Dns service failed

Some general diagnostic and health checking advice appreciated please. Yesterday my mail server became unavailable due to the dns service being stopped - it would not resolve internet hostnames. i rebooted and server was ok. Later the same day my other email server (different domain) became unavailable also. I was unable to do any detailed diagnosis (phone login) so just rebooted and it was fine. 2 questions stem from this:

  1. is there a basic method to follow for diagnosing faults on a linux server - im slightly overwhelmed by the number of logs and am looking for some clever starting points! My main concern is that the system has been violated by an intruder

  2. is there a method for building a secondary mail server for each domain (there must be!). I assume it uses the same accounts provider but am not sure what “mail” config is required to make this work

thanks in advance

AFAIK dns runs after two services, you should stop to restart the server it doesn’t help to diagnostic what it is occurring

systemctl status unbound dnsmasq

after that you have to look after /var/log/messages and also /var/log/maillog

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you have some command to test and debug dns