Technical Tip: How to minimize occurrence of unexpected closure of putty sessions to FortiGate

Steps to configure Putty.

1. Open PuTTY.
2. Load saved connection settings, if available.
3. Navigate to the 'Connection' section.
4. In the 'Sending of null packets to keep session active' field, set it to 6 seconds. If experiencing network outages, setting it to 300 seconds might work better; see the image below for more info.

Rationale.

FortiGate monitors connections and may assume a session is inactive if no data is exchanged for a specific period. If this happens, FortiGate might unexpectedly close the PuTTY session due to the lack of traffic.

To avoid this, PuTTY offers a 'keepalive' feature (‘Seconds between keepalives’) that lets one set an interval for sending data through the session without interrupting the terminal activity. If FortiGate is disconnecting idle connections, try setting a non-zero value in this field. The value is in seconds; for example, if the FortiGate disconnects after ten minutes, set it to 300 seconds (5 minutes) to keep the session alive.