Network address translation
Network address translation (NAT) is a method of mapping an IP address space into another by modifying network address information in the IP header of packets while they are in transit across a traffic routing device.[1] The technique was originally used to bypass the need to assign a new address to every host when a network was moved, or when the upstream Internet service provider was replaced, but could not route the network's address space. It has become a popular and essential tool in conserving global address space in the face of IPv4 address exhaustion. One Internet-routable IP address of a NAT gateway can be used for an entire private network.[2]
As network address translation modifies the IP address information in packets, NAT implementations may vary in their specific behavior in various addressing cases and their effect on network traffic. The specifics of NAT behavior are not commonly documented by vendors of equipment containing NAT implementations.[2]
History
IPv4 uses 32-bit addresses, capable of uniquely addressing about 4.4 billion devices. By 1992 it became evident that that would not be enough. The 1994
Basic NAT
The simplest type of NAT provides a one-to-one translation of IP addresses (RFC 1631).
One-to-many NAT
The majority of network address translators map multiple private hosts to one publicly exposed IP address.
Here is a typical configuration:
- A local network uses one of the designated private IP address subnets (RFC 1918[4]).
- The network has a router having both a private and a public address. The private address is used by the router for communicating with other devices in the private local network. The public address (typically assigned by an Internet service provider) is used by the router for communicating with the rest of the Internet.
- As traffic passes from the network to the Internet, the router translates the source address in each packet from a private address to the router's own public address. The router tracks basic data about each active connection (particularly the destination address and port). When the router receives inbound traffic from the Internet, it uses the connection tracking data it stored during the outbound phase to determine to which private address (if any) it should forward the reply.[2]
All IP packets have a source IP address and a destination IP address. Typically, packets passing from the private network to the public network will have their source address modified, while packets passing from the public network back to the private network will have their destination address modified. To avoid ambiguity in how replies are translated, further modifications to the packets are required. The vast bulk of Internet traffic uses
This method allows communication through the router only when the conversation originates in the private network, since the initial originating transmission is what establishes the required information in the translation tables. Thus a web browser within the private network would be able to browse websites that are outside the network, whereas web browsers outside the network would be unable to browse a website hosted within.[a] Protocols not based on TCP and UDP require other translation techniques.
An additional benefit of one-to-many NAT is that it mitigates IPv4 address exhaustion by allowing entire networks to be connected to the Internet using a single public IP address.[b]
Methods of translation
Network address and port translation may be implemented in several ways. Some applications that use IP address information may need to determine the external address of a network address translator. This is the address that its communication peers in the external network detect. Furthermore, it may be necessary to examine and categorize the type of mapping in use, for example when it is desired to set up a direct communication path between two clients both of which are behind separate NAT gateways.
For this purpose, RFC 3489 specified a protocol called Simple Traversal of UDP over NATs (STUN) in 2003. It classified NAT implementations as full-cone NAT, (address) restricted-cone NAT, port-restricted cone NAT or symmetric NAT, and proposed a methodology for testing a device accordingly. However, these procedures have since been deprecated from standards status, as the methods are inadequate to correctly assess many devices. RFC 5389 standardized new methods in 2008 and the acronym STUN now represents the new title of the specification: Session Traversal Utilities for NAT.
Full-cone NAT, also known as one-to-one NAT
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(Address)-restricted-cone NAT
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Port-restricted cone NAT Like an address restricted cone NAT, but the restriction includes port numbers.
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Symmetric NAT
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Many NAT implementations combine these types, so it is better to refer to specific individual NAT behavior instead of using the Cone/Symmetric terminology. RFC 4787 attempts to alleviate confusion by introducing standardized terminology for observed behaviors. For the first bullet in each row of the above table, the RFC would characterize Full-Cone, Restricted-Cone, and Port-Restricted Cone NATs as having an Endpoint-Independent Mapping, whereas it would characterize a Symmetric NAT as having an Address- and Port-Dependent Mapping. For the second bullet in each row of the above table, RFC 4787 would also label Full-Cone NAT as having an Endpoint-Independent Filtering, Restricted-Cone NAT as having an Address-Dependent Filtering, Port-Restricted Cone NAT as having an Address and Port-Dependent Filtering, and Symmetric NAT as having either an Address-Dependent Filtering or Address and Port-Dependent Filtering. Other classifications of NAT behavior mentioned in the RFC include whether they preserve ports, when and how mappings are refreshed, whether external mappings can be used by internal hosts (i.e., its
NAT mapping vs NAT filtering
RFC 4787[2] makes a distinction between NAT mapping and NAT filtering.
