Network Working Group P. Srisuresh Request for Comments: 2663 M. Holdrege Category: Informational Lucent Technologies August 1999 IP Network Address Translator (NAT) Terminology and Considerations Status of this Memo This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (1999). All Rights Reserved. Preface The motivation behind this document is to provide clarity to the terms used in conjunction with Network Address Translators. The term "Network Address Translator" means different things in different contexts. The intent of this document is to define the various flavors of NAT and standardize the meaning of terms used. The authors listed are editors for this document and owe the content to contributions from members of the working group. Large chunks of the document titled, "IP Network Address Translator (NAT)" were extracted almost as is, to form the initial basis for this document. The editors would like to thank the authors Pyda Srisuresh and Kjeld Egevang for the same. The editors would like to thank Praveen Akkiraju for his contributions in describing NAT deployment scenarios. The editors would also like to thank the IESG members Scott Bradner, Vern Paxson and Thomas Narten for their detailed review of the document and adding clarity to the text.
AbstractNetwork Address Translation is a method by which IP addresses are mapped from one realm to another, in an attempt to provide transparent routing to hosts. Traditionally, NAT devices are used to connect an isolated address realm with private unregistered addresses to an external realm with globally unique registered addresses. This document attempts to describe the operation of NAT devices and the associated considerations in general, and to define the terminology used to identify various flavors of NAT.
sections 8 and 9) hosts in a private network to transparently communicate with destinations on an external network and vice versa. There are a variety of flavors of NAT and terms to match them. This document attempts to define the terminology used and to identify various flavors of NAT. The document also attempts to describe other considerations applicable to NAT devices in general. Note, however, this document is not intended to describe the operations of individual NAT variations or the applicability of NAT devices. NAT devices attempt to provide a transparent routing solution to end hosts trying to communicate from disparate address realms. This is achieved by modifying end node addresses en-route and maintaining state for these updates so that datagrams pertaining to a session are routed to the right end-node in either realm. This solution only works when the applications do not use the IP addresses as part of the protocol itself. For example, identifying endpoints using DNS names rather than addresses makes applications less dependent of the actual addresses that NAT chooses and avoids the need to also translate payload contents when NAT changes an IP address. The NAT function cannot by itself support all applications transparently and often must co-exist with application level gateways (ALGs) for this reason. People looking to deploy NAT based solutions need to determine their application requirements first and assess the NAT extensions (i.e., ALGs) necessary to provide application transparency for their environment. IPsec techniques which are intended to preserve the Endpoint addresses of an IP packet will not work with NAT enroute for most applications in practice. Techniques such as AH and ESP protect the contents of the IP headers (including the source and destination addresses) from modification. Yet, NAT's fundamental role is to alter the addresses in the IP header of a packet.
Section 3.2 has a detailed description of transparent routing.
Note, there is no guarantee that the idea of a session, determined as above by NAT, will coincide with the application's idea of a session. An application might view a bundle of sessions (as viewed by NAT) as a single session and might not even view its communication with its peers as a session. Not all applications are guaranteed to work across realms, even with an ALG (defined below in section 2.9) enroute. RFC 1700 [Ref 2]. section 2.3) as constituting the start of new session.
