Thx for the replies guys.
I got the right syntax figured out now which is to use a "!" in order to suppress certain log messages.
Example:
:msg, !contains, "Connection closed by xxx.xxx.xxx.xxx port"
The working example I used is this:
/var/etc/syslog.d/myssh.conf
!sshd
:msg, !contains, "Connection closed by xxx.xxx.xxx.xxx port"
*.* /var/log/myssh.log
This logs now everything related to the sshd except the above defined string.
Eventually, I am pretty sure that this will not work with pfSense.
The reason for this is that the automatically created pfSense log config (based on the GUI) is also stored in the same dir: /var/etc/syslog.d/pfSense.conf
In this config, all auth message are logged:
# Automatically generated, do not edit!
!*
auth.*;authpriv.* /var/log/auth.log
Changing this is not possible as all changes are reverted once the syslogd is restarted. Therefore all sshd activities will be logged despite what is defined in additional config files in /var/etc/syslog.d/
Regarding the Check_MK check interval of 1min:
this is the default check interval for the active checks. To change this of a single check is not possible to my knowledge, but only for ALL active checks for a dedicated host. But maybe this is the way to investigate for me.
Appreciate the help here!