While there are a few reasons for using NAT, the primary reason that networks use RFC 1918 addressing is to reduce IP address consumption. Routable IP addresses are expensive and limited in number. A specific form of NAT called Overload NAT provides a useful solution to this problem. Overload NAT, also known as Port Address Translation (PAT), works differently from normal one-to-one NAT, whereby each RFC 1918 address is converted to its own unique routable IP address. In Overload NAT, the RFC 1918 addresses are translated to a small number of routable IP addresses (often just one routable IP address, frequently that of the router's external interface).
The device doing PAT distinguishes between the traffic destined for the various RFC 1918 addresses by tracking the source TCP or UDP ports used when the connection is initiated. Figure 6-1 shows how PAT works.
NAT is sometimes confused with proxy servers. However, these are two completely different entities. NAT is a Layer 3 occurrence that uses Layer 4 information when doing PAT. On the other hand, proxy servers usually work on Layer 4 or higher of the OSI model. The most significant difference between the two mechanisms is the way they work. Although NAT is completely transparent to the source and destination devices, proxy servers require the source machine to be configured to make requests to the proxy server, which then facilitates the connection on behalf of the source machine. This is completely different from NAT, in which neither the source nor the destination machines need to know about the device doing the NAT.
Security Benefits of Network Address Translation
NAT used in PAT mode can be a source of security for the network that is using PAT to translate its private addresses.
To understand this, assume that the device doing the NAT is a router that is sitting on the edge of the network, with one interface connected to the RFC 1918 private network and another interface connected to the Internet. When a device sitting behind the router wants to go out to the Internet, it sends packets to the router. The router then translates the source address into a globally routable address and puts the source IP address and source TCP or UDP port number in its NAT tables.
When the reply packets are delivered to the router, destined for the globally routable IP address, the router looks at the destination port number (remember, the source and destination port numbers are flipped in the return packet). Based on a lookup of its NAT table, the router determines which RFC 1918 address to send the packet to. It changes the destination address to the private address found in its NAT table and forwards the packet appropriately. The important point to realize in this operation is that the router can change back the IP address only if it has an entry for the destination port number in its NAT table. If for some reason that entry got cleared out and the router received a packet destined for the globally routable address, it would simply discard it, because it would not know where to send the packet. This property is at the crux of PAT's secure nature. Unless a NAT (PAT) entry is created on the router in its NAT table that contains the port number and private to global address mapping, the router does not forward any packets to the RFC 1918 network. Therefore, any connections not initiated from the inside are not allowed through the PAT device. This is a significant measure of security. However, you will see in the next section why this is not the type of security you can really rely on.
Disadvantages of Relying on NAT for Security
Although NAT provides some level of protection to the networks sitting behind it, it is important to understand that it is by no means a comprehensive security solution.
The following sections outline some of the most obvious reasons why NAT should not be considered a security mechanism, despite the illusion of security it provides.
No Tracking of Protocol Information Other Than the Port Number Information
The NAT table, created on the NAT device that is used to track the outgoing and incoming connections, does not track any of the other information contained in the packets. Information such as the packet sequence numbers, the TCP handshake, and UDP progress-based timers are some of the pieces of information that most firewalls track in order to prevent the usage of the connections established through the firewall by attackers spoofing IP addresses. NAT does not track any of this information, because it does not need to for the purposes of creating and maintaining NAT translations. Chapter 8, "PIX Firewall," contains a detailed discussion of the algorithm that PIX Firewall uses to provide security.
No Restriction on the Type of Content Flowing Based on PAT Tables
NAT also does not concern itself with protecting the hosts from malicious data being sent on the NAT connections established by the hosts themselves. You can only protect your network from such malicious content by having a firewall and an intrusion detection system in place.
Limited Control on Initial Connections
NAT does not have any real control over who can initiate connections from the inside network to the outside network. Although an access list is configured to define which hosts can initiate NAT connections, this is a rudimentary measure at best.
By using route maps and extended access control lists, you can put further restraints on what traffic can be NATed. However, this is not the ideal way to restrict traffic. It is difficult to implement with the same granularity as standard access control mechanisms, and it can be resource-intensive for the router.
In light of this, NAT is a useful mechanism for increasing the available IP address space. It can also be a convenient tool for some other network design aspects. However, it should not be relied on to provide security. When used on a security device such as a firewall in conjunction with other security features, NAT provides definite enhancements to the security provided by the firewall. However, it should not be used in isolation as a security mechanism. It does provide some measure of security. However, this always needs to be enhanced with additional tools and products designed specifically with security in mind, such as firewalls and intrusion detection systems.
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