Section 4.1 of the RFC covers NAT mapping and specifies how an external IP address and port number should be translated into an internal IP address and port number. It defines Endpoint-Independent Mapping, Address-Dependent Mapping and Address and Port-Dependent Mapping, explains that these three possible choices do not relate to the security of the NAT as security is determined by the filtering behavior and then specifies 'A NAT MUST have an "Endpoint-Independent Mapping" behavior.'
Section 5 of the RFC covers NAT filtering and describes what criteria are used by the NAT to filter packets originating from specific external endpoints. The options are Endpoint-Independent Filtering, Address-Dependent Filtering and Address and Port-Dependent Filtering. Endpoint-Independent Filtering is recommended where maximum application transparency is required while Address-Dependent Filtering is recommended where more stringent filtering behavior is most important.
Some NAT devices are not yet compliant with RFC 4787 as they treat NAT mapping and filtering in the same way so that their configuration option for changing the NAT filtering method also changes the NAT mapping method (e.g. Netgate TNSR). The PF firewall has a patch available to enable RFC 4787 support but this has not yet been merged.
Type of NAT and NAT traversal, role of port preservation for TCP
The NAT traversal problem arises when peers behind different NATs try to communicate. One way to solve this problem is to use port forwarding. Another way is to use various NAT traversal techniques. The most popular technique for TCP NAT traversal is TCP hole punching.
TCP hole punching requires the NAT to follow the port preservation design for TCP. For a given outgoing TCP communication, the same port numbers are used on both sides of the NAT. NAT port preservation for outgoing TCP connections is crucial for TCP NAT traversal because, under TCP, one port can only be used for one communication at a time, so programs bind distinct TCP sockets to
On the other hand, for UDP, NATs do not need port preservation. Indeed, multiple UDP communications (each with a distinct endpoint) can occur on the same source port, and applications usually reuse the same UDP socket to send packets to distinct hosts. This makes port prediction straightforward, as it is the same source port for each packet.
Furthermore, port preservation in NAT for TCP allows P2P protocols to offer less complexity and less latency because there is no need to use a third party (like STUN) to discover the NAT port since the application itself already knows the NAT port.[2][5]
However, if two internal hosts attempt to communicate with the same external host using the same port number, the NAT may attempt to use a different external IP address for the second connection or may need to forgo port preservation and remap the port.[2]: 9
As of 2006[update], roughly 70% of the clients in P2P networks employed some form of NAT.[6]
Implementation
Establishing two-way communication
Every TCP and UDP packet contains a source port number and a destination port number. Each of those packets is encapsulated in an IP packet, whose IP header contains a source IP address and a destination IP address. The IP address/protocol/port number triple defines an association with a network socket.
For publicly accessible services such as web and mail servers the port number is important. For example, port 80 connects through a socket to the
Private IP addresses as described in RFC 1918 are usable only on private networks not directly connected to the internet. Ports are endpoints of communication unique to that host, so a connection through the NAT device is maintained by the combined mapping of port and IP address. A private address on the inside of the NAT is mapped to an external public address. Port address translation (PAT) resolves conflicts that arise when multiple hosts happen to use the same source port number to establish different external connections at the same time.
Telephone number extension analogy
A NAT device is similar to a phone system at an office that has one public telephone number and multiple extensions. Outbound phone calls made from the office all appear to come from the same telephone number. However, an incoming call that does not specify an extension cannot be automatically transferred to an individual inside the office. In this scenario, the office is a private LAN, the main phone number is the public IP address, and the individual extensions are unique port numbers.[7]
Translation process
With NAT, all communications sent to external hosts actually contain the external IP address and port information of the NAT device instead of internal host IP addresses or port numbers. NAT only translates IP addresses and ports of its internal hosts, hiding the true endpoint of an internal host on a private network.