4 minutes subsequent to this detection. The need for this extended wait period is described in RFC 793 [Ref 7], which suggests a TIME- WAIT duration of 2 * MSL (Maximum Segment Lifetime) or 4 minutes. Note that it is also possible for a TCP connection to terminate without the NAT device becoming aware of the event (e.g., in the case where one or both peers reboot). Consequently, garbage collection is necessary on NAT devices to clean up unused state about TCP sessions that no longer exist. However, it is not possible in the general case to distinguish between connections that have been idle for an extended period of time from those that no longer exist. In the case of UDP-based sessions, there is no single way to determine when a session ends, since UDP-based protocols are application specific. Many heuristic approaches are used to terminate sessions. You can make the assumption that TCP sessions that have not been used for say, 24 hours, and non-TCP sessions that have not been used for a couple of minutes, are terminated. Often this assumption works, but sometimes it doesn't. These idle period session timeouts vary a great deal both from application to application and for different sessions of the same application. Consequently, session timeouts must be configurable. Even so, there is no guarantee that a satisfactory value can be found. Further, as stated in section 2.3, there is no guarantee that NAT's view of session termination will coincide with that of the application. Another way to handle session terminations is to timestamp entries and keep them as long as possible and retire the longest idle session when it becomes necessary. RFC 1918 [Ref 1] has recommendations on address space allocation for private networks. Internet Assigned Numbers Authority (IANA) has three blocks of IP address space, namely 10/8, 172.16/12, and 192.168/16 set aside for private internets. In pre-CIDR notation, the
first block is nothing but a single class A network number, while the second block is a set of 16 contiguous class B networks, and the third block is a set of 256 contiguous class C networks. An organization that decides to use IP addresses in the address space defined above can do so without coordination with IANA or any other Internet registry such as APNIC, RIPE and ARIN. The address space can thus be used privately by many independent organizations at the same time. However, if those independent organizations later decide they wish to communicate with each other or the public Internet, they will either have to renumber their networks or enable NAT on their border routers.
a) Transparent Address assignment. b) Transparent routing through address translation. (routing here refers to forwarding packets, and not exchanging routing information) c) ICMP error packet payload translation. Below is a diagram illustrating a scenario in which NAT is enabled on a stub domain border router, connected to the Internet through a regional router made available by a service provider. \ | / . / +---------------+ WAN . +-----------------+/ |Regional Router|----------------------|Stub Router w/NAT|--- +---------------+ . +-----------------+\ . | \ . | LAN . --------------- Stub border Figure 1: A typical NAT operation scenario
section 2.6 for some heuristic ways to handle session terminations.
________________ ( ) ( External ) +--+ ( Address Realm )-- |__| ( (N-Ext) ) /____\ (________________) Host-X | (Addr-X) |(Addr-Nx) +--------------+ | | | NAT router | | | +--------------+ |(Addr-Np) | ---------------- ( ) +--+ ( Private ) |__|------( Address Realm ) /____\ ( (N-pri) ) Host-A (________________) (Addr-A) Figure 2: A base model to illustrate NAT terms. section 4.2. The following is a description of the properties of realms supported by traditional NAT. IP addresses of hosts in external network are unique and valid in external as well as private networks. However, the addresses of hosts in private network are unique only within the private network and may not be valid in the external network. In other words, NAT would not advertise private networks to the external realm. But, networks from the external realm may be advertised within the private network. The addresses used within private network must not overlap with the external addresses. Any given address must either be a private address or an external address; not both.
A traditional NAT router in figure 2 would allow Host-A to initiate sessions to Host-X, but not the other way around. Also, N-Ext is routable from within N-Pri, whereas N-Pri may not be routable from N-Ext. Traditional NAT is primarily used by sites using private addresses that wish to allow outbound sessions from their site. There are two variations to traditional NAT, namely Basic NAT and NAPT (Network Address Port Translation). These are discussed in the following sub-sections.
A NAPT router in figure 2 may be configured to translate sessions originated from N-Pri into a single external address, say Addr-i. Very often, the external interface address Addr-Nx of NAPT router is used as the address to map N-Pri to.
