Define a Hostgroup Debian
for all your Debian 9 client and Server
 #  vi /etc/icinga2/conf.d/groups.conf   object HostGroup "debian9" {
display_name = "Debian 9"

assign where host.vars.os == "debian9"
}
 
         
add definition to a Host #  cd /etc/icinga2/zones.d/master/ open your hostfile endig with *.conf object Host "client.kozo.ch" {

check_command = "hostalive" //check is executed on the master
display_name = "client name text"
address = "10.147.42.63"
vars.os = "debian9"

vars.client_endpoint = "name" //follows the convention that host name == endpoint name
}
 
         
do a check if it is OK #  icinga2 daemon -C      
and if OK - restart #   systemctl reload icinga2      
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NOW --> add service to a hostgroup
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