Our servers usually behave nicely, but these are some tools to use to make sure we know if they aren't.
Recently, after an update of an Ubuntu 16.04 LTS VPS, we got the following connection issue:
$ nc -zvn IPADDRESS PORT
nc: connectx to IPADDRESS port PORT (tcp) failed: Connection refused
The following command yields more information:
"Missing priviledge separation directory: /var/run/sshd"
The fix in this case was easy enough:
(no issues reported)
service sshd restart