same address as a host within the local site. If that address were to appear in a packet, it would be forwarded to the internal node rather than through the NAT device to the external realm. Twice-NAT attempts to bridge these realms by translating both source and destination address of an IP packet, as the packet transitions realms. Twice-NAT works as follows. When Host-A wishes to initiate a session to Host-X, it issues a DNS query for Host-X. A DNS-ALG intercepts the DNS query, and in the response returned to Host-A the DNS-ALG replaces the address for Host-X with one that is properly routable in the local site (say Host-XPRIME). Host A then initiates communication with Host-XPRIME. When the packets traverse the NAT device, the source IP address is translated (as in the case of traditional NAT) and the destination address is translated to Host-X. A similar translation is performed on return packets coming from Host-X. The following is a description of the properties of realms supported by Twice-NAT. Network address of hosts in external network are unique in external networks, but not within private network. Likewise, the network address of hosts in private network are unique only within the private network. In other words, the address space used in private network to locate hosts in private and public networks is unrelated to the address space used in public network to locate hosts in private and public networks. Twice NAT would not be allowed to advertise local networks to the external network or vice versa. A Twice NAT router in figure 2 would allow Host-A to initiate sessions to Host-X, and Host-X to initiate sessions to Host-A. However, N-Ext (or a subset of N-Ext) is not routable from within N- Pri, and N-Pri is not routable from N-Ext. Twice NAT is typically used when address space used in a Private network overlaps with addresses used in the Public space. For example, say a private site uses the 184.108.40.206/24 address space which is officially assigned to another site in the public internet. Host_A (220.127.116.11) in Private space seeks to connect to Host_X (18.104.22.168) in Public space. In order to make this connection work, Host_X's address is mapped to a different address for Host_A and vice versa. The twice NAT located at the Private site border may be configured as follows:
Private to Public : 22.214.171.124/24 -> 126.96.36.199/24 Public to Private : 188.8.131.52/24 -> 172.16.1.0/24 Datagram flow : Host_A(Private) -> Host_X(Public) a) Within private network DA: 172.16.1.100 SA: 184.108.40.206 b) After twice-NAT translation DA: 220.127.116.11 SA: 18.104.22.168 Datagram flow Host_X (Public) -> Host_A (Private) a) Within Public network DA: 22.214.171.124 SA: 126.96.36.199 b) After twice-NAT translation, in private network SA: 188.8.131.52 DA: 172.16.1.100
Multiple NAT boxes or multiple links on the same NAT box, sharing the same NAT configuration can provide fail-safe backup for each other. In such a case, it is necessary for backup NAT device to exchange state information so that a backup NAT can take on session load transparently when the primary NAT fails. NAT backup becomes simpler, when configuration is based on static maps.
normal between the border router and the remote destination. Note, the tunnel from the client TO the border router may not be necessary. You might be able to just forward the packet directly. This should work so long as your internal network isn't filtering packets based on source addresses (which in this case would be external addresses). As an example, Host-A in figure 2 above, could assume an address Addr-k from the external realm and act as RSA-IP-Client to allow end-to-end sessions between Addr-k and Addr-X. Traversal of end-to- end packets within private realm may be illustrated as follows: First method, using NAT router enroute to translate: =================================================== Host-A NAT router Host-X ------ ----------- ------ <Outer IP header, with src=Addr-A, Dest=Addr-X>, embedding <End-to-end packet, with src=Addr-k, Dest=Addr-X> -----------------------------> <Outer IP header, with src=Addr-k, Dest=Addr-X>, embedding <End-to-end packet, with src=Addr-k, Dest=Addr-X> ---------------------------> . . . <Outer IP header, with src=Addr-X, Dest=Addr-k>, embedding <End-to-end packet, with src=Addr-X, Dest=Addr-k> <--------------------------------- <Outer IP header, with src=Addr-X, Dest=Addr-A>, embedding <End-to-end packet, with src=Addr-X, Dest=Addr-k> <--------------------------------------
Second method, using a tunnel within private realm: ================================================== Host-A NAT router Host-X ------ ----------- ------ <Outer IP header, with src=Addr-A, Dest=Addr-Np>, embedding <End-to-end packet, with src=Addr-k, Dest=Addr-X> -----------------------------> <End-to-end packet, with src=Addr-k, Dest=Addr-X> -------------------------------> . . . <End-to-end packet, with src=Addr-X, Dest=Addr-k> <-------------------------------- <Outer IP header, with src=Addr-Np, Dest=Addr-A>, embedding <End-to-end packet, with src=Addr-X, Dest=Addr-k> <---------------------------------- There may be other approaches to pursue. An RSA-IP-Client has the following characteristics. The collective set of operations performed by an RSA-IP-Client may be termed "RSA- IP". 1. Aware of the realm to which its peer nodes belong. 2. Assumes an address from external realm when communicating with hosts in that realm. Such an address may be assigned statically or obtained dynamically (through a yet-to-be-defined protocol) from a node capable of assigning addresses from external realm. RSA-IP-Server could be the node coordinating external realm address assignment.
