I configured monit to monitor the TLS certificate validity of all of our
highly productive websites. To all websites, the unnecessary full
certificate (without root CA) was installed. However, on 30th of May
2020 one of the chain certificates (COMODO) ran out of its validity
period. Obviously monit only checks for the server certificate, that's
why the check did not notice this, and such a check is completely
pointless. It led to a massive damage to my company, and since I was to
deal with monitoring as well as TLS certificates, I had to move on to
find a new job.
During the notice period, I implemented an own check in PHP and let
monit execute this PHP program to check TLS certificates. This PHP
program did not just check the entire chain, but also the chain against
the system's own trust store (in /etc/ssl/certs). I think it would be an
interesting feature to deal with TLS certificates like this in monit in
order to avoid more people losing the jobs.