Nagios

Nagios monitors your entire IT infrastructure to ensure systems, applications, services, and business processes are functioning properly. In the event of a failure, Nagios can alert technical staff of the problem, allowing them to begin remediation processes before outages affect business processes, end-users, or customers.

sample configuration:

1. host/host-group

    define host {
    use                linux-server ### Inherit default values from a template/template.cfg (placed at <nagios-home-dir>/etc/)
    host_name        jenkins ## Name of host
    alias              jenkins
    address            192.168.0.105 
    parents            MAC  ### check Nagios doc
    hostgroups    Linux ## optional

   }

   define hostgroup {
      hostgroup_name          Linux           ; The name of the hostgroup
      alias                   Linux            ; Long name of the group
      members                 localhost,jenkins               ;(Optional) Comma separated list of hosts that belong to          this group

   }

1. services/services-group:

      define service {
        use                     local-service           ; Name of service template to use
        hostgroup_name            Linux 
        service_description     PING
        check_command           check_ping!100.0,20%!500.0,60%  ; ! to specify arguments to commands
      }
      define servicegroup {
        servicegroup_name            HTTP
        alias                            HTTP
        members                        localhost,HTTP_80,DOCKER_HOST,HTTP_80,jenkins,HTTP_8080 ; host_name,service,host_name,service.....
      }

1. Commands:

   define command {
      command_name    check_ping
      command_line    $USER1$/check_ping -H $HOSTADDRESS$ -w $ARG1$ -c $ARG2$ -p 5 
    }

1. Command check_ping execute:
   /libexec/check_ping -H -w 100.0,20% -c 500.0,60% -p 5
   output: PING OK - Packet loss = 0%, RTA = 0.65 ms|rta=0.655000ms;100.000000;500.000000;0.000000 pl=0%;20;60;0
   Above output used by nagios server.

Note: Place services, commands and hosts file in /etc/.
Add below lines in nagios.cfg (/etc/)

         # Definitions for monitoring the  (Linux) host
          cfg_dir=/usr/local/nagios/etc/hosts
          # Definitions for service file
          cfg_dir=/usr/local/nagios/etc/services
          # commands files
          cfg_dir=/usr/local/nagios/etc/commands

Link: https://www.slashroot.in/what-nagios-introduction-enterprise-level-server-monitoring