About Dynamic NAT Source IP Addresses

In the default dynamic NAT configuration, the XTM device changes the source IP address for traffic that goes out an external interface to the primary IP address of the external interface the traffic leaves. You can optionally configure dynamic NAT to use a different source IP address. You can set the dynamic NAT source IP address in a network NAT rule or in the NAT settings for a policy. When you select a source IP address, dynamic NAT uses the specified source IP address for any traffic that matches the dynamic NAT rule or policy.

Whether you specify the source IP address in a network dynamic NAT rule or in a policy, it is important that the source IP address is on the same subnet as the primary or secondary IP address of the interface from which the traffic is sent. It is also important to make sure that the traffic the rule applies to goes out through only one interface.

If the dynamic NAT source IP address is not on the same subnet as the primary or secondary IP address of the outgoing interface for that traffic, the XTM device does not change the source IP address for each packet to the source IP address specified in the dynamic NAT rule. Instead, it changes the source IP address to the primary IP address of the interface from which the packet is sent.

Set the Dynamic NAT Source IP Address in a Network Dynamic NAT rule

If you have a WatchGuard XTM 21, 22, or 23 device, this feature is not available for your device.

If you want to set the source IP address for traffic that matches a dynamic NAT rule, regardless of any policies that apply to the traffic, add a network dynamic NAT rule that specifies the source IP address. The source IP address you specify must be on the same subnet as the primary or secondary IP address of the interface the traffic leaves.

If the To location in the network dynamic NAT rule specifies an alias, such as Any-External, that includes more than one interface, the source IP address is used only for traffic that leaves an interface that has an IP address on the same subnet as the source IP address.

For example, if:

The result is:

For more information, see Add Network Dynamic NAT Rules.

Set the Dynamic NAT Source IP Address in a Policy

If you want to set the source IP address for traffic handled by a specific policy, configure the source IP address in the network settings of the policy. The source IP address you specify must be on the same subnet as the primary or secondary IP address of the interface you specified for outgoing traffic in the policy.

We recommend that you do not use the Set source IP option in a policy if you have more than one external interface configured on your XTM device. If you use the Set source IP option in a policy, do not enable policy-based routing with failover in the policy settings.

For more information, see Configure Policy-Based Dynamic NAT.

See also

About Dynamic NAT

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