3. Route packets to external hosts using an approach amenable to RSA-IP-Server. In all cases, RSA-IP-Client will likely need to act as a tunnel end-point, capable of encapsulating end-to-end packets while forwarding and decapsulating in the return path. "Realm Specific Address IP Server" (RSA-IP server) is a node resident on both private and external realms, that facilitates routing of external realm packets specific to RSA-IP clients inside a private realm. An RSA-IP-Server may be described as having the following characteristics. 1. May be configured to assign addresses from external realm to RSA-IP-Clients, either statically or dynamically. 2. Must be a router resident on both the private and external address realms. 3. Must be able to provide a mechanism to route external realm packets within private realm. Of the two approaches described, the first approach requires RSA-IP-Server to be a NAT router providing transparent routing for the outer header. This approach requires the external peer to be a tunnel end-point. With the second approach, an RSA-IP-Server could be any router (including a NAT router) that can be a tunnel end-point with RSA-IP-Clients. It would detunnel end-to-end packets outbound from RSA-IP-Clients and forward to external hosts. On the return path, it would locate RSA-IP-Client tunnel, based on the destination address of the end-to-end packet and encapsulate the packet in a tunnel to forward to RSA-IP-Client. RSA-IP-Clients may pursue any of the IPsec techniques, namely transport or tunnel mode Authentication and confidentiality using AH and ESP headers on the embedded packets. Any of the tunneling techniques may be adapted for encapsulation between RSA-IP-Client and RSA-IP-Server or between RSA-IP-Client and external host. For example, IPsec tunnel mode encapsulation is a valid type of encapsulation that ensures IPsec authentication and confidentiality for the embedded end-to-end packets.
"RSAP-IP-Client" may be defined similar to RSA-IP-Client with the variation that RSAP-IP-Client assumes a tuple of (external address, transport Identifier) when connecting to hosts in external realm to pursue end-to-end communication. As such, communication with external nodes for an RSAP-IP-Client may be limited to TCP, UDP and ICMP sessions. "RSAP-IP-Server" is similar to RSA-IP-Server in that it facilitates routing of external realm packets specific to RSAP-IP clients inside a private realm. Typically, an RSAP-IP-Server would also be the one to assign transport tuples to RSAP-IP-Clients. A NAPT router enroute could serve as RSAP-IP-Server, when the outer encapsulation is TCP/UDP based and is addressed between the RSAP-IP- Client and external peer. This approach requires the external peer to be the end-point of TCP/UDP based tunnel. Using this approach, RSAP-IP-Clients may pursue any of the IPsec techniques, namely transport or tunnel mode authentication and confidentiality using AH and ESP headers on the embedded packets. Note however, IPsec tunnel mode is not a valid type of encapsulation, as a NAPT router cannot provide routing transparency to AH and ESP protocols. Alternately, packets may be tunneled between RSAP-IP-Client and RSAP-IP-Server such that RSAP-IP-Server would detunnel packets outbound from RSAP-IP-Clients and forward to external hosts. On the return path, RSAP-IP-Server would locate RSAP-IP-Client tunnel, based on the tuple of (destination address, transport Identifier) and encapsulate the original packet within a tunnel to forward to RSAP- IP-Client. With this approach, there is no limitation on the tunneling technique employed between RSAP-IP-Client and RSAP-IP- Server. However, there are limitations to applying IPsec based security on end-to-end packets. Transport mode based authentication and integrity may be attained. But, confidentiality cannot be permitted because RSAP-IP-Server must be able to examine the destination transport Identifier in order to identify the RSAP-IP- tunnel to forward inbound packets to. For this reason, only the transport mode TCP, UDP and ICMP packets protected by AH and ESP- authentication can traverse a RSAP-IP-Server using this approach. As an example, say Host-A in figure 2 above, obtains a tuple of (Addr-Nx, TCP port T-Nx) from NAPT router to act as RSAP-IP-Client to initiate end-to-end TCP sessions with Host-X. Traversal of end-to- end packets within private realm may be illustrated as follows. In the first method, outer layer of the outgoing packet from Host-A uses (private address Addr-A, source port T-Na) as source tuple to communicate with Host-X. NAPT router enroute translates this tuple into (Addr-Nx, Port T-Nxa). This translation is independent of RSAP- IP-Client tuple parameters used in the embedded packet.
