Policy NAT vs Central NAT mode

There are 2 NAT modes in FortiGate: policy NAT mode and central NAT mode. Policy NAT mode requires NATs to be configured inside firewall policies, which is the default mode that FortiGate uses. Central NAT mode separates NATs and policies into 2 independent modules so policies do not reference NAT objects.

FortiConverter provides the option Enable Central NAT merge to control the NAT modes for the conversion of some 3rd party vendors, and the recommended mode is different depending on the vendor of the source configuration. When the recommended mode of each vendor is selected, the NAT conversion is more straightforward. It means that the NATs would be similar between the source and converted configuration. Hence, the number of policies and NAT objects do not change a lot, and it would be easier to review the conversion result.

In Juniper SSG and Forcepoint Sidewinder, NATs are configured inside firewall policies, which is similar to policy NAT mode. Therefore, the option is disabled by default. WatchGuard allows NATs to be configured both inside policies and in an independent list at the same time. Currently, FortiConverter only converts it into the policy NAT mode.

In Cisco, Check Point, Juniper SRX, Palo Alto, SonicWALL, Sophos, Huawei, and Forcepoint Stonesoft, NATs and policies are configured separately. Therefore, the option is enabled by default. On the contrary, the number of policies may greatly increase after converting these vendors into the policy mode because FortiConverter applies the “NAT merge” process to match the traffic of each NAT and each policy. This may create extra policies to perform the NAT behavior when the traffic overlaps. It is possible to get 2 or 3 times of policies after the NAT merge. For more details about NAT merge, please see the examples in Check Point and Cisco. In order to prevent users from reviewing a much larger policy list, central NAT mode should be the first choice.

However, in central NAT mode, FortiGate doesn’t allow dynamic NAT rules to translate a single internal address into different external addresses based on different services. For example, if there are 2 dynamic NATs in the source configuration, one translates 10.10.10.1 with HTTP into 20.10.10.1, and the other translates 10.10.10.1 with SMTP into 20.10.10.2, then there is no way to distinguish these NATs under central NAT mode. If there are many such dynamic NATs in the source configuration, please select policy mode instead.

The following table shows the difference between the 2 NAT modes:

	Policy NAT mode	Central NAT mode
Description	NATs are configured in policies.	NATs and policies are separated.
Related categories for dynamic NAT	config firewall ippool config firewall policy	config firewall ippool config firewall central-snat-map
Related categories for static NAT	config firewall vip config firewall policy	config firewall vip
Recommended in vendors	Juniper SSG, Forcepoint Sidewinder, WatchGuard	Cisco, Check Point, Juniper SRX, Palo Alto, SonicWALL, Sophos, Huawei, Forcepoint Stonesoft
Supported in vendors	Cisco, Check Point, Juniper, Palo Alto, SonicWALL, Sophos, WatchGuard, Forcepoint	Cisco, Check Point, Juniper, Palo Alto, SonicWALL, Sophos, Huawei, Forcepoint
Allow dynamic NAT based on services	Yes	No
May greatly increase the number of policies	Yes for Cisco, Check Point, Juniper SRX, Palo Alto, SonicWALL, Sophos, Huawei and Forcepoint Stonesoft	No

For more information about central NAT mode, please refer to the links(in FortiOS 7.2.4) below:

Central SNAT:

https://docs.fortinet.com/document/fortigate/7.2.4/administration-guide/421028/central-snat

Central DNAT:

https://docs.fortinet.com/document/fortigate/7.2.4/administration-guide/448790/central-dnat