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Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →Enabling Central NAT moves source NAT out of individual IPv4 firewall policies and into the Central SNAT table. The firewall policy still decides whether traffic is allowed; a matching central SNAT rule decides how that permitted traffic is source-translated. Before switching modes, recreate the required SNAT behavior centrally and unassign any VIPs attached to firewall policies. Then change the setting and validate representative traffic paths.
What changes when you enable Central NAT?
Source NAT moves to the Central SNAT table
With Central NAT enabled, FortiOS skips the NAT option under IPv4 firewall policies. Source NAT must instead be configured through central-snat-map. A policy can still allow a flow without supplying its source translation, so enabling the mode without corresponding central SNAT rules can leave expected outbound traffic untranslated. Fortinet documents this behavior for FortiOS 7.6.6 in its Central SNAT guide.
Central SNAT rules can match interfaces, addresses, and optional protocol or port details. FortiOS evaluates them top-down after security policy processing and uses the first matching rule. Put narrow matches ahead of broader rules to avoid an early catch-all selecting an unintended translation. The Fortinet FortiOS 7.4.7 CLI reference documents the central SNAT map fields; confirm the available fields and labels for your own release.
VIPs become separate DNAT objects
In central mode, destination NAT is represented by standalone VIP objects rather than being configured as part of a firewall policy. The firewall policy remains necessary to permit traffic to the translated destination. Fortinet’s FortiOS 7.6.2 static VIP guide places this configuration under Policy & Objects > DNAT & Virtual IPs. UI wording may vary by FortiOS build.
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Keep the three functions distinct: the security policy permits or denies the flow, the VIP supplies destination translation for an inbound flow, and the central SNAT rule supplies source translation when a rule matches.
What can break or behave differently?
Why did my outbound NAT stop working?
Existing per-policy IPv4 NAT settings are skipped in central mode. Do not assume FortiOS automatically creates equivalent central SNAT entries from those settings; the reviewed documentation does not promise such a conversion. Inventory each policy NAT use and create the intended central rules before relying on those flows.
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Do I need to remove VIPs from policies first?
Yes. Fortinet states that a VIP assigned to a firewall policy in non-central NAT mode must be unassigned before switching to central mode. The VIP object can carry over; remove its policy assignment rather than deleting the object. See Fortinet’s FortiOS 7.0.18 Central DNAT guide.
Could a central SNAT rule match the wrong traffic?
Yes. Since rules are checked top-down and evaluation stops at a match, an overly broad entry above a specific one can select the wrong translation. Review the order and match criteria together, especially when adding a catch-all.
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What about ICMP and explicit port mapping?
An explicit source-port mapping cannot match a portless protocol such as ICMP. Fortinet also limits explicit mapping with IP pools to Overload pools in the FortiOS 7.6.6 Central SNAT documentation. Check port-sensitive rules and pool type against the protocols you actually carry.
Why does a VIP flow still fail after translation?
DNAT changes the destination before routing, so a correct VIP alone is not enough. Confirm the mapped destination is reachable through routing and that a firewall policy permits the resulting flow. FortiManager’s FortiManager 7.2.6 central DNAT procedure describes DNAT as occurring before routing.
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Will existing sessions survive, and how long will a change take?
The cited Fortinet material does not establish a universal session-survival, outage-duration, or rollback guarantee across releases and topologies. Treat session impact as device- and deployment-specific; do not promise a hitless change based on the mode toggle alone.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Recommended order for switching to Central NAT
This is a cautious change sequence based on the documented mechanics, not a universal vendor migration procedure. Confirm the exact FortiOS release and NGFW mode on the device first; documentation here spans FortiOS 7.0.18, 7.4.7, 7.6.2, and 7.6.6, and labels or behavior may differ.
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- Record the current state. Export or back up the configuration. Inventory IPv4 policies with NAT enabled, IP pools, VIPs and their policy assignments, interfaces, address objects, and relevant routes.
- Translate current SNAT intent into rules. For each outbound NAT case, note source and destination, ingress and egress interfaces, protocol and ports, and whether translation should use the outgoing-interface address or an IP pool. Draft the corresponding central SNAT entries and place specific matches above broad ones.
- Prepare VIPs and permitting policies. Review each VIP mapping and the firewall policy that allows the resulting traffic. Unassign VIPs from policies while still in non-central mode, as Fortinet requires before the switch; retain the VIP objects you need.
- Check exceptions. Verify that explicit port mappings apply only to protocols with ports and that any associated IP pool type supports the mapping. Include ICMP and other portless traffic in the review.
- Enable the mode in a controlled change window. Fortinet documents enabling Central SNAT through System > Settings / System Operations Settings and applying the change, or with CLI:
config system settings, thenset central-nat enable. Check the exact UI path and CLI behavior for the installed release in the Central SNAT guide. - Validate representative paths. Test outbound traffic using interface NAT and each relevant IP pool, inbound VIP and port-forwarded flows, non-NAT traffic, and relevant ICMP. Confirm the selected central SNAT rule, firewall policy decision, route, and translated destination or source. Verification commands differ by release, so use the device’s matching Fortinet guide.
- Keep a release-specific rollback plan. Decide in advance how you will disable Central NAT and restore the recorded configuration if validation fails. The documentation shows the enable/disable setting but does not define one universal rollback sequence or session impact; account for the device and topology.
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