First method, using NAPT router enroute to translate: ==================================================== Host-A NAPT router Host-X ------ ----------- ------ <Outer TCP/UDP packet, with src=Addr-A, Src Port=T-Na, Dest=Addr-X>, embedding <End-to-end packet, with src=Addr-Nx, Src Port=T-Nx, Dest=Addr-X> -----------------------------> <Outer TCP/UDP packet, with src=Addr-Nx, Src Port=T-Nxa, Dest=Addr-X>, embedding <End-to-end packet, with src=Addr-Nx, Src Port=T-Nx, Dest=Addr-X> ---------------------------------------> . . . <Outer TCP/UDP packet with src=Addr-X, Dest=Addr-Nx, Dest Port=T-Nxa>, embedding <End-to-end packet, with src=Addr-X, Dest=Addr-Nx, Dest Port=T-Nx> <---------------------------------- <Outer TCP/UDP packet, with src=Addr-X, Dest=Addr-A, Dest Port=T-Na>, embedding <End-to-end packet, with src=Addr-X, Dest=Addr-Nx, Dest Port=T-Nx> <-----------------------------------
Second method, using a tunnel within private realm: ================================================== Host-A NAPT router Host-X ------ ----------- ------ <Outer IP header, with src=Addr-A, Dest=Addr-Np>, embedding <End-to-end packet, with src=Addr-Nx, Src Port=T-Nx, Dest=Addr-X> -----------------------------> <End-to-end packet, with src=Addr-Nx, Src Port=T-Nx, Dest=Addr-X> --------------------------------> . . . <End-to-end packet, with src=Addr-X, Dest=Addr-Nx, Dest Port=T-Nx> <---------------------------------- <Outer IP header, with src=Addr-Np, Dest=Addr-A>, embedding <End-to-end packet, with src=Addr-X, Dest=Addr-Nx, Dest Port=T-Nx> <----------------------------------
RFC 2385 [Ref 17] do not. In IPSec transport mode, both AH and ESP have an integrity check covering the entire payload. When the payload is TCP or UDP, the TCP/UDP checksum is covered by the integrity check. When a NAT device modifies an address the checksum is no longer valid with respect to the new address. Normally, NAT also updates the checksum, but this is ineffective when AH and ESP are used. Consequently, receivers will discard a packet either because it fails the IPSec integrity check (if the NAT device updates the checksum), or because the checksum is invalid (if the NAT device leaves the checksum unmodified). Note that IPsec tunnel mode ESP is permissible so long as the embedded packet contents are unaffected by the outer IP header translation. Although this technique does not work in traditional NAT deployments (i.e., where hosts are unaware that NATs are present), the technique is applicable to Realm-Specific IP as described in Section 5.0. Note also that end-to-end ESP based transport mode authentication and confidentiality are permissible for packets such as ICMP, whose IP payload content is unaffected by the outer IP header translation. NAT devices also break fundamental assumptions by public key distribution infrastructures such as Secure DNS RFC 2535 [Ref 18] and X.509 certificates with signed public keys. In the case of Secure
DNS, each DNS RRset is signed with a key from within the zone. Moreover, the authenticity of a specific key is verified by following a chain of trust that goes all the way to the DNS root. When a DNS- ALG modifies addresses (e.g., as in the case of Twice-NAT), verification of signatures fails. It may be of interest to note that IKE (Session key negotiation protocol) is a UDP based session layer protocol and is not protected by network based IPsec security. Only a portion of the individual payloads within IKE are protected. As a result, IKE sessions are permissible across NAT, so long as IKE payload does not contain addresses and/or transport IDs specific to one realm and not the other. Given that IKE is used to setup IPSec associations, and there are at present no known ways of making IPSec work through a NAT function, it is a future work item to take advantage of IKE through a NAT box. One of the most popular internet applications "FTP" would not work with the definition of NAT as described. The following sub-section is devoted to describing how FTP is supported on NAT devices. FTP ALG is an integral part of most NAT implementations. Some vendors may choose to include additional ALGs to custom support other applications on the NAT device.
