remote site, thereby violating the integrity and confidentiality of the contents relating to that application. In the case a corporate address resource overlaps with the router on the remote site, the VPN user could loose connectivity entirely if requests to the router address are redirected to the VPN. Likewise, if a corporate address resource overlaps with the DHCP server on the remote site, the VPN user could also loose connectivity if requests to the DHCP server are redirected to the VPN. 3.3. Recommendations Remote access VPN clients work best when the external client IP address (on the physical network interface) does not overlap with the address space from the corporate VPN address pool. However, this cannot be assumed always. Employees often need to work from locations such as hotel rooms and conferences that use arbitrary blocks of private address spaces, which neither the employee nor the corporate network administrator has any control over. In these situations, the following recommendations will help ensure that a complete "network meltdown" is prevented. 1. The VPN client's external IP address and subnet (at the remote site) should not fall within the VIP address pool assigned by the VPN server. 2. The VPN users should not attempt to access services on the remote site and services on the corporate site simultaneously. Specifically, when the VPN is connected, the VPN user should not attempt to access services on the remote site. Unavoidable services such as the routing and DHCP service at the remote site are exempted. 3. VPN servers should not permit access to corporate services that are running on an IP address that match the following entities at the remote site. a) client's external IP address, b) client's next-hop router IP address used to access the VPN server, and c) DHCP server providing address lease on the external interface. The good news is that all these three essential services are Srisuresh & Ford [Page 14]
Internet-Draft Complications from NAT Deployments March 2006 on the same subnet on the external interface. As a general practice, it is advisable to disallow access to any corporate network resource that overlaps the client's external IP subnet. For example, if the PC's external network interface is configured with 10.1.1.1/24, disallow access to the corporate network that overlaps this subnet from the remote access VPN client. The above recommendations do not guarantee that the remote employee will be able to gain complete access to the corporate network he needs to if there is address overlap. Below are some recommendations to ensure the employee is always able to access mission critical application on the corporate network. 1. Even if most of the private corporate network uses RFC1918 address space, allocate global IP addresses at least for the pool of IP addresses assigned to remote VPN clients, and for the critical servers on the corporate network that the remote VPN clients typically need to access. This will ensure that the remote VPN clients can always access those critical servers regardless of the private address space used at the remote attachment point. 2. We suggest that there be two IP address pools on the VPN server, (or) two VPN servers with different address pools so the address pool which is used for a VPN client doesn't ever conflict with the physical Network interface IP address. For example, the VPN server might detect a conflict and inform the user that he/she should try to connect to the "other" VPN server or IP address pool. Ideally, the VPN client and server could cooperate to perform this negotiation automatically. 3. The subnet mask used on the hotels be as small as possible (say, /29) and the hotels have a centralized DHCP-server that covers multiple small subnets. By doing this, the likelihood of conflict with corporate services is minimized. Perhaps, if the VPN server identified the overlap of the remote IP network and notified the VPN-client of the loss of connectivity to that portion of the corporate world, the VPN-client could do something about it - such as talk to the local admin about assigning himself an IP address from a different subnet (say, plugging to a different plug point) if the hotel has such a facility. 4. Finally, the VPN-client could come in as bump in the stack and redirect all relevant packets in the subnet (with the exception of those that match with the router and DHCP server) over the Srisuresh & Ford [Page 15]
Internet-Draft Complications from NAT Deployments March 2006 VPN. This at least reduces the size of the "black hole" on the corporate network from a whole subnet to merely the specific services running on the DHCP server and the next-hop router. 4. Security Considerations Sections 2.2.1 and 3.2 specify the potential security violations that can arise when there are IP address conflicts from topological deployments. Sections 2.3 and 3.3 recommend ways to protect the users from these security violations. 5. Informational References [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot,G. and Lear, E., "Address Allocation for Private Internets", BCP 5, RFC 1918, February 1996. [KEGEL] Dan Kegel, "NAT and Peer-to-Peer Networking", July 1999. http://www.alumni.caltech.edu/~dank/peer-nat.html [NAT-PROT] M. Holdrege and P. Srisuresh, "Protocol Complications with the IP Network Address Translator", RFC 3027, January 2001. [NAT-PT] G. Tsirtsis and P. Srisuresh, "Network Address Translation - Protocol Translation (NAT-PT)", RFC 2766, February 2000. [NAT-TERM] P. Srisuresh and M. Holdrege, "IP Network Address Translator (NAT) Terminology and Considerations", RFC 2663, August 1999. [NAT-TRAD] P. Srisuresh and K. Egevang, "Traditional IP Network Address Translator (Traditional NAT)", RFC 3022, January 2001. Authors' Addresses: Pyda Srisuresh Consultant 20072 Pacifica Dr. Cupertino, CA 95014 U.S.A. Phone: (408) 836-4773 E-mail: srisuresh@yahoo.com Bryan Ford Srisuresh & Ford [Page 16]
Internet-Draft Complications from NAT Deployments March 2006 Computer Science and Artificial Intelligence Laboratory Massachusetts Institute of Technology 77 Massachusetts Ave. Cambridge, MA 02139 U.S.A. Phone: (617) 253-5261 E-mail: baford@mit.edu Web: http://www.brynosaurus.com/ Full Copyright Statement Copyright (C) The Internet Society (2006). 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 rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Srisuresh & Ford [Page 17]