When SSL is enable in Webmin it creates a self-signed certificate which is stored in the file /etc/webmin/miniserv.pem. I recently upgraded a server from Debian 7 to Debian 9 and discovered that Webmin would no longer start.
Category: system administration
Waiting to Bite: BIND and Invalid Zone Files
It looks like Webmin lets you create DNS entries which BIND 9 does not like. When it sees one it refuses to load the whole zone and keeps what it has in RAM. This may go unnoticed for months until the server is rebooted. BIND then restarts with that zone completely empty!
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