Central SNAT Policy Table Desynchronization Following FortiManager Push
Following policy package pushes containing reordered or newly inserted Central SNAT rules, a race condition in FortiOS desynchronizes the kernel policy engine, corrupting IP pool bindings and dropping outbound Internet traffic.
Technical Root Cause Analysis
Enterprises deploying FortiOS Central NAT decouple Source NAT definitions from the firewall policy list, centralizing all IP translation logic into the firewall central-snat-map database. In this design, egress packets evaluate the Central SNAT map sequentially to determine whether to perform PAT via outgoing interface IP or translate using a defined ippool object.
In FortiOS 7.2.3 through 7.2.6, when FortiManager pushes an updated policy package with modified Central SNAT rule priorities, the configuration engine (fcnfg) performs rapid sequential delete-and-reinsert operations to update rule indices. However, an internal race condition between fcnfg and the kernel policy acceleration engine (iprope) fails to preserve pointer bindings between the reordered rule IDs and the allocated IP pool structures. Consequently, matching outbound sessions trigger kernel NAT resolution faults (error: -ENOENT), and packets are dropped with central_snat_failed. Outbound web traffic, cloud SaaS connectivity, and internal server egress fail abruptly across enterprise subnets.
Affected Firmware & Blast Radius Matrix
| FortiOS Branch | Vulnerable Builds | Confirmed Clean Build | Status & Workaround |
|---|---|---|---|
| FortiOS 7.2 | 7.2.3 – 7.2.6 | 7.2.7+ | Re-index Central SNAT table via CLI |
| FortiOS 7.4 | 7.4.0 – 7.4.1 | 7.4.2+ | Atomic iprope pointer sync enabled |
| FortiOS 7.0 | Not Affected | 7.0.x Branch Clean | Legacy transactional locking active |
Platform Impact: Affects enterprise and data center FortiGate models (FG-100F up to FG-3000F) utilizing Central SNAT architecture and managed via FortiManager.
Step 01: Free Verification CLI (Safe Read-Only)
Execute these diagnostic commands to inspect active kernel NAT indices, verify central-snat-map configurations, and detect corrupted IP pool bindings:
Diagnostic Commands
# 1. Inspect kernel iprope rule table and active SNAT mapping indices
diagnose firewall iprope list 100008
# 2. Display configured Central SNAT map rules, policy IDs, and IP pool assignments
show firewall central-snat-map
# 3. Verify IP pool utilization, allocated addresses, and binding health
diagnose firewall ippool list
# 4. View recent FGFM script execution and Central SNAT deployment logs
diagnose test application fgfmd 9Remediation & Workaround Steps (Teaser Preview)
Review the complete non-disruptive kernel re-indexing protocol and FortiManager alignment guide in the Ferrite platform:
Frequently Asked Questions
During policy installations that reorder Central SNAT rules, a timing flaw between fcnfg and iprope unlinks IP pool pointers from the kernel policy table, dropping matching egress packets.
A packet trace using diagnose debug flow will reveal outbound drops with the explicit failure reason action=drop, reason=central_snat_failed.
Permanent fixes with atomic pointer binding synchronization are available in FortiOS 7.2.7 and 7.4.2+.