D-Link NetDefend DFL-210 User Manual page 299

7.2. NAT
The Source IP Address Used for Translation
There are three options for how NetDefendOS determines the source IP address that will be used for
NAT:
• Use the IP Address of the Interface
When a new connection is established, the routing table is consulted to resolve the outbound
interface for the connection. The IP address of that resolved interface is then used as the new
source IP address when NetDefendOS performs the address translation. This is the default way
that the IP address is determined.
• Specify a Specific IP Address
A specific IP address can be specified as the new source IP address. The specified IP address
needs to have a matching ARP Publish entry configured for the outbound interface. Otherwise,
the return traffic will not be received by the NetDefend Firewall. This technique might be used
when the source IP is to differ based on the source of the traffic. For example, an ISP that is
using NAT, might use different IP addresses for different customers.
• Use an IP Address from a NAT Pool
A NAT Pool, which is a set of IP addresses defined by the administrator, can be used. The next
available address from the pool can be used as the IP address used for NAT. There can be one or
many NAT pools and a single pool can be used in more than one NAT rule. This topic is
discussed further in Section 7.3, "NAT Pools".
Applying NAT Translation
The following illustrates how NAT is applied in practice on a new connection:
1. The sender, for example 192.168.1.5, sends a packet from a dynamically assigned port, for
instance, port 1038, to a server, for example 195.55.66.77 port 80.
2. In this example, the Use Interface Address option is used, and we will use 195.11.22.33 as the
interface address. In addition, the source port is changed to a free port on the NetDefend
Firewall, usually one above 32768. In this example, we will use port 32789. The packet is then
sent to its destination.
3. The recipient server then processes the packet and sends its response.
Tip: Use NAT pools to get around the connection limit
The connection maximum per unique IP pair is normally adequate for all but the most
extreme scenarios. However, to increase the number of NAT connections that can exist
between the NetDefend Firewall and a particular external host IP, the NetDefendOS
NAT pools feature can be used which can automatically make use of additional IP
addresses on the firewall.
See Section 7.3, "NAT Pools" for more information about this topic.
192.168.1.5:1038 => 195.55.66.77:80
195.11.22.33:32789 => 195.55.66.77:80
195.55.66.77:80 => 195.11.22.33:32789
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Chapter 7. Address Translation