techniques is not attainable with NAT devices in between. One of the ends must be a NAT box. Refer section 7.0 for a discussion on why end-to-end IPsec security cannot be assured with NAT devices along the route. The combination of NAT functionality, ALGs and firewalls will provide a transparent working environment for a private networking domain. With the exception of RSIP, end-to-end network security assured by IPsec cannot be attained for end-hosts within the private network (Refer section 5.0 for RSIP operation). In all other cases, if you want to use end-to-end IPsec, there cannot be a NAT device in the path. If we make the assumption that NAT devices are part of a trusted boundary, tunnel mode IPsec can be accomplished with NAT router (or a combination of NAT, ALGs and firewall) serving as tunnel end point. NAT devices, when combined with ALGs, can ensure that the datagrams injected into Internet have no private addresses in headers or payload. Applications that do not meet these requirements may be dropped using firewall filters. For this reason, it is not uncommon to find NAT, ALG and firewall functions co-exist to provide security at the borders of a private network. NAT gateways can be used as tunnel end points to provide secure VPN transport of packet data across an external network domain. Below are some additional security considerations associated with NAT routers. 1. UDP sessions are inherently unsafe. Responses to a datagram could come from an address different from the target address used by sender ([Ref 4]). As a result, an incoming UDP packet might match the outbound session of a traditional NAT router only in part (the destination address and UDP port number of the packet match, but the source address and port number may not). In such a case, there is a potential security compromise for the NAT device in permitting inbound packets with partial match. This UDP security issue is also inherent to firewalls. Traditional NAT implementations that do not track datagrams on a per-session basis but lump states of multiple UDP sessions using the same address binding into a single unified session could compromise the security even further. This is because, the granularity of packet matching would be further limited to just the destination address of the inbound UDP packets. 2. Multicast sessions (UDP based) are another source for security weakness for traditional-NAT routers. Once again, firewalls face the same security dilemma as the NAT routers.
Say, a host on private network initiated a multicast session. Datagram sent by the private host could trigger responses in the reverse direction from multiple external hosts. Traditional-NAT implementations that use a single state to track a multicast session cannot determine for certain if the incoming UDP packet is in response to an existing multicast session or the start of new UDP session initiated by an attacker. 3. NAT devices can be a target for attacks. Since NAT devices are Internet hosts they can be the target of a number of different attacks, such as SYN flood and ping flood attacks. NAT devices should employ the same sort of protection techniques as Internet-based servers do.  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot,G. and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, February 1996.  Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700, October, 1994.  Braden, R., "Requirements for Internet Hosts -- Communication Layers", STD 3, RFC 1122, October 1989.  Braden, R., "Requirements for Internet Hosts -- Application and Support", STD 3, RFC 1123, October 1989.  Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, June 1995.  Postel, J. and J. Reynolds, "File Transfer Protocol (FTP)", STD 9, RFC 959, October 1985.  Postel, J., "Transmission Control Protocol (TCP) Specification", STD 7, RFC 793, September 1981.  Postel, J., "Internet Control Message Protocol Specification" STD 5, RFC 792, September 1981.  Postel, J., "User Datagram Protocol (UDP)", STD 6, RFC 768, August 1980.  Mogul, J. and J. Postel, "Internet Standard Subnetting Procedure", STD 5, RFC 950, August 1985.
 Carpenter, B., Crowcroft, J. and Y. Rekhter, "IPv4 Address Behavior Today", RFC 2101, February 1997.  Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998.  Kent, S. and R. Atkinson, "IP Encapsulating Security Payload (ESP)", RFC 2406, November 1998.  Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402, November 1998.  Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", RFC 2409, November 1998.  Piper, D., "The Internet IP Security Domain of Interpretation for ISAKMP", RFC 2407, November 1998.  Heffernan, A., "Protection of BGP Sessions via the TCP MD5 Signature Option", RFC 2385, August 1998.  Eastlake, D., "Domain Name System Security Extensions", RFC 2535, March 1999.
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