When a computer on the private (internal) network sends an IP packet to the external network, the NAT device replaces the internal source IP address in the packet header with the external IP address of the NAT device. PAT may then assign the connection a port number from a pool of available ports, inserting this port number in the source port field. The packet is then forwarded to the external network. The NAT device then makes an entry in a translation table containing the internal IP address, original source port, and the translated source port. Subsequent packets from the same internal source IP address and port number are translated to the same external source IP address and port number. The computer receiving a packet that has undergone NAT establishes a connection to the port and IP address specified in the altered packet, oblivious to the fact that the supplied address is being translated.
Upon receiving a packet from the external network, the NAT device searches the translation table based on the destination port in the packet header. If a match is found, the destination IP address and port number is replaced with the values found in the table and the packet is forwarded to the inside network. Otherwise, if the destination port number of the incoming packet is not found in the translation table, the packet is dropped or rejected because the PAT device doesn't know where to send it.
Applications
- Routing
- Network address translation can be used to mitigate IP address overlap.[8][9] Address overlap occurs when hosts in different networks with the same IP address space try to reach the same destination host. This is most often a misconfiguration and may result from the merger of two networks or subnets, especially when using RFC 1918 private network addressing. The destination host experiences traffic apparently arriving from the same network, and intermediate routers have no way to determine where reply traffic should be sent to. The solution is either renumbering to eliminate overlap or network address translation.
- Load balancing
- In client–server applications, load balancers forward client requests to a set of server computers to manage the workload of each server. Network address translation may be used to map a representative IP address of the server cluster to specific hosts that service the request.[10][11][12][13]
Related techniques
Issues and limitations
Hosts behind NAT-enabled routers do not have
End-to-end connectivity has been a core principle of the Internet, supported, for example, by the Internet Architecture Board. Current Internet architectural documents observe that NAT is a violation of the end-to-end principle, but that NAT does have a valid role in careful design.[15] There is considerably more concern with the use of IPv6 NAT, and many IPv6 architects believe IPv6 was intended to remove the need for NAT.[16]
An implementation that only tracks ports can be quickly depleted by internal applications that use multiple simultaneous connections such as an HTTP request for a web page with many embedded objects. This problem can be mitigated by tracking the destination IP address in addition to the port thus sharing a single local port with many remote hosts. This additional tracking increases implementation complexity and computing resources at the translation device.
Because the internal addresses are all disguised behind one publicly accessible address, it is impossible for external hosts to directly initiate a connection to a particular internal host. Applications such as
Fragmentation and checksums
Pure NAT, operating on IP alone, may or may not correctly parse protocols with payloads containing information about IP, such as ICMP. This depends on whether the payload is interpreted by a host on the inside or outside of the translation. Basic protocols as TCP and UDP cannot function properly unless NAT takes action beyond the network layer.
IP packets have a checksum in each packet header, which provides error detection only for the header. IP datagrams may become fragmented and it is necessary for a NAT to reassemble these fragments to allow correct recalculation of higher-level checksums and correct tracking of which packets belong to which connection.
TCP and UDP, have a checksum that covers all the data they carry, as well as the TCP or UDP header, plus a pseudo-header that contains the source and destination IP addresses of the packet carrying the TCP or UDP header. For an originating NAT to pass TCP or UDP successfully, it must recompute the TCP or UDP header checksum based on the translated IP addresses, not the original ones, and put that checksum into the TCP or UDP header of the first packet of the fragmented set of packets.
Alternatively, the originating host may perform path MTU Discovery to determine the packet size that can be transmitted without fragmentation and then set the don't fragment (DF) bit in the appropriate packet header field. This is only a one-way solution, because the responding host can send packets of any size, which may be fragmented before reaching the NAT.
Variant terms
DNAT
Destination network address translation (DNAT) is a technique for transparently changing the destination IP address of a routed packet and performing the inverse function for any replies. Any router situated between two endpoints can perform this transformation of the packet.
DNAT is commonly used to publish a service located in a private network on a publicly accessible IP address. This use of DNAT is also called port forwarding, or DMZ when used on an entire server, which becomes exposed to the WAN, becoming analogous to an undefended military demilitarized zone (DMZ).
SNAT
The meaning of the term SNAT varies by vendor:[17][18][19]
- source NAT is a common expansion and is the counterpart of destination NAT (DNAT). This is used to describe one-to-many NAT; NAT for outgoing connections to public services.
- stateful NAT is used by Cisco Systems[20]
- static NAT is used by WatchGuard[21]
- secure NAT is used by ISA Server)
Secure network address translation (SNAT) is part of Microsoft's
Dynamic network address translation
Dynamic NAT, just like static NAT, is not common in smaller networks but is found within larger corporations with complex networks. Where static NAT provides a one-to-one internal to public static IP address mapping, dynamic NAT uses a group of public IP addresses.[23][24]
NAT hairpinning
NAT hairpinning, also known as NAT loopback or NAT reflection,
The following describes an example network:
- Public address: 203.0.113.1. This is the address of the WAN interface on the router.
- Internal address of router: 192.168.1.1
- Address of the server: 192.168.1.2
- Address of a local computer: 192.168.1.100
If a packet is sent to 203.0.113.1 by a computer at 192.168.1.100, the packet would normally be routed to the default gateway (the router)[d] A router with the NAT loopback feature detects that 203.0.113.1 is the address of its WAN interface, and treats the packet as if coming from that interface. It determines the destination for that packet, based on DNAT (port forwarding) rules for the destination. If the data were sent to port 80 and a DNAT rule exists for port 80 directed to 192.168.1.2, then the host at that address receives the packet.
If no applicable DNAT rule is available, the router drops the packet. An
NAT in IPv6
Network address translation is not commonly used in IPv6 because one of the design goals of IPv6 is to restore end-to-end network connectivity.[27] The large addressing space of IPv6 obviates the need to conserve addresses and every device can be given a unique globally routable address. Use of unique local addresses in combination with network prefix translation can achieve results similar to NAT.
The large addressing space of IPv6 can still be defeated depending on the actual prefix length given by the carrier. It is not uncommon to be handed a /64 prefix – the smallest recommended subnet – for an entire home network, requiring a variety of techniques to be used to manually subdivide the range for all devices to remain reachable.[28] Even actual IPv6-to-IPv6 NAT, NAT66, can turn out useful at times: the APNIC blog outlines a case where the author was only provided a single address (/128).[29]
Applications affected by NAT
Some
Another possible solution to this problem is to use NAT traversal techniques using protocols such as STUN or Interactive Connectivity Establishment (ICE), or proprietary approaches in a session border controller. NAT traversal is possible in both TCP- and UDP-based applications, but the UDP-based technique is simpler, more widely understood, and more compatible with legacy NATs.[citation needed] In either case, the high-level protocol must be designed with NAT traversal in mind, and it does not work reliably across symmetric NATs or other poorly behaved legacy NATs.
Other possibilities are Port Control Protocol (PCP),[30] NAT Port Mapping Protocol (NAT-PMP), or Internet Gateway Device Protocol but these require the NAT device to implement that protocol.
Most client–server protocols (FTP being the main exception[e]), however, do not send layer 3 contact information and do not require any special treatment by NATs. In fact, avoiding NAT complications is practically a requirement when designing new higher-layer protocols today.
NATs can also cause problems where
Interactive Connectivity Establishment is a NAT traversal technique that does not rely on ALG support.
The DNS protocol vulnerability announced by
Examples of NAT software
- Internet Connection Sharing (ICS): NAT & DHCP implementation included with Windows desktop operating systems
- IPFilter: included with (OpenSolaris, FreeBSD and NetBSD, available for many other Unix-like operating systems
- ipfirewall (ipfw): FreeBSD-native packet filter
- Netfilter with iptables/nftables: the Linux packet filter
- NPF: NetBSD-native packet filter
- PF: OpenBSD-native packet filter
- Routing and Remote Access Service (RRAS): routing implementation included with Windows Server operating systems
- user spacepacket forwarding implementation for Linux
- WinGate: third-party routing implementation for Windows
See also
- Anything In Anything (AYIYA) – IPv6 over IPv4 UDP, thus working IPv6 tunneling over most NATs
- Carrier-grade NAT – NAT behind NAT within ISP.
- Gateway (telecommunications) – Connection between two network systems
- Internet Gateway Device Protocol (UPnP IGD) NAT-traversal method
- Middlebox – Intermediary box on the data path between a source host and destination host
- NAT Port Mapping Protocol (NAT-PMP) NAT-traversal method
- Port Control Protocol (PCP) NAT-traversal method
- Port triggering – NAT traversal mechanism
- Subnetwork– Logical subdivision of an IP network
- Teredo tunneling – NAT traversal using IPv6
Notes
- DMZ hostwhich passes all traffic received on the external interface (on any port number) to an internal IP address while preserving the destination port. Both types may be available in the same NAT device.
