Many network engineers consider v6.0 the "sweet spot" for the 100D.
The FortiGate 100D reached End of Engineering (EOE) on September 29, 2022 and End of Support (EOS) on March 31, 2024. This means:
Using the phrase "BEST" with firmware downloads is concerning — there is no "best" unofficial source. Only official Fortinet downloads are legitimate and safe. Fortigate 100d Firmware Download BEST
This is older but stable and often easier to find on archive mirrors. It is less secure (missing fixes for Log4j and Heartbleed variants), but if the device is behind another firewall, it may suffice.
After downloading your "BEST" firmware, you must verify it. Fortinet publishes SHA256 checksums on the download page. Many network engineers consider v6
Example for FGT_100D-v6.0.15-build0330-FORTINET.out:
If the hash does not match, DELETE THE FILE. It is corrupted or malicious. The FortiGate 100D reached End of Engineering (EOE)
The BEST method requires a valid support contract. However, there is a nuance for EOL devices.