Network address translation (NAT) is a technique of transparently mapping an IP address or range to another IP address or range. Any routing device situated between two endpoints can perform this transformation of the packet. Network designers must however take one key element under consideration when laying out a network with NAT in mind. The router(s) performing NAT must have an opportunity to rewrite the packet upon entry to the network and upon exit from the network ^[29].

Because network address translation manipulates the addressing of a packet, the NAT transformation becomes a passive but critical part of the conversation between hosts exchanging packets. NAT is by necessity transparent to the application layer endpoints and operates on any type of IP packet. There are some application and even network layer protocols which will break as a result of this rewriting. Consult Section 5.2, “Application Layer Protocols with Embedded Network Information” for a discussion of these cases.

Here are a few common reasons to consider NAT along with potential NAT solution candidates shown in parentheses.

These are the commonest reasons to consider and implement NAT. Other niche applications of NAT, notably as part of load balancing systems, exist although this chapter will concentrate on the use of NAT to hide, isolate or renumber networks. It will also cover inbound connections, leaving the discussion of many-to-one NAT, SNAT and masquerading for Chapter 6, Masquerading and Source Network Address Translation.

One motivator for deploying NAT in a network is the benefit of virtualizing the network. By isolating services provided in one network from changes in other networks, the effects of such changes can be minimized. The disadvantage of virtualizing the network in this way is the increased reliance on the NAT device.

Providing inbound services via NAT can be accomplished in several different ways. Two common techniques are to use iproute2 NAT and netfilter DNAT. Less common (and possibly less desirable) one can use port redirection tools. Depending on which tool is employed, different characteristics of a packet can trigger the address transformation.

The simplest form of NAT under linux is iproute2 NAT. This type of NAT requires two matching commands, one to cause the kernel to rewrite the inbound packets (ip route add nat $NATIP via $REAL) and one to rewrite the outbound packets (ip rule add from $REAL nat $NATIP). The router configured in this fashion will retain no state for connections. It will simply transform any packets passing through. By contrast, netfilter is capable of retaining state on connections passing through the router and selecting packets more granularly than is possible with only iproute2 tools.

Before the advent of the netfilter engine in the linux kernel, there were several tools available to administer NAT, DNAT and PAT. These tools were not included in many distributions and weren't adopted broadly in the community. Although you may find references to ipmasqadm, ipnatadm and ipportfw across the Internet in older documentation, these tools have been superseded in functionality and widespread deployment by the netfilter engine and its userspace partner, iptables.

The netfilter engine provides a more flexible language for selection of packets to be transformed than that provided by the iproute2 suite and kernel routing functionality. Additionally, any NAT services provided by the netfilter engine come with the labor-saving and resource-consuming connection tracking mechanism. DNAT translates the address on an inbound packet and creates an entry in the connection tracking state table. For even modest machines, the connection tracking resource consumption should not be problematic.

Netfilter DNAT allows the user to select packets based on characteristics such as destination port. This blurs the distinction between network address translation and port address translation. NAT always transforms the layer 3 contents of a packet. Port redirection operates at layer 4. From a practical perspective, there is little difference between a port redirection and a netfilter DNAT which has selected a single port. The manner in which the packet and contents are retransmitted, however, is tremendously different.

One other less common technique for furnishing inbound services is the use of port redirection. Although there are higher layer tools which can perform transparent application layer proxying (e.g. Squid), these are outside the scope of this documentation.

There are a number of IP addresses involved in any NAT transformations or connection states. The following list identifies these names and the convention used to describe each IP address. Beware that the prevalance of NAT to publish services on the Internet via public IP addresses has lead to the server/client lingo common in discussions of NAT.

The above terms will be used below and in general discussions of NAT.