- ^ The more common arrangement is having computers that require end-to-end connectivity supplied with a routable IP address, while having others that do not provide services to outside users behind NAT with only a few IP addresses used to enable Internet access.
- ^ The port numbers are 16-bit integers. The total number of internal addresses that can be translated to one external address could theoretically be as high as 65,536 per IP address. Realistically, the number of ports that can be assigned a single IP address is around 4000.
- ^ Unless an explicit route is set in the computer's routing tables.
- ^ This issue can be avoided by using SFTP instead of FTP
References
- ISBN 9780974094526. Retrieved 2014-09-16.
- ^ .
- ^ Geoff Huston (September 2004). "Anatomy: A Look Inside Network Address Translators" (PDF). The Internet Protocol Journal.
- ^ S2CID 31082389.
- ^ "Characterization and Measurement of TCP Traversal through NATs and Firewalls". December 2006.
- ^ "Illuminating the shadows: Opportunistic network and web measurement". December 2006. Archived from the original on 2010-07-24.
- ^ "The Audio over IP Instant Expert Guide" (PDF). Tieline. January 2010. Archived from the original (PDF) on 2011-10-08. Retrieved 2011-08-19.
- ^ "Using NAT in Overlapping Networks". August 2005.
- ^ "VPNs with Overlapping Subnets Problem Scenario". September 2017.
- RFC 2391.
- ^ "What Is Layer 4 Load Balancing?". June 2020.
- ^ "What is load balancing?". November 2018.
- ^ "Configure Server Load Balancing Using Dynamic NAT". June 2018.
- S2CID 7657883.
- .
- .
- ^ "Enhanced IP Resiliency Using Cisco Stateful NAT". Cisco.
- ^ "Use NAT for Public Accessto Servers with Private IP Addresses on the Private Network (WatchGuard configuration example)" (PDF). www.watchguard.com. Archived from the original (PDF) on 2013-01-17.
- ^ "K7820: Overview of SNAT features". AskF5. August 28, 2007. Retrieved February 24, 2019.
- ^ "Enhanced IP Resiliency Using Cisco Stateful NAT". Cisco.
- ^ "Use NAT for Public Accessto Servers with Private IP Addresses on the Private Network (WatchGuard configuration example)" (PDF). www.watchguard.com. Archived from the original (PDF) on 2013-01-17.
- ^ "K7820: Overview of SNAT features". AskF5. August 28, 2007. Retrieved February 24, 2019.
- ^ "Dynamic NAT". 26 January 2016. Retrieved 2022-04-19.
- ^ "Dynamic NAT". Retrieved 2022-04-19.
- ^ "What is NAT Reflection/NAT Loopback/NAT Hairpinning?". NYC Networkers. 2014-11-09. Retrieved 2017-04-27.
- ^ "NAT Loopback Routers – OpenSim" (MediaWiki). OpenSimulator. 2013-10-21. Retrieved 2014-02-21.
- ^ Iljitsch van Beijnum (2008-07-23). "After staunch resistance, NAT may come to IPv6 after all". Ars Technica. Retrieved 2014-04-24.
- ^ Dupont, Kasper (Aug 18, 2015). "subnet - IPv6 subnetting a /64 - what will break, and how to work around it?". Server Fault. Retrieved 2023-04-20.
- ^ Cilloni, Marco (2018-02-01). "NAT66: The good, the bad, the ugly". APNIC Blog. Retrieved 2023-04-20.
- .
- Network World. Archived from the originalon 2009-02-13. Retrieved 14 June 2021.
External links
- Characterization of different TCP NATs at the Wayback Machine (archived 2006-01-11) – Paper discussing the different types of NAT
- Anatomy: A Look Inside Network Address Translators – Volume 7, Issue 3, September 2004
- Jeff Tyson, HowStuffWorks: How Network Address Translation Works
- Routing with NAT at archive.today (archived 2013-01-03) (Part of the documentation for the IBM iSeries)
- Network Address Translation (NAT) FAQ – Cisco Systems