Found the answer:
The master server polls the storage node web server. (/fog/status/bandwidth.php?dev=eth-device-name)
If the master server is HTTPS enabled, the storage nodes must expose the APIs in HTTPS too.
I recently upgraded to 1.5.9 and turned on HTTPS on our Fog server (with the --force-https flag and a custom certificate from our internal PKI). I also updated the storage nodes to 1.5.9. Everything seems to work fine so far.
However, the dashboard does not report the disk usage statistics of my storage nodes.
Could you please explain how is this information obtained by